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Who says that a modern tank needs a 1,800 mm wide turret ring? Modern tanks have large turret ring diameters in order to gain the internal space required for the crew and components in the interior. By moving to an unmanned turret - or placing the gun in a overhead mount - one does not need a turret ring diameter of 1,800 mm or more. Just look at some of the UDES testsbeds or the Marder VTS.

 

vts-1.jpg

 

Cqp2i.jpg

 

I also don't see any reason why a tranversly mounted engine needs to be placed at the front of the vehicle. One can get the same reduction in hull length by placing the engine at the rear. Given that a frontally mounted engine will still lead to additional side skirts being required for crew protection, the weight savings seem to be greater when mounting the engine at the rear.

As long as we use a conventional turret, we still need a turret ring diamteter of around 2000mm to fit the gun, armor and to have space for recoil length and such. 

 

Alternativly we could use a turret which is not mounted in the hull, but on top of the hull like you suggested. Though, I am not a big fan of the UDES XX20. The loading mechanism and turret in general seems very hard to armor and leaves much exposed to small arms fire. A piviot turret is not really the best choise around. A type of occilaiting turret would work much better in my opinion. 

 

But may I ask, how do you suggest we design a autoloader for the system above?  The swedish system seems overly complicated and exposed. 

 

And about the engine mount. I argued why I would have the engine transversly mounted, not why it is better to have it transversly mounted in the front. I think we can stop the disscusion about front mounted engines now, since we have gotten both the good and the bad sides out, and the rest is subjecvtive. 

 

 

 

No, we don't need to stop it completely. But relying on (thin) steel plates at high obliquity should stop. The protection provided by a thin steel plate, such as the 40-50 mm used on M1 Abrams, Leopard 2, K1, Type 90 and similar tanks, is not enough to stop modern 120/125 mm APFSDS. Throw the new 130 mm gun from Rheinmetall and the Russian 152 mm gun into the mix and even more armor is required. Given that there are limitations to weight and (high-hardness) steel plate thickness, the armor has to be thicker and made of composite armor in order to stay at an acceptable weight.

 

So high obliquity composite plate of around 200-300mm?

 

 

 

 

If you already need a ~250-300 mm glacis, why not use it as roof protection for the crew? The hatch design doesn't need to be very complicated. Sure, simpler designs might have a few issues with armor coverage at the hinge/sliding mechanisms... but then again current hatches are also considered a weakspot on tanks like Abrams.

For an engine having an engine cover that can only be lifted using a large crane seems to be another drawback. Yes, usually the engine covers are always lifted by a crane, but in case of emergency the crew can still use less potent/non-military equipment to deal with it.

 

In your suggested layout, the frontal armor acts as roof armor only for the engine, which frankly doesn't require the same level as protection for the crew. The turret will only protect the crew compartment, if it has a large, overhanging turret bustle (which is capable of providing enough crew protection and not just external storage boxes as most of the turret bustles of the Leopard 2A5/6/7 and the Challenger 2) and the turret is turned towards the front. Want to fight against an enemy at the side? Well, then you have to expose the crew to top-attack weapons...

 

No, because it is not possible to provide the same level of protection to all parts of the tank. In ideal case every area of the tank would have armor as thick as the turret front - but that's not possible due to weight, size, useability and other factors. The engine is always a structural weakspot due to needing a cooling solution and exhaust systems. So if some place should be less armored, it should be the area at the engine (above and below), which doesn't protect the crew as much as the front, aswell as the roof and belly of the crew compartment. Just look at current tanks: the armor at the sides of the engine of a Leopard 2 and M1 Abrams is 40 mm thick at best. The armor at the front is multiple times thicker, because it protects the crew. The same applies to the roof armor. The engine covers of the tanks are usually 20 to 25 mm thick, whereas the sloped glacis and turret roof are 40 mm or more thick.

Considering the fact about the highly sloped plates, a rear mounted engine would make more sense maintenance wise. 

 

 

 

 The photo is a bit misleading, because it compares the vehicles at different angles. The CV90 appears to be a lot smaller mainly due to it's smaller turret. The height hull itself doesn't seem to be any smaller than that of a Marder or Warrior IFV if you take other factors (such as the ~60-70 mm thicker, spaced roof armor of the Marder 1A3 hull) into account. The "Puma" is actually a Marder 1A3 with the Kuka M12 turret, which was offered in the export market to Norway, Switzerland and a few selected other countries.

I cannot comment on your esimations, because I have never been inside a CV90 myself. However there are a few things that I can still add to this part of the discussion. In SteelBeasts at least the CV90 hull is a tad higher than the Leopard 2 engine compartment, by about 100 mm. If we take the ground clearance into account - which is between 50 and 100 mm less for the CV90 - the CV90 rear compartment is about 150 - 200 mm taller than the engine compartment of the Leopard 2.

Now, I only looked at the Norwegian CV90, the Swedish version has (afaik) a slightly lower height at the rear compartment. But the Swedish version isn't exactly the standard configuration of the CV90, being not designed for compability with all NATO standards. On the CV90 Mk II (CV9030) the roof is raised by 140 mm, one the CV90 Mk III (CV9035 operated by Denmark and the Netherlands) it is raised even more. I don't know exactly if this is a NATO standard yet, but German AFVs are always designed with a compability from the 5th to 95th percentile (only the smallest 5% and the tallest 5% of the male adults are incompatible with the vehicle).

I am pretty sure you are wrong about the MKII being 140mm taller at the rear compartment. I heard no account of that. Unless the entire hull roof was raised, it is wrong, since the hull goes flat all the way to the back. 

 

About the raised crew compartment, in the C2, the troop compartment is raised by 170mm, to provide headspace in case of a blast mine or IED. 

 

I do not know how comfortable a Marder is, but the the CV90's troop compartment is quite tight from what I have heard from the people serving in Kampeskvadronen. But not BMP tight. 

 

 

And, when you say the Leopard 2s engine compartment, do you mean from the highest point or the lowest point?

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Overhead turrets make sense maybe up to 30mm. At 40mm and above, it gets bulky and has to take up hull space.

 

The only 40mm turret that is planned to be overhead, is the one designed for the Eitan IFV. However, there's still the option for 30mm, so it may or may not prove to be feasible.

 

Overhead cannons don't seem to be the way to go for large calibers either, as while less vulnerable than unmanned ones in theory, they are doomed to fail in urban combat or even prove too vulnerable to artillery.

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Every other front-engined MBT design proposal I can think of came from a country that had the luxury of choosing an engine that would fit in a smaller frontal hull. Liquid-cooled Vs (STRV-2000, NKPz), Boxer diesels (Object 416), even turbines (Object 299 and some US FMBT concepts) have been proposed.

A common factor of the vehicles mentioned by you is an autoloader, which the Merkava tanks don't have. Just look at how much smaller a T-90 or Leclerc/K2 is compared to an Abrams and a Leopard 2. The vehicles mentioned by you being smaller than a Merkava is also affected by the autoloader. The Merkava 4 has the MT883 engine, which is a lot smaller than the Leopard 2's MB873 - still it is a larger vehicle in every aspect.

I also wouldn't mention the NKPz as proof of front-mounted engines causing less issues when using more/better technology than what was available to the IDF; the NKPz doesn't have composite armor at the glacis, while having slightly worse slope at the glacis than a Leopard 2 or Abrams.

 

As long as we use a conventional turret, we still need a turret ring diamteter of around 2000mm to fit the gun, armor and to have space for recoil length and such.

Yes, for a conventional turret we need at least ~2,000 mm. For a 130 mm or a 152 mm gun there might even be a greater space requirement.

 

Alternativly we could use a turret which is not mounted in the hull, but on top of the hull like you suggested. Though, I am not a big fan of the UDES XX20. The loading mechanism and turret in general seems very hard to armor and leaves much exposed to small arms fire. A piviot turret is not really the best choise around. A type of occilaiting turret would work much better in my opinion.

The Experimentalwanne Gesamtschutz (EGS) testbed made in Germany in 1989 was designed to have a gun in an overhead mount. While it wasn't actually fitted with a gun (due to being an armor testbed), the EGS had an armored cover around were the gun would have been located.

 

But may I ask, how do you suggest we design a autoloader for the system above? The swedish system seems overly complicated and exposed.

There are many different options. One could use a conveyor-belt or revolver autoloader in a small turret similar to the Jordanian Falcon 2 turret or the Stryker MGS turret. In ideal case the autoloader would be fixed to the gun, so that it would follow the same movements. In Germany and Switzerland there were concepts that used an autoloader located in the hull, which then loaded the gun via inserting the ammunition with an "arm" from the outside. Such a design was used on a Bofors-designed tank in 1977 (UDES-18/19?) Another option is to have a conveyor-belt autoloader located in the hull, which can rotate with the turret. This would require a slightly larger turret ring, enough to transport a single round (about 1,000 mm for 120 mm ammunition) with a conveyor belt. Krauss-Maffei designed such a system for a tank concept in the 1970s, Rheinmetall and Wegmann later used broadly similar designs in other concepts.

The US ELKE testbed with the 75 mm ARES gun had an autoloader, which stored the ammunition vertically in the hull, the rounds were then rotated by 90° in the turret/gun. This might be a bit problematic with current and next-generation ammunition though.

The German company MaK submitted a design to the Kampfpanzer 3 project in 1974, which had an overhead mount for the gun, which essentially had the size of a turret and stored 40 rounds of 120 mm ammunition inside an autoloader. The turret ring was less than a metre in diameter.

 

So high obliquity composite plate of around 200-300mm?

Yes, this should be sufficient to protect against current tank ammunition.

 

I am pretty sure you are wrong about the MKII being 140mm taller at the rear compartment. I heard no account of that. Unless the entire hull roof was raised, it is wrong, since the hull goes flat all the way to the back.

About the raised crew compartment, in the C2, the troop compartment is raised by 170mm, to provide headspace in case of a blast mine or IED.

The first CV90 Mk II was the Schützenpanzer 2000 (CV9030CH) for the Swiss Army. The original CV9030 procured by Norway was still the Mk I model. You can see the raised roof of the rear compartment of the CV9030CH here:

csm_Schuetzenpanzer_2_gross_fc32caffac.j

Current Norwegian models (the later production models at least) have the same raised roof at the rear compartment:

innsats5_cv90.jpg

The same applies to the CV9030FIN:

original_image.jpg?1456467680

 

I do not know how comfortable a Marder is, but the the CV90's troop compartment is quite tight from what I have heard from the people serving in Kampeskvadronen. But not BMP tight.

I guess it is depending on opinion, but I'd consider the Marder also as rather cramped.

 

And, when you say the Leopard 2s engine compartment, do you mean from the highest point or the lowest point?

In SteelBeasts the rear section of the CV9040 has about the same height as the rear section section of the Leopard 2 engine compartment - maybe not exactly the highest point of the engine compartment, but quite similar. The Leopard 2's engine bay is a bit more sloped than the CV90 hull, the frontal section of the engine bay has a lower height than the CV9040 rear compartment. The CV90 Mk II's rear compartment seems to have a greater height than the engine compartment of Leopard 2.

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Ok folks! A idea from Xoon's crazy lab! 

 

In AFVs we have a lot of heavy wiring. This problem is partially solved when it comes to control and signal cables with fiber optics. Put when it comes to power supply we still use copper cables. 

FiberOpticCable78e9320edd2d4aaca30ab18fc

 

 

So I have a idea, what about using aluminum cables instead on the main power lines?

615380.jpg

 

 

 

 

Aluminum needs to be 1,5 times thicker to transfer the same amount of current, but it turn it is twice as light for the same amount of current. 

This would mean that if you have say, 500 kg worth of power supply cables, you could reduce the weight to 250 kg.  Another advantage is that aluminum costs only 1/3 to 1/5 for the same amount of copper, and it would reduce the usage of strategic resources. 

 

There are drawbacks of course, the number one drawback is that you need a adopter between the copper and the aluminum, since they react with each other. This would add weight and cost, but this should be off sett by the savings the cable in itself provides:

aluminium-copper-bi-metal-terminals.jpg

 

 

 

The other downside is the tab bit bigger volume it would take. 

For example, a 4mmcopper cable can supply the same amount of amperage as a 6mm2 aluminum cable. 

 

I think this can come in handy when considering things like a electric turret drive and elevation mechanism, a APU and a diesel-electric drive train.

 

 

Mvh
Xoon

 

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Since when has Zuk been on this forum, lol. 

 

The first diagram Met749 posted is probably the most accurate in regards to T-14. It would make no sense for the layout to be otherwise (ie heavily sloped like the T-64 and onward) because of the new use of ceramic composite armour. I suspect that the external ERA is mounted upon a thin outer layer of metal/armour, which is then followed by spacing and finally the composite. It makes the most sense. 

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From what I have gathered, these seem like the 3 main gun mount types:
7eUYYE3.png

 

 

In short, a over turret ring mount, inside turret ring mount or a casemate mount.

 

 

Casemate provides the greatest amount of fire rate inside the smallest possible silhouette, with the least gun overhang and the simplest of the 3.

​Conventional turret is a conventional turret

Over turret ring mount requires the smallest turret ring, and does not protrude into the hull, but lacks ammunition capacity and is more complex.

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Has anyone looked at temperature controlled ammunition storage?  The ammo storage on Iowa-class battleships was refrigerated, which slightly improved accuracy and slightly reduced cookoff risk.  I would think there would be similar benefits in a tank.

 

I've heard claims that chieftain had such, my gut says it must have been peltier based. Wet ammo stowage is convenient for that, although I could see it working well with the M1's isolated stowage. Like a fridge with blow-out panels

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I've heard claims that chieftain had such, my gut says it must have been peltier based. Wet ammo stowage is convenient for that, although I could see it working well with the M1's isolated stowage. Like a fridge with blow-out panels

 

Xlucine posted, a new years miracle!

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So here's a video of a T-72 (?) having a catastrophic failure (At least 2 crew members survive, unsure about the third): https://twitter.com/RamiAILoIah/status/815694957746987008

 

Does anyone know what could have caused this? I've been thinking about it a bit and I have no idea. It seems like a failure during loading since the breech was open, but (part of) the crew had time to get out. Which makes me think it wasn't an immediate catastrophic failure and thus something not related to loading.

 

Anyone any idea what could have caused this?

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quick guesses: primer failure resulting in a dud. Although would still require an open breech.

excessive barrel bending causing fuze setting inside the barrel.

Cracks in the breech causing rupture.

Could be either, or none.

Oh and that's a T-55. Old platform coupled with poor maintenance can have deadly results.

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I've heard claims that chieftain had such, my gut says it must have been peltier based. Wet ammo stowage is convenient for that, although I could see it working well with the M1's isolated stowage. Like a fridge with blow-out panels

 

Wet stowage would definitely make it work better thanks to the extra thermal mass.

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I was thinking of the coupling between the cooling engine and the charge - water is nice and conductive, and you don't need airtight stowage for the rounds (that would get in the way of loading).

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I was thinking of the coupling between the cooling engine and the charge - water is nice and conductive, and you don't need airtight stowage for the rounds (that would get in the way of loading).

 

I was thinking of how fast the system could cool down new ammo that was loaded into the tank.

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I have been wondering how short you can make a MBT and I came up with this:
cJrVFeY.png

 

 

Using a unmanned turret and placing all crew in the front reduces the height of the tank by:

1.Removing the loader, thereby not needing the height of comfortable loading.

2. Moving the crew away from the turret, removing the need for head space for the crew and need for thick turret roof armor. 

 

And by using a reclined position, we can further reduce the height of the hull.

Next would be to use hydropeumatic suspension to remove the floor space needed by the torsion bars.

By sloping the UFP at 80 degrees, the barrel gets extra space to depress, reducing the height needed for 10 degree depression. 

Using a conventional turret, which allows the turret to depress into the hull when elevating. 

Only having 450mm ground clearance makes the vehicle as low as possible without losing off road mobility. 

And only using thick roof and belly armor around the crew reduces the height of the hull. 

 

 

Of course other solutions could be used too like:

*Reducing the depression and elevation to -5 and +15.

*Only using thin roof armor (like 30mm roof and 25mm thick belly).

*Using shorter ammunition and gun, reducing the turret roof height needed to depression. 

*Adding a muzzle break to reduce recoil, which would allow a shorter recoil distance, which would reduce the height of the turret roof 

 

needed to depress the gun.

*Making the crew go prone. 
*Using a rotating breach.

 

 

So, any realistic ideas to make it even lower?

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I present to you........

 

Knøtpanser model 1!:

uIfD4C2.png

 

Commander and driver located in the turret, with the gun in between them and a soviet style autoloader supplying the gun, or a Leclerc style autoloader.

 

Front wheel drive, however rear wheel drive could work just as well. 

 

 

The vehicle could be even lower with a lower powerpack, and maybe a cleft turret. However, the vehicle will probably not be much shorter than 1,1mm, simply because there is only so much you can recline the crew, and armor and stuff is a thing. 

 

Max would be 1,55m tall with when taking ground clearance into account.

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19 hours ago, Alzoc said:

I just meant a tandem of an AC in combination with an ATGM, not a twin AC or something like that.

May have phrased that one poorly.

In general if you want to use a full pressure gun, you better use a tracked vehicle since it will save weight on the suspension, and the vehicle will also be smaller.

But at the same time a tracked vehicle will often exceed 20 metric ton anyway.

Tracks are more efficient weight-wise but they  automatically put the vehicle above a  minimal weight.

 

The M8 is an exception since it was designed to be just light enough to be squeezed inside a C-130, and I guess that some serious compromises were made for that.

 

Light tanks can potentially be airlifted by tactical aircrafts but have, generally, a greater logistical trail than wheeled vehicles (higher fuel consumption and no parts commonality with APC and IFVs deployed alongside them) which is also a problem for an expeditionary force. Also their effective range will be smaller.

 

A Centauro II will barely fit in an A400M and for the US army to airlift such a vehicle would require the use of a C-17 (which is not as flexible as a C-130 in term of possible landing zone).

The MGS will fit in a C-130 thanks to it's unmanned turret but it's nowhere near the capability of a Centauro II (and most likely of a B1 Centauro as well, especially in the AT department) but it's quite an old design anyway.

If it were to be remade nowadays, I think it would end up heavier and larger.


In the end I think that there is two school:

 

-The European one which use heavy (25-30 metric ton) IFV-based vehicles in combination with the A400M (which is a sort of heavy tactical aircraft). The vehicles may use either a gun (which is not the best idea for wheeled vehicles) or an AC+ATGM combo

-The US that use lighter and less protected wheeled vehicles (Stryker family) and if a bigger vehicle is needed will just use a C-17 and land it on a better airstrip.

 

In the end it mostly comes down to the US having access to a heavy lift strategical aircraft and having vastly superior logistics than European country, while the European will have access to a better tactical aircraft.

 

 

I am posting this here so I don't derail the ammunition thread.  How exactly are tracks more efficient weight-wise?  Tracks are quite heavy; usually 10% of the total mass of a tracked vehicle while the suspension for another 10% at least.  Surely that's lighter than wheels in almost all cases.

The advantage of tracks is lower ground pressure, and less intrusion into the hull.

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1 hour ago, Collimatrix said:

 

 

I am posting this here so I don't derail the ammunition thread.  How exactly are tracks more efficient weight-wise?  Tracks are quite heavy; usually 10% of the total mass of a tracked vehicle while the suspension for another 10% at least.  Surely that's lighter than wheels in almost all cases.

The advantage of tracks is lower ground pressure, and less intrusion into the hull.

 

I will dig back the book where I found that tonight when coming back from work, but as far as I remember the explanation was the following.:

 

Tracks themselves are inherently heavy, as well as their suspensions (be it torsion bars, coil springs, or hydro-pneumatic) so as soon a you choose to use that type propulsion you have to cope with the weight.

But as you said they intrude less in the hull, which mean a smaller vehicle and so a lighter vehicle at equivalent level of protection.

 

As for wheeled vehicles, past a certain weight (10 ton is a rule of thumb) wheels suspension need to become more complex, the wheels gets bigger to keep the ground pressure to a reasonable level.

That mean that if you want to keep a decent vertical travel your vehicle will be taller and the suspensions more complex (and generally heavier).

 

Now if you want to use high pressure gun on a wheeled IFV, you have to deal with the recoil and part of it will have to be absorbed by the suspensions.

Especially when you want to fire your gun with the turret at 90° from the hull, you have to deal with a force applied on top of a very tall vehicle, suspensions will have to be rather complex to deal with the recoil and prevent the vehicle from tipping over.

Tracks having a bigger ground contact area and making the vehicle lower have less problem dealing with side shot (force applied closer to the center of gravity of the vehicle and more friction with the ground).

 

This is really a technological problem, the higher the weight of a wheeled vehicle, the more complex it's suspensions, and in that case more complexity often mean more weight.

Suspensions for tracked vehicles are much less sensitive to weight change, the technology used scales up much better.

 

In the past the rule of thumb was:

 

m < 10 ton : Wheels

10 < m < 20 ton : Tracks or wheels, depending on the usual track vs wheel debate (Cost, tactical mobility vs strategical mobility, type of terrain, etc)

m > 20 ton : Tracks

 

But with technological advance the lines are getting blurred and the grey area is expanding, you have behemoth wheeled vehicles like the VBCI or the Centauro II using fancy supsensions and reaching around 30 ton and on the other end of the spectrum you have light IFV  that start using rubber tracks (vastly reducing the inherent weight of a tracked system).

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This image is kind of weird, but the part in the center is the part on the far left, zoomed in, showing you where the lever and the opposite side's torsion bar are attached. As you can see, road wheels in a torsion bar suspension are going to be a little off on one side, unlike what you're used to on cars and such.     Now, since torsion bars are metal bars on the floor, they are going to make your tank taller. If you want a tank that's as short as possible at the expense of width, you may want to consider a Christie like suspension. Here, much like in torsion bars, the pressure is transferred inside the tank, but instead of a bar to absorb it, it's a spring in a vertical (or angled) tube. In most tanks with this kind of suspension, the springs are on the inside, but if you want to make the tank roomier on the inside, you can have them on the outside too. If you're really fancy, you can put a spring within the spring like in this diagram.     Since this is a Soviet tank book, you gotta have a huge T-34 diagram. Here it is.     The T-34 uses Christie springs, which you can see in the diagram. The road wheel configuration is a mix of the externally dampened and internally dampened "Stalingrad type" road wheels. The former have more rubber for absorbing hits from terrain, but the latter use less rubber. When you're in Stalingrad and you have to make tanks with a rubber deficit, that's the kind you want. When road wheels from other factories were available, they would go in the front and then the back to absorb most of the impact from harsh terrain features, and the steel-rimmed wheels went in the middle. The diagram shows how both types of wheels work.   Rubber can't really take too much punishment, so the KV, being a heavy tank, went with internally dampened road wheels from the very beginning, with a ring of rubber on the inside around the axle.     And finally, idlers. If you don't have big Christie type wheels, you gotta have idlers so your saggy track doesn't fall off. This diagram shows the rubber coating on an idler, and also how the rear idler can adjust to tighten the track. A loose track makes more noise, gets worn more, and is liable to slip off.     Keep those tracks tight, and you'll be zooming towards glorious victory in no time flat!     Now, the book ends and my own stuff begins. I mentioned rubber, but not what a headache it was to tank designers. In hot weather, the rubber in your tracks and wheels tends to fall apart. If you go fast enough, tires that don't have proper ventilation are going to melt too. There was a lot of pre-war panic in the USSR about the German PzIII being able to do 70 kph on tracks, but once the Soviets started building SU-76Is on the PzIII chassis they found out that the speed had to be limited to a whopping 25 kph to keep the wear to a reasonable level.
    • By Collimatrix
      Tank design is often conceptualized as a balance between mobility, protection and firepower.  This is, at best, a messy and imprecise conceptualization.  It is messy because these three traits cannot be completely separated from each other.  An APC, for example, that provides basic protection against small arms fire and shell fragments is effectively more mobile than an open-topped vehicle because the APC can traverse areas swept by artillery fires that are closed off entirely to the open-topped vehicle.  It is an imprecise conceptualization because broad ideas like "mobility" are very complex in practice.  The M1 Abrams burns more fuel than the Leo 2, but the Leo 2 requires diesel fuel, while the omnivorous AGT-1500 will run happily on anything liquid and flammable.  Which has better strategic mobility?  Soviet rail gauge was slightly wider than Western European standard; 3.32 vs 3.15 meters.  But Soviet tanks in the Cold War were generally kept lighter and smaller, and had to be in order to be moved in large numbers on a rail and road network that was not as robust as that further west.  So if NATO and the Warsaw Pact had switched tanks in the late 1950s, they would both have downgraded the strategic mobility of their forces, as the Soviet tanks would be slightly too wide for unrestricted movement on rails in the free world, and the NATO tanks would have demanded more logistical support per tank than evil atheist commie formations were designed to provide.
       

       
      So instead of wading into a deep and subtle subject, I am going to write about something that is extremely simple and easy to describe in mathematical terms; the top speed of a tank moving in a straight line.  Because it is so simple and straightforward to understand, it is also nearly meaningless in terms of the combat performance of a tank.
       
      In short, the top speed of a tank is limited by three things; the gear ratio limit, the power limit and the suspension limit.  The tank's maximum speed will be whichever of these limits is the lowest on a given terrain.  The top speed of a tank is of limited significance, even from a tactical perspective, because the tank's ability to exploit its top speed is constrained by other factors.  A high top speed, however, looks great on sales brochures, and there are examples of tanks that were designed with pointlessly high top speeds in order to overawe people who needed impressing.
       

      When this baby hits 88 miles per hour, you're going to see some serious shit.
       
      The Gear Ratio Limit
       
      Every engine has a maximum speed at which it can turn.  Often, the engine is artificially governed to a maximum speed slightly less than what it is mechanically capable of in order to reduce wear.  Additionally, most piston engines develop their maximum power at slightly less than their maximum speed due to valve timing issues:
       

      A typical power/speed relationship for an Otto Cycle engine.  Otto Cycle engines are primitive devices that are only used when the Brayton Cycle Master Race is unavailable.
       
      Most tanks have predominantly or purely mechanical drivetrains, which exchange rotational speed for torque by easily measurable ratios.  The maximum rotational speed of the engine, multiplied by the gear ratio of the highest gear in the transmission multiplied by the gear ratio of the final drives multiplied by the circumference of the drive sprocket will equal the gear ratio limit of the tank.  The tank is unable to achieve higher speeds than the gear ratio limit because it physically cannot spin its tracks around any faster.
       
      Most spec sheets don't actually give out the transmission ratios in different gears, but such excessively detailed specification sheets are provided in Germany's Tiger Tanks by Hilary Doyle and Thomas Jentz.  The gear ratios, final drive ratios, and maximum engine RPM of the Tiger II are all provided, along with a handy table of the vehicle's maximum speed in each gear.  In eighth gear, the top speed is given as 41.5 KPH, but that is at an engine speed of 3000 RPM, and in reality the German tank engines were governed to less than that in order to conserve their service life.  At a more realistic 2500 RPM, the mighty Tiger II would have managed 34.6 KPH.
       
      In principle there are analogous limits for electrical and hydraulic drive components based on free speeds and stall torques, but they are a little more complicated to actually calculate.
       

      Part of the transmission from an M4 Sherman, picture from Jeeps_Guns_Tanks' great Sherman website
       
      The Power Limit
       
      So a Tiger II could totally go 34.6 KPH in combat, right?  Well, perhaps.  And by "perhaps," I mean "lolololololol, fuck no."  I defy you to find me a test report where anybody manages to get a Tiger II over 33 KPH.  While the meticulous engineers of Henschel did accurately transcribe the gear ratios of the transmission and final drive accurately, and did manage to use their tape measures correctly when measuring the drive sprockets, their rosy projections of the top speed did not account for the power limit.
       
      As a tank moves, power from the engine is wasted in various ways and so is unavailable to accelerate the tank.  As the tank goes faster and faster, the magnitude of these power-wasting phenomena grows, until there is no surplus power to accelerate the tank any more.  The system reaches equilibrium, and the tank maxes out at some top speed where it hits its power limit (unless, of course, the tank hits its gear ratio limit first).
       
      The actual power available to a tank is not the same as the gross power of the motor.  Some of the gross horsepower of the motor has to be directed to fans to cool the engine (except, of course, in the case of the Brayton Cycle Master Race, whose engines are almost completely self-cooling).  The transmission and final drives are not perfectly efficient either, and waste a significant amount of the power flowing through them as heat.  As a result of this, the actual power available at the sprocket is typically between 61% and 74% of the engine's quoted gross power.
       
      Once the power does hit the drive sprocket, it is wasted in overcoming the friction of the tank's tracks, in churning up the ground the tank is on, and in aerodynamic drag.  I have helpfully listed these in the order of decreasing importance.
       
      The drag coefficient of a cube (which is a sufficiently accurate physical representation of a Tiger II) is .8. This, multiplied by half the fluid density of air (1.2 kg/m^3) times the velocity (9.4 m/s) squared times a rough frontal area of 3.8 by 3 meters gives a force of 483 newtons of drag.  This multiplied by the velocity of the tiger II gives 4.5 kilowatts, or about six horsepower lost to drag.  With the governor installed, the HL 230 could put out about 580 horsepower, which would be four hundred something horses at the sprocket, so the aerodynamic drag would be 1.5% of the total available power.  Negligible.  Tanks are just too slow to lose much power to aerodynamic effects.
       
      Losses to the soil can be important, depending on the surface the tank is operating on.  On a nice, hard surface like a paved road there will be minimal losses between the tank's tracks and the surface.  Off-road, however, the tank's tracks will start to sink into soil or mud, and more power will be wasted in churning up the soil.  If the soil is loose or boggy enough, the tank will simply sink in and be immobilized.  Tanks that spread their weight out over a larger area will lose less power, and be able to traverse soft soils at higher speed.  This paper from the UK shows the relationship between mean maximum pressure (MMP), and the increase in rolling resistance on various soils and sands in excruciating detail.  In general, tanks with more track area, with more and bigger road wheels, and with longer track pitch will have lower MMP, and will sink into soft soils less and therefore lose less top speed.
       
      The largest loss of power usually comes from friction within the tracks themselves.  This is sometimes called rolling resistance, but this term is also used to mean other, subtly different things, so it pays to be precise.  Compared to wheeled vehicles, tracked vehicles have extremely high rolling resistance, and lose a lot of power just keeping the tracks turning.  Rolling resistance is generally expressed as a dimensionless coefficient, CR, which multiplied against vehicle weight gives the force of friction.  This chart from R.M. Ogorkiewicz' Technology of Tanks shows experimentally determined rolling resistance coefficients for various tracked vehicles:
       

       
      The rolling resistance coefficients given here show that a tracked vehicle going on ideal testing ground conditions is about as efficient as a car driving over loose gravel.  It also shows that the rolling resistance increases with vehicle speed.  A rough approximation of this increase in CR is given by the equation CR=A+BV, where A and B are constants and V is vehicle speed.  Ogorkiewicz explains:
       
       
      It should be noted that the lubricated needle bearing track joints of which he speaks were only ever used by the Germans in WWII because they were insanely complicated.  Band tracks have lower rolling resistance than metal link tracks, but they really aren't practical for vehicles much above thirty tonnes.  Other ways of reducing rolling resistance include using larger road wheels, omitting return rollers, and reducing track tension.  Obviously, there are practical limits to these approaches.
       
      To calculate power losses due to rolling resistance, multiply vehicle weight by CR by vehicle velocity to get power lost.  The velocity at which the power lost to rolling resistance equals the power available at the sprocket is the power limit on the speed of the tank.
       
      The Suspension Limit
       
      The suspension limit on speed is starting to get dangerously far away from the world of spherical, frictionless horses where everything is easy to calculate using simple algebra, so I will be brief.  In addition to the continents of the world not being completely comprised of paved surfaces that minimize rolling resistance, the continents of the world are also not perfectly flat.  This means that in order to travel at high speed off road, tanks require some sort of suspension or else they would shake their crews into jelly.  If the crew is being shaken too much to operate effectively, then it doesn't really matter if a tank has a high enough gear ratio limit or power limit to go faster.  This is also particularly obnoxious because suspension performance is difficult to quantify, as it involves resonance frequencies, damping coefficients, and a bunch of other complicated shit.
       
      Suffice it to say, then, that a very rough estimate of the ride-smoothing qualities of a tank's suspension can be made from the total travel of its road wheels:
       

       
      This chart from Technology of Tanks is helpful.  A more detailed discussion of the subject of tank suspension can be found here.
       
      The Real World Rudely Intrudes
       
      So, how useful is high top speed in a tank in messy, hard-to-mathematically-express reality?  The answer might surprise you!
       

      A Wehrmacht M.A.N. combustotron Ausf G
       
      We'll take some whacks at everyone's favorite whipping boy; the Panther.
       
      A US report on a captured Panther Ausf G gives its top speed on roads as an absolutely blistering 60 KPH on roads.  The Soviets could only get their captured Ausf D to do 50 KPH, but compared to a Sherman, which is generally only credited with 40 KPH on roads, that's alarmingly fast.
       
      So, would this mean that the Panther enjoyed a mobility advantage over the Sherman?  Would this mean that it was better able to make quick advances and daring flanking maneuvers during a battle?
       
      No.
       
      In field tests the British found the panther to have lower off-road speed than a Churchill VII (the panther had a slightly busted transmission though).  In the same American report that credits the Panther Ausf G with a 60 KPH top speed on roads, it was found that off road the panther was almost exactly as fast as an M4A376W, with individual Shermans slightly outpacing the big cat or lagging behind it slightly.  Another US report from January 1945 states that over courses with many turns and curves, the Sherman would pull out ahead because the Sherman lost less speed negotiating corners.  Clearly, the Panther's advantage in straight line speed did not translate into better mobility in any combat scenario that did not involve drag racing.
       
      So what was going on with the Panther?  How could it leave everything but light tanks in the dust on a straight highway, but be outpaced by the ponderous Churchill heavy tank in actual field tests?
       

      Panther Ausf A tanks captured by the Soviets
       
      A British report from 1946 on the Panther's transmission explains what's going on.  The Panther's transmission had seven forward gears, but off-road it really couldn't make it out of fifth.  In other words, the Panther had an extremely high gear ratio limit that allowed it exceptional speed on roads.  However, the Panther's mediocre power to weight ratio (nominally 13 hp/ton for the RPM limited HL 230) meant that once the tank was off road and fighting mud, it only had a mediocre power limit.  Indeed, it is a testament to the efficiency of the Panther's running gear that it could keep up with Shermans at all, since the Panther's power to weight ratio was about 20% lower than that particular variant of Sherman.
       
      There were other factors limiting the Panther's speed in practical circumstances.  The geared steering system used in the Panther had different steering radii based on what gear the Panther was in.  The higher the gear, the wider the turn.  In theory this was excellent, but in practice the designers chose too wide a turn radius for each gear, which meant that for any but the gentlest turns the Panther's drive would need to slow down and downshift in order to complete the turn, thus sacrificing any speed advantage his tank enjoyed.
       
      So why would a tank be designed in such a strange fashion?  The British thought that the Panther was originally designed to be much lighter, and that the transmission had never been re-designed in order to compensate.  Given the weight gain that the Panther experienced early in development, this explanation seems like it may be partially true.  However, when interrogated, Ernst Kniepkamp, a senior engineer in Germany's wartime tank development bureaucracy, stated that the additional gears were there simply to give the Panther a high speed on roads, because it looked good to senior generals.
       
      So, this is the danger in evaluating tanks based on extremely simplistic performance metrics that look good on paper.  They may be simple to digest and simple to calculate, but in the messy real world, they may mean simply nothing.
    • By Collimatrix
      Since we've got the new AFV design competition going and not everyone has solidworks, I thought I would share this information from Technology of Tanks so those who do not have CAD/CAM programs could come up with a reasonable accounting of what a tank ought to weigh:
      -Armor usually contribute between 35% and 51% of the total mass of the vehicle. The lower figure is typical for light tanks, the higher for MBTs. If the armor were reduced to the minimum necessary for structural purposes it would still be about 20% of the total mass. The highest figure on record is 57% for the armor of the IS-3.
      -The tracks contribute about 8% to 10% of the mass of the vehicle in the case of steel link tracks. On a fast track-laying combat vehicle the tracks are getting slung around over all sorts of rocks and whatnot, so they need to be tough, which means that they're heavy. Band tracks weigh 25%-50% less than steel link tracks, but band tracks can only be used on lighter vehicles. The heaviest vehicle I know of that uses band tracks is the Turkish Tulpar IFV at 32 tonnes.
      -Suspensions contribute about 8% to 10% of the total mass of the vehicle. Hydropneumatic suspensions are the lightest, but not by an enormous margin. Higher performance suspensions weigh more.
      -The power pack, that is the engine and the transmission together, account for about 12% of the vehicle's mass.
      -Guns typically contribute 3% to 7% of the total mass of the vehicle, although cramming the very largest gun possible into the very smallest tank possible can bring this up to about 10%.
      -Ammo generally weighs less than the gun. Fuel weighs about the same as ammo.
      On any fictional or notional tank design, I'll be looking to see if the weight of the components are within these bounds. If they're not there had better be a damned good explanation.
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