The Universal Tank

My only question after reading the wiki is - if its an 84mm gun sleeved down to take a 76mm shell, arent the breech/trunnion dimensions going to be rather hefty as in, equivalent to the 25lb? I know it gives you 20% over the trusty soixante-quinze, but the US seem to have got even the short barreled m2 up to 588m/s with 6.3kg shot so perhaps it is worth thinking about a cheap-as-chips 75mm, maybe bored out to 3in for better ammo compatability although Vickers were making 75mm shells for their AA gun.

75x350R would certainly be a winner on the export market if one were to visualise a "Vickers Commercial Heavy"...
 
My only concern is the recoil is 24" - but given that the subsequent AAA guns had half that recoil I'm going to assume that 'that' later tech can be applied to the lighter gun and the recoil being nearer 12"

Compare the recoil travel of the original French 75 to the later 1941 M3 75mm to the final 1944 M6 75mm used in the Chaffee.

Travel was about halved at each step
 
In 1938, the War Office had issued a requirement for a new, better armoured "heavy" cruiser tank. From this was developed the Covenantor. The "30 mm armoured standard" referred to any vertical plate having to be 30 mm thick. Angled surfaces (through the principles of sloped armour) could be thinner, provided they were at least as effective as a 30 mm thick vertical plate. The turret was polygonal—with sides that sloped out then in again—to give maximum space on the limited turret ring diameter. In Australia design work began in November 1940, the AC1 with a turret built closely along the lines of a British Crusader with a QF 6 pounder. This turret was cast and 65mm thick all round. Would this turret fit our Valentine based universal tank?
 
Would there be any benefit in installing the Bristol Mercury or A/S Cheetah on the 'universal tank'?
Radial engines did prove effective on US tanks, but they increased the height of the tank, and the bottom cylinders were constantly exposed to fouling because of where they were mounted (though that problem was overcome). The US later abandoned this route when such a tall tank made it difficult to armor.

It seems the UK decided to go with separate tanks after the A6 and A7 tanks. If the British hadn't made that decision here they probably would have developed decent Universal Tanks by the beginning of WWII. To make a decision that Universal Tanks were viable in the 1920's, they would have to have a decent all-purpose gun and a decent engine, instead of their OTL Armstrong Siddeley and Liberty engines. A decent gun such as that described here would be much better than the OTL 3-pounder and 2-pounder, which would obviate the need for a separate infantry support tank with a howitzer.

For engines, there are several options to choose from, but an inline engine is preferred due to its compactness and low height. The engines that could produce enough power for the tanks of the day would be the Rolls-Royce Kestrel and the Napier Lion. If more power is still required, the Rolls-Royce Buzzard (the engine developed into the famous Rolls-Royce R) could also be used. I would prefer the Napier Lion, as it is older and thus surplus engines are likely to be common. Without a supercharger (which a tank engine is likely to lack), it produces about the same horsepower as the Kestrel because it has a similar engine displacement, and weighs about the same. It allows a shorter engine compartment than a Kestrel because it only has 4 cylinder banks, though the engine would be wider than a similar V-12.
 
Radial engines did prove effective on US tanks, but they increased the height of the tank, and the bottom cylinders were constantly exposed to fouling because of where they were mounted (though that problem was overcome). The US later abandoned this route when such a tall tank made it difficult to armor

Radials could have been run at an angle, as shown by the S-58
1721534.jpg


Or in horizontal 'Pancake' Mode
radial2.jpg
 
On second thought, the Napier Lion will probably need some additional modifications to work well in a tank. This source states that it was tested for the Cruiser Mk III, and the engine was prone to severe detonation when running at full throttle at low speeds with Grade III 65 octane fuel. This was despite lowering its compression ratio to be less than that of its 2 competitors (the Liberty L-12 and Thornycroft RY/12), which did not have this problem. The other engines had both smaller and larger bores, and both smaller and larger displacements than the modified Lion, so that shouldn't have been the cause of its problems. Both engines had 2 valves per cylinder compared to the Napier Lion's 4, but this doesn't seem to have much to do with detonation or compression ratio, so it probably wasn't the cause of detonation. This leaves just the stroke- 7 inches for the other engines and 5 1/8 inches for the Lion. This meant the competing engines were long-stroke (their stroke was longer than their bore diameter) while the Lion was short-stroke (its bore diameter was longer than its stroke). As little as I understand this, this is the only design feature I can think of that would cause the detonation problems.

To fix this problem, a tank engine may have to be based on an earlier version of the Lion, as the variant that was tested was the Lion XIA, the last major variant built for commercial use (as opposed to racing use). Since the Lion was designed in 1917 before the advent of octane rating fuels, earlier versions, designed to run on low-octane WWI fuels, may adapt better to Grade III fuel as it is more similar to the fuel they were designed for. Another option would be to modify the engine to increase its stroke in the hopes that this can cure its problems with low-octane fuel. Finally, the spark plugs and carburetors should be changed to more suitable models for tank use in any case, as was done with the Nuffield Liberty OTL after it was selected.

Radials could have been run at an angle, as shown by the S-58

Or in horizontal 'Pancake' Mode
That might work, just remember that radials, being air-cooled, will require more space for airflow and cooling fans than their physical size would suggest, and that angled or horizontal layouts will require more complex transmissions to connect the engine shaft to the drive sprockets.
 
Radial engines did prove effective on US tanks, but they increased the height of the tank, and the bottom cylinders were constantly exposed to fouling because of where they were mounted (though that problem was overcome). The US later abandoned this route when such a tall tank made it difficult to armor.
...

There was the Sherman Jumbo, with increased armor protection. Hence - even if it was tall (it was), it was possible to up-armor the Sherman.
The height of Sherman tank was due to not having the intermediate gear, that lowers the height of driveshaft and thus the hull, like it was done on the M18 Hellcat. The fouling of bottom cylinders was not an issue.

But still, UK have had enough of en-bloc or 'composite' V12 engines that should be better choice for a tank.
 
Just to throw another item into the fray. Armoured Regiments had their own supply line (thus they had no problem with 7.92 BESA ammunition). If their fuel could also be 'armour dedicated' then one could look to updating Lions, Kestrels or whatever is the universal tank engine by issuing them with aviation 100 octane fuel which would allow extra power and torque via increased compression ratios and/or supercharging. IOTL the Meteor went from @600bhp on pool petrol to nearly 850bhp on 1960's fuel and assorted other developments. Supercharging comes with induction heating issues but water/air intercoolers and/or water injection were well within the period technology. Before anyone looks to Merlin centrifugal compressors they live in a very different environment and some sort of Roots would be a better tank fit where low speed torque is more of a tactical limit than top end power. Despite the Battle of the Atlantic etc. Britain was awash (I exaggerate slightly)with 87 and 100 octane petrol.
 
The Americans used supercharged radial engines in their tanks, variants of the Wright Whirlwind in Lee/Grant/Sherman, and Cyclone in the M6 heavy tank. So - nothing prevents British doing the same, both for liquid cooled and radials. Just use 'medium supercharging', ie. choose such S/C gearing that it won't push too much of manifold pressure, while providing a substantial increase in power. We could get Kestrel probably at close to 500 HP even with 87 oct fuel, after all the low level Kestrels were doing 620 HP even with 77 oct fuel, and 730 HP on 87 oct. In other words, no need for 100 oct fuel. Also ther is no need for intercoolers and/or water injection for +2.625 psig manifold pressure that 87 oct fuel allowed.
Note about compression ratio - in supercharged engines, lower CR is better than higher CR.
 
With pressure charged engines read effective compression ration which is dependent upon the mechanical one and the intake pressure.
 
The effective compression ratio (ECR) is the mechanical compression ratio (CR) plus the pressure of the blown intake thus a CR of 6:1 becomes an ECR of 9:1 with an over pressure of 1.5. Having been involved in pressure charging for motor racing I am aware that the true effective compression ratio is far more complex than that. The mechanical compression ratio is a simple volumetric measure of the cylinder and chamber and takes no account of the pumping efficiency etc. of even a naturally aspirated motor.
 
Despite the Battle of the Atlantic etc. Britain was awash (I exaggerate slightly)with 87 and 100 octane petrol.

Even during the Battle of Britain there was about 150,000 tons of 100 octane, 350,000 tons of 87 octane and 300,000 tons of Pool petrol in storage. The RAF was using about 2,000 tons per week of all types of petrol.
 
On second thought, the Napier Lion will probably need some additional modifications to work well in a tank. This source states that it was tested for the Cruiser Mk III, and the engine was prone to severe detonation when running at full throttle at low speeds with Grade III 65 octane fuel.
The fix is to use slightly higher octane fuel than Pool Petrol, as the US and Soviets did for for using aero engines in tanks.

The real fix for the Lion was ignition timing, easier done with the Liberty than the Lion, as the Lion used a standard aero magneto rather than battery/points that most versions the Liberty used

That might work, just remember that radials, being air-cooled, will require more space for airflow and cooling fans than their physical size would suggest, and that angled or horizontal layouts will require more complex transmissions to connect the engine shaft to the drive sprockets.

Not much different than requiring radiators and the ducting for that. Offset gearing at other than 90 degrees isn't really an issue.
 
The effective compression ratio (ECR) is the mechanical compression ratio (CR) plus the pressure of the blown intake thus a CR of 6:1 becomes an ECR of 9:1 with an over pressure of 1.5. Having been involved in pressure charging for motor racing I am aware that the true effective compression ratio is far more complex than that. The mechanical compression ratio is a simple volumetric measure of the cylinder and chamber and takes no account of the pumping efficiency etc. of even a naturally aspirated motor.

Not as in the bolded part, but rather 'multiplied by the pressure of the blown intake?
An any rate - I was talking about the plain vanilla compression ratio, or 'static compression ratio', as I've just learned :)

With the supercharged Kestrel, we basically have the tank engine capable for 600+ HP on 77 oct fuel, thus need for the Meteor is much resuced. While even saving some weight.
 
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