Damian

Now about some good stuff, heavy armor.

T-72M1, we have around 350 these tanks in total, with around 159 in active service. No upgrades are planned for them, and are meant to be replaced as soon as it will be possible.

PT-91 "Twardy", Poland have around 232 of these in active service. Improvements over standard T-72M1 include, new 850HP diesel S-12U, new explosive reactive armor ERAWA-1 and ERAWA-2 (I will post separate informations about them later), fire control system Drawa or it's improved variant Radew, improved TPD-K1 day sight and also thermal sight, there are two variants of thermal sights, older Israeli made TES, and new Polish 2nd gen FLIR KLW-1 "Asteria". Vehicle is also equipped with self defense system SSP-1 Obra-3 which includes both laser warning receivers, and new smoke dischargers Tellur mounted on the both sides of turret. Other major improvements were only included in export variants PT-91Ex, PT-91M and PT-91P. There was also a technology demonstrator called PT-16 that is a proposal for T-72M1 and PT-91 tanks upgrade, however the proposal contains two solutions for turret, either rebuild + addon armor for existing turret, which was presented on the technology demonstrator, and second, probably new welded low profile turret based on the UMPG Anders design.


PT-16 technology demonstrator.

Poland purchased from Germany 142 Leopard 2A4 tanks, currently upgrade codenamed Leopard 2PL is developed by Rhinemetall and polish industry.

Leopard 2PL technology dekonstrator.

Besides Leopard 2A4's, also 105 Leopard 2A5's are in service.

BWP-1 as BMP-1 is called here, we still have around 1268 of these antiques in service, tough replacement for them is currently in development, codename "Borsuk". It is still uncertain if "Borsuk" will be based on domestically designed chassis, or on foreign one purchased via license, both options are currently considered, also issue with amphibious capabilities are debated, initially there was requirement for new IFV to be amphibious, but after intelligence reports from Ukraine, now it's considered that perhaps heavier armor is a better choice.
 
Also new IFV "Borsuk" will be equipped with unmanned turret ZSSW-30. Armed with 30mm ATK Mk44, 7,62mm coax, and Spike ATGM's. Same turret is also intended as upgrade for Rosomak wheeled IFV.


Early concept model of IFV "Borsuk".

Now about Rosomak, this is the basic Rosomak M1 variant which is literally just wheeled IFV, equipped with manned turret Hitfist-30P armed with 30mm Mk44 and 7,62mm coax, there is also planned upgrade for these turrets to equip them with Spike ATGM's launchers.

Here is also IFV variant with unmanned turret ZSSW-30 prototype on the firing range.
 
There were many more specialized variants of Rosomak developed. For example fire support variant with 105mm gun or 120mm gun, yet these are not ordered by Polish Army, tough 120mm variant might be eventually ordered.

Rosomak/Wilk armed with 120mm gun.

Rosomak-M with upgraded Hitfist-30P turret, as well as new addon armor acting also as buoyancy elements improving vehicles amphibious capabilities.

Experimental Rosomak-XP with improved suspension and armor protection + some minor improvements.

Rosomak WRT, you can consider it as ARV variant of Rosomak family.

Rosomak WEM.
 
Of course there are many more variants in development.
 
Another interesting R&D program was UMPG Anders for modular tracked platform.

Single technology demonstrator was build and later to prove it's modularity reconfigured in to several variants, these were.

Light tank with 120mm gun in low profile manned turret.

Light tank with 105mm gun in CT-CV turret.

IFV variant with Hitfist-30P turret.

IFV with unmanned Hitfist-OWS turret.

Unmanned autonomous tank destroyer with Spike ATGM's in AMUR weapons module.

Improved IFV variant with new version of Hitfist-30P turret with Spike ATGM launchers.
 
 
 

With all of the above, we finally have a formula for why NERA is so prolific:
 
The basic issue is long rods and shaped charges penetrate, to a first approximation, in proportion to their length. Accordingly, the first order of business is to deform or break up the penetrator. This is why NERA and ERA are so good - because only reactive armours can even attempt to do this. It also helps that, as a byproduct of moving towards the penetrator, the reactive armour will tend to feed material into it and effectively increase the volume of material that the penetrator has to plough through.
This does not mean, however, that material properties are useless beyond mere density - high hardness can provide a 30-40% improvement in TE at 'lower' velocities (ie: sub-3km/s). Even where density is concerned, there are optima to be found which can increase ME and TE significantly.
 
So here, then, is the recipe for NERA - an array of modules arranged at a significant angle to the penetrator, the modules being composed of two plates seperated by an incompressible solid (or a liquid, if you can find a way to keep it from leaking). The penetrator, striking the module, forces the plates away from each other due to the action of the incompressible middle layer. Both, in deforming, will tend to feed material into the path of the penetrator, with the facing plate potentially applying a shearing force to the penetrator as well. Finally; where long rods are concerned the rod will tend to bend towards the module during penetration - something which may be less pronounced where the rod is more rigid.
As to the materials and exact composition: equally-sized RHA plates seperated by rubber will do (rubber being the solid which comes closest to being incompressible), but my guess is that unevenly-sized plates would provide an advantage in terms of optimising their function. Here my guess is a lighter (thinner) facing plate and heavier (thicker) backing plate, as this will tend to maximise the shear forces applied. While doing so, the best approach might also be to use a high-hardness titanium facing plate in order to get the highest mass efficiency and plate velocity.
Further improvements might be to put a thin skin of high-density material in front of the modules, to eat up the highest-velocity portions of a shaped charge tip. A ceramic sandwich array might also be a good option. Behind the array, it might be a good idea to have a high mass-efficiency plate (possibly a high-hardness aluminium alloy) to deal with residual penetration. This, in turn, might be backed by a final layer of RHA to provide structural support and a last, high-ME barrier. The whole arrangement; modules, front and back plates etc; should ideally be enclosed in an armoured box for ease of repair and replacement.

So yeah, maybe I gonna show here some of the more recent development. Let's start with our latest projects, the MSBS and PR-15.
MSBS (Modułowy System Broni Strzeleckiej - Modular Small Arms System) as it's codename says is program focused on developing new family of modular small arms for military, law enforcement and civilian use.
The first variant is developed for NATO 5.56x45mm cartridge.
What is interesting is that MSBS is based on a single, common upper receiver to which various modules can be attached thus creating several weapons types in both classic and bullpup configuration.


1. From top to bottom in classic configuration.
- Carbine
- Rifle
- Rifle + under barrel granade launcher
- Magazine fed LMG
- Marksman rifle
- Ceremonial rifle
2. From top to bottom in bullpup configuration.
- Carbine
- Rifle
- Rifle + under barrel granade launcher
- Magazine fed LMG
- Marksman rifle

Also MSBS for 7.62x51mm cartridge is being in development, both as marksman rifle to replace the SVD, and also as a battle rifle.

MSBS was also adopted for 7.62x39mm cartridge seen here compared to 5.56x45mm variant.

Also a 12 gauge shotgun variant is considered and seems to be in early development stage.
Worth to mention is that also a 40mm UBGL was redesigned.

Original design was replaced with new, more classic and more compact one.



Also older GPBO-40 and other types of UBGL can be used.



 
Here is episode of Our Army where we can see a great presentation of MSBS ergonomics and functionality. It starts at 3:20.
Few words of my own experience with the rifle, it's a bit heavier than standard wz.96 Beryl rifle I am now issued with as a reservist in the army, however MSBS is extremely ergonomic, in terms of ergonomy it's better than AR15 or G36 I also had chance to get some time with.

Also during last MSPO I talked with one of the FB engineers working on the project and it seems everything is going well, rifle meet and as I was said even exceeded reliability requirements.

MSBS will also receive new magazines despite a fact it can use standard STANAG mags.

A standard 30 round magazine.

10 round magazine.

Special magazine for blanks only for use in ceremonial rifle variant.
Another new weapon designed by Fabryka Broni is new service pistol PR-15 it's general design can be called evolution of MAG-95/98 pistol, also designed by Fabryka Broni, it's fully ambidextreous, and is a really nice pistol, also had it in my hands during last MSPO and it feels good, very ergonomic.


 
That's all for now.

So yeah, maybe I gonna show here some of the more recent development. Let's start with our latest projects, the MSBS and PR-15.
MSBS (Modułowy System Broni Strzeleckiej - Modular Small Arms System) as it's codename says is program focused on developing new family of modular small arms for military, law enforcement and civilian use.
The first variant is developed for NATO 5.56x45mm cartridge.
What is interesting is that MSBS is based on a single, common upper receiver to which various modules can be attached thus creating several weapons types in both classic and bullpup configuration.


1. From top to bottom in classic configuration.
- Carbine
- Rifle
- Rifle + under barrel granade launcher
- Magazine fed LMG
- Marksman rifle
- Ceremonial rifle
2. From top to bottom in bullpup configuration.
- Carbine
- Rifle
- Rifle + under barrel granade launcher
- Magazine fed LMG
- Marksman rifle

Also MSBS for 7.62x51mm cartridge is being in development, both as marksman rifle to replace the SVD, and also as a battle rifle.

MSBS was also adopted for 7.62x39mm cartridge seen here compared to 5.56x45mm variant.

Also a 12 gauge shotgun variant is considered and seems to be in early development stage.
Worth to mention is that also a 40mm UBGL was redesigned.

Original design was replaced with new, more classic and more compact one.



Also older GPBO-40 and other types of UBGL can be used.



 
Here is episode of Our Army where we can see a great presentation of MSBS ergonomics and functionality. It starts at 3:20.
Few words of my own experience with the rifle, it's a bit heavier than standard wz.96 Beryl rifle I am now issued with as a reservist in the army, however MSBS is extremely ergonomic, in terms of ergonomy it's better than AR15 or G36 I also had chance to get some time with.

Also during last MSPO I talked with one of the FB engineers working on the project and it seems everything is going well, rifle meet and as I was said even exceeded reliability requirements.

MSBS will also receive new magazines despite a fact it can use standard STANAG mags.

A standard 30 round magazine.

10 round magazine.

Special magazine for blanks only for use in ceremonial rifle variant.
Another new weapon designed by Fabryka Broni is new service pistol PR-15 it's general design can be called evolution of MAG-95/98 pistol, also designed by Fabryka Broni, it's fully ambidextreous, and is a really nice pistol, also had it in my hands during last MSPO and it feels good, very ergonomic.


 
That's all for now.

It's an older video but worth to watch.
 
I will describe situation with our heavy armor a bit later, but now also very important topic, artillery.
 
Let's start with new 155mm self propelled howitzers. There are two projects, wheeled and tracked, let's start with the later one.
 
155mm AHS Krab is a tracked self propelled howitzer based on modified british AS90/52 turret and south korean K9 chassis, initially desire was to use domestically designed and produced UPG chassis, but because it was based on T-72 tank components that were designed for vehicle weighting below 50 metric tons, and Krab weighted 55 metric tons, it's performance was rather poor. Also quality of that chassis didn't meet army requirements so decision was made to use foreign chassis in the end. Only 8 AHS Krab vehicles on older UPG chassis were made, and are currently used for crews training.
This is old Krab on UPG chassis.
And this is new Krab on K9 chassis.
Currently AHS Krab passed all factory and state trails and is mass produced at the moment with first units being inducted in to service. Hulls are now made in South Korea, but production of these hulls will be moved to Poland. The good thing is we purchased full license, which means we can also modify these hulls design and use for other applications, as well as reexport them. Also if Norway or other European countries would purchase South Korean K9 Thunder, it's possible that these would have made hulls in Poland, and Poland would become service center for them.
 
AHS Krab will replace 122mm AHS 2S1 Gvozdika and partially 152mm AHS Dana.
Here you can watch production capabilities of the HSW factory.
Another project, directly to replace AHS Dana is AHS Kryl.
AHS Kryl is designed as lightweight and very mobile system to supplement heavier AHS Krab. While Krab is intended to support heavy formations like Armored and Armored Cavalry Brigades, Kryl will support mainly medium formations builded around family of wheeled combat vehicles Rosomak. Kryl uses Israeli ATMOS 2000 howitzer based on new model of Jelcz truck.
 
To supplement these, there is also developed universal turret module with 120mm mortar Rak with autoloader, there will be two variants, wheeled SMK 120 Rak-K, and tracked SMG 120 Rak-G. Right now there are performed final preparations for starting mass production of SMK 120 Rak-K based on Rosomak chassis.
As I mentioned, turret module is universal, so can be installed on wheeled and tracked platforms, but also on static platforms/fortifications or navy vessels.
 
Now about rocket artillery.
 
The primary MLRS currently used is WR-40 Langusta, which is modrnized BM-21 Grad, wich increased range, crew protection, mobility, communications etc.
There was also project of WR-40 Langusta-2 which had additional missile rack with autoloader for rapid reload.

It was however not purchased yet by Polish Army, it might happen when more WR-40's will be ordered to replace last BM-21's and RM-70's.
 
The last project is WR-300 Homar, right now the prefered offer is polonized US M142 HIMARS system.

This is how it can look more or less, using the same Jelcz truck model as 155mm AHS Kryl.
 
Additional double purpose system are NSM missiles used in Coastal Missile Squadron (currently one in service, second on order), besides capability to attack enemy vessels, it have also secondary mode, that gives it capability to attack and destroy ground targets.

It's an older video but worth to watch.
 
I will describe situation with our heavy armor a bit later, but now also very important topic, artillery.
 
Let's start with new 155mm self propelled howitzers. There are two projects, wheeled and tracked, let's start with the later one.
 
155mm AHS Krab is a tracked self propelled howitzer based on modified british AS90/52 turret and south korean K9 chassis, initially desire was to use domestically designed and produced UPG chassis, but because it was based on T-72 tank components that were designed for vehicle weighting below 50 metric tons, and Krab weighted 55 metric tons, it's performance was rather poor. Also quality of that chassis didn't meet army requirements so decision was made to use foreign chassis in the end. Only 8 AHS Krab vehicles on older UPG chassis were made, and are currently used for crews training.
This is old Krab on UPG chassis.
And this is new Krab on K9 chassis.
Currently AHS Krab passed all factory and state trails and is mass produced at the moment with first units being inducted in to service. Hulls are now made in South Korea, but production of these hulls will be moved to Poland. The good thing is we purchased full license, which means we can also modify these hulls design and use for other applications, as well as reexport them. Also if Norway or other European countries would purchase South Korean K9 Thunder, it's possible that these would have made hulls in Poland, and Poland would become service center for them.
 
AHS Krab will replace 122mm AHS 2S1 Gvozdika and partially 152mm AHS Dana.
Here you can watch production capabilities of the HSW factory.
Another project, directly to replace AHS Dana is AHS Kryl.
AHS Kryl is designed as lightweight and very mobile system to supplement heavier AHS Krab. While Krab is intended to support heavy formations like Armored and Armored Cavalry Brigades, Kryl will support mainly medium formations builded around family of wheeled combat vehicles Rosomak. Kryl uses Israeli ATMOS 2000 howitzer based on new model of Jelcz truck.
 
To supplement these, there is also developed universal turret module with 120mm mortar Rak with autoloader, there will be two variants, wheeled SMK 120 Rak-K, and tracked SMG 120 Rak-G. Right now there are performed final preparations for starting mass production of SMK 120 Rak-K based on Rosomak chassis.
As I mentioned, turret module is universal, so can be installed on wheeled and tracked platforms, but also on static platforms/fortifications or navy vessels.
 
Now about rocket artillery.
 
The primary MLRS currently used is WR-40 Langusta, which is modrnized BM-21 Grad, wich increased range, crew protection, mobility, communications etc.
There was also project of WR-40 Langusta-2 which had additional missile rack with autoloader for rapid reload.

It was however not purchased yet by Polish Army, it might happen when more WR-40's will be ordered to replace last BM-21's and RM-70's.
 
The last project is WR-300 Homar, right now the prefered offer is polonized US M142 HIMARS system.

This is how it can look more or less, using the same Jelcz truck model as 155mm AHS Kryl.
 
Additional double purpose system are NSM missiles used in Coastal Missile Squadron (currently one in service, second on order), besides capability to attack enemy vessels, it have also secondary mode, that gives it capability to attack and destroy ground targets.

It's an older video but worth to watch.
 
I will describe situation with our heavy armor a bit later, but now also very important topic, artillery.
 
Let's start with new 155mm self propelled howitzers. There are two projects, wheeled and tracked, let's start with the later one.
 
155mm AHS Krab is a tracked self propelled howitzer based on modified british AS90/52 turret and south korean K9 chassis, initially desire was to use domestically designed and produced UPG chassis, but because it was based on T-72 tank components that were designed for vehicle weighting below 50 metric tons, and Krab weighted 55 metric tons, it's performance was rather poor. Also quality of that chassis didn't meet army requirements so decision was made to use foreign chassis in the end. Only 8 AHS Krab vehicles on older UPG chassis were made, and are currently used for crews training.
This is old Krab on UPG chassis.
And this is new Krab on K9 chassis.
Currently AHS Krab passed all factory and state trails and is mass produced at the moment with first units being inducted in to service. Hulls are now made in South Korea, but production of these hulls will be moved to Poland. The good thing is we purchased full license, which means we can also modify these hulls design and use for other applications, as well as reexport them. Also if Norway or other European countries would purchase South Korean K9 Thunder, it's possible that these would have made hulls in Poland, and Poland would become service center for them.
 
AHS Krab will replace 122mm AHS 2S1 Gvozdika and partially 152mm AHS Dana.
Here you can watch production capabilities of the HSW factory.
Another project, directly to replace AHS Dana is AHS Kryl.
AHS Kryl is designed as lightweight and very mobile system to supplement heavier AHS Krab. While Krab is intended to support heavy formations like Armored and Armored Cavalry Brigades, Kryl will support mainly medium formations builded around family of wheeled combat vehicles Rosomak. Kryl uses Israeli ATMOS 2000 howitzer based on new model of Jelcz truck.
 
To supplement these, there is also developed universal turret module with 120mm mortar Rak with autoloader, there will be two variants, wheeled SMK 120 Rak-K, and tracked SMG 120 Rak-G. Right now there are performed final preparations for starting mass production of SMK 120 Rak-K based on Rosomak chassis.
As I mentioned, turret module is universal, so can be installed on wheeled and tracked platforms, but also on static platforms/fortifications or navy vessels.
 
Now about rocket artillery.
 
The primary MLRS currently used is WR-40 Langusta, which is modrnized BM-21 Grad, wich increased range, crew protection, mobility, communications etc.
There was also project of WR-40 Langusta-2 which had additional missile rack with autoloader for rapid reload.

It was however not purchased yet by Polish Army, it might happen when more WR-40's will be ordered to replace last BM-21's and RM-70's.
 
The last project is WR-300 Homar, right now the prefered offer is polonized US M142 HIMARS system.

This is how it can look more or less, using the same Jelcz truck model as 155mm AHS Kryl.
 
Additional double purpose system are NSM missiles used in Coastal Missile Squadron (currently one in service, second on order), besides capability to attack enemy vessels, it have also secondary mode, that gives it capability to attack and destroy ground targets.

Merkava Mk1, Mk2 and Mk3 have indeed a magazine on the turret floor.
Here you can see how ammo is stored inside vehicle.
In case of Merkava Mk4, ready magazine is isolated behind armored bulkhead in turret bustle.

 
As for how ammo is again loaded in to the ready racks, it's simple, if there is time, loader will take one round from storage container at a time, and load it in to the ready rack.

I guess in case of emergency, main gun can be directly reloaded with ammo from storage containers.

Merkava Mk1, Mk2 and Mk3 have indeed a magazine on the turret floor.
Here you can see how ammo is stored inside vehicle.
In case of Merkava Mk4, ready magazine is isolated behind armored bulkhead in turret bustle.

 
As for how ammo is again loaded in to the ready racks, it's simple, if there is time, loader will take one round from storage container at a time, and load it in to the ready rack.

I guess in case of emergency, main gun can be directly reloaded with ammo from storage containers.

Merkava Mk1, Mk2 and Mk3 have indeed a magazine on the turret floor.
Here you can see how ammo is stored inside vehicle.
In case of Merkava Mk4, ready magazine is isolated behind armored bulkhead in turret bustle.

 
As for how ammo is again loaded in to the ready racks, it's simple, if there is time, loader will take one round from storage container at a time, and load it in to the ready rack.

I guess in case of emergency, main gun can be directly reloaded with ammo from storage containers.

Its funny, I never noticed that bit about the possibility of the transverse mounted AVDS-1790 in an Abrams before.  I know I had asked my dad once about transverse mounting the AVDS and he said Teledyne had asked Allison long ago about designing a transmission for such a mounting and Allison said there wouldn't be enough room for the linkages.  The reality was that Allison didn't want to do it.  For Allison, it was more important that they didn't piss off General Dynamics, who would not have appreciated Allison helping out their competitor, Teledyne Continental. 

We've had this link in the resources section for a while on TMEPS.  The engine was not "a standard AGT-1500," as the document explains, it had a new high pressure turbine rotor, new high pressure turbine cylinder, new recuperator, new power turbines, a new electronic control unit and a new auxiliary power take off.

Ok, so let's start to share here some informations and fight with some myths. I will start with M1, people that know me, know this is my black horse. So let's start with history and then move on. Historical description is a quote from Richard P. Hunnicutt Abrams - A History Of The American Main Battle Tank Vol.2.
M1 Abrams
History
Below is the LK 10322 design concept with heavy protection, rear engine, and a conventional turret.

Although Congress cancelled the XM803 program in late 1971, the Army's need for a new tank was recognized. To meet this requirement, 20 million dollars were authorized for the procurement of two prototypes of a new main battle tank. This was in addition to the 20 million dollars allocated to phase out the XM803. Although the funds were provided specifically for two industrial prototype tanks, the Army initially preferred to concentrate on component development until a specification could be prepared for a new main battle tank. At that time, there was considerable controversy within the Army itself as to what form the tank should take. In February 1972, the Main Battle Tank Task Force (MBTTF) was established at Fort Knox, Kentucky under the leadership of Major General William R. Desobry. Its objective was to define the characteristics of the new MET and to prepare the Material Need (MN) document. Technical support for the Task Force was provided by the Advanced Concepts Branch of the Tank Automotive Command (TACOM). Eventually, this work resulted in a series of eight main concept studies prepared by J. B. Gilvydis. The first three of these studies covered the work performed for the Task Force during its period of operation from February to August 1972. The later five were conducted for the Main Battle Tank Project Manager's Office, after it was established in September, and for the Army Materiel Command (AMC). This work continued until March 1973. On 5 February 1972, General Henry A. Miley, Jr., the Commanding General of AMC, concluded in a letter to the Department of the Army that tank system concept studies by industry would be of value to the Army and would help meet the directions of Congress. Following this, contracts were awarded to Chrysler Corporation and General Motors Corporation for such design studies to be completed by September 1972.
 

 
The cross section at the left shows the spaced armor arrangement incorporated in some of the new designs.

Drawing at the bottom of the page depict a General Motors tank concept powered by the AGT-1500 gas turbine. The estimated weight of this vehicle was about 47 tons.
The Chrysler program was to cover new evolutionary design concepts based on the M60A1 tank. The General Motors contract was to study the design of a new MBT using advanced design components based on the experience of the XM803 program. Both projects provided concept studies to support the work of the MBT Task Force. Initially, the new main battle tank was designated as the XM815, but with the revision of the nomenclature system, this was soon changed to the XML TACOM presented the results of their first study to the MBTTF at Fort Knox on 25 February 1972. The initial concept, designated as LK 10322, was basically a conventional arrangement with the engine in the rear, the commander, gunner, and loader centrally located in the turret, and the driver in the front center of the hull. The total armored volume of this concept was about 540 cubic feet. This compared to an armored volume of approximately 425 cubic feet for the Soviet T55 and 650 cubic feet for the M60A1. Armed with the 105mm gun M68, the LK 10322 concept was used in this study to analyse the effect on the combat weight of the different thicknesses of armor required to defeat various enemy threats. Fourteen different levels of protection were considered in this first study with the estimated vehicle weights ranging from about 34 tons to approximately 57 tons.

The General Motors 400K tank proposal is sketched below.

Above is the General Motors 500K tank concept.

The loading sequence for the rounds stowed behind the armored bulkhead in the turret bustle is shown at the left. 

The full size mock-up corresponding to the LK 10372 design can be seen in the photographs on this page. Note the coaxial Bushmaster weapon.
The armor ranged from all round protection against only 23mm armor piercing projectiles to frontal protection against 120mm shaped charge rounds. Various composite armor arrays developed by the Ballistic Research Laboratory were utilized in different versions of the concept. Since the design of the armor was rapidly developing, frequent changes were required to adapt the constantly changing arrays to the tank concept studies. It is interesting to note that the initial drawing for the LK 10322 concept featured the AGT-1500 gas turbine, the power plant eventually selected for the Ml production tank. With six road wheels per track, a tube-over-bar suspension was specified for the LK 10322 to improve the cross-country performance. During April and May, a second TACOM study investigated modified versions of the LK 10322 concept in an effort to reduce the weight. Initially, the hull design remained the same, but the turret was slightly modified. The side walls were tapered five degrees to the rear and the slope of the right side was increased from 14 to 20 degrees from the vertical. A new low profile commander's station was introduced with an externally mounted 7.62mm machine gun and the 7.62mm coaxial machine gun was replaced by a .50 caliber weapon. These modifications resulted in estimated weight reductions ranging from a little less than three tons to over three and one half tons depending upon the protection level under consideration. Later, the hull of the modified LK 10322 was lengthened from 270 inches to 275 inches which allowed an increase in fuel capacity from 300 to 350 gallons. This hull also could accept either the AGT 1500 turbine with the XHM-1500 transmission or the AVCR-1100 diesel engine with the X-1100 transmission. The average weight increase with these hull changes was about 600 pounds. On 20 June 1972, the MET Task Force requested TACOM to perform a third weight and protection analysis. This study was to evaluate the effect of 72 different component combinations applied to the modified LK 10322 concept. The following component list was supplied by the Task Force for the analysis.

1. Primary Armament
a. 105mm gun M68
b. 110mm gun, United Kingdom
c. 120mm gun, Federal Republic of Germany
2. Fire Control
a. XM803 system (without commander's day/night
sight)
b. M60A1 (with turret integrated night thermal sight)
3. Power Plants
a. AVCR-1100 diesel engine with the X-1100 transmission
b. DB1500 diesel engine with the Renk HSWL 354
transmission
c. AGT-1500 gas turbine with the XHM-1500-2 transmission
4. Suspension
a. Tube-over-bar
b. Piston hydropneumatic
5. Tracks
a. T142 or Diehl
b. Lightweight, 28 inch width

This study used two different approaches. The first calculated the armor weight for the hull and turret and determined the protection level obtained with total vehicle weights of 43, 45, 47, and 49 tons. The second approach analysed the effect of 72 combinations of components on the total vehicle weight at a single level of protection. The protection level selected was that required to defeat the combined threat of the Soviet 115mm APFSDS projectile at 800 meters range and the 3.2 inch diameter HEAT round on the front and up to 30 degrees to each flank. In addition to the 72 variations of the modified LK 10322 concept from the third TACOM study, the MET Task Force reviewed fifteen additional designs. Numbers 73 through 80 were submitted by General Motors Corporation and 81 through 83 came from Chrysler. Candidates 84 through 87 were product improved versions of the M60A1. In reviewing the 72 candidates from the third TACOM study, the Task Force eliminated those armed with the United Kingdom 110mm rifled gun and the 120mm smooth bore weapon from the Federal Republic of Germany. The 110mm gun was not considered sufficiently superior to the 105mm gun M68 and the 120mm smooth bore was not expected to be available in time to meet the proposed production schedule. At that time, the Task Force also considered the AGT-1500 gas turbine to be a high risk
power plant and it was dropped from the concepts under review. This reduced the number of proposals in the TACOM study from 72 to 16.

General Motors XM1 prototype, note placement of ammunition. There were two large exposed ammo racks in crew compartment, one with rounds stored in vertical position next to the loader in turret, and second on the right side of the driver. This was probably one of several reasons why General Motors prototype was discarded in favor of Chrysler XM1.
Those remaining differed primarily in the engine and transmission with minor variations in the fire control systems as well as the suspensions and tracks. As a result, the 16 candidates were reduced to two featuring the two power packages considered acceptable. Thus one was powered by the air-cooled Continental AVCR-1100 engine with the X-1100 transmission. The other was fitted with the Daimler Benz (later MTU) DB1500 liquid-cooled diesel with the Renk transmission. These two concepts were included by the Task Force among the five candidates for the final trade off analysis. Of the eight General Motors proposals, two were powered by the AGT-1500 gas turbine using the X-1100 transmission, one utilized the DB1500 diesel with the Renk transmission, and two were fitted with the AVCR-1100-3B with the X-1100 transmission. The remaining three had a twin engine arrangement consisting of two General Motors 8V71T liquid-cooled diesels, again with the X-1100 transmission. All eight concepts used a high strength torsion bar suspension and were armed with the M68 105mm gun. Estimated weights of these vehicles ranged from 47 to 67 tons depending upon their level of protection. The Task Force rejected the three proposals powered by the twin 8V71T diesels because of anticipated increased maintenance time and reduced reliability. With the elimination of the AGT-1500 in two of the concepts, the number of General Motors candidates was reduced to three. One of these also was rejected because of its excessive weight of 67 tons. One of the remaining two was selected as the General Motors candidate even though it weighed 56 tons because it came closest to meeting the MN requirements for ballistic protection. One of the three Chrysler concepts was selected as superior to the other two in several important areas and essentially equal in other respects. It was powered by the AVCR-1100 engine with the X-1100 transmission and had an estimated combat weight of 48.5 tons. However, like the other candidates that met the MN weight requirements, it did not provide the desired protection level. None of the four product improved MoOAls met the M N requirements for ballistic protection or sustained speeds on 10 per cent and 30 per cent grades. Of the four, numbers 84 and 85 provided the highest level of protection. Since number 85 featured the more advanced fire control system, it was selected as one of the five candidates for the final trade off analysis. The most obvious conclusion from the review of these concept studies was that the level of ballistic protection required by the proposed MN document was incompatible with the weight limitation specified. This problem was outlined in a fact sheet forwarded from the Task Force to the Commanding General of AMC, dated 3 August 1972.

This fact sheet concluded that the only practical solution was to either reduce the level of ballistic protection or increase the 49 ton weight limitation in the proposed MN document. The latter alternative was subsequently adopted. Although General Motors and Chrysler provided support to the MET Task Force under their study contracts for new tank prototypes, the final reports on both of these contracts were not submitted until October 1972. In their report, General Motors proposed two concepts. They were based on estimated average production unit costs of $400,000 and $500,000, both in fiscal year 1972 dollars. Referred to as the 400K and 500K tanks, the latter was a growth version of the former with improvements in ballistic protection, firepower, and night vision. Both were manned by a crew of four with the driver in the left front hull. The usual arrangement of the three man turret crew was reversed placing the gunner and tank commander on the left side of the cannon with the loader at the right rear. The turret on both vehicles utilized the large bearing ring from the XM803. Both tanks were armed with a stabilized 105mm gun M68. Forty rounds of 105mm ammunition were stowed in the hull below the level of the turret ring without separate compartmentation. The 400K tank was protected by welded plate armor arrays on the turret front. The sides and rear of the turret consisted of single plate rolled homogeneous armor. The hull front was a single plate of high obliquity armor. Spaced armor skirts were provided on the forward sides of the hull. The tank was powered by the previously proposed twin General Motors 8V71T diesel engines connected to a modified X-1100 transmission through a transfer case. With this arrangement, the tank could be driven on one engine in an emergency. The twin engine installation was rated at 1200 gross horsepower. The estimated combat weight of the 400K concept was 52 tons. Although the armor protection was superior to the M60A3, it did not meet the requirements for the new MET.
 
The running gear consisted of an advanced high strength torsion bar suspension with six 30 inch diameter road wheels per side running on a 22 inch wide, double pin, flat track. A 7.62mm coaxial machine gun was located on the right side of the cannon and a 40mm high velocity grenade launcher was installed on the commander's hatch. The independently stabilized gunner's day/night sight included a laser range finder. The combat weight of the 500K tank was increased to 55 tons providing improved ballistic protection. The front hull now consisted of spaced rolled homogeneous armor and steel/aluminum array skirts were installed on the sides. On the turret, a cast armor trunnion support was joined to frontal spaced armor arrays. The sides and rear also consisted of spaced armor. Although the M68 105mm gun was retained in the 500K tank, the mount was strengthened to permit future adaptation of a 120mm gun firing kinetic energy ammunition. The 7.62mm coaxial machine gun in the 400K tank was replaced by the Bushmaster (20mm-30mm) weapon system then under development. The 40mm high velocity grenade launcher was moved to the loader's station and replaced on the commander's hatch by a .50 caliber M85 machine gun. The commander's weapon was now mounted on a powered slew ring. The gunner's stabilized sight with its laser range finder also now included a far infrared imager for night operations. Individual displays were provided for both the gunner and the tank commander. With the heavier weight, the 500K tank was powered by the AVCR-1100-3B engine rated at 1450 gross horsepower using the modified X-1100 transmission. The number of road wheels was raised from six to seven per side and the width of the double pin flat track was increased to 24 inches. Chrysler's report on the New Evolutionary Tank reviewed eight different vehicle concepts as well as the development status of various design features and components. Main armament on all eight concept studies was the M68 105mm gun. In one case, this was supplemented by a TOW missile installation. On two other concepts, the Shillelagh II or Swifty gun launched missile was specified. This was a proposed development of the Shillelagh reduced in diameter to permit launching from the 105mm gun. The missile velocity also was increased to reduce the flight time by about 50 per cent and to extend the range by approximately 25 per cent. However, the reduced diameter would have decreased the effectiveness of its shaped charge warhead. The estimated combat weight of the eight Chrysler proposals ranged from about 49 tons to 57 tons. After considering the eight concept studies, the Chrysler report proposed a final configuration combining the best components from two of them. With an estimated combat weight of 57.2 tons, this tank had a conventional layout. The driver was in the center front hull and the three man turret located the gunner and the tank commander on the right side of the 105mm gun with the loader on the left. The 105mm ammunition was stowed in compartments separated from the crew space. These compartments in the turret bustle and the hull were designed to fail away from the crew space. Thus any ammunition explosion resulting from a penetration of the armor would be directed either to the outside or into the engine compartment away from the crew. 

AGT-1500 and AVCR-1360.
The 105mm M68 gun was selected as the main armament with possible future upgrading through improved ammunition or the introduction of a gun launched missile such as the Swifty. A .50 caliber M 85 coaxial machine gun was specified, but the design permitted its replacement by the Bushmaster weapon when it became available. The fire control equipment was a modified M60A3 system with the turret integrated night thermal sight (TINTS). Final selection of the power plant was not made at that time with the German MTU MB 873 Ka 500 (formerly the Daimler-Benz DB1500) 1500 horsepower liquidcooled diesel, the AVCR-1360-1 air-cooled diesel, and the AGT-1500 gas turbine all being considered. It was proposed to select two of these power plants for test rig evaluation. The X-1100 transmission was to be used in all three cases. The advanced torsion bar suspension was selected. As mentioned earlier, this system utilized high strength torsion bars permitting a much greater twist before exceeding the elastic limit of the material. Thus a much greater wheel travel could be obtained. As with other proposed tank concepts, it was obvious that the combat weight would greatly exceed the original target if satisfactory protection levels were to be achieved. A new concept, designated LK 10352, was proposed by TACOM in a fourth study dated July 1972. The main objective of this study was to determine the effect on the gross vehicle weight if the fuel capacity was increased from 300 to 350 gallons. This was similar to the earlier examination of the same increase on the modified LK 10322. The hull was lengthened to 276 inches with the extra six inches between the crew compartment bulkhead and the engine being used to accommodate a tank for the additional 50 gallons of fuel. The lower glacis was redesigned on this new concept and the angle with the vertical increased from 55 degrees to 60 degrees. The upper glacis remained at 65 degrees from the vertical. No changes were made to the turret. The effect of these modifications on the gross vehicle weight was calculated based on the protection required to defeat two threat levels. The greatest of these was the combined threat of the Soviet 115mm APFSDS kinetic energy projectile at a range of 800 meters and a 3.2 inch diameter precision HEAT shaped charge round. The calculation indicated that a vehicle weight of over 52 tons would be required for protection directly from the front out to an angle of 30 degrees to each flank. In August and September, the LK 10352 concept was modified in a fifth study to determine the effect of second generation armor arrays on the gross vehicle weight. The early steel-glass-steel composite armor was replaced by multiple steel and aluminum plate combinations. For this analysis, three threat levels and the armor arrays required to defeat them were supplied by the Ballistic Research Laboratory. The light threat consisted only of the kinetic energy projectile representated by the Soviet 115mm APFSDS at a range of 800 meters. The medium level of protection was required to defeat both the kinetic energy threat and the 3.2 inch diameter HEAT shaped charge round. A new heavy threat was introduced at this time. It consisted of both the 115mm kinetic energy projectile and a 5.0 inch diameter HEAT round.

Chrysler design hull mockup with AVCR-1360 diesel.
This was the threat against which the new MBT was to be protected in future TACOM studies. For the front hull armor array, the upper glacis was angled at 35 degrees and the lower glacis at 70 degrees, both from the vertical. The nose also was extended 2.5 inches to give more room to the driver. The turret was modified from the earlier concept by changing the left front from a curved to a flat surface angled back at 30 degrees to the rear and sloped at 60 degrees from the vertical. The slope of the right front was changed from 60 to 55 degrees from the vertical to provide more room for the gunner. This side already had a flat surface angled to the rear at 30 degrees. The turret sides were unchanged, but the bustle was extended by 12 inches to obtain more stowage space. The .50 caliber coaxial machine gun was replaced by the Bushmaster and the .50 caliber weapon was relocated to the commander's hatch. The study indicated that a combat weight of over 59 tons would be required to provide protection against the heavy threat from the front and at 30 degrees to each flank. A sixth study by TACOM from October 1972 to January 1973 covered the preparation and weight analyses of new design concepts and the building of a full size wooden mock-up. The rapid development of composite armor technology continued to cause repeated changes in the tank design concepts. In fact, according to TACOM personnel at that time, the Ballistic Research Laboratory was cranking out new armor arrays faster than new concept designs based on them could be drawn and analyzed. After a preliminary study, it was decided to draw two different versions of the new tank. One version was to have the main gun ammunition stowed in the crew space as in earlier tanks. In the other, it was to be separated from the crew in ammunition compartments. Two different turret designs were prepared. One turret stowed the ammunition separate from the crew in a bustle compartment. The second turret had a short bustle with no ammunition stowage. Two designs were considered for the tank with compartmented ammunition. In the first type, 18 out of a total of 40 105mm rounds were stowed in automated racks in the turret bustle behind a one inch thick aluminum alloy bulkhead. These racks were similar to those in the XM803 and the rounds were retrieved through a single round door in the bulkhead. Sixteen rounds were in two revolving drum type racks in the front hull, one on each side of the driver. These rounds were retrieved through small round doors in the one inch thick bulkhead facing the turret basket.

Chrysler mobility test rig.
The remaining six rounds were stowed in a non-automated rack in a two inch thick box underneath the cannon in the turret basket. The second design of the tank with the compartmented ammunition eliminated the automated racks. They were replaced by simple lightweight racks and some rounds were relocated. The eighteen rounds remained in the turret bustle, but they were now retrieved through sliding aluminum doors. The six rounds previously stowed in the turret basket were moved to the front hull. The compartments on each side of the driver now contained 11 rounds each and they could be reached through sliding doors. In the non-compartmented version of the new tank, all 40 rounds of main gun ammunition were stowed in the crew compartment below the turret ring. Twentyeight were in two racks, one on each side of the driver. The remaining 12 were located in the turret basket. These were considered to be ready rounds and were in easy reach of the loader. The compartmented and non-compartmented concepts were quite similar in external appearance except for slight changes in the turret design. In addition to the shorter bustle, the non-compartmented version retained the five degree taper to the rear on the turret sides. On the compartmented turret, the side walls were parallel to the center line. This provided more stowage space for ammunition in the bustle. Both sides of the turret front were angled back 30 degrees to the rear and sloped at 45 degrees from the vertical. The left and right sides of the turret were sloped at 35 degrees and 20 degrees from the vertical respectively. The roof was flat and the rear of the bustle was angled at 40 degrees from the vertical. As in the previous concepts, the turret ring diameter was 85 inches. 

Chrysler and General Motors XM1's compared to M60A1 Main Battle Tank.
The hull design was identical for both concepts, except for the ammunition racks. The upper and lower front glacises were angled at 35 and 70 degrees from the vertical respectively. Side skirts extended from the front idler to the rear sprocket protecting both the crew and engine compartments. The power plant was the AVCR-1360 engine with the X-1100 transmission. All of the fuel was located in the engine compartment. A tube-over-bar suspension system with six road wheels per side was used with 26 inch wide tracks. For the commander, a low rotatable station even with the turret roof line was used for both concepts. It was fitted with a .50 caliber M85 machine gun which could be operated from either inside or outside the tank. In addition to the machine gun sight, the commander was provided with six periscopes for all around vision. A Bushmaster weapon system was installed on the left side of the 105mm gun. A day/night sight including a laser range finder was mounted in front of the gunner's station with separate controls for the gunner and the tank commander. Weight analyses of the two concepts armored against the heavy threat indicated that the tank with the compartmented ammunition weighed slightly over 60 tons compared to under 57 tons for the non-compartmented version. The tank with the compartmented ammunition and armored against the heavy threat was selected for the construction of a full size wooden mock-up. The mock-up was modified in several ways from the original concept drawings. The day/night sight for the gunner and the tank commander was relocated to the right side of the turret. This was done to eliminate a weak ballistic area on the turret caused by the previous mounting of the sight. To provide protection against HEAT rounds, armor arrays extending alongside the entire crew compartment were installed on the hull side areas over the sponsons. The overhanging frontal sections of the turret were lowered by about four inches leaving only a minimum clearance for the driver's head when driving with his head exposed. This was to improve the ballistic protection by reducing the shot trap in this area. The turret roof was slightly crowned to increase its rigidity and to eliminate any possibility of buckling. This was achieved by raising the center line one inch and sloping the roof toward the sides at five degrees from the horizontal. After completion of the mock-up, a new concept drawing, designated LK 10372, was prepared incorporating all of the changes.
 
Data on improved armor arrays were received from the Ballistic Research Laboratory at the conclusion of the previous study. To evaluate the effect of these new arrays on the vehicle weight, a new concept was prepared including the changes applied to the mock-up as well as several new features. This design, designated as LK 10379, was covered in the seventh study from TACOM in February 1973. From this point on, the analyses considered only the protection level required to defeat the heavy threat. This was still the Soviet 115mm APFSDS projectile at 800 meters and the 5.0 inch diameter HEAT round. The 40 rounds of 105mm ammunition in the LK 10379 concept were located in the hull below the turret ring and were not stowed in separate compartments. The Bushmaster weapon system was removed from inside the turret and remounted on the left side exterior near the bustle. The day/night sight for the gunner and the tank commander was installed through the right side of the turret as on the mock-up. The gun shield was narrowed from a width of 38 inches to 12 inches. The five degree taper to the rear of the turret side walls was eliminated making the sides parallel to the turret center line. This provided more space for the tank commander and simplified the design of the Bushmaster mount. The slope of the left and right sides of the turret remained at 35 and 20 degrees from the vertical respectively. The angle of the turret frontal sections toward the rear was increased from 30 to 40 degrees, but they remained sloped at 45 degrees from the vertical. The turret roof was crowned as on the mock-up. The hull front on the LK 10379 was redesigned with a 55 degree angle from the vertical on both the upper and lower glacises. This increased the hull length making the six road wheel suspension marginal for good mobility. Therefore, a new tube-over-bar suspension was specified with seven road wheels per track. As on the mockup, armor arrays were installed on the hull sides above the sponsons. The combat weight of the LK 10379 concept was calculated to be a little over 60 tons. The analysis showed that by reducing the protection on certain areas of the hull and lowering the fuel capacity from 350 to 300 gallons, the weight could be reduced to about 58 tons. The eighth and last in the main series of new XM1 concept studies was performed by TACOM in March 1973. Its objective was to explore ways to reduce the weight of the proposed tank to below 58 tons. To achieve this goal, a new concept drawing, LK 10382, was prepared with several changes from the previous design. However, the same armor arrays were used to permit accurate comparisons. The front hull was changed back to the earlier design with the upper and lower glacis plates sloped at 35 and 70 degrees from the vertical respectively. This reduced the length of the hull by 15 inches and lowered the weight by about 900 pounds. The hull height was reduced from 42 to 40 inches and the sides above the sponsons were sloped at a greater angle from the vertical. The fuel tank behind the crew compartment bulkhead was removed and the engine was moved four inches forward. This dropped the fuel capacity back to 300 gallons. The turret ring diameter was reduced from 85 to 83 inches which also shortened the hull by an additional two inches.
 
With the shorter hull, the suspension was changed back to six road wheels per side. The ground clearance was increased from 17 to 18 inches for better mobility and mine protection. Except for the smaller ring, the turret design remained the same, but the Bushmaster was moved farther to the rear, lowered, and mounted closer to the turret wall. The ammunition feeding system for the Bushmaster was changed so that it fed directly through the turret side armor. This eliminated the partially exposed ammunition chutes which formerly were routed through the turret roof. The estimated combat weight of the LK 10382 design was between 57 and 58 tons depending upon weights assumed for the suspension and the possible reduction in thickness of the hull bottom plate under the engine compartment. After the complexity and high cost that resulted in the cancellation of the XM803, Congress required that these factors be tightly controlled in the new tank program. Thus a design to unit cost ceiling was established at the beginning of the project. The specification requirements in the Materiel Need (MN) document were presented as ranges permitting the contractors some leeway in meeting the design objectives. As the Army later observed during a Congressional hearing in April 1974, "We would like to have the maximum performance, but we are sticking to a design to unit cost and we therefore have to give the contractor the ability to make trade offs to meet the design to cost goal". As mentioned earlier, the weight limitation in the original MN document was incompatible with the protection level required. Since crew survivability had the highest priority on the list of characteristics for the new tank, the maximum weight was raised to 58 tons to permit adequate armor. Later, this would be Increased to 60 tons. During the initial studies in 1972, the design to unit cost goal was set at $400,000. However, by the time that the request for proposal was issued, this had increased to $507,790, also in 1972 dollars. This compared to estimated unit costs of $339,000 for the M60A1, $432,000 for the M60A3, and $611,000 for the cancelled XM803. The new program was divided into three phases. The first of these was a competitive advanced development validation phase during which contracts would be awarded to two sources. Each of these contractors would design, build, and test one complete prototype tank and one automotive test rig. They also were to furnish an additional hull and turret for ballistic evaluation. This 34 month advanced development phase would end with a combined developmental/operational test (DT/OT-I) by the Army. These tests would be used to validate the contractors' proposals for phase 2, which would be a sole source full scale engineering development (FSED) program. Phase 3 would put the new tank into production.
 
The various tank concepts being considered by the Army at this time reflected a change in thinking since the period of the MBT70 and the XM803. Although some sources regarded the appearance of the long range antitank missile as sounding the death knell of the tank, this was not the case. The main effect of the new missiles was to release the tank from the defensive role since other units were now capable of defending themselves against armored attack. The tank could now resume its place as the key offensive weapon in the combined arms team. In that role, the missile firing gun-launcher system was less attractive as the main tank weapon than the fast shooting 105mm gun,
particularly with improved ammunition. Thus when proposals were requested for the development of the new tank, the 105mm gun with its quick reaction, multitarget performance, and immediate availability was the obvious candidate despite its shorter range. Proposals for the advanced development validation phase were received from Chrysler and General Motors in May 1973. Originally, Ford Motor Company had been expected to participate, but they declined to enter the competition. As a result, after review, contracts were awarded to Chrysler and to the Detroit Diesel Allison Division of General Motors on 28 June 1973. As mentioned earlier, these contracts required the design, construction, and testing of a completely integrated prototype tank. It was to include all subsystems except for the night vision equipment. An automotive test rig also was to be constructed for use in the test program and an additional ballistic hull and turret were to be provided for ballistic tests at Aberdeen Proving Ground. These cost plus incentive fee (CPIF) contracts eventually amounted to $68,999,000 for Chrysler and $87,969,000 for General Motors. Chrysler proposed a conventional arrangement for its prototype tank with a three man turret crew and the driver in the center front hull. The gunner and the tank commander were in the usual location on the right side of the M68 105mm gun with the loader on the left. A coaxial Bushmaster weapon was fitted on the left side of the cannon. An M85 .50 caliber machine gun was mounted on the tank commander's station and either a 7.62mm M60D machine gun or a 40mm high velocity grenade launcher was installed on the turret roof in front of the loader's hatch.
 
A simplified line of sight stabilization system was expected to provide performance equal to at least 90 per cent of a dual axis system with a cost saving of about $3000. The 105mm ammunition was stowed in separate compartments vented away from the crew space. New composite special armor was incorporated into the frontal areas of the tank and side skirts extended from the front idler to the rear sprocket partially shielding the hydromechanical suspension. This was actually a tube-over-bar suspension with seven road wheels per track using rotary hydraulic shock absorbers. The term hydromechanical suspension was selected to avoid confusion with the earlier tube-over-bar suspension installed experimentally on the M60A1 which lacked the performance of the new design. It also was noted that the same name could be applied to the advanced high strength torsion bar suspension if it replaced the tube-over bar design in the future. At the time of the proposal, the advanced torsion bar was considered a high risk item, but the option of using high strength steel or titanium torsion bars was kept open for the future. In regard to the power plant, Dr. Philip W. Lett Jr., the leader of the Chrysler team, made a bold decision. Despite the fact that the Army MET Task Force had considered the gas turbine to be a high risk item, he selected the AGT-1500 with the X-1100 transmission to power their prototype. In retrospect, this appears to have been a crucial decision in the design of the new tank. Subsequent events were to prove that the AGT-1500 would develop into a highly reliable power plant and it had several advantages over its diesel competitors. Although both the AGT-1500 and the AVCR-1360 diesel were rated at 1500 gross horsepower, the former utilized only 30 horsepower for cooling compared to 160 horsepower for the latter. Also, a growth potential to 2000 gross horsepower was estimated for the turbine compared to 1800 gross horsepower for the variable compression ratio diesel. The less complex design of the turbine engine with its fewer parts was expected to provide higher reliability and reduced maintenance in the future. Its lighter weight also permitted increased armor protection without exceeding the overall weight limit for the tank.
 
The gas turbine did not generate smoke during operation and it was capable of using a wider range of fuels than the diesel. Also, the turbine was easier to start at low temperatures. The greatest disadvantage of the gas turbine was its higher fuel consumption. This was particularly obvious at low vehicle speeds. However, the small size of the
power plant allowed space for an increase in fuel capacity to achieve the specified cruising range. Another problem was the high volume of combustion air used by the gas turbine. This required a filter system with approximately three times the capacity of that necessary for the diesel engine. The turbine, of course, did not require the high volume of cooling air used by the air-cooled diesel. The development of the gas turbine tank engine resulted from recommendations of the Committee on Gas Turbine and Electric Propulsion for Tanks, chaired by Mr. James C. Zeder. When presented in 1965 to the Mobility Advisory Group, the recommendations of the Zeder Committee were initially rejected because of cost. However, the program was subsequently approved. After the evaluation of several proposals from industry, the Lycoming Division of AVCO was selected to develop the
new turbine, designated as the AGT-1500. At that time, it was considered as a possible power plant for the MBT70. However, after the selection of the diesel engine for that program, attempts were made to cancel the AGT-1500 and to transfer its funds elsewhere. These attempts were prevented by the efforts of Lieutenant Colonel John W. Wiss, Chief of the Army Tank Automotive Command Laboratories, and Mr. Wayne S. Anderson who directed the Propulsion Systems Laboratory. Thus the AGT-1500 was available when needed for the XM1 program and its state of development was not far from that of the AVCR-1360 variable compression ratio diesel. Although the initial cost of the gas turbine exceeded that of the diesel engine, it was anticipated that the life cycle cost would be competitive because of reduced maintenance requirements.
 
The early General Motors concept for the XM1 validation phase tank also was armed with the 105mm gun M68 and a Bushmaster. However, the latter was located in a separate mount on the left side of the turret. The turret crew rode in the usual positions with the gunner and tank commander on the right side of the cannon and the loader on the left. A .50 caliber M85 machine gun was provided for the tank commander and a 7.62mm machine gun was installed at the loader's position. The main gun ammunition was separated from the crew space in compartments. The driver was located in the left front hull and special armor was incorporated in the frontal areas of the tank. The Continental AVCR-1360 diesel engine drove the tank through the General Motors X-1100 transmission. As mentioned earlier, the jWCR-1360 diesel was a 1500 horsepower development of the AVCR-1100 used in the MBT70 and the XM803. The suspension on the General Motors tank was fitted with six road wheels per track. This was a hybrid suspension with road wheels 1, 2, and 6 fitted with the National Water Lift hydropneumatic suspension units. Road wheels 3 and 5 utilized high strength torsion bars. In early July 1973, Major General Robert J. Baer, the XM1 Project Manager, and personnel from the Ballistic Research Laboratory escorted representatives from Chrysler and General Motors to the United Kingdom for an update on the British developed Chobham special armor.
 
During this trip they observed the proposed design of a new United Kingdom vehicle utilizing this special armor as well as the manufacturing processes required for its
production. Based upon the newly obtained data, both contractors reevaluated their proposed armor configurations. Chrysler made a number of changes, but they retained the basic design in their original proposal. However, General Motors made major changes in the configuration of their proposed tank. The most obvious change was in the turret where the vertical front and side walls were replaced by sloped armor. The modifications by both contractors required additional work by the Ballistic Research Laboratory to develop new special armor combinations. By January 1974, both Chrysler and General Motors completed the final design of their special armor configurations and some of the hull and turret armor structures had been delivered to Aberdeen Proving Ground for ballistic tests. Some changes in the specifications for the new tank resulted from a study of the Israeli experience during the October 1973 war in the Middle East. One change which was applied to both of the validation phase prototypes was the replacement of the Bushmaster weapon system with a 7.62mm coaxial machine gun. The original intention had been to utilize the Bushmaster against lightly armored vehicles thus reducing the stowage requirement for the expensive main gun rounds. However, battle experience showed that the tank crews would invariably use the main gun against these vehicles. Also, there was a need for a coaxial machine gun for use against infantry and to suppress antitank weapons. Elimination of the bulky Bushmaster and its ammunition provided space to increase the number of 105mm rounds to 55. Some of the modifications resulting from the study of battlefield experience were applied at a later date to the full scale engineering development (FSED) tank proposals. These included changes in the ammunition compartments to accommodate the larger number of 105mm rounds and the use of a less flammable hydraulic fluid.

Early and late FSED designs, notice changes in the turret design.
The complete validation phase prototypes were delivered to Aberdeen Proving Ground for the combined developmental/operational test (DT/OT-I) which ran from 31 January to 7 May 1976. During this same period, the competing automotive test rigs were driven more than 3000 miles over a variety of terrain. Both prototypes were displayed to the public for the first time on 3 February 1976. The results of the DT/OT-I indicated that both the Chrysler and the General Motors prototypes met the specified requirements for the new tank. The Chrysler tank with its gas turbine power plant showed slightly greater acceleration, but the performance was satisfactory for both vehicles. The operational portion of the tests included two weeks of simulated combat operations with the tanks manned by crews from Fort Knox and Fort Hood. After the evaluation of the competing prototypes, the selection of the contractor for the full scale engineering development phase was scheduled for July 1976. 

From left to right: Early Chrysler XM1 FSED with AGT-1500 gas turbine, final XM1 FSED with AGT-1500, and final XM1 FSED with AVCR-1360 diesel.
After the cancellation of the MBT70 program, the Federal Republic of Germany continued with the development of their new Leopard II tank. In line with efforts to standardize materiel in NATO, Germany proposed that the Leopard II be modified to meet American requirements. In December 1974, a Memorandum of Understanding between the United States and the Federal Republic of Germany was signed in which the U.S. agreed to evaluate the Leopard II under the same test conditions as the XM1 candidates. At that time, it was estimated that the Leopard II would cost about one million 1975 dollars compared to approximately $750,000 for the XML The modified version, designated as the Leopard II AV was armed with the 105mm gun. It had improved armor protection and a simplified fire control system to reduce costs. Tests of the Leopard II AV were carried out at Aberdeen Proving Ground from September to December 1976. Previously, on 22 July 1975, FMC Corporation had been awarded a contract for a study to determine the costs and technical problems involved in manufacturing the Leopard II AV in the United States. The tests at Aberdeen indicated that the Leopard II AV met or exceeded most of the U.S. requirements. The accuracy of its fire control system was slightly higher than that of the XM1, but it was more expensive. The armor protection, ammunition compartmentation, and gun movement were considered somewhat inferior to that on the American tank. Rather than continue the evaluation of the complete tank, it was agreed in January 1977 that only certain major components would be considered for adoption to enhance NATO standardization.
 
Shortly before the end of the XM1 validation test program, a new requirement was introduced for inclusion in the proposals for the full scale engineering development/ producibility engineering and planning (FSED/PEP) contract. These proposals were now to include firm price ceilings for the first two production options. These were for 110 low initial rate production (LIRP) tanks in fiscal year 1979 and 352 tanks in fiscal year 1980. The winner of the competition for the XM1 FSED/PEP contract was selected on schedule in July 1976, but the results were not released by the Army. Information published later in the press indicated that General Motors won with a bid of 208 million dollars compared to Chrysler's bid of 221 million dollars. However, these bids were based upon the Chrysler and General Motors tanks being powered by the AGT-1500 turbine and the AVCR-1360 diesel respectively. The Army, by this time, favored the gas turbine and although awarding the contract to General Motors, planned to change the power plant to the AGT-1500. However, this approach was rejected by the Office of the Secretary of Defense. As the Deputy Secretary of Defense later testified before Congress: "I was unalterably opposed to making these changes in a sole source environment after the contractor was selected. I wanted to know what those unit costs were, what the delays in the program might be, if any, and have those costs determined competitively between the two contractors." Also, a decision had been made to arm the new tank with a 120mm gun at some time in the future. Therefore, the turret had to be capable of mounting either the 105mm gun M68 or a 120mm gun without extensive modification. The 120mm guns under consideration were the British rifled weapon and the German Rheinmetall smooth bore gun used in the Leopard II. As a result of the new requirements, a revised request for proposal was issued, with the award of the FSED/PEP contract now rescheduled to November 1976. Both contractors submitted proposals by 28 October for versions of their tanks with a hybrid turret capable of mounting either a 105mm gun or a 120mm gun. In the interest of NATO standardization, provision was made for the use of the gunner's auxiliary sight from the Leopard II as well as metric fasteners at crew serviced interfaces. General Motors presented a version of their tank powered by the AGT-1500 gas turbine and Chrysler proposed an alternate design using the AVCR-1360 diesel engine. However, they recommended their turbine driven tank as the primary choice. All of the proposed tanks were fitted with British developed smoke grenade launchers.
 
Prior to the submission of the final proposals on 28 October, Chrysler made a number of additional changes to enhance the survivability of their candidate and to reduce the cost. The arrangement of the special armor was modified and a special armor gun shield replaced the casting on the earlier proposal. A new minilaser range finder was incorporated and an extension from the gunner's primary sight was provided for the tank commander removing the need for a separate sight. Other cost reductions included the elimination of the power elevation for the commander's weapon and the driver's built in test equipment (BITE) panel. The latter was replaced by pop-up indicators and other gages in the engine compartment and at the crew stations. 
 
At a news conference on 12 November 1976, it was announced that Chrysler had won the FSED/PEP contract. Published reports indicated that the cost reduction efforts at Chrysler had reduced their bid to 196 million dollars compared to a 232 million dollar bid from General Motors. The increase in the latter was, no doubt, partially due to the higher initial cost of the AGT-1500 turbine compared to the AVCR-1360 diesel engine. However, Chrysler's diesel powered proposal for the FSED/PEP program was priced at 186 million dollars. The five month delay in awarding the contract had permitted further refinement of the design and better determination of the manufacturing costs. For
example, the design to unit cost for Chrysler's tank decreased from $574,000 for the original FSED version to $422,000 for the final turbine powered tank proposal. The unit cost for Chrysler's diesel tank design was $376,000. All of these prices were in 1972 dollars for comparison with the earlier estimates. The actual unit price of the winning tank was expected to be about $754,000 in 1976 dollars. Under the 36 month FSED/PEP contract, Chrysler was to manufacture 11 new pilot tanks incorporating the latest changes resulting from the validation phase tests and other experience. The validation phase prototype and the automotive test rig also were to be refurbished for use in the test program. Evaluation of these vehicles would determine the details of the final design for low initial rate production (LIRP). All of the 11 new pilot tanks were manufactured at the Detroit Army Tank Plant with the first one being delivered in February 1978. The last of the 11 was completed the following July. Developmental Test II (DT-II) extended from February 1978 to September 1979 at Aberdeen Proving Ground. Operational Test II (OT-II) at Fort Bliss, Texas lasted from April 1978 to February 1979 with the tanks manned by the 2nd Squadron, 3rd Armored Cavalry Regiment. Three of the tanks, after testing at Fort Bliss, were refurbished at Detroit, fitted with the latest configuration hardware, and shipped to Fort Knox in June 1979 for additional durability and reliability testing. This program was conducted by H Company, 2nd Squadron, 6th Cavalry under the direction of the Armor and Engineer Board. During these tests, the reliability, availability, maintainability-durability (RAM-D) goal was 272 mean miles between failures (MMBF). The XM1 reached 326 MMBF. Initially, Chrysler personnel provided the maintenance, but in August the organizational maintenance was taken over by the soldiers of H Company for the remainder of the test program. During the OT-II at Fort Bliss, many of the power train failures were caused by dust being ingested into the engine. After modification of the seals in the air filtration system, there were no further failures from this cause during all of the tests at Fort Knox. The tests at Fort Bliss also revealed a tendency for the tracks to be thrown by the buildup of soil between the sprocket hub and the bottom of the rear sponson. Some tracks also were thrown by rapid backing and turning when changing firing positions. This problem was solved by the combination of some additional suspension hardware and the redistribution of the ground pressure with an increase in track tension. The hardware change consisted of a sprocket hub with discharge ports, a mud scraper, steel blocks welded to the lower rear hull to prevent inside throws, a steel plate mounted above the final drive to maintain track alignment, and a retaining ring on the outside of the sprocket. Satisfactory results were obtained with these modifications during further tests at Fort Bliss in October 1979.

Major General Baer (far left) and Dr. P. W. Lett (at right) are with a group observing the AGT-1500.
The survivability of the XM1 was demonstrated during DT-II at Aberdeen using pilot vehicle 11 (PV 11). The tank was fully loaded with fuel and ammunition with dummies placed in the various crew positions. Sensors were installed throughout the tank and, with the engine idling, it was fired upon using various types of ammunition at typical combat ranges. The tank was not destroyed and it was driven away under its own power after the tests. PV 11 also was subjected to a blast from an antitank mine which badly damaged the suspension. However, after short tracking one side of the vehicle, the tank was started and driven away from the test site. On 7 May 1979, the XM1 was approved by the Secretary of Defense for low initial rate production (LIRP) at the newly equipped Lima Army Tank Plant. As mentioned previously, this was for 110 tanks with the first two being delivered at a special acceptance ceremony on 28 February 1980. At this time, the new tank was named the Abrams in honor of the late General Creighton W. Abrams, an outstanding armor commander and former Army Chief of Staff. The General's wife and three sons attended the ceremony and Mrs. Abrams christened the first production tank.
 
The LIRP tanks began developmental testing (DT-III) in March 1980 and it continued until September 1981. The major part was conducted at Aberdeen Proving Ground for RAM-D testing. Other work was at Yuma, Arizona for desert tests, Eglin Air Force Base, Florida and the Alaska Cold Region Test Center for cold room and arctic environment testing, and the White Sands Missile Range, New Mexico for electromagnetic radiation and nuclear vulnerability evaluation. The operational tests (OT-III) started at both Fort Knox and Fort Hood during September 1980 and continued until May 1981. At Fort Hood, the tanks were operated and maintained by the 2/5th Cavalry and the 27th Maintenance Battalion. The new tank was enthusiastically received by the troops who greatly appreciated its speed and agility. The operational tests revealed that the cruising range of the production tank was about 20 to 25 miles less than the objective of 275 miles at 25 miles per hour. This was attributed partially to the suspension modifications made to reduce the track throwing experienced during DT/OT-H. However, it was noted that the turbine operated more efficiently at higher speeds and that an increase in the tank test speed from 25 to only 27 miles per hour produced an improvement in cruising range. Thus by operating at higher speeds, the required cruising range could be obtained.
 
In February 1981, production was approved for 7058 tanks at a rate of 30 per month. The vehicle was classified as standard on 17 February 1981 as the 105mm gun full tracked combat tank M1. Production problems at AVCO Lycoming resulted in their failure to meet the deliveryf schedule and also produced a large number of defects in the engines that were delivered. Poor quality control and shortages of skilled personnel were the primary causes. Eventually these problems were solved and the number of defective engines was minimized as production increased. Early tank production also suffered from delays in obtaining the thermal imaging systems and from some fabrication difficulties with the hull and turret at the Lima plant. These problems also were solved and the production of the new tank soon reached the required level.
 
Unlike the M48 and M60 tank series, the M1 did not make use of large armor steel castings. The hull and turret were assembled by welding sections of rolled homogeneous steel armor plate to which the special armor was attached. These steel sections were cut to size using precision flame cutting equipment. The hull was divided into three sections with the crew compartment in the center separated from the fuel cells in the front and the engine compartment in the rear by two bulkheads. In addition to the two fuel cells adjacent to the driver's station, two were located in the engine compartment and two in the rear sponsons. A Halon fire extinguisher system with infrared sensors was installed in both the crew and engine compartments. It could detect a fire in 4-5 milliseconds and extinguish it in about two tenths of a second. The hatch above the driver's seat was fitted with three periscopes providing an overlapping field of vision exceeding 120 degrees. Like on the XM803, there was no floor escape hatch. The driver's T-bar steering control incorporated motorcycle type hand throttles eliminating the need for a floor accelerator pedal thus providing more foot space. The transmission shift selector and push buttons for the intercom system also were located on the T-bar assembly. The service brake pedal was in the center of the floor allowing braking with either foot. The driver's seat was adjustable for driving with the hatch either open or closed and the seat height could be set to four different positions. A large knob on an adjustable lower lumbar support allowed the driver to control the amount of lower back support. The aluminum turret basket floor was suspended from five posts, two of which mounted the seats for the tank commander and the loader. A door in the basket floor provided access to components in the bottom of the hull. The commander's station permitted observation in the seated position through both the extension from the gunner's primary sight and the sight for the .50 caliber machine gun. Six periscopes permitted 360 degree vision. The three positions of the commander's platform allowed him to observe standing with the hatch in the protected open position or at two heights with the hatch fully open. The commander's .50 caliber machine gun was electrically powered in azimuth, but it was elevated manually. The loader at the left rear of the cannon had an eight position swiveling seat that was adjustable in the vertical direction. An eight round armored hull ammunition compartment was located on the right side of the tank behind the engine compartment bulkhead. Blowout panels in the top and bottom of the hull vented any ammunition explosion to the outside and away from the crew. Fortyfour 105mm rounds of ammunition were stowed in the turret bustle behind sliding armor plate doors. Twentytwo of these rounds could be reached by the loader without leaving his seat. A knee switch operated the bustle compartment door covering these 22 ready rounds on the left side of the turret. The door also could be operated manually in an emergency. Blowout panels in the bustle roof vented any ammunition explosion to the outside. Three additional 105mm rounds were stowed on the turret basket floor to the left of the cannon in spall protection covers. This brought the total 105mm ammunition stowage to 55 rounds.
 
The loader's hatch cover was fitted with a rotatable periscope mount. A 7.62mm M240 machine gun was skate mounted around the outer edge of the loader's hatch. Another 7.62mm M240 machine gun was installed coaxially on the right side of the 105mm gun M68E1 above the gunner's auxiliary sight. The gunner's seat was pedestal mounted on the turret basket floor and it was adjustable in both the vertical and horizontal directions. No adjustments were required when the gunner shifted from the primary sight to the auxiliary sight. The gunner's primary sight with its extension for the tank commander was the main optical sighting instrument. It also included the laser range finder, the thermal vision system, and the gyro stabilized line of sight platform. The sight was protected by an armor steel cover with doors operable from within the turret. The gunner's auxiliary sight was an eight power articulated telescope installed in the right side of the gun mount below the coaxial machine gun. The heart of the fire control system was the digital computer which received signals from the sensors as well as inputs from various controls. Data from the laser range finder, the cant sensor, and the wind sensor were received automatically, but other inputs were entered into the computer manually. The gunner used the muzzle reference system to determine tube bend corrections which were then entered into the computer. Atmospheric temperature and pressure as well as ammunition temperature and tube wear also were manual inputs to the computer.

General Motors XM1.
The powerpack consisted of the AGT-1500 turbine engine, the X-1100-3B transmission, final drives, air cleaner, scavenging blower, and cooling system. The powerpack weighed about 8500 pounds and could be easily removed as a unit using a five ton wrecker. The sprockets at the rear of the vehicle drove the 25 inch wide tracks and the tank rode on seven dual road wheels per side. The 25 inch diameter compensating idler installed at the front of each track was interchangeable with the road wheels. It was mounted on a pivoting idler arm connected to the number one road wheel arm by a track adjusting link. Adjustment of the static track tension was by hand pumping grease into the link. When the proper tension was reached, a pressure relief valve opened releasing any additional grease.
 
The vehicle was sprung by high strength steel torsion bars accessible from the outside on both sides of the tank. Thus, if a torsion bar fractured, both pieces could be more easily extracted. There were only two track support rollers per track compared to three on the validation phase prototype. The production tank was governed for a maximum level speed of 45 miles per hour and it could operate cross country at speeds up to 30 miles per hour. 
 
On 19 February 1982, it was announced that a preliminary agreement had been reached between Chrysler and General Dynamics Corporation for the latter to purchase the Chrysler Defense Division. The deal was completed in March and the organization was renamed as the General Dynamics Land Systems Division. The personnel remained
the same with Dr. Philip W. Lett, Jr. becoming Vice President of Research and Engineering. Thus Chrysler severed its connection with the development and production of tanks which had existed since before World War II.
 
It had always been intended to produce the M1 at both the Lima and the Detroit tank plants. In September 1981, the production of 60 tanks per month was authorized with 30 at Lima and 30 at Detroit. This was based on a single work shift at both plants. A total of 150 tanks per month was possible with multi-shift operation. Production of the M1 began at Detroit alongside the M60A3 during March 1982 and the first tank was accepted by the Army at a ceremony on 31 March. Production of the basic M1 continued at both plants for a total run of 2374 tanks. The last basic M1 was completed in January 1985.

This photograph was taken at the rollout of the first XM1 pilot tank at Detroit in February 1978. In the view above, the two center figures
are Dr. P. W. Lett (left) and Eugene Trapp (right).
*Note - all vehicle weights are provided in short tons.
 
PS. I will update this post a bit later with drawings and photos. And then move on.

Ok, so let's start to share here some informations and fight with some myths. I will start with M1, people that know me, know this is my black horse. So let's start with history and then move on. Historical description is a quote from Richard P. Hunnicutt Abrams - A History Of The American Main Battle Tank Vol.2.
M1 Abrams
History
Below is the LK 10322 design concept with heavy protection, rear engine, and a conventional turret.

Although Congress cancelled the XM803 program in late 1971, the Army's need for a new tank was recognized. To meet this requirement, 20 million dollars were authorized for the procurement of two prototypes of a new main battle tank. This was in addition to the 20 million dollars allocated to phase out the XM803. Although the funds were provided specifically for two industrial prototype tanks, the Army initially preferred to concentrate on component development until a specification could be prepared for a new main battle tank. At that time, there was considerable controversy within the Army itself as to what form the tank should take. In February 1972, the Main Battle Tank Task Force (MBTTF) was established at Fort Knox, Kentucky under the leadership of Major General William R. Desobry. Its objective was to define the characteristics of the new MET and to prepare the Material Need (MN) document. Technical support for the Task Force was provided by the Advanced Concepts Branch of the Tank Automotive Command (TACOM). Eventually, this work resulted in a series of eight main concept studies prepared by J. B. Gilvydis. The first three of these studies covered the work performed for the Task Force during its period of operation from February to August 1972. The later five were conducted for the Main Battle Tank Project Manager's Office, after it was established in September, and for the Army Materiel Command (AMC). This work continued until March 1973. On 5 February 1972, General Henry A. Miley, Jr., the Commanding General of AMC, concluded in a letter to the Department of the Army that tank system concept studies by industry would be of value to the Army and would help meet the directions of Congress. Following this, contracts were awarded to Chrysler Corporation and General Motors Corporation for such design studies to be completed by September 1972.
 

 
The cross section at the left shows the spaced armor arrangement incorporated in some of the new designs.

Drawing at the bottom of the page depict a General Motors tank concept powered by the AGT-1500 gas turbine. The estimated weight of this vehicle was about 47 tons.
The Chrysler program was to cover new evolutionary design concepts based on the M60A1 tank. The General Motors contract was to study the design of a new MBT using advanced design components based on the experience of the XM803 program. Both projects provided concept studies to support the work of the MBT Task Force. Initially, the new main battle tank was designated as the XM815, but with the revision of the nomenclature system, this was soon changed to the XML TACOM presented the results of their first study to the MBTTF at Fort Knox on 25 February 1972. The initial concept, designated as LK 10322, was basically a conventional arrangement with the engine in the rear, the commander, gunner, and loader centrally located in the turret, and the driver in the front center of the hull. The total armored volume of this concept was about 540 cubic feet. This compared to an armored volume of approximately 425 cubic feet for the Soviet T55 and 650 cubic feet for the M60A1. Armed with the 105mm gun M68, the LK 10322 concept was used in this study to analyse the effect on the combat weight of the different thicknesses of armor required to defeat various enemy threats. Fourteen different levels of protection were considered in this first study with the estimated vehicle weights ranging from about 34 tons to approximately 57 tons.

The General Motors 400K tank proposal is sketched below.

Above is the General Motors 500K tank concept.

The loading sequence for the rounds stowed behind the armored bulkhead in the turret bustle is shown at the left. 

The full size mock-up corresponding to the LK 10372 design can be seen in the photographs on this page. Note the coaxial Bushmaster weapon.
The armor ranged from all round protection against only 23mm armor piercing projectiles to frontal protection against 120mm shaped charge rounds. Various composite armor arrays developed by the Ballistic Research Laboratory were utilized in different versions of the concept. Since the design of the armor was rapidly developing, frequent changes were required to adapt the constantly changing arrays to the tank concept studies. It is interesting to note that the initial drawing for the LK 10322 concept featured the AGT-1500 gas turbine, the power plant eventually selected for the Ml production tank. With six road wheels per track, a tube-over-bar suspension was specified for the LK 10322 to improve the cross-country performance. During April and May, a second TACOM study investigated modified versions of the LK 10322 concept in an effort to reduce the weight. Initially, the hull design remained the same, but the turret was slightly modified. The side walls were tapered five degrees to the rear and the slope of the right side was increased from 14 to 20 degrees from the vertical. A new low profile commander's station was introduced with an externally mounted 7.62mm machine gun and the 7.62mm coaxial machine gun was replaced by a .50 caliber weapon. These modifications resulted in estimated weight reductions ranging from a little less than three tons to over three and one half tons depending upon the protection level under consideration. Later, the hull of the modified LK 10322 was lengthened from 270 inches to 275 inches which allowed an increase in fuel capacity from 300 to 350 gallons. This hull also could accept either the AGT 1500 turbine with the XHM-1500 transmission or the AVCR-1100 diesel engine with the X-1100 transmission. The average weight increase with these hull changes was about 600 pounds. On 20 June 1972, the MET Task Force requested TACOM to perform a third weight and protection analysis. This study was to evaluate the effect of 72 different component combinations applied to the modified LK 10322 concept. The following component list was supplied by the Task Force for the analysis.

1. Primary Armament
a. 105mm gun M68
b. 110mm gun, United Kingdom
c. 120mm gun, Federal Republic of Germany
2. Fire Control
a. XM803 system (without commander's day/night
sight)
b. M60A1 (with turret integrated night thermal sight)
3. Power Plants
a. AVCR-1100 diesel engine with the X-1100 transmission
b. DB1500 diesel engine with the Renk HSWL 354
transmission
c. AGT-1500 gas turbine with the XHM-1500-2 transmission
4. Suspension
a. Tube-over-bar
b. Piston hydropneumatic
5. Tracks
a. T142 or Diehl
b. Lightweight, 28 inch width

This study used two different approaches. The first calculated the armor weight for the hull and turret and determined the protection level obtained with total vehicle weights of 43, 45, 47, and 49 tons. The second approach analysed the effect of 72 combinations of components on the total vehicle weight at a single level of protection. The protection level selected was that required to defeat the combined threat of the Soviet 115mm APFSDS projectile at 800 meters range and the 3.2 inch diameter HEAT round on the front and up to 30 degrees to each flank. In addition to the 72 variations of the modified LK 10322 concept from the third TACOM study, the MET Task Force reviewed fifteen additional designs. Numbers 73 through 80 were submitted by General Motors Corporation and 81 through 83 came from Chrysler. Candidates 84 through 87 were product improved versions of the M60A1. In reviewing the 72 candidates from the third TACOM study, the Task Force eliminated those armed with the United Kingdom 110mm rifled gun and the 120mm smooth bore weapon from the Federal Republic of Germany. The 110mm gun was not considered sufficiently superior to the 105mm gun M68 and the 120mm smooth bore was not expected to be available in time to meet the proposed production schedule. At that time, the Task Force also considered the AGT-1500 gas turbine to be a high risk
power plant and it was dropped from the concepts under review. This reduced the number of proposals in the TACOM study from 72 to 16.

General Motors XM1 prototype, note placement of ammunition. There were two large exposed ammo racks in crew compartment, one with rounds stored in vertical position next to the loader in turret, and second on the right side of the driver. This was probably one of several reasons why General Motors prototype was discarded in favor of Chrysler XM1.
Those remaining differed primarily in the engine and transmission with minor variations in the fire control systems as well as the suspensions and tracks. As a result, the 16 candidates were reduced to two featuring the two power packages considered acceptable. Thus one was powered by the air-cooled Continental AVCR-1100 engine with the X-1100 transmission. The other was fitted with the Daimler Benz (later MTU) DB1500 liquid-cooled diesel with the Renk transmission. These two concepts were included by the Task Force among the five candidates for the final trade off analysis. Of the eight General Motors proposals, two were powered by the AGT-1500 gas turbine using the X-1100 transmission, one utilized the DB1500 diesel with the Renk transmission, and two were fitted with the AVCR-1100-3B with the X-1100 transmission. The remaining three had a twin engine arrangement consisting of two General Motors 8V71T liquid-cooled diesels, again with the X-1100 transmission. All eight concepts used a high strength torsion bar suspension and were armed with the M68 105mm gun. Estimated weights of these vehicles ranged from 47 to 67 tons depending upon their level of protection. The Task Force rejected the three proposals powered by the twin 8V71T diesels because of anticipated increased maintenance time and reduced reliability. With the elimination of the AGT-1500 in two of the concepts, the number of General Motors candidates was reduced to three. One of these also was rejected because of its excessive weight of 67 tons. One of the remaining two was selected as the General Motors candidate even though it weighed 56 tons because it came closest to meeting the MN requirements for ballistic protection. One of the three Chrysler concepts was selected as superior to the other two in several important areas and essentially equal in other respects. It was powered by the AVCR-1100 engine with the X-1100 transmission and had an estimated combat weight of 48.5 tons. However, like the other candidates that met the MN weight requirements, it did not provide the desired protection level. None of the four product improved MoOAls met the M N requirements for ballistic protection or sustained speeds on 10 per cent and 30 per cent grades. Of the four, numbers 84 and 85 provided the highest level of protection. Since number 85 featured the more advanced fire control system, it was selected as one of the five candidates for the final trade off analysis. The most obvious conclusion from the review of these concept studies was that the level of ballistic protection required by the proposed MN document was incompatible with the weight limitation specified. This problem was outlined in a fact sheet forwarded from the Task Force to the Commanding General of AMC, dated 3 August 1972.

This fact sheet concluded that the only practical solution was to either reduce the level of ballistic protection or increase the 49 ton weight limitation in the proposed MN document. The latter alternative was subsequently adopted. Although General Motors and Chrysler provided support to the MET Task Force under their study contracts for new tank prototypes, the final reports on both of these contracts were not submitted until October 1972. In their report, General Motors proposed two concepts. They were based on estimated average production unit costs of $400,000 and $500,000, both in fiscal year 1972 dollars. Referred to as the 400K and 500K tanks, the latter was a growth version of the former with improvements in ballistic protection, firepower, and night vision. Both were manned by a crew of four with the driver in the left front hull. The usual arrangement of the three man turret crew was reversed placing the gunner and tank commander on the left side of the cannon with the loader at the right rear. The turret on both vehicles utilized the large bearing ring from the XM803. Both tanks were armed with a stabilized 105mm gun M68. Forty rounds of 105mm ammunition were stowed in the hull below the level of the turret ring without separate compartmentation. The 400K tank was protected by welded plate armor arrays on the turret front. The sides and rear of the turret consisted of single plate rolled homogeneous armor. The hull front was a single plate of high obliquity armor. Spaced armor skirts were provided on the forward sides of the hull. The tank was powered by the previously proposed twin General Motors 8V71T diesel engines connected to a modified X-1100 transmission through a transfer case. With this arrangement, the tank could be driven on one engine in an emergency. The twin engine installation was rated at 1200 gross horsepower. The estimated combat weight of the 400K concept was 52 tons. Although the armor protection was superior to the M60A3, it did not meet the requirements for the new MET.
 
The running gear consisted of an advanced high strength torsion bar suspension with six 30 inch diameter road wheels per side running on a 22 inch wide, double pin, flat track. A 7.62mm coaxial machine gun was located on the right side of the cannon and a 40mm high velocity grenade launcher was installed on the commander's hatch. The independently stabilized gunner's day/night sight included a laser range finder. The combat weight of the 500K tank was increased to 55 tons providing improved ballistic protection. The front hull now consisted of spaced rolled homogeneous armor and steel/aluminum array skirts were installed on the sides. On the turret, a cast armor trunnion support was joined to frontal spaced armor arrays. The sides and rear also consisted of spaced armor. Although the M68 105mm gun was retained in the 500K tank, the mount was strengthened to permit future adaptation of a 120mm gun firing kinetic energy ammunition. The 7.62mm coaxial machine gun in the 400K tank was replaced by the Bushmaster (20mm-30mm) weapon system then under development. The 40mm high velocity grenade launcher was moved to the loader's station and replaced on the commander's hatch by a .50 caliber M85 machine gun. The commander's weapon was now mounted on a powered slew ring. The gunner's stabilized sight with its laser range finder also now included a far infrared imager for night operations. Individual displays were provided for both the gunner and the tank commander. With the heavier weight, the 500K tank was powered by the AVCR-1100-3B engine rated at 1450 gross horsepower using the modified X-1100 transmission. The number of road wheels was raised from six to seven per side and the width of the double pin flat track was increased to 24 inches. Chrysler's report on the New Evolutionary Tank reviewed eight different vehicle concepts as well as the development status of various design features and components. Main armament on all eight concept studies was the M68 105mm gun. In one case, this was supplemented by a TOW missile installation. On two other concepts, the Shillelagh II or Swifty gun launched missile was specified. This was a proposed development of the Shillelagh reduced in diameter to permit launching from the 105mm gun. The missile velocity also was increased to reduce the flight time by about 50 per cent and to extend the range by approximately 25 per cent. However, the reduced diameter would have decreased the effectiveness of its shaped charge warhead. The estimated combat weight of the eight Chrysler proposals ranged from about 49 tons to 57 tons. After considering the eight concept studies, the Chrysler report proposed a final configuration combining the best components from two of them. With an estimated combat weight of 57.2 tons, this tank had a conventional layout. The driver was in the center front hull and the three man turret located the gunner and the tank commander on the right side of the 105mm gun with the loader on the left. The 105mm ammunition was stowed in compartments separated from the crew space. These compartments in the turret bustle and the hull were designed to fail away from the crew space. Thus any ammunition explosion resulting from a penetration of the armor would be directed either to the outside or into the engine compartment away from the crew. 

AGT-1500 and AVCR-1360.
The 105mm M68 gun was selected as the main armament with possible future upgrading through improved ammunition or the introduction of a gun launched missile such as the Swifty. A .50 caliber M 85 coaxial machine gun was specified, but the design permitted its replacement by the Bushmaster weapon when it became available. The fire control equipment was a modified M60A3 system with the turret integrated night thermal sight (TINTS). Final selection of the power plant was not made at that time with the German MTU MB 873 Ka 500 (formerly the Daimler-Benz DB1500) 1500 horsepower liquidcooled diesel, the AVCR-1360-1 air-cooled diesel, and the AGT-1500 gas turbine all being considered. It was proposed to select two of these power plants for test rig evaluation. The X-1100 transmission was to be used in all three cases. The advanced torsion bar suspension was selected. As mentioned earlier, this system utilized high strength torsion bars permitting a much greater twist before exceeding the elastic limit of the material. Thus a much greater wheel travel could be obtained. As with other proposed tank concepts, it was obvious that the combat weight would greatly exceed the original target if satisfactory protection levels were to be achieved. A new concept, designated LK 10352, was proposed by TACOM in a fourth study dated July 1972. The main objective of this study was to determine the effect on the gross vehicle weight if the fuel capacity was increased from 300 to 350 gallons. This was similar to the earlier examination of the same increase on the modified LK 10322. The hull was lengthened to 276 inches with the extra six inches between the crew compartment bulkhead and the engine being used to accommodate a tank for the additional 50 gallons of fuel. The lower glacis was redesigned on this new concept and the angle with the vertical increased from 55 degrees to 60 degrees. The upper glacis remained at 65 degrees from the vertical. No changes were made to the turret. The effect of these modifications on the gross vehicle weight was calculated based on the protection required to defeat two threat levels. The greatest of these was the combined threat of the Soviet 115mm APFSDS kinetic energy projectile at a range of 800 meters and a 3.2 inch diameter precision HEAT shaped charge round. The calculation indicated that a vehicle weight of over 52 tons would be required for protection directly from the front out to an angle of 30 degrees to each flank. In August and September, the LK 10352 concept was modified in a fifth study to determine the effect of second generation armor arrays on the gross vehicle weight. The early steel-glass-steel composite armor was replaced by multiple steel and aluminum plate combinations. For this analysis, three threat levels and the armor arrays required to defeat them were supplied by the Ballistic Research Laboratory. The light threat consisted only of the kinetic energy projectile representated by the Soviet 115mm APFSDS at a range of 800 meters. The medium level of protection was required to defeat both the kinetic energy threat and the 3.2 inch diameter HEAT shaped charge round. A new heavy threat was introduced at this time. It consisted of both the 115mm kinetic energy projectile and a 5.0 inch diameter HEAT round.

Chrysler design hull mockup with AVCR-1360 diesel.
This was the threat against which the new MBT was to be protected in future TACOM studies. For the front hull armor array, the upper glacis was angled at 35 degrees and the lower glacis at 70 degrees, both from the vertical. The nose also was extended 2.5 inches to give more room to the driver. The turret was modified from the earlier concept by changing the left front from a curved to a flat surface angled back at 30 degrees to the rear and sloped at 60 degrees from the vertical. The slope of the right front was changed from 60 to 55 degrees from the vertical to provide more room for the gunner. This side already had a flat surface angled to the rear at 30 degrees. The turret sides were unchanged, but the bustle was extended by 12 inches to obtain more stowage space. The .50 caliber coaxial machine gun was replaced by the Bushmaster and the .50 caliber weapon was relocated to the commander's hatch. The study indicated that a combat weight of over 59 tons would be required to provide protection against the heavy threat from the front and at 30 degrees to each flank. A sixth study by TACOM from October 1972 to January 1973 covered the preparation and weight analyses of new design concepts and the building of a full size wooden mock-up. The rapid development of composite armor technology continued to cause repeated changes in the tank design concepts. In fact, according to TACOM personnel at that time, the Ballistic Research Laboratory was cranking out new armor arrays faster than new concept designs based on them could be drawn and analyzed. After a preliminary study, it was decided to draw two different versions of the new tank. One version was to have the main gun ammunition stowed in the crew space as in earlier tanks. In the other, it was to be separated from the crew in ammunition compartments. Two different turret designs were prepared. One turret stowed the ammunition separate from the crew in a bustle compartment. The second turret had a short bustle with no ammunition stowage. Two designs were considered for the tank with compartmented ammunition. In the first type, 18 out of a total of 40 105mm rounds were stowed in automated racks in the turret bustle behind a one inch thick aluminum alloy bulkhead. These racks were similar to those in the XM803 and the rounds were retrieved through a single round door in the bulkhead. Sixteen rounds were in two revolving drum type racks in the front hull, one on each side of the driver. These rounds were retrieved through small round doors in the one inch thick bulkhead facing the turret basket.

Chrysler mobility test rig.
The remaining six rounds were stowed in a non-automated rack in a two inch thick box underneath the cannon in the turret basket. The second design of the tank with the compartmented ammunition eliminated the automated racks. They were replaced by simple lightweight racks and some rounds were relocated. The eighteen rounds remained in the turret bustle, but they were now retrieved through sliding aluminum doors. The six rounds previously stowed in the turret basket were moved to the front hull. The compartments on each side of the driver now contained 11 rounds each and they could be reached through sliding doors. In the non-compartmented version of the new tank, all 40 rounds of main gun ammunition were stowed in the crew compartment below the turret ring. Twentyeight were in two racks, one on each side of the driver. The remaining 12 were located in the turret basket. These were considered to be ready rounds and were in easy reach of the loader. The compartmented and non-compartmented concepts were quite similar in external appearance except for slight changes in the turret design. In addition to the shorter bustle, the non-compartmented version retained the five degree taper to the rear on the turret sides. On the compartmented turret, the side walls were parallel to the center line. This provided more stowage space for ammunition in the bustle. Both sides of the turret front were angled back 30 degrees to the rear and sloped at 45 degrees from the vertical. The left and right sides of the turret were sloped at 35 degrees and 20 degrees from the vertical respectively. The roof was flat and the rear of the bustle was angled at 40 degrees from the vertical. As in the previous concepts, the turret ring diameter was 85 inches. 

Chrysler and General Motors XM1's compared to M60A1 Main Battle Tank.
The hull design was identical for both concepts, except for the ammunition racks. The upper and lower front glacises were angled at 35 and 70 degrees from the vertical respectively. Side skirts extended from the front idler to the rear sprocket protecting both the crew and engine compartments. The power plant was the AVCR-1360 engine with the X-1100 transmission. All of the fuel was located in the engine compartment. A tube-over-bar suspension system with six road wheels per side was used with 26 inch wide tracks. For the commander, a low rotatable station even with the turret roof line was used for both concepts. It was fitted with a .50 caliber M85 machine gun which could be operated from either inside or outside the tank. In addition to the machine gun sight, the commander was provided with six periscopes for all around vision. A Bushmaster weapon system was installed on the left side of the 105mm gun. A day/night sight including a laser range finder was mounted in front of the gunner's station with separate controls for the gunner and the tank commander. Weight analyses of the two concepts armored against the heavy threat indicated that the tank with the compartmented ammunition weighed slightly over 60 tons compared to under 57 tons for the non-compartmented version. The tank with the compartmented ammunition and armored against the heavy threat was selected for the construction of a full size wooden mock-up. The mock-up was modified in several ways from the original concept drawings. The day/night sight for the gunner and the tank commander was relocated to the right side of the turret. This was done to eliminate a weak ballistic area on the turret caused by the previous mounting of the sight. To provide protection against HEAT rounds, armor arrays extending alongside the entire crew compartment were installed on the hull side areas over the sponsons. The overhanging frontal sections of the turret were lowered by about four inches leaving only a minimum clearance for the driver's head when driving with his head exposed. This was to improve the ballistic protection by reducing the shot trap in this area. The turret roof was slightly crowned to increase its rigidity and to eliminate any possibility of buckling. This was achieved by raising the center line one inch and sloping the roof toward the sides at five degrees from the horizontal. After completion of the mock-up, a new concept drawing, designated LK 10372, was prepared incorporating all of the changes.
 
Data on improved armor arrays were received from the Ballistic Research Laboratory at the conclusion of the previous study. To evaluate the effect of these new arrays on the vehicle weight, a new concept was prepared including the changes applied to the mock-up as well as several new features. This design, designated as LK 10379, was covered in the seventh study from TACOM in February 1973. From this point on, the analyses considered only the protection level required to defeat the heavy threat. This was still the Soviet 115mm APFSDS projectile at 800 meters and the 5.0 inch diameter HEAT round. The 40 rounds of 105mm ammunition in the LK 10379 concept were located in the hull below the turret ring and were not stowed in separate compartments. The Bushmaster weapon system was removed from inside the turret and remounted on the left side exterior near the bustle. The day/night sight for the gunner and the tank commander was installed through the right side of the turret as on the mock-up. The gun shield was narrowed from a width of 38 inches to 12 inches. The five degree taper to the rear of the turret side walls was eliminated making the sides parallel to the turret center line. This provided more space for the tank commander and simplified the design of the Bushmaster mount. The slope of the left and right sides of the turret remained at 35 and 20 degrees from the vertical respectively. The angle of the turret frontal sections toward the rear was increased from 30 to 40 degrees, but they remained sloped at 45 degrees from the vertical. The turret roof was crowned as on the mock-up. The hull front on the LK 10379 was redesigned with a 55 degree angle from the vertical on both the upper and lower glacises. This increased the hull length making the six road wheel suspension marginal for good mobility. Therefore, a new tube-over-bar suspension was specified with seven road wheels per track. As on the mockup, armor arrays were installed on the hull sides above the sponsons. The combat weight of the LK 10379 concept was calculated to be a little over 60 tons. The analysis showed that by reducing the protection on certain areas of the hull and lowering the fuel capacity from 350 to 300 gallons, the weight could be reduced to about 58 tons. The eighth and last in the main series of new XM1 concept studies was performed by TACOM in March 1973. Its objective was to explore ways to reduce the weight of the proposed tank to below 58 tons. To achieve this goal, a new concept drawing, LK 10382, was prepared with several changes from the previous design. However, the same armor arrays were used to permit accurate comparisons. The front hull was changed back to the earlier design with the upper and lower glacis plates sloped at 35 and 70 degrees from the vertical respectively. This reduced the length of the hull by 15 inches and lowered the weight by about 900 pounds. The hull height was reduced from 42 to 40 inches and the sides above the sponsons were sloped at a greater angle from the vertical. The fuel tank behind the crew compartment bulkhead was removed and the engine was moved four inches forward. This dropped the fuel capacity back to 300 gallons. The turret ring diameter was reduced from 85 to 83 inches which also shortened the hull by an additional two inches.
 
With the shorter hull, the suspension was changed back to six road wheels per side. The ground clearance was increased from 17 to 18 inches for better mobility and mine protection. Except for the smaller ring, the turret design remained the same, but the Bushmaster was moved farther to the rear, lowered, and mounted closer to the turret wall. The ammunition feeding system for the Bushmaster was changed so that it fed directly through the turret side armor. This eliminated the partially exposed ammunition chutes which formerly were routed through the turret roof. The estimated combat weight of the LK 10382 design was between 57 and 58 tons depending upon weights assumed for the suspension and the possible reduction in thickness of the hull bottom plate under the engine compartment. After the complexity and high cost that resulted in the cancellation of the XM803, Congress required that these factors be tightly controlled in the new tank program. Thus a design to unit cost ceiling was established at the beginning of the project. The specification requirements in the Materiel Need (MN) document were presented as ranges permitting the contractors some leeway in meeting the design objectives. As the Army later observed during a Congressional hearing in April 1974, "We would like to have the maximum performance, but we are sticking to a design to unit cost and we therefore have to give the contractor the ability to make trade offs to meet the design to cost goal". As mentioned earlier, the weight limitation in the original MN document was incompatible with the protection level required. Since crew survivability had the highest priority on the list of characteristics for the new tank, the maximum weight was raised to 58 tons to permit adequate armor. Later, this would be Increased to 60 tons. During the initial studies in 1972, the design to unit cost goal was set at $400,000. However, by the time that the request for proposal was issued, this had increased to $507,790, also in 1972 dollars. This compared to estimated unit costs of $339,000 for the M60A1, $432,000 for the M60A3, and $611,000 for the cancelled XM803. The new program was divided into three phases. The first of these was a competitive advanced development validation phase during which contracts would be awarded to two sources. Each of these contractors would design, build, and test one complete prototype tank and one automotive test rig. They also were to furnish an additional hull and turret for ballistic evaluation. This 34 month advanced development phase would end with a combined developmental/operational test (DT/OT-I) by the Army. These tests would be used to validate the contractors' proposals for phase 2, which would be a sole source full scale engineering development (FSED) program. Phase 3 would put the new tank into production.
 
The various tank concepts being considered by the Army at this time reflected a change in thinking since the period of the MBT70 and the XM803. Although some sources regarded the appearance of the long range antitank missile as sounding the death knell of the tank, this was not the case. The main effect of the new missiles was to release the tank from the defensive role since other units were now capable of defending themselves against armored attack. The tank could now resume its place as the key offensive weapon in the combined arms team. In that role, the missile firing gun-launcher system was less attractive as the main tank weapon than the fast shooting 105mm gun,
particularly with improved ammunition. Thus when proposals were requested for the development of the new tank, the 105mm gun with its quick reaction, multitarget performance, and immediate availability was the obvious candidate despite its shorter range. Proposals for the advanced development validation phase were received from Chrysler and General Motors in May 1973. Originally, Ford Motor Company had been expected to participate, but they declined to enter the competition. As a result, after review, contracts were awarded to Chrysler and to the Detroit Diesel Allison Division of General Motors on 28 June 1973. As mentioned earlier, these contracts required the design, construction, and testing of a completely integrated prototype tank. It was to include all subsystems except for the night vision equipment. An automotive test rig also was to be constructed for use in the test program and an additional ballistic hull and turret were to be provided for ballistic tests at Aberdeen Proving Ground. These cost plus incentive fee (CPIF) contracts eventually amounted to $68,999,000 for Chrysler and $87,969,000 for General Motors. Chrysler proposed a conventional arrangement for its prototype tank with a three man turret crew and the driver in the center front hull. The gunner and the tank commander were in the usual location on the right side of the M68 105mm gun with the loader on the left. A coaxial Bushmaster weapon was fitted on the left side of the cannon. An M85 .50 caliber machine gun was mounted on the tank commander's station and either a 7.62mm M60D machine gun or a 40mm high velocity grenade launcher was installed on the turret roof in front of the loader's hatch.
 
A simplified line of sight stabilization system was expected to provide performance equal to at least 90 per cent of a dual axis system with a cost saving of about $3000. The 105mm ammunition was stowed in separate compartments vented away from the crew space. New composite special armor was incorporated into the frontal areas of the tank and side skirts extended from the front idler to the rear sprocket partially shielding the hydromechanical suspension. This was actually a tube-over-bar suspension with seven road wheels per track using rotary hydraulic shock absorbers. The term hydromechanical suspension was selected to avoid confusion with the earlier tube-over-bar suspension installed experimentally on the M60A1 which lacked the performance of the new design. It also was noted that the same name could be applied to the advanced high strength torsion bar suspension if it replaced the tube-over bar design in the future. At the time of the proposal, the advanced torsion bar was considered a high risk item, but the option of using high strength steel or titanium torsion bars was kept open for the future. In regard to the power plant, Dr. Philip W. Lett Jr., the leader of the Chrysler team, made a bold decision. Despite the fact that the Army MET Task Force had considered the gas turbine to be a high risk item, he selected the AGT-1500 with the X-1100 transmission to power their prototype. In retrospect, this appears to have been a crucial decision in the design of the new tank. Subsequent events were to prove that the AGT-1500 would develop into a highly reliable power plant and it had several advantages over its diesel competitors. Although both the AGT-1500 and the AVCR-1360 diesel were rated at 1500 gross horsepower, the former utilized only 30 horsepower for cooling compared to 160 horsepower for the latter. Also, a growth potential to 2000 gross horsepower was estimated for the turbine compared to 1800 gross horsepower for the variable compression ratio diesel. The less complex design of the turbine engine with its fewer parts was expected to provide higher reliability and reduced maintenance in the future. Its lighter weight also permitted increased armor protection without exceeding the overall weight limit for the tank.
 
The gas turbine did not generate smoke during operation and it was capable of using a wider range of fuels than the diesel. Also, the turbine was easier to start at low temperatures. The greatest disadvantage of the gas turbine was its higher fuel consumption. This was particularly obvious at low vehicle speeds. However, the small size of the
power plant allowed space for an increase in fuel capacity to achieve the specified cruising range. Another problem was the high volume of combustion air used by the gas turbine. This required a filter system with approximately three times the capacity of that necessary for the diesel engine. The turbine, of course, did not require the high volume of cooling air used by the air-cooled diesel. The development of the gas turbine tank engine resulted from recommendations of the Committee on Gas Turbine and Electric Propulsion for Tanks, chaired by Mr. James C. Zeder. When presented in 1965 to the Mobility Advisory Group, the recommendations of the Zeder Committee were initially rejected because of cost. However, the program was subsequently approved. After the evaluation of several proposals from industry, the Lycoming Division of AVCO was selected to develop the
new turbine, designated as the AGT-1500. At that time, it was considered as a possible power plant for the MBT70. However, after the selection of the diesel engine for that program, attempts were made to cancel the AGT-1500 and to transfer its funds elsewhere. These attempts were prevented by the efforts of Lieutenant Colonel John W. Wiss, Chief of the Army Tank Automotive Command Laboratories, and Mr. Wayne S. Anderson who directed the Propulsion Systems Laboratory. Thus the AGT-1500 was available when needed for the XM1 program and its state of development was not far from that of the AVCR-1360 variable compression ratio diesel. Although the initial cost of the gas turbine exceeded that of the diesel engine, it was anticipated that the life cycle cost would be competitive because of reduced maintenance requirements.
 
The early General Motors concept for the XM1 validation phase tank also was armed with the 105mm gun M68 and a Bushmaster. However, the latter was located in a separate mount on the left side of the turret. The turret crew rode in the usual positions with the gunner and tank commander on the right side of the cannon and the loader on the left. A .50 caliber M85 machine gun was provided for the tank commander and a 7.62mm machine gun was installed at the loader's position. The main gun ammunition was separated from the crew space in compartments. The driver was located in the left front hull and special armor was incorporated in the frontal areas of the tank. The Continental AVCR-1360 diesel engine drove the tank through the General Motors X-1100 transmission. As mentioned earlier, the jWCR-1360 diesel was a 1500 horsepower development of the AVCR-1100 used in the MBT70 and the XM803. The suspension on the General Motors tank was fitted with six road wheels per track. This was a hybrid suspension with road wheels 1, 2, and 6 fitted with the National Water Lift hydropneumatic suspension units. Road wheels 3 and 5 utilized high strength torsion bars. In early July 1973, Major General Robert J. Baer, the XM1 Project Manager, and personnel from the Ballistic Research Laboratory escorted representatives from Chrysler and General Motors to the United Kingdom for an update on the British developed Chobham special armor.
 
During this trip they observed the proposed design of a new United Kingdom vehicle utilizing this special armor as well as the manufacturing processes required for its
production. Based upon the newly obtained data, both contractors reevaluated their proposed armor configurations. Chrysler made a number of changes, but they retained the basic design in their original proposal. However, General Motors made major changes in the configuration of their proposed tank. The most obvious change was in the turret where the vertical front and side walls were replaced by sloped armor. The modifications by both contractors required additional work by the Ballistic Research Laboratory to develop new special armor combinations. By January 1974, both Chrysler and General Motors completed the final design of their special armor configurations and some of the hull and turret armor structures had been delivered to Aberdeen Proving Ground for ballistic tests. Some changes in the specifications for the new tank resulted from a study of the Israeli experience during the October 1973 war in the Middle East. One change which was applied to both of the validation phase prototypes was the replacement of the Bushmaster weapon system with a 7.62mm coaxial machine gun. The original intention had been to utilize the Bushmaster against lightly armored vehicles thus reducing the stowage requirement for the expensive main gun rounds. However, battle experience showed that the tank crews would invariably use the main gun against these vehicles. Also, there was a need for a coaxial machine gun for use against infantry and to suppress antitank weapons. Elimination of the bulky Bushmaster and its ammunition provided space to increase the number of 105mm rounds to 55. Some of the modifications resulting from the study of battlefield experience were applied at a later date to the full scale engineering development (FSED) tank proposals. These included changes in the ammunition compartments to accommodate the larger number of 105mm rounds and the use of a less flammable hydraulic fluid.

Early and late FSED designs, notice changes in the turret design.
The complete validation phase prototypes were delivered to Aberdeen Proving Ground for the combined developmental/operational test (DT/OT-I) which ran from 31 January to 7 May 1976. During this same period, the competing automotive test rigs were driven more than 3000 miles over a variety of terrain. Both prototypes were displayed to the public for the first time on 3 February 1976. The results of the DT/OT-I indicated that both the Chrysler and the General Motors prototypes met the specified requirements for the new tank. The Chrysler tank with its gas turbine power plant showed slightly greater acceleration, but the performance was satisfactory for both vehicles. The operational portion of the tests included two weeks of simulated combat operations with the tanks manned by crews from Fort Knox and Fort Hood. After the evaluation of the competing prototypes, the selection of the contractor for the full scale engineering development phase was scheduled for July 1976. 

From left to right: Early Chrysler XM1 FSED with AGT-1500 gas turbine, final XM1 FSED with AGT-1500, and final XM1 FSED with AVCR-1360 diesel.
After the cancellation of the MBT70 program, the Federal Republic of Germany continued with the development of their new Leopard II tank. In line with efforts to standardize materiel in NATO, Germany proposed that the Leopard II be modified to meet American requirements. In December 1974, a Memorandum of Understanding between the United States and the Federal Republic of Germany was signed in which the U.S. agreed to evaluate the Leopard II under the same test conditions as the XM1 candidates. At that time, it was estimated that the Leopard II would cost about one million 1975 dollars compared to approximately $750,000 for the XML The modified version, designated as the Leopard II AV was armed with the 105mm gun. It had improved armor protection and a simplified fire control system to reduce costs. Tests of the Leopard II AV were carried out at Aberdeen Proving Ground from September to December 1976. Previously, on 22 July 1975, FMC Corporation had been awarded a contract for a study to determine the costs and technical problems involved in manufacturing the Leopard II AV in the United States. The tests at Aberdeen indicated that the Leopard II AV met or exceeded most of the U.S. requirements. The accuracy of its fire control system was slightly higher than that of the XM1, but it was more expensive. The armor protection, ammunition compartmentation, and gun movement were considered somewhat inferior to that on the American tank. Rather than continue the evaluation of the complete tank, it was agreed in January 1977 that only certain major components would be considered for adoption to enhance NATO standardization.
 
Shortly before the end of the XM1 validation test program, a new requirement was introduced for inclusion in the proposals for the full scale engineering development/ producibility engineering and planning (FSED/PEP) contract. These proposals were now to include firm price ceilings for the first two production options. These were for 110 low initial rate production (LIRP) tanks in fiscal year 1979 and 352 tanks in fiscal year 1980. The winner of the competition for the XM1 FSED/PEP contract was selected on schedule in July 1976, but the results were not released by the Army. Information published later in the press indicated that General Motors won with a bid of 208 million dollars compared to Chrysler's bid of 221 million dollars. However, these bids were based upon the Chrysler and General Motors tanks being powered by the AGT-1500 turbine and the AVCR-1360 diesel respectively. The Army, by this time, favored the gas turbine and although awarding the contract to General Motors, planned to change the power plant to the AGT-1500. However, this approach was rejected by the Office of the Secretary of Defense. As the Deputy Secretary of Defense later testified before Congress: "I was unalterably opposed to making these changes in a sole source environment after the contractor was selected. I wanted to know what those unit costs were, what the delays in the program might be, if any, and have those costs determined competitively between the two contractors." Also, a decision had been made to arm the new tank with a 120mm gun at some time in the future. Therefore, the turret had to be capable of mounting either the 105mm gun M68 or a 120mm gun without extensive modification. The 120mm guns under consideration were the British rifled weapon and the German Rheinmetall smooth bore gun used in the Leopard II. As a result of the new requirements, a revised request for proposal was issued, with the award of the FSED/PEP contract now rescheduled to November 1976. Both contractors submitted proposals by 28 October for versions of their tanks with a hybrid turret capable of mounting either a 105mm gun or a 120mm gun. In the interest of NATO standardization, provision was made for the use of the gunner's auxiliary sight from the Leopard II as well as metric fasteners at crew serviced interfaces. General Motors presented a version of their tank powered by the AGT-1500 gas turbine and Chrysler proposed an alternate design using the AVCR-1360 diesel engine. However, they recommended their turbine driven tank as the primary choice. All of the proposed tanks were fitted with British developed smoke grenade launchers.
 
Prior to the submission of the final proposals on 28 October, Chrysler made a number of additional changes to enhance the survivability of their candidate and to reduce the cost. The arrangement of the special armor was modified and a special armor gun shield replaced the casting on the earlier proposal. A new minilaser range finder was incorporated and an extension from the gunner's primary sight was provided for the tank commander removing the need for a separate sight. Other cost reductions included the elimination of the power elevation for the commander's weapon and the driver's built in test equipment (BITE) panel. The latter was replaced by pop-up indicators and other gages in the engine compartment and at the crew stations. 
 
At a news conference on 12 November 1976, it was announced that Chrysler had won the FSED/PEP contract. Published reports indicated that the cost reduction efforts at Chrysler had reduced their bid to 196 million dollars compared to a 232 million dollar bid from General Motors. The increase in the latter was, no doubt, partially due to the higher initial cost of the AGT-1500 turbine compared to the AVCR-1360 diesel engine. However, Chrysler's diesel powered proposal for the FSED/PEP program was priced at 186 million dollars. The five month delay in awarding the contract had permitted further refinement of the design and better determination of the manufacturing costs. For
example, the design to unit cost for Chrysler's tank decreased from $574,000 for the original FSED version to $422,000 for the final turbine powered tank proposal. The unit cost for Chrysler's diesel tank design was $376,000. All of these prices were in 1972 dollars for comparison with the earlier estimates. The actual unit price of the winning tank was expected to be about $754,000 in 1976 dollars. Under the 36 month FSED/PEP contract, Chrysler was to manufacture 11 new pilot tanks incorporating the latest changes resulting from the validation phase tests and other experience. The validation phase prototype and the automotive test rig also were to be refurbished for use in the test program. Evaluation of these vehicles would determine the details of the final design for low initial rate production (LIRP). All of the 11 new pilot tanks were manufactured at the Detroit Army Tank Plant with the first one being delivered in February 1978. The last of the 11 was completed the following July. Developmental Test II (DT-II) extended from February 1978 to September 1979 at Aberdeen Proving Ground. Operational Test II (OT-II) at Fort Bliss, Texas lasted from April 1978 to February 1979 with the tanks manned by the 2nd Squadron, 3rd Armored Cavalry Regiment. Three of the tanks, after testing at Fort Bliss, were refurbished at Detroit, fitted with the latest configuration hardware, and shipped to Fort Knox in June 1979 for additional durability and reliability testing. This program was conducted by H Company, 2nd Squadron, 6th Cavalry under the direction of the Armor and Engineer Board. During these tests, the reliability, availability, maintainability-durability (RAM-D) goal was 272 mean miles between failures (MMBF). The XM1 reached 326 MMBF. Initially, Chrysler personnel provided the maintenance, but in August the organizational maintenance was taken over by the soldiers of H Company for the remainder of the test program. During the OT-II at Fort Bliss, many of the power train failures were caused by dust being ingested into the engine. After modification of the seals in the air filtration system, there were no further failures from this cause during all of the tests at Fort Knox. The tests at Fort Bliss also revealed a tendency for the tracks to be thrown by the buildup of soil between the sprocket hub and the bottom of the rear sponson. Some tracks also were thrown by rapid backing and turning when changing firing positions. This problem was solved by the combination of some additional suspension hardware and the redistribution of the ground pressure with an increase in track tension. The hardware change consisted of a sprocket hub with discharge ports, a mud scraper, steel blocks welded to the lower rear hull to prevent inside throws, a steel plate mounted above the final drive to maintain track alignment, and a retaining ring on the outside of the sprocket. Satisfactory results were obtained with these modifications during further tests at Fort Bliss in October 1979.

Major General Baer (far left) and Dr. P. W. Lett (at right) are with a group observing the AGT-1500.
The survivability of the XM1 was demonstrated during DT-II at Aberdeen using pilot vehicle 11 (PV 11). The tank was fully loaded with fuel and ammunition with dummies placed in the various crew positions. Sensors were installed throughout the tank and, with the engine idling, it was fired upon using various types of ammunition at typical combat ranges. The tank was not destroyed and it was driven away under its own power after the tests. PV 11 also was subjected to a blast from an antitank mine which badly damaged the suspension. However, after short tracking one side of the vehicle, the tank was started and driven away from the test site. On 7 May 1979, the XM1 was approved by the Secretary of Defense for low initial rate production (LIRP) at the newly equipped Lima Army Tank Plant. As mentioned previously, this was for 110 tanks with the first two being delivered at a special acceptance ceremony on 28 February 1980. At this time, the new tank was named the Abrams in honor of the late General Creighton W. Abrams, an outstanding armor commander and former Army Chief of Staff. The General's wife and three sons attended the ceremony and Mrs. Abrams christened the first production tank.
 
The LIRP tanks began developmental testing (DT-III) in March 1980 and it continued until September 1981. The major part was conducted at Aberdeen Proving Ground for RAM-D testing. Other work was at Yuma, Arizona for desert tests, Eglin Air Force Base, Florida and the Alaska Cold Region Test Center for cold room and arctic environment testing, and the White Sands Missile Range, New Mexico for electromagnetic radiation and nuclear vulnerability evaluation. The operational tests (OT-III) started at both Fort Knox and Fort Hood during September 1980 and continued until May 1981. At Fort Hood, the tanks were operated and maintained by the 2/5th Cavalry and the 27th Maintenance Battalion. The new tank was enthusiastically received by the troops who greatly appreciated its speed and agility. The operational tests revealed that the cruising range of the production tank was about 20 to 25 miles less than the objective of 275 miles at 25 miles per hour. This was attributed partially to the suspension modifications made to reduce the track throwing experienced during DT/OT-H. However, it was noted that the turbine operated more efficiently at higher speeds and that an increase in the tank test speed from 25 to only 27 miles per hour produced an improvement in cruising range. Thus by operating at higher speeds, the required cruising range could be obtained.
 
In February 1981, production was approved for 7058 tanks at a rate of 30 per month. The vehicle was classified as standard on 17 February 1981 as the 105mm gun full tracked combat tank M1. Production problems at AVCO Lycoming resulted in their failure to meet the deliveryf schedule and also produced a large number of defects in the engines that were delivered. Poor quality control and shortages of skilled personnel were the primary causes. Eventually these problems were solved and the number of defective engines was minimized as production increased. Early tank production also suffered from delays in obtaining the thermal imaging systems and from some fabrication difficulties with the hull and turret at the Lima plant. These problems also were solved and the production of the new tank soon reached the required level.
 
Unlike the M48 and M60 tank series, the M1 did not make use of large armor steel castings. The hull and turret were assembled by welding sections of rolled homogeneous steel armor plate to which the special armor was attached. These steel sections were cut to size using precision flame cutting equipment. The hull was divided into three sections with the crew compartment in the center separated from the fuel cells in the front and the engine compartment in the rear by two bulkheads. In addition to the two fuel cells adjacent to the driver's station, two were located in the engine compartment and two in the rear sponsons. A Halon fire extinguisher system with infrared sensors was installed in both the crew and engine compartments. It could detect a fire in 4-5 milliseconds and extinguish it in about two tenths of a second. The hatch above the driver's seat was fitted with three periscopes providing an overlapping field of vision exceeding 120 degrees. Like on the XM803, there was no floor escape hatch. The driver's T-bar steering control incorporated motorcycle type hand throttles eliminating the need for a floor accelerator pedal thus providing more foot space. The transmission shift selector and push buttons for the intercom system also were located on the T-bar assembly. The service brake pedal was in the center of the floor allowing braking with either foot. The driver's seat was adjustable for driving with the hatch either open or closed and the seat height could be set to four different positions. A large knob on an adjustable lower lumbar support allowed the driver to control the amount of lower back support. The aluminum turret basket floor was suspended from five posts, two of which mounted the seats for the tank commander and the loader. A door in the basket floor provided access to components in the bottom of the hull. The commander's station permitted observation in the seated position through both the extension from the gunner's primary sight and the sight for the .50 caliber machine gun. Six periscopes permitted 360 degree vision. The three positions of the commander's platform allowed him to observe standing with the hatch in the protected open position or at two heights with the hatch fully open. The commander's .50 caliber machine gun was electrically powered in azimuth, but it was elevated manually. The loader at the left rear of the cannon had an eight position swiveling seat that was adjustable in the vertical direction. An eight round armored hull ammunition compartment was located on the right side of the tank behind the engine compartment bulkhead. Blowout panels in the top and bottom of the hull vented any ammunition explosion to the outside and away from the crew. Fortyfour 105mm rounds of ammunition were stowed in the turret bustle behind sliding armor plate doors. Twentytwo of these rounds could be reached by the loader without leaving his seat. A knee switch operated the bustle compartment door covering these 22 ready rounds on the left side of the turret. The door also could be operated manually in an emergency. Blowout panels in the bustle roof vented any ammunition explosion to the outside. Three additional 105mm rounds were stowed on the turret basket floor to the left of the cannon in spall protection covers. This brought the total 105mm ammunition stowage to 55 rounds.
 
The loader's hatch cover was fitted with a rotatable periscope mount. A 7.62mm M240 machine gun was skate mounted around the outer edge of the loader's hatch. Another 7.62mm M240 machine gun was installed coaxially on the right side of the 105mm gun M68E1 above the gunner's auxiliary sight. The gunner's seat was pedestal mounted on the turret basket floor and it was adjustable in both the vertical and horizontal directions. No adjustments were required when the gunner shifted from the primary sight to the auxiliary sight. The gunner's primary sight with its extension for the tank commander was the main optical sighting instrument. It also included the laser range finder, the thermal vision system, and the gyro stabilized line of sight platform. The sight was protected by an armor steel cover with doors operable from within the turret. The gunner's auxiliary sight was an eight power articulated telescope installed in the right side of the gun mount below the coaxial machine gun. The heart of the fire control system was the digital computer which received signals from the sensors as well as inputs from various controls. Data from the laser range finder, the cant sensor, and the wind sensor were received automatically, but other inputs were entered into the computer manually. The gunner used the muzzle reference system to determine tube bend corrections which were then entered into the computer. Atmospheric temperature and pressure as well as ammunition temperature and tube wear also were manual inputs to the computer.

General Motors XM1.
The powerpack consisted of the AGT-1500 turbine engine, the X-1100-3B transmission, final drives, air cleaner, scavenging blower, and cooling system. The powerpack weighed about 8500 pounds and could be easily removed as a unit using a five ton wrecker. The sprockets at the rear of the vehicle drove the 25 inch wide tracks and the tank rode on seven dual road wheels per side. The 25 inch diameter compensating idler installed at the front of each track was interchangeable with the road wheels. It was mounted on a pivoting idler arm connected to the number one road wheel arm by a track adjusting link. Adjustment of the static track tension was by hand pumping grease into the link. When the proper tension was reached, a pressure relief valve opened releasing any additional grease.
 
The vehicle was sprung by high strength steel torsion bars accessible from the outside on both sides of the tank. Thus, if a torsion bar fractured, both pieces could be more easily extracted. There were only two track support rollers per track compared to three on the validation phase prototype. The production tank was governed for a maximum level speed of 45 miles per hour and it could operate cross country at speeds up to 30 miles per hour. 
 
On 19 February 1982, it was announced that a preliminary agreement had been reached between Chrysler and General Dynamics Corporation for the latter to purchase the Chrysler Defense Division. The deal was completed in March and the organization was renamed as the General Dynamics Land Systems Division. The personnel remained
the same with Dr. Philip W. Lett, Jr. becoming Vice President of Research and Engineering. Thus Chrysler severed its connection with the development and production of tanks which had existed since before World War II.
 
It had always been intended to produce the M1 at both the Lima and the Detroit tank plants. In September 1981, the production of 60 tanks per month was authorized with 30 at Lima and 30 at Detroit. This was based on a single work shift at both plants. A total of 150 tanks per month was possible with multi-shift operation. Production of the M1 began at Detroit alongside the M60A3 during March 1982 and the first tank was accepted by the Army at a ceremony on 31 March. Production of the basic M1 continued at both plants for a total run of 2374 tanks. The last basic M1 was completed in January 1985.

This photograph was taken at the rollout of the first XM1 pilot tank at Detroit in February 1978. In the view above, the two center figures
are Dr. P. W. Lett (left) and Eugene Trapp (right).
*Note - all vehicle weights are provided in short tons.
 
PS. I will update this post a bit later with drawings and photos. And then move on.

Survivability
Armor protection of the M1 is very interesting subject because it's not entirely well understood what happend with requirements when US joined the British "Burlington" program.
One is certain, that original requirements for 115mm APFSDS at 800m and 127mm HEAT warhead over frontal arc, are requirements for early armor types developed by BRL, and are not connected with later development.
Recently the CIA declassified some drawings of the original M1 armor.

This confirms that vehicle was protected by "Burlington" type armor. Similarities can be observed when we compare these drawings with the British drawings of the "Burlington" armors developed for Chieftain. I strongly recommend here articles by Polish historian Paweł Przeździecki.
http://wceo.com.pl/images/Dokumenty/WBBH/PHW/PHW_3_2011.pdf  -> PDF reader page 112.

http://wceo.com.pl/images/Dokumenty/WBBH/PHW/PHW_4_2011.pdf  -> PDF reader page 106.

Another interesting factors is that declassified document that says one of the armor types used in M1 turret, offered KE protection = 400mm RHAe and CE protection = 750mm RHAe at front.



Another document is also interesting because it says.


This fragment is very interesting when taken in to account later fragment of this post when I will talk a bit about armor types. Because it makes situation confusing. We know that BRL tested two armor types, BRL-1 that is assumed to be the original M1 armor, and heavier, better performing BRL-2. The question is, are both armors related? And what type of armor was used by M1IP variant, did it used simply the same armor as M1, and only improvement was added through increasing turret front armor thickness? Or perhaps M1IP used the same armor type as M1A1?
When it comes to the armor itself, this is based on my estimations and measurements I made on the real thing.

 
Besides that:

Hull sides over crew compartment - 40mm + 30/40mm = 70-80mm.
Hull sides over engine compartment - 40mm.
Hull sides around suspension attachement points - 40mm.
Hull rear - 40mm.
Upper front plate and drivers hatch - 50mm at 82-85 degrees = 359.26/573.69mm.
Turret roof front - 50mm at 82-85 degrees = 359.26/573.69mm.
Turret roof around hatches and turret bustle - 25mm.
Engine compartment roof - 25mm.
Hull belly - 25mm.
Heavy ballistic skirts - 70-80mm.
Non ballistic skirts - 5-10mm.
 

Before we move to the armor itself it's also worth to note something about hulls and turrets.
Because there is no official classification I needed to create one by myself. However there are very clear difference between various generations of tank variants.
Turrets:
Type 1 "Short Turret", used only in original M1, it have thinner frontal armor that have effective armor thickness ~700-750mm, and a bit different roof over turret bustle with 3 cutouts for blow off panels.
Type 2 "Interim Long Turret", used only in M1IP variant, have thicker frontal armor, effective thickness ~800-900mm (Steven Zaloga in one of his books said that armor thickness was increased by ~200mm), no other structural changes.
Type 3 "Long Turret", this is the final turret variant used in M1A1 and M1A2, older turrets can't be upgraded to this variant due to significant structural changes thus all these turrets are new builds, even if M1 or M1IP was upgraded to M1A1 or M1A2 standard. Front armor effective thickness is also ~800-900mm.
Hulls:
Type 1 used in M1 and M1IP, it lacks the overpressure NBC protection system, probably have some minor differences with the next type. Front armor effective thickness where special armor is mounted is ~650-700mm and did not changed till this day.
Type 2 used in M1A1 and M1A2 variants, major difference is that overpressure NBC protection system is present, some other minor differences are present depending on tank variant, Type 1 hulls can be upgraded to Type 2, however it requires complete disassembly, cutting and welding in some places.
About special armor itself, there were several types used:
BRL-1 (1980) used in original M1.
BRL-2 (1984) used in M1IP and M1A1 (Not certain!).
HAP 1st generation (1987/1988) used in early production M1A1HA.
HAP 2nd generation (1990) used in late production M1A1HA as well as new M1A1HC and M1A2.
HAP 3rd generation (1999/2000) used in M1A1SA, M1A1FEP, M1A2SEPv1 and M1A2SEPv2 (there is possibility of some upgrades for this armor in 2010+ period of time).
NGAP/NEA (2017+) used in newest M1A2SEPv3.
EAP (?) used in all export variants for Arab states.
BRL stands for Ballistic Research Laboratory, these were designations of lighter and heavier armor variants based on British research, that were developed by BRL in US, it's assumed that BRL-1 was for original M1, while BRL-2 was improved, heavier version for M1IP and M1A1 however as mentioned before, this is not certain.

HAP stands for Heavy Armor Package that uses components made from depleted uranium alloy, we know that there are at least 3 generations of this armor, there is also special variant for Australia where depleted uranium was replaced by something else, perhaps tungsten alloy, but Australian documents I found suggest that it's protection levels are equal to 3rd generation Heavy Armor Package, originaly it was said this type of armor is used in turret front only, however I found mentions in official sources saying it is used also in hull front and turret sides.

EAP or Export Armor Package is meant for non NATO/Close allies (like Australia) of the US, used in tanks provided to Arab states, some theoretize it might be equivalent to BRL-2 but it's probably heavier and offer better protection, but lower than HAP armor package.
One more important thing to note, today we also know that at least the newest M1's also uses Titanium alloy in their armor and several other components.
http://c.ymcdn.com/sites/www.titanium.org/resource/resmgr/2005_2009_papers/Gooch_Final_2007.pdf
http://c.ymcdn.com/sites/www.titanium.org/resource/resmgr/2010_2014_papers/GoochWilliam_2010_MilitaryGr.pdf


There are also two types of serial numbers. One for the turret, and one for the hull, serial numbers on turrets says us what type of armor vehicle uses, and on the hull where it was made. X will equal digit.

Turret:

XXXX - BRL-1 or BRL-2 armor package.
XXXXU - Heavy Armor Package.
XXXXM - Heavy Armor Package (some US tanks have it, don't know why that change in letter).
XXXXA - Heavy Armor Package variant for Australia.
XXXXE - Export Armor Package for Arab states.

Hull:

XXXXD - Tank was made in Detroit Tank Arsenal.
XXXXL - Tank was made in Lima Army Tank Plant/Joint Systems Manufacturing Center.

When it comes to armor itself, today we know it is indeed some type of advanced NERA or NxRA armor, however we only know more or less the structure of the turret side armor, and still only of older variant probably M1A1HA or M1A1HC.

This is how probably turret side armor over crew compartment looks like.

And this is how turret bustle side armor looks like.

And it makes sense, on one of the photos we can see some turrets after armor replacement/upgrade work in Joint Systems Manufacturing Center Lima, Ohio. We can notice that their frontal and side armor was replaced, but turret bustle was untouched.

With such evidence we can assume that turret armor is only replaced over crew compartment, while turret bustle armor might be untouched, and it makes sense from armor protection distribution point of view, where majority of protection increase and thus weight increase we want around the crew and for vehicle frontal 60 degree arc.

Here some more sources.

And here US Army Weapons Hand Book 2016.

https://www.army.mil/e2/c/downloads/431298.pdf

At page 14 in PDF reader we can read for M1A1SA and M1A2SEP "Survivability improvements include frontal armor and turret side-armor upgrades.", of course term "frontal armor" means most likely both turret and hull, and we have confirmation of side turret armor upgrades. And it also seems that M1A1SA and M1A2SEP shares the same armor type, which makes sense because due to reduced costs, simplicity etc. at one specific time you manufacture only one specific type of armor and it makes sense for MBT's even in various but not that much different variants to share the same armor protection. USMC M1A1FEP is also technically equivalent to US Army/ARNG M1A1SA with some minor differences, thus it's safe to assume that it also shares the same armor.

Good question is how exactly DU is applied to the armor design? We do not know, but perhaps it might be part of the NERA array itself like in this prototype British armor (perhaps one of several armor types developed within "Burlington" program?) which uses DU alloy plates with polycarbonate as a reactive layer?



When it comes to armor steel alloys used in the vehicles construction and armor, most likely these standard NATO steel types are used with such range of thickness and hardness.




Another thing is that M1's armor, like all modern 3rd generation MBT's armor is modular or semi modular, which makes it's replacement or repair relatively easy, quick and cheap. Another factor is that M1's turret and hull have very simple geometrical shapes making both manufacturing relatively simple, quick and cheap, as well as armor replacement/repair will be such relatively simple, quick and cheap task.

This also means that frontal protection is uniform for both hull and turret, and have very small weak zones.


Some additional protection over hull front is added by massive fuel tanks (visible on one of the drawing above), Israeli data says that 70mm of fuel is equivalent for 10mm of armored steel, thus we can assume that front hull protection where fuel tanks are is significantly increased.

However the largest improvement in protection will be made in M1A2SEPv3, photographs of one of the 9 prototypes shows us vehicle with massive weight simulators on turret front and hull front. This prototype still uses 3rd generation HAP armor package because it was originally M1A2SEPv2 converted in to M1A2SEPv3 prototype.


Each turret cheek have 4x welded plates, each looks to be ~50mm thick at most. Hull front have 3x welded plates, also each looks to be ~50mm thick at most. This means very significant weight increase over the previous generation armor, so we can assume also very significant protection levels increase over the front. There are also weight simulators packs welded to the turret roof, two have 4x 50mm plates and one have 3x 50mm plates.

http://www.dote.osd.mil/pub/reports/FY2015/pdf/army/2015m1a2sep3.pdf

Some additional informations about vehicle survivability improvements can be found here.

Final armor protection improvement are M19 ARAT-1 and M32 ARAT-2 Explosive Reactive Armor modules.

M19 ARAT-1 is modern, multilayer explosive reactive armor, which might have some sort of anti-tandem warheads capabilities.








From these photos we can see angled layers of ERA inside the module, and perhaps some sort of passive component?

Also to M19 modules, can be attached M32 modules.


It's very unusual design for ERA, and it's interesting it can be combined with M19 modules in to a single, multilayer type of ERA protection. M32 modules can also be attached alone to the turret sides.

In such configuration both turret and hull sides have very good, right ERA cover. The system is modular and can be arranged per need.

Of course besides it's armor M1 have other survivability meassures, primary one is mentioned earlier complete isolation of the main gun ammunition storage. It needs to be noted that the biggest tank and crew killer is not the enemy ammunition penetrating the armor but own ammunition cooking off.

The original M1 and M1IP had such ammo storage:

Turret bustle ready rack - 22 rounds.
Turret bustle semi ready rack - 22 rounds.
Turret basket floor armored box - 3 rounds.
Hull ammunition rack - 8 rounds.

M1A1 and M1A2 have such ammo such ammo storage:

Turret bustle ready rack - 17 rounds with old type of racks and 18 with new type of racks.
Turret bustle semi ready rack - 17 rounds with old type of racks and 18 with new type of racks.
Hull ammunition rack - 6 rounds.
 
 




Blow off panels and ammo racks in turret are connected, so if possible during ammunition cook off, blow off panels will be able at least to some degree pull out ammo racks with burning ammo outside, thus minimizing the damage.



These are armored blast doors for the hull ammunition magazines.



Hull blow off panels are placed in top (we can see large opening for main blow off panel) and belly of the hull (we can see two smaller openings for blow off panels).

In future another upgrade that will increase M1's already fantastic protection and survivability will be hard kill active protection system, however and it's not widely known, M1's had in the past, but not used often a soft kill active protection system or actually two, the AN/VLQ-6 and AN/VLQ-8, both are sort of IR dazzlers which work similiar to Russian TShU-1-7 Shtora-1 and Ukrainian Varta.





In future also M1 will be equipped with Hard Kill Active Protection System, most likely Israeli Trophy HV system as a mid term solution before new Modular Active Protection System will be ready.
 

And finally, another part of survivability improvements are multispectral camouflage nets and covers.

US Army is purchasing Saab Barracuda MCS.



And also uses a set of camouflage nets.

Survivability
Armor protection of the M1 is very interesting subject because it's not entirely well understood what happend with requirements when US joined the British "Burlington" program.
One is certain, that original requirements for 115mm APFSDS at 800m and 127mm HEAT warhead over frontal arc, are requirements for early armor types developed by BRL, and are not connected with later development.
Recently the CIA declassified some drawings of the original M1 armor.

This confirms that vehicle was protected by "Burlington" type armor. Similarities can be observed when we compare these drawings with the British drawings of the "Burlington" armors developed for Chieftain. I strongly recommend here articles by Polish historian Paweł Przeździecki.
http://wceo.com.pl/images/Dokumenty/WBBH/PHW/PHW_3_2011.pdf  -> PDF reader page 112.

http://wceo.com.pl/images/Dokumenty/WBBH/PHW/PHW_4_2011.pdf  -> PDF reader page 106.

Another interesting factors is that declassified document that says one of the armor types used in M1 turret, offered KE protection = 400mm RHAe and CE protection = 750mm RHAe at front.



Another document is also interesting because it says.


This fragment is very interesting when taken in to account later fragment of this post when I will talk a bit about armor types. Because it makes situation confusing. We know that BRL tested two armor types, BRL-1 that is assumed to be the original M1 armor, and heavier, better performing BRL-2. The question is, are both armors related? And what type of armor was used by M1IP variant, did it used simply the same armor as M1, and only improvement was added through increasing turret front armor thickness? Or perhaps M1IP used the same armor type as M1A1?
When it comes to the armor itself, this is based on my estimations and measurements I made on the real thing.

 
Besides that:

Hull sides over crew compartment - 40mm + 30/40mm = 70-80mm.
Hull sides over engine compartment - 40mm.
Hull sides around suspension attachement points - 40mm.
Hull rear - 40mm.
Upper front plate and drivers hatch - 50mm at 82-85 degrees = 359.26/573.69mm.
Turret roof front - 50mm at 82-85 degrees = 359.26/573.69mm.
Turret roof around hatches and turret bustle - 25mm.
Engine compartment roof - 25mm.
Hull belly - 25mm.
Heavy ballistic skirts - 70-80mm.
Non ballistic skirts - 5-10mm.
 

Before we move to the armor itself it's also worth to note something about hulls and turrets.
Because there is no official classification I needed to create one by myself. However there are very clear difference between various generations of tank variants.
Turrets:
Type 1 "Short Turret", used only in original M1, it have thinner frontal armor that have effective armor thickness ~700-750mm, and a bit different roof over turret bustle with 3 cutouts for blow off panels.
Type 2 "Interim Long Turret", used only in M1IP variant, have thicker frontal armor, effective thickness ~800-900mm (Steven Zaloga in one of his books said that armor thickness was increased by ~200mm), no other structural changes.
Type 3 "Long Turret", this is the final turret variant used in M1A1 and M1A2, older turrets can't be upgraded to this variant due to significant structural changes thus all these turrets are new builds, even if M1 or M1IP was upgraded to M1A1 or M1A2 standard. Front armor effective thickness is also ~800-900mm.
Hulls:
Type 1 used in M1 and M1IP, it lacks the overpressure NBC protection system, probably have some minor differences with the next type. Front armor effective thickness where special armor is mounted is ~650-700mm and did not changed till this day.
Type 2 used in M1A1 and M1A2 variants, major difference is that overpressure NBC protection system is present, some other minor differences are present depending on tank variant, Type 1 hulls can be upgraded to Type 2, however it requires complete disassembly, cutting and welding in some places.
About special armor itself, there were several types used:
BRL-1 (1980) used in original M1.
BRL-2 (1984) used in M1IP and M1A1 (Not certain!).
HAP 1st generation (1987/1988) used in early production M1A1HA.
HAP 2nd generation (1990) used in late production M1A1HA as well as new M1A1HC and M1A2.
HAP 3rd generation (1999/2000) used in M1A1SA, M1A1FEP, M1A2SEPv1 and M1A2SEPv2 (there is possibility of some upgrades for this armor in 2010+ period of time).
NGAP/NEA (2017+) used in newest M1A2SEPv3.
EAP (?) used in all export variants for Arab states.
BRL stands for Ballistic Research Laboratory, these were designations of lighter and heavier armor variants based on British research, that were developed by BRL in US, it's assumed that BRL-1 was for original M1, while BRL-2 was improved, heavier version for M1IP and M1A1 however as mentioned before, this is not certain.

HAP stands for Heavy Armor Package that uses components made from depleted uranium alloy, we know that there are at least 3 generations of this armor, there is also special variant for Australia where depleted uranium was replaced by something else, perhaps tungsten alloy, but Australian documents I found suggest that it's protection levels are equal to 3rd generation Heavy Armor Package, originaly it was said this type of armor is used in turret front only, however I found mentions in official sources saying it is used also in hull front and turret sides.

EAP or Export Armor Package is meant for non NATO/Close allies (like Australia) of the US, used in tanks provided to Arab states, some theoretize it might be equivalent to BRL-2 but it's probably heavier and offer better protection, but lower than HAP armor package.
One more important thing to note, today we also know that at least the newest M1's also uses Titanium alloy in their armor and several other components.
http://c.ymcdn.com/sites/www.titanium.org/resource/resmgr/2005_2009_papers/Gooch_Final_2007.pdf
http://c.ymcdn.com/sites/www.titanium.org/resource/resmgr/2010_2014_papers/GoochWilliam_2010_MilitaryGr.pdf


There are also two types of serial numbers. One for the turret, and one for the hull, serial numbers on turrets says us what type of armor vehicle uses, and on the hull where it was made. X will equal digit.

Turret:

XXXX - BRL-1 or BRL-2 armor package.
XXXXU - Heavy Armor Package.
XXXXM - Heavy Armor Package (some US tanks have it, don't know why that change in letter).
XXXXA - Heavy Armor Package variant for Australia.
XXXXE - Export Armor Package for Arab states.

Hull:

XXXXD - Tank was made in Detroit Tank Arsenal.
XXXXL - Tank was made in Lima Army Tank Plant/Joint Systems Manufacturing Center.

When it comes to armor itself, today we know it is indeed some type of advanced NERA or NxRA armor, however we only know more or less the structure of the turret side armor, and still only of older variant probably M1A1HA or M1A1HC.

This is how probably turret side armor over crew compartment looks like.

And this is how turret bustle side armor looks like.

And it makes sense, on one of the photos we can see some turrets after armor replacement/upgrade work in Joint Systems Manufacturing Center Lima, Ohio. We can notice that their frontal and side armor was replaced, but turret bustle was untouched.

With such evidence we can assume that turret armor is only replaced over crew compartment, while turret bustle armor might be untouched, and it makes sense from armor protection distribution point of view, where majority of protection increase and thus weight increase we want around the crew and for vehicle frontal 60 degree arc.

Here some more sources.

And here US Army Weapons Hand Book 2016.

https://www.army.mil/e2/c/downloads/431298.pdf

At page 14 in PDF reader we can read for M1A1SA and M1A2SEP "Survivability improvements include frontal armor and turret side-armor upgrades.", of course term "frontal armor" means most likely both turret and hull, and we have confirmation of side turret armor upgrades. And it also seems that M1A1SA and M1A2SEP shares the same armor type, which makes sense because due to reduced costs, simplicity etc. at one specific time you manufacture only one specific type of armor and it makes sense for MBT's even in various but not that much different variants to share the same armor protection. USMC M1A1FEP is also technically equivalent to US Army/ARNG M1A1SA with some minor differences, thus it's safe to assume that it also shares the same armor.

Good question is how exactly DU is applied to the armor design? We do not know, but perhaps it might be part of the NERA array itself like in this prototype British armor (perhaps one of several armor types developed within "Burlington" program?) which uses DU alloy plates with polycarbonate as a reactive layer?



When it comes to armor steel alloys used in the vehicles construction and armor, most likely these standard NATO steel types are used with such range of thickness and hardness.




Another thing is that M1's armor, like all modern 3rd generation MBT's armor is modular or semi modular, which makes it's replacement or repair relatively easy, quick and cheap. Another factor is that M1's turret and hull have very simple geometrical shapes making both manufacturing relatively simple, quick and cheap, as well as armor replacement/repair will be such relatively simple, quick and cheap task.

This also means that frontal protection is uniform for both hull and turret, and have very small weak zones.


Some additional protection over hull front is added by massive fuel tanks (visible on one of the drawing above), Israeli data says that 70mm of fuel is equivalent for 10mm of armored steel, thus we can assume that front hull protection where fuel tanks are is significantly increased.

However the largest improvement in protection will be made in M1A2SEPv3, photographs of one of the 9 prototypes shows us vehicle with massive weight simulators on turret front and hull front. This prototype still uses 3rd generation HAP armor package because it was originally M1A2SEPv2 converted in to M1A2SEPv3 prototype.


Each turret cheek have 4x welded plates, each looks to be ~50mm thick at most. Hull front have 3x welded plates, also each looks to be ~50mm thick at most. This means very significant weight increase over the previous generation armor, so we can assume also very significant protection levels increase over the front. There are also weight simulators packs welded to the turret roof, two have 4x 50mm plates and one have 3x 50mm plates.

http://www.dote.osd.mil/pub/reports/FY2015/pdf/army/2015m1a2sep3.pdf

Some additional informations about vehicle survivability improvements can be found here.

Final armor protection improvement are M19 ARAT-1 and M32 ARAT-2 Explosive Reactive Armor modules.

M19 ARAT-1 is modern, multilayer explosive reactive armor, which might have some sort of anti-tandem warheads capabilities.








From these photos we can see angled layers of ERA inside the module, and perhaps some sort of passive component?

Also to M19 modules, can be attached M32 modules.


It's very unusual design for ERA, and it's interesting it can be combined with M19 modules in to a single, multilayer type of ERA protection. M32 modules can also be attached alone to the turret sides.

In such configuration both turret and hull sides have very good, right ERA cover. The system is modular and can be arranged per need.

Of course besides it's armor M1 have other survivability meassures, primary one is mentioned earlier complete isolation of the main gun ammunition storage. It needs to be noted that the biggest tank and crew killer is not the enemy ammunition penetrating the armor but own ammunition cooking off.

The original M1 and M1IP had such ammo storage:

Turret bustle ready rack - 22 rounds.
Turret bustle semi ready rack - 22 rounds.
Turret basket floor armored box - 3 rounds.
Hull ammunition rack - 8 rounds.

M1A1 and M1A2 have such ammo such ammo storage:

Turret bustle ready rack - 17 rounds with old type of racks and 18 with new type of racks.
Turret bustle semi ready rack - 17 rounds with old type of racks and 18 with new type of racks.
Hull ammunition rack - 6 rounds.
 
 




Blow off panels and ammo racks in turret are connected, so if possible during ammunition cook off, blow off panels will be able at least to some degree pull out ammo racks with burning ammo outside, thus minimizing the damage.



These are armored blast doors for the hull ammunition magazines.



Hull blow off panels are placed in top (we can see large opening for main blow off panel) and belly of the hull (we can see two smaller openings for blow off panels).

In future another upgrade that will increase M1's already fantastic protection and survivability will be hard kill active protection system, however and it's not widely known, M1's had in the past, but not used often a soft kill active protection system or actually two, the AN/VLQ-6 and AN/VLQ-8, both are sort of IR dazzlers which work similiar to Russian TShU-1-7 Shtora-1 and Ukrainian Varta.





In future also M1 will be equipped with Hard Kill Active Protection System, most likely Israeli Trophy HV system as a mid term solution before new Modular Active Protection System will be ready.
 

And finally, another part of survivability improvements are multispectral camouflage nets and covers.

US Army is purchasing Saab Barracuda MCS.



And also uses a set of camouflage nets.

Firepower
When it comes to M1's main armament, there is a bit of confusion as well. We can see in many sources that it's often said that both 105mm rifled M68/M68A1/M68A2 gun is a license copy of the British 105mm rifled L7 gun, or that 120mm smoothbore M256 gun is a license copy of the German, 120mm smoothbore Rh120.
The M68 series were actually 100% designed in US, and the experimental variant was called T254, it only happens that because of US-British agreements, bith M68 and L7 can fire the same ammo types and have interchangeable barrels. Also US used the bore evacuator of the assymetric design from the L7, while T254 in it's original form had symetrical bore evacuator, that caused some problems when gun was fired at minimum depression.
Informations can be found in Richard P. Hunnicutt Patton - A History Of The American Main Battle Tank Vol.1 and Abrams - A History Of The American Main Battle Tank Vol.2.
The primary characteristic of US gun is the concentric recoil mechanism that allows the gun to be more compact, so it's either easier to fit it in to a smaller gun mantlet or a smaller turret. While many other guns designs use eccentric recoil mechanism that takes a bit more space.

Also the gun breech is different in M68 series and L7 series. We can see it on videos showing interior of the M60A3 and Centurion.
Similiar story with the 120mm smoothbore M256. It's based on the Rheinmetall gun technology, but it's not license copy of the Rh120. Both guns can fire the same ammunition but are not interchangeable, simply Rh120 would not fit the M1A1/M1A2's gun mantlet. Also recoil mechanism is different, breech is slightly different, different are MRS, cradle, thermal shroud and bore evacuator.


This is the M256 in M1A1/M1A2.

And this is Rh120 in Leopard 2.

What is interesting tough, majority of countries using 120mm smoothbore guns, base their design on the M256 not Rh120, probably because M256 offers this compactness.
For example Israeli MG251 and MG253 guns are based on M256. Also South Korean and Turkish 120mm L55 guns seems to be based on M256 (or rather M256E1 that used L55 barrel).


Turkish MKEK 120mm smoothbore L55 gun.

As for source for Israeli guns, my source is this book written by Paweł Przeździecki.



As for M256E1, we have two photos that might actually present this gun, one mounted in M1, that probably served as a test bed, and second mounted in M1A2.




The new gun developed for M1A1/M1A2 is the XM360E1, however it's not certain if it will be used altough US Army showed some interest.


http://www.dtic.mil/ndia/2010armament/WednesdayReunionDavidSmith.pdf

 
It's a further development of the XM360 120mm smoohbore low recoil gun for the XM1202 MCS light tank. And there is some interesting observations we can make from CAD models and official statements. It's definately lighter gun, more compact than the previous generation guns. Also we know that XM360 was able to fire exactly the same ammunition with the same performance as M256, and here interesting bit, as we can see on screenshot, it is said, M360E1 compared to XM360 can fire ammunition with higher impulse and higher chamber pressure. Which means it's more pwoerfull gun than M256 or Rh120 or other comparable 120mm smoothbore guns. And it's kinda similiar to for example what Russians did with their new 2A82-1M 125mm smoothbore gun that can fire rounds at higher chamber pressure than previous generation 125mm smoothbores.

Another things we can note is that XM360E1 is longer without muzzle break than XM360. AFAIK XM360 had a L48 barrel, so this can mean that XM360E1 have L50 barrel, maybe longer, something like that. And another interesting bit about these guns are locking lugs, XM360 and XM360E1 have 6 locking lugs, compared to M256 or Rh120 2 locking lugs. Again suggests that these guns were designed with higher chamber pressure in mind.

We can also note that both XM360 and XM360E1 does not have reactive bore evacuator, so perhaps compressed air is used in both?

But returning for a moment to locking lugs, I found that 6 locking lugs is nothing new, actually it was present in some of the previous development like 120/140mm XM291 smoothbore gun and 155mm XM297E2 gun-howitzer for XM2001 Crusader.


XM291, photo is of bad quality but we can see more than 2 locking lugs.
 
Video of XM2001 compared to M109A6, but we can clearly see breech of the XM297E1 gun-howitzer with the same pattern of 6 locking lugs as in XM360/XM360E1.

So what about the future? As I said earlier and I found digging through various official US Army websites in various documents that, indeed, there are some consideration of replacing M256 with XM360 (or rather XM360E1), but what I also found interesting, they also consider using autoloader for the M1, and here it also starts to be interesting. There were two designs of autoloaders for M1A1 and M1A2. First was developed by General Dynamics and was codenamed FASTDRAW.



 
More about it can be read in ARMOR Magazine issue from 1995 page 18 in PDF reader.
http://www.benning.army.mil/armor/eARMOR/content/issues/1995/MAR_APR/ArmorMarchApril1995web.pdf
In general I recommend to take a look at various issues of ARMOR Magazine -> http://www.benning.army.mil/armor/eA...ackIssues.html
Another type of autoloader was developed by Meggitt Defense, and it's called Compact Autoloader. Unique feature of this autoloader type is a fact that it have two parts, it's automated magazine and the autoloader itself, and automated magazine can used without autoloader, so it would work the way that it automatically move selected round to the ammo port where human loader can take it and load it in to main gun.
https://www.meggittdefense.com/images/stories/pdf/PD_120mmCompactAutoloader.pdf
https://www.meggittdefense.com/images/stories/pdf/PD_120mmAutoloaderMagazineNEW.pdf
 
Some other unique features of US made tank autoloaders like FASTDRAW or Compact Autoloader is that they can both load and unload the main gun, as well as their high ammo capacity compared to other systems. 
 

Survivability
Armor protection of the M1 is very interesting subject because it's not entirely well understood what happend with requirements when US joined the British "Burlington" program.
One is certain, that original requirements for 115mm APFSDS at 800m and 127mm HEAT warhead over frontal arc, are requirements for early armor types developed by BRL, and are not connected with later development.
Recently the CIA declassified some drawings of the original M1 armor.

This confirms that vehicle was protected by "Burlington" type armor. Similarities can be observed when we compare these drawings with the British drawings of the "Burlington" armors developed for Chieftain. I strongly recommend here articles by Polish historian Paweł Przeździecki.
http://wceo.com.pl/images/Dokumenty/WBBH/PHW/PHW_3_2011.pdf  -> PDF reader page 112.

http://wceo.com.pl/images/Dokumenty/WBBH/PHW/PHW_4_2011.pdf  -> PDF reader page 106.

Another interesting factors is that declassified document that says one of the armor types used in M1 turret, offered KE protection = 400mm RHAe and CE protection = 750mm RHAe at front.



Another document is also interesting because it says.


This fragment is very interesting when taken in to account later fragment of this post when I will talk a bit about armor types. Because it makes situation confusing. We know that BRL tested two armor types, BRL-1 that is assumed to be the original M1 armor, and heavier, better performing BRL-2. The question is, are both armors related? And what type of armor was used by M1IP variant, did it used simply the same armor as M1, and only improvement was added through increasing turret front armor thickness? Or perhaps M1IP used the same armor type as M1A1?
When it comes to the armor itself, this is based on my estimations and measurements I made on the real thing.

 
Besides that:

Hull sides over crew compartment - 40mm + 30/40mm = 70-80mm.
Hull sides over engine compartment - 40mm.
Hull sides around suspension attachement points - 40mm.
Hull rear - 40mm.
Upper front plate and drivers hatch - 50mm at 82-85 degrees = 359.26/573.69mm.
Turret roof front - 50mm at 82-85 degrees = 359.26/573.69mm.
Turret roof around hatches and turret bustle - 25mm.
Engine compartment roof - 25mm.
Hull belly - 25mm.
Heavy ballistic skirts - 70-80mm.
Non ballistic skirts - 5-10mm.
 

Before we move to the armor itself it's also worth to note something about hulls and turrets.
Because there is no official classification I needed to create one by myself. However there are very clear difference between various generations of tank variants.
Turrets:
Type 1 "Short Turret", used only in original M1, it have thinner frontal armor that have effective armor thickness ~700-750mm, and a bit different roof over turret bustle with 3 cutouts for blow off panels.
Type 2 "Interim Long Turret", used only in M1IP variant, have thicker frontal armor, effective thickness ~800-900mm (Steven Zaloga in one of his books said that armor thickness was increased by ~200mm), no other structural changes.
Type 3 "Long Turret", this is the final turret variant used in M1A1 and M1A2, older turrets can't be upgraded to this variant due to significant structural changes thus all these turrets are new builds, even if M1 or M1IP was upgraded to M1A1 or M1A2 standard. Front armor effective thickness is also ~800-900mm.
Hulls:
Type 1 used in M1 and M1IP, it lacks the overpressure NBC protection system, probably have some minor differences with the next type. Front armor effective thickness where special armor is mounted is ~650-700mm and did not changed till this day.
Type 2 used in M1A1 and M1A2 variants, major difference is that overpressure NBC protection system is present, some other minor differences are present depending on tank variant, Type 1 hulls can be upgraded to Type 2, however it requires complete disassembly, cutting and welding in some places.
About special armor itself, there were several types used:
BRL-1 (1980) used in original M1.
BRL-2 (1984) used in M1IP and M1A1 (Not certain!).
HAP 1st generation (1987/1988) used in early production M1A1HA.
HAP 2nd generation (1990) used in late production M1A1HA as well as new M1A1HC and M1A2.
HAP 3rd generation (1999/2000) used in M1A1SA, M1A1FEP, M1A2SEPv1 and M1A2SEPv2 (there is possibility of some upgrades for this armor in 2010+ period of time).
NGAP/NEA (2017+) used in newest M1A2SEPv3.
EAP (?) used in all export variants for Arab states.
BRL stands for Ballistic Research Laboratory, these were designations of lighter and heavier armor variants based on British research, that were developed by BRL in US, it's assumed that BRL-1 was for original M1, while BRL-2 was improved, heavier version for M1IP and M1A1 however as mentioned before, this is not certain.

HAP stands for Heavy Armor Package that uses components made from depleted uranium alloy, we know that there are at least 3 generations of this armor, there is also special variant for Australia where depleted uranium was replaced by something else, perhaps tungsten alloy, but Australian documents I found suggest that it's protection levels are equal to 3rd generation Heavy Armor Package, originaly it was said this type of armor is used in turret front only, however I found mentions in official sources saying it is used also in hull front and turret sides.

EAP or Export Armor Package is meant for non NATO/Close allies (like Australia) of the US, used in tanks provided to Arab states, some theoretize it might be equivalent to BRL-2 but it's probably heavier and offer better protection, but lower than HAP armor package.
One more important thing to note, today we also know that at least the newest M1's also uses Titanium alloy in their armor and several other components.
http://c.ymcdn.com/sites/www.titanium.org/resource/resmgr/2005_2009_papers/Gooch_Final_2007.pdf
http://c.ymcdn.com/sites/www.titanium.org/resource/resmgr/2010_2014_papers/GoochWilliam_2010_MilitaryGr.pdf


There are also two types of serial numbers. One for the turret, and one for the hull, serial numbers on turrets says us what type of armor vehicle uses, and on the hull where it was made. X will equal digit.

Turret:

XXXX - BRL-1 or BRL-2 armor package.
XXXXU - Heavy Armor Package.
XXXXM - Heavy Armor Package (some US tanks have it, don't know why that change in letter).
XXXXA - Heavy Armor Package variant for Australia.
XXXXE - Export Armor Package for Arab states.

Hull:

XXXXD - Tank was made in Detroit Tank Arsenal.
XXXXL - Tank was made in Lima Army Tank Plant/Joint Systems Manufacturing Center.

When it comes to armor itself, today we know it is indeed some type of advanced NERA or NxRA armor, however we only know more or less the structure of the turret side armor, and still only of older variant probably M1A1HA or M1A1HC.

This is how probably turret side armor over crew compartment looks like.

And this is how turret bustle side armor looks like.

And it makes sense, on one of the photos we can see some turrets after armor replacement/upgrade work in Joint Systems Manufacturing Center Lima, Ohio. We can notice that their frontal and side armor was replaced, but turret bustle was untouched.

With such evidence we can assume that turret armor is only replaced over crew compartment, while turret bustle armor might be untouched, and it makes sense from armor protection distribution point of view, where majority of protection increase and thus weight increase we want around the crew and for vehicle frontal 60 degree arc.

Here some more sources.

And here US Army Weapons Hand Book 2016.

https://www.army.mil/e2/c/downloads/431298.pdf

At page 14 in PDF reader we can read for M1A1SA and M1A2SEP "Survivability improvements include frontal armor and turret side-armor upgrades.", of course term "frontal armor" means most likely both turret and hull, and we have confirmation of side turret armor upgrades. And it also seems that M1A1SA and M1A2SEP shares the same armor type, which makes sense because due to reduced costs, simplicity etc. at one specific time you manufacture only one specific type of armor and it makes sense for MBT's even in various but not that much different variants to share the same armor protection. USMC M1A1FEP is also technically equivalent to US Army/ARNG M1A1SA with some minor differences, thus it's safe to assume that it also shares the same armor.

Good question is how exactly DU is applied to the armor design? We do not know, but perhaps it might be part of the NERA array itself like in this prototype British armor (perhaps one of several armor types developed within "Burlington" program?) which uses DU alloy plates with polycarbonate as a reactive layer?



When it comes to armor steel alloys used in the vehicles construction and armor, most likely these standard NATO steel types are used with such range of thickness and hardness.




Another thing is that M1's armor, like all modern 3rd generation MBT's armor is modular or semi modular, which makes it's replacement or repair relatively easy, quick and cheap. Another factor is that M1's turret and hull have very simple geometrical shapes making both manufacturing relatively simple, quick and cheap, as well as armor replacement/repair will be such relatively simple, quick and cheap task.

This also means that frontal protection is uniform for both hull and turret, and have very small weak zones.


Some additional protection over hull front is added by massive fuel tanks (visible on one of the drawing above), Israeli data says that 70mm of fuel is equivalent for 10mm of armored steel, thus we can assume that front hull protection where fuel tanks are is significantly increased.

However the largest improvement in protection will be made in M1A2SEPv3, photographs of one of the 9 prototypes shows us vehicle with massive weight simulators on turret front and hull front. This prototype still uses 3rd generation HAP armor package because it was originally M1A2SEPv2 converted in to M1A2SEPv3 prototype.


Each turret cheek have 4x welded plates, each looks to be ~50mm thick at most. Hull front have 3x welded plates, also each looks to be ~50mm thick at most. This means very significant weight increase over the previous generation armor, so we can assume also very significant protection levels increase over the front. There are also weight simulators packs welded to the turret roof, two have 4x 50mm plates and one have 3x 50mm plates.

http://www.dote.osd.mil/pub/reports/FY2015/pdf/army/2015m1a2sep3.pdf

Some additional informations about vehicle survivability improvements can be found here.

Final armor protection improvement are M19 ARAT-1 and M32 ARAT-2 Explosive Reactive Armor modules.

M19 ARAT-1 is modern, multilayer explosive reactive armor, which might have some sort of anti-tandem warheads capabilities.








From these photos we can see angled layers of ERA inside the module, and perhaps some sort of passive component?

Also to M19 modules, can be attached M32 modules.


It's very unusual design for ERA, and it's interesting it can be combined with M19 modules in to a single, multilayer type of ERA protection. M32 modules can also be attached alone to the turret sides.

In such configuration both turret and hull sides have very good, right ERA cover. The system is modular and can be arranged per need.

Of course besides it's armor M1 have other survivability meassures, primary one is mentioned earlier complete isolation of the main gun ammunition storage. It needs to be noted that the biggest tank and crew killer is not the enemy ammunition penetrating the armor but own ammunition cooking off.

The original M1 and M1IP had such ammo storage:

Turret bustle ready rack - 22 rounds.
Turret bustle semi ready rack - 22 rounds.
Turret basket floor armored box - 3 rounds.
Hull ammunition rack - 8 rounds.

M1A1 and M1A2 have such ammo such ammo storage:

Turret bustle ready rack - 17 rounds with old type of racks and 18 with new type of racks.
Turret bustle semi ready rack - 17 rounds with old type of racks and 18 with new type of racks.
Hull ammunition rack - 6 rounds.
 
 




Blow off panels and ammo racks in turret are connected, so if possible during ammunition cook off, blow off panels will be able at least to some degree pull out ammo racks with burning ammo outside, thus minimizing the damage.



These are armored blast doors for the hull ammunition magazines.



Hull blow off panels are placed in top (we can see large opening for main blow off panel) and belly of the hull (we can see two smaller openings for blow off panels).

In future another upgrade that will increase M1's already fantastic protection and survivability will be hard kill active protection system, however and it's not widely known, M1's had in the past, but not used often a soft kill active protection system or actually two, the AN/VLQ-6 and AN/VLQ-8, both are sort of IR dazzlers which work similiar to Russian TShU-1-7 Shtora-1 and Ukrainian Varta.





In future also M1 will be equipped with Hard Kill Active Protection System, most likely Israeli Trophy HV system as a mid term solution before new Modular Active Protection System will be ready.
 

And finally, another part of survivability improvements are multispectral camouflage nets and covers.

US Army is purchasing Saab Barracuda MCS.



And also uses a set of camouflage nets.

Ok, so let's start to share here some informations and fight with some myths. I will start with M1, people that know me, know this is my black horse. So let's start with history and then move on. Historical description is a quote from Richard P. Hunnicutt Abrams - A History Of The American Main Battle Tank Vol.2.
M1 Abrams
History
Below is the LK 10322 design concept with heavy protection, rear engine, and a conventional turret.

Although Congress cancelled the XM803 program in late 1971, the Army's need for a new tank was recognized. To meet this requirement, 20 million dollars were authorized for the procurement of two prototypes of a new main battle tank. This was in addition to the 20 million dollars allocated to phase out the XM803. Although the funds were provided specifically for two industrial prototype tanks, the Army initially preferred to concentrate on component development until a specification could be prepared for a new main battle tank. At that time, there was considerable controversy within the Army itself as to what form the tank should take. In February 1972, the Main Battle Tank Task Force (MBTTF) was established at Fort Knox, Kentucky under the leadership of Major General William R. Desobry. Its objective was to define the characteristics of the new MET and to prepare the Material Need (MN) document. Technical support for the Task Force was provided by the Advanced Concepts Branch of the Tank Automotive Command (TACOM). Eventually, this work resulted in a series of eight main concept studies prepared by J. B. Gilvydis. The first three of these studies covered the work performed for the Task Force during its period of operation from February to August 1972. The later five were conducted for the Main Battle Tank Project Manager's Office, after it was established in September, and for the Army Materiel Command (AMC). This work continued until March 1973. On 5 February 1972, General Henry A. Miley, Jr., the Commanding General of AMC, concluded in a letter to the Department of the Army that tank system concept studies by industry would be of value to the Army and would help meet the directions of Congress. Following this, contracts were awarded to Chrysler Corporation and General Motors Corporation for such design studies to be completed by September 1972.
 

 
The cross section at the left shows the spaced armor arrangement incorporated in some of the new designs.

Drawing at the bottom of the page depict a General Motors tank concept powered by the AGT-1500 gas turbine. The estimated weight of this vehicle was about 47 tons.
The Chrysler program was to cover new evolutionary design concepts based on the M60A1 tank. The General Motors contract was to study the design of a new MBT using advanced design components based on the experience of the XM803 program. Both projects provided concept studies to support the work of the MBT Task Force. Initially, the new main battle tank was designated as the XM815, but with the revision of the nomenclature system, this was soon changed to the XML TACOM presented the results of their first study to the MBTTF at Fort Knox on 25 February 1972. The initial concept, designated as LK 10322, was basically a conventional arrangement with the engine in the rear, the commander, gunner, and loader centrally located in the turret, and the driver in the front center of the hull. The total armored volume of this concept was about 540 cubic feet. This compared to an armored volume of approximately 425 cubic feet for the Soviet T55 and 650 cubic feet for the M60A1. Armed with the 105mm gun M68, the LK 10322 concept was used in this study to analyse the effect on the combat weight of the different thicknesses of armor required to defeat various enemy threats. Fourteen different levels of protection were considered in this first study with the estimated vehicle weights ranging from about 34 tons to approximately 57 tons.

The General Motors 400K tank proposal is sketched below.

Above is the General Motors 500K tank concept.

The loading sequence for the rounds stowed behind the armored bulkhead in the turret bustle is shown at the left. 

The full size mock-up corresponding to the LK 10372 design can be seen in the photographs on this page. Note the coaxial Bushmaster weapon.
The armor ranged from all round protection against only 23mm armor piercing projectiles to frontal protection against 120mm shaped charge rounds. Various composite armor arrays developed by the Ballistic Research Laboratory were utilized in different versions of the concept. Since the design of the armor was rapidly developing, frequent changes were required to adapt the constantly changing arrays to the tank concept studies. It is interesting to note that the initial drawing for the LK 10322 concept featured the AGT-1500 gas turbine, the power plant eventually selected for the Ml production tank. With six road wheels per track, a tube-over-bar suspension was specified for the LK 10322 to improve the cross-country performance. During April and May, a second TACOM study investigated modified versions of the LK 10322 concept in an effort to reduce the weight. Initially, the hull design remained the same, but the turret was slightly modified. The side walls were tapered five degrees to the rear and the slope of the right side was increased from 14 to 20 degrees from the vertical. A new low profile commander's station was introduced with an externally mounted 7.62mm machine gun and the 7.62mm coaxial machine gun was replaced by a .50 caliber weapon. These modifications resulted in estimated weight reductions ranging from a little less than three tons to over three and one half tons depending upon the protection level under consideration. Later, the hull of the modified LK 10322 was lengthened from 270 inches to 275 inches which allowed an increase in fuel capacity from 300 to 350 gallons. This hull also could accept either the AGT 1500 turbine with the XHM-1500 transmission or the AVCR-1100 diesel engine with the X-1100 transmission. The average weight increase with these hull changes was about 600 pounds. On 20 June 1972, the MET Task Force requested TACOM to perform a third weight and protection analysis. This study was to evaluate the effect of 72 different component combinations applied to the modified LK 10322 concept. The following component list was supplied by the Task Force for the analysis.

1. Primary Armament
a. 105mm gun M68
b. 110mm gun, United Kingdom
c. 120mm gun, Federal Republic of Germany
2. Fire Control
a. XM803 system (without commander's day/night
sight)
b. M60A1 (with turret integrated night thermal sight)
3. Power Plants
a. AVCR-1100 diesel engine with the X-1100 transmission
b. DB1500 diesel engine with the Renk HSWL 354
transmission
c. AGT-1500 gas turbine with the XHM-1500-2 transmission
4. Suspension
a. Tube-over-bar
b. Piston hydropneumatic
5. Tracks
a. T142 or Diehl
b. Lightweight, 28 inch width

This study used two different approaches. The first calculated the armor weight for the hull and turret and determined the protection level obtained with total vehicle weights of 43, 45, 47, and 49 tons. The second approach analysed the effect of 72 combinations of components on the total vehicle weight at a single level of protection. The protection level selected was that required to defeat the combined threat of the Soviet 115mm APFSDS projectile at 800 meters range and the 3.2 inch diameter HEAT round on the front and up to 30 degrees to each flank. In addition to the 72 variations of the modified LK 10322 concept from the third TACOM study, the MET Task Force reviewed fifteen additional designs. Numbers 73 through 80 were submitted by General Motors Corporation and 81 through 83 came from Chrysler. Candidates 84 through 87 were product improved versions of the M60A1. In reviewing the 72 candidates from the third TACOM study, the Task Force eliminated those armed with the United Kingdom 110mm rifled gun and the 120mm smooth bore weapon from the Federal Republic of Germany. The 110mm gun was not considered sufficiently superior to the 105mm gun M68 and the 120mm smooth bore was not expected to be available in time to meet the proposed production schedule. At that time, the Task Force also considered the AGT-1500 gas turbine to be a high risk
power plant and it was dropped from the concepts under review. This reduced the number of proposals in the TACOM study from 72 to 16.

General Motors XM1 prototype, note placement of ammunition. There were two large exposed ammo racks in crew compartment, one with rounds stored in vertical position next to the loader in turret, and second on the right side of the driver. This was probably one of several reasons why General Motors prototype was discarded in favor of Chrysler XM1.
Those remaining differed primarily in the engine and transmission with minor variations in the fire control systems as well as the suspensions and tracks. As a result, the 16 candidates were reduced to two featuring the two power packages considered acceptable. Thus one was powered by the air-cooled Continental AVCR-1100 engine with the X-1100 transmission. The other was fitted with the Daimler Benz (later MTU) DB1500 liquid-cooled diesel with the Renk transmission. These two concepts were included by the Task Force among the five candidates for the final trade off analysis. Of the eight General Motors proposals, two were powered by the AGT-1500 gas turbine using the X-1100 transmission, one utilized the DB1500 diesel with the Renk transmission, and two were fitted with the AVCR-1100-3B with the X-1100 transmission. The remaining three had a twin engine arrangement consisting of two General Motors 8V71T liquid-cooled diesels, again with the X-1100 transmission. All eight concepts used a high strength torsion bar suspension and were armed with the M68 105mm gun. Estimated weights of these vehicles ranged from 47 to 67 tons depending upon their level of protection. The Task Force rejected the three proposals powered by the twin 8V71T diesels because of anticipated increased maintenance time and reduced reliability. With the elimination of the AGT-1500 in two of the concepts, the number of General Motors candidates was reduced to three. One of these also was rejected because of its excessive weight of 67 tons. One of the remaining two was selected as the General Motors candidate even though it weighed 56 tons because it came closest to meeting the MN requirements for ballistic protection. One of the three Chrysler concepts was selected as superior to the other two in several important areas and essentially equal in other respects. It was powered by the AVCR-1100 engine with the X-1100 transmission and had an estimated combat weight of 48.5 tons. However, like the other candidates that met the MN weight requirements, it did not provide the desired protection level. None of the four product improved MoOAls met the M N requirements for ballistic protection or sustained speeds on 10 per cent and 30 per cent grades. Of the four, numbers 84 and 85 provided the highest level of protection. Since number 85 featured the more advanced fire control system, it was selected as one of the five candidates for the final trade off analysis. The most obvious conclusion from the review of these concept studies was that the level of ballistic protection required by the proposed MN document was incompatible with the weight limitation specified. This problem was outlined in a fact sheet forwarded from the Task Force to the Commanding General of AMC, dated 3 August 1972.

This fact sheet concluded that the only practical solution was to either reduce the level of ballistic protection or increase the 49 ton weight limitation in the proposed MN document. The latter alternative was subsequently adopted. Although General Motors and Chrysler provided support to the MET Task Force under their study contracts for new tank prototypes, the final reports on both of these contracts were not submitted until October 1972. In their report, General Motors proposed two concepts. They were based on estimated average production unit costs of $400,000 and $500,000, both in fiscal year 1972 dollars. Referred to as the 400K and 500K tanks, the latter was a growth version of the former with improvements in ballistic protection, firepower, and night vision. Both were manned by a crew of four with the driver in the left front hull. The usual arrangement of the three man turret crew was reversed placing the gunner and tank commander on the left side of the cannon with the loader at the right rear. The turret on both vehicles utilized the large bearing ring from the XM803. Both tanks were armed with a stabilized 105mm gun M68. Forty rounds of 105mm ammunition were stowed in the hull below the level of the turret ring without separate compartmentation. The 400K tank was protected by welded plate armor arrays on the turret front. The sides and rear of the turret consisted of single plate rolled homogeneous armor. The hull front was a single plate of high obliquity armor. Spaced armor skirts were provided on the forward sides of the hull. The tank was powered by the previously proposed twin General Motors 8V71T diesel engines connected to a modified X-1100 transmission through a transfer case. With this arrangement, the tank could be driven on one engine in an emergency. The twin engine installation was rated at 1200 gross horsepower. The estimated combat weight of the 400K concept was 52 tons. Although the armor protection was superior to the M60A3, it did not meet the requirements for the new MET.
 
The running gear consisted of an advanced high strength torsion bar suspension with six 30 inch diameter road wheels per side running on a 22 inch wide, double pin, flat track. A 7.62mm coaxial machine gun was located on the right side of the cannon and a 40mm high velocity grenade launcher was installed on the commander's hatch. The independently stabilized gunner's day/night sight included a laser range finder. The combat weight of the 500K tank was increased to 55 tons providing improved ballistic protection. The front hull now consisted of spaced rolled homogeneous armor and steel/aluminum array skirts were installed on the sides. On the turret, a cast armor trunnion support was joined to frontal spaced armor arrays. The sides and rear also consisted of spaced armor. Although the M68 105mm gun was retained in the 500K tank, the mount was strengthened to permit future adaptation of a 120mm gun firing kinetic energy ammunition. The 7.62mm coaxial machine gun in the 400K tank was replaced by the Bushmaster (20mm-30mm) weapon system then under development. The 40mm high velocity grenade launcher was moved to the loader's station and replaced on the commander's hatch by a .50 caliber M85 machine gun. The commander's weapon was now mounted on a powered slew ring. The gunner's stabilized sight with its laser range finder also now included a far infrared imager for night operations. Individual displays were provided for both the gunner and the tank commander. With the heavier weight, the 500K tank was powered by the AVCR-1100-3B engine rated at 1450 gross horsepower using the modified X-1100 transmission. The number of road wheels was raised from six to seven per side and the width of the double pin flat track was increased to 24 inches. Chrysler's report on the New Evolutionary Tank reviewed eight different vehicle concepts as well as the development status of various design features and components. Main armament on all eight concept studies was the M68 105mm gun. In one case, this was supplemented by a TOW missile installation. On two other concepts, the Shillelagh II or Swifty gun launched missile was specified. This was a proposed development of the Shillelagh reduced in diameter to permit launching from the 105mm gun. The missile velocity also was increased to reduce the flight time by about 50 per cent and to extend the range by approximately 25 per cent. However, the reduced diameter would have decreased the effectiveness of its shaped charge warhead. The estimated combat weight of the eight Chrysler proposals ranged from about 49 tons to 57 tons. After considering the eight concept studies, the Chrysler report proposed a final configuration combining the best components from two of them. With an estimated combat weight of 57.2 tons, this tank had a conventional layout. The driver was in the center front hull and the three man turret located the gunner and the tank commander on the right side of the 105mm gun with the loader on the left. The 105mm ammunition was stowed in compartments separated from the crew space. These compartments in the turret bustle and the hull were designed to fail away from the crew space. Thus any ammunition explosion resulting from a penetration of the armor would be directed either to the outside or into the engine compartment away from the crew. 

AGT-1500 and AVCR-1360.
The 105mm M68 gun was selected as the main armament with possible future upgrading through improved ammunition or the introduction of a gun launched missile such as the Swifty. A .50 caliber M 85 coaxial machine gun was specified, but the design permitted its replacement by the Bushmaster weapon when it became available. The fire control equipment was a modified M60A3 system with the turret integrated night thermal sight (TINTS). Final selection of the power plant was not made at that time with the German MTU MB 873 Ka 500 (formerly the Daimler-Benz DB1500) 1500 horsepower liquidcooled diesel, the AVCR-1360-1 air-cooled diesel, and the AGT-1500 gas turbine all being considered. It was proposed to select two of these power plants for test rig evaluation. The X-1100 transmission was to be used in all three cases. The advanced torsion bar suspension was selected. As mentioned earlier, this system utilized high strength torsion bars permitting a much greater twist before exceeding the elastic limit of the material. Thus a much greater wheel travel could be obtained. As with other proposed tank concepts, it was obvious that the combat weight would greatly exceed the original target if satisfactory protection levels were to be achieved. A new concept, designated LK 10352, was proposed by TACOM in a fourth study dated July 1972. The main objective of this study was to determine the effect on the gross vehicle weight if the fuel capacity was increased from 300 to 350 gallons. This was similar to the earlier examination of the same increase on the modified LK 10322. The hull was lengthened to 276 inches with the extra six inches between the crew compartment bulkhead and the engine being used to accommodate a tank for the additional 50 gallons of fuel. The lower glacis was redesigned on this new concept and the angle with the vertical increased from 55 degrees to 60 degrees. The upper glacis remained at 65 degrees from the vertical. No changes were made to the turret. The effect of these modifications on the gross vehicle weight was calculated based on the protection required to defeat two threat levels. The greatest of these was the combined threat of the Soviet 115mm APFSDS kinetic energy projectile at a range of 800 meters and a 3.2 inch diameter precision HEAT shaped charge round. The calculation indicated that a vehicle weight of over 52 tons would be required for protection directly from the front out to an angle of 30 degrees to each flank. In August and September, the LK 10352 concept was modified in a fifth study to determine the effect of second generation armor arrays on the gross vehicle weight. The early steel-glass-steel composite armor was replaced by multiple steel and aluminum plate combinations. For this analysis, three threat levels and the armor arrays required to defeat them were supplied by the Ballistic Research Laboratory. The light threat consisted only of the kinetic energy projectile representated by the Soviet 115mm APFSDS at a range of 800 meters. The medium level of protection was required to defeat both the kinetic energy threat and the 3.2 inch diameter HEAT shaped charge round. A new heavy threat was introduced at this time. It consisted of both the 115mm kinetic energy projectile and a 5.0 inch diameter HEAT round.

Chrysler design hull mockup with AVCR-1360 diesel.
This was the threat against which the new MBT was to be protected in future TACOM studies. For the front hull armor array, the upper glacis was angled at 35 degrees and the lower glacis at 70 degrees, both from the vertical. The nose also was extended 2.5 inches to give more room to the driver. The turret was modified from the earlier concept by changing the left front from a curved to a flat surface angled back at 30 degrees to the rear and sloped at 60 degrees from the vertical. The slope of the right front was changed from 60 to 55 degrees from the vertical to provide more room for the gunner. This side already had a flat surface angled to the rear at 30 degrees. The turret sides were unchanged, but the bustle was extended by 12 inches to obtain more stowage space. The .50 caliber coaxial machine gun was replaced by the Bushmaster and the .50 caliber weapon was relocated to the commander's hatch. The study indicated that a combat weight of over 59 tons would be required to provide protection against the heavy threat from the front and at 30 degrees to each flank. A sixth study by TACOM from October 1972 to January 1973 covered the preparation and weight analyses of new design concepts and the building of a full size wooden mock-up. The rapid development of composite armor technology continued to cause repeated changes in the tank design concepts. In fact, according to TACOM personnel at that time, the Ballistic Research Laboratory was cranking out new armor arrays faster than new concept designs based on them could be drawn and analyzed. After a preliminary study, it was decided to draw two different versions of the new tank. One version was to have the main gun ammunition stowed in the crew space as in earlier tanks. In the other, it was to be separated from the crew in ammunition compartments. Two different turret designs were prepared. One turret stowed the ammunition separate from the crew in a bustle compartment. The second turret had a short bustle with no ammunition stowage. Two designs were considered for the tank with compartmented ammunition. In the first type, 18 out of a total of 40 105mm rounds were stowed in automated racks in the turret bustle behind a one inch thick aluminum alloy bulkhead. These racks were similar to those in the XM803 and the rounds were retrieved through a single round door in the bulkhead. Sixteen rounds were in two revolving drum type racks in the front hull, one on each side of the driver. These rounds were retrieved through small round doors in the one inch thick bulkhead facing the turret basket.

Chrysler mobility test rig.
The remaining six rounds were stowed in a non-automated rack in a two inch thick box underneath the cannon in the turret basket. The second design of the tank with the compartmented ammunition eliminated the automated racks. They were replaced by simple lightweight racks and some rounds were relocated. The eighteen rounds remained in the turret bustle, but they were now retrieved through sliding aluminum doors. The six rounds previously stowed in the turret basket were moved to the front hull. The compartments on each side of the driver now contained 11 rounds each and they could be reached through sliding doors. In the non-compartmented version of the new tank, all 40 rounds of main gun ammunition were stowed in the crew compartment below the turret ring. Twentyeight were in two racks, one on each side of the driver. The remaining 12 were located in the turret basket. These were considered to be ready rounds and were in easy reach of the loader. The compartmented and non-compartmented concepts were quite similar in external appearance except for slight changes in the turret design. In addition to the shorter bustle, the non-compartmented version retained the five degree taper to the rear on the turret sides. On the compartmented turret, the side walls were parallel to the center line. This provided more stowage space for ammunition in the bustle. Both sides of the turret front were angled back 30 degrees to the rear and sloped at 45 degrees from the vertical. The left and right sides of the turret were sloped at 35 degrees and 20 degrees from the vertical respectively. The roof was flat and the rear of the bustle was angled at 40 degrees from the vertical. As in the previous concepts, the turret ring diameter was 85 inches. 

Chrysler and General Motors XM1's compared to M60A1 Main Battle Tank.
The hull design was identical for both concepts, except for the ammunition racks. The upper and lower front glacises were angled at 35 and 70 degrees from the vertical respectively. Side skirts extended from the front idler to the rear sprocket protecting both the crew and engine compartments. The power plant was the AVCR-1360 engine with the X-1100 transmission. All of the fuel was located in the engine compartment. A tube-over-bar suspension system with six road wheels per side was used with 26 inch wide tracks. For the commander, a low rotatable station even with the turret roof line was used for both concepts. It was fitted with a .50 caliber M85 machine gun which could be operated from either inside or outside the tank. In addition to the machine gun sight, the commander was provided with six periscopes for all around vision. A Bushmaster weapon system was installed on the left side of the 105mm gun. A day/night sight including a laser range finder was mounted in front of the gunner's station with separate controls for the gunner and the tank commander. Weight analyses of the two concepts armored against the heavy threat indicated that the tank with the compartmented ammunition weighed slightly over 60 tons compared to under 57 tons for the non-compartmented version. The tank with the compartmented ammunition and armored against the heavy threat was selected for the construction of a full size wooden mock-up. The mock-up was modified in several ways from the original concept drawings. The day/night sight for the gunner and the tank commander was relocated to the right side of the turret. This was done to eliminate a weak ballistic area on the turret caused by the previous mounting of the sight. To provide protection against HEAT rounds, armor arrays extending alongside the entire crew compartment were installed on the hull side areas over the sponsons. The overhanging frontal sections of the turret were lowered by about four inches leaving only a minimum clearance for the driver's head when driving with his head exposed. This was to improve the ballistic protection by reducing the shot trap in this area. The turret roof was slightly crowned to increase its rigidity and to eliminate any possibility of buckling. This was achieved by raising the center line one inch and sloping the roof toward the sides at five degrees from the horizontal. After completion of the mock-up, a new concept drawing, designated LK 10372, was prepared incorporating all of the changes.
 
Data on improved armor arrays were received from the Ballistic Research Laboratory at the conclusion of the previous study. To evaluate the effect of these new arrays on the vehicle weight, a new concept was prepared including the changes applied to the mock-up as well as several new features. This design, designated as LK 10379, was covered in the seventh study from TACOM in February 1973. From this point on, the analyses considered only the protection level required to defeat the heavy threat. This was still the Soviet 115mm APFSDS projectile at 800 meters and the 5.0 inch diameter HEAT round. The 40 rounds of 105mm ammunition in the LK 10379 concept were located in the hull below the turret ring and were not stowed in separate compartments. The Bushmaster weapon system was removed from inside the turret and remounted on the left side exterior near the bustle. The day/night sight for the gunner and the tank commander was installed through the right side of the turret as on the mock-up. The gun shield was narrowed from a width of 38 inches to 12 inches. The five degree taper to the rear of the turret side walls was eliminated making the sides parallel to the turret center line. This provided more space for the tank commander and simplified the design of the Bushmaster mount. The slope of the left and right sides of the turret remained at 35 and 20 degrees from the vertical respectively. The angle of the turret frontal sections toward the rear was increased from 30 to 40 degrees, but they remained sloped at 45 degrees from the vertical. The turret roof was crowned as on the mock-up. The hull front on the LK 10379 was redesigned with a 55 degree angle from the vertical on both the upper and lower glacises. This increased the hull length making the six road wheel suspension marginal for good mobility. Therefore, a new tube-over-bar suspension was specified with seven road wheels per track. As on the mockup, armor arrays were installed on the hull sides above the sponsons. The combat weight of the LK 10379 concept was calculated to be a little over 60 tons. The analysis showed that by reducing the protection on certain areas of the hull and lowering the fuel capacity from 350 to 300 gallons, the weight could be reduced to about 58 tons. The eighth and last in the main series of new XM1 concept studies was performed by TACOM in March 1973. Its objective was to explore ways to reduce the weight of the proposed tank to below 58 tons. To achieve this goal, a new concept drawing, LK 10382, was prepared with several changes from the previous design. However, the same armor arrays were used to permit accurate comparisons. The front hull was changed back to the earlier design with the upper and lower glacis plates sloped at 35 and 70 degrees from the vertical respectively. This reduced the length of the hull by 15 inches and lowered the weight by about 900 pounds. The hull height was reduced from 42 to 40 inches and the sides above the sponsons were sloped at a greater angle from the vertical. The fuel tank behind the crew compartment bulkhead was removed and the engine was moved four inches forward. This dropped the fuel capacity back to 300 gallons. The turret ring diameter was reduced from 85 to 83 inches which also shortened the hull by an additional two inches.
 
With the shorter hull, the suspension was changed back to six road wheels per side. The ground clearance was increased from 17 to 18 inches for better mobility and mine protection. Except for the smaller ring, the turret design remained the same, but the Bushmaster was moved farther to the rear, lowered, and mounted closer to the turret wall. The ammunition feeding system for the Bushmaster was changed so that it fed directly through the turret side armor. This eliminated the partially exposed ammunition chutes which formerly were routed through the turret roof. The estimated combat weight of the LK 10382 design was between 57 and 58 tons depending upon weights assumed for the suspension and the possible reduction in thickness of the hull bottom plate under the engine compartment. After the complexity and high cost that resulted in the cancellation of the XM803, Congress required that these factors be tightly controlled in the new tank program. Thus a design to unit cost ceiling was established at the beginning of the project. The specification requirements in the Materiel Need (MN) document were presented as ranges permitting the contractors some leeway in meeting the design objectives. As the Army later observed during a Congressional hearing in April 1974, "We would like to have the maximum performance, but we are sticking to a design to unit cost and we therefore have to give the contractor the ability to make trade offs to meet the design to cost goal". As mentioned earlier, the weight limitation in the original MN document was incompatible with the protection level required. Since crew survivability had the highest priority on the list of characteristics for the new tank, the maximum weight was raised to 58 tons to permit adequate armor. Later, this would be Increased to 60 tons. During the initial studies in 1972, the design to unit cost goal was set at $400,000. However, by the time that the request for proposal was issued, this had increased to $507,790, also in 1972 dollars. This compared to estimated unit costs of $339,000 for the M60A1, $432,000 for the M60A3, and $611,000 for the cancelled XM803. The new program was divided into three phases. The first of these was a competitive advanced development validation phase during which contracts would be awarded to two sources. Each of these contractors would design, build, and test one complete prototype tank and one automotive test rig. They also were to furnish an additional hull and turret for ballistic evaluation. This 34 month advanced development phase would end with a combined developmental/operational test (DT/OT-I) by the Army. These tests would be used to validate the contractors' proposals for phase 2, which would be a sole source full scale engineering development (FSED) program. Phase 3 would put the new tank into production.
 
The various tank concepts being considered by the Army at this time reflected a change in thinking since the period of the MBT70 and the XM803. Although some sources regarded the appearance of the long range antitank missile as sounding the death knell of the tank, this was not the case. The main effect of the new missiles was to release the tank from the defensive role since other units were now capable of defending themselves against armored attack. The tank could now resume its place as the key offensive weapon in the combined arms team. In that role, the missile firing gun-launcher system was less attractive as the main tank weapon than the fast shooting 105mm gun,
particularly with improved ammunition. Thus when proposals were requested for the development of the new tank, the 105mm gun with its quick reaction, multitarget performance, and immediate availability was the obvious candidate despite its shorter range. Proposals for the advanced development validation phase were received from Chrysler and General Motors in May 1973. Originally, Ford Motor Company had been expected to participate, but they declined to enter the competition. As a result, after review, contracts were awarded to Chrysler and to the Detroit Diesel Allison Division of General Motors on 28 June 1973. As mentioned earlier, these contracts required the design, construction, and testing of a completely integrated prototype tank. It was to include all subsystems except for the night vision equipment. An automotive test rig also was to be constructed for use in the test program and an additional ballistic hull and turret were to be provided for ballistic tests at Aberdeen Proving Ground. These cost plus incentive fee (CPIF) contracts eventually amounted to $68,999,000 for Chrysler and $87,969,000 for General Motors. Chrysler proposed a conventional arrangement for its prototype tank with a three man turret crew and the driver in the center front hull. The gunner and the tank commander were in the usual location on the right side of the M68 105mm gun with the loader on the left. A coaxial Bushmaster weapon was fitted on the left side of the cannon. An M85 .50 caliber machine gun was mounted on the tank commander's station and either a 7.62mm M60D machine gun or a 40mm high velocity grenade launcher was installed on the turret roof in front of the loader's hatch.
 
A simplified line of sight stabilization system was expected to provide performance equal to at least 90 per cent of a dual axis system with a cost saving of about $3000. The 105mm ammunition was stowed in separate compartments vented away from the crew space. New composite special armor was incorporated into the frontal areas of the tank and side skirts extended from the front idler to the rear sprocket partially shielding the hydromechanical suspension. This was actually a tube-over-bar suspension with seven road wheels per track using rotary hydraulic shock absorbers. The term hydromechanical suspension was selected to avoid confusion with the earlier tube-over-bar suspension installed experimentally on the M60A1 which lacked the performance of the new design. It also was noted that the same name could be applied to the advanced high strength torsion bar suspension if it replaced the tube-over bar design in the future. At the time of the proposal, the advanced torsion bar was considered a high risk item, but the option of using high strength steel or titanium torsion bars was kept open for the future. In regard to the power plant, Dr. Philip W. Lett Jr., the leader of the Chrysler team, made a bold decision. Despite the fact that the Army MET Task Force had considered the gas turbine to be a high risk item, he selected the AGT-1500 with the X-1100 transmission to power their prototype. In retrospect, this appears to have been a crucial decision in the design of the new tank. Subsequent events were to prove that the AGT-1500 would develop into a highly reliable power plant and it had several advantages over its diesel competitors. Although both the AGT-1500 and the AVCR-1360 diesel were rated at 1500 gross horsepower, the former utilized only 30 horsepower for cooling compared to 160 horsepower for the latter. Also, a growth potential to 2000 gross horsepower was estimated for the turbine compared to 1800 gross horsepower for the variable compression ratio diesel. The less complex design of the turbine engine with its fewer parts was expected to provide higher reliability and reduced maintenance in the future. Its lighter weight also permitted increased armor protection without exceeding the overall weight limit for the tank.
 
The gas turbine did not generate smoke during operation and it was capable of using a wider range of fuels than the diesel. Also, the turbine was easier to start at low temperatures. The greatest disadvantage of the gas turbine was its higher fuel consumption. This was particularly obvious at low vehicle speeds. However, the small size of the
power plant allowed space for an increase in fuel capacity to achieve the specified cruising range. Another problem was the high volume of combustion air used by the gas turbine. This required a filter system with approximately three times the capacity of that necessary for the diesel engine. The turbine, of course, did not require the high volume of cooling air used by the air-cooled diesel. The development of the gas turbine tank engine resulted from recommendations of the Committee on Gas Turbine and Electric Propulsion for Tanks, chaired by Mr. James C. Zeder. When presented in 1965 to the Mobility Advisory Group, the recommendations of the Zeder Committee were initially rejected because of cost. However, the program was subsequently approved. After the evaluation of several proposals from industry, the Lycoming Division of AVCO was selected to develop the
new turbine, designated as the AGT-1500. At that time, it was considered as a possible power plant for the MBT70. However, after the selection of the diesel engine for that program, attempts were made to cancel the AGT-1500 and to transfer its funds elsewhere. These attempts were prevented by the efforts of Lieutenant Colonel John W. Wiss, Chief of the Army Tank Automotive Command Laboratories, and Mr. Wayne S. Anderson who directed the Propulsion Systems Laboratory. Thus the AGT-1500 was available when needed for the XM1 program and its state of development was not far from that of the AVCR-1360 variable compression ratio diesel. Although the initial cost of the gas turbine exceeded that of the diesel engine, it was anticipated that the life cycle cost would be competitive because of reduced maintenance requirements.
 
The early General Motors concept for the XM1 validation phase tank also was armed with the 105mm gun M68 and a Bushmaster. However, the latter was located in a separate mount on the left side of the turret. The turret crew rode in the usual positions with the gunner and tank commander on the right side of the cannon and the loader on the left. A .50 caliber M85 machine gun was provided for the tank commander and a 7.62mm machine gun was installed at the loader's position. The main gun ammunition was separated from the crew space in compartments. The driver was located in the left front hull and special armor was incorporated in the frontal areas of the tank. The Continental AVCR-1360 diesel engine drove the tank through the General Motors X-1100 transmission. As mentioned earlier, the jWCR-1360 diesel was a 1500 horsepower development of the AVCR-1100 used in the MBT70 and the XM803. The suspension on the General Motors tank was fitted with six road wheels per track. This was a hybrid suspension with road wheels 1, 2, and 6 fitted with the National Water Lift hydropneumatic suspension units. Road wheels 3 and 5 utilized high strength torsion bars. In early July 1973, Major General Robert J. Baer, the XM1 Project Manager, and personnel from the Ballistic Research Laboratory escorted representatives from Chrysler and General Motors to the United Kingdom for an update on the British developed Chobham special armor.
 
During this trip they observed the proposed design of a new United Kingdom vehicle utilizing this special armor as well as the manufacturing processes required for its
production. Based upon the newly obtained data, both contractors reevaluated their proposed armor configurations. Chrysler made a number of changes, but they retained the basic design in their original proposal. However, General Motors made major changes in the configuration of their proposed tank. The most obvious change was in the turret where the vertical front and side walls were replaced by sloped armor. The modifications by both contractors required additional work by the Ballistic Research Laboratory to develop new special armor combinations. By January 1974, both Chrysler and General Motors completed the final design of their special armor configurations and some of the hull and turret armor structures had been delivered to Aberdeen Proving Ground for ballistic tests. Some changes in the specifications for the new tank resulted from a study of the Israeli experience during the October 1973 war in the Middle East. One change which was applied to both of the validation phase prototypes was the replacement of the Bushmaster weapon system with a 7.62mm coaxial machine gun. The original intention had been to utilize the Bushmaster against lightly armored vehicles thus reducing the stowage requirement for the expensive main gun rounds. However, battle experience showed that the tank crews would invariably use the main gun against these vehicles. Also, there was a need for a coaxial machine gun for use against infantry and to suppress antitank weapons. Elimination of the bulky Bushmaster and its ammunition provided space to increase the number of 105mm rounds to 55. Some of the modifications resulting from the study of battlefield experience were applied at a later date to the full scale engineering development (FSED) tank proposals. These included changes in the ammunition compartments to accommodate the larger number of 105mm rounds and the use of a less flammable hydraulic fluid.

Early and late FSED designs, notice changes in the turret design.
The complete validation phase prototypes were delivered to Aberdeen Proving Ground for the combined developmental/operational test (DT/OT-I) which ran from 31 January to 7 May 1976. During this same period, the competing automotive test rigs were driven more than 3000 miles over a variety of terrain. Both prototypes were displayed to the public for the first time on 3 February 1976. The results of the DT/OT-I indicated that both the Chrysler and the General Motors prototypes met the specified requirements for the new tank. The Chrysler tank with its gas turbine power plant showed slightly greater acceleration, but the performance was satisfactory for both vehicles. The operational portion of the tests included two weeks of simulated combat operations with the tanks manned by crews from Fort Knox and Fort Hood. After the evaluation of the competing prototypes, the selection of the contractor for the full scale engineering development phase was scheduled for July 1976. 

From left to right: Early Chrysler XM1 FSED with AGT-1500 gas turbine, final XM1 FSED with AGT-1500, and final XM1 FSED with AVCR-1360 diesel.
After the cancellation of the MBT70 program, the Federal Republic of Germany continued with the development of their new Leopard II tank. In line with efforts to standardize materiel in NATO, Germany proposed that the Leopard II be modified to meet American requirements. In December 1974, a Memorandum of Understanding between the United States and the Federal Republic of Germany was signed in which the U.S. agreed to evaluate the Leopard II under the same test conditions as the XM1 candidates. At that time, it was estimated that the Leopard II would cost about one million 1975 dollars compared to approximately $750,000 for the XML The modified version, designated as the Leopard II AV was armed with the 105mm gun. It had improved armor protection and a simplified fire control system to reduce costs. Tests of the Leopard II AV were carried out at Aberdeen Proving Ground from September to December 1976. Previously, on 22 July 1975, FMC Corporation had been awarded a contract for a study to determine the costs and technical problems involved in manufacturing the Leopard II AV in the United States. The tests at Aberdeen indicated that the Leopard II AV met or exceeded most of the U.S. requirements. The accuracy of its fire control system was slightly higher than that of the XM1, but it was more expensive. The armor protection, ammunition compartmentation, and gun movement were considered somewhat inferior to that on the American tank. Rather than continue the evaluation of the complete tank, it was agreed in January 1977 that only certain major components would be considered for adoption to enhance NATO standardization.
 
Shortly before the end of the XM1 validation test program, a new requirement was introduced for inclusion in the proposals for the full scale engineering development/ producibility engineering and planning (FSED/PEP) contract. These proposals were now to include firm price ceilings for the first two production options. These were for 110 low initial rate production (LIRP) tanks in fiscal year 1979 and 352 tanks in fiscal year 1980. The winner of the competition for the XM1 FSED/PEP contract was selected on schedule in July 1976, but the results were not released by the Army. Information published later in the press indicated that General Motors won with a bid of 208 million dollars compared to Chrysler's bid of 221 million dollars. However, these bids were based upon the Chrysler and General Motors tanks being powered by the AGT-1500 turbine and the AVCR-1360 diesel respectively. The Army, by this time, favored the gas turbine and although awarding the contract to General Motors, planned to change the power plant to the AGT-1500. However, this approach was rejected by the Office of the Secretary of Defense. As the Deputy Secretary of Defense later testified before Congress: "I was unalterably opposed to making these changes in a sole source environment after the contractor was selected. I wanted to know what those unit costs were, what the delays in the program might be, if any, and have those costs determined competitively between the two contractors." Also, a decision had been made to arm the new tank with a 120mm gun at some time in the future. Therefore, the turret had to be capable of mounting either the 105mm gun M68 or a 120mm gun without extensive modification. The 120mm guns under consideration were the British rifled weapon and the German Rheinmetall smooth bore gun used in the Leopard II. As a result of the new requirements, a revised request for proposal was issued, with the award of the FSED/PEP contract now rescheduled to November 1976. Both contractors submitted proposals by 28 October for versions of their tanks with a hybrid turret capable of mounting either a 105mm gun or a 120mm gun. In the interest of NATO standardization, provision was made for the use of the gunner's auxiliary sight from the Leopard II as well as metric fasteners at crew serviced interfaces. General Motors presented a version of their tank powered by the AGT-1500 gas turbine and Chrysler proposed an alternate design using the AVCR-1360 diesel engine. However, they recommended their turbine driven tank as the primary choice. All of the proposed tanks were fitted with British developed smoke grenade launchers.
 
Prior to the submission of the final proposals on 28 October, Chrysler made a number of additional changes to enhance the survivability of their candidate and to reduce the cost. The arrangement of the special armor was modified and a special armor gun shield replaced the casting on the earlier proposal. A new minilaser range finder was incorporated and an extension from the gunner's primary sight was provided for the tank commander removing the need for a separate sight. Other cost reductions included the elimination of the power elevation for the commander's weapon and the driver's built in test equipment (BITE) panel. The latter was replaced by pop-up indicators and other gages in the engine compartment and at the crew stations. 
 
At a news conference on 12 November 1976, it was announced that Chrysler had won the FSED/PEP contract. Published reports indicated that the cost reduction efforts at Chrysler had reduced their bid to 196 million dollars compared to a 232 million dollar bid from General Motors. The increase in the latter was, no doubt, partially due to the higher initial cost of the AGT-1500 turbine compared to the AVCR-1360 diesel engine. However, Chrysler's diesel powered proposal for the FSED/PEP program was priced at 186 million dollars. The five month delay in awarding the contract had permitted further refinement of the design and better determination of the manufacturing costs. For
example, the design to unit cost for Chrysler's tank decreased from $574,000 for the original FSED version to $422,000 for the final turbine powered tank proposal. The unit cost for Chrysler's diesel tank design was $376,000. All of these prices were in 1972 dollars for comparison with the earlier estimates. The actual unit price of the winning tank was expected to be about $754,000 in 1976 dollars. Under the 36 month FSED/PEP contract, Chrysler was to manufacture 11 new pilot tanks incorporating the latest changes resulting from the validation phase tests and other experience. The validation phase prototype and the automotive test rig also were to be refurbished for use in the test program. Evaluation of these vehicles would determine the details of the final design for low initial rate production (LIRP). All of the 11 new pilot tanks were manufactured at the Detroit Army Tank Plant with the first one being delivered in February 1978. The last of the 11 was completed the following July. Developmental Test II (DT-II) extended from February 1978 to September 1979 at Aberdeen Proving Ground. Operational Test II (OT-II) at Fort Bliss, Texas lasted from April 1978 to February 1979 with the tanks manned by the 2nd Squadron, 3rd Armored Cavalry Regiment. Three of the tanks, after testing at Fort Bliss, were refurbished at Detroit, fitted with the latest configuration hardware, and shipped to Fort Knox in June 1979 for additional durability and reliability testing. This program was conducted by H Company, 2nd Squadron, 6th Cavalry under the direction of the Armor and Engineer Board. During these tests, the reliability, availability, maintainability-durability (RAM-D) goal was 272 mean miles between failures (MMBF). The XM1 reached 326 MMBF. Initially, Chrysler personnel provided the maintenance, but in August the organizational maintenance was taken over by the soldiers of H Company for the remainder of the test program. During the OT-II at Fort Bliss, many of the power train failures were caused by dust being ingested into the engine. After modification of the seals in the air filtration system, there were no further failures from this cause during all of the tests at Fort Knox. The tests at Fort Bliss also revealed a tendency for the tracks to be thrown by the buildup of soil between the sprocket hub and the bottom of the rear sponson. Some tracks also were thrown by rapid backing and turning when changing firing positions. This problem was solved by the combination of some additional suspension hardware and the redistribution of the ground pressure with an increase in track tension. The hardware change consisted of a sprocket hub with discharge ports, a mud scraper, steel blocks welded to the lower rear hull to prevent inside throws, a steel plate mounted above the final drive to maintain track alignment, and a retaining ring on the outside of the sprocket. Satisfactory results were obtained with these modifications during further tests at Fort Bliss in October 1979.

Major General Baer (far left) and Dr. P. W. Lett (at right) are with a group observing the AGT-1500.
The survivability of the XM1 was demonstrated during DT-II at Aberdeen using pilot vehicle 11 (PV 11). The tank was fully loaded with fuel and ammunition with dummies placed in the various crew positions. Sensors were installed throughout the tank and, with the engine idling, it was fired upon using various types of ammunition at typical combat ranges. The tank was not destroyed and it was driven away under its own power after the tests. PV 11 also was subjected to a blast from an antitank mine which badly damaged the suspension. However, after short tracking one side of the vehicle, the tank was started and driven away from the test site. On 7 May 1979, the XM1 was approved by the Secretary of Defense for low initial rate production (LIRP) at the newly equipped Lima Army Tank Plant. As mentioned previously, this was for 110 tanks with the first two being delivered at a special acceptance ceremony on 28 February 1980. At this time, the new tank was named the Abrams in honor of the late General Creighton W. Abrams, an outstanding armor commander and former Army Chief of Staff. The General's wife and three sons attended the ceremony and Mrs. Abrams christened the first production tank.
 
The LIRP tanks began developmental testing (DT-III) in March 1980 and it continued until September 1981. The major part was conducted at Aberdeen Proving Ground for RAM-D testing. Other work was at Yuma, Arizona for desert tests, Eglin Air Force Base, Florida and the Alaska Cold Region Test Center for cold room and arctic environment testing, and the White Sands Missile Range, New Mexico for electromagnetic radiation and nuclear vulnerability evaluation. The operational tests (OT-III) started at both Fort Knox and Fort Hood during September 1980 and continued until May 1981. At Fort Hood, the tanks were operated and maintained by the 2/5th Cavalry and the 27th Maintenance Battalion. The new tank was enthusiastically received by the troops who greatly appreciated its speed and agility. The operational tests revealed that the cruising range of the production tank was about 20 to 25 miles less than the objective of 275 miles at 25 miles per hour. This was attributed partially to the suspension modifications made to reduce the track throwing experienced during DT/OT-H. However, it was noted that the turbine operated more efficiently at higher speeds and that an increase in the tank test speed from 25 to only 27 miles per hour produced an improvement in cruising range. Thus by operating at higher speeds, the required cruising range could be obtained.
 
In February 1981, production was approved for 7058 tanks at a rate of 30 per month. The vehicle was classified as standard on 17 February 1981 as the 105mm gun full tracked combat tank M1. Production problems at AVCO Lycoming resulted in their failure to meet the deliveryf schedule and also produced a large number of defects in the engines that were delivered. Poor quality control and shortages of skilled personnel were the primary causes. Eventually these problems were solved and the number of defective engines was minimized as production increased. Early tank production also suffered from delays in obtaining the thermal imaging systems and from some fabrication difficulties with the hull and turret at the Lima plant. These problems also were solved and the production of the new tank soon reached the required level.
 
Unlike the M48 and M60 tank series, the M1 did not make use of large armor steel castings. The hull and turret were assembled by welding sections of rolled homogeneous steel armor plate to which the special armor was attached. These steel sections were cut to size using precision flame cutting equipment. The hull was divided into three sections with the crew compartment in the center separated from the fuel cells in the front and the engine compartment in the rear by two bulkheads. In addition to the two fuel cells adjacent to the driver's station, two were located in the engine compartment and two in the rear sponsons. A Halon fire extinguisher system with infrared sensors was installed in both the crew and engine compartments. It could detect a fire in 4-5 milliseconds and extinguish it in about two tenths of a second. The hatch above the driver's seat was fitted with three periscopes providing an overlapping field of vision exceeding 120 degrees. Like on the XM803, there was no floor escape hatch. The driver's T-bar steering control incorporated motorcycle type hand throttles eliminating the need for a floor accelerator pedal thus providing more foot space. The transmission shift selector and push buttons for the intercom system also were located on the T-bar assembly. The service brake pedal was in the center of the floor allowing braking with either foot. The driver's seat was adjustable for driving with the hatch either open or closed and the seat height could be set to four different positions. A large knob on an adjustable lower lumbar support allowed the driver to control the amount of lower back support. The aluminum turret basket floor was suspended from five posts, two of which mounted the seats for the tank commander and the loader. A door in the basket floor provided access to components in the bottom of the hull. The commander's station permitted observation in the seated position through both the extension from the gunner's primary sight and the sight for the .50 caliber machine gun. Six periscopes permitted 360 degree vision. The three positions of the commander's platform allowed him to observe standing with the hatch in the protected open position or at two heights with the hatch fully open. The commander's .50 caliber machine gun was electrically powered in azimuth, but it was elevated manually. The loader at the left rear of the cannon had an eight position swiveling seat that was adjustable in the vertical direction. An eight round armored hull ammunition compartment was located on the right side of the tank behind the engine compartment bulkhead. Blowout panels in the top and bottom of the hull vented any ammunition explosion to the outside and away from the crew. Fortyfour 105mm rounds of ammunition were stowed in the turret bustle behind sliding armor plate doors. Twentytwo of these rounds could be reached by the loader without leaving his seat. A knee switch operated the bustle compartment door covering these 22 ready rounds on the left side of the turret. The door also could be operated manually in an emergency. Blowout panels in the bustle roof vented any ammunition explosion to the outside. Three additional 105mm rounds were stowed on the turret basket floor to the left of the cannon in spall protection covers. This brought the total 105mm ammunition stowage to 55 rounds.
 
The loader's hatch cover was fitted with a rotatable periscope mount. A 7.62mm M240 machine gun was skate mounted around the outer edge of the loader's hatch. Another 7.62mm M240 machine gun was installed coaxially on the right side of the 105mm gun M68E1 above the gunner's auxiliary sight. The gunner's seat was pedestal mounted on the turret basket floor and it was adjustable in both the vertical and horizontal directions. No adjustments were required when the gunner shifted from the primary sight to the auxiliary sight. The gunner's primary sight with its extension for the tank commander was the main optical sighting instrument. It also included the laser range finder, the thermal vision system, and the gyro stabilized line of sight platform. The sight was protected by an armor steel cover with doors operable from within the turret. The gunner's auxiliary sight was an eight power articulated telescope installed in the right side of the gun mount below the coaxial machine gun. The heart of the fire control system was the digital computer which received signals from the sensors as well as inputs from various controls. Data from the laser range finder, the cant sensor, and the wind sensor were received automatically, but other inputs were entered into the computer manually. The gunner used the muzzle reference system to determine tube bend corrections which were then entered into the computer. Atmospheric temperature and pressure as well as ammunition temperature and tube wear also were manual inputs to the computer.

General Motors XM1.
The powerpack consisted of the AGT-1500 turbine engine, the X-1100-3B transmission, final drives, air cleaner, scavenging blower, and cooling system. The powerpack weighed about 8500 pounds and could be easily removed as a unit using a five ton wrecker. The sprockets at the rear of the vehicle drove the 25 inch wide tracks and the tank rode on seven dual road wheels per side. The 25 inch diameter compensating idler installed at the front of each track was interchangeable with the road wheels. It was mounted on a pivoting idler arm connected to the number one road wheel arm by a track adjusting link. Adjustment of the static track tension was by hand pumping grease into the link. When the proper tension was reached, a pressure relief valve opened releasing any additional grease.
 
The vehicle was sprung by high strength steel torsion bars accessible from the outside on both sides of the tank. Thus, if a torsion bar fractured, both pieces could be more easily extracted. There were only two track support rollers per track compared to three on the validation phase prototype. The production tank was governed for a maximum level speed of 45 miles per hour and it could operate cross country at speeds up to 30 miles per hour. 
 
On 19 February 1982, it was announced that a preliminary agreement had been reached between Chrysler and General Dynamics Corporation for the latter to purchase the Chrysler Defense Division. The deal was completed in March and the organization was renamed as the General Dynamics Land Systems Division. The personnel remained
the same with Dr. Philip W. Lett, Jr. becoming Vice President of Research and Engineering. Thus Chrysler severed its connection with the development and production of tanks which had existed since before World War II.
 
It had always been intended to produce the M1 at both the Lima and the Detroit tank plants. In September 1981, the production of 60 tanks per month was authorized with 30 at Lima and 30 at Detroit. This was based on a single work shift at both plants. A total of 150 tanks per month was possible with multi-shift operation. Production of the M1 began at Detroit alongside the M60A3 during March 1982 and the first tank was accepted by the Army at a ceremony on 31 March. Production of the basic M1 continued at both plants for a total run of 2374 tanks. The last basic M1 was completed in January 1985.

This photograph was taken at the rollout of the first XM1 pilot tank at Detroit in February 1978. In the view above, the two center figures
are Dr. P. W. Lett (left) and Eugene Trapp (right).
*Note - all vehicle weights are provided in short tons.
 
PS. I will update this post a bit later with drawings and photos. And then move on.

Survivability
Armor protection of the M1 is very interesting subject because it's not entirely well understood what happend with requirements when US joined the British "Burlington" program.
One is certain, that original requirements for 115mm APFSDS at 800m and 127mm HEAT warhead over frontal arc, are requirements for early armor types developed by BRL, and are not connected with later development.
Recently the CIA declassified some drawings of the original M1 armor.

This confirms that vehicle was protected by "Burlington" type armor. Similarities can be observed when we compare these drawings with the British drawings of the "Burlington" armors developed for Chieftain. I strongly recommend here articles by Polish historian Paweł Przeździecki.
http://wceo.com.pl/images/Dokumenty/WBBH/PHW/PHW_3_2011.pdf  -> PDF reader page 112.

http://wceo.com.pl/images/Dokumenty/WBBH/PHW/PHW_4_2011.pdf  -> PDF reader page 106.

Another interesting factors is that declassified document that says one of the armor types used in M1 turret, offered KE protection = 400mm RHAe and CE protection = 750mm RHAe at front.



Another document is also interesting because it says.


This fragment is very interesting when taken in to account later fragment of this post when I will talk a bit about armor types. Because it makes situation confusing. We know that BRL tested two armor types, BRL-1 that is assumed to be the original M1 armor, and heavier, better performing BRL-2. The question is, are both armors related? And what type of armor was used by M1IP variant, did it used simply the same armor as M1, and only improvement was added through increasing turret front armor thickness? Or perhaps M1IP used the same armor type as M1A1?
When it comes to the armor itself, this is based on my estimations and measurements I made on the real thing.

 
Besides that:

Hull sides over crew compartment - 40mm + 30/40mm = 70-80mm.
Hull sides over engine compartment - 40mm.
Hull sides around suspension attachement points - 40mm.
Hull rear - 40mm.
Upper front plate and drivers hatch - 50mm at 82-85 degrees = 359.26/573.69mm.
Turret roof front - 50mm at 82-85 degrees = 359.26/573.69mm.
Turret roof around hatches and turret bustle - 25mm.
Engine compartment roof - 25mm.
Hull belly - 25mm.
Heavy ballistic skirts - 70-80mm.
Non ballistic skirts - 5-10mm.
 

Before we move to the armor itself it's also worth to note something about hulls and turrets.
Because there is no official classification I needed to create one by myself. However there are very clear difference between various generations of tank variants.
Turrets:
Type 1 "Short Turret", used only in original M1, it have thinner frontal armor that have effective armor thickness ~700-750mm, and a bit different roof over turret bustle with 3 cutouts for blow off panels.
Type 2 "Interim Long Turret", used only in M1IP variant, have thicker frontal armor, effective thickness ~800-900mm (Steven Zaloga in one of his books said that armor thickness was increased by ~200mm), no other structural changes.
Type 3 "Long Turret", this is the final turret variant used in M1A1 and M1A2, older turrets can't be upgraded to this variant due to significant structural changes thus all these turrets are new builds, even if M1 or M1IP was upgraded to M1A1 or M1A2 standard. Front armor effective thickness is also ~800-900mm.
Hulls:
Type 1 used in M1 and M1IP, it lacks the overpressure NBC protection system, probably have some minor differences with the next type. Front armor effective thickness where special armor is mounted is ~650-700mm and did not changed till this day.
Type 2 used in M1A1 and M1A2 variants, major difference is that overpressure NBC protection system is present, some other minor differences are present depending on tank variant, Type 1 hulls can be upgraded to Type 2, however it requires complete disassembly, cutting and welding in some places.
About special armor itself, there were several types used:
BRL-1 (1980) used in original M1.
BRL-2 (1984) used in M1IP and M1A1 (Not certain!).
HAP 1st generation (1987/1988) used in early production M1A1HA.
HAP 2nd generation (1990) used in late production M1A1HA as well as new M1A1HC and M1A2.
HAP 3rd generation (1999/2000) used in M1A1SA, M1A1FEP, M1A2SEPv1 and M1A2SEPv2 (there is possibility of some upgrades for this armor in 2010+ period of time).
NGAP/NEA (2017+) used in newest M1A2SEPv3.
EAP (?) used in all export variants for Arab states.
BRL stands for Ballistic Research Laboratory, these were designations of lighter and heavier armor variants based on British research, that were developed by BRL in US, it's assumed that BRL-1 was for original M1, while BRL-2 was improved, heavier version for M1IP and M1A1 however as mentioned before, this is not certain.

HAP stands for Heavy Armor Package that uses components made from depleted uranium alloy, we know that there are at least 3 generations of this armor, there is also special variant for Australia where depleted uranium was replaced by something else, perhaps tungsten alloy, but Australian documents I found suggest that it's protection levels are equal to 3rd generation Heavy Armor Package, originaly it was said this type of armor is used in turret front only, however I found mentions in official sources saying it is used also in hull front and turret sides.

EAP or Export Armor Package is meant for non NATO/Close allies (like Australia) of the US, used in tanks provided to Arab states, some theoretize it might be equivalent to BRL-2 but it's probably heavier and offer better protection, but lower than HAP armor package.
One more important thing to note, today we also know that at least the newest M1's also uses Titanium alloy in their armor and several other components.
http://c.ymcdn.com/sites/www.titanium.org/resource/resmgr/2005_2009_papers/Gooch_Final_2007.pdf
http://c.ymcdn.com/sites/www.titanium.org/resource/resmgr/2010_2014_papers/GoochWilliam_2010_MilitaryGr.pdf


There are also two types of serial numbers. One for the turret, and one for the hull, serial numbers on turrets says us what type of armor vehicle uses, and on the hull where it was made. X will equal digit.

Turret:

XXXX - BRL-1 or BRL-2 armor package.
XXXXU - Heavy Armor Package.
XXXXM - Heavy Armor Package (some US tanks have it, don't know why that change in letter).
XXXXA - Heavy Armor Package variant for Australia.
XXXXE - Export Armor Package for Arab states.

Hull:

XXXXD - Tank was made in Detroit Tank Arsenal.
XXXXL - Tank was made in Lima Army Tank Plant/Joint Systems Manufacturing Center.

When it comes to armor itself, today we know it is indeed some type of advanced NERA or NxRA armor, however we only know more or less the structure of the turret side armor, and still only of older variant probably M1A1HA or M1A1HC.

This is how probably turret side armor over crew compartment looks like.

And this is how turret bustle side armor looks like.

And it makes sense, on one of the photos we can see some turrets after armor replacement/upgrade work in Joint Systems Manufacturing Center Lima, Ohio. We can notice that their frontal and side armor was replaced, but turret bustle was untouched.

With such evidence we can assume that turret armor is only replaced over crew compartment, while turret bustle armor might be untouched, and it makes sense from armor protection distribution point of view, where majority of protection increase and thus weight increase we want around the crew and for vehicle frontal 60 degree arc.

Here some more sources.

And here US Army Weapons Hand Book 2016.

https://www.army.mil/e2/c/downloads/431298.pdf

At page 14 in PDF reader we can read for M1A1SA and M1A2SEP "Survivability improvements include frontal armor and turret side-armor upgrades.", of course term "frontal armor" means most likely both turret and hull, and we have confirmation of side turret armor upgrades. And it also seems that M1A1SA and M1A2SEP shares the same armor type, which makes sense because due to reduced costs, simplicity etc. at one specific time you manufacture only one specific type of armor and it makes sense for MBT's even in various but not that much different variants to share the same armor protection. USMC M1A1FEP is also technically equivalent to US Army/ARNG M1A1SA with some minor differences, thus it's safe to assume that it also shares the same armor.

Good question is how exactly DU is applied to the armor design? We do not know, but perhaps it might be part of the NERA array itself like in this prototype British armor (perhaps one of several armor types developed within "Burlington" program?) which uses DU alloy plates with polycarbonate as a reactive layer?



When it comes to armor steel alloys used in the vehicles construction and armor, most likely these standard NATO steel types are used with such range of thickness and hardness.




Another thing is that M1's armor, like all modern 3rd generation MBT's armor is modular or semi modular, which makes it's replacement or repair relatively easy, quick and cheap. Another factor is that M1's turret and hull have very simple geometrical shapes making both manufacturing relatively simple, quick and cheap, as well as armor replacement/repair will be such relatively simple, quick and cheap task.

This also means that frontal protection is uniform for both hull and turret, and have very small weak zones.


Some additional protection over hull front is added by massive fuel tanks (visible on one of the drawing above), Israeli data says that 70mm of fuel is equivalent for 10mm of armored steel, thus we can assume that front hull protection where fuel tanks are is significantly increased.

However the largest improvement in protection will be made in M1A2SEPv3, photographs of one of the 9 prototypes shows us vehicle with massive weight simulators on turret front and hull front. This prototype still uses 3rd generation HAP armor package because it was originally M1A2SEPv2 converted in to M1A2SEPv3 prototype.


Each turret cheek have 4x welded plates, each looks to be ~50mm thick at most. Hull front have 3x welded plates, also each looks to be ~50mm thick at most. This means very significant weight increase over the previous generation armor, so we can assume also very significant protection levels increase over the front. There are also weight simulators packs welded to the turret roof, two have 4x 50mm plates and one have 3x 50mm plates.

http://www.dote.osd.mil/pub/reports/FY2015/pdf/army/2015m1a2sep3.pdf

Some additional informations about vehicle survivability improvements can be found here.

Final armor protection improvement are M19 ARAT-1 and M32 ARAT-2 Explosive Reactive Armor modules.

M19 ARAT-1 is modern, multilayer explosive reactive armor, which might have some sort of anti-tandem warheads capabilities.








From these photos we can see angled layers of ERA inside the module, and perhaps some sort of passive component?

Also to M19 modules, can be attached M32 modules.


It's very unusual design for ERA, and it's interesting it can be combined with M19 modules in to a single, multilayer type of ERA protection. M32 modules can also be attached alone to the turret sides.

In such configuration both turret and hull sides have very good, right ERA cover. The system is modular and can be arranged per need.

Of course besides it's armor M1 have other survivability meassures, primary one is mentioned earlier complete isolation of the main gun ammunition storage. It needs to be noted that the biggest tank and crew killer is not the enemy ammunition penetrating the armor but own ammunition cooking off.

The original M1 and M1IP had such ammo storage:

Turret bustle ready rack - 22 rounds.
Turret bustle semi ready rack - 22 rounds.
Turret basket floor armored box - 3 rounds.
Hull ammunition rack - 8 rounds.

M1A1 and M1A2 have such ammo such ammo storage:

Turret bustle ready rack - 17 rounds with old type of racks and 18 with new type of racks.
Turret bustle semi ready rack - 17 rounds with old type of racks and 18 with new type of racks.
Hull ammunition rack - 6 rounds.
 
 




Blow off panels and ammo racks in turret are connected, so if possible during ammunition cook off, blow off panels will be able at least to some degree pull out ammo racks with burning ammo outside, thus minimizing the damage.



These are armored blast doors for the hull ammunition magazines.



Hull blow off panels are placed in top (we can see large opening for main blow off panel) and belly of the hull (we can see two smaller openings for blow off panels).

In future another upgrade that will increase M1's already fantastic protection and survivability will be hard kill active protection system, however and it's not widely known, M1's had in the past, but not used often a soft kill active protection system or actually two, the AN/VLQ-6 and AN/VLQ-8, both are sort of IR dazzlers which work similiar to Russian TShU-1-7 Shtora-1 and Ukrainian Varta.





In future also M1 will be equipped with Hard Kill Active Protection System, most likely Israeli Trophy HV system as a mid term solution before new Modular Active Protection System will be ready.
 

And finally, another part of survivability improvements are multispectral camouflage nets and covers.

US Army is purchasing Saab Barracuda MCS.



And also uses a set of camouflage nets.

Ok, so let's start to share here some informations and fight with some myths. I will start with M1, people that know me, know this is my black horse. So let's start with history and then move on. Historical description is a quote from Richard P. Hunnicutt Abrams - A History Of The American Main Battle Tank Vol.2.
M1 Abrams
History
Below is the LK 10322 design concept with heavy protection, rear engine, and a conventional turret.

Although Congress cancelled the XM803 program in late 1971, the Army's need for a new tank was recognized. To meet this requirement, 20 million dollars were authorized for the procurement of two prototypes of a new main battle tank. This was in addition to the 20 million dollars allocated to phase out the XM803. Although the funds were provided specifically for two industrial prototype tanks, the Army initially preferred to concentrate on component development until a specification could be prepared for a new main battle tank. At that time, there was considerable controversy within the Army itself as to what form the tank should take. In February 1972, the Main Battle Tank Task Force (MBTTF) was established at Fort Knox, Kentucky under the leadership of Major General William R. Desobry. Its objective was to define the characteristics of the new MET and to prepare the Material Need (MN) document. Technical support for the Task Force was provided by the Advanced Concepts Branch of the Tank Automotive Command (TACOM). Eventually, this work resulted in a series of eight main concept studies prepared by J. B. Gilvydis. The first three of these studies covered the work performed for the Task Force during its period of operation from February to August 1972. The later five were conducted for the Main Battle Tank Project Manager's Office, after it was established in September, and for the Army Materiel Command (AMC). This work continued until March 1973. On 5 February 1972, General Henry A. Miley, Jr., the Commanding General of AMC, concluded in a letter to the Department of the Army that tank system concept studies by industry would be of value to the Army and would help meet the directions of Congress. Following this, contracts were awarded to Chrysler Corporation and General Motors Corporation for such design studies to be completed by September 1972.
 

 
The cross section at the left shows the spaced armor arrangement incorporated in some of the new designs.

Drawing at the bottom of the page depict a General Motors tank concept powered by the AGT-1500 gas turbine. The estimated weight of this vehicle was about 47 tons.
The Chrysler program was to cover new evolutionary design concepts based on the M60A1 tank. The General Motors contract was to study the design of a new MBT using advanced design components based on the experience of the XM803 program. Both projects provided concept studies to support the work of the MBT Task Force. Initially, the new main battle tank was designated as the XM815, but with the revision of the nomenclature system, this was soon changed to the XML TACOM presented the results of their first study to the MBTTF at Fort Knox on 25 February 1972. The initial concept, designated as LK 10322, was basically a conventional arrangement with the engine in the rear, the commander, gunner, and loader centrally located in the turret, and the driver in the front center of the hull. The total armored volume of this concept was about 540 cubic feet. This compared to an armored volume of approximately 425 cubic feet for the Soviet T55 and 650 cubic feet for the M60A1. Armed with the 105mm gun M68, the LK 10322 concept was used in this study to analyse the effect on the combat weight of the different thicknesses of armor required to defeat various enemy threats. Fourteen different levels of protection were considered in this first study with the estimated vehicle weights ranging from about 34 tons to approximately 57 tons.

The General Motors 400K tank proposal is sketched below.

Above is the General Motors 500K tank concept.

The loading sequence for the rounds stowed behind the armored bulkhead in the turret bustle is shown at the left. 

The full size mock-up corresponding to the LK 10372 design can be seen in the photographs on this page. Note the coaxial Bushmaster weapon.
The armor ranged from all round protection against only 23mm armor piercing projectiles to frontal protection against 120mm shaped charge rounds. Various composite armor arrays developed by the Ballistic Research Laboratory were utilized in different versions of the concept. Since the design of the armor was rapidly developing, frequent changes were required to adapt the constantly changing arrays to the tank concept studies. It is interesting to note that the initial drawing for the LK 10322 concept featured the AGT-1500 gas turbine, the power plant eventually selected for the Ml production tank. With six road wheels per track, a tube-over-bar suspension was specified for the LK 10322 to improve the cross-country performance. During April and May, a second TACOM study investigated modified versions of the LK 10322 concept in an effort to reduce the weight. Initially, the hull design remained the same, but the turret was slightly modified. The side walls were tapered five degrees to the rear and the slope of the right side was increased from 14 to 20 degrees from the vertical. A new low profile commander's station was introduced with an externally mounted 7.62mm machine gun and the 7.62mm coaxial machine gun was replaced by a .50 caliber weapon. These modifications resulted in estimated weight reductions ranging from a little less than three tons to over three and one half tons depending upon the protection level under consideration. Later, the hull of the modified LK 10322 was lengthened from 270 inches to 275 inches which allowed an increase in fuel capacity from 300 to 350 gallons. This hull also could accept either the AGT 1500 turbine with the XHM-1500 transmission or the AVCR-1100 diesel engine with the X-1100 transmission. The average weight increase with these hull changes was about 600 pounds. On 20 June 1972, the MET Task Force requested TACOM to perform a third weight and protection analysis. This study was to evaluate the effect of 72 different component combinations applied to the modified LK 10322 concept. The following component list was supplied by the Task Force for the analysis.

1. Primary Armament
a. 105mm gun M68
b. 110mm gun, United Kingdom
c. 120mm gun, Federal Republic of Germany
2. Fire Control
a. XM803 system (without commander's day/night
sight)
b. M60A1 (with turret integrated night thermal sight)
3. Power Plants
a. AVCR-1100 diesel engine with the X-1100 transmission
b. DB1500 diesel engine with the Renk HSWL 354
transmission
c. AGT-1500 gas turbine with the XHM-1500-2 transmission
4. Suspension
a. Tube-over-bar
b. Piston hydropneumatic
5. Tracks
a. T142 or Diehl
b. Lightweight, 28 inch width

This study used two different approaches. The first calculated the armor weight for the hull and turret and determined the protection level obtained with total vehicle weights of 43, 45, 47, and 49 tons. The second approach analysed the effect of 72 combinations of components on the total vehicle weight at a single level of protection. The protection level selected was that required to defeat the combined threat of the Soviet 115mm APFSDS projectile at 800 meters range and the 3.2 inch diameter HEAT round on the front and up to 30 degrees to each flank. In addition to the 72 variations of the modified LK 10322 concept from the third TACOM study, the MET Task Force reviewed fifteen additional designs. Numbers 73 through 80 were submitted by General Motors Corporation and 81 through 83 came from Chrysler. Candidates 84 through 87 were product improved versions of the M60A1. In reviewing the 72 candidates from the third TACOM study, the Task Force eliminated those armed with the United Kingdom 110mm rifled gun and the 120mm smooth bore weapon from the Federal Republic of Germany. The 110mm gun was not considered sufficiently superior to the 105mm gun M68 and the 120mm smooth bore was not expected to be available in time to meet the proposed production schedule. At that time, the Task Force also considered the AGT-1500 gas turbine to be a high risk
power plant and it was dropped from the concepts under review. This reduced the number of proposals in the TACOM study from 72 to 16.

General Motors XM1 prototype, note placement of ammunition. There were two large exposed ammo racks in crew compartment, one with rounds stored in vertical position next to the loader in turret, and second on the right side of the driver. This was probably one of several reasons why General Motors prototype was discarded in favor of Chrysler XM1.
Those remaining differed primarily in the engine and transmission with minor variations in the fire control systems as well as the suspensions and tracks. As a result, the 16 candidates were reduced to two featuring the two power packages considered acceptable. Thus one was powered by the air-cooled Continental AVCR-1100 engine with the X-1100 transmission. The other was fitted with the Daimler Benz (later MTU) DB1500 liquid-cooled diesel with the Renk transmission. These two concepts were included by the Task Force among the five candidates for the final trade off analysis. Of the eight General Motors proposals, two were powered by the AGT-1500 gas turbine using the X-1100 transmission, one utilized the DB1500 diesel with the Renk transmission, and two were fitted with the AVCR-1100-3B with the X-1100 transmission. The remaining three had a twin engine arrangement consisting of two General Motors 8V71T liquid-cooled diesels, again with the X-1100 transmission. All eight concepts used a high strength torsion bar suspension and were armed with the M68 105mm gun. Estimated weights of these vehicles ranged from 47 to 67 tons depending upon their level of protection. The Task Force rejected the three proposals powered by the twin 8V71T diesels because of anticipated increased maintenance time and reduced reliability. With the elimination of the AGT-1500 in two of the concepts, the number of General Motors candidates was reduced to three. One of these also was rejected because of its excessive weight of 67 tons. One of the remaining two was selected as the General Motors candidate even though it weighed 56 tons because it came closest to meeting the MN requirements for ballistic protection. One of the three Chrysler concepts was selected as superior to the other two in several important areas and essentially equal in other respects. It was powered by the AVCR-1100 engine with the X-1100 transmission and had an estimated combat weight of 48.5 tons. However, like the other candidates that met the MN weight requirements, it did not provide the desired protection level. None of the four product improved MoOAls met the M N requirements for ballistic protection or sustained speeds on 10 per cent and 30 per cent grades. Of the four, numbers 84 and 85 provided the highest level of protection. Since number 85 featured the more advanced fire control system, it was selected as one of the five candidates for the final trade off analysis. The most obvious conclusion from the review of these concept studies was that the level of ballistic protection required by the proposed MN document was incompatible with the weight limitation specified. This problem was outlined in a fact sheet forwarded from the Task Force to the Commanding General of AMC, dated 3 August 1972.

This fact sheet concluded that the only practical solution was to either reduce the level of ballistic protection or increase the 49 ton weight limitation in the proposed MN document. The latter alternative was subsequently adopted. Although General Motors and Chrysler provided support to the MET Task Force under their study contracts for new tank prototypes, the final reports on both of these contracts were not submitted until October 1972. In their report, General Motors proposed two concepts. They were based on estimated average production unit costs of $400,000 and $500,000, both in fiscal year 1972 dollars. Referred to as the 400K and 500K tanks, the latter was a growth version of the former with improvements in ballistic protection, firepower, and night vision. Both were manned by a crew of four with the driver in the left front hull. The usual arrangement of the three man turret crew was reversed placing the gunner and tank commander on the left side of the cannon with the loader at the right rear. The turret on both vehicles utilized the large bearing ring from the XM803. Both tanks were armed with a stabilized 105mm gun M68. Forty rounds of 105mm ammunition were stowed in the hull below the level of the turret ring without separate compartmentation. The 400K tank was protected by welded plate armor arrays on the turret front. The sides and rear of the turret consisted of single plate rolled homogeneous armor. The hull front was a single plate of high obliquity armor. Spaced armor skirts were provided on the forward sides of the hull. The tank was powered by the previously proposed twin General Motors 8V71T diesel engines connected to a modified X-1100 transmission through a transfer case. With this arrangement, the tank could be driven on one engine in an emergency. The twin engine installation was rated at 1200 gross horsepower. The estimated combat weight of the 400K concept was 52 tons. Although the armor protection was superior to the M60A3, it did not meet the requirements for the new MET.
 
The running gear consisted of an advanced high strength torsion bar suspension with six 30 inch diameter road wheels per side running on a 22 inch wide, double pin, flat track. A 7.62mm coaxial machine gun was located on the right side of the cannon and a 40mm high velocity grenade launcher was installed on the commander's hatch. The independently stabilized gunner's day/night sight included a laser range finder. The combat weight of the 500K tank was increased to 55 tons providing improved ballistic protection. The front hull now consisted of spaced rolled homogeneous armor and steel/aluminum array skirts were installed on the sides. On the turret, a cast armor trunnion support was joined to frontal spaced armor arrays. The sides and rear also consisted of spaced armor. Although the M68 105mm gun was retained in the 500K tank, the mount was strengthened to permit future adaptation of a 120mm gun firing kinetic energy ammunition. The 7.62mm coaxial machine gun in the 400K tank was replaced by the Bushmaster (20mm-30mm) weapon system then under development. The 40mm high velocity grenade launcher was moved to the loader's station and replaced on the commander's hatch by a .50 caliber M85 machine gun. The commander's weapon was now mounted on a powered slew ring. The gunner's stabilized sight with its laser range finder also now included a far infrared imager for night operations. Individual displays were provided for both the gunner and the tank commander. With the heavier weight, the 500K tank was powered by the AVCR-1100-3B engine rated at 1450 gross horsepower using the modified X-1100 transmission. The number of road wheels was raised from six to seven per side and the width of the double pin flat track was increased to 24 inches. Chrysler's report on the New Evolutionary Tank reviewed eight different vehicle concepts as well as the development status of various design features and components. Main armament on all eight concept studies was the M68 105mm gun. In one case, this was supplemented by a TOW missile installation. On two other concepts, the Shillelagh II or Swifty gun launched missile was specified. This was a proposed development of the Shillelagh reduced in diameter to permit launching from the 105mm gun. The missile velocity also was increased to reduce the flight time by about 50 per cent and to extend the range by approximately 25 per cent. However, the reduced diameter would have decreased the effectiveness of its shaped charge warhead. The estimated combat weight of the eight Chrysler proposals ranged from about 49 tons to 57 tons. After considering the eight concept studies, the Chrysler report proposed a final configuration combining the best components from two of them. With an estimated combat weight of 57.2 tons, this tank had a conventional layout. The driver was in the center front hull and the three man turret located the gunner and the tank commander on the right side of the 105mm gun with the loader on the left. The 105mm ammunition was stowed in compartments separated from the crew space. These compartments in the turret bustle and the hull were designed to fail away from the crew space. Thus any ammunition explosion resulting from a penetration of the armor would be directed either to the outside or into the engine compartment away from the crew. 

AGT-1500 and AVCR-1360.
The 105mm M68 gun was selected as the main armament with possible future upgrading through improved ammunition or the introduction of a gun launched missile such as the Swifty. A .50 caliber M 85 coaxial machine gun was specified, but the design permitted its replacement by the Bushmaster weapon when it became available. The fire control equipment was a modified M60A3 system with the turret integrated night thermal sight (TINTS). Final selection of the power plant was not made at that time with the German MTU MB 873 Ka 500 (formerly the Daimler-Benz DB1500) 1500 horsepower liquidcooled diesel, the AVCR-1360-1 air-cooled diesel, and the AGT-1500 gas turbine all being considered. It was proposed to select two of these power plants for test rig evaluation. The X-1100 transmission was to be used in all three cases. The advanced torsion bar suspension was selected. As mentioned earlier, this system utilized high strength torsion bars permitting a much greater twist before exceeding the elastic limit of the material. Thus a much greater wheel travel could be obtained. As with other proposed tank concepts, it was obvious that the combat weight would greatly exceed the original target if satisfactory protection levels were to be achieved. A new concept, designated LK 10352, was proposed by TACOM in a fourth study dated July 1972. The main objective of this study was to determine the effect on the gross vehicle weight if the fuel capacity was increased from 300 to 350 gallons. This was similar to the earlier examination of the same increase on the modified LK 10322. The hull was lengthened to 276 inches with the extra six inches between the crew compartment bulkhead and the engine being used to accommodate a tank for the additional 50 gallons of fuel. The lower glacis was redesigned on this new concept and the angle with the vertical increased from 55 degrees to 60 degrees. The upper glacis remained at 65 degrees from the vertical. No changes were made to the turret. The effect of these modifications on the gross vehicle weight was calculated based on the protection required to defeat two threat levels. The greatest of these was the combined threat of the Soviet 115mm APFSDS kinetic energy projectile at a range of 800 meters and a 3.2 inch diameter precision HEAT shaped charge round. The calculation indicated that a vehicle weight of over 52 tons would be required for protection directly from the front out to an angle of 30 degrees to each flank. In August and September, the LK 10352 concept was modified in a fifth study to determine the effect of second generation armor arrays on the gross vehicle weight. The early steel-glass-steel composite armor was replaced by multiple steel and aluminum plate combinations. For this analysis, three threat levels and the armor arrays required to defeat them were supplied by the Ballistic Research Laboratory. The light threat consisted only of the kinetic energy projectile representated by the Soviet 115mm APFSDS at a range of 800 meters. The medium level of protection was required to defeat both the kinetic energy threat and the 3.2 inch diameter HEAT shaped charge round. A new heavy threat was introduced at this time. It consisted of both the 115mm kinetic energy projectile and a 5.0 inch diameter HEAT round.

Chrysler design hull mockup with AVCR-1360 diesel.
This was the threat against which the new MBT was to be protected in future TACOM studies. For the front hull armor array, the upper glacis was angled at 35 degrees and the lower glacis at 70 degrees, both from the vertical. The nose also was extended 2.5 inches to give more room to the driver. The turret was modified from the earlier concept by changing the left front from a curved to a flat surface angled back at 30 degrees to the rear and sloped at 60 degrees from the vertical. The slope of the right front was changed from 60 to 55 degrees from the vertical to provide more room for the gunner. This side already had a flat surface angled to the rear at 30 degrees. The turret sides were unchanged, but the bustle was extended by 12 inches to obtain more stowage space. The .50 caliber coaxial machine gun was replaced by the Bushmaster and the .50 caliber weapon was relocated to the commander's hatch. The study indicated that a combat weight of over 59 tons would be required to provide protection against the heavy threat from the front and at 30 degrees to each flank. A sixth study by TACOM from October 1972 to January 1973 covered the preparation and weight analyses of new design concepts and the building of a full size wooden mock-up. The rapid development of composite armor technology continued to cause repeated changes in the tank design concepts. In fact, according to TACOM personnel at that time, the Ballistic Research Laboratory was cranking out new armor arrays faster than new concept designs based on them could be drawn and analyzed. After a preliminary study, it was decided to draw two different versions of the new tank. One version was to have the main gun ammunition stowed in the crew space as in earlier tanks. In the other, it was to be separated from the crew in ammunition compartments. Two different turret designs were prepared. One turret stowed the ammunition separate from the crew in a bustle compartment. The second turret had a short bustle with no ammunition stowage. Two designs were considered for the tank with compartmented ammunition. In the first type, 18 out of a total of 40 105mm rounds were stowed in automated racks in the turret bustle behind a one inch thick aluminum alloy bulkhead. These racks were similar to those in the XM803 and the rounds were retrieved through a single round door in the bulkhead. Sixteen rounds were in two revolving drum type racks in the front hull, one on each side of the driver. These rounds were retrieved through small round doors in the one inch thick bulkhead facing the turret basket.

Chrysler mobility test rig.
The remaining six rounds were stowed in a non-automated rack in a two inch thick box underneath the cannon in the turret basket. The second design of the tank with the compartmented ammunition eliminated the automated racks. They were replaced by simple lightweight racks and some rounds were relocated. The eighteen rounds remained in the turret bustle, but they were now retrieved through sliding aluminum doors. The six rounds previously stowed in the turret basket were moved to the front hull. The compartments on each side of the driver now contained 11 rounds each and they could be reached through sliding doors. In the non-compartmented version of the new tank, all 40 rounds of main gun ammunition were stowed in the crew compartment below the turret ring. Twentyeight were in two racks, one on each side of the driver. The remaining 12 were located in the turret basket. These were considered to be ready rounds and were in easy reach of the loader. The compartmented and non-compartmented concepts were quite similar in external appearance except for slight changes in the turret design. In addition to the shorter bustle, the non-compartmented version retained the five degree taper to the rear on the turret sides. On the compartmented turret, the side walls were parallel to the center line. This provided more stowage space for ammunition in the bustle. Both sides of the turret front were angled back 30 degrees to the rear and sloped at 45 degrees from the vertical. The left and right sides of the turret were sloped at 35 degrees and 20 degrees from the vertical respectively. The roof was flat and the rear of the bustle was angled at 40 degrees from the vertical. As in the previous concepts, the turret ring diameter was 85 inches. 

Chrysler and General Motors XM1's compared to M60A1 Main Battle Tank.
The hull design was identical for both concepts, except for the ammunition racks. The upper and lower front glacises were angled at 35 and 70 degrees from the vertical respectively. Side skirts extended from the front idler to the rear sprocket protecting both the crew and engine compartments. The power plant was the AVCR-1360 engine with the X-1100 transmission. All of the fuel was located in the engine compartment. A tube-over-bar suspension system with six road wheels per side was used with 26 inch wide tracks. For the commander, a low rotatable station even with the turret roof line was used for both concepts. It was fitted with a .50 caliber M85 machine gun which could be operated from either inside or outside the tank. In addition to the machine gun sight, the commander was provided with six periscopes for all around vision. A Bushmaster weapon system was installed on the left side of the 105mm gun. A day/night sight including a laser range finder was mounted in front of the gunner's station with separate controls for the gunner and the tank commander. Weight analyses of the two concepts armored against the heavy threat indicated that the tank with the compartmented ammunition weighed slightly over 60 tons compared to under 57 tons for the non-compartmented version. The tank with the compartmented ammunition and armored against the heavy threat was selected for the construction of a full size wooden mock-up. The mock-up was modified in several ways from the original concept drawings. The day/night sight for the gunner and the tank commander was relocated to the right side of the turret. This was done to eliminate a weak ballistic area on the turret caused by the previous mounting of the sight. To provide protection against HEAT rounds, armor arrays extending alongside the entire crew compartment were installed on the hull side areas over the sponsons. The overhanging frontal sections of the turret were lowered by about four inches leaving only a minimum clearance for the driver's head when driving with his head exposed. This was to improve the ballistic protection by reducing the shot trap in this area. The turret roof was slightly crowned to increase its rigidity and to eliminate any possibility of buckling. This was achieved by raising the center line one inch and sloping the roof toward the sides at five degrees from the horizontal. After completion of the mock-up, a new concept drawing, designated LK 10372, was prepared incorporating all of the changes.
 
Data on improved armor arrays were received from the Ballistic Research Laboratory at the conclusion of the previous study. To evaluate the effect of these new arrays on the vehicle weight, a new concept was prepared including the changes applied to the mock-up as well as several new features. This design, designated as LK 10379, was covered in the seventh study from TACOM in February 1973. From this point on, the analyses considered only the protection level required to defeat the heavy threat. This was still the Soviet 115mm APFSDS projectile at 800 meters and the 5.0 inch diameter HEAT round. The 40 rounds of 105mm ammunition in the LK 10379 concept were located in the hull below the turret ring and were not stowed in separate compartments. The Bushmaster weapon system was removed from inside the turret and remounted on the left side exterior near the bustle. The day/night sight for the gunner and the tank commander was installed through the right side of the turret as on the mock-up. The gun shield was narrowed from a width of 38 inches to 12 inches. The five degree taper to the rear of the turret side walls was eliminated making the sides parallel to the turret center line. This provided more space for the tank commander and simplified the design of the Bushmaster mount. The slope of the left and right sides of the turret remained at 35 and 20 degrees from the vertical respectively. The angle of the turret frontal sections toward the rear was increased from 30 to 40 degrees, but they remained sloped at 45 degrees from the vertical. The turret roof was crowned as on the mock-up. The hull front on the LK 10379 was redesigned with a 55 degree angle from the vertical on both the upper and lower glacises. This increased the hull length making the six road wheel suspension marginal for good mobility. Therefore, a new tube-over-bar suspension was specified with seven road wheels per track. As on the mockup, armor arrays were installed on the hull sides above the sponsons. The combat weight of the LK 10379 concept was calculated to be a little over 60 tons. The analysis showed that by reducing the protection on certain areas of the hull and lowering the fuel capacity from 350 to 300 gallons, the weight could be reduced to about 58 tons. The eighth and last in the main series of new XM1 concept studies was performed by TACOM in March 1973. Its objective was to explore ways to reduce the weight of the proposed tank to below 58 tons. To achieve this goal, a new concept drawing, LK 10382, was prepared with several changes from the previous design. However, the same armor arrays were used to permit accurate comparisons. The front hull was changed back to the earlier design with the upper and lower glacis plates sloped at 35 and 70 degrees from the vertical respectively. This reduced the length of the hull by 15 inches and lowered the weight by about 900 pounds. The hull height was reduced from 42 to 40 inches and the sides above the sponsons were sloped at a greater angle from the vertical. The fuel tank behind the crew compartment bulkhead was removed and the engine was moved four inches forward. This dropped the fuel capacity back to 300 gallons. The turret ring diameter was reduced from 85 to 83 inches which also shortened the hull by an additional two inches.
 
With the shorter hull, the suspension was changed back to six road wheels per side. The ground clearance was increased from 17 to 18 inches for better mobility and mine protection. Except for the smaller ring, the turret design remained the same, but the Bushmaster was moved farther to the rear, lowered, and mounted closer to the turret wall. The ammunition feeding system for the Bushmaster was changed so that it fed directly through the turret side armor. This eliminated the partially exposed ammunition chutes which formerly were routed through the turret roof. The estimated combat weight of the LK 10382 design was between 57 and 58 tons depending upon weights assumed for the suspension and the possible reduction in thickness of the hull bottom plate under the engine compartment. After the complexity and high cost that resulted in the cancellation of the XM803, Congress required that these factors be tightly controlled in the new tank program. Thus a design to unit cost ceiling was established at the beginning of the project. The specification requirements in the Materiel Need (MN) document were presented as ranges permitting the contractors some leeway in meeting the design objectives. As the Army later observed during a Congressional hearing in April 1974, "We would like to have the maximum performance, but we are sticking to a design to unit cost and we therefore have to give the contractor the ability to make trade offs to meet the design to cost goal". As mentioned earlier, the weight limitation in the original MN document was incompatible with the protection level required. Since crew survivability had the highest priority on the list of characteristics for the new tank, the maximum weight was raised to 58 tons to permit adequate armor. Later, this would be Increased to 60 tons. During the initial studies in 1972, the design to unit cost goal was set at $400,000. However, by the time that the request for proposal was issued, this had increased to $507,790, also in 1972 dollars. This compared to estimated unit costs of $339,000 for the M60A1, $432,000 for the M60A3, and $611,000 for the cancelled XM803. The new program was divided into three phases. The first of these was a competitive advanced development validation phase during which contracts would be awarded to two sources. Each of these contractors would design, build, and test one complete prototype tank and one automotive test rig. They also were to furnish an additional hull and turret for ballistic evaluation. This 34 month advanced development phase would end with a combined developmental/operational test (DT/OT-I) by the Army. These tests would be used to validate the contractors' proposals for phase 2, which would be a sole source full scale engineering development (FSED) program. Phase 3 would put the new tank into production.
 
The various tank concepts being considered by the Army at this time reflected a change in thinking since the period of the MBT70 and the XM803. Although some sources regarded the appearance of the long range antitank missile as sounding the death knell of the tank, this was not the case. The main effect of the new missiles was to release the tank from the defensive role since other units were now capable of defending themselves against armored attack. The tank could now resume its place as the key offensive weapon in the combined arms team. In that role, the missile firing gun-launcher system was less attractive as the main tank weapon than the fast shooting 105mm gun,
particularly with improved ammunition. Thus when proposals were requested for the development of the new tank, the 105mm gun with its quick reaction, multitarget performance, and immediate availability was the obvious candidate despite its shorter range. Proposals for the advanced development validation phase were received from Chrysler and General Motors in May 1973. Originally, Ford Motor Company had been expected to participate, but they declined to enter the competition. As a result, after review, contracts were awarded to Chrysler and to the Detroit Diesel Allison Division of General Motors on 28 June 1973. As mentioned earlier, these contracts required the design, construction, and testing of a completely integrated prototype tank. It was to include all subsystems except for the night vision equipment. An automotive test rig also was to be constructed for use in the test program and an additional ballistic hull and turret were to be provided for ballistic tests at Aberdeen Proving Ground. These cost plus incentive fee (CPIF) contracts eventually amounted to $68,999,000 for Chrysler and $87,969,000 for General Motors. Chrysler proposed a conventional arrangement for its prototype tank with a three man turret crew and the driver in the center front hull. The gunner and the tank commander were in the usual location on the right side of the M68 105mm gun with the loader on the left. A coaxial Bushmaster weapon was fitted on the left side of the cannon. An M85 .50 caliber machine gun was mounted on the tank commander's station and either a 7.62mm M60D machine gun or a 40mm high velocity grenade launcher was installed on the turret roof in front of the loader's hatch.
 
A simplified line of sight stabilization system was expected to provide performance equal to at least 90 per cent of a dual axis system with a cost saving of about $3000. The 105mm ammunition was stowed in separate compartments vented away from the crew space. New composite special armor was incorporated into the frontal areas of the tank and side skirts extended from the front idler to the rear sprocket partially shielding the hydromechanical suspension. This was actually a tube-over-bar suspension with seven road wheels per track using rotary hydraulic shock absorbers. The term hydromechanical suspension was selected to avoid confusion with the earlier tube-over-bar suspension installed experimentally on the M60A1 which lacked the performance of the new design. It also was noted that the same name could be applied to the advanced high strength torsion bar suspension if it replaced the tube-over bar design in the future. At the time of the proposal, the advanced torsion bar was considered a high risk item, but the option of using high strength steel or titanium torsion bars was kept open for the future. In regard to the power plant, Dr. Philip W. Lett Jr., the leader of the Chrysler team, made a bold decision. Despite the fact that the Army MET Task Force had considered the gas turbine to be a high risk item, he selected the AGT-1500 with the X-1100 transmission to power their prototype. In retrospect, this appears to have been a crucial decision in the design of the new tank. Subsequent events were to prove that the AGT-1500 would develop into a highly reliable power plant and it had several advantages over its diesel competitors. Although both the AGT-1500 and the AVCR-1360 diesel were rated at 1500 gross horsepower, the former utilized only 30 horsepower for cooling compared to 160 horsepower for the latter. Also, a growth potential to 2000 gross horsepower was estimated for the turbine compared to 1800 gross horsepower for the variable compression ratio diesel. The less complex design of the turbine engine with its fewer parts was expected to provide higher reliability and reduced maintenance in the future. Its lighter weight also permitted increased armor protection without exceeding the overall weight limit for the tank.
 
The gas turbine did not generate smoke during operation and it was capable of using a wider range of fuels than the diesel. Also, the turbine was easier to start at low temperatures. The greatest disadvantage of the gas turbine was its higher fuel consumption. This was particularly obvious at low vehicle speeds. However, the small size of the
power plant allowed space for an increase in fuel capacity to achieve the specified cruising range. Another problem was the high volume of combustion air used by the gas turbine. This required a filter system with approximately three times the capacity of that necessary for the diesel engine. The turbine, of course, did not require the high volume of cooling air used by the air-cooled diesel. The development of the gas turbine tank engine resulted from recommendations of the Committee on Gas Turbine and Electric Propulsion for Tanks, chaired by Mr. James C. Zeder. When presented in 1965 to the Mobility Advisory Group, the recommendations of the Zeder Committee were initially rejected because of cost. However, the program was subsequently approved. After the evaluation of several proposals from industry, the Lycoming Division of AVCO was selected to develop the
new turbine, designated as the AGT-1500. At that time, it was considered as a possible power plant for the MBT70. However, after the selection of the diesel engine for that program, attempts were made to cancel the AGT-1500 and to transfer its funds elsewhere. These attempts were prevented by the efforts of Lieutenant Colonel John W. Wiss, Chief of the Army Tank Automotive Command Laboratories, and Mr. Wayne S. Anderson who directed the Propulsion Systems Laboratory. Thus the AGT-1500 was available when needed for the XM1 program and its state of development was not far from that of the AVCR-1360 variable compression ratio diesel. Although the initial cost of the gas turbine exceeded that of the diesel engine, it was anticipated that the life cycle cost would be competitive because of reduced maintenance requirements.
 
The early General Motors concept for the XM1 validation phase tank also was armed with the 105mm gun M68 and a Bushmaster. However, the latter was located in a separate mount on the left side of the turret. The turret crew rode in the usual positions with the gunner and tank commander on the right side of the cannon and the loader on the left. A .50 caliber M85 machine gun was provided for the tank commander and a 7.62mm machine gun was installed at the loader's position. The main gun ammunition was separated from the crew space in compartments. The driver was located in the left front hull and special armor was incorporated in the frontal areas of the tank. The Continental AVCR-1360 diesel engine drove the tank through the General Motors X-1100 transmission. As mentioned earlier, the jWCR-1360 diesel was a 1500 horsepower development of the AVCR-1100 used in the MBT70 and the XM803. The suspension on the General Motors tank was fitted with six road wheels per track. This was a hybrid suspension with road wheels 1, 2, and 6 fitted with the National Water Lift hydropneumatic suspension units. Road wheels 3 and 5 utilized high strength torsion bars. In early July 1973, Major General Robert J. Baer, the XM1 Project Manager, and personnel from the Ballistic Research Laboratory escorted representatives from Chrysler and General Motors to the United Kingdom for an update on the British developed Chobham special armor.
 
During this trip they observed the proposed design of a new United Kingdom vehicle utilizing this special armor as well as the manufacturing processes required for its
production. Based upon the newly obtained data, both contractors reevaluated their proposed armor configurations. Chrysler made a number of changes, but they retained the basic design in their original proposal. However, General Motors made major changes in the configuration of their proposed tank. The most obvious change was in the turret where the vertical front and side walls were replaced by sloped armor. The modifications by both contractors required additional work by the Ballistic Research Laboratory to develop new special armor combinations. By January 1974, both Chrysler and General Motors completed the final design of their special armor configurations and some of the hull and turret armor structures had been delivered to Aberdeen Proving Ground for ballistic tests. Some changes in the specifications for the new tank resulted from a study of the Israeli experience during the October 1973 war in the Middle East. One change which was applied to both of the validation phase prototypes was the replacement of the Bushmaster weapon system with a 7.62mm coaxial machine gun. The original intention had been to utilize the Bushmaster against lightly armored vehicles thus reducing the stowage requirement for the expensive main gun rounds. However, battle experience showed that the tank crews would invariably use the main gun against these vehicles. Also, there was a need for a coaxial machine gun for use against infantry and to suppress antitank weapons. Elimination of the bulky Bushmaster and its ammunition provided space to increase the number of 105mm rounds to 55. Some of the modifications resulting from the study of battlefield experience were applied at a later date to the full scale engineering development (FSED) tank proposals. These included changes in the ammunition compartments to accommodate the larger number of 105mm rounds and the use of a less flammable hydraulic fluid.

Early and late FSED designs, notice changes in the turret design.
The complete validation phase prototypes were delivered to Aberdeen Proving Ground for the combined developmental/operational test (DT/OT-I) which ran from 31 January to 7 May 1976. During this same period, the competing automotive test rigs were driven more than 3000 miles over a variety of terrain. Both prototypes were displayed to the public for the first time on 3 February 1976. The results of the DT/OT-I indicated that both the Chrysler and the General Motors prototypes met the specified requirements for the new tank. The Chrysler tank with its gas turbine power plant showed slightly greater acceleration, but the performance was satisfactory for both vehicles. The operational portion of the tests included two weeks of simulated combat operations with the tanks manned by crews from Fort Knox and Fort Hood. After the evaluation of the competing prototypes, the selection of the contractor for the full scale engineering development phase was scheduled for July 1976. 

From left to right: Early Chrysler XM1 FSED with AGT-1500 gas turbine, final XM1 FSED with AGT-1500, and final XM1 FSED with AVCR-1360 diesel.
After the cancellation of the MBT70 program, the Federal Republic of Germany continued with the development of their new Leopard II tank. In line with efforts to standardize materiel in NATO, Germany proposed that the Leopard II be modified to meet American requirements. In December 1974, a Memorandum of Understanding between the United States and the Federal Republic of Germany was signed in which the U.S. agreed to evaluate the Leopard II under the same test conditions as the XM1 candidates. At that time, it was estimated that the Leopard II would cost about one million 1975 dollars compared to approximately $750,000 for the XML The modified version, designated as the Leopard II AV was armed with the 105mm gun. It had improved armor protection and a simplified fire control system to reduce costs. Tests of the Leopard II AV were carried out at Aberdeen Proving Ground from September to December 1976. Previously, on 22 July 1975, FMC Corporation had been awarded a contract for a study to determine the costs and technical problems involved in manufacturing the Leopard II AV in the United States. The tests at Aberdeen indicated that the Leopard II AV met or exceeded most of the U.S. requirements. The accuracy of its fire control system was slightly higher than that of the XM1, but it was more expensive. The armor protection, ammunition compartmentation, and gun movement were considered somewhat inferior to that on the American tank. Rather than continue the evaluation of the complete tank, it was agreed in January 1977 that only certain major components would be considered for adoption to enhance NATO standardization.
 
Shortly before the end of the XM1 validation test program, a new requirement was introduced for inclusion in the proposals for the full scale engineering development/ producibility engineering and planning (FSED/PEP) contract. These proposals were now to include firm price ceilings for the first two production options. These were for 110 low initial rate production (LIRP) tanks in fiscal year 1979 and 352 tanks in fiscal year 1980. The winner of the competition for the XM1 FSED/PEP contract was selected on schedule in July 1976, but the results were not released by the Army. Information published later in the press indicated that General Motors won with a bid of 208 million dollars compared to Chrysler's bid of 221 million dollars. However, these bids were based upon the Chrysler and General Motors tanks being powered by the AGT-1500 turbine and the AVCR-1360 diesel respectively. The Army, by this time, favored the gas turbine and although awarding the contract to General Motors, planned to change the power plant to the AGT-1500. However, this approach was rejected by the Office of the Secretary of Defense. As the Deputy Secretary of Defense later testified before Congress: "I was unalterably opposed to making these changes in a sole source environment after the contractor was selected. I wanted to know what those unit costs were, what the delays in the program might be, if any, and have those costs determined competitively between the two contractors." Also, a decision had been made to arm the new tank with a 120mm gun at some time in the future. Therefore, the turret had to be capable of mounting either the 105mm gun M68 or a 120mm gun without extensive modification. The 120mm guns under consideration were the British rifled weapon and the German Rheinmetall smooth bore gun used in the Leopard II. As a result of the new requirements, a revised request for proposal was issued, with the award of the FSED/PEP contract now rescheduled to November 1976. Both contractors submitted proposals by 28 October for versions of their tanks with a hybrid turret capable of mounting either a 105mm gun or a 120mm gun. In the interest of NATO standardization, provision was made for the use of the gunner's auxiliary sight from the Leopard II as well as metric fasteners at crew serviced interfaces. General Motors presented a version of their tank powered by the AGT-1500 gas turbine and Chrysler proposed an alternate design using the AVCR-1360 diesel engine. However, they recommended their turbine driven tank as the primary choice. All of the proposed tanks were fitted with British developed smoke grenade launchers.
 
Prior to the submission of the final proposals on 28 October, Chrysler made a number of additional changes to enhance the survivability of their candidate and to reduce the cost. The arrangement of the special armor was modified and a special armor gun shield replaced the casting on the earlier proposal. A new minilaser range finder was incorporated and an extension from the gunner's primary sight was provided for the tank commander removing the need for a separate sight. Other cost reductions included the elimination of the power elevation for the commander's weapon and the driver's built in test equipment (BITE) panel. The latter was replaced by pop-up indicators and other gages in the engine compartment and at the crew stations. 
 
At a news conference on 12 November 1976, it was announced that Chrysler had won the FSED/PEP contract. Published reports indicated that the cost reduction efforts at Chrysler had reduced their bid to 196 million dollars compared to a 232 million dollar bid from General Motors. The increase in the latter was, no doubt, partially due to the higher initial cost of the AGT-1500 turbine compared to the AVCR-1360 diesel engine. However, Chrysler's diesel powered proposal for the FSED/PEP program was priced at 186 million dollars. The five month delay in awarding the contract had permitted further refinement of the design and better determination of the manufacturing costs. For
example, the design to unit cost for Chrysler's tank decreased from $574,000 for the original FSED version to $422,000 for the final turbine powered tank proposal. The unit cost for Chrysler's diesel tank design was $376,000. All of these prices were in 1972 dollars for comparison with the earlier estimates. The actual unit price of the winning tank was expected to be about $754,000 in 1976 dollars. Under the 36 month FSED/PEP contract, Chrysler was to manufacture 11 new pilot tanks incorporating the latest changes resulting from the validation phase tests and other experience. The validation phase prototype and the automotive test rig also were to be refurbished for use in the test program. Evaluation of these vehicles would determine the details of the final design for low initial rate production (LIRP). All of the 11 new pilot tanks were manufactured at the Detroit Army Tank Plant with the first one being delivered in February 1978. The last of the 11 was completed the following July. Developmental Test II (DT-II) extended from February 1978 to September 1979 at Aberdeen Proving Ground. Operational Test II (OT-II) at Fort Bliss, Texas lasted from April 1978 to February 1979 with the tanks manned by the 2nd Squadron, 3rd Armored Cavalry Regiment. Three of the tanks, after testing at Fort Bliss, were refurbished at Detroit, fitted with the latest configuration hardware, and shipped to Fort Knox in June 1979 for additional durability and reliability testing. This program was conducted by H Company, 2nd Squadron, 6th Cavalry under the direction of the Armor and Engineer Board. During these tests, the reliability, availability, maintainability-durability (RAM-D) goal was 272 mean miles between failures (MMBF). The XM1 reached 326 MMBF. Initially, Chrysler personnel provided the maintenance, but in August the organizational maintenance was taken over by the soldiers of H Company for the remainder of the test program. During the OT-II at Fort Bliss, many of the power train failures were caused by dust being ingested into the engine. After modification of the seals in the air filtration system, there were no further failures from this cause during all of the tests at Fort Knox. The tests at Fort Bliss also revealed a tendency for the tracks to be thrown by the buildup of soil between the sprocket hub and the bottom of the rear sponson. Some tracks also were thrown by rapid backing and turning when changing firing positions. This problem was solved by the combination of some additional suspension hardware and the redistribution of the ground pressure with an increase in track tension. The hardware change consisted of a sprocket hub with discharge ports, a mud scraper, steel blocks welded to the lower rear hull to prevent inside throws, a steel plate mounted above the final drive to maintain track alignment, and a retaining ring on the outside of the sprocket. Satisfactory results were obtained with these modifications during further tests at Fort Bliss in October 1979.

Major General Baer (far left) and Dr. P. W. Lett (at right) are with a group observing the AGT-1500.
The survivability of the XM1 was demonstrated during DT-II at Aberdeen using pilot vehicle 11 (PV 11). The tank was fully loaded with fuel and ammunition with dummies placed in the various crew positions. Sensors were installed throughout the tank and, with the engine idling, it was fired upon using various types of ammunition at typical combat ranges. The tank was not destroyed and it was driven away under its own power after the tests. PV 11 also was subjected to a blast from an antitank mine which badly damaged the suspension. However, after short tracking one side of the vehicle, the tank was started and driven away from the test site. On 7 May 1979, the XM1 was approved by the Secretary of Defense for low initial rate production (LIRP) at the newly equipped Lima Army Tank Plant. As mentioned previously, this was for 110 tanks with the first two being delivered at a special acceptance ceremony on 28 February 1980. At this time, the new tank was named the Abrams in honor of the late General Creighton W. Abrams, an outstanding armor commander and former Army Chief of Staff. The General's wife and three sons attended the ceremony and Mrs. Abrams christened the first production tank.
 
The LIRP tanks began developmental testing (DT-III) in March 1980 and it continued until September 1981. The major part was conducted at Aberdeen Proving Ground for RAM-D testing. Other work was at Yuma, Arizona for desert tests, Eglin Air Force Base, Florida and the Alaska Cold Region Test Center for cold room and arctic environment testing, and the White Sands Missile Range, New Mexico for electromagnetic radiation and nuclear vulnerability evaluation. The operational tests (OT-III) started at both Fort Knox and Fort Hood during September 1980 and continued until May 1981. At Fort Hood, the tanks were operated and maintained by the 2/5th Cavalry and the 27th Maintenance Battalion. The new tank was enthusiastically received by the troops who greatly appreciated its speed and agility. The operational tests revealed that the cruising range of the production tank was about 20 to 25 miles less than the objective of 275 miles at 25 miles per hour. This was attributed partially to the suspension modifications made to reduce the track throwing experienced during DT/OT-H. However, it was noted that the turbine operated more efficiently at higher speeds and that an increase in the tank test speed from 25 to only 27 miles per hour produced an improvement in cruising range. Thus by operating at higher speeds, the required cruising range could be obtained.
 
In February 1981, production was approved for 7058 tanks at a rate of 30 per month. The vehicle was classified as standard on 17 February 1981 as the 105mm gun full tracked combat tank M1. Production problems at AVCO Lycoming resulted in their failure to meet the deliveryf schedule and also produced a large number of defects in the engines that were delivered. Poor quality control and shortages of skilled personnel were the primary causes. Eventually these problems were solved and the number of defective engines was minimized as production increased. Early tank production also suffered from delays in obtaining the thermal imaging systems and from some fabrication difficulties with the hull and turret at the Lima plant. These problems also were solved and the production of the new tank soon reached the required level.
 
Unlike the M48 and M60 tank series, the M1 did not make use of large armor steel castings. The hull and turret were assembled by welding sections of rolled homogeneous steel armor plate to which the special armor was attached. These steel sections were cut to size using precision flame cutting equipment. The hull was divided into three sections with the crew compartment in the center separated from the fuel cells in the front and the engine compartment in the rear by two bulkheads. In addition to the two fuel cells adjacent to the driver's station, two were located in the engine compartment and two in the rear sponsons. A Halon fire extinguisher system with infrared sensors was installed in both the crew and engine compartments. It could detect a fire in 4-5 milliseconds and extinguish it in about two tenths of a second. The hatch above the driver's seat was fitted with three periscopes providing an overlapping field of vision exceeding 120 degrees. Like on the XM803, there was no floor escape hatch. The driver's T-bar steering control incorporated motorcycle type hand throttles eliminating the need for a floor accelerator pedal thus providing more foot space. The transmission shift selector and push buttons for the intercom system also were located on the T-bar assembly. The service brake pedal was in the center of the floor allowing braking with either foot. The driver's seat was adjustable for driving with the hatch either open or closed and the seat height could be set to four different positions. A large knob on an adjustable lower lumbar support allowed the driver to control the amount of lower back support. The aluminum turret basket floor was suspended from five posts, two of which mounted the seats for the tank commander and the loader. A door in the basket floor provided access to components in the bottom of the hull. The commander's station permitted observation in the seated position through both the extension from the gunner's primary sight and the sight for the .50 caliber machine gun. Six periscopes permitted 360 degree vision. The three positions of the commander's platform allowed him to observe standing with the hatch in the protected open position or at two heights with the hatch fully open. The commander's .50 caliber machine gun was electrically powered in azimuth, but it was elevated manually. The loader at the left rear of the cannon had an eight position swiveling seat that was adjustable in the vertical direction. An eight round armored hull ammunition compartment was located on the right side of the tank behind the engine compartment bulkhead. Blowout panels in the top and bottom of the hull vented any ammunition explosion to the outside and away from the crew. Fortyfour 105mm rounds of ammunition were stowed in the turret bustle behind sliding armor plate doors. Twentytwo of these rounds could be reached by the loader without leaving his seat. A knee switch operated the bustle compartment door covering these 22 ready rounds on the left side of the turret. The door also could be operated manually in an emergency. Blowout panels in the bustle roof vented any ammunition explosion to the outside. Three additional 105mm rounds were stowed on the turret basket floor to the left of the cannon in spall protection covers. This brought the total 105mm ammunition stowage to 55 rounds.
 
The loader's hatch cover was fitted with a rotatable periscope mount. A 7.62mm M240 machine gun was skate mounted around the outer edge of the loader's hatch. Another 7.62mm M240 machine gun was installed coaxially on the right side of the 105mm gun M68E1 above the gunner's auxiliary sight. The gunner's seat was pedestal mounted on the turret basket floor and it was adjustable in both the vertical and horizontal directions. No adjustments were required when the gunner shifted from the primary sight to the auxiliary sight. The gunner's primary sight with its extension for the tank commander was the main optical sighting instrument. It also included the laser range finder, the thermal vision system, and the gyro stabilized line of sight platform. The sight was protected by an armor steel cover with doors operable from within the turret. The gunner's auxiliary sight was an eight power articulated telescope installed in the right side of the gun mount below the coaxial machine gun. The heart of the fire control system was the digital computer which received signals from the sensors as well as inputs from various controls. Data from the laser range finder, the cant sensor, and the wind sensor were received automatically, but other inputs were entered into the computer manually. The gunner used the muzzle reference system to determine tube bend corrections which were then entered into the computer. Atmospheric temperature and pressure as well as ammunition temperature and tube wear also were manual inputs to the computer.

General Motors XM1.
The powerpack consisted of the AGT-1500 turbine engine, the X-1100-3B transmission, final drives, air cleaner, scavenging blower, and cooling system. The powerpack weighed about 8500 pounds and could be easily removed as a unit using a five ton wrecker. The sprockets at the rear of the vehicle drove the 25 inch wide tracks and the tank rode on seven dual road wheels per side. The 25 inch diameter compensating idler installed at the front of each track was interchangeable with the road wheels. It was mounted on a pivoting idler arm connected to the number one road wheel arm by a track adjusting link. Adjustment of the static track tension was by hand pumping grease into the link. When the proper tension was reached, a pressure relief valve opened releasing any additional grease.
 
The vehicle was sprung by high strength steel torsion bars accessible from the outside on both sides of the tank. Thus, if a torsion bar fractured, both pieces could be more easily extracted. There were only two track support rollers per track compared to three on the validation phase prototype. The production tank was governed for a maximum level speed of 45 miles per hour and it could operate cross country at speeds up to 30 miles per hour. 
 
On 19 February 1982, it was announced that a preliminary agreement had been reached between Chrysler and General Dynamics Corporation for the latter to purchase the Chrysler Defense Division. The deal was completed in March and the organization was renamed as the General Dynamics Land Systems Division. The personnel remained
the same with Dr. Philip W. Lett, Jr. becoming Vice President of Research and Engineering. Thus Chrysler severed its connection with the development and production of tanks which had existed since before World War II.
 
It had always been intended to produce the M1 at both the Lima and the Detroit tank plants. In September 1981, the production of 60 tanks per month was authorized with 30 at Lima and 30 at Detroit. This was based on a single work shift at both plants. A total of 150 tanks per month was possible with multi-shift operation. Production of the M1 began at Detroit alongside the M60A3 during March 1982 and the first tank was accepted by the Army at a ceremony on 31 March. Production of the basic M1 continued at both plants for a total run of 2374 tanks. The last basic M1 was completed in January 1985.

This photograph was taken at the rollout of the first XM1 pilot tank at Detroit in February 1978. In the view above, the two center figures
are Dr. P. W. Lett (left) and Eugene Trapp (right).
*Note - all vehicle weights are provided in short tons.
 
PS. I will update this post a bit later with drawings and photos. And then move on.

Survivability
Armor protection of the M1 is very interesting subject because it's not entirely well understood what happend with requirements when US joined the British "Burlington" program.
One is certain, that original requirements for 115mm APFSDS at 800m and 127mm HEAT warhead over frontal arc, are requirements for early armor types developed by BRL, and are not connected with later development.
Recently the CIA declassified some drawings of the original M1 armor.

This confirms that vehicle was protected by "Burlington" type armor. Similarities can be observed when we compare these drawings with the British drawings of the "Burlington" armors developed for Chieftain. I strongly recommend here articles by Polish historian Paweł Przeździecki.
http://wceo.com.pl/images/Dokumenty/WBBH/PHW/PHW_3_2011.pdf  -> PDF reader page 112.

http://wceo.com.pl/images/Dokumenty/WBBH/PHW/PHW_4_2011.pdf  -> PDF reader page 106.

Another interesting factors is that declassified document that says one of the armor types used in M1 turret, offered KE protection = 400mm RHAe and CE protection = 750mm RHAe at front.



Another document is also interesting because it says.


This fragment is very interesting when taken in to account later fragment of this post when I will talk a bit about armor types. Because it makes situation confusing. We know that BRL tested two armor types, BRL-1 that is assumed to be the original M1 armor, and heavier, better performing BRL-2. The question is, are both armors related? And what type of armor was used by M1IP variant, did it used simply the same armor as M1, and only improvement was added through increasing turret front armor thickness? Or perhaps M1IP used the same armor type as M1A1?
When it comes to the armor itself, this is based on my estimations and measurements I made on the real thing.

 
Besides that:

Hull sides over crew compartment - 40mm + 30/40mm = 70-80mm.
Hull sides over engine compartment - 40mm.
Hull sides around suspension attachement points - 40mm.
Hull rear - 40mm.
Upper front plate and drivers hatch - 50mm at 82-85 degrees = 359.26/573.69mm.
Turret roof front - 50mm at 82-85 degrees = 359.26/573.69mm.
Turret roof around hatches and turret bustle - 25mm.
Engine compartment roof - 25mm.
Hull belly - 25mm.
Heavy ballistic skirts - 70-80mm.
Non ballistic skirts - 5-10mm.
 

Before we move to the armor itself it's also worth to note something about hulls and turrets.
Because there is no official classification I needed to create one by myself. However there are very clear difference between various generations of tank variants.
Turrets:
Type 1 "Short Turret", used only in original M1, it have thinner frontal armor that have effective armor thickness ~700-750mm, and a bit different roof over turret bustle with 3 cutouts for blow off panels.
Type 2 "Interim Long Turret", used only in M1IP variant, have thicker frontal armor, effective thickness ~800-900mm (Steven Zaloga in one of his books said that armor thickness was increased by ~200mm), no other structural changes.
Type 3 "Long Turret", this is the final turret variant used in M1A1 and M1A2, older turrets can't be upgraded to this variant due to significant structural changes thus all these turrets are new builds, even if M1 or M1IP was upgraded to M1A1 or M1A2 standard. Front armor effective thickness is also ~800-900mm.
Hulls:
Type 1 used in M1 and M1IP, it lacks the overpressure NBC protection system, probably have some minor differences with the next type. Front armor effective thickness where special armor is mounted is ~650-700mm and did not changed till this day.
Type 2 used in M1A1 and M1A2 variants, major difference is that overpressure NBC protection system is present, some other minor differences are present depending on tank variant, Type 1 hulls can be upgraded to Type 2, however it requires complete disassembly, cutting and welding in some places.
About special armor itself, there were several types used:
BRL-1 (1980) used in original M1.
BRL-2 (1984) used in M1IP and M1A1 (Not certain!).
HAP 1st generation (1987/1988) used in early production M1A1HA.
HAP 2nd generation (1990) used in late production M1A1HA as well as new M1A1HC and M1A2.
HAP 3rd generation (1999/2000) used in M1A1SA, M1A1FEP, M1A2SEPv1 and M1A2SEPv2 (there is possibility of some upgrades for this armor in 2010+ period of time).
NGAP/NEA (2017+) used in newest M1A2SEPv3.
EAP (?) used in all export variants for Arab states.
BRL stands for Ballistic Research Laboratory, these were designations of lighter and heavier armor variants based on British research, that were developed by BRL in US, it's assumed that BRL-1 was for original M1, while BRL-2 was improved, heavier version for M1IP and M1A1 however as mentioned before, this is not certain.

HAP stands for Heavy Armor Package that uses components made from depleted uranium alloy, we know that there are at least 3 generations of this armor, there is also special variant for Australia where depleted uranium was replaced by something else, perhaps tungsten alloy, but Australian documents I found suggest that it's protection levels are equal to 3rd generation Heavy Armor Package, originaly it was said this type of armor is used in turret front only, however I found mentions in official sources saying it is used also in hull front and turret sides.

EAP or Export Armor Package is meant for non NATO/Close allies (like Australia) of the US, used in tanks provided to Arab states, some theoretize it might be equivalent to BRL-2 but it's probably heavier and offer better protection, but lower than HAP armor package.
One more important thing to note, today we also know that at least the newest M1's also uses Titanium alloy in their armor and several other components.
http://c.ymcdn.com/sites/www.titanium.org/resource/resmgr/2005_2009_papers/Gooch_Final_2007.pdf
http://c.ymcdn.com/sites/www.titanium.org/resource/resmgr/2010_2014_papers/GoochWilliam_2010_MilitaryGr.pdf


There are also two types of serial numbers. One for the turret, and one for the hull, serial numbers on turrets says us what type of armor vehicle uses, and on the hull where it was made. X will equal digit.

Turret:

XXXX - BRL-1 or BRL-2 armor package.
XXXXU - Heavy Armor Package.
XXXXM - Heavy Armor Package (some US tanks have it, don't know why that change in letter).
XXXXA - Heavy Armor Package variant for Australia.
XXXXE - Export Armor Package for Arab states.

Hull:

XXXXD - Tank was made in Detroit Tank Arsenal.
XXXXL - Tank was made in Lima Army Tank Plant/Joint Systems Manufacturing Center.

When it comes to armor itself, today we know it is indeed some type of advanced NERA or NxRA armor, however we only know more or less the structure of the turret side armor, and still only of older variant probably M1A1HA or M1A1HC.

This is how probably turret side armor over crew compartment looks like.

And this is how turret bustle side armor looks like.

And it makes sense, on one of the photos we can see some turrets after armor replacement/upgrade work in Joint Systems Manufacturing Center Lima, Ohio. We can notice that their frontal and side armor was replaced, but turret bustle was untouched.

With such evidence we can assume that turret armor is only replaced over crew compartment, while turret bustle armor might be untouched, and it makes sense from armor protection distribution point of view, where majority of protection increase and thus weight increase we want around the crew and for vehicle frontal 60 degree arc.

Here some more sources.

And here US Army Weapons Hand Book 2016.

https://www.army.mil/e2/c/downloads/431298.pdf

At page 14 in PDF reader we can read for M1A1SA and M1A2SEP "Survivability improvements include frontal armor and turret side-armor upgrades.", of course term "frontal armor" means most likely both turret and hull, and we have confirmation of side turret armor upgrades. And it also seems that M1A1SA and M1A2SEP shares the same armor type, which makes sense because due to reduced costs, simplicity etc. at one specific time you manufacture only one specific type of armor and it makes sense for MBT's even in various but not that much different variants to share the same armor protection. USMC M1A1FEP is also technically equivalent to US Army/ARNG M1A1SA with some minor differences, thus it's safe to assume that it also shares the same armor.

Good question is how exactly DU is applied to the armor design? We do not know, but perhaps it might be part of the NERA array itself like in this prototype British armor (perhaps one of several armor types developed within "Burlington" program?) which uses DU alloy plates with polycarbonate as a reactive layer?



When it comes to armor steel alloys used in the vehicles construction and armor, most likely these standard NATO steel types are used with such range of thickness and hardness.




Another thing is that M1's armor, like all modern 3rd generation MBT's armor is modular or semi modular, which makes it's replacement or repair relatively easy, quick and cheap. Another factor is that M1's turret and hull have very simple geometrical shapes making both manufacturing relatively simple, quick and cheap, as well as armor replacement/repair will be such relatively simple, quick and cheap task.

This also means that frontal protection is uniform for both hull and turret, and have very small weak zones.


Some additional protection over hull front is added by massive fuel tanks (visible on one of the drawing above), Israeli data says that 70mm of fuel is equivalent for 10mm of armored steel, thus we can assume that front hull protection where fuel tanks are is significantly increased.

However the largest improvement in protection will be made in M1A2SEPv3, photographs of one of the 9 prototypes shows us vehicle with massive weight simulators on turret front and hull front. This prototype still uses 3rd generation HAP armor package because it was originally M1A2SEPv2 converted in to M1A2SEPv3 prototype.


Each turret cheek have 4x welded plates, each looks to be ~50mm thick at most. Hull front have 3x welded plates, also each looks to be ~50mm thick at most. This means very significant weight increase over the previous generation armor, so we can assume also very significant protection levels increase over the front. There are also weight simulators packs welded to the turret roof, two have 4x 50mm plates and one have 3x 50mm plates.

http://www.dote.osd.mil/pub/reports/FY2015/pdf/army/2015m1a2sep3.pdf

Some additional informations about vehicle survivability improvements can be found here.

Final armor protection improvement are M19 ARAT-1 and M32 ARAT-2 Explosive Reactive Armor modules.

M19 ARAT-1 is modern, multilayer explosive reactive armor, which might have some sort of anti-tandem warheads capabilities.








From these photos we can see angled layers of ERA inside the module, and perhaps some sort of passive component?

Also to M19 modules, can be attached M32 modules.


It's very unusual design for ERA, and it's interesting it can be combined with M19 modules in to a single, multilayer type of ERA protection. M32 modules can also be attached alone to the turret sides.

In such configuration both turret and hull sides have very good, right ERA cover. The system is modular and can be arranged per need.

Of course besides it's armor M1 have other survivability meassures, primary one is mentioned earlier complete isolation of the main gun ammunition storage. It needs to be noted that the biggest tank and crew killer is not the enemy ammunition penetrating the armor but own ammunition cooking off.

The original M1 and M1IP had such ammo storage:

Turret bustle ready rack - 22 rounds.
Turret bustle semi ready rack - 22 rounds.
Turret basket floor armored box - 3 rounds.
Hull ammunition rack - 8 rounds.

M1A1 and M1A2 have such ammo such ammo storage:

Turret bustle ready rack - 17 rounds with old type of racks and 18 with new type of racks.
Turret bustle semi ready rack - 17 rounds with old type of racks and 18 with new type of racks.
Hull ammunition rack - 6 rounds.
 
 




Blow off panels and ammo racks in turret are connected, so if possible during ammunition cook off, blow off panels will be able at least to some degree pull out ammo racks with burning ammo outside, thus minimizing the damage.



These are armored blast doors for the hull ammunition magazines.



Hull blow off panels are placed in top (we can see large opening for main blow off panel) and belly of the hull (we can see two smaller openings for blow off panels).

In future another upgrade that will increase M1's already fantastic protection and survivability will be hard kill active protection system, however and it's not widely known, M1's had in the past, but not used often a soft kill active protection system or actually two, the AN/VLQ-6 and AN/VLQ-8, both are sort of IR dazzlers which work similiar to Russian TShU-1-7 Shtora-1 and Ukrainian Varta.





In future also M1 will be equipped with Hard Kill Active Protection System, most likely Israeli Trophy HV system as a mid term solution before new Modular Active Protection System will be ready.
 

And finally, another part of survivability improvements are multispectral camouflage nets and covers.

US Army is purchasing Saab Barracuda MCS.



And also uses a set of camouflage nets.

Ok, so let's start to share here some informations and fight with some myths. I will start with M1, people that know me, know this is my black horse. So let's start with history and then move on. Historical description is a quote from Richard P. Hunnicutt Abrams - A History Of The American Main Battle Tank Vol.2.
M1 Abrams
History
Below is the LK 10322 design concept with heavy protection, rear engine, and a conventional turret.

Although Congress cancelled the XM803 program in late 1971, the Army's need for a new tank was recognized. To meet this requirement, 20 million dollars were authorized for the procurement of two prototypes of a new main battle tank. This was in addition to the 20 million dollars allocated to phase out the XM803. Although the funds were provided specifically for two industrial prototype tanks, the Army initially preferred to concentrate on component development until a specification could be prepared for a new main battle tank. At that time, there was considerable controversy within the Army itself as to what form the tank should take. In February 1972, the Main Battle Tank Task Force (MBTTF) was established at Fort Knox, Kentucky under the leadership of Major General William R. Desobry. Its objective was to define the characteristics of the new MET and to prepare the Material Need (MN) document. Technical support for the Task Force was provided by the Advanced Concepts Branch of the Tank Automotive Command (TACOM). Eventually, this work resulted in a series of eight main concept studies prepared by J. B. Gilvydis. The first three of these studies covered the work performed for the Task Force during its period of operation from February to August 1972. The later five were conducted for the Main Battle Tank Project Manager's Office, after it was established in September, and for the Army Materiel Command (AMC). This work continued until March 1973. On 5 February 1972, General Henry A. Miley, Jr., the Commanding General of AMC, concluded in a letter to the Department of the Army that tank system concept studies by industry would be of value to the Army and would help meet the directions of Congress. Following this, contracts were awarded to Chrysler Corporation and General Motors Corporation for such design studies to be completed by September 1972.
 

 
The cross section at the left shows the spaced armor arrangement incorporated in some of the new designs.

Drawing at the bottom of the page depict a General Motors tank concept powered by the AGT-1500 gas turbine. The estimated weight of this vehicle was about 47 tons.
The Chrysler program was to cover new evolutionary design concepts based on the M60A1 tank. The General Motors contract was to study the design of a new MBT using advanced design components based on the experience of the XM803 program. Both projects provided concept studies to support the work of the MBT Task Force. Initially, the new main battle tank was designated as the XM815, but with the revision of the nomenclature system, this was soon changed to the XML TACOM presented the results of their first study to the MBTTF at Fort Knox on 25 February 1972. The initial concept, designated as LK 10322, was basically a conventional arrangement with the engine in the rear, the commander, gunner, and loader centrally located in the turret, and the driver in the front center of the hull. The total armored volume of this concept was about 540 cubic feet. This compared to an armored volume of approximately 425 cubic feet for the Soviet T55 and 650 cubic feet for the M60A1. Armed with the 105mm gun M68, the LK 10322 concept was used in this study to analyse the effect on the combat weight of the different thicknesses of armor required to defeat various enemy threats. Fourteen different levels of protection were considered in this first study with the estimated vehicle weights ranging from about 34 tons to approximately 57 tons.

The General Motors 400K tank proposal is sketched below.

Above is the General Motors 500K tank concept.

The loading sequence for the rounds stowed behind the armored bulkhead in the turret bustle is shown at the left. 

The full size mock-up corresponding to the LK 10372 design can be seen in the photographs on this page. Note the coaxial Bushmaster weapon.
The armor ranged from all round protection against only 23mm armor piercing projectiles to frontal protection against 120mm shaped charge rounds. Various composite armor arrays developed by the Ballistic Research Laboratory were utilized in different versions of the concept. Since the design of the armor was rapidly developing, frequent changes were required to adapt the constantly changing arrays to the tank concept studies. It is interesting to note that the initial drawing for the LK 10322 concept featured the AGT-1500 gas turbine, the power plant eventually selected for the Ml production tank. With six road wheels per track, a tube-over-bar suspension was specified for the LK 10322 to improve the cross-country performance. During April and May, a second TACOM study investigated modified versions of the LK 10322 concept in an effort to reduce the weight. Initially, the hull design remained the same, but the turret was slightly modified. The side walls were tapered five degrees to the rear and the slope of the right side was increased from 14 to 20 degrees from the vertical. A new low profile commander's station was introduced with an externally mounted 7.62mm machine gun and the 7.62mm coaxial machine gun was replaced by a .50 caliber weapon. These modifications resulted in estimated weight reductions ranging from a little less than three tons to over three and one half tons depending upon the protection level under consideration. Later, the hull of the modified LK 10322 was lengthened from 270 inches to 275 inches which allowed an increase in fuel capacity from 300 to 350 gallons. This hull also could accept either the AGT 1500 turbine with the XHM-1500 transmission or the AVCR-1100 diesel engine with the X-1100 transmission. The average weight increase with these hull changes was about 600 pounds. On 20 June 1972, the MET Task Force requested TACOM to perform a third weight and protection analysis. This study was to evaluate the effect of 72 different component combinations applied to the modified LK 10322 concept. The following component list was supplied by the Task Force for the analysis.

1. Primary Armament
a. 105mm gun M68
b. 110mm gun, United Kingdom
c. 120mm gun, Federal Republic of Germany
2. Fire Control
a. XM803 system (without commander's day/night
sight)
b. M60A1 (with turret integrated night thermal sight)
3. Power Plants
a. AVCR-1100 diesel engine with the X-1100 transmission
b. DB1500 diesel engine with the Renk HSWL 354
transmission
c. AGT-1500 gas turbine with the XHM-1500-2 transmission
4. Suspension
a. Tube-over-bar
b. Piston hydropneumatic
5. Tracks
a. T142 or Diehl
b. Lightweight, 28 inch width

This study used two different approaches. The first calculated the armor weight for the hull and turret and determined the protection level obtained with total vehicle weights of 43, 45, 47, and 49 tons. The second approach analysed the effect of 72 combinations of components on the total vehicle weight at a single level of protection. The protection level selected was that required to defeat the combined threat of the Soviet 115mm APFSDS projectile at 800 meters range and the 3.2 inch diameter HEAT round on the front and up to 30 degrees to each flank. In addition to the 72 variations of the modified LK 10322 concept from the third TACOM study, the MET Task Force reviewed fifteen additional designs. Numbers 73 through 80 were submitted by General Motors Corporation and 81 through 83 came from Chrysler. Candidates 84 through 87 were product improved versions of the M60A1. In reviewing the 72 candidates from the third TACOM study, the Task Force eliminated those armed with the United Kingdom 110mm rifled gun and the 120mm smooth bore weapon from the Federal Republic of Germany. The 110mm gun was not considered sufficiently superior to the 105mm gun M68 and the 120mm smooth bore was not expected to be available in time to meet the proposed production schedule. At that time, the Task Force also considered the AGT-1500 gas turbine to be a high risk
power plant and it was dropped from the concepts under review. This reduced the number of proposals in the TACOM study from 72 to 16.

General Motors XM1 prototype, note placement of ammunition. There were two large exposed ammo racks in crew compartment, one with rounds stored in vertical position next to the loader in turret, and second on the right side of the driver. This was probably one of several reasons why General Motors prototype was discarded in favor of Chrysler XM1.
Those remaining differed primarily in the engine and transmission with minor variations in the fire control systems as well as the suspensions and tracks. As a result, the 16 candidates were reduced to two featuring the two power packages considered acceptable. Thus one was powered by the air-cooled Continental AVCR-1100 engine with the X-1100 transmission. The other was fitted with the Daimler Benz (later MTU) DB1500 liquid-cooled diesel with the Renk transmission. These two concepts were included by the Task Force among the five candidates for the final trade off analysis. Of the eight General Motors proposals, two were powered by the AGT-1500 gas turbine using the X-1100 transmission, one utilized the DB1500 diesel with the Renk transmission, and two were fitted with the AVCR-1100-3B with the X-1100 transmission. The remaining three had a twin engine arrangement consisting of two General Motors 8V71T liquid-cooled diesels, again with the X-1100 transmission. All eight concepts used a high strength torsion bar suspension and were armed with the M68 105mm gun. Estimated weights of these vehicles ranged from 47 to 67 tons depending upon their level of protection. The Task Force rejected the three proposals powered by the twin 8V71T diesels because of anticipated increased maintenance time and reduced reliability. With the elimination of the AGT-1500 in two of the concepts, the number of General Motors candidates was reduced to three. One of these also was rejected because of its excessive weight of 67 tons. One of the remaining two was selected as the General Motors candidate even though it weighed 56 tons because it came closest to meeting the MN requirements for ballistic protection. One of the three Chrysler concepts was selected as superior to the other two in several important areas and essentially equal in other respects. It was powered by the AVCR-1100 engine with the X-1100 transmission and had an estimated combat weight of 48.5 tons. However, like the other candidates that met the MN weight requirements, it did not provide the desired protection level. None of the four product improved MoOAls met the M N requirements for ballistic protection or sustained speeds on 10 per cent and 30 per cent grades. Of the four, numbers 84 and 85 provided the highest level of protection. Since number 85 featured the more advanced fire control system, it was selected as one of the five candidates for the final trade off analysis. The most obvious conclusion from the review of these concept studies was that the level of ballistic protection required by the proposed MN document was incompatible with the weight limitation specified. This problem was outlined in a fact sheet forwarded from the Task Force to the Commanding General of AMC, dated 3 August 1972.

This fact sheet concluded that the only practical solution was to either reduce the level of ballistic protection or increase the 49 ton weight limitation in the proposed MN document. The latter alternative was subsequently adopted. Although General Motors and Chrysler provided support to the MET Task Force under their study contracts for new tank prototypes, the final reports on both of these contracts were not submitted until October 1972. In their report, General Motors proposed two concepts. They were based on estimated average production unit costs of $400,000 and $500,000, both in fiscal year 1972 dollars. Referred to as the 400K and 500K tanks, the latter was a growth version of the former with improvements in ballistic protection, firepower, and night vision. Both were manned by a crew of four with the driver in the left front hull. The usual arrangement of the three man turret crew was reversed placing the gunner and tank commander on the left side of the cannon with the loader at the right rear. The turret on both vehicles utilized the large bearing ring from the XM803. Both tanks were armed with a stabilized 105mm gun M68. Forty rounds of 105mm ammunition were stowed in the hull below the level of the turret ring without separate compartmentation. The 400K tank was protected by welded plate armor arrays on the turret front. The sides and rear of the turret consisted of single plate rolled homogeneous armor. The hull front was a single plate of high obliquity armor. Spaced armor skirts were provided on the forward sides of the hull. The tank was powered by the previously proposed twin General Motors 8V71T diesel engines connected to a modified X-1100 transmission through a transfer case. With this arrangement, the tank could be driven on one engine in an emergency. The twin engine installation was rated at 1200 gross horsepower. The estimated combat weight of the 400K concept was 52 tons. Although the armor protection was superior to the M60A3, it did not meet the requirements for the new MET.
 
The running gear consisted of an advanced high strength torsion bar suspension with six 30 inch diameter road wheels per side running on a 22 inch wide, double pin, flat track. A 7.62mm coaxial machine gun was located on the right side of the cannon and a 40mm high velocity grenade launcher was installed on the commander's hatch. The independently stabilized gunner's day/night sight included a laser range finder. The combat weight of the 500K tank was increased to 55 tons providing improved ballistic protection. The front hull now consisted of spaced rolled homogeneous armor and steel/aluminum array skirts were installed on the sides. On the turret, a cast armor trunnion support was joined to frontal spaced armor arrays. The sides and rear also consisted of spaced armor. Although the M68 105mm gun was retained in the 500K tank, the mount was strengthened to permit future adaptation of a 120mm gun firing kinetic energy ammunition. The 7.62mm coaxial machine gun in the 400K tank was replaced by the Bushmaster (20mm-30mm) weapon system then under development. The 40mm high velocity grenade launcher was moved to the loader's station and replaced on the commander's hatch by a .50 caliber M85 machine gun. The commander's weapon was now mounted on a powered slew ring. The gunner's stabilized sight with its laser range finder also now included a far infrared imager for night operations. Individual displays were provided for both the gunner and the tank commander. With the heavier weight, the 500K tank was powered by the AVCR-1100-3B engine rated at 1450 gross horsepower using the modified X-1100 transmission. The number of road wheels was raised from six to seven per side and the width of the double pin flat track was increased to 24 inches. Chrysler's report on the New Evolutionary Tank reviewed eight different vehicle concepts as well as the development status of various design features and components. Main armament on all eight concept studies was the M68 105mm gun. In one case, this was supplemented by a TOW missile installation. On two other concepts, the Shillelagh II or Swifty gun launched missile was specified. This was a proposed development of the Shillelagh reduced in diameter to permit launching from the 105mm gun. The missile velocity also was increased to reduce the flight time by about 50 per cent and to extend the range by approximately 25 per cent. However, the reduced diameter would have decreased the effectiveness of its shaped charge warhead. The estimated combat weight of the eight Chrysler proposals ranged from about 49 tons to 57 tons. After considering the eight concept studies, the Chrysler report proposed a final configuration combining the best components from two of them. With an estimated combat weight of 57.2 tons, this tank had a conventional layout. The driver was in the center front hull and the three man turret located the gunner and the tank commander on the right side of the 105mm gun with the loader on the left. The 105mm ammunition was stowed in compartments separated from the crew space. These compartments in the turret bustle and the hull were designed to fail away from the crew space. Thus any ammunition explosion resulting from a penetration of the armor would be directed either to the outside or into the engine compartment away from the crew. 

AGT-1500 and AVCR-1360.
The 105mm M68 gun was selected as the main armament with possible future upgrading through improved ammunition or the introduction of a gun launched missile such as the Swifty. A .50 caliber M 85 coaxial machine gun was specified, but the design permitted its replacement by the Bushmaster weapon when it became available. The fire control equipment was a modified M60A3 system with the turret integrated night thermal sight (TINTS). Final selection of the power plant was not made at that time with the German MTU MB 873 Ka 500 (formerly the Daimler-Benz DB1500) 1500 horsepower liquidcooled diesel, the AVCR-1360-1 air-cooled diesel, and the AGT-1500 gas turbine all being considered. It was proposed to select two of these power plants for test rig evaluation. The X-1100 transmission was to be used in all three cases. The advanced torsion bar suspension was selected. As mentioned earlier, this system utilized high strength torsion bars permitting a much greater twist before exceeding the elastic limit of the material. Thus a much greater wheel travel could be obtained. As with other proposed tank concepts, it was obvious that the combat weight would greatly exceed the original target if satisfactory protection levels were to be achieved. A new concept, designated LK 10352, was proposed by TACOM in a fourth study dated July 1972. The main objective of this study was to determine the effect on the gross vehicle weight if the fuel capacity was increased from 300 to 350 gallons. This was similar to the earlier examination of the same increase on the modified LK 10322. The hull was lengthened to 276 inches with the extra six inches between the crew compartment bulkhead and the engine being used to accommodate a tank for the additional 50 gallons of fuel. The lower glacis was redesigned on this new concept and the angle with the vertical increased from 55 degrees to 60 degrees. The upper glacis remained at 65 degrees from the vertical. No changes were made to the turret. The effect of these modifications on the gross vehicle weight was calculated based on the protection required to defeat two threat levels. The greatest of these was the combined threat of the Soviet 115mm APFSDS kinetic energy projectile at a range of 800 meters and a 3.2 inch diameter precision HEAT shaped charge round. The calculation indicated that a vehicle weight of over 52 tons would be required for protection directly from the front out to an angle of 30 degrees to each flank. In August and September, the LK 10352 concept was modified in a fifth study to determine the effect of second generation armor arrays on the gross vehicle weight. The early steel-glass-steel composite armor was replaced by multiple steel and aluminum plate combinations. For this analysis, three threat levels and the armor arrays required to defeat them were supplied by the Ballistic Research Laboratory. The light threat consisted only of the kinetic energy projectile representated by the Soviet 115mm APFSDS at a range of 800 meters. The medium level of protection was required to defeat both the kinetic energy threat and the 3.2 inch diameter HEAT shaped charge round. A new heavy threat was introduced at this time. It consisted of both the 115mm kinetic energy projectile and a 5.0 inch diameter HEAT round.

Chrysler design hull mockup with AVCR-1360 diesel.
This was the threat against which the new MBT was to be protected in future TACOM studies. For the front hull armor array, the upper glacis was angled at 35 degrees and the lower glacis at 70 degrees, both from the vertical. The nose also was extended 2.5 inches to give more room to the driver. The turret was modified from the earlier concept by changing the left front from a curved to a flat surface angled back at 30 degrees to the rear and sloped at 60 degrees from the vertical. The slope of the right front was changed from 60 to 55 degrees from the vertical to provide more room for the gunner. This side already had a flat surface angled to the rear at 30 degrees. The turret sides were unchanged, but the bustle was extended by 12 inches to obtain more stowage space. The .50 caliber coaxial machine gun was replaced by the Bushmaster and the .50 caliber weapon was relocated to the commander's hatch. The study indicated that a combat weight of over 59 tons would be required to provide protection against the heavy threat from the front and at 30 degrees to each flank. A sixth study by TACOM from October 1972 to January 1973 covered the preparation and weight analyses of new design concepts and the building of a full size wooden mock-up. The rapid development of composite armor technology continued to cause repeated changes in the tank design concepts. In fact, according to TACOM personnel at that time, the Ballistic Research Laboratory was cranking out new armor arrays faster than new concept designs based on them could be drawn and analyzed. After a preliminary study, it was decided to draw two different versions of the new tank. One version was to have the main gun ammunition stowed in the crew space as in earlier tanks. In the other, it was to be separated from the crew in ammunition compartments. Two different turret designs were prepared. One turret stowed the ammunition separate from the crew in a bustle compartment. The second turret had a short bustle with no ammunition stowage. Two designs were considered for the tank with compartmented ammunition. In the first type, 18 out of a total of 40 105mm rounds were stowed in automated racks in the turret bustle behind a one inch thick aluminum alloy bulkhead. These racks were similar to those in the XM803 and the rounds were retrieved through a single round door in the bulkhead. Sixteen rounds were in two revolving drum type racks in the front hull, one on each side of the driver. These rounds were retrieved through small round doors in the one inch thick bulkhead facing the turret basket.

Chrysler mobility test rig.
The remaining six rounds were stowed in a non-automated rack in a two inch thick box underneath the cannon in the turret basket. The second design of the tank with the compartmented ammunition eliminated the automated racks. They were replaced by simple lightweight racks and some rounds were relocated. The eighteen rounds remained in the turret bustle, but they were now retrieved through sliding aluminum doors. The six rounds previously stowed in the turret basket were moved to the front hull. The compartments on each side of the driver now contained 11 rounds each and they could be reached through sliding doors. In the non-compartmented version of the new tank, all 40 rounds of main gun ammunition were stowed in the crew compartment below the turret ring. Twentyeight were in two racks, one on each side of the driver. The remaining 12 were located in the turret basket. These were considered to be ready rounds and were in easy reach of the loader. The compartmented and non-compartmented concepts were quite similar in external appearance except for slight changes in the turret design. In addition to the shorter bustle, the non-compartmented version retained the five degree taper to the rear on the turret sides. On the compartmented turret, the side walls were parallel to the center line. This provided more stowage space for ammunition in the bustle. Both sides of the turret front were angled back 30 degrees to the rear and sloped at 45 degrees from the vertical. The left and right sides of the turret were sloped at 35 degrees and 20 degrees from the vertical respectively. The roof was flat and the rear of the bustle was angled at 40 degrees from the vertical. As in the previous concepts, the turret ring diameter was 85 inches. 

Chrysler and General Motors XM1's compared to M60A1 Main Battle Tank.
The hull design was identical for both concepts, except for the ammunition racks. The upper and lower front glacises were angled at 35 and 70 degrees from the vertical respectively. Side skirts extended from the front idler to the rear sprocket protecting both the crew and engine compartments. The power plant was the AVCR-1360 engine with the X-1100 transmission. All of the fuel was located in the engine compartment. A tube-over-bar suspension system with six road wheels per side was used with 26 inch wide tracks. For the commander, a low rotatable station even with the turret roof line was used for both concepts. It was fitted with a .50 caliber M85 machine gun which could be operated from either inside or outside the tank. In addition to the machine gun sight, the commander was provided with six periscopes for all around vision. A Bushmaster weapon system was installed on the left side of the 105mm gun. A day/night sight including a laser range finder was mounted in front of the gunner's station with separate controls for the gunner and the tank commander. Weight analyses of the two concepts armored against the heavy threat indicated that the tank with the compartmented ammunition weighed slightly over 60 tons compared to under 57 tons for the non-compartmented version. The tank with the compartmented ammunition and armored against the heavy threat was selected for the construction of a full size wooden mock-up. The mock-up was modified in several ways from the original concept drawings. The day/night sight for the gunner and the tank commander was relocated to the right side of the turret. This was done to eliminate a weak ballistic area on the turret caused by the previous mounting of the sight. To provide protection against HEAT rounds, armor arrays extending alongside the entire crew compartment were installed on the hull side areas over the sponsons. The overhanging frontal sections of the turret were lowered by about four inches leaving only a minimum clearance for the driver's head when driving with his head exposed. This was to improve the ballistic protection by reducing the shot trap in this area. The turret roof was slightly crowned to increase its rigidity and to eliminate any possibility of buckling. This was achieved by raising the center line one inch and sloping the roof toward the sides at five degrees from the horizontal. After completion of the mock-up, a new concept drawing, designated LK 10372, was prepared incorporating all of the changes.
 
Data on improved armor arrays were received from the Ballistic Research Laboratory at the conclusion of the previous study. To evaluate the effect of these new arrays on the vehicle weight, a new concept was prepared including the changes applied to the mock-up as well as several new features. This design, designated as LK 10379, was covered in the seventh study from TACOM in February 1973. From this point on, the analyses considered only the protection level required to defeat the heavy threat. This was still the Soviet 115mm APFSDS projectile at 800 meters and the 5.0 inch diameter HEAT round. The 40 rounds of 105mm ammunition in the LK 10379 concept were located in the hull below the turret ring and were not stowed in separate compartments. The Bushmaster weapon system was removed from inside the turret and remounted on the left side exterior near the bustle. The day/night sight for the gunner and the tank commander was installed through the right side of the turret as on the mock-up. The gun shield was narrowed from a width of 38 inches to 12 inches. The five degree taper to the rear of the turret side walls was eliminated making the sides parallel to the turret center line. This provided more space for the tank commander and simplified the design of the Bushmaster mount. The slope of the left and right sides of the turret remained at 35 and 20 degrees from the vertical respectively. The angle of the turret frontal sections toward the rear was increased from 30 to 40 degrees, but they remained sloped at 45 degrees from the vertical. The turret roof was crowned as on the mock-up. The hull front on the LK 10379 was redesigned with a 55 degree angle from the vertical on both the upper and lower glacises. This increased the hull length making the six road wheel suspension marginal for good mobility. Therefore, a new tube-over-bar suspension was specified with seven road wheels per track. As on the mockup, armor arrays were installed on the hull sides above the sponsons. The combat weight of the LK 10379 concept was calculated to be a little over 60 tons. The analysis showed that by reducing the protection on certain areas of the hull and lowering the fuel capacity from 350 to 300 gallons, the weight could be reduced to about 58 tons. The eighth and last in the main series of new XM1 concept studies was performed by TACOM in March 1973. Its objective was to explore ways to reduce the weight of the proposed tank to below 58 tons. To achieve this goal, a new concept drawing, LK 10382, was prepared with several changes from the previous design. However, the same armor arrays were used to permit accurate comparisons. The front hull was changed back to the earlier design with the upper and lower glacis plates sloped at 35 and 70 degrees from the vertical respectively. This reduced the length of the hull by 15 inches and lowered the weight by about 900 pounds. The hull height was reduced from 42 to 40 inches and the sides above the sponsons were sloped at a greater angle from the vertical. The fuel tank behind the crew compartment bulkhead was removed and the engine was moved four inches forward. This dropped the fuel capacity back to 300 gallons. The turret ring diameter was reduced from 85 to 83 inches which also shortened the hull by an additional two inches.
 
With the shorter hull, the suspension was changed back to six road wheels per side. The ground clearance was increased from 17 to 18 inches for better mobility and mine protection. Except for the smaller ring, the turret design remained the same, but the Bushmaster was moved farther to the rear, lowered, and mounted closer to the turret wall. The ammunition feeding system for the Bushmaster was changed so that it fed directly through the turret side armor. This eliminated the partially exposed ammunition chutes which formerly were routed through the turret roof. The estimated combat weight of the LK 10382 design was between 57 and 58 tons depending upon weights assumed for the suspension and the possible reduction in thickness of the hull bottom plate under the engine compartment. After the complexity and high cost that resulted in the cancellation of the XM803, Congress required that these factors be tightly controlled in the new tank program. Thus a design to unit cost ceiling was established at the beginning of the project. The specification requirements in the Materiel Need (MN) document were presented as ranges permitting the contractors some leeway in meeting the design objectives. As the Army later observed during a Congressional hearing in April 1974, "We would like to have the maximum performance, but we are sticking to a design to unit cost and we therefore have to give the contractor the ability to make trade offs to meet the design to cost goal". As mentioned earlier, the weight limitation in the original MN document was incompatible with the protection level required. Since crew survivability had the highest priority on the list of characteristics for the new tank, the maximum weight was raised to 58 tons to permit adequate armor. Later, this would be Increased to 60 tons. During the initial studies in 1972, the design to unit cost goal was set at $400,000. However, by the time that the request for proposal was issued, this had increased to $507,790, also in 1972 dollars. This compared to estimated unit costs of $339,000 for the M60A1, $432,000 for the M60A3, and $611,000 for the cancelled XM803. The new program was divided into three phases. The first of these was a competitive advanced development validation phase during which contracts would be awarded to two sources. Each of these contractors would design, build, and test one complete prototype tank and one automotive test rig. They also were to furnish an additional hull and turret for ballistic evaluation. This 34 month advanced development phase would end with a combined developmental/operational test (DT/OT-I) by the Army. These tests would be used to validate the contractors' proposals for phase 2, which would be a sole source full scale engineering development (FSED) program. Phase 3 would put the new tank into production.
 
The various tank concepts being considered by the Army at this time reflected a change in thinking since the period of the MBT70 and the XM803. Although some sources regarded the appearance of the long range antitank missile as sounding the death knell of the tank, this was not the case. The main effect of the new missiles was to release the tank from the defensive role since other units were now capable of defending themselves against armored attack. The tank could now resume its place as the key offensive weapon in the combined arms team. In that role, the missile firing gun-launcher system was less attractive as the main tank weapon than the fast shooting 105mm gun,
particularly with improved ammunition. Thus when proposals were requested for the development of the new tank, the 105mm gun with its quick reaction, multitarget performance, and immediate availability was the obvious candidate despite its shorter range. Proposals for the advanced development validation phase were received from Chrysler and General Motors in May 1973. Originally, Ford Motor Company had been expected to participate, but they declined to enter the competition. As a result, after review, contracts were awarded to Chrysler and to the Detroit Diesel Allison Division of General Motors on 28 June 1973. As mentioned earlier, these contracts required the design, construction, and testing of a completely integrated prototype tank. It was to include all subsystems except for the night vision equipment. An automotive test rig also was to be constructed for use in the test program and an additional ballistic hull and turret were to be provided for ballistic tests at Aberdeen Proving Ground. These cost plus incentive fee (CPIF) contracts eventually amounted to $68,999,000 for Chrysler and $87,969,000 for General Motors. Chrysler proposed a conventional arrangement for its prototype tank with a three man turret crew and the driver in the center front hull. The gunner and the tank commander were in the usual location on the right side of the M68 105mm gun with the loader on the left. A coaxial Bushmaster weapon was fitted on the left side of the cannon. An M85 .50 caliber machine gun was mounted on the tank commander's station and either a 7.62mm M60D machine gun or a 40mm high velocity grenade launcher was installed on the turret roof in front of the loader's hatch.
 
A simplified line of sight stabilization system was expected to provide performance equal to at least 90 per cent of a dual axis system with a cost saving of about $3000. The 105mm ammunition was stowed in separate compartments vented away from the crew space. New composite special armor was incorporated into the frontal areas of the tank and side skirts extended from the front idler to the rear sprocket partially shielding the hydromechanical suspension. This was actually a tube-over-bar suspension with seven road wheels per track using rotary hydraulic shock absorbers. The term hydromechanical suspension was selected to avoid confusion with the earlier tube-over-bar suspension installed experimentally on the M60A1 which lacked the performance of the new design. It also was noted that the same name could be applied to the advanced high strength torsion bar suspension if it replaced the tube-over bar design in the future. At the time of the proposal, the advanced torsion bar was considered a high risk item, but the option of using high strength steel or titanium torsion bars was kept open for the future. In regard to the power plant, Dr. Philip W. Lett Jr., the leader of the Chrysler team, made a bold decision. Despite the fact that the Army MET Task Force had considered the gas turbine to be a high risk item, he selected the AGT-1500 with the X-1100 transmission to power their prototype. In retrospect, this appears to have been a crucial decision in the design of the new tank. Subsequent events were to prove that the AGT-1500 would develop into a highly reliable power plant and it had several advantages over its diesel competitors. Although both the AGT-1500 and the AVCR-1360 diesel were rated at 1500 gross horsepower, the former utilized only 30 horsepower for cooling compared to 160 horsepower for the latter. Also, a growth potential to 2000 gross horsepower was estimated for the turbine compared to 1800 gross horsepower for the variable compression ratio diesel. The less complex design of the turbine engine with its fewer parts was expected to provide higher reliability and reduced maintenance in the future. Its lighter weight also permitted increased armor protection without exceeding the overall weight limit for the tank.
 
The gas turbine did not generate smoke during operation and it was capable of using a wider range of fuels than the diesel. Also, the turbine was easier to start at low temperatures. The greatest disadvantage of the gas turbine was its higher fuel consumption. This was particularly obvious at low vehicle speeds. However, the small size of the
power plant allowed space for an increase in fuel capacity to achieve the specified cruising range. Another problem was the high volume of combustion air used by the gas turbine. This required a filter system with approximately three times the capacity of that necessary for the diesel engine. The turbine, of course, did not require the high volume of cooling air used by the air-cooled diesel. The development of the gas turbine tank engine resulted from recommendations of the Committee on Gas Turbine and Electric Propulsion for Tanks, chaired by Mr. James C. Zeder. When presented in 1965 to the Mobility Advisory Group, the recommendations of the Zeder Committee were initially rejected because of cost. However, the program was subsequently approved. After the evaluation of several proposals from industry, the Lycoming Division of AVCO was selected to develop the
new turbine, designated as the AGT-1500. At that time, it was considered as a possible power plant for the MBT70. However, after the selection of the diesel engine for that program, attempts were made to cancel the AGT-1500 and to transfer its funds elsewhere. These attempts were prevented by the efforts of Lieutenant Colonel John W. Wiss, Chief of the Army Tank Automotive Command Laboratories, and Mr. Wayne S. Anderson who directed the Propulsion Systems Laboratory. Thus the AGT-1500 was available when needed for the XM1 program and its state of development was not far from that of the AVCR-1360 variable compression ratio diesel. Although the initial cost of the gas turbine exceeded that of the diesel engine, it was anticipated that the life cycle cost would be competitive because of reduced maintenance requirements.
 
The early General Motors concept for the XM1 validation phase tank also was armed with the 105mm gun M68 and a Bushmaster. However, the latter was located in a separate mount on the left side of the turret. The turret crew rode in the usual positions with the gunner and tank commander on the right side of the cannon and the loader on the left. A .50 caliber M85 machine gun was provided for the tank commander and a 7.62mm machine gun was installed at the loader's position. The main gun ammunition was separated from the crew space in compartments. The driver was located in the left front hull and special armor was incorporated in the frontal areas of the tank. The Continental AVCR-1360 diesel engine drove the tank through the General Motors X-1100 transmission. As mentioned earlier, the jWCR-1360 diesel was a 1500 horsepower development of the AVCR-1100 used in the MBT70 and the XM803. The suspension on the General Motors tank was fitted with six road wheels per track. This was a hybrid suspension with road wheels 1, 2, and 6 fitted with the National Water Lift hydropneumatic suspension units. Road wheels 3 and 5 utilized high strength torsion bars. In early July 1973, Major General Robert J. Baer, the XM1 Project Manager, and personnel from the Ballistic Research Laboratory escorted representatives from Chrysler and General Motors to the United Kingdom for an update on the British developed Chobham special armor.
 
During this trip they observed the proposed design of a new United Kingdom vehicle utilizing this special armor as well as the manufacturing processes required for its
production. Based upon the newly obtained data, both contractors reevaluated their proposed armor configurations. Chrysler made a number of changes, but they retained the basic design in their original proposal. However, General Motors made major changes in the configuration of their proposed tank. The most obvious change was in the turret where the vertical front and side walls were replaced by sloped armor. The modifications by both contractors required additional work by the Ballistic Research Laboratory to develop new special armor combinations. By January 1974, both Chrysler and General Motors completed the final design of their special armor configurations and some of the hull and turret armor structures had been delivered to Aberdeen Proving Ground for ballistic tests. Some changes in the specifications for the new tank resulted from a study of the Israeli experience during the October 1973 war in the Middle East. One change which was applied to both of the validation phase prototypes was the replacement of the Bushmaster weapon system with a 7.62mm coaxial machine gun. The original intention had been to utilize the Bushmaster against lightly armored vehicles thus reducing the stowage requirement for the expensive main gun rounds. However, battle experience showed that the tank crews would invariably use the main gun against these vehicles. Also, there was a need for a coaxial machine gun for use against infantry and to suppress antitank weapons. Elimination of the bulky Bushmaster and its ammunition provided space to increase the number of 105mm rounds to 55. Some of the modifications resulting from the study of battlefield experience were applied at a later date to the full scale engineering development (FSED) tank proposals. These included changes in the ammunition compartments to accommodate the larger number of 105mm rounds and the use of a less flammable hydraulic fluid.

Early and late FSED designs, notice changes in the turret design.
The complete validation phase prototypes were delivered to Aberdeen Proving Ground for the combined developmental/operational test (DT/OT-I) which ran from 31 January to 7 May 1976. During this same period, the competing automotive test rigs were driven more than 3000 miles over a variety of terrain. Both prototypes were displayed to the public for the first time on 3 February 1976. The results of the DT/OT-I indicated that both the Chrysler and the General Motors prototypes met the specified requirements for the new tank. The Chrysler tank with its gas turbine power plant showed slightly greater acceleration, but the performance was satisfactory for both vehicles. The operational portion of the tests included two weeks of simulated combat operations with the tanks manned by crews from Fort Knox and Fort Hood. After the evaluation of the competing prototypes, the selection of the contractor for the full scale engineering development phase was scheduled for July 1976. 

From left to right: Early Chrysler XM1 FSED with AGT-1500 gas turbine, final XM1 FSED with AGT-1500, and final XM1 FSED with AVCR-1360 diesel.
After the cancellation of the MBT70 program, the Federal Republic of Germany continued with the development of their new Leopard II tank. In line with efforts to standardize materiel in NATO, Germany proposed that the Leopard II be modified to meet American requirements. In December 1974, a Memorandum of Understanding between the United States and the Federal Republic of Germany was signed in which the U.S. agreed to evaluate the Leopard II under the same test conditions as the XM1 candidates. At that time, it was estimated that the Leopard II would cost about one million 1975 dollars compared to approximately $750,000 for the XML The modified version, designated as the Leopard II AV was armed with the 105mm gun. It had improved armor protection and a simplified fire control system to reduce costs. Tests of the Leopard II AV were carried out at Aberdeen Proving Ground from September to December 1976. Previously, on 22 July 1975, FMC Corporation had been awarded a contract for a study to determine the costs and technical problems involved in manufacturing the Leopard II AV in the United States. The tests at Aberdeen indicated that the Leopard II AV met or exceeded most of the U.S. requirements. The accuracy of its fire control system was slightly higher than that of the XM1, but it was more expensive. The armor protection, ammunition compartmentation, and gun movement were considered somewhat inferior to that on the American tank. Rather than continue the evaluation of the complete tank, it was agreed in January 1977 that only certain major components would be considered for adoption to enhance NATO standardization.
 
Shortly before the end of the XM1 validation test program, a new requirement was introduced for inclusion in the proposals for the full scale engineering development/ producibility engineering and planning (FSED/PEP) contract. These proposals were now to include firm price ceilings for the first two production options. These were for 110 low initial rate production (LIRP) tanks in fiscal year 1979 and 352 tanks in fiscal year 1980. The winner of the competition for the XM1 FSED/PEP contract was selected on schedule in July 1976, but the results were not released by the Army. Information published later in the press indicated that General Motors won with a bid of 208 million dollars compared to Chrysler's bid of 221 million dollars. However, these bids were based upon the Chrysler and General Motors tanks being powered by the AGT-1500 turbine and the AVCR-1360 diesel respectively. The Army, by this time, favored the gas turbine and although awarding the contract to General Motors, planned to change the power plant to the AGT-1500. However, this approach was rejected by the Office of the Secretary of Defense. As the Deputy Secretary of Defense later testified before Congress: "I was unalterably opposed to making these changes in a sole source environment after the contractor was selected. I wanted to know what those unit costs were, what the delays in the program might be, if any, and have those costs determined competitively between the two contractors." Also, a decision had been made to arm the new tank with a 120mm gun at some time in the future. Therefore, the turret had to be capable of mounting either the 105mm gun M68 or a 120mm gun without extensive modification. The 120mm guns under consideration were the British rifled weapon and the German Rheinmetall smooth bore gun used in the Leopard II. As a result of the new requirements, a revised request for proposal was issued, with the award of the FSED/PEP contract now rescheduled to November 1976. Both contractors submitted proposals by 28 October for versions of their tanks with a hybrid turret capable of mounting either a 105mm gun or a 120mm gun. In the interest of NATO standardization, provision was made for the use of the gunner's auxiliary sight from the Leopard II as well as metric fasteners at crew serviced interfaces. General Motors presented a version of their tank powered by the AGT-1500 gas turbine and Chrysler proposed an alternate design using the AVCR-1360 diesel engine. However, they recommended their turbine driven tank as the primary choice. All of the proposed tanks were fitted with British developed smoke grenade launchers.
 
Prior to the submission of the final proposals on 28 October, Chrysler made a number of additional changes to enhance the survivability of their candidate and to reduce the cost. The arrangement of the special armor was modified and a special armor gun shield replaced the casting on the earlier proposal. A new minilaser range finder was incorporated and an extension from the gunner's primary sight was provided for the tank commander removing the need for a separate sight. Other cost reductions included the elimination of the power elevation for the commander's weapon and the driver's built in test equipment (BITE) panel. The latter was replaced by pop-up indicators and other gages in the engine compartment and at the crew stations. 
 
At a news conference on 12 November 1976, it was announced that Chrysler had won the FSED/PEP contract. Published reports indicated that the cost reduction efforts at Chrysler had reduced their bid to 196 million dollars compared to a 232 million dollar bid from General Motors. The increase in the latter was, no doubt, partially due to the higher initial cost of the AGT-1500 turbine compared to the AVCR-1360 diesel engine. However, Chrysler's diesel powered proposal for the FSED/PEP program was priced at 186 million dollars. The five month delay in awarding the contract had permitted further refinement of the design and better determination of the manufacturing costs. For
example, the design to unit cost for Chrysler's tank decreased from $574,000 for the original FSED version to $422,000 for the final turbine powered tank proposal. The unit cost for Chrysler's diesel tank design was $376,000. All of these prices were in 1972 dollars for comparison with the earlier estimates. The actual unit price of the winning tank was expected to be about $754,000 in 1976 dollars. Under the 36 month FSED/PEP contract, Chrysler was to manufacture 11 new pilot tanks incorporating the latest changes resulting from the validation phase tests and other experience. The validation phase prototype and the automotive test rig also were to be refurbished for use in the test program. Evaluation of these vehicles would determine the details of the final design for low initial rate production (LIRP). All of the 11 new pilot tanks were manufactured at the Detroit Army Tank Plant with the first one being delivered in February 1978. The last of the 11 was completed the following July. Developmental Test II (DT-II) extended from February 1978 to September 1979 at Aberdeen Proving Ground. Operational Test II (OT-II) at Fort Bliss, Texas lasted from April 1978 to February 1979 with the tanks manned by the 2nd Squadron, 3rd Armored Cavalry Regiment. Three of the tanks, after testing at Fort Bliss, were refurbished at Detroit, fitted with the latest configuration hardware, and shipped to Fort Knox in June 1979 for additional durability and reliability testing. This program was conducted by H Company, 2nd Squadron, 6th Cavalry under the direction of the Armor and Engineer Board. During these tests, the reliability, availability, maintainability-durability (RAM-D) goal was 272 mean miles between failures (MMBF). The XM1 reached 326 MMBF. Initially, Chrysler personnel provided the maintenance, but in August the organizational maintenance was taken over by the soldiers of H Company for the remainder of the test program. During the OT-II at Fort Bliss, many of the power train failures were caused by dust being ingested into the engine. After modification of the seals in the air filtration system, there were no further failures from this cause during all of the tests at Fort Knox. The tests at Fort Bliss also revealed a tendency for the tracks to be thrown by the buildup of soil between the sprocket hub and the bottom of the rear sponson. Some tracks also were thrown by rapid backing and turning when changing firing positions. This problem was solved by the combination of some additional suspension hardware and the redistribution of the ground pressure with an increase in track tension. The hardware change consisted of a sprocket hub with discharge ports, a mud scraper, steel blocks welded to the lower rear hull to prevent inside throws, a steel plate mounted above the final drive to maintain track alignment, and a retaining ring on the outside of the sprocket. Satisfactory results were obtained with these modifications during further tests at Fort Bliss in October 1979.

Major General Baer (far left) and Dr. P. W. Lett (at right) are with a group observing the AGT-1500.
The survivability of the XM1 was demonstrated during DT-II at Aberdeen using pilot vehicle 11 (PV 11). The tank was fully loaded with fuel and ammunition with dummies placed in the various crew positions. Sensors were installed throughout the tank and, with the engine idling, it was fired upon using various types of ammunition at typical combat ranges. The tank was not destroyed and it was driven away under its own power after the tests. PV 11 also was subjected to a blast from an antitank mine which badly damaged the suspension. However, after short tracking one side of the vehicle, the tank was started and driven away from the test site. On 7 May 1979, the XM1 was approved by the Secretary of Defense for low initial rate production (LIRP) at the newly equipped Lima Army Tank Plant. As mentioned previously, this was for 110 tanks with the first two being delivered at a special acceptance ceremony on 28 February 1980. At this time, the new tank was named the Abrams in honor of the late General Creighton W. Abrams, an outstanding armor commander and former Army Chief of Staff. The General's wife and three sons attended the ceremony and Mrs. Abrams christened the first production tank.
 
The LIRP tanks began developmental testing (DT-III) in March 1980 and it continued until September 1981. The major part was conducted at Aberdeen Proving Ground for RAM-D testing. Other work was at Yuma, Arizona for desert tests, Eglin Air Force Base, Florida and the Alaska Cold Region Test Center for cold room and arctic environment testing, and the White Sands Missile Range, New Mexico for electromagnetic radiation and nuclear vulnerability evaluation. The operational tests (OT-III) started at both Fort Knox and Fort Hood during September 1980 and continued until May 1981. At Fort Hood, the tanks were operated and maintained by the 2/5th Cavalry and the 27th Maintenance Battalion. The new tank was enthusiastically received by the troops who greatly appreciated its speed and agility. The operational tests revealed that the cruising range of the production tank was about 20 to 25 miles less than the objective of 275 miles at 25 miles per hour. This was attributed partially to the suspension modifications made to reduce the track throwing experienced during DT/OT-H. However, it was noted that the turbine operated more efficiently at higher speeds and that an increase in the tank test speed from 25 to only 27 miles per hour produced an improvement in cruising range. Thus by operating at higher speeds, the required cruising range could be obtained.
 
In February 1981, production was approved for 7058 tanks at a rate of 30 per month. The vehicle was classified as standard on 17 February 1981 as the 105mm gun full tracked combat tank M1. Production problems at AVCO Lycoming resulted in their failure to meet the deliveryf schedule and also produced a large number of defects in the engines that were delivered. Poor quality control and shortages of skilled personnel were the primary causes. Eventually these problems were solved and the number of defective engines was minimized as production increased. Early tank production also suffered from delays in obtaining the thermal imaging systems and from some fabrication difficulties with the hull and turret at the Lima plant. These problems also were solved and the production of the new tank soon reached the required level.
 
Unlike the M48 and M60 tank series, the M1 did not make use of large armor steel castings. The hull and turret were assembled by welding sections of rolled homogeneous steel armor plate to which the special armor was attached. These steel sections were cut to size using precision flame cutting equipment. The hull was divided into three sections with the crew compartment in the center separated from the fuel cells in the front and the engine compartment in the rear by two bulkheads. In addition to the two fuel cells adjacent to the driver's station, two were located in the engine compartment and two in the rear sponsons. A Halon fire extinguisher system with infrared sensors was installed in both the crew and engine compartments. It could detect a fire in 4-5 milliseconds and extinguish it in about two tenths of a second. The hatch above the driver's seat was fitted with three periscopes providing an overlapping field of vision exceeding 120 degrees. Like on the XM803, there was no floor escape hatch. The driver's T-bar steering control incorporated motorcycle type hand throttles eliminating the need for a floor accelerator pedal thus providing more foot space. The transmission shift selector and push buttons for the intercom system also were located on the T-bar assembly. The service brake pedal was in the center of the floor allowing braking with either foot. The driver's seat was adjustable for driving with the hatch either open or closed and the seat height could be set to four different positions. A large knob on an adjustable lower lumbar support allowed the driver to control the amount of lower back support. The aluminum turret basket floor was suspended from five posts, two of which mounted the seats for the tank commander and the loader. A door in the basket floor provided access to components in the bottom of the hull. The commander's station permitted observation in the seated position through both the extension from the gunner's primary sight and the sight for the .50 caliber machine gun. Six periscopes permitted 360 degree vision. The three positions of the commander's platform allowed him to observe standing with the hatch in the protected open position or at two heights with the hatch fully open. The commander's .50 caliber machine gun was electrically powered in azimuth, but it was elevated manually. The loader at the left rear of the cannon had an eight position swiveling seat that was adjustable in the vertical direction. An eight round armored hull ammunition compartment was located on the right side of the tank behind the engine compartment bulkhead. Blowout panels in the top and bottom of the hull vented any ammunition explosion to the outside and away from the crew. Fortyfour 105mm rounds of ammunition were stowed in the turret bustle behind sliding armor plate doors. Twentytwo of these rounds could be reached by the loader without leaving his seat. A knee switch operated the bustle compartment door covering these 22 ready rounds on the left side of the turret. The door also could be operated manually in an emergency. Blowout panels in the bustle roof vented any ammunition explosion to the outside. Three additional 105mm rounds were stowed on the turret basket floor to the left of the cannon in spall protection covers. This brought the total 105mm ammunition stowage to 55 rounds.
 
The loader's hatch cover was fitted with a rotatable periscope mount. A 7.62mm M240 machine gun was skate mounted around the outer edge of the loader's hatch. Another 7.62mm M240 machine gun was installed coaxially on the right side of the 105mm gun M68E1 above the gunner's auxiliary sight. The gunner's seat was pedestal mounted on the turret basket floor and it was adjustable in both the vertical and horizontal directions. No adjustments were required when the gunner shifted from the primary sight to the auxiliary sight. The gunner's primary sight with its extension for the tank commander was the main optical sighting instrument. It also included the laser range finder, the thermal vision system, and the gyro stabilized line of sight platform. The sight was protected by an armor steel cover with doors operable from within the turret. The gunner's auxiliary sight was an eight power articulated telescope installed in the right side of the gun mount below the coaxial machine gun. The heart of the fire control system was the digital computer which received signals from the sensors as well as inputs from various controls. Data from the laser range finder, the cant sensor, and the wind sensor were received automatically, but other inputs were entered into the computer manually. The gunner used the muzzle reference system to determine tube bend corrections which were then entered into the computer. Atmospheric temperature and pressure as well as ammunition temperature and tube wear also were manual inputs to the computer.

General Motors XM1.
The powerpack consisted of the AGT-1500 turbine engine, the X-1100-3B transmission, final drives, air cleaner, scavenging blower, and cooling system. The powerpack weighed about 8500 pounds and could be easily removed as a unit using a five ton wrecker. The sprockets at the rear of the vehicle drove the 25 inch wide tracks and the tank rode on seven dual road wheels per side. The 25 inch diameter compensating idler installed at the front of each track was interchangeable with the road wheels. It was mounted on a pivoting idler arm connected to the number one road wheel arm by a track adjusting link. Adjustment of the static track tension was by hand pumping grease into the link. When the proper tension was reached, a pressure relief valve opened releasing any additional grease.
 
The vehicle was sprung by high strength steel torsion bars accessible from the outside on both sides of the tank. Thus, if a torsion bar fractured, both pieces could be more easily extracted. There were only two track support rollers per track compared to three on the validation phase prototype. The production tank was governed for a maximum level speed of 45 miles per hour and it could operate cross country at speeds up to 30 miles per hour. 
 
On 19 February 1982, it was announced that a preliminary agreement had been reached between Chrysler and General Dynamics Corporation for the latter to purchase the Chrysler Defense Division. The deal was completed in March and the organization was renamed as the General Dynamics Land Systems Division. The personnel remained
the same with Dr. Philip W. Lett, Jr. becoming Vice President of Research and Engineering. Thus Chrysler severed its connection with the development and production of tanks which had existed since before World War II.
 
It had always been intended to produce the M1 at both the Lima and the Detroit tank plants. In September 1981, the production of 60 tanks per month was authorized with 30 at Lima and 30 at Detroit. This was based on a single work shift at both plants. A total of 150 tanks per month was possible with multi-shift operation. Production of the M1 began at Detroit alongside the M60A3 during March 1982 and the first tank was accepted by the Army at a ceremony on 31 March. Production of the basic M1 continued at both plants for a total run of 2374 tanks. The last basic M1 was completed in January 1985.

This photograph was taken at the rollout of the first XM1 pilot tank at Detroit in February 1978. In the view above, the two center figures
are Dr. P. W. Lett (left) and Eugene Trapp (right).
*Note - all vehicle weights are provided in short tons.
 
PS. I will update this post a bit later with drawings and photos. And then move on.

Ok, so let's start to share here some informations and fight with some myths. I will start with M1, people that know me, know this is my black horse. So let's start with history and then move on. Historical description is a quote from Richard P. Hunnicutt Abrams - A History Of The American Main Battle Tank Vol.2.
M1 Abrams
History
Below is the LK 10322 design concept with heavy protection, rear engine, and a conventional turret.

Although Congress cancelled the XM803 program in late 1971, the Army's need for a new tank was recognized. To meet this requirement, 20 million dollars were authorized for the procurement of two prototypes of a new main battle tank. This was in addition to the 20 million dollars allocated to phase out the XM803. Although the funds were provided specifically for two industrial prototype tanks, the Army initially preferred to concentrate on component development until a specification could be prepared for a new main battle tank. At that time, there was considerable controversy within the Army itself as to what form the tank should take. In February 1972, the Main Battle Tank Task Force (MBTTF) was established at Fort Knox, Kentucky under the leadership of Major General William R. Desobry. Its objective was to define the characteristics of the new MET and to prepare the Material Need (MN) document. Technical support for the Task Force was provided by the Advanced Concepts Branch of the Tank Automotive Command (TACOM). Eventually, this work resulted in a series of eight main concept studies prepared by J. B. Gilvydis. The first three of these studies covered the work performed for the Task Force during its period of operation from February to August 1972. The later five were conducted for the Main Battle Tank Project Manager's Office, after it was established in September, and for the Army Materiel Command (AMC). This work continued until March 1973. On 5 February 1972, General Henry A. Miley, Jr., the Commanding General of AMC, concluded in a letter to the Department of the Army that tank system concept studies by industry would be of value to the Army and would help meet the directions of Congress. Following this, contracts were awarded to Chrysler Corporation and General Motors Corporation for such design studies to be completed by September 1972.
 

 
The cross section at the left shows the spaced armor arrangement incorporated in some of the new designs.

Drawing at the bottom of the page depict a General Motors tank concept powered by the AGT-1500 gas turbine. The estimated weight of this vehicle was about 47 tons.
The Chrysler program was to cover new evolutionary design concepts based on the M60A1 tank. The General Motors contract was to study the design of a new MBT using advanced design components based on the experience of the XM803 program. Both projects provided concept studies to support the work of the MBT Task Force. Initially, the new main battle tank was designated as the XM815, but with the revision of the nomenclature system, this was soon changed to the XML TACOM presented the results of their first study to the MBTTF at Fort Knox on 25 February 1972. The initial concept, designated as LK 10322, was basically a conventional arrangement with the engine in the rear, the commander, gunner, and loader centrally located in the turret, and the driver in the front center of the hull. The total armored volume of this concept was about 540 cubic feet. This compared to an armored volume of approximately 425 cubic feet for the Soviet T55 and 650 cubic feet for the M60A1. Armed with the 105mm gun M68, the LK 10322 concept was used in this study to analyse the effect on the combat weight of the different thicknesses of armor required to defeat various enemy threats. Fourteen different levels of protection were considered in this first study with the estimated vehicle weights ranging from about 34 tons to approximately 57 tons.

The General Motors 400K tank proposal is sketched below.

Above is the General Motors 500K tank concept.

The loading sequence for the rounds stowed behind the armored bulkhead in the turret bustle is shown at the left. 

The full size mock-up corresponding to the LK 10372 design can be seen in the photographs on this page. Note the coaxial Bushmaster weapon.
The armor ranged from all round protection against only 23mm armor piercing projectiles to frontal protection against 120mm shaped charge rounds. Various composite armor arrays developed by the Ballistic Research Laboratory were utilized in different versions of the concept. Since the design of the armor was rapidly developing, frequent changes were required to adapt the constantly changing arrays to the tank concept studies. It is interesting to note that the initial drawing for the LK 10322 concept featured the AGT-1500 gas turbine, the power plant eventually selected for the Ml production tank. With six road wheels per track, a tube-over-bar suspension was specified for the LK 10322 to improve the cross-country performance. During April and May, a second TACOM study investigated modified versions of the LK 10322 concept in an effort to reduce the weight. Initially, the hull design remained the same, but the turret was slightly modified. The side walls were tapered five degrees to the rear and the slope of the right side was increased from 14 to 20 degrees from the vertical. A new low profile commander's station was introduced with an externally mounted 7.62mm machine gun and the 7.62mm coaxial machine gun was replaced by a .50 caliber weapon. These modifications resulted in estimated weight reductions ranging from a little less than three tons to over three and one half tons depending upon the protection level under consideration. Later, the hull of the modified LK 10322 was lengthened from 270 inches to 275 inches which allowed an increase in fuel capacity from 300 to 350 gallons. This hull also could accept either the AGT 1500 turbine with the XHM-1500 transmission or the AVCR-1100 diesel engine with the X-1100 transmission. The average weight increase with these hull changes was about 600 pounds. On 20 June 1972, the MET Task Force requested TACOM to perform a third weight and protection analysis. This study was to evaluate the effect of 72 different component combinations applied to the modified LK 10322 concept. The following component list was supplied by the Task Force for the analysis.

1. Primary Armament
a. 105mm gun M68
b. 110mm gun, United Kingdom
c. 120mm gun, Federal Republic of Germany
2. Fire Control
a. XM803 system (without commander's day/night
sight)
b. M60A1 (with turret integrated night thermal sight)
3. Power Plants
a. AVCR-1100 diesel engine with the X-1100 transmission
b. DB1500 diesel engine with the Renk HSWL 354
transmission
c. AGT-1500 gas turbine with the XHM-1500-2 transmission
4. Suspension
a. Tube-over-bar
b. Piston hydropneumatic
5. Tracks
a. T142 or Diehl
b. Lightweight, 28 inch width

This study used two different approaches. The first calculated the armor weight for the hull and turret and determined the protection level obtained with total vehicle weights of 43, 45, 47, and 49 tons. The second approach analysed the effect of 72 combinations of components on the total vehicle weight at a single level of protection. The protection level selected was that required to defeat the combined threat of the Soviet 115mm APFSDS projectile at 800 meters range and the 3.2 inch diameter HEAT round on the front and up to 30 degrees to each flank. In addition to the 72 variations of the modified LK 10322 concept from the third TACOM study, the MET Task Force reviewed fifteen additional designs. Numbers 73 through 80 were submitted by General Motors Corporation and 81 through 83 came from Chrysler. Candidates 84 through 87 were product improved versions of the M60A1. In reviewing the 72 candidates from the third TACOM study, the Task Force eliminated those armed with the United Kingdom 110mm rifled gun and the 120mm smooth bore weapon from the Federal Republic of Germany. The 110mm gun was not considered sufficiently superior to the 105mm gun M68 and the 120mm smooth bore was not expected to be available in time to meet the proposed production schedule. At that time, the Task Force also considered the AGT-1500 gas turbine to be a high risk
power plant and it was dropped from the concepts under review. This reduced the number of proposals in the TACOM study from 72 to 16.

General Motors XM1 prototype, note placement of ammunition. There were two large exposed ammo racks in crew compartment, one with rounds stored in vertical position next to the loader in turret, and second on the right side of the driver. This was probably one of several reasons why General Motors prototype was discarded in favor of Chrysler XM1.
Those remaining differed primarily in the engine and transmission with minor variations in the fire control systems as well as the suspensions and tracks. As a result, the 16 candidates were reduced to two featuring the two power packages considered acceptable. Thus one was powered by the air-cooled Continental AVCR-1100 engine with the X-1100 transmission. The other was fitted with the Daimler Benz (later MTU) DB1500 liquid-cooled diesel with the Renk transmission. These two concepts were included by the Task Force among the five candidates for the final trade off analysis. Of the eight General Motors proposals, two were powered by the AGT-1500 gas turbine using the X-1100 transmission, one utilized the DB1500 diesel with the Renk transmission, and two were fitted with the AVCR-1100-3B with the X-1100 transmission. The remaining three had a twin engine arrangement consisting of two General Motors 8V71T liquid-cooled diesels, again with the X-1100 transmission. All eight concepts used a high strength torsion bar suspension and were armed with the M68 105mm gun. Estimated weights of these vehicles ranged from 47 to 67 tons depending upon their level of protection. The Task Force rejected the three proposals powered by the twin 8V71T diesels because of anticipated increased maintenance time and reduced reliability. With the elimination of the AGT-1500 in two of the concepts, the number of General Motors candidates was reduced to three. One of these also was rejected because of its excessive weight of 67 tons. One of the remaining two was selected as the General Motors candidate even though it weighed 56 tons because it came closest to meeting the MN requirements for ballistic protection. One of the three Chrysler concepts was selected as superior to the other two in several important areas and essentially equal in other respects. It was powered by the AVCR-1100 engine with the X-1100 transmission and had an estimated combat weight of 48.5 tons. However, like the other candidates that met the MN weight requirements, it did not provide the desired protection level. None of the four product improved MoOAls met the M N requirements for ballistic protection or sustained speeds on 10 per cent and 30 per cent grades. Of the four, numbers 84 and 85 provided the highest level of protection. Since number 85 featured the more advanced fire control system, it was selected as one of the five candidates for the final trade off analysis. The most obvious conclusion from the review of these concept studies was that the level of ballistic protection required by the proposed MN document was incompatible with the weight limitation specified. This problem was outlined in a fact sheet forwarded from the Task Force to the Commanding General of AMC, dated 3 August 1972.

This fact sheet concluded that the only practical solution was to either reduce the level of ballistic protection or increase the 49 ton weight limitation in the proposed MN document. The latter alternative was subsequently adopted. Although General Motors and Chrysler provided support to the MET Task Force under their study contracts for new tank prototypes, the final reports on both of these contracts were not submitted until October 1972. In their report, General Motors proposed two concepts. They were based on estimated average production unit costs of $400,000 and $500,000, both in fiscal year 1972 dollars. Referred to as the 400K and 500K tanks, the latter was a growth version of the former with improvements in ballistic protection, firepower, and night vision. Both were manned by a crew of four with the driver in the left front hull. The usual arrangement of the three man turret crew was reversed placing the gunner and tank commander on the left side of the cannon with the loader at the right rear. The turret on both vehicles utilized the large bearing ring from the XM803. Both tanks were armed with a stabilized 105mm gun M68. Forty rounds of 105mm ammunition were stowed in the hull below the level of the turret ring without separate compartmentation. The 400K tank was protected by welded plate armor arrays on the turret front. The sides and rear of the turret consisted of single plate rolled homogeneous armor. The hull front was a single plate of high obliquity armor. Spaced armor skirts were provided on the forward sides of the hull. The tank was powered by the previously proposed twin General Motors 8V71T diesel engines connected to a modified X-1100 transmission through a transfer case. With this arrangement, the tank could be driven on one engine in an emergency. The twin engine installation was rated at 1200 gross horsepower. The estimated combat weight of the 400K concept was 52 tons. Although the armor protection was superior to the M60A3, it did not meet the requirements for the new MET.
 
The running gear consisted of an advanced high strength torsion bar suspension with six 30 inch diameter road wheels per side running on a 22 inch wide, double pin, flat track. A 7.62mm coaxial machine gun was located on the right side of the cannon and a 40mm high velocity grenade launcher was installed on the commander's hatch. The independently stabilized gunner's day/night sight included a laser range finder. The combat weight of the 500K tank was increased to 55 tons providing improved ballistic protection. The front hull now consisted of spaced rolled homogeneous armor and steel/aluminum array skirts were installed on the sides. On the turret, a cast armor trunnion support was joined to frontal spaced armor arrays. The sides and rear also consisted of spaced armor. Although the M68 105mm gun was retained in the 500K tank, the mount was strengthened to permit future adaptation of a 120mm gun firing kinetic energy ammunition. The 7.62mm coaxial machine gun in the 400K tank was replaced by the Bushmaster (20mm-30mm) weapon system then under development. The 40mm high velocity grenade launcher was moved to the loader's station and replaced on the commander's hatch by a .50 caliber M85 machine gun. The commander's weapon was now mounted on a powered slew ring. The gunner's stabilized sight with its laser range finder also now included a far infrared imager for night operations. Individual displays were provided for both the gunner and the tank commander. With the heavier weight, the 500K tank was powered by the AVCR-1100-3B engine rated at 1450 gross horsepower using the modified X-1100 transmission. The number of road wheels was raised from six to seven per side and the width of the double pin flat track was increased to 24 inches. Chrysler's report on the New Evolutionary Tank reviewed eight different vehicle concepts as well as the development status of various design features and components. Main armament on all eight concept studies was the M68 105mm gun. In one case, this was supplemented by a TOW missile installation. On two other concepts, the Shillelagh II or Swifty gun launched missile was specified. This was a proposed development of the Shillelagh reduced in diameter to permit launching from the 105mm gun. The missile velocity also was increased to reduce the flight time by about 50 per cent and to extend the range by approximately 25 per cent. However, the reduced diameter would have decreased the effectiveness of its shaped charge warhead. The estimated combat weight of the eight Chrysler proposals ranged from about 49 tons to 57 tons. After considering the eight concept studies, the Chrysler report proposed a final configuration combining the best components from two of them. With an estimated combat weight of 57.2 tons, this tank had a conventional layout. The driver was in the center front hull and the three man turret located the gunner and the tank commander on the right side of the 105mm gun with the loader on the left. The 105mm ammunition was stowed in compartments separated from the crew space. These compartments in the turret bustle and the hull were designed to fail away from the crew space. Thus any ammunition explosion resulting from a penetration of the armor would be directed either to the outside or into the engine compartment away from the crew. 

AGT-1500 and AVCR-1360.
The 105mm M68 gun was selected as the main armament with possible future upgrading through improved ammunition or the introduction of a gun launched missile such as the Swifty. A .50 caliber M 85 coaxial machine gun was specified, but the design permitted its replacement by the Bushmaster weapon when it became available. The fire control equipment was a modified M60A3 system with the turret integrated night thermal sight (TINTS). Final selection of the power plant was not made at that time with the German MTU MB 873 Ka 500 (formerly the Daimler-Benz DB1500) 1500 horsepower liquidcooled diesel, the AVCR-1360-1 air-cooled diesel, and the AGT-1500 gas turbine all being considered. It was proposed to select two of these power plants for test rig evaluation. The X-1100 transmission was to be used in all three cases. The advanced torsion bar suspension was selected. As mentioned earlier, this system utilized high strength torsion bars permitting a much greater twist before exceeding the elastic limit of the material. Thus a much greater wheel travel could be obtained. As with other proposed tank concepts, it was obvious that the combat weight would greatly exceed the original target if satisfactory protection levels were to be achieved. A new concept, designated LK 10352, was proposed by TACOM in a fourth study dated July 1972. The main objective of this study was to determine the effect on the gross vehicle weight if the fuel capacity was increased from 300 to 350 gallons. This was similar to the earlier examination of the same increase on the modified LK 10322. The hull was lengthened to 276 inches with the extra six inches between the crew compartment bulkhead and the engine being used to accommodate a tank for the additional 50 gallons of fuel. The lower glacis was redesigned on this new concept and the angle with the vertical increased from 55 degrees to 60 degrees. The upper glacis remained at 65 degrees from the vertical. No changes were made to the turret. The effect of these modifications on the gross vehicle weight was calculated based on the protection required to defeat two threat levels. The greatest of these was the combined threat of the Soviet 115mm APFSDS kinetic energy projectile at a range of 800 meters and a 3.2 inch diameter precision HEAT shaped charge round. The calculation indicated that a vehicle weight of over 52 tons would be required for protection directly from the front out to an angle of 30 degrees to each flank. In August and September, the LK 10352 concept was modified in a fifth study to determine the effect of second generation armor arrays on the gross vehicle weight. The early steel-glass-steel composite armor was replaced by multiple steel and aluminum plate combinations. For this analysis, three threat levels and the armor arrays required to defeat them were supplied by the Ballistic Research Laboratory. The light threat consisted only of the kinetic energy projectile representated by the Soviet 115mm APFSDS at a range of 800 meters. The medium level of protection was required to defeat both the kinetic energy threat and the 3.2 inch diameter HEAT shaped charge round. A new heavy threat was introduced at this time. It consisted of both the 115mm kinetic energy projectile and a 5.0 inch diameter HEAT round.

Chrysler design hull mockup with AVCR-1360 diesel.
This was the threat against which the new MBT was to be protected in future TACOM studies. For the front hull armor array, the upper glacis was angled at 35 degrees and the lower glacis at 70 degrees, both from the vertical. The nose also was extended 2.5 inches to give more room to the driver. The turret was modified from the earlier concept by changing the left front from a curved to a flat surface angled back at 30 degrees to the rear and sloped at 60 degrees from the vertical. The slope of the right front was changed from 60 to 55 degrees from the vertical to provide more room for the gunner. This side already had a flat surface angled to the rear at 30 degrees. The turret sides were unchanged, but the bustle was extended by 12 inches to obtain more stowage space. The .50 caliber coaxial machine gun was replaced by the Bushmaster and the .50 caliber weapon was relocated to the commander's hatch. The study indicated that a combat weight of over 59 tons would be required to provide protection against the heavy threat from the front and at 30 degrees to each flank. A sixth study by TACOM from October 1972 to January 1973 covered the preparation and weight analyses of new design concepts and the building of a full size wooden mock-up. The rapid development of composite armor technology continued to cause repeated changes in the tank design concepts. In fact, according to TACOM personnel at that time, the Ballistic Research Laboratory was cranking out new armor arrays faster than new concept designs based on them could be drawn and analyzed. After a preliminary study, it was decided to draw two different versions of the new tank. One version was to have the main gun ammunition stowed in the crew space as in earlier tanks. In the other, it was to be separated from the crew in ammunition compartments. Two different turret designs were prepared. One turret stowed the ammunition separate from the crew in a bustle compartment. The second turret had a short bustle with no ammunition stowage. Two designs were considered for the tank with compartmented ammunition. In the first type, 18 out of a total of 40 105mm rounds were stowed in automated racks in the turret bustle behind a one inch thick aluminum alloy bulkhead. These racks were similar to those in the XM803 and the rounds were retrieved through a single round door in the bulkhead. Sixteen rounds were in two revolving drum type racks in the front hull, one on each side of the driver. These rounds were retrieved through small round doors in the one inch thick bulkhead facing the turret basket.

Chrysler mobility test rig.
The remaining six rounds were stowed in a non-automated rack in a two inch thick box underneath the cannon in the turret basket. The second design of the tank with the compartmented ammunition eliminated the automated racks. They were replaced by simple lightweight racks and some rounds were relocated. The eighteen rounds remained in the turret bustle, but they were now retrieved through sliding aluminum doors. The six rounds previously stowed in the turret basket were moved to the front hull. The compartments on each side of the driver now contained 11 rounds each and they could be reached through sliding doors. In the non-compartmented version of the new tank, all 40 rounds of main gun ammunition were stowed in the crew compartment below the turret ring. Twentyeight were in two racks, one on each side of the driver. The remaining 12 were located in the turret basket. These were considered to be ready rounds and were in easy reach of the loader. The compartmented and non-compartmented concepts were quite similar in external appearance except for slight changes in the turret design. In addition to the shorter bustle, the non-compartmented version retained the five degree taper to the rear on the turret sides. On the compartmented turret, the side walls were parallel to the center line. This provided more stowage space for ammunition in the bustle. Both sides of the turret front were angled back 30 degrees to the rear and sloped at 45 degrees from the vertical. The left and right sides of the turret were sloped at 35 degrees and 20 degrees from the vertical respectively. The roof was flat and the rear of the bustle was angled at 40 degrees from the vertical. As in the previous concepts, the turret ring diameter was 85 inches. 

Chrysler and General Motors XM1's compared to M60A1 Main Battle Tank.
The hull design was identical for both concepts, except for the ammunition racks. The upper and lower front glacises were angled at 35 and 70 degrees from the vertical respectively. Side skirts extended from the front idler to the rear sprocket protecting both the crew and engine compartments. The power plant was the AVCR-1360 engine with the X-1100 transmission. All of the fuel was located in the engine compartment. A tube-over-bar suspension system with six road wheels per side was used with 26 inch wide tracks. For the commander, a low rotatable station even with the turret roof line was used for both concepts. It was fitted with a .50 caliber M85 machine gun which could be operated from either inside or outside the tank. In addition to the machine gun sight, the commander was provided with six periscopes for all around vision. A Bushmaster weapon system was installed on the left side of the 105mm gun. A day/night sight including a laser range finder was mounted in front of the gunner's station with separate controls for the gunner and the tank commander. Weight analyses of the two concepts armored against the heavy threat indicated that the tank with the compartmented ammunition weighed slightly over 60 tons compared to under 57 tons for the non-compartmented version. The tank with the compartmented ammunition and armored against the heavy threat was selected for the construction of a full size wooden mock-up. The mock-up was modified in several ways from the original concept drawings. The day/night sight for the gunner and the tank commander was relocated to the right side of the turret. This was done to eliminate a weak ballistic area on the turret caused by the previous mounting of the sight. To provide protection against HEAT rounds, armor arrays extending alongside the entire crew compartment were installed on the hull side areas over the sponsons. The overhanging frontal sections of the turret were lowered by about four inches leaving only a minimum clearance for the driver's head when driving with his head exposed. This was to improve the ballistic protection by reducing the shot trap in this area. The turret roof was slightly crowned to increase its rigidity and to eliminate any possibility of buckling. This was achieved by raising the center line one inch and sloping the roof toward the sides at five degrees from the horizontal. After completion of the mock-up, a new concept drawing, designated LK 10372, was prepared incorporating all of the changes.
 
Data on improved armor arrays were received from the Ballistic Research Laboratory at the conclusion of the previous study. To evaluate the effect of these new arrays on the vehicle weight, a new concept was prepared including the changes applied to the mock-up as well as several new features. This design, designated as LK 10379, was covered in the seventh study from TACOM in February 1973. From this point on, the analyses considered only the protection level required to defeat the heavy threat. This was still the Soviet 115mm APFSDS projectile at 800 meters and the 5.0 inch diameter HEAT round. The 40 rounds of 105mm ammunition in the LK 10379 concept were located in the hull below the turret ring and were not stowed in separate compartments. The Bushmaster weapon system was removed from inside the turret and remounted on the left side exterior near the bustle. The day/night sight for the gunner and the tank commander was installed through the right side of the turret as on the mock-up. The gun shield was narrowed from a width of 38 inches to 12 inches. The five degree taper to the rear of the turret side walls was eliminated making the sides parallel to the turret center line. This provided more space for the tank commander and simplified the design of the Bushmaster mount. The slope of the left and right sides of the turret remained at 35 and 20 degrees from the vertical respectively. The angle of the turret frontal sections toward the rear was increased from 30 to 40 degrees, but they remained sloped at 45 degrees from the vertical. The turret roof was crowned as on the mock-up. The hull front on the LK 10379 was redesigned with a 55 degree angle from the vertical on both the upper and lower glacises. This increased the hull length making the six road wheel suspension marginal for good mobility. Therefore, a new tube-over-bar suspension was specified with seven road wheels per track. As on the mockup, armor arrays were installed on the hull sides above the sponsons. The combat weight of the LK 10379 concept was calculated to be a little over 60 tons. The analysis showed that by reducing the protection on certain areas of the hull and lowering the fuel capacity from 350 to 300 gallons, the weight could be reduced to about 58 tons. The eighth and last in the main series of new XM1 concept studies was performed by TACOM in March 1973. Its objective was to explore ways to reduce the weight of the proposed tank to below 58 tons. To achieve this goal, a new concept drawing, LK 10382, was prepared with several changes from the previous design. However, the same armor arrays were used to permit accurate comparisons. The front hull was changed back to the earlier design with the upper and lower glacis plates sloped at 35 and 70 degrees from the vertical respectively. This reduced the length of the hull by 15 inches and lowered the weight by about 900 pounds. The hull height was reduced from 42 to 40 inches and the sides above the sponsons were sloped at a greater angle from the vertical. The fuel tank behind the crew compartment bulkhead was removed and the engine was moved four inches forward. This dropped the fuel capacity back to 300 gallons. The turret ring diameter was reduced from 85 to 83 inches which also shortened the hull by an additional two inches.
 
With the shorter hull, the suspension was changed back to six road wheels per side. The ground clearance was increased from 17 to 18 inches for better mobility and mine protection. Except for the smaller ring, the turret design remained the same, but the Bushmaster was moved farther to the rear, lowered, and mounted closer to the turret wall. The ammunition feeding system for the Bushmaster was changed so that it fed directly through the turret side armor. This eliminated the partially exposed ammunition chutes which formerly were routed through the turret roof. The estimated combat weight of the LK 10382 design was between 57 and 58 tons depending upon weights assumed for the suspension and the possible reduction in thickness of the hull bottom plate under the engine compartment. After the complexity and high cost that resulted in the cancellation of the XM803, Congress required that these factors be tightly controlled in the new tank program. Thus a design to unit cost ceiling was established at the beginning of the project. The specification requirements in the Materiel Need (MN) document were presented as ranges permitting the contractors some leeway in meeting the design objectives. As the Army later observed during a Congressional hearing in April 1974, "We would like to have the maximum performance, but we are sticking to a design to unit cost and we therefore have to give the contractor the ability to make trade offs to meet the design to cost goal". As mentioned earlier, the weight limitation in the original MN document was incompatible with the protection level required. Since crew survivability had the highest priority on the list of characteristics for the new tank, the maximum weight was raised to 58 tons to permit adequate armor. Later, this would be Increased to 60 tons. During the initial studies in 1972, the design to unit cost goal was set at $400,000. However, by the time that the request for proposal was issued, this had increased to $507,790, also in 1972 dollars. This compared to estimated unit costs of $339,000 for the M60A1, $432,000 for the M60A3, and $611,000 for the cancelled XM803. The new program was divided into three phases. The first of these was a competitive advanced development validation phase during which contracts would be awarded to two sources. Each of these contractors would design, build, and test one complete prototype tank and one automotive test rig. They also were to furnish an additional hull and turret for ballistic evaluation. This 34 month advanced development phase would end with a combined developmental/operational test (DT/OT-I) by the Army. These tests would be used to validate the contractors' proposals for phase 2, which would be a sole source full scale engineering development (FSED) program. Phase 3 would put the new tank into production.
 
The various tank concepts being considered by the Army at this time reflected a change in thinking since the period of the MBT70 and the XM803. Although some sources regarded the appearance of the long range antitank missile as sounding the death knell of the tank, this was not the case. The main effect of the new missiles was to release the tank from the defensive role since other units were now capable of defending themselves against armored attack. The tank could now resume its place as the key offensive weapon in the combined arms team. In that role, the missile firing gun-launcher system was less attractive as the main tank weapon than the fast shooting 105mm gun,
particularly with improved ammunition. Thus when proposals were requested for the development of the new tank, the 105mm gun with its quick reaction, multitarget performance, and immediate availability was the obvious candidate despite its shorter range. Proposals for the advanced development validation phase were received from Chrysler and General Motors in May 1973. Originally, Ford Motor Company had been expected to participate, but they declined to enter the competition. As a result, after review, contracts were awarded to Chrysler and to the Detroit Diesel Allison Division of General Motors on 28 June 1973. As mentioned earlier, these contracts required the design, construction, and testing of a completely integrated prototype tank. It was to include all subsystems except for the night vision equipment. An automotive test rig also was to be constructed for use in the test program and an additional ballistic hull and turret were to be provided for ballistic tests at Aberdeen Proving Ground. These cost plus incentive fee (CPIF) contracts eventually amounted to $68,999,000 for Chrysler and $87,969,000 for General Motors. Chrysler proposed a conventional arrangement for its prototype tank with a three man turret crew and the driver in the center front hull. The gunner and the tank commander were in the usual location on the right side of the M68 105mm gun with the loader on the left. A coaxial Bushmaster weapon was fitted on the left side of the cannon. An M85 .50 caliber machine gun was mounted on the tank commander's station and either a 7.62mm M60D machine gun or a 40mm high velocity grenade launcher was installed on the turret roof in front of the loader's hatch.
 
A simplified line of sight stabilization system was expected to provide performance equal to at least 90 per cent of a dual axis system with a cost saving of about $3000. The 105mm ammunition was stowed in separate compartments vented away from the crew space. New composite special armor was incorporated into the frontal areas of the tank and side skirts extended from the front idler to the rear sprocket partially shielding the hydromechanical suspension. This was actually a tube-over-bar suspension with seven road wheels per track using rotary hydraulic shock absorbers. The term hydromechanical suspension was selected to avoid confusion with the earlier tube-over-bar suspension installed experimentally on the M60A1 which lacked the performance of the new design. It also was noted that the same name could be applied to the advanced high strength torsion bar suspension if it replaced the tube-over bar design in the future. At the time of the proposal, the advanced torsion bar was considered a high risk item, but the option of using high strength steel or titanium torsion bars was kept open for the future. In regard to the power plant, Dr. Philip W. Lett Jr., the leader of the Chrysler team, made a bold decision. Despite the fact that the Army MET Task Force had considered the gas turbine to be a high risk item, he selected the AGT-1500 with the X-1100 transmission to power their prototype. In retrospect, this appears to have been a crucial decision in the design of the new tank. Subsequent events were to prove that the AGT-1500 would develop into a highly reliable power plant and it had several advantages over its diesel competitors. Although both the AGT-1500 and the AVCR-1360 diesel were rated at 1500 gross horsepower, the former utilized only 30 horsepower for cooling compared to 160 horsepower for the latter. Also, a growth potential to 2000 gross horsepower was estimated for the turbine compared to 1800 gross horsepower for the variable compression ratio diesel. The less complex design of the turbine engine with its fewer parts was expected to provide higher reliability and reduced maintenance in the future. Its lighter weight also permitted increased armor protection without exceeding the overall weight limit for the tank.
 
The gas turbine did not generate smoke during operation and it was capable of using a wider range of fuels than the diesel. Also, the turbine was easier to start at low temperatures. The greatest disadvantage of the gas turbine was its higher fuel consumption. This was particularly obvious at low vehicle speeds. However, the small size of the
power plant allowed space for an increase in fuel capacity to achieve the specified cruising range. Another problem was the high volume of combustion air used by the gas turbine. This required a filter system with approximately three times the capacity of that necessary for the diesel engine. The turbine, of course, did not require the high volume of cooling air used by the air-cooled diesel. The development of the gas turbine tank engine resulted from recommendations of the Committee on Gas Turbine and Electric Propulsion for Tanks, chaired by Mr. James C. Zeder. When presented in 1965 to the Mobility Advisory Group, the recommendations of the Zeder Committee were initially rejected because of cost. However, the program was subsequently approved. After the evaluation of several proposals from industry, the Lycoming Division of AVCO was selected to develop the
new turbine, designated as the AGT-1500. At that time, it was considered as a possible power plant for the MBT70. However, after the selection of the diesel engine for that program, attempts were made to cancel the AGT-1500 and to transfer its funds elsewhere. These attempts were prevented by the efforts of Lieutenant Colonel John W. Wiss, Chief of the Army Tank Automotive Command Laboratories, and Mr. Wayne S. Anderson who directed the Propulsion Systems Laboratory. Thus the AGT-1500 was available when needed for the XM1 program and its state of development was not far from that of the AVCR-1360 variable compression ratio diesel. Although the initial cost of the gas turbine exceeded that of the diesel engine, it was anticipated that the life cycle cost would be competitive because of reduced maintenance requirements.
 
The early General Motors concept for the XM1 validation phase tank also was armed with the 105mm gun M68 and a Bushmaster. However, the latter was located in a separate mount on the left side of the turret. The turret crew rode in the usual positions with the gunner and tank commander on the right side of the cannon and the loader on the left. A .50 caliber M85 machine gun was provided for the tank commander and a 7.62mm machine gun was installed at the loader's position. The main gun ammunition was separated from the crew space in compartments. The driver was located in the left front hull and special armor was incorporated in the frontal areas of the tank. The Continental AVCR-1360 diesel engine drove the tank through the General Motors X-1100 transmission. As mentioned earlier, the jWCR-1360 diesel was a 1500 horsepower development of the AVCR-1100 used in the MBT70 and the XM803. The suspension on the General Motors tank was fitted with six road wheels per track. This was a hybrid suspension with road wheels 1, 2, and 6 fitted with the National Water Lift hydropneumatic suspension units. Road wheels 3 and 5 utilized high strength torsion bars. In early July 1973, Major General Robert J. Baer, the XM1 Project Manager, and personnel from the Ballistic Research Laboratory escorted representatives from Chrysler and General Motors to the United Kingdom for an update on the British developed Chobham special armor.
 
During this trip they observed the proposed design of a new United Kingdom vehicle utilizing this special armor as well as the manufacturing processes required for its
production. Based upon the newly obtained data, both contractors reevaluated their proposed armor configurations. Chrysler made a number of changes, but they retained the basic design in their original proposal. However, General Motors made major changes in the configuration of their proposed tank. The most obvious change was in the turret where the vertical front and side walls were replaced by sloped armor. The modifications by both contractors required additional work by the Ballistic Research Laboratory to develop new special armor combinations. By January 1974, both Chrysler and General Motors completed the final design of their special armor configurations and some of the hull and turret armor structures had been delivered to Aberdeen Proving Ground for ballistic tests. Some changes in the specifications for the new tank resulted from a study of the Israeli experience during the October 1973 war in the Middle East. One change which was applied to both of the validation phase prototypes was the replacement of the Bushmaster weapon system with a 7.62mm coaxial machine gun. The original intention had been to utilize the Bushmaster against lightly armored vehicles thus reducing the stowage requirement for the expensive main gun rounds. However, battle experience showed that the tank crews would invariably use the main gun against these vehicles. Also, there was a need for a coaxial machine gun for use against infantry and to suppress antitank weapons. Elimination of the bulky Bushmaster and its ammunition provided space to increase the number of 105mm rounds to 55. Some of the modifications resulting from the study of battlefield experience were applied at a later date to the full scale engineering development (FSED) tank proposals. These included changes in the ammunition compartments to accommodate the larger number of 105mm rounds and the use of a less flammable hydraulic fluid.

Early and late FSED designs, notice changes in the turret design.
The complete validation phase prototypes were delivered to Aberdeen Proving Ground for the combined developmental/operational test (DT/OT-I) which ran from 31 January to 7 May 1976. During this same period, the competing automotive test rigs were driven more than 3000 miles over a variety of terrain. Both prototypes were displayed to the public for the first time on 3 February 1976. The results of the DT/OT-I indicated that both the Chrysler and the General Motors prototypes met the specified requirements for the new tank. The Chrysler tank with its gas turbine power plant showed slightly greater acceleration, but the performance was satisfactory for both vehicles. The operational portion of the tests included two weeks of simulated combat operations with the tanks manned by crews from Fort Knox and Fort Hood. After the evaluation of the competing prototypes, the selection of the contractor for the full scale engineering development phase was scheduled for July 1976. 

From left to right: Early Chrysler XM1 FSED with AGT-1500 gas turbine, final XM1 FSED with AGT-1500, and final XM1 FSED with AVCR-1360 diesel.
After the cancellation of the MBT70 program, the Federal Republic of Germany continued with the development of their new Leopard II tank. In line with efforts to standardize materiel in NATO, Germany proposed that the Leopard II be modified to meet American requirements. In December 1974, a Memorandum of Understanding between the United States and the Federal Republic of Germany was signed in which the U.S. agreed to evaluate the Leopard II under the same test conditions as the XM1 candidates. At that time, it was estimated that the Leopard II would cost about one million 1975 dollars compared to approximately $750,000 for the XML The modified version, designated as the Leopard II AV was armed with the 105mm gun. It had improved armor protection and a simplified fire control system to reduce costs. Tests of the Leopard II AV were carried out at Aberdeen Proving Ground from September to December 1976. Previously, on 22 July 1975, FMC Corporation had been awarded a contract for a study to determine the costs and technical problems involved in manufacturing the Leopard II AV in the United States. The tests at Aberdeen indicated that the Leopard II AV met or exceeded most of the U.S. requirements. The accuracy of its fire control system was slightly higher than that of the XM1, but it was more expensive. The armor protection, ammunition compartmentation, and gun movement were considered somewhat inferior to that on the American tank. Rather than continue the evaluation of the complete tank, it was agreed in January 1977 that only certain major components would be considered for adoption to enhance NATO standardization.
 
Shortly before the end of the XM1 validation test program, a new requirement was introduced for inclusion in the proposals for the full scale engineering development/ producibility engineering and planning (FSED/PEP) contract. These proposals were now to include firm price ceilings for the first two production options. These were for 110 low initial rate production (LIRP) tanks in fiscal year 1979 and 352 tanks in fiscal year 1980. The winner of the competition for the XM1 FSED/PEP contract was selected on schedule in July 1976, but the results were not released by the Army. Information published later in the press indicated that General Motors won with a bid of 208 million dollars compared to Chrysler's bid of 221 million dollars. However, these bids were based upon the Chrysler and General Motors tanks being powered by the AGT-1500 turbine and the AVCR-1360 diesel respectively. The Army, by this time, favored the gas turbine and although awarding the contract to General Motors, planned to change the power plant to the AGT-1500. However, this approach was rejected by the Office of the Secretary of Defense. As the Deputy Secretary of Defense later testified before Congress: "I was unalterably opposed to making these changes in a sole source environment after the contractor was selected. I wanted to know what those unit costs were, what the delays in the program might be, if any, and have those costs determined competitively between the two contractors." Also, a decision had been made to arm the new tank with a 120mm gun at some time in the future. Therefore, the turret had to be capable of mounting either the 105mm gun M68 or a 120mm gun without extensive modification. The 120mm guns under consideration were the British rifled weapon and the German Rheinmetall smooth bore gun used in the Leopard II. As a result of the new requirements, a revised request for proposal was issued, with the award of the FSED/PEP contract now rescheduled to November 1976. Both contractors submitted proposals by 28 October for versions of their tanks with a hybrid turret capable of mounting either a 105mm gun or a 120mm gun. In the interest of NATO standardization, provision was made for the use of the gunner's auxiliary sight from the Leopard II as well as metric fasteners at crew serviced interfaces. General Motors presented a version of their tank powered by the AGT-1500 gas turbine and Chrysler proposed an alternate design using the AVCR-1360 diesel engine. However, they recommended their turbine driven tank as the primary choice. All of the proposed tanks were fitted with British developed smoke grenade launchers.
 
Prior to the submission of the final proposals on 28 October, Chrysler made a number of additional changes to enhance the survivability of their candidate and to reduce the cost. The arrangement of the special armor was modified and a special armor gun shield replaced the casting on the earlier proposal. A new minilaser range finder was incorporated and an extension from the gunner's primary sight was provided for the tank commander removing the need for a separate sight. Other cost reductions included the elimination of the power elevation for the commander's weapon and the driver's built in test equipment (BITE) panel. The latter was replaced by pop-up indicators and other gages in the engine compartment and at the crew stations. 
 
At a news conference on 12 November 1976, it was announced that Chrysler had won the FSED/PEP contract. Published reports indicated that the cost reduction efforts at Chrysler had reduced their bid to 196 million dollars compared to a 232 million dollar bid from General Motors. The increase in the latter was, no doubt, partially due to the higher initial cost of the AGT-1500 turbine compared to the AVCR-1360 diesel engine. However, Chrysler's diesel powered proposal for the FSED/PEP program was priced at 186 million dollars. The five month delay in awarding the contract had permitted further refinement of the design and better determination of the manufacturing costs. For
example, the design to unit cost for Chrysler's tank decreased from $574,000 for the original FSED version to $422,000 for the final turbine powered tank proposal. The unit cost for Chrysler's diesel tank design was $376,000. All of these prices were in 1972 dollars for comparison with the earlier estimates. The actual unit price of the winning tank was expected to be about $754,000 in 1976 dollars. Under the 36 month FSED/PEP contract, Chrysler was to manufacture 11 new pilot tanks incorporating the latest changes resulting from the validation phase tests and other experience. The validation phase prototype and the automotive test rig also were to be refurbished for use in the test program. Evaluation of these vehicles would determine the details of the final design for low initial rate production (LIRP). All of the 11 new pilot tanks were manufactured at the Detroit Army Tank Plant with the first one being delivered in February 1978. The last of the 11 was completed the following July. Developmental Test II (DT-II) extended from February 1978 to September 1979 at Aberdeen Proving Ground. Operational Test II (OT-II) at Fort Bliss, Texas lasted from April 1978 to February 1979 with the tanks manned by the 2nd Squadron, 3rd Armored Cavalry Regiment. Three of the tanks, after testing at Fort Bliss, were refurbished at Detroit, fitted with the latest configuration hardware, and shipped to Fort Knox in June 1979 for additional durability and reliability testing. This program was conducted by H Company, 2nd Squadron, 6th Cavalry under the direction of the Armor and Engineer Board. During these tests, the reliability, availability, maintainability-durability (RAM-D) goal was 272 mean miles between failures (MMBF). The XM1 reached 326 MMBF. Initially, Chrysler personnel provided the maintenance, but in August the organizational maintenance was taken over by the soldiers of H Company for the remainder of the test program. During the OT-II at Fort Bliss, many of the power train failures were caused by dust being ingested into the engine. After modification of the seals in the air filtration system, there were no further failures from this cause during all of the tests at Fort Knox. The tests at Fort Bliss also revealed a tendency for the tracks to be thrown by the buildup of soil between the sprocket hub and the bottom of the rear sponson. Some tracks also were thrown by rapid backing and turning when changing firing positions. This problem was solved by the combination of some additional suspension hardware and the redistribution of the ground pressure with an increase in track tension. The hardware change consisted of a sprocket hub with discharge ports, a mud scraper, steel blocks welded to the lower rear hull to prevent inside throws, a steel plate mounted above the final drive to maintain track alignment, and a retaining ring on the outside of the sprocket. Satisfactory results were obtained with these modifications during further tests at Fort Bliss in October 1979.

Major General Baer (far left) and Dr. P. W. Lett (at right) are with a group observing the AGT-1500.
The survivability of the XM1 was demonstrated during DT-II at Aberdeen using pilot vehicle 11 (PV 11). The tank was fully loaded with fuel and ammunition with dummies placed in the various crew positions. Sensors were installed throughout the tank and, with the engine idling, it was fired upon using various types of ammunition at typical combat ranges. The tank was not destroyed and it was driven away under its own power after the tests. PV 11 also was subjected to a blast from an antitank mine which badly damaged the suspension. However, after short tracking one side of the vehicle, the tank was started and driven away from the test site. On 7 May 1979, the XM1 was approved by the Secretary of Defense for low initial rate production (LIRP) at the newly equipped Lima Army Tank Plant. As mentioned previously, this was for 110 tanks with the first two being delivered at a special acceptance ceremony on 28 February 1980. At this time, the new tank was named the Abrams in honor of the late General Creighton W. Abrams, an outstanding armor commander and former Army Chief of Staff. The General's wife and three sons attended the ceremony and Mrs. Abrams christened the first production tank.
 
The LIRP tanks began developmental testing (DT-III) in March 1980 and it continued until September 1981. The major part was conducted at Aberdeen Proving Ground for RAM-D testing. Other work was at Yuma, Arizona for desert tests, Eglin Air Force Base, Florida and the Alaska Cold Region Test Center for cold room and arctic environment testing, and the White Sands Missile Range, New Mexico for electromagnetic radiation and nuclear vulnerability evaluation. The operational tests (OT-III) started at both Fort Knox and Fort Hood during September 1980 and continued until May 1981. At Fort Hood, the tanks were operated and maintained by the 2/5th Cavalry and the 27th Maintenance Battalion. The new tank was enthusiastically received by the troops who greatly appreciated its speed and agility. The operational tests revealed that the cruising range of the production tank was about 20 to 25 miles less than the objective of 275 miles at 25 miles per hour. This was attributed partially to the suspension modifications made to reduce the track throwing experienced during DT/OT-H. However, it was noted that the turbine operated more efficiently at higher speeds and that an increase in the tank test speed from 25 to only 27 miles per hour produced an improvement in cruising range. Thus by operating at higher speeds, the required cruising range could be obtained.
 
In February 1981, production was approved for 7058 tanks at a rate of 30 per month. The vehicle was classified as standard on 17 February 1981 as the 105mm gun full tracked combat tank M1. Production problems at AVCO Lycoming resulted in their failure to meet the deliveryf schedule and also produced a large number of defects in the engines that were delivered. Poor quality control and shortages of skilled personnel were the primary causes. Eventually these problems were solved and the number of defective engines was minimized as production increased. Early tank production also suffered from delays in obtaining the thermal imaging systems and from some fabrication difficulties with the hull and turret at the Lima plant. These problems also were solved and the production of the new tank soon reached the required level.
 
Unlike the M48 and M60 tank series, the M1 did not make use of large armor steel castings. The hull and turret were assembled by welding sections of rolled homogeneous steel armor plate to which the special armor was attached. These steel sections were cut to size using precision flame cutting equipment. The hull was divided into three sections with the crew compartment in the center separated from the fuel cells in the front and the engine compartment in the rear by two bulkheads. In addition to the two fuel cells adjacent to the driver's station, two were located in the engine compartment and two in the rear sponsons. A Halon fire extinguisher system with infrared sensors was installed in both the crew and engine compartments. It could detect a fire in 4-5 milliseconds and extinguish it in about two tenths of a second. The hatch above the driver's seat was fitted with three periscopes providing an overlapping field of vision exceeding 120 degrees. Like on the XM803, there was no floor escape hatch. The driver's T-bar steering control incorporated motorcycle type hand throttles eliminating the need for a floor accelerator pedal thus providing more foot space. The transmission shift selector and push buttons for the intercom system also were located on the T-bar assembly. The service brake pedal was in the center of the floor allowing braking with either foot. The driver's seat was adjustable for driving with the hatch either open or closed and the seat height could be set to four different positions. A large knob on an adjustable lower lumbar support allowed the driver to control the amount of lower back support. The aluminum turret basket floor was suspended from five posts, two of which mounted the seats for the tank commander and the loader. A door in the basket floor provided access to components in the bottom of the hull. The commander's station permitted observation in the seated position through both the extension from the gunner's primary sight and the sight for the .50 caliber machine gun. Six periscopes permitted 360 degree vision. The three positions of the commander's platform allowed him to observe standing with the hatch in the protected open position or at two heights with the hatch fully open. The commander's .50 caliber machine gun was electrically powered in azimuth, but it was elevated manually. The loader at the left rear of the cannon had an eight position swiveling seat that was adjustable in the vertical direction. An eight round armored hull ammunition compartment was located on the right side of the tank behind the engine compartment bulkhead. Blowout panels in the top and bottom of the hull vented any ammunition explosion to the outside and away from the crew. Fortyfour 105mm rounds of ammunition were stowed in the turret bustle behind sliding armor plate doors. Twentytwo of these rounds could be reached by the loader without leaving his seat. A knee switch operated the bustle compartment door covering these 22 ready rounds on the left side of the turret. The door also could be operated manually in an emergency. Blowout panels in the bustle roof vented any ammunition explosion to the outside. Three additional 105mm rounds were stowed on the turret basket floor to the left of the cannon in spall protection covers. This brought the total 105mm ammunition stowage to 55 rounds.
 
The loader's hatch cover was fitted with a rotatable periscope mount. A 7.62mm M240 machine gun was skate mounted around the outer edge of the loader's hatch. Another 7.62mm M240 machine gun was installed coaxially on the right side of the 105mm gun M68E1 above the gunner's auxiliary sight. The gunner's seat was pedestal mounted on the turret basket floor and it was adjustable in both the vertical and horizontal directions. No adjustments were required when the gunner shifted from the primary sight to the auxiliary sight. The gunner's primary sight with its extension for the tank commander was the main optical sighting instrument. It also included the laser range finder, the thermal vision system, and the gyro stabilized line of sight platform. The sight was protected by an armor steel cover with doors operable from within the turret. The gunner's auxiliary sight was an eight power articulated telescope installed in the right side of the gun mount below the coaxial machine gun. The heart of the fire control system was the digital computer which received signals from the sensors as well as inputs from various controls. Data from the laser range finder, the cant sensor, and the wind sensor were received automatically, but other inputs were entered into the computer manually. The gunner used the muzzle reference system to determine tube bend corrections which were then entered into the computer. Atmospheric temperature and pressure as well as ammunition temperature and tube wear also were manual inputs to the computer.

General Motors XM1.
The powerpack consisted of the AGT-1500 turbine engine, the X-1100-3B transmission, final drives, air cleaner, scavenging blower, and cooling system. The powerpack weighed about 8500 pounds and could be easily removed as a unit using a five ton wrecker. The sprockets at the rear of the vehicle drove the 25 inch wide tracks and the tank rode on seven dual road wheels per side. The 25 inch diameter compensating idler installed at the front of each track was interchangeable with the road wheels. It was mounted on a pivoting idler arm connected to the number one road wheel arm by a track adjusting link. Adjustment of the static track tension was by hand pumping grease into the link. When the proper tension was reached, a pressure relief valve opened releasing any additional grease.
 
The vehicle was sprung by high strength steel torsion bars accessible from the outside on both sides of the tank. Thus, if a torsion bar fractured, both pieces could be more easily extracted. There were only two track support rollers per track compared to three on the validation phase prototype. The production tank was governed for a maximum level speed of 45 miles per hour and it could operate cross country at speeds up to 30 miles per hour. 
 
On 19 February 1982, it was announced that a preliminary agreement had been reached between Chrysler and General Dynamics Corporation for the latter to purchase the Chrysler Defense Division. The deal was completed in March and the organization was renamed as the General Dynamics Land Systems Division. The personnel remained
the same with Dr. Philip W. Lett, Jr. becoming Vice President of Research and Engineering. Thus Chrysler severed its connection with the development and production of tanks which had existed since before World War II.
 
It had always been intended to produce the M1 at both the Lima and the Detroit tank plants. In September 1981, the production of 60 tanks per month was authorized with 30 at Lima and 30 at Detroit. This was based on a single work shift at both plants. A total of 150 tanks per month was possible with multi-shift operation. Production of the M1 began at Detroit alongside the M60A3 during March 1982 and the first tank was accepted by the Army at a ceremony on 31 March. Production of the basic M1 continued at both plants for a total run of 2374 tanks. The last basic M1 was completed in January 1985.

This photograph was taken at the rollout of the first XM1 pilot tank at Detroit in February 1978. In the view above, the two center figures
are Dr. P. W. Lett (left) and Eugene Trapp (right).
*Note - all vehicle weights are provided in short tons.
 
PS. I will update this post a bit later with drawings and photos. And then move on.