Trade-offs in WWII Tank Design

[hobbes154]

Inspired by Collimatrix's excellent topic in the Aviation section, my attempt at doing the same for tanks. 

Tank design is often represented as a trade-off between firepower, armour, and speed. But this ignores many other, equally important variables. Furthermore, this formula doesn't explain why the trade-offs are there in the first place. So here is my attempt to make things a bit more complicated.

The Constraint: Compact, powerful and reliable engines – and the ability to use them
Firepower and armour are the obvious features that made a 1940 tank obsolete in 1945. Yet even a 1945 tank is trivial compared to a WWI battleship. Or more modestly, the German 88mm, Soviet 85mm and US 90mm AA guns were prewar designs. Why not put them on a tank from the beginning? Were people just stupid back then?

Up to a point, I would say ‘yes’. While it is unrealistic to think of 1945 combat aircraft, radar, or nuclear weapons in 1940 as the result of anything short of time travel, 1940 tanks could have been significantly better without anachronistic scientific or engineering breakthroughs. Like the assault rifle, this really was a case where the right people just didn't see the need or spend the money.

 

However, it's not the only reason. A tank with a big gun and thick armour that can't move is just a pillbox. Like aircraft, WW2 tanks were fundamentally limited by their engine power. This is less obvious because additional power was typically used to ‘buy’ weight rather than speed. Yet the trend is clear. Shown below are the improvements in engine power for the German and British (cruiser) tank lines, which were in the war the longest.

German Panzers

I	100 hp
II	138 hp
III, IV	250-300 hp
Tiger, Panther	690 hp

British Cruisers

I, II	150 hp
III, IV, Crusader	340 hp (Liberty)
Cromwell, Comet	600 hp (Meteor)

 

Arguably, more powerful tank engines could and should have been introduced much earlier (the Liberty was first used in a tank as early as 1918).  I will leave that aside, noting only that, relative to aircraft engines, tank engines were forced to use lower octane fuel for economic reasons (preventing tank use of the Napier Lion), and are harder to cool due to being inside a slow moving armoured box (this was a particular challenge with the Merlin's conversion to the Meteor). 

 

There's also the choice of diesel (compression ignition) vs. petrol/gasoline (spark ignition) engines. Diesels had higher torque and lower fuel consumption, but lower specific power, were heavier and cost more, and meant an extra type of fuel in your logistics train. Both Germany and the US decided against diesels for fear sufficient fuel would not be available.

Once you have an engine, you still need to put power to the wheels through what the British call a ‘transmission’ and Americans call a ‘drivetrain’, which also does the steering since almost all tanks turn by making one track spin faster than the other. And you need a track that won't fall apart and a suspension that will stop the occupants falling apart. These were big problems during WW1 – the first tanks had no suspension at all! – and into the 1920s, but by the 1930s you could more or less use the power of the available engines. Even in 1945, however, the Panther and Comet were deliberately speed limited to around 30 mph.

TRADE-OFFS
OK, we have an engine of a given horsepower, and the ability to turn that horsepower into forward motion off-road with an acceptable degree of unreliability and discomfort. What choices do we have to make now?

1. Weight vs. mobility
This is almost self-explanatory, but mobility is more than speed. Very roughly, multiplying the hp/ton ratio by two gives an approximate top road speed in mph, although looking at individual types this correlation is surprisingly loose. The 15 mph of the British infantry tanks was annoyingly slow, the 25 mph of most German tanks seemed good enough, and as mentioned above, anything over 30 mph arguably wore out the running gear and the occupants to little benefit. A high power-weight ratio was also useful to provide rapid acceleration to dash from cover to cover.

 

But weight has other penalties that are less amenable to increased engine power:

• Reliability and maintenance time – pushing around more weight means more parts everywhere from the engine to the suspension will break. (US tracks lasted about 6000km on light tanks but only 2400km on medium tanks. See Exercise Dracula for the effect of maintenance downtime on overall mobility.)
• Fuel consumption – less range for the individual tank, more for the logistics train to haul.
• Bridging – if you can’t cross a bridge without breaking it, you may have to go a much longer long way round.
• Shipping – the M6 heavy tank was not adopted partly because it exceeded the 40-ton limit on many dockyard cranes.

Tanks that were kept in service a long time such as T-34 and Panzers III and IV tended to creep up in weight as bigger guns and frontal armour were added, but for new designs bigger engines and better transmission, steering and track technology (and bridge building) roughly kept pace. Except when they didn't.

2. For a given weight: armour vs. internal volume 
The basic choice is a smaller box with thicker armour or a bigger box with thinner armour. Similarly, sloped armour will give more protection for a given weight, but reduces the internal volume. At the start of the war, most countries tried to armour the front, sides, and even rear to a similar standard, using mostly vertical armour.  As (anti-)tank guns got more powerful, this became impractical, and focus shifted to improving the front armour, both by increasing thickness and sloping (sloping all round reduced the volume of the tank excessively, as in the pyramid shaped early T-34s). 

3a. For a given volume:  guns vs. crew vs. ammo vs. suspension...

Everyone wants a bigger gun, but you need to fit other things in too. For example, a three-man turret crew (commander, gunner, loader) worked better than one or two men, because everyone could focus on one job. But when the British upgunned their Crusader and Valentine tanks from 2-pdr to 6-pdr guns, there was not enough room in the turret for the third man.

Of course, if you run out of ammo, or your crew are bumping into something every time they move, your tank will not fight very well either. (A bigger gun has a double penalty: it reduces the room for other things, including ammo, and also makes each round bigger. The IS-2 looks stunning on paper, but remember that 122mm gun only has 28 rounds and has a slower rate of fire due to its separate loading ammunition.) The Soviets limited the height of their tank crews for this reason. 

If you just want to shoot an enemy soldier or two, the main gun is overkill, so almost all tanks have a ‘coaxial’ machine gun next to it in the turret. Is it worth having a second machine gun in the hull and someone to shoot it? This was nearly universal during the war but fell out of fashion soon afterwards, as bigger guns needed more room for ammunition. (I also imagine most people trying to sneak up on a tank didn't do so from the front.) A few designs even had little secondary MG turrets, a hangover from prewar, but these were quickly abandoned.

Tank suspension is a whole topic of its own. Broadly the choice was between types that allowed more independent wheel movement for a better ride but took up valuable room inside the tank (and were harder to repair in the field) like Christie and torsion bar, and types that gave a worse ride but were completely external (and easier to repair) like VVSS/HVSS, leaf spring and Horstmann. Also, lots of small wheels are better to spread weight evenly and not sink into mud or snow, but fewer bigger wheels are better if you want to drive fast over bumps. The Germans tried to have the best of both worlds with many overlapping large wheels, which was complicated and tended to freeze solid in the Russian winter.

And obviously you need fuel, and a radio or two, a boiler for tea if you're British, compressed air tanks for cold weather starts if you're Russian, and other stuff I haven't mentioned or thought of...

3b. For a given volume: height vs. width
A taller tank is easier for the enemy to spot. A wider tank lets you have a bigger turret ring and therefore a bigger gun. So it seems like a low, wide tank is the ideal. But make your tank too wide and it can't fit on railways – the British were particularly constrained with a narrow railway loading gauge, but even the Germans had to put narrower tracks on their Tigers and Panthers to transport them by rail – or narrow roads and bridges.  Also, height allows more ground clearance to get over obstacles and greater gun depression to shoot at the enemy while hull down.

Finally, height can actually substitute for width to some extent in fitting a bigger gun: a tall hull as in the Sherman allows the turret ring to be extended over the tracks, or a tall turret as in the Challenger (and later Strv 74) allows the gun to recoil (and the crew to squeeze in) above the turret ring. 

Length tends to be the residual in the equation, within limits - too long and you can't steer, too short and the crew gets motion sickness. The Sherman was stretched as required to go from short radial to longer inline engines. Similarly the Challenger was basically a stretched Cromwell.

4. For a given sized turret/gun: AP vs. HE
Tanks sometimes shoot at enemy tanks, but mostly at other, softer things. If you need to punch through armour with AP rounds, you want a high velocity gun (penetration increases with roughly the square of the velocity, but only linearly in calibre). If you want to blow things up, it's all about calibre (HE capacity increases with the cube of calibre, or even a bit more when you consider the minimum size of the fuze and thickness of the shell wall). So for a gun that will fit in a given sized turret, you can have a smaller calibre high velocity hole puncher or a larger calibre, low velocity HE lobber. While you can build specialised guns for each job (and even different tanks to put them in, as the Germans did, which is going a bit far), it's better to have one kind of gun that can do both reasonably well in most of your tanks, since you never know what they will run into.

Conveniently, while tank armour increased throughout the war, common building materials and the human body stayed the same. Therefore, while AP rounds needed ever greater performance, HE didn't. Around 3 inch calibre, with increasing velocity as the war went on, proved a good compromise. The Americans and British both picked the medium velocity 75mm over the high velocity 6-pdr, as did the Soviets with the 76.2mm over their own 57mm AT gun. The higher velocity 76mm, 17-pdr and 77mm then gave the needed AP upgrade while the Soviets went for 85mm (probably because they already had the AA gun rather than any ideal calibre calculation). The Germans also ended up with high velocity 75mm guns on most of their late war tanks (except the 88s on the Tigers, again copied from the AA calibre).

5. For a given budget: quality vs. quantity
Obviously, a bigger tank uses more steel and other resources, and fancy gadgets like better radios, optics and steering systems have a cost. The Tiger was hugely expensive compared to the German mediums (and, more speculatively, other countries' tanks). The Panther was surprisingly cheap for its size, but partly by skimping on the final drive, which crueled its reliability. 

This trade-off applies to distribution as well as production. If you need to move your tanks across the ocean like the Americans, or even by rail across the steppe like the Germans and Soviets, a bigger and better tank at the factory gate meant fewer delivered to the battlefield for the same freight tonnage. So we are back where we started with weight vs. mobility, except in terms of numbers rather than the individual tank's capability.

[hobbes154]

Some random opinions based on the above

1. By the simple firepower/armour/speed trinity, the outstanding tanks of WW2 were, chronologically, the SOMUA S35, T-34, and Panther. But they all lost their first, and except for the T-34, basically all of their battles. Of course there were many reasons for that, but I would argue they had flaws as tanks that meant they were not the best of their time. While the first two often lacked radios, which is a huge weakness, they could carry them, so you can't really call that a tradeoff except in the budgetary sense. Similarly with the poor visibility out of early T-34 turrets - that was just bad design and quickly corrected at little cost. Their 1- and 2-man turrets, however, were a tradeoff with tangible benefits - better armour at a given weight. It just turned out to be a bad tradeoff. The Panther's reliability problems, caused by a combination of weight and penny pinching, I have mentioned already.

2. I find horsepower to be an interesting way to compare different tanks, rather than the more usual classification by weight. E.g.

• Of course the T-34/KV and then Tiger/Panther were outstanding. They were working with far more horsepower than everyone else at the time.
• The SOMUA had to be weak in some way with only 200 hp. Similarly the M26's 500 hp is not a lot for a late war heavy tank.
• Although British tanks are often criticised, the Valentine made outstanding use of 135 hp (less than the Pz II!). OTOH the entire cruiser line was pretty wasteful considering what the Pz IV, Sherman and Centurion did with comparable power.

3. The Germans started the war with the best balanced, but still undergunned, tanks. The trauma of meeting the heavier Char B, Matilda, T-34 and KV-1 led to the emphasis on gun power and armour in the big cats, which in turn traumatised the British leading to the postwar Centurion, Conqueror and Chieftain. Ironically the Germans realised they had gone too far and went back to mobility with the Leopard.

4. Leaving aside the engine question, could better use have been made of the available horsepower early war? Both the British and Germans had already worked out the need for 3 man turrets and radio. But the British divided a pitifully small development budget between too slow infantry tanks and pointlessly fast, thin skinned cruiser tanks (both with needlessly small turret rings even given their restrictive loading gauge), and as a result started the war with mostly even worse light tanks. Meanwhile the Germans built not one but two medium tanks with nice big turret rings but stuck peashooters in them until experience taught otherwise. (Hitler's infatuation with monstrosities like the Maus and Ratte is rightly mocked, but it had a perfectly reasonable beginning in trying to get the Panzer III upgunned.) All the pieces for a 20 tonner with 300 hp and medium velocity 75mm/3 inch were there, but no-one put them together. 

The interesting contrast is, I wouldn't say the same about the fighter aircraft of 1940 even with all the hindsight in the world. I think they were about as good as you could hope for.

5. No matter what tradeoffs you pick, someone will complain about it. Here is a German commander complaining about the Panther relative to the Sherman in Normandy! 

[Domus Acipenseris]

 

"The interesting contrast is, I wouldn't say the same about the fighter aircraft of 1940 even with all the hindsight in the world. I think they were about as good as you could hope for."

 

I've wondered about this.  How many good fighters are there vs how many good tanks?  It seems as if to make a good fighter required mainly a good wing and a good engine.  The good tank required so much more.  Is it that standards for rating fighters are lower, is it easier to design a fighter than a tank or is it the relative quality of the designers?

[holoween]

18 hours ago, hobbes154 said:

Some random opinions based on the above

1. By the simple firepower/armour/speed trinity, the outstanding tanks of WW2 were, chronologically, the SOMUA S35, T-34, and Panther. But they all lost their first, and except for the T-34, basically all of their battles. Of course there were many reasons for that, but I would argue they had flaws as tanks that meant they were not the best of their time. While the first two often lacked radios, which is a huge weakness, they could carry them, so you can't really call that a tradeoff except in the budgetary sense. Similarly with the poor visibility out of early T-34 turrets - that was just bad design and quickly corrected at little cost. Their 1- and 2-man turrets, however, were a tradeoff with tangible benefits - better armour at a given weight. It just turned out to be a bad tradeoff. The Panther's reliability problems, caused by a combination of weight and penny pinching, I have mentioned already.

 That progression only holds up with loads of specifications. If you allow heavy tanks in it they completely dominate it. KW1, Tiger, IS2 have not much worse hp/ton than contemporary mediums but completely outclass them in armour and firepower.

And even if we look at the mediums their loosing runs are consistently when outnumbered operationally, under enemy air superiority and with generally worse trained crews and units.

 

18 hours ago, hobbes154 said:

2. I find horsepower to be an interesting way to compare different tanks, rather than the more usual classification by weight. E.g.

• Of course the T-34/KV and then Tiger/Panther were outstanding. They were working with far more horsepower than everyone else at the time.
• The SOMUA had to be weak in some way with only 200 hp. Similarly the M26's 500 hp is not a lot for a late war heavy tank.
• Although British tanks are often criticised, the Valentine made outstanding use of 135 hp (less than the Pz II!). OTOH the entire cruiser line was pretty wasteful considering what the Pz IV, Sherman and Centurion did with comparable power.

Well similar to aircraft having a better engine makes a better tank

 

18 hours ago, hobbes154 said:

3. The Germans started the war with the best balanced, but still undergunned, tanks. The trauma of meeting the heavier Char B, Matilda, T-34 and KV-1 led to the emphasis on gun power and armour in the big cats, which in turn traumatised the British leading to the postwar Centurion, Conqueror and Chieftain. Ironically the Germans realised they had gone too far and went back to mobility with the Leopard.

For the time they were designed they had good guns. But they got introdiced at the start of an arms race that send medium tanks from around 15 tons to 45 tons in 10 years.

 

18 hours ago, hobbes154 said:

4. Leaving aside the engine question, could better use have been made of the available horsepower early war? Both the British and Germans had already worked out the need for 3 man turrets and radio. But the British divided a pitifully small development budget between too slow infantry tanks and pointlessly fast, thin skinned cruiser tanks (both with needlessly small turret rings even given their restrictive loading gauge), and as a result started the war with mostly even worse light tanks. Meanwhile the Germans built not one but two medium tanks with nice big turret rings but stuck peashooters in them until experience taught otherwise. (Hitler's infatuation with monstrosities like the Maus and Ratte is rightly mocked, but it had a perfectly reasonable beginning in trying to get the Panzer III upgunned.) All the pieces for a 20 tonner with 300 hp and medium velocity 75mm/3 inch were there, but no-one put them together. 

The only tank reaching your 20tons, 300hp, med velocity 75 is the pz4f2 and it was at that point at its practical weight limit (though circumstance forced it to keep growing)

 

18 hours ago, hobbes154 said:

The interesting contrast is, I wouldn't say the same about the fighter aircraft of 1940 even with all the hindsight in the world. I think they were about as good as you could hope for.

 

Only with hindsight on where the development would go could you do much better. There are some details that could be improved but the vast majority of design decisions that we would maake different today were made different as a result of a different technological level

 

18 hours ago, hobbes154 said:

5. No matter what tradeoffs you pick, someone will complain about it. Here is a German commander complaining about the Panther relative to the Sherman in Normandy! 

 

Well that commander is right though. The sherman is better suited for bocage country and the panther isnt exactly reliable.

[domobran7]

On 1/15/2022 at 5:48 AM, hobbes154 said:

2. I find horsepower to be an interesting way to compare different tanks, rather than the more usual classification by weight. E.g.

•  

I think better measure is horsepower per ton. When it comes to overcoming obstacles especially, engine has to fight against tank's own weight. By that measure, tank power is:

 

T-34 M1941: 18,9 hp/t

T-34/85: 15,6 hp/t

Panther: 13,8 hp/t

Tiger I: 12,8 hp/t

Panzer IV Ausf.H: 11,8 hp/t

Sherman M4A6: 11,8 hp/t

 

Weight however is still important. Heavier tank will usually have better firepower and armor, but worse mobility: even using more powerful engine and wider tracks does not remove the problems that heavy weight can create in certain terrains and when crossing bridges.

 

[hobbes154]

On 1/16/2022 at 3:22 AM, Domus Acipenseris said:

It seems as if to make a good fighter required mainly a good wing and a good engine.  The good tank required so much more.  Is it that standards for rating fighters are lower, is it easier to design a fighter than a tank or is it the relative quality of the designers?

Not sure about that, both needed good guns and armour (though in different ways, I wouldn't say the aircraft ones were easier just because they were smaller/lighter), and propellers are roughly analagous to transmission. Plus wings and engine are hugely complicated, there's no challenge for a tank as complex as aerodynamics, and tank engines are often derated or obsolescent aircraft engines. I think the difference is just they put much more effort into aircraft than tanks, partly because there was more of a civilian market, but mainly because the military budgets were much bigger.

[hobbes154]

18 hours ago, pukovnik7 said:

I think better measure is horsepower per ton.

That is a better predictor of (some aspects of) performance, but if you are thinking about design tradeoffs, I think it is interesting to think of a total horsepower "budget" and how to allocate that between tons and hp/ton.

 

"even using more powerful engine and wider tracks does not remove the problems that heavy weight can create in certain terrains and when crossing bridges."

Yes exactly.

[hobbes154]

On 1/16/2022 at 11:27 AM, holoween said:

If you allow heavy tanks in it they completely dominate it. KW1, Tiger, IS2 have not much worse hp/ton than contemporary mediums but completely outclass them in armour and firepower.

Depends what you mean by "not much worse": KV and IS vs T-34 is same engine and roughly 50 or 45 tons vs 30. Similarly Panther vs Tiger I/II is same engine and 45 vs 55 or 70 tons. But supposing you think that trade is worth it, I'd say it proves my point even more, which is that top speed is a very poor proxy for operational mobility, so the standard trinity is a really poor way of evaluating tanks from this period.

 

On 1/16/2022 at 11:27 AM, holoween said:

For the time they were designed they had good guns. But they got introdiced at the start of an arms race that send medium tanks from around 15 tons to 45 tons in 10 years.

The German 37mm was significantly inferior to the British 2-pdr, French 47mm or Soviet 45mm. But the "start of an arms race" thing is exactly what I am getting at. It shouldn't have required a genius to predict an arms race in guns and armour, and plenty of tanks at the time had the horsepower and weight to carry much more powerful guns than they actually did. In the Germans' case they even had the turret ring to fit them. Admittedly the Kwk 40 was an impressive bit of work but something closer to the Allied 75mm would have been easy.

 

On 1/16/2022 at 11:27 AM, holoween said:

The only tank reaching your 20tons, 300hp, med velocity 75 is the pz4f2

Hence my "noone put them together" comment.

 

On 1/16/2022 at 11:27 AM, holoween said:

There are some details that could be improved but the vast majority of design decisions that we would maake different today were made different as a result of a different technological level

Aircraft, yes. Tanks, no. Britain and Germany had both worked out 3 man turrets. Fitting the biggest practical gun firing both AP and HE should have been a no-brainer. Focusing on (sloped) front armour is a bit more hindsight but doesn't need new technology.

 

"The sherman is better suited for bocage country and the panther isnt exactly reliable."

Sure, I just find it amusing because it is the opposite of the usual stereotype (Panther awesome! Sherman crap!).

[Toxn]

On 1/15/2022 at 6:22 PM, Domus Acipenseris said:

 

"The interesting contrast is, I wouldn't say the same about the fighter aircraft of 1940 even with all the hindsight in the world. I think they were about as good as you could hope for."

 

I've wondered about this.  How many good fighters are there vs how many good tanks?  It seems as if to make a good fighter required mainly a good wing and a good engine.  The good tank required so much more.  Is it that standards for rating fighters are lower, is it easier to design a fighter than a tank or is it the relative quality of the designers?

There were many more fighter programs than tank programs, many of them producing dogs that never went into service. Of the ones that went into service, most were a disappointment in some way. Of the few that weren't, only one or two were outstanding. This gives you a good idea of the numbers involved: around 240 types used or tested, including foreign types, trainers, utility aircraft etc. Of those, maybe half were used in any great numbers in service. Of that 100-ish aircraft, perhaps two dozen rose above the level of mediocre. And of that two dozen, a handful are considered superlative in their class.

 

Aircraft design is very fiddly, and requires a mix of easily-ascertained factors (power-to-weight ratio, wing loading, armament etc.), hard-to-ascertain factors (top speed, turn times in various configurations, landing speeds) and factors which defied empirical modelling and could only be found by experiment (stability, stall characteristics, maintenance and service niggles, random engine/landing gear/aerodynamic bugs etc).

 

Making a good aircraft in WW2 was as much alchemy as science, and resulted in a lot of dead test pilots. Tanks were actually comparatively easier to design, and accordingly got designed by lesser talents on lower budgets (see, again, the example of British tank building in WW2, which was the product of a bare handful of second-tier engineers). Even today, the best mechanical engineers are mostly doing aviation and aerospace. 

 

 

[domobran7]

On 1/17/2022 at 8:18 AM, hobbes154 said:

Sure, I just find it amusing because it is the opposite of the usual stereotype (Panther awesome! Sherman crap!).

Isn't that kinda true, but mostly because of the situation they found themselves in? Shermans in Normandy were forced to do the duty of heavy tanks (breakthrough) despite being designed as mobile medium tanks. Meanwhile, Panthers were in a defensive situation where mobility wasn't that important, meaning that they could capitalize on Panther's good aspects (excellent gun, good frontal armor) while not being that much held back by Panther's lack of reliability and disadvantages it did have.

 

Plus, I think early Shermans had some issues with ammunition stowage and tank bursting into flames when hit, which were solved later, but created a lasting perception of M4s being only good at roasting their own crews. But it's been a while since I've read about it, so don't take me at my word.

On 1/17/2022 at 7:57 AM, hobbes154 said:

That is a better predictor of (some aspects of) performance, but if you are thinking about design tradeoffs, I think it is interesting to think of a total horsepower "budget" and how to allocate that between tons and hp/ton.

 

Agreed. Although hp/ton is still important because it tells you how much weight you can have with a given engine, or else how powerful of an engine you need for a given weight.

 

[Sturgeon]

On 1/14/2022 at 11:40 PM, hobbes154 said:

Inspired by Collimatrix's excellent topic in the Aviation section, my attempt at doing the same for tanks. 

Tank design is often represented as a trade-off between firepower, armour, and speed. But this ignores many other, equally important variables. Furthermore, this formula doesn't explain why the trade-offs are there in the first place. So here is my attempt to make things a bit more complicated.

The Constraint: Compact, powerful and reliable engines – and the ability to use them
Firepower and armour are the obvious features that made a 1940 tank obsolete in 1945. Yet even a 1945 tank is trivial compared to a WWI battleship. Or more modestly, the German 88mm, Soviet 85mm and US 90mm AA guns were prewar designs. Why not put them on a tank from the beginning? Were people just stupid back then?

Up to a point, I would say ‘yes’. While it is unrealistic to think of 1945 combat aircraft, radar, or nuclear weapons in 1940 as the result of anything short of time travel, 1940 tanks could have been significantly better without anachronistic scientific or engineering breakthroughs. Like the assault rifle, this really was a case where the right people just didn't see the need or spend the money.

 

However, it's not the only reason. A tank with a big gun and thick armour that can't move is just a pillbox. Like aircraft, WW2 tanks were fundamentally limited by their engine power. This is less obvious because additional power was typically used to ‘buy’ weight rather than speed. Yet the trend is clear. Shown below are the improvements in engine power for the German and British (cruiser) tank lines, which were in the war the longest.

German Panzers

I	100 hp
II	138 hp
III, IV	250-300 hp
Tiger, Panther	690 hp

British Cruisers

I, II	150 hp
III, IV, Crusader	340 hp (Liberty)
Cromwell, Comet	600 hp (Meteor)

 

Arguably, more powerful tank engines could and should have been introduced much earlier (the Liberty was first used in a tank as early as 1918).  I will leave that aside, noting only that, relative to aircraft engines, tank engines were forced to use lower octane fuel for economic reasons (preventing tank use of the Napier Lion), and are harder to cool due to being inside a slow moving armoured box (this was a particular challenge with the Merlin's conversion to the Meteor). 

 

There's also the choice of diesel (compression ignition) vs. petrol/gasoline (spark ignition) engines. Diesels had higher torque and lower fuel consumption, but lower specific power, were heavier and cost more, and meant an extra type of fuel in your logistics train. Both Germany and the US decided against diesels for fear sufficient fuel would not be available.

Once you have an engine, you still need to put power to the wheels through what the British call a ‘transmission’ and Americans call a ‘drivetrain’, which also does the steering since almost all tanks turn by making one track spin faster than the other. And you need a track that won't fall apart and a suspension that will stop the occupants falling apart. These were big problems during WW1 – the first tanks had no suspension at all! – and into the 1920s, but by the 1930s you could more or less use the power of the available engines. Even in 1945, however, the Panther and Comet were deliberately speed limited to around 30 mph.

TRADE-OFFS
OK, we have an engine of a given horsepower, and the ability to turn that horsepower into forward motion off-road with an acceptable degree of unreliability and discomfort. What choices do we have to make now?

1. Weight vs. mobility
This is almost self-explanatory, but mobility is more than speed. Very roughly, multiplying the hp/ton ratio by two gives an approximate top road speed in mph, although looking at individual types this correlation is surprisingly loose. The 15 mph of the British infantry tanks was annoyingly slow, the 25 mph of most German tanks seemed good enough, and as mentioned above, anything over 30 mph arguably wore out the running gear and the occupants to little benefit. A high power-weight ratio was also useful to provide rapid acceleration to dash from cover to cover.

 

But weight has other penalties that are less amenable to increased engine power:

• Reliability and maintenance time – pushing around more weight means more parts everywhere from the engine to the suspension will break. (US tracks lasted about 6000km on light tanks but only 2400km on medium tanks. See Exercise Dracula for the effect of maintenance downtime on overall mobility.)
• Fuel consumption – less range for the individual tank, more for the logistics train to haul.
• Bridging – if you can’t cross a bridge without breaking it, you may have to go a much longer long way round.
• Shipping – the M6 heavy tank was not adopted partly because it exceeded the 40-ton limit on many dockyard cranes.

Tanks that were kept in service a long time such as T-34 and Panzers III and IV tended to creep up in weight as bigger guns and frontal armour were added, but for new designs bigger engines and better transmission, steering and track technology (and bridge building) roughly kept pace. Except when they didn't.

2. For a given weight: armour vs. internal volume 
The basic choice is a smaller box with thicker armour or a bigger box with thinner armour. Similarly, sloped armour will give more protection for a given weight, but reduces the internal volume. At the start of the war, most countries tried to armour the front, sides, and even rear to a similar standard, using mostly vertical armour.  As (anti-)tank guns got more powerful, this became impractical, and focus shifted to improving the front armour, both by increasing thickness and sloping (sloping all round reduced the volume of the tank excessively, as in the pyramid shaped early T-34s). 

3a. For a given volume:  guns vs. crew vs. ammo vs. suspension...

Everyone wants a bigger gun, but you need to fit other things in too. For example, a three-man turret crew (commander, gunner, loader) worked better than one or two men, because everyone could focus on one job. But when the British upgunned their Crusader and Valentine tanks from 2-pdr to 6-pdr guns, there was not enough room in the turret for the third man.

Of course, if you run out of ammo, or your crew are bumping into something every time they move, your tank will not fight very well either. (A bigger gun has a double penalty: it reduces the room for other things, including ammo, and also makes each round bigger. The IS-2 looks stunning on paper, but remember that 122mm gun only has 28 rounds and has a slower rate of fire due to its separate loading ammunition.) The Soviets limited the height of their tank crews for this reason. 

If you just want to shoot an enemy soldier or two, the main gun is overkill, so almost all tanks have a ‘coaxial’ machine gun next to it in the turret. Is it worth having a second machine gun in the hull and someone to shoot it? This was nearly universal during the war but fell out of fashion soon afterwards, as bigger guns needed more room for ammunition. (I also imagine most people trying to sneak up on a tank didn't do so from the front.) A few designs even had little secondary MG turrets, a hangover from prewar, but these were quickly abandoned.

Tank suspension is a whole topic of its own. Broadly the choice was between types that allowed more independent wheel movement for a better ride but took up valuable room inside the tank (and were harder to repair in the field) like Christie and torsion bar, and types that gave a worse ride but were completely external (and easier to repair) like VVSS/HVSS, leaf spring and Horstmann. Also, lots of small wheels are better to spread weight evenly and not sink into mud or snow, but fewer bigger wheels are better if you want to drive fast over bumps. The Germans tried to have the best of both worlds with many overlapping large wheels, which was complicated and tended to freeze solid in the Russian winter.

And obviously you need fuel, and a radio or two, a boiler for tea if you're British, compressed air tanks for cold weather starts if you're Russian, and other stuff I haven't mentioned or thought of...

3b. For a given volume: height vs. width
A taller tank is easier for the enemy to spot. A wider tank lets you have a bigger turret ring and therefore a bigger gun. So it seems like a low, wide tank is the ideal. But make your tank too wide and it can't fit on railways – the British were particularly constrained with a narrow railway loading gauge, but even the Germans had to put narrower tracks on their Tigers and Panthers to transport them by rail – or narrow roads and bridges.  Also, height allows more ground clearance to get over obstacles and greater gun depression to shoot at the enemy while hull down.

Finally, height can actually substitute for width to some extent in fitting a bigger gun: a tall hull as in the Sherman allows the turret ring to be extended over the tracks, or a tall turret as in the Challenger (and later Strv 74) allows the gun to recoil (and the crew to squeeze in) above the turret ring. 

Length tends to be the residual in the equation, within limits - too long and you can't steer, too short and the crew gets motion sickness. The Sherman was stretched as required to go from short radial to longer inline engines. Similarly the Challenger was basically a stretched Cromwell.

4. For a given sized turret/gun: AP vs. HE
Tanks sometimes shoot at enemy tanks, but mostly at other, softer things. If you need to punch through armour with AP rounds, you want a high velocity gun (penetration increases with roughly the square of the velocity, but only linearly in calibre). If you want to blow things up, it's all about calibre (HE capacity increases with the cube of calibre, or even a bit more when you consider the minimum size of the fuze and thickness of the shell wall). So for a gun that will fit in a given sized turret, you can have a smaller calibre high velocity hole puncher or a larger calibre, low velocity HE lobber. While you can build specialised guns for each job (and even different tanks to put them in, as the Germans did, which is going a bit far), it's better to have one kind of gun that can do both reasonably well in most of your tanks, since you never know what they will run into.

Conveniently, while tank armour increased throughout the war, common building materials and the human body stayed the same. Therefore, while AP rounds needed ever greater performance, HE didn't. Around 3 inch calibre, with increasing velocity as the war went on, proved a good compromise. The Americans and British both picked the medium velocity 75mm over the high velocity 6-pdr, as did the Soviets with the 76.2mm over their own 57mm AT gun. The higher velocity 76mm, 17-pdr and 77mm then gave the needed AP upgrade while the Soviets went for 85mm (probably because they already had the AA gun rather than any ideal calibre calculation). The Germans also ended up with high velocity 75mm guns on most of their late war tanks (except the 88s on the Tigers, again copied from the AA calibre).

5. For a given budget: quality vs. quantity
Obviously, a bigger tank uses more steel and other resources, and fancy gadgets like better radios, optics and steering systems have a cost. The Tiger was hugely expensive compared to the German mediums (and, more speculatively, other countries' tanks). The Panther was surprisingly cheap for its size, but partly by skimping on the final drive, which crueled its reliability. 

This trade-off applies to distribution as well as production. If you need to move your tanks across the ocean like the Americans, or even by rail across the steppe like the Germans and Soviets, a bigger and better tank at the factory gate meant fewer delivered to the battlefield for the same freight tonnage. So we are back where we started with weight vs. mobility, except in terms of numbers rather than the individual tank's capability.

 

The various design competitions and the judgings thereof are a good resource for seeing how to approach tank design (especially in a fictional context).

Something to think about is that the tank that won the "WWII-era" (Cascadia) competition ended up a lot like a T-55. In theory you could have been making T-55s from 1939 on. They didn't, because actual chronological development doesn't work that way. In 1939, what is a T-55 designed to kill?

[hobbes154]

3 hours ago, pukovnik7 said:

I think early Shermans had some issues with ammunition stowage and tank bursting into flames when hit, which were solved later

My understanding is basically every tank had that problem once it was penetrated - might be better to say the later wet stowage Shermans were unusually safe.

 

I can't find a definitive source but see Table VIII here and the discussion here.

 

Edit: also this https://www.tankarchives.ca/2016/03/tank-crew-losses.html

Edited January 19, 2022 by hobbes154
Added link

[hobbes154]

21 minutes ago, Sturgeon said:

In 1939, what is a T-55 designed to kill?

Who was suggesting a T-55? In 1940, the Germans were unable to penetrate the Char B, Matilda, SOMUA and even struggling with the more common Hotchkiss and Renaults. 

 

(And I'm not sure the more boring stuff like tracks, gearboxes etc. were ready in 1939 even if the engine and gun were.)

[Sturgeon]

19 minutes ago, hobbes154 said:

Who was suggesting a T-55? In 1940, the Germans were unable to penetrate the Char B, Matilda, SOMUA and even struggling with the more common Hotchkiss and Renaults. 

 

(And I'm not sure the more boring stuff like tracks, gearboxes etc. were ready in 1939 even if the engine and gun were.)

Re-read my post again, slowly this time.

[Sturgeon]

Also @hobbes154 this forum works a little bit differently, you're expected to pay your dues by posting primary source documents before anyone will accept you as an authority with the right to make big effort posts like that.

[Toxn]

So, Sherman vs Panther is a topic that has been chewed over on this forum until only gristle remains. I accordingly have very little to add except to urge the newer members to dig into some of our older threads.

 

In terms of chronological progression vs what hindsight tells us - as @Sturgeon has stated, a T-44/T-54 was entirely within the state of the art in 1939. If aircraft seem to have more quickly arrived at a local optimum, it's partly a function of more resources being poured into them than tanks*, partly a function of the relative utility of outdated models^, and partly a function of different operational and strategic tradeoffs.

 

Tanks are rigidly constrained by fuel supply lines, bridge sizing, tunnel width and train gauges. The result is that you want to get along with the smallest, lightest, most mobile vehicle you can until such time as it isn't tenable any more. With aircraft, the major limitation of runways only kicks in at the very frontline, and accordingly puts hard constraints only on shorter-ranged types such as interceptors and tactical support aircraft. Even then, this mostly bites around the point where jet aircraft become common and landing speeds start to balloon.

 

 

*Resources put into tank vs. aircraft production in WW2 are uniformly almost impossible to directly quantify given wildly fluctuating budgets, the different strategic resources needed by each, the inaccuracies of stated prices, and the fact that all the services kept their own accounts. On the Nazi side of things, wild swings in allocation were frequent but the luftwaffe nearly always ended up with the lion's share of resources (especially scarce resources such as aluminium). As for the Army, only around 20% of their budget went into tanks. The production figures of all combatant nations reflect this: around two aircraft were produced for every tank.

 

 

^An outdated tank can still provide valuable frontline service, while an outdated fighter or bomber is dead weight.