M6 Heavy Tank

Heavy tank (1942-45) U.S.A. - about 45 built

The first US WW2 heavy tank

Little known, the M6 was the first modern US heavy tank. The only other heavy tank built in series was the Liberty or Mark VII, a joint US-British design, produced in 1918, of the WWI-era lozenge type. When the Second World War broke out, the successful German campaign of 1940 gave a spur to US tank development. Already, on 20 May 1940, the Chief of Infantry recommended to the US Army Ordnance Corps to study a 50-ton tank design. The first one was a projected multi-turreted model with a main 75 mm (2.95 in) turret, a secondary one with a 37 mm (1.46 in) gun and coaxial M1919A4, and another, smaller turret with a 20 mm (0.79 in) gun and coaxial cal.30 (7.62 mm), plus four cal.30 machine guns in each corner of the tank in ball mounts. In was approved on 11 June 1940 under the ordnance prototype registration T1 heavy tank. The T1 had to be fitted with a cast hull and hydramatic transmission. However, this design already appeared outdated. In October, this was changed for a more conventional approach, the T1E1.

T1E1 and T1E2 prototypes (1942)

This second design had a single three-man turret, with a vertically-stabilized 3 in gun (75 mm) coupled with a coaxial 37 mm (1.46 in) gun. The turret was cast and had an electric drive and manual traverse. There was also a bow mount with twin cal.50 (12.7 mm) machine guns handled by the assistant driver. The commander cupola and some other parts were borrowed from the M3 Lee. The suspension was also borrowed from the M3, being a standard vertical volute spring (VVSS), with four bogies per side, but the tracks were doubled in width, with two rubber shoes instead of one. The upper tracks reposed on a guide rail, and the entire side was protected by large, one-piece side skirt. The armor ranged from 83 mm (3.27 in) to 44 mm (1.73 in), while the hull was sloped. The overall weight was a staggering 50+ tons. Due to this, the main issues were the engine and transmission.

The Society of Automotive Engineers was charged with finding a suitable combination. The Wright G-200 air-cooled radial gasoline engine was initially selected, and served with an electric transmission, a hydramatic one or with a torque converter. Three prototypes were ordered, but only two were delivered early in 1942 by Baldwin Locomotive Works, using the electric transmission and torque converter. One differed by having a cast hull, while the other had a welded one. On 26 May 1942, other prototypes of both series were ordered under the T1E1 designation (unofficially M6A2), with electric transmission, welded hull, 20 being built, and T1E2 (M6), with the torque converter, with a cast hull, 8 built.

The M6 and M6A1

Eventually, after many unit trials, the Ordnance Corp advised to concentrate only on the torque converter variants, and a third improved series, the T1E3 (M6A1) was ordered. Recommendations included that 115 M6s would be built for US army service and 115 others for the Allies, starting in December 1942. These series vehicles were given a two-piece cupola hatch and ring mount for an AA cal.50 (12.7 mm). The left front machine-gun was removed. However, the Armored Corps had quickly grasped the utility of having a single tank type in service, cheaper and easier to transport, such as the Sherman, and the production was stopped. The development, however, did not end there.

M6A1E2

In early 1944 the M6A1 was successfully tested with the new experimental T7 90 mm (3.54 in), but a new turret layout was urgently needed. In August 1944, the Ordnance Corps recommended an experimental series of 15 T1E1s with a new turret, later known as the M6A1E2.