The Medium Tank M1921 (Medium A) was the first new tank design to be built by the US Ordnance department after the First World War, and suffered from a lack of engine power.
In June 1919 the Ordnance Department issued a work order approving the design of a medium tank that was to be similar to the British Medium D. The first formal specifications were drawn up by Brigadier General S. D. Rockenbach (then chief of the Tank Corps and with experience of commanding armoured units in France in 1918) on 18 August 1919 and called for a tank weighing less than 18 tons, with at least a 180hp engine, a top speed of 12 mph and cruising radius of 60 miles, one light cannon, two machine guns and armour that provide protection against rifle calibre armour piercing bullets. On 4 November 1919 the Ordnance Committee upped the armour protection to be proof against.50in calibre armour piercing rounds fired at close rounds, but otherwise approved the specifications.
The Ordnance department produced a design and then built a mock up, which was inspected by the Tank Corps Technical Board on 2 April 1920. Production of a pilot tank was approved, with the designation Medium A. On 13 April this was altered to the production of two medium tanks, and soon afterwards this was altered again, to the production of tank to the original design (the M1921) and one using a new flexible cable suspension system then under development in Britain (the M1922).
The M1921 was built around a simple box, with a slightly sloping front plate. It had a circular turret with vertical sides, armed with a 57mm gun and coaxial 0.30in machine gun. A smaller independently rotating turret was carried on top of the main turret, armed with a second 0.30in machine gun. When seen from the front the main turret roof had a flat centre and sloping sides. A similar design was used on the M1922. The 57mm gun was carried in a semi-spherical mounting. It was powered by a Murray and Tregurtha marine engine, originally designed for large speed boats.
The M1921 was built at the Rock Island Arsenal, and was completed by December 1921. It went to the Aberdeen Proving Ground on 20 February 1922 where it underwent tests. The prototype weighed 41,000lb when fully loaded. The biggest problem with the design turned out to be the engine, which was meant to provide 220hp at 1,200 rpm, but in practise only reached 195hp at 1,250rpm.
The M1921 was used for tests until 1926. During this period it was given a 338hp Liberty 12 engine, but although this provided an increase in power it also caused an increased number of failures in the power train, which couldn't cope. In 1926 a newly developed Packard 8-cylinder was installed as part of the development of the Medium Tank T1 - with this engine the M1921 was known as the Phase 1 medium tank. The M1921 was also used to test out stereoscopic vision devices, the Winkley odograph and the Sperry gyroscopic tank compass. In June 1925 it was used in tests with a medium pontoon bridge, successfully crossing the bridge at a test weight of 23 tons.
Hull Length: 21ft 7in
Hull Width: 8ft 0in
Height: 9ft 6in
Weight: 23 tons
Engine: Murray-Tregurtha marine 6 cylinder water cooled engine. 220hp in theory, 195hp in practise
Max Speed: 10mph
Armament: One 57mm gun, two machine guns
Armour: 3/8th - 1in