BTW I spoke to someone who had looked into Napiers for a book (abandoned) on their aero engines and he tells me that they were selling Sea Lions late in the 1930s but made from old stock and new major parts manufacturing had long stopped by then. The surplus RAF ones were scrap price for a good reason as the RAF had cannibalised anything that would keep their Lions going as long as possible so they would need a lot of work to make them fit and there were not enough parts for that and those in hand at Napiers were earmarked to the Sea Lion programme. Napier, of course, would make new Lions for a big enough order but much machinery was already moved onto the Rapier/Dagger/Sabre work so would have to cover the cost of new machinery. You could have spent the entire tank budget on just the engines.
He also pointed out that Miles were sucking up all the surplus Kestrels to keep Master trainer production going and these soon ran out, hence the substitution of the Mercury and Twin Wasp.
So what was everyone else making in what would be a sort universal tank class of midish 1930s?
The PzIV was at 16 tons, the S35 Somua 20 tons, Bt7 14 tons, M3(Stuart not Lee) 15 tons and the A10 at 14 tons so a we would be looking at a weight in the order of 20 tons and expect a growth to 25 tons..
These were dragged along by 200bhp for the S35, 220bhp for the M3, 300 for the PzIV, 450 for the BT7 and a mighty 150bhp for the A10 which tells you an awful lot about British tank design using road engines. The use of the M3 is perhaps unfair as it was not pre war and the USA was looking to numbers that justified an aero engine but even they had to resort to road engines to supplement the aero ones but they were doubling Cadillac V8s not a couple of modest bus engines. So to pull up to 20 tons 300bhp would work and 450 would allow for weight growth. Hence the enthusiasm for Lion and Kestrel based units. The Nuffield licence built Liberty was @350bhp and pushed too far at 450 bhp. The Russians did a better job with their Liberty V12 at 450bhp. However the Nuffield Liberty did not exist pre war. The Valentine was a sort of mini universal tank which topped out at 16 tons pulled by 200bhp which sets a lower limit. So some sort of POD would be needed to afford a converted aero engine. History suggests 3 possible sources of free capital. One would be Lord Nuffield investing in the Liberty earlier. Another would be the government ordering enough Lions to justify reopening/maintaining a Lion production line for a tank Lion and thirdly Rolls Royce seeing an opportunity to support the development of the Kestrel into the Peregrine by offering a tank Kestrel which would morph into a tank Peregrine later on and may maintain Peregrines for the Whirlwind. All of these would make in the region of 400 bhp from @24 litres.
So the initial Universal tank would weigh @20tons and have @350bhp to power it. I would suggest that a model for it might be a larger Valentine which would allow for a larger turret ring and allow a 3 man turret with the future 6 pounder/75mm ROF gun. The speed would not increase much on the road but the extra torque would let the suspension carry it at a good cross country speed.
As to armament at entry to service it would need to kill opposing tanks and to kill AA guns and dug in MGs. One contemporary scheme was a hull mounted HE gun and a turret mounted AP gun. The other was something AP in @45mm with an HE round and turret mounted for which Vickers might enhance the 3 pounder.
It might be nice to do something about track design. British tracks were prone to breaking and shedding. The Germans found that captured A10s did better on PvII tracks than their own.
Oh yes, make the designers aware that they will be the maintenance and repair team for the official trials and a copy hull/turret will be set on fire with them in it to demonstrate the ease of egress..........I bet that will do wonders for the actual crews in wartime.
