January 1936, Vickers Engineering. (part 1)
With the way international relations seemed to be deteriorating, and the consequent possibilities for Vickers in arms sales, a number of meetings of the various areas of responsibility had been arranged for the start of the year. The one that most concerned Sir John Carden was the armoured vehicle meeting.
He'd been giving considerable thought to the problems of meeting the specifications the Army had been coming up with, the main problem being that the army had unreasonable ideas about just what could be built within the weight limits they specified. He'd decided that he was going to use the license they'd agreed to on his designing the new Cruiser and infantry tank to show them what they needed to do the jobs they asked for, rather than let them just waste time trying to squeeze a quart into a pint pot. Hopefully they would agree to adjust the requirement to something more believable.
The first item up for discussion was the request from the Army to look into putting a cannon of some sort on the MkVI light tank. This was seen as actually fairly easy; a version of their light tank had been sold to Latvia with such a cannon, and the Russian T-26 was a turreted light tank based on their original design. The Latvian order was due to be delivered next year, and they hadn't actually built any yet, but the design of the turret was done and in fact they'd built a wooden mock-up to check everything would fit. So it was really just a matter of working out the maximum size gun that would fit, then seeing what the Army wanted that would fall inside that parameter. The light tank was pretty easy to produce, and some of Vickers associate firms could build them as well as Vickers themselves. Whether such a tank was any use of not, a number of the experts had disputed, but giving the Army a cost for them probably wouldn't hurt.
Given the limited size, and the fact they wanted an urgent fix, gun discussion had centred on models already available. This had come down to three possibilities. First, the current low velocity 3pdr. As this was the current fitting for the medium tanks, some spares were available, and the production line was still active. Second was the new 2pdr gun. This was a better anti-tank gun, but they weren't sure if the army would be happy to have the initial production diverted, which is what would be needed. The third option was the COW gun. While lighter than the other two, and with inferior penetration, it was still more than capable of killing a tankette. While this gun wasn't being used by the army, they had a few models available, and the production line could be re-opened. The gun was clip fed, which might be a useful addition allowing a burst of rounds against an evading target. One advantage of this gun was the engineers could fit it in place of the 0.5" machinegun, although this would make the turret cramped. Realistically the slowest part of any modification would be the turret, so whichever gun was chosen availability wouldn't be an issue.
The other decision they needed the army to make was if they wanted to convert existing models, or build new. 22 of the Mk V were being built, and they would soon be delivering the Mk VI - orders for 51 of this tank had already been placed. They could modify existing Mk V's, although the Army weren't plentifully supplied. One other possibility that had been raised was a version of the 6-tone Vickers tank. This had been made in a version with a turret, and successfully sold abroad, although the British Army hadn't bought any. They still had the plans, and if new build tanks were required they could offer this as an alternative.
Once the easy questions had been got out of the way, discussion turned to a more contentious topic - the proposed Cruiser and Infantry tanks. Sir John and his team had been looking seriously at these for some months, starting from the A9 and A10 designs, neither of which would fill the new requirements.
The initial and biggest problem was the engine needed. The current bus engines weren't up to driving even the cruiser tank at anything like the speed required, and the heavy infantry tank was even worse. The problem was that the Army was quite keen on some designs soon, so a long wait would be unacceptable if they suggested a custom-built engine. Given the small numbers the Army tended to order in, this would likely be too expensive anyway.
So Sir John and his team had started to look around, and hold conversations with various people about what engines were actually available, or could be modified or put into production at a relatively low cost. Based on the weights expected and the performance the Army wanted, Sir John had estimated they would need something delivering 300 - 400hp. A diesel might be a better option, assuming they had a choice, as it would power a tank more efficiently and me more economical on fuel. A big engine would require a correspondingly large fuel tank.
The first option was to use two of the existing bus engines together. None of the engineers really liked the idea. Yes, two coupled diesel engines would give somewhere around 350hp, depending on which engines they started with. But there was the issue of the cost and complexity of all the mechanicals necessary to do that coupling, the loss of efficiency, and the high maintenance cost. Such a solution was considered to be a fallback one if they couldn't find a more suitable one.
The engines that did produce the sort of power output they needed were normally aircraft engines or marine diesels. Aircraft engines, while light and usually small, were expensive, and rather fragile by tank standards. Marine engines were robust, but heavy and often large, things that were of less importance in a ship that in a tank.
Sir John had had some interesting conversations with his friend Harry Ricardo. He'd designed the diesels used in the WW1 tanks, as well as pioneering many modern diesel developments, and he'd wanted to get his input. He knew that Ricardo had modified one of Rolls-Royces Kestrel aero engines to run as a diesel for the 'Flying Spray' car, which was soon to make an effort at some land speed records. He was particularly interested in this as the car and engine had been designed for endurance records rather than pure short-term speed, so would be more likely to stand up to use in a tank. Ricardo had modified an old Kestrel Rolls-Royce had let him have, reducing the capacity and adding sleeve valves while turning it into a diesel. Ricardo told him that yes, he saw no reason why the engine shouldn't work as a tank engine. With the modifications he'd made, the output had been reduced from around 500 to 340hp, but he felt that while the un-supercharged petrol version would in theory get close to 500hp, there would be big issues with the low-quality pool petrol used in the Army. Part of the reduction in performance had been caused by the reduction in the valve bore size to allow for the insertion of a sleeve valve. If a little more power was needed, he thought it would be possible to increase the bore a little to retain the old volume even with a sleeve valve, which would give around 380hp, although this would mean new machinery to make the engine. The aero engine wasn't cheap, a standard Kestrel was about £2,000, but he felt that but leaving off all the unneeded parts, and possible replace some parts with lower cost, heavier alternatives would allow this to be reduced. The problem was production. Rolls-Royce was very busy with the production of their aero engines, and realistically a new production line would have to be set up, perhaps elsewhere with Rolls-Royces cooperation.
The next engine to be looked at had been the engine used in one of the Medium Mk III prototypes. Initially a 180hp engine had been fitted, but this had shown to be underpowered, and in the third prototype a Thornycroft 6V 500 hp, a slow revving marine engine had been fitted. This would certainly provide the power necessary, maybe even more than necessary, although the engine was rather heavy. It was noted that it would be worthwhile to see if the weight and cost could be reduced even if this reduced the power. Even with a reduction in power, this engine was a good candidate for the heavy infantry tank.
The final promising engine was a version of the Paxman R-series marine diesel. Ricardo had mentioned this to him, as the way they were built allowed customisation of the number of cylinders. They had a number of advantages; they were built in a robust form (they supplied a version for the Navy to use in Submarines) that were resistant to shock, which would be a valuable trait for a tank engine. They could be produced to give around 500hp. The main issues would be weight and cost. Ricardo was very familiar with the design - they used the Comet head he'd designed in some of their engines- and he pointed out that setting up for a series production run would likely reduce the costs considerably.
While there were obvious problems with all the likely candidates, the problem now looked solvable. A decision was taken to investigate the three promising candidates more fully, and costs estimated for a run of either 250 or 500 engines. Even if the Army didn't want them all, the foreign market would likely be interested in more powerful tanks, and if not they could sell them for marine use. This was to be done as soon as possible so a decision could be made on usage. For his part, Sir John liked the look of the diesel Kestrel, if production could be arranged. Given its origin of Rolls-Royce and Ricardo, he felt reliability, a constant problem with high powered engines, wouldn't be an issue. 340hp should drive his idea of a Cruiser tank nicely, and one of the higher power engines would be a good fit for his Infantry tank idea. Given the much heavier weight the infantry tank was likely to be, he was starting to think that a common engine, while good for production, wouldn't be the best technical solution.