On the 2nd of September 1941, a single tank was completed and drove under its own power for the first time at Aberdeen Proving Ground (APG) in Maryland. It was the first of nearly 50,000 and the first U.S. designed tank to employ the now standard concept of a three man turret crew. Only 13 months later it would see its first action in the North African desert in the second battle of El Alamein with the British Army and would remain in frontline service for another 30 years in various forms with a variety of countries. The tank was the T6 and it would become the legendary Medium Tank M4 or as the British named it, the Sherman.
On the 31st of August 1940, the United States Army Armored Force submitted detailed characteristics for a medium tank to replace the Medium Tank M3, even though the design of the M3 hadn’t been finalised and was not expected to be in production before the following summer. The M3 was a stop gap design and its shortcomings were already apparent. The most obvious shortcoming of the M3 was the location of the main 75mm gun in a limited traverse hull mounting, so the highest priority was given to designing a suitable fully rotating turret for 75mm main gun. The Armored Force also listed in its requirements a lowering of the overall height of the tank in comparison to the M3 and a provision for anti-aircraft protection.
As the design process of the M3 was fully occupying the design team at Aberdeen, the new tank had to wait until the M3 design was completed. On the 1st of February 1941, the Chief of Ordnance released a directive to proceed immediately with the design of the M3 replacement.
At a conference at APG on the 18th of April 1941, the major features were confirmed for the new design. The basic chassis of the M3 was to be retained including the lower hull, engine, gearbox and final drive, suspension and tracks, most of which had already been carried over from the previous Medium Tank M2. Two main reasons for the M2/M3 carryovers were an ease of transition on the production lines from the M3 to the new tank and because the M2 chassis dimensions were already designed with mass transportation in mind. The original M2 was built to fit a standard gauge railway flatcar. The U.S. was under no illusion that it would be fighting a major war in the continental United States, so priority was given for a mass produced vehicle able to be shipped across oceans with as little alteration to existing transportation infrastructure as possible.
Although the design was based around the continued use of the Wright R975 radial engine, the engine bay was designed with enough space to accept larger engines in the future as they became available.
The new upper hull was to be either cast or welded and was to use as many existing components from the M3 design as possible. The turret ring was increased to 69” (1752.6mm) and the armor thickness was to be a maximum of 3” (76.2mm) on the glacis. The twin fixed .30 caliber machine guns (operated by the driver) were to be retained from the M3 design as well as a new bow mounted .30 caliber machine gun (operated by the co-driver, also known as the bow gunner or BoG) on the right hand side of the glacis.
Two escape hatches were to be placed (one on either side) of the hull sponson plates to allow the crew the ability to bail out of a damaged vehicle on the leeward side of any incoming fire.
The turret was to have a removable plate to allow the fitting of a selection of armament combinations. Five possibilities were considered:
(1) One 75mm (2.95”) gun M2 with a .30” (7.62mm) coaxial machine gun.
(2) Two 37mm (1.45”) guns M6 with a .30” (7.62mm) coaxial machine gun.
(3) One 105mm (4.13”) howitzer with a .30” (7.62mm) coaxial machine gun.
(4) Three .50” (12.7mm) machine guns mounted for high angle anti-aircraft fire.
(5) One British Qf 6pdr (57mm) high velocity gun with a .30” (7.62mm) coaxial machine gun.
Although not listed in available reference material, one other combination was obviously considered at some point as a picture depicts the wooden mock up with a different style turret with a 75mm M3 main gun and a 37mm M6 coaxially mounted. The commander’s cupola from the M3 with its high angle .30 caliber anti-aircraft machine gun was to be retained. It was hoped that this design would allow a single vehicle design to be equipped to meet a number of tactical missions.
The crew was to be reduced to five with a driver and co-driver in the front of the lower hull and three man turret crew in the arrangement that would become the standard for all future U.S. tank designs. The gunner was to be on the right of the gun at the front of the turret with the vehicle commander behind and slightly above him under the cupola. The loader was on the left of the main gun with the .30 caliber coaxial machine gun to his front. The turret crew would share a single roof hatch and the two hull crew would share a single roof hatch on the left side of the hull and a floor escape hatch just behind the co-driver’s seat (these were obviously augmented by the two sponson escape doors).
The T6 full size wooden mockup, to aid speed the lower hull and running gear was reused from the wooden mock up of the M3.
In May 1941 the Ordnance Committee recommended a full size wooden mockup and a pilot tank to be built. This was approved in June 1941 and the new design was designated Medium Tank T6. Once the wooden mock up was completed, Aberdeen was instructed to build a pilot model using a cast upper hull. The casting of both hull and turret was to be done by the General Steel Castings Corporation of Granite City, Illinois. At the same time the Rock Island Arsenal was instructed to build a welded hull version but this was delayed because of modifications to the design to reduce the number of plates and the length of the welded joints.
The T6 Pilot
The pilot T6 was completed at Aberdeen on the 2nd of September 1941, and was inspected by representatives of both the Armored Force and the Ordnance Department. The pilot used a donor lower hull, suspension and tracks from an M3 to aid speed to the project.
The T6 pilot on presentation day, original features that were soon removed include 1) the M3 commander’s cupola 2) side grab handles and 3) the circular armored cover for the antenna base. Picture credit: sherman_minutia
As built, the T6 had side doors but without the vision devices as seen in similar doors on the M3. The bow mounted .30 caliber was linked to a sight rotor in the upper hull casting and both hull crew positions were equipped with direct vision slots with hinged armored covers. The driver also had a periscope mounted in the hatch above his head and his seat was height adjustable allowing him to drive with his head out of the hatch when open, or lower down with the hatch closed using either the periscope of direct vision slot for observation. The twin .30 caliber machine guns were operated by the driver with a limited elevation.
A pistol port with a protectoscope (an armored glass vision port) was located on each side of the turret, and the gunner’s sight rotor was linked in the same way as the bow machine gun. This linkage only moved the top mirror in the periscope which if damaged could be replaced from within the tank by way of a mirror magazine that was integral to the sight.
The gun mount (T48) was designed for the longer barreled M3 75mm (2.95”) gun (L40) but as none were available the shorter M2 75mm (L31) gun was installed. This was breech heavy in this mount and prevented the gyrostabilizer from working correctly. Counterbalance blocks on the muzzle were required to remedy this and were installed shortly after presentation day. The gyrostabilizer was essentially the same as the unit used on the M6 37mm (1.45”) in the M3 Medium Tank.
The T6 was fitted with two radios, one in the front right of the hull, operated by the co-driver (an SCR 506) and one in the turret bustle for use of the commander (an SCR 508). The radio brackets in the turret were also designed to take the British No. 9 radio set.
The co-driver’s position showing twin bow machine guns and the linkage bar to the rotor sight with the SCR506 radio behind.
At a conference on 3rd September, a number of key changes to the T6 were agreed for any further production models, these included;
(1) Removal of the M3 style commander’s cupola to be replaced with a commander’s split hatch that would become standard on the M4.
(2) Removal of the hull sponson doors, as they were considered to be too great a compromise of the armor integrity and also restricted the amount of main gun ammunition that could be stowed in the sponson racks.
(3) A rotating periscope would be retrofitted in the turret roof above the loader’s position.
(4) The rotor mount bow machine gun was to be replaced with a ball mount.
(5) If possible a .50 caliber (12.7mm) anti-aircraft machine gun be installed.
(6) A gun shield should be added to protect the gun mount from splash damage.
The conference confirmed that the T6 would be standardized as the Medium Tank M4.
The M3 style cupola was removed very soon afterwards as it only appears on pictures taken on the 15th of September and is gone in photos from the 16th of September.
Further changes include the addition of an armored cover for the M3 air intake on the engine deck. A change in the casting of the rear of the hull and the replacement of the pepperpot exhaust, which were ill suited to the task and caused problems of serious overheating of the rear decks of M3s. These changes required an adjustment in the tool stowage positions.
The rear deck showing the unarmored air intake inherited from the M3.
The co-driver also received a roof hatch with the removal of the rotor sight. The hinges of the hull hatches were moved forward from their original position at the rear of the hatch. The circular armor protection for the antenna mount above the SRC 506 radio was replaced with a hull ventilator and another ventilator was added just behind the turret and another fitted in the turret roof.
The welded hull pilot constructed by the Rock Island Arsenal has become somewhat of a mystery, no photos have (to date) been found of the completed pilot and only one picture seems to exist of a scale model of the vehicle. Interestingly, it includes the co-drivers hatch and does not have the side sponson doors and also included the later M34 gun shield.
It should be noted that the T6 was not the only horse in the race to replace the Medium Tank M3. In June 1941, three months before the T6 was completed, the Canadian built pilot of the Cruiser Tank Ram was completed and arrived for testing at APG in August. Before the first M4 had rolled off the production lines, 110 Rams had been built by the end of February 1942.
The Ram used the entire lower hull and running gear of the M3 with little or no changes. It utilized a smaller turret ring than the T6, only 60” (1524 mm) as opposed to the 69” (1752.6 mm) in the T6, which left the turret described as “cramped” and unsuitable for anything larger than the QF 6pdr fitted. As such, the Ram was considered unsuitable for combat by both the Americans and the British after trials at APG, but it served successfully as a training vehicle and as a base for a number of specialist vehicles.
T6 Medium prototype by David Bocquelet
On the 11th of December 1941, the welded hull version was designated the Medium Tank M4 and the cast hull the Medium Tank M4A1. Construction of the initial production pilots commenced in November 1941 and full production of the Medium Tank M4 began in February 1942. The last known reference to the T6 was in February 1947 in a picture showing it fitted with a potential field modification for additional armor protection for the differential housing. It is likely that the T6 was scrapped during the Korean War era “scrap drives,” but there remains a small hope that it survived and is still waiting to be rediscovered in some forgotten corner of APG.
The Canadian built Ram pilot undergoing automotive trials at APG.
Sherman: A History of the American Medium Tank, R.P. Hunnicutt
The T6 Medium Tank on The Shadock
Armored Thunderbolt, Steven Zaloga
AFV/G2 T6 Mockups, Chris Benedict
Medium Tank T6 specifications
|Dimensions||5.6 x 2.6 x 2.9 m|
18’4” x 8’6” x 9’6”
|Total weight, battle ready||27.2 tonnes (60,058 lbs)|
|Crew||5 (Commander, Driver, Co-Driver, Gunner and Loader)|
|Propulsion||Wright (Continental) R975 EC2, 9 cylinder radial|
|Speed (road)||38.6 km/h (24 mph)|
|Range||193 km (120 miles)|
|Armament||75 mm Gun M2 with 75 rounds|
5 x .30 MG M1919A4 machine-guns with 10,000 rounds
|Armor||Maximum 76.2 mm (3”)|
|For information about abbreviations check the Lexical Index|
This picture dated 7th April 1941 (although this date cannot be confirmed and May would be more likely) of the wooden mock up showing the little known 75mm and 37mm combination and a wider turret than the final design. The cut out in the upper right front of turret is possible a gunner’s sighting periscope. Although the twin bow machine gun apertures are present there is no flexible bow machine gun and no obvious vision device in the front right hull it would seem at this stage no co-driver was planned. The driver’s hatch appears to roll backwards unlike the later hinged hatches.
In this right side view of the early version T6 the right hand side sponson door is missing further adding to the argument for no co-driver.
The last known picture of the T6 taken on the 18th February 1947 with a prototype field expedient armor upgrade for the differential cover.
The new turret design with the pistol port and Protectoscope vision device.
The only known picture of the Rock Island Arsenal welded hull pilot (albeit a scale model) showing some of the design changes from the original cast hull pilot.
A practical demonstration of the floor escape hatch located behind the co-driver’s seat.
Rear view shower the pepperpot exhausts (1) that would soon be discontinued due to overheating of the rear deck area and the original straight edge casting that would be changed on the production models.
This picture clearly shows the reason for the removal of the forward hull mounted antenna which could easily be entangled with the main gun when traversing the turret.