Could the British Have Fielded a Balanced Tank in 1939?

Instead of fielding heavily armoured but slow infantry tanks and lightly armoured fast cruisers, could Britain have deployed a well-armoured, fast 6-pdr tank with a three-man turret in 1939? Two things need to pass, first the early launch of the 6-pdr over the 2-pdr to give us a HE punch and a turret diameter able to hold 3 men (ideally for update to the 17-pdr). Second, and most importantly, we need a 550-600 hp engine to be in development (and thus deemed necessary) by 1936, or a twin-engine solution running through a combining gearbox. My vote's on the Kestrel, (de-rated, delete reduction gear/supercharger, heavy-duty cooling and dry-sump). There's also the issue of a wide turret ring impacting loading gauge, needed for easy transport by rail.

The loading gauge was not an obstacle to A24/27 vehicles which otherwise match your requirements.

Other than that, the Kestrel is a logical option since it was discussed in 1933-35, or a new engine. But even the Liberty would meet the power reqs.

The balance of armament, turret size and protection can be achieved with a 25 ton basis.

But to achieve what you want, the British overall need to seriously invest in tank research so that heavy duty but sufficiently compact and high speed automotive components can be obtained in time, to obtain the 6 pdr in time, and so that they can enable the 25 ton bridging limit on a large scale.

The sea-level rated (ie no supercharger) Kestrel had an international rating of 585 BHP at 2500 rpm in the late-1930s. This was with a compression ratio of 6:1 using 87 octane. It should not be a problem to reduce the compression ratio to 5.5:1 or even 5:1 and run on 65-70 octane motor pool fuel, putting out ~530-480 BHP at 2500 rpm. But IMO you really only need around 450 BHP for the 1939 period as this would give 20 BHP/ton for a 22.5 ton tank. The 18 ton variants of the Valentine only had a power:weight ratio of ~9.2 BHP/ton.

Beef up the Vickers Slow Motion suspension to maintain the 15 mph (UK rating) to 20 mph (USSR rating) road speed and 12 mph cross-country speed at 22.5 tons.

The Valentine with the 6pdr or 75mm in a 2-man turret weighed in at just under 18 tons, which was a little under 2 tons more than the original Mk I model with the 2pdr. The original front and side armour basis of 2.36" would be considered light for the mid- and late-war, but it was good enough in the 1939-41 period to stop the German 37mm and short 50mm APC at any reasonable combat range.

The Valentine was a small tank, only 6' 8" to the turret roof. Adding another 12" between the upper hull structure over the tracks (at least in the area of the turret) and the height of the turret to allow a larger turret ring and provide more room for the crew would not increase the height beyond what was tactically sound. This would of course add weight to the vehicle, as would a properly designed larger turret needed to house a larger gun and the 3-man crew.

Adopting welding for the main structure of the turret and hull would reduce the weight gain to a degree, as would using more effectively sloped armour on the front of the hull and turret. Welding was used on the hulls of many later production Valentines - but I have not been able to find any info on how much weight this saved over the bolted and riveted method.

Increasing the effective frontal armour basis to 3.36" would only cost about 1 ton. This leaves 3.5 tons for the rest of the "mods".

NOTE Weights are in long tons at 2240 lbs/ton.

[edited to correct height to turret roof, from 7'6" (actually the height to the top of the blade sight) to the correct value of 6' 8" to the turret roof, plus a few bits of info - including the info about later production tanks use of welding for the hull construction, and use of long tons in weights]

Hi,
I think that the requirements as laid out might be a bit too ambitious for the timeframe noted.

Specifically, in 1936 I believe that the Panzer III was only using a Maybach HL108 TR -cylinder gasoline engine producing about 250 hp, while the Panzer IV didn't enter service until 1939 with a Maybach HL120 TRM developing about 300 hp (I think). And similarly, I believe that at the time the armor on either tank wasn't anything that would be considered too great a challenge for the existing 2pdrs being deployed. Though ideally, designing to a tank with a turret suitable for up-gunning in the future probably wouldn't be a bad idea, if you could accommodate a larger turret ring in the baseline design.

However, seeing the real world issues that the UK had with either trying to fit the ~340hp Meadows DAV Flat-12 engine (and its cooling system) into the Covenanter tank, as well as the reliability issues that they appeared to have with the Liberty engine in heavy use in harsh environments, I wouldn't be surprised if trying to design a tank around an even larger 500-550hp engine in the 1936-39 timeframe wouldn't experience even more issues, which may delay the UKs ability to get effective tanks into service in late 1939 to early 1940/41 even more difficult.

As such you might end up with a situation where "striving for something better than you need in the near term getting in the way of meeting your immediate pressing needs".

FWIW The Kestrel (1296 in3) without a supercharger was actually smaller than the Liberty (1649 in3) - in both displacement and in length, width & height - although the weights were about the same in aircraft installation form (around 850-900 lbs).

Playing the part of the obstinate forum contrarian:

• The AP performance of the 2pdr was perfectly adequate early in the war. Replacing that with the historical QF 6pdr, a design similarly focused on AP performance and thus high muzzle velocity would have required a bigger and heavier vehicle to carry it. Rather make something roughly 6pdr sized but with more modest MV enabling some weight savings? (Similar to how later the 6pdr was adapted to fire the French/US 75mm ammo, giving much better HE punch for about the same weight. ) Alternatively if you really want a small tanks as possible, equip part of them with the 2pdr and part with some low velocity ~75mm gun for HE support? Save some R&D and logistics headaches by using the same tank for both roles.
• RR wasn't interested in adapting the Kestrel for ground use, as they were focused on aero engines, in particular the Merlin. Arguably the right call, considering how important the Merlin would turn out to be. Heck, they had to be dragged kicking and screaming into the Meteor project, which ultimately was given to Rover. The Lion might be a more realistic alternative (though I read somewhere that the Lion was very expensive, any truth in that? If so, might put some damper on the enthusiasm to adopt it..). Or then just hold your nose and pick the Liberty?
• Abandoning Christie in favor of torsion bars would have helped wrt width concerns.
• The separation into cruiser and infantry tanks was a result of doctrine which hadn't been tested in battle against peer opponents or even much combined arms exercises. They would have needed much more resources for experiments in the early-mid thirties.

z42 said:

The separation into cruiser and infantry tanks was a result of doctrine which hadn't been tested in battle against peer opponents or even much combined arms exercises. They would have needed much more resources for experiments in the early-mid thirties.

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What Britain and France need is a quick war against Italy to test and refine tank doctrine. The Abyssinian Crisis (1935–1936) seems like a good launch point, but with British-French security guarantees to Ethiopia (then Abyssinia). Before the kickoff, Britain and France both sends tanks to both Abyssinia and to France's frontier with Italy. But then again, IIRC Italy doesn't have any tanks in 1935 beyond the tiny Carro Veloce CV-35. Can that be true?