By
Ryan Crierie
Copyright 2010
A very special thanks to: Richard P. Hunnicutt, without whose marvelous book Firepower, on US Heavy Tank Development, I could not have written this article.
NOTE: As mentioned above, this article is based heavily on Firepower -- but since I now have located the Record Group at NARA responsible for US WWII tank design, I can do my own research into US tank design, instead of relying on Hunnicutt. This article was written several years ago, I just cleaned it up to post on SDN.
The M6 Heavy Tank Program
US heavy tank design in World War II began with the German invasion of France on May 10, 1940. The success of the German Panzer arm beyond anyone's wildest expectations led to the Chief of Infantry of the US Army recommending that a requirement be established for a new heavy tank in the weight range of either 50 tons or 80 tons.
Two days later, moving with remarkable rapidity, the Ordnance Committee responded to that recommendation, by supplying it's own recommendation for a 50 ton tank development project.
By July 11, 1940, a preliminary design was approved and designated the Heavy Tank, T1.
This design was a multi-turreted vehicle very similar to the Soviet T-28 and T-35 tanks.
Armament consisted of the following:
- Two primary turrets armed with the low velocity T6 75mm gun (Combined total coverage of 250 degrees)
- One secondary turret armed with 37mm Gun and .30 Caliber Coaxial MG
- One secondary turret armed with a 20mm Cannon and a .30 Caliber Coaxial MG
- Two .30 Caliber MGs in ball mounts on frontal armor plate
- Two .30 Caliber MGs in ball mounts on diagonal armor plates in the rear hull area.
The revised T1 was now equipped with a version of the 3-inch T9 Anti-aircraft gun, with a 37mm M5E1 gun as the coaxial weapon. Both weapons had power elevation and a gyrostabilizer early on, but the power elevation feature was deleted as the design progressed.
Contrary to later US Tank design practices, the tank commander sat on the left side of the main gun.
Secondary armament consisted of a .30 Caliber MG in the tank commander's cupola, which was of the same type as used on the M3 Medium Tank.
The loader operated a .50 Caliber machine gun from his position. The driver had control of two .30 caliber machine guns mounted on the front armor, which he fired electrically. The assistant driver controlled twin .50 Caliber machine guns in a flexible mount.
Early on in the development cycle, a crew of six to seven men was proposed. When the tank was standardized, it was specified at six. They were:
1.) Commander
2.) Gunner
3.) Loader
4.) Ammunition Passer
5.) Driver
6.) Assistant Driver
Mobility was provided by a Wright G-200 aircraft radial, which developed 960 hp at 2,300 rpm. To successfully transmit the power from this very high-powered engine to the tracks required an entirely new transmission as no existing tank transmission could do the job.
After some deliberation, a new Hydramatic transmission proposed by Oldsmobile was chosen. As a fallback, torque converter proposals by Schneider Hydraulic, Twin Disc Clutch, and Borg-Warner were to be studied as alternatives to the Hydramatic transmission.
A General Electric Gas-Electric transmission was also approved for a single vehicle designated T1E2, due to recent studies by GE indicating that it would only add three tons to the tank's weight (as opposed to earlier studies during the transmission selection process which estimated added weight at 5 tons).
By August 1940, contracts had been placed with Baldwin Locomotive Works for the pilot construction and 50 production models. Due to unforseen problems with the Hydramatic transmission resulting in it missing it's delivery date of May 1941, the pilot tank was completed using Twin Disc's torque converter as the T1E2.
Baldwin began testing the T1E2 during August 1941 as it prepared it's plans for mass production of the design. The usual problems with such a new and heavy vehicle were found during this test, such as transmission problems and steering brake overheating.
Finally, on December 8, 1941, the T1E2 was officially presented to the US Army at the Baldwin Locomotive Works during a demonstration which showed it alongside a M3 medium tank, which had been designed in parallel with the T1. Many features such as fixed machine guns used by the driver, and a commander's cupola with a .30 caliber machine gun were shared across both designs.
The production version of the T1 benefited from experience gained in tank operation and design by eliminating the left fixed machine gun, as well as reducing the size of the driver's door in the frontal glacis (eliminating it completely and going to a roof hatch would have required too much in terms of delaying production). The commander's cupola was removed, and replaced with a hatch similar to that on the production M4 Medium, with a rotating ring allowing fitting of anti-aircraft machine gun.
With America now at war, it was decided to get the new tank into production as quickly as possible, and not wait for the test program to finish. Any changes that needed to be made due to experience could be introduced into later production marks; as the US was already doing with the M3 Medium.
In order to meet the expected numbers of tanks that would be needed by the Army, on February 14, 1942, several different versions of the T1 were officially named. They were:
- T1 - Cast hull, Wright G-200 engine, Hydramatic transmission
- T1E1 - Cast hull, Wright G-200 engine, GE electric drive transmission
- T1E2 - Cast hull, Wright G-200 engine, Twin Disc torque converter transmission
- T1E3 - Welded hull, Wright G-200 engine, Twin Disc torque converter transmission
- T1E4 - Welded Hull, Four GM 6-71 Diesels, two Hydramatic transmissions
Despite funding having been allocated for 1,084 heavy tanks, the changing requirements of the Armored Force reduced the number authorized to just 115. With these reduced requirements, and the standardization of the M6 and M6A1, the T1 and T1E4 were cancelled on June 11, 1942.
The T1E1 program was allowed to continue informally, due to the pilot tank showing exceptional performance; and it's standardization as the M6A2 was proposed, but not approved. On August 10, 1942, the Army recommended that the T1E1 be classified as a limited procurement type to allow 115 to be manufactured for extended service test.
Due to the procurement of heavy tanks having been increased from 115 vehicles to 230 in June; the result was that all the T1E1s would be sent to the U.S. Army, with the M6 and M6A1s sent overseas via lend-lease.
By September 1942, heavy tank production was expected to consist of 50 M6s and 65 M6A1s for Britain, and 115 T1E1s for the US. Production was expected to start in either October or November of that year.
However, the opinion of the Army was shifting against the heavy tank. On December 7, 1942; the Commanding General of the Armored Force, General Jacob Devers, wrote to the Commanding General of the Army Ground Forces; stating 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".
This was also accompanied by general opinion in the Armored Force that it was better to use the available shipping to carry two 30-ton medium tanks instead of one 60-ton heavy tank.
In light of the Armored Force's opinion, the Services of Supply approved the termination of the heavy tank production program after 40 units had been produced.
The first M6 production model was delivered in December 1942; while Fisher/General Motors produced just a single M6A1 pilot. Due to the cancellation of the Fisher contract, all production tanks ended up being built by Baldwin Locomotive Works. The last production tank, a T1E1, was delivered in February of 1944.
M6 and M5 at Fort Knox, showing disparity in size
Breakdown of M6 series production
- 1 - T1E1 Pilot
- 20 - T1E1
- 1 - T1E2 Pilot
- 8 - M6
- 1 - Fisher M6A1 Pilot
- 12 - M6A1
- Total: 43
Among the problems encountered were:
- Crew positions were awkward, making it difficult to operate the main gun and secondary machine guns.
- The Direct sight telescope was located so close to the 3" gun, that the gunner could only use it with his left eye.
- The tank commander had no way to deliver machine gun fire to the sides and rear without exposing himself due to the removal of the .50 caliber machine gun cupola from production turrets.
Experiments and Modifications to the M6 Series.
Despite the program being effectively dead in the water, this did not stop the Army from further developing the M6 series experimentally.
The 90mm Gun Armed T1E1
Due to the Armored Force now considering the 3 inch gun as inadequate for a heavy tank, the Ordnance Department began a test program in which the pilot T1E1 at Aberdeen had the 90mm gun T7 installed experimentally. This gun was eventually standardized as the 90mm gun M3 and installed in the Pershing.
Firing tests showed that the T1E1 provided a stable gun platform for the 90mm gun, but the turret was in need of a redesign to accomodate the efficient operation of the new gun.
However, by the time the firing tests had been done and the reports written up, the Army Ground Forces had cancelled the heavy tank program, and the 90mm program did not proceed any further.
The 105mm Gun Armed M6A2E1
Shortly after D-Day, there was some thought in the Ordnance Department that there would be a need for a small number of tanks with thick armor and a powerful gun to break through fortified areas.
To this end, they put forth a proposal to modify several existing T1E1s by doing the following tasks:
- Removal of the Bow Machine Gun Mounts
- Removal of the Driver's Vision Door
- Welding additional steel plate onto the front to bring frontal protection up to the equivalent of 7.5 inches of vertical armor.
- A new turret mounting the 105mm gun T5E1.
- Modifying the turret ring from 69" to 80" to accept the new turret.
An OCM of August 14, 1944 recommended the modification of the 15 tanks and gave them a tentative designation of Heavy Tank, M6A2E1. The hoped-for goal was delivery by November 15th.
However, the Army Ground Forces had been indifferent to the project, and the Army Service Forces referred the matter of the program to Eisenhower in a cablegram of August 2, 1944. His reply of August 18th stated that the 15 modified tanks weren't wanted as they were considered impractical for the ETO. Upon this, the project was effectively killed off for the better. Early tests at Aberdeen with a T1E1 loaded to the 77-ton weight of the M6A2E1 showed it couldn't even climb a 40 percent slope, and since the final drive reduction gearing would not have been changed in the very short time available for conversion, the modified tanks would have been limited to operating on very favorable terrain.
The T29/M6A2E1 Hybrids
After the M6A2E1 project died, it was requested that two T1E1s be converted anyway to quasi-M6A2E1 standards to test the new heavy tank T29's armament and turret. Following approval of the request, the modifications were carried through, albeit without the hull armor from the original M6A2E1 program, as it was not needed for a demonstrator vehicle.
The End of the M6 Program
On December 14th, 1944 the M6, M6A1, and T1E1 were classified as obsolete.
All were scrapped except a single T1E1 which has remained at Aberdeen Proving Ground, MD for the last sixty-five years, and recently was restored at least externally. It is likely that it will be moved down south to the new U.S. Army Ordnance Museum complex in Virginia as part of BRAC, which will see most of Aberdeen's existing exhibits (about 60%) move down south.
The following photos were taken in September of 2008.
Technical Data of M6 Series
T1E1
63.5 ton combat weight
6 man crew
3" M7 Main Gun with 75 Rounds
37mm M6 Coaxial with 202 Rounds
2 x .50 Caliber M2HB MGs in Fixed Front Hull Mount with 6,900 rounds
.30 Caliber M1919A4 MG in fixed Bow Mount with 5,500 rounds (total .30 cal)
.30 Caliber M1919A4 MG in flexible Anti-Aircraft Mount
Front Upper Hull: 83mm at 30 degrees
Front Lower Hull: 102-70mm at 0 to 60 degrees
Side Upper Hull: 44mm at 20 degrees
Side Lower Hull: 70mm at 0 degrees (includes track armor)
Rear Hull: 41mm at 17 degrees
Top Hull: 25mm at 90 degrees
Hull Floor: 25mm at 90 degrees
Gun Mantlet: 102mm at 0 degrees
Front Turret: 83mm at 7 degrees
Side Turret: 83mm at 0 degrees
Rear Turret: 83mm at 0 degrees
Top Turret: 25mm at 90 degrees
Engine
Wright G-200 Radial 960 hp @ 2,300 RPM
464 gallons of 80 octane gasoline providing a cruising range of 100 miles on roads
Top speed 20~ MPH on roads.
T1E2
63 tons combat weight
6 or 7 man crew
3" T12 Main Gun with 75 Rounds
37mm M5E1 Coaxial with 200 Rounds
2 x .50 Caliber M2HBs in Flexible Twin Hull Mount with 8,000 rounds (total .50 cal)
2 x .30 Caliber M1919A4 MG in fixed Bow Mount with 10,000 rounds (total .30 cal)
.30 Caliber M1919A4 in commander's cupola
.50 Caliber M2HB in flexible Anti-Aircraft Mount
Front Hull: 76mm at 0 degrees
Front Side Hull: 64mm at 0 degrees
Rear Side Hull: 51mm at 0 degrees
Rear Hull: 51mm at 0 degrees
Top Hull: 25mm at 90 degrees
Hull Floor: 25mm at 90 degrees
Gun Mantlet: 76mm at 0 degrees
Front Turret: 76mm at 0 degrees
Side Turret: 76mm at 0 degrees
Rear Turret: 76mm at 0 degrees
Top Turret: 25mm at 90 degrees
Engine
Wright G-200 Radial 960 hp @ 2,300 RPM
477 gallons of 80 octane gasoline providing a cruising range of 100 miles on roads
Top speed 22~ MPH on roads.
M6
63.25 ton combat weight
6 man crew
3" M7 Main Gun with 75 Rounds
37mm M6 Coaxial with 202 Rounds
2 x .50 Caliber M2HBs in Flexible Twin Hull Mount with 6,900 rounds
.30 Caliber M1919A4 MG in fixed Bow Mount with 5,500 rounds (total .30 cal)
.30 Caliber M1919A4 in flexible Anti-Aircraft Mount (or .50 M2HB)
Front Upper Hull: 83mm at 30 degrees
Front Lower Hull: 102-70mm at 0 to 60 degrees
Upper Side Hull: 44mm at 20 degrees
Lower Side Hull: 70mm at 0 degrees (including track armor)
Rear Hull: 41mm at 17 degrees
Top Hull: 25mm at 90 degrees
Hull Floor: 25mm at 90 degrees
Gun Mantlet: 102mm at 0 degrees
Front Turret: 83mm at 7 degrees
Side Turret: 83mm at 0 degrees
Rear Turret: 83mm at 0 degrees
Top Turret: 25mm at 90 degrees
Engine
Wright G-200 Radial 960 hp @ 2,300 RPM
477 gallons of 80 octane gasoline providing a cruising range of 100 miles on roads
Top speed 22~ MPH on roads.
M6A1
63.15 ton combat weight
6 man crew
3" M7 Main Gun with 75 Rounds
37mm M6 Coaxial with 202 Rounds
2 x .50 Caliber M2HBs in Flexible Twin Hull Mount with 6,900 rounds
.30 Caliber M1919A4 MG in fixed Bow Mount with 5,500 rounds (total .30 cal)
.30 Caliber M1919A4 in flexible Anti-Aircraft Mount (or .50 M2HB)
Front Upper Hull: 83mm at 30 degrees
Front Lower Hull: 102-70mm at 0 to 60 degrees
Upper Side Hull: 44mm at 20 degrees
Lower Side Hull: 70mm at 0 degrees (including track armor)
Rear Hull: 41mm at 17 degrees
Top Hull: 25mm at 90 degrees
Hull Floor: 25mm at 90 degrees
Gun Mantlet: 102mm at 0 degrees
Front Turret: 83mm at 7 degrees
Side Turret: 83mm at 0 degrees
Rear Turret: 83mm at 0 degrees
Top Turret: 25mm at 90 degrees
Engine
Wright G-200 Radial 960 hp @ 2,300 RPM
477 gallons of 80 octane gasoline providing a cruising range of 100 miles on roads
Top speed 22~ MPH on roads.
M6A2E1
77 ton combat weight
5 man crew
Armament Data
105mm T5E1 Main Gun with 60 rounds
.30 caliber M1919A4 Coaxial with 4,000 rounds
.50 Caliber M2HB in flexible Anti-Aircraft Mount with 600 rounds
Front Hull: 191mm at 0 degrees
Upper Side Hull: 44mm at 20 degrees
Lower Side Hull: 70mm at 0 degrees (including track armor)
Rear Hull: 41mm at 17 degrees
Top Hull: 25mm at 90 degrees
Hull Floor: 25mm at 90 degrees
Gun Mantlet: 191mm at 0 degrees
Front Turret: 191mm at 0 degrees
Side Turret: 89mm at 0 degrees
Rear Turret: 208mm at 0 degrees
Top Turret: 25mm at 90 degrees
Engine
Wright G-200 Radial 960 hp @ 2,300 RPM
464 gallons of 80 octane gasoline providing a cruising range of 100 miles on roads
Top speed 18~ MPH on roads.
