Heavy Tank M6/ Heavy Tank T1

The Heavy Tank M6 (Heavy Tank T1) was the first American heavy tank to come close to production during the Second World War, but was rejected by the Armored Force and only a handful were ever completed.

In late May 1940 the Chief of Infantry (then responsible for American tank designs) gave a list of the type of tanks the United States would require in response to the stunning German successes in the Low Countries and in France, and in particular the appearance of a small number of Panzer IVs armed with a 75mm gun. At the time the Medium Tank M2 was the newest US tank, but this was only armed with a 37mm gun and was suddenly obsolete. Two types were suggested - a medium tank armed with a 75mm gun (this led to the Medium Tank M3/ Grant/ Lee and then to the Medium Tank M4 Sherman) and a 80 ton heavy tank.

The first design for the heavy tank was a ghastly machine. It was armed with two primary and two secondary turrets. The sub-committee on Automotive Equipment Item 15842 (OCM 15842) laid out a design for a 50 ton tank, to be armed with:

'2 - Primary turrets, each mounting a 75mm Gun, T6, with power traverse to cover approximately 250 degrees.
2 - Secondard (sic) turrets with power traverse, one to have a 37mm gun anc (six) calibre .30 machine gun in combination mount, the other a 20mm gun in combination mount. These weapons cover 360 degrees'.

No plans of this version of the attack appear to have survived, and it is possible than none were ever drawn up, as the first wooden model was built to a different design. Quite how the four turrets would have been laid out is thus unclear, and much depends on what was meant by the specifications for traverse range. If they refer to the combined coverage of the two turrets of each type, then this would be two main turrets, each with a traverse of 125 degrees (possibly buried into the hull, otherwise the limited traverse is hard to understand). Each of the main turrets would have one of the secondary turrets on top, capable of covering 360 degrees between them.

If the traverse range refers to the individual turrets, then this would give us two normal turrets, each capable of rotating through 250 degrees (allowing the tank to fire a two-gun broadside), but each unable to fire over the other, giving each a 110 degree blind spot. The secondary turrets must still have been mounted on top of the main turrets, presumably with the capacity for one to fire directly over the top of the other.

OCM 15946 of 11 July 1940 approved the development of a new heavy tank, with the designation Heavy Tank T1. In August 1940 the Baldwin Locomotive Works was give an contract to produce one pilot and fifty production tanks.

During this period the original multi-turret design was abandoned in favour of a more conventional type. A wooden model of this new design had been completed by 4 October 1940, when a report based on this design was sent to the Armored Force, and the details were laid out in OCM 16200 of 24 October 1940. This called for a 50 ton tank, to be armed with a 3" (76.2mm) Anti-Aircraft Gun T9 and a 37mm Tank Gun M5E1 in a fully traversing turret. One .30in machine gun and one .50in machine gun were to be mounted on the turret roof, two .50in machine guns in the bow (for use against ground and air targets) and two .03in fixed forward firing machine guns (with limited elevation) in the front.  Side and rear armour was to be 2.5in thick, front, vertical and turret walls to be 3in thick.  No engine was specified at this point.

The T1 emerged as a fairly boxy tank. The tank drive wheel and idler were level with the return track, with the idler wheel at the front higher than the drive wheel at the back, so the return track sloped down towards the back of the tank. There were sixteen road wheels in dual bogies on each side, using a Horizonal Volute Spring suspension system. The superstructure had a sloped front. The top of the superstructure was flat. At the side sponsons with sloped sides extended over the tracks. The turret was mounted at the front of the superstructure. The engine was in the back. 

The new design was approved on 22 November 1940. The original aim was to use a 1000hp engine and a Wright Cyclone air-cooled radial was chosen. This high powered engine required a new form of transmission, and a Hydramatic transmission, designed by the Oldsmobile Division of General Motors was chosen. Work was also approved on a electric drive and stearing mechanism, to be used in place of the Hydramatic system. When equipped with this system the tank would be the T1E1.

The new tank ended up with a crew of six - commander, gunner, loader, ammunition passer, driver and assistant driver. The first three were in the turret, the last three in the hull.

In February 1941 approval was given for four pilot models, with plans to produce 100 machines per month. Again the contract went to Baldwin. The four pilots were to test different combinations of transmissions and hull. In February 1942 the various types were officially given designations. The T1 was the original version with a cast hull and Hydramatic transmission. T1E1 would have a cast hull and electric transmission. T1E2 had a cast hull and a torque converter transmission. T1E3 had a welded hull and torque converter. T1E4 had a welded hull and two diesel motors with torque converters (this version was cancelled because the diesel engine would have required too much time to develop).

The original plan called for the Hydramatic transmission to be ready to be installed in the T1 by May 1941. The new transmission was repeatedly delayed, and the first version to be completed was thus the T1E2, with a Twin Disc torque converter. This model was ready for tests at Baldwin by the end of August 1942. A number of problems emerged at this point, including vapour lock in the engine, problems with the transmission and overheating of the steering brakes. 

The T1E2 was presented to the Ordnance Department on 8 December 1941, the day after the Japanese attack on Pearl Harbor. This presentation didn't go entirely according to plan - for the last three miles the hydraulics failed, so the power steering and gear shifts failed leaving the crew to use the backup mechanical steering system. After the presentation the T1E2 was largely rebuilt, in an attempt to deal with many of the problems revealed in these early tests.

A number of design changes were made at this point. The turret cupola was removed and replaced with a double hatch with a rotating ring that could carry a .30in or .50in anti-aircraft gun. The second turret machine gun, which had been mounted at the rear-left of the turret, was eliminated. One of the fixed machine guns was removed and a new mount was designed for the .50in guns in the hull front. 

The T1E3 was the second model to be completed and tested. The only difference between this and the T1E2 was the use of a welded hull.

The T1E1, with its electric drive, was the last to be completed, and wasn't ready for testing until June 1943. This version was never standardised, but it was semi-officially called the M6A2.

Soon after the American entry into the Second World War in December 1941 the decision was made to put the T1E2 into production before the test program had been completed. By February 1942 orders had been placed for the T1E2 with cast hull and twin disc torque converter and the T1E2 with welded hull and twin disc torque converter. On 13 April 1942 OCM 18059 recommended standardising these as the M6 and M6A1 respectively, and this was approved on 26 May 1942. However at the same time the number to be built was reduced from 1,084 (with the potential to expand to 5,000), down to only 115.

The T1 and T1E4 were both cancelled on 11 June 1942, meaning that the Hydramatic transmission was never actually installed. The electric drive system was so promising that 27 extra systems were ordered, for delivery by the end of 1942. In June 1942 the number of heavy tanks to be produced was increased to 230, with the extra 115 tanks to go to Lend-Lease. On 10 August 1942 OCM 18984 recommended that the T1E1 should become a 'limited procurement' type, and 115 should be ordered for extensive tests. The idea now was that the T1E1 would be used for tests in the US, and the fifty M6s and sixty-five M6A1s would go to lend-lease. Production was to start in October or November 1942.

By now the Army had a chance to test the pilots, and they weren't impressed. On 7 December 1942 General Devers, commander of the Armored Force, stated that 'Due to its tremendous weight and limited tactical use, there is no requirement in the Armored Force for the heavy tank. The increase in the power of the armament of the heavy tank does not compensate for the heavier armor'. The view was that two 30-ton medium tanks would be more useful than one 60-ton heavy tank. This general opposition to the heavy tank persisted beyond the point when it could be in any way justified. Even as US forces on the ground in North Africa and Italy were encountering the heavy Tiger tank and urgently requesting heavier tanks of their own, the US based establishment was resisting the development of the M26 Pershing Tank. This opposition finally ended after the Battle of the Bulge, when even the most stubborn opponent of the heavy tank had to admit that they were urgently needed.

In March 1943 the production programme was reduced to 40 tanks. The first production M6 had already been delivered, in December 1942, so the US army could have had a useful heavy tank in large numbers for the battles of 1943-44. Instead only 8 M6s, 12 M6A1s and 20 T1E1s were completed, with production ending in February 1944. A total of 43 tanks were thus built - the T1E1, T1E2, production pilot for the M6A1 and the forty production tanks.

The first M6 went to Fort Knox for evaluation in January 1943, and the first M6A1 followed in March. These tests lasted until late April, and the results weren't at all positive. The internal layout was criticised, as it made it hard to use both the main gun and secondary armament. The lack of any rear firing machine guns was a problem, while the 37mm gun was now obsolete. The 3in gun was also considered to be too light for a heavy tank.

After the D-Day landings the Ordnance Department considered using the T1E1s as the basis of a heavy assault tank. This would have involved fitting the tank with the turret then being designed for the Heavy Tank T29. this carried a high velocity 150mm gun T5E1. The hull would have been modified to expand the 69in turret ring to 80in. The bow machine guns and driver's door in the front would have been removed, and extra armour installed to give 7.5in of frontal armour. Fifteen of the twenty T1E1s could have been converted to the new design. The idea was officially suggested on 14 August 1944, as the M6A2E1, with the tanks to be delivered by mid-November. In August General Eisenhower made it clear that he didn’t consider the type to the practical in Europe. Tests on a T1E1 carrying extra weights to simulate the new design suggested that it would have been largely immobile.

In December 1944 the M6 series was declared obsolete.

The M6A2E1 was partially revived in mid-1945 in order to help with the development of the Heavy Tank T29. Two M6A2s were given the T29 turret, gun and fighting compartments, and used for test firing at the Aberdeen Proving Ground. This new turret had a large bustle, and the forward mounted turret meant that the massive 105mm gun jutted out massively from the front of the tank.

Stats (M6)
Production: 40
Hull Length: 27ft 8in (including gun); 24ft 9in (without gun)
Hull Width: 10ft 2.5in
Height: 10ft 7in
Crew: 6
Battle Weight: 126,500lb
Engine: 800hp Wright G-200 9 air cooled cylinder radial
Max Speed: 22mph
Max Range: 100 miles road radius
Armament: One 3in (76.2mm) M7 and one 37mm M6 in turret, two .30 Browning machine guns, one .50 anti-aircraft gun
Armour: 25-100mm

Armour

Armour

Front

Side

Rear

Top/ Bottom

Turret

83mm

83mm

83mm

25mm

Hull

70-102mm

44-70mm

41mm

25mm

Gun shield

102mm

 

 

 

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How to cite this article: Rickard, J (8 March 2017), Heavy Tank M6/ Heavy Tank T1 , http://www.historyofwar.org/articles/weapons_heavy_tank_M6.html

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