A20

On 1st September, 1939 the General Staff issued a requirement for a slow, heavily armoured tank with an extremely good cross country performance to traverse heavily shelled areas of Northern Europe during the winter season. The reasoning being that requirement was that it was anticipated that static warfare would develop between the Siegfried Line and the Maginot Line. The Initial specification stipulated that the tank must be transportable by rail and that the armour should be "reasonable" to obtain protection against the German 37 mm. Anti-tank gun. The armour should provide protection for the crew, engine and fuel but not necessarily for the tracks. The maximum speed required was 10 m.p.h. and a very low ground pressure was necessary. What the General Staff had in mind was something along the lines of the Mark VIII tank of World War I. This is apparent from their mention of the incorporation of an unditching beam, equipment which had proved an important factor in the conditions experienced in the Somme but which implied a turretless, all round track vehicle. The General Staff therefore suggested that side sponsons were acceptable to house the armament. Each sponson was to carry a 2-pr. and a co-axle Besa.

The Mechanisation Board were entrusted with the design and by the 10th of October 1939 had produced an initial design. The Board concluded that the ground pressure requirements of the General Staff specification were not technically possible and so "interpreted the requirement on a broad basis" and produced a design based on A.12 that used the A.13 MkIII (Covenanter) power plant and transmission with an armour basis of 60mm. This design included WWI type high horns but also had an A.12 turret containing a mount for a 2-pdr. The Board clearly felt that a turret with 360 degree traverse was essential even if it did rule out the General Staff's favoured unditching beam. In addition to the turret armament there was a machine gun in the front hull as well as another two mounted in sponsons each side. It is not clear if the hull mounted 3" howitzer was present at this time but if not this was added pretty soon afterward as it at the Mechanisation Board meeting of 17th November Dr. Merritt asked if the howitzer barrel length could be shortened.

On the 28th of October the Director of Mechanisation approached Harland and Wolff of Belfast. Harland & Wolff were told that with the outline drawings practically complete the General Staff now required them to complete the design and make working drawings with an aim to produce 50 to 100 tanks, production to commence in early 1941. There was no time to permit the building of pilot, the only concession to which was to make the first few tanks of the batch in mild steel. As encouragement for Harland & Wolff to take on the project mention was made that the diesel engine that the company had been developing could be fitted as an alternative to the Meadows engine. It is hard to believe that this was a serious proposal from the Mechanisation Board as when the General Staff had expressed a preference for a diesel engine they had been quite indifferent.

The decision to use Harland & Wolff is perhaps a curious one given the nature of this project and it was soon discovered that there were real problems in having a design department in England and the manufacturer in Belfast. Indeed there were several proposals made in the following months to promote closer integration of the two, included a proposal to set up a special Design Department in Liverpool or Southampton specifically to facilitate liaison, and offers/requests from/to Harland & Wolff to send draftsmen to Woolwich and Staines. There was even a suggestion that a pilot vehicle should be built in England. Matters were not helped following a particularly 'harrowing' flight made by the president of the Mechanisation Board to Belfast during which the pilot lost his way and nearly ended up in the Irish Free State.

By early December 1939, financial approval was given for 100 tanks, the order to include four mild steel pilot models, however by the 12th of December this order had been withdrawn due to modifications being made to the original specification. Quite what these changes were I'm not sure but we do know that at about this time the General Staff were asking whether it was too late to consider the replacement of the 2-pr. gun by the new 6-pdr, the design of which was virtually complete. Subsequent investigation revealed that the size of the mount meant that it could not be fitted into the proposed A20 turret and the overall length meant that if mounted in the hull the muzzle would project some two feet beyond the horns. Any frontal overhang would seriously compromise the obstacle crossing abilities, which was after all a primary design requirement. Not deterred the General Staff then proposed the mounting of a 2-pdr. in the turret and a shortened 6-pr. between the horns, the weapon they now had in mind being a Naval 6-pdr used on Anti Motor Torpedo Boats. With hindsight it seems doubtful that the Navel gun was any more suitable than the anti-tank gun and after investigation it was established that the ballistics were much the same and that a shortening of barrel length would inevitably result a serious loss in muzzle velocity. The Ordnance Board confirmed this and added that it would be impossible to get any regularity in performance and that the 2-pr., with its higher velocity, would probably be just as effective in destroying obstacles. At this point any consideration of a hull mounted 6-pdr was abandoned however this didn't stop the search for alternative weapons with the French 75mm. gun and even twin 2-prs, firing simultaneously being discussed.

In January, the Mechanisation Board became concerned at the number of man hours involved in the production of the A20 suspension. In addition expressing these concerns to Harland & Wolff they also approached Vauxhalls for advice. Vauxhall proposed a re-design and it would seem that the Mechanisation Board were impressed by Vauxhalls as they proceeded to ask them not only to provide complete drawings but also to manufacture some trial units and also to consider the production of four complete sets. Vauxhall's involvement was taken even further on the 12th February, 1940 when at a meeting at the company's Luton works Vauxhall were approached with regard to the possibility of them manufacturing the Mk.IV Infantry tank (A20). Such was the desire to get Vauxhall's on board that it was proposed that if Vauxhalls would consider to manufacture this tank, then the Ministry of Supply were prepared to build and equip a factory to be run under Vauxhall's administration.

By the end of February the design was reviewed and it was confirmed that it generally met all the requirements of the original General Staff specification with the exception of the requirement of 45 climb which was impossible owing to track slip, the assumed climb was 35. The design barely met the limitations of the railway loading gauge and consequently the tank traveled by train the cupola would have to be removed. It was noted that in a new design of turret this item would be omitted.

By April the design of A.20 was practically complete except for details of the cooling system, however at a meeting held on the 9th April, 1940, the main armament was again called into question this time with regard to a new turret design. Clearly the General Staff were still pushing for a 6-pdr gun and the only way this could be achieved was to design a new turret. Once again once the subject of armament became a subject for debate and the Board found themselves discussing several alternatives such as the French 75mm and 47mm guns. All were rejected and it was finally decided that the new turret should mount the 6-pdr. It would appear that the alternate turret design went ahead as pilot models 3 and 4, were to have a turret with an electrical power traverse, whereas pilot Nos. 1 and 2 would be fitted with A.12 turrets complete with hydraulic traverse. Meanwhile the English Steel Corporation had succeeded in producing two cast A.20 turrets and had reported no difficulty in obtaining the thin sections required. Presumably this refers to a larger 6-pdr turret with the thin casting required to keep the overall weight in check.

Despite the progress made, on the 7th June, 1940, it was decided that the order for 100 A.20s should be stopped. This presumably was because A22 was now in the pipeline and was of a similar specification. Work did proceed on the pilot models on the basis that they would be useful for the testing of the Merritt transmission. Confirmation of this policy was provided on the 25th June, 1940, when it was definitely established that the A.20 pilot tanks should be used only as vehicles for the testing of components which were to be used in A.22.

In July Sir Albert Stern was invited to inspect a mock-up of A.20 at Woolwich and though in his opinion the machine was a relatively small target he had many criticisms as to space allotted to the drivers, which was cramped. His final and most devastating criticism was that Dr. Merritt's gear box had a clash change which, Sir Albert declared, would be hopeless in soft mud. Sir Albert was wrong and the Merritt gearbox went on to become one of the few successes of British WWII tank design and was used in nearly all later tank designs.

 

© C. Shillito 2001