Monday, 4 October 2021

On the Way to the Most Numerous American Tank

The Americans met the start of WWII with about a dozen Convertible Medium Tanks T4 and a handful of Medium Tanks M2. Despite this poor state of readiness, American industry made some important steps even before the war that let them make up this gap very quickly. While battles were raging in Europe, the US launched first the Medium Tank M2A1 then the Medium Tank M3 into production. Solutions worked out on these vehicles allowed the development of a new generation medium tank: the Medium Tank T6. This vehicle was standardized as the Medium Tank M4 on September 5th, 1941.

Gun in a turret

Unlike light tanks, which the Americans were doing well with, the situation with medium tanks was dire. To be fair, medium tanks were in a difficult position in more than half of major tank building nations in the late 1930s. The search for an optimal medium tank took a long time, but the Americans were in a situation where there was simply no need for one. An attempt to use the Christie chassis resulted in a tank that did not differ from its light cousins in effectiveness.

The situation only changed during the development of the Medium Tank T5, the precursor to the Medium Tank M2. This tank was quite good for its time, but was not mass produced for a number of reasons. First of all, Americans also kept an eye on the situation in Europe and made proper conclusions. Second, the tank was built at Rock Island Arsenal, a factory intended to build artillery. It was simply not suited for building even the 17.3 ton Medium Tank M2, let alone the 18.7 ton M2A1.

The first experience in building a fully cast turret platform and engine deck. Areas where bolts were to be installed can be seen. This is a feature inherited from the SOMUA S 35.

A decision to build a tank factory in Detroit was made in mid-June of 1940. However, the Medium Tank M2A1 did not remain a priority product for long. The Ordnance Committee developed requirements for a new medium tank with a 75 mm gun on June 13th, 1940. The tank was based on the M2A1 chassis, but this time the weapon was not fitted in a rotating turret. American industry was not ready for such a task, in part due to the cutting equipment required to make the turret ring. The largest turret ring they could provide was a 1380 mm wide. As a result, the Medium Tank M3 accepted into service on July 11th was in many ways a compromise. This tank was standardized before it was ever built in metal since the task was urgent and the army was in desperate need of medium tanks. The 75 mm M2 gun was located in a sponson on the right side of the hull. The tank's turret mounted a 37 mm M5 gun. Despite this strange design, the M3 was not a bad tank. It was quite an adequate vehicle for its time, especially if you compare it to what the British were building.

The final design for a cast upper hull. The influence of the Ram tank can be seen. The same overall concept was used for the Medium Tank T6.

The fact that the M3 was only a temporary measure is underscored by the new requirements composed on August 31st, 1940. Armored Forces command wanted a new medium tank with a 75 mm gun in a full turret. However, these requirements were nothing more than an intention to move in a new direction. All the resources of the Ordnance Department were aimed at development of the Medium Tank M3, which at that point was nothing more than a scale model. However, it's hard to say that no work was done at all. The new tank was slowly coming together with help from the Canadians and the French. There were plans to build the M3 in Canada with the General Steel Castings Corporation as a subcontractor. This factory was built in the late 1920s and specialized in locomotives. Like its founder, Baldwin Locomotive Works, the factory was pulled into tank building in 1940. Since the factory was already equipped for casting steel, it was reasonable to use it to supply cast hulls and turrets. This is where French experience came in handy.

Martin Preval, a former employee of SOMUA, ended up in Canada after the fall of France. He knew more about cast tank hulls than any man alive. Preval ended up being extremely useful for both Canada and the US. General Steel presented a prototype turret platform and engine deck for the Medium Tank M3 that were fully cast in September of 1940. Bolt connections characteristic of the SOMUA S 35 were very prominent. However, they were later discarded since they were difficult to produce and weakened the hull. Preval's work continued, resulting in the Canadian Ram tank but also some side effects that benefited the Americans.

A full scale model of the future Medium Tank T6. This model shows an exotic set of armaments: coaxial 75 and 37 mm guns.

Work on the M3's replacement began only in February of 1941. By then the Medium Tank M3 prototype had been built and it was clearer what its replacement would have to look like. General Steel had also finished the hull for the future Ram tank. Unlike the Canadian vehicle, this tank would not have a machine gun cupola, but the same side hatches were present. Requirements for the tank were still being debated as of April of 1941. The overall requirement to put the gun in the turret and reduce the height of the tank remained, but final decisions were still not made about the rest of the tank.

The final mockup. Work on the real thing began in May of 1941.

A large scale meeting with representatives of the Ordnance Department, Armored Force command, and other stakeholders took place on April 18th, 1941, at the Aberdeen Proving Grounds. A full sized model of the tank was already finished by then, but it didn't have a designation yet. The armament was the same as on the Medium Tank M3, but both guns were installed in the turret. The Heavy Tank T1 with a similar gun mount was also coming together at the time. The turret ring was widened to 1753 mm to give the crew proper working conditions, which came in handy later during the evolution of the Medium Tank M4. The idea of using these two guns disappeared quickly, but there were still plenty of options. In addition to reasonable ones (105 mm howitzer or 57 mm 6-pounder gun with a coaxial machine gun) there were more exotic options: two 12.7 mm machine guns or two 37 mm M6 guns plus a machine gun. The approved variant was far more conventional: the 75 mm M2 gun with a coaxial machine gun. The gunner was located to the right of the gun, not the left.

This model used the M3 gun, but the prototype had the shorter M2. No other option was available.

The configuration of the hull was also hotly debated. A welded variant was discussed in addition to the cast one. Even though welded hulls were not yet picked up by American tank building, work in that direction was already underway. More powerful engines were also discussed, including the Wright R-1820 Cyclone. The G100 variant put out 1100 hp (or 775 hp for a reduced power version) and the G200 put out 1200 hp. The idea of a new engine was rejected, since it would require changes to other components. The armament in the hull was also changed. The assistant driver was given a machine gun in a ball mount. Changes to the M3 chassis were minimal. The AA machine gun cupola from the M3 tank was also installed on the turret without changes.

The Medium Tank T6 was built with a cast hull, but Rock Island Arsenal also prepared a welded design.

The Ordnance Committee authorized the building of a new full sized model in May of 1941. This model would include all the changes required of its predecessor. The model was finished in June. The top of the hull was still cast, but now the assistant driver had his own observation device (although still no hatch). The shape of the turret changed and the gun mount took its final form. The reason for preserving the casting was partially economical: each tank with this hull cost $3000 less, or a savings of 10%. On June 19th the Ordnance Committee approved the cast hull variant as the main type. The model was also approved. The replacement for the Medium Tank M3 finally received a name: Medium Tank T6.

The bottom part of the hull was unchanged compared to the Medium Tank M3.

This was not the end of changes to the model. Even though the M2 gun was quite decent and work on a stabilizer was already underway, the idea of building an even more powerful gun already came up. This was a future-proofing idea, as for 1941 the M2's AP round was powerful enough. The 40-caliber version of the gun was indexed T8 and later standardized as M3. A model of the T8 was installed in the model of the Medium Tank T6 in August of 1941, although the prototype was still built with the M2 gun.


The Ordnance Committee split up the work on the Medium Tank T6. Just because the cast upper hull was higher in priority didn't mean that the welded hull was no longer necessary. This solution was considered promising, but it would take time to set up welding in production. Work on the welded hull was transferred to Rock Island Arsenal. They were supposed to build a welded hull with no turret. The idea here was to work out issues with welding.

Chassis of the Medium Tank T6 in the process of assembly. The driver's seat allowed him to look out of his hatch while driving.

The Aberdeen Proving Grounds was no doubt the key player in the Medium Tank T6 program. Its abilities permitted it to build not only wooden mockups, but also functional prototypes. All that was needed was subcontractors to make components. In case of the hull, this was provided by General Steel. In the summer of 1941 they were preparing for production of hulls for the Canadian Ram tank. The two vehicles were similar: they were both built on the Medium Tank M3 chassis with a riveted lower hull and cast upper hull with evacuation hatches in the sides. This was unlikely to be an accident, since the first Ram I had arrived at the Aberdeen Proving Grounds by August of 1941. Assembly of the Medium Tank T6 prototype was coming to an end and the presence of its Canadian cousin was very helpful.

The driver had two vision devices: a direct vision slit and a periscope.

The Medium Tank T6 began to come together in mid-summer of 1941. The vehicle already looked like the M4A1 that would enter production in February of 1942, but the resemblance was superficial. The lower part of the vehicle was closer to that of the M3, plus as mentioned above the work on welding hulls was still in early stages, and so the hull did not use welding at all. The lower hull was assembled with rivets. The engine and transmission were the same. Despite the wishes of the Ordnance Department and other organizations to have a more powerful engine, there was not yet an alternative available to the Continental R975 EC2. The transmission remained the same, but the driver's compartment was different.

The assistant driver's station. The SCR 506 radio to his right partially covered the evacuation hatch.

The driver was no longer perched on top of the crankshaft, but seated to the left of it. His vision port disappeared, but his vision was hardly worse. First of all, in combat the driver of the M3 looked through a vision slit covered only with bulletproof glass. The T6's driver also had an armoured shutter. This was safer and did not impact his vision. There was also a periscope located in the driver's hatch flap, plus the driver could elevate his seat and look out of the hatch during travel. This made it much easier to drive this tank than the M3.

The experimental prototype only had a hatch for the driver.

The assistant driver's seat also changed radically. He now sat to the right of the driver in a proper workspace rather than an awkward corner. He had no hatch at first, but two observation devices. One difference from the M3 was that the assistant driver was also the machine gunner with three machine guns to service as well as a radio operator. The Medium Tank T6 had two radios: an SCR 508 in the turret and SCR 506 in the hull. The SCR 506 had a longer range. It was located in the sponson to the right of the assistant driver. The antenna port was located on the roof of the driver's compartment. Issues with the evacuation hatches arose at this point, since the radio partially covered one of them.

Turret of the Medium Tank M4A1. The T6 had a nearly identical one. The turret basket greatly hampered access to the driver's compartment.

The fighting compartment also changed radically. Like before, three men sat in the turret: the commander, gunner, and loader. This time the turret was much larger to accommodate the 75 mm gun M2. Like the M3, the gun was equipped with a periscopic sight. The gunner sat to the right of the gun. The commander sat behind him. He inherited the M3's machine gun cupola. The loader was located to the left, and at first he had no observation devices other than the pistol port. Like the Medium Tank M3, the turret of the T6 had hydraulically powered traverse which could make a complete rotation in only 15 seconds. The commander also operated the radio. The SCR 508 was located in the turret bustle like on the Grant tank. 

SCR 508 radio in the turret bustle.

The new fighting compartment was noticeably roomier than on the Medium Tank M3, but not without its quirks. The turret and its components evolved from analogues used on the M3, which included the turret basket. The M3 had special access ports to enter and exit the basket. The T6 also had ports, but they were much smaller. One could still move between the fighting and driver's compartments with some effort if they really wanted to, but only if the turret was in a certain position. The ports were not meant for that, but rather for accessing the ammunition stored in the sponsons.

The cast upper hull.

The turret basket also called into question the use of evacuation hatches in the sides of the hull. Only the driver and the assistant driver could really use them, but they already had a hatch in the floor. Theoretically, if the turret crew managed to squeeze into the driver's compartment then they would also use this hatch. It was dirtier, but much safer. Another drawback of the turret basket was the limit to the height of the fighting compartment. Despite the height of the tank (measured up to the turret roof) being 2642 mm, the turret crew could not work while standing.

The Medium Tank T6 was shown at the Aberdeen Proving Grounds like this on September 3rd, 1941.

Assembly of the Medium Tank T6 began closer to August of 1941. The delays were the fault of General Steel, which was responsible for the hull and turret. Final assembly was completed by September 3rd, 1941, which is when the tank was presented to a commission from the Ordnance Department and Armored Force command. The tank weighed 27,241 kg, less than the Medium Tank M3. The armour was the same (51 mm in the front and 38 on the sides), but the turret armour was now 76 mm. The same engine gave the tank comparable mobility to its predecessor. The top speed was 38.4 kph for short bursts and 33.6 kph at normal power.

The tank changed by mid-September. The commander's cupola disappeared and a counterweight was added to the gun.

The difference between the prototype Medium Tank M3 and Medium Tank T6 was stunning, even though they were built less than half a year apart. While the M3 was inferior to the leading medium tanks of the time due to its unconventional gun placement, the T6 caught up with them. It had a powerful gun with a stabilizer. The armour was quite competitive, especially the turret. The protection was superior to that of the Pz.Kpfw.III and Pz.Kpfw.IV. This was also true for the tank's armament, even before the M2 gun was replaced with the even more powerful T8 (M3). The T48 gun mount with a coaxial machine gun was initially planned for this longer gun. The mobility was also high. In summary, the idea of building a new tank on an existing chassis was quite wise.

The rear of the experimental vehicle was identical to the model.

A meeting was held on the same day that the Medium Tank T6 was demonstrated to discuss overall wishes for improving the tank. For starters, the evacuation hatches in the sides were considered unnecessary and were removed. The Medium Tank M3 cupola was also considered unnecessary, and a much smaller hatch with a periscope in one of the flaps was installed instead. The fact that a 12.7 mm AA machine gun was needed was also voiced. The loader needed a rotating periscope. The T48 gun mount was good, but its protection could be improved. The overall list of changes was short and the Medium Tank T6 was considered quite good. The issue of immediate standardization was raised, and on September 5th, 1941, the tank was standardized as the Medium Tank M4.

Mobility trials continue, fall 1941.

Conversions began immediately after the official presentation. The cupola was replaced with the new hatch on September 15th. A counterweight was added to the barrel to allow the stabilizer to function. The tank entered trials in this form. The trials also checked the operation of the radio. Issues were raised with the radio antenna in the hull, although this radio was only present on commanders' tanks. The list of desired improvements kept growing during trials, now it included improvement of the machine gun ball mount, need for extra turret ring protection, and hatch for the assistant driver. The list grew further and the Medium Tank T6 changed gradually. The production Medium Tank M4 would have been even more different.

Medium Tank T6 in February of 1942. Instead of the M2 gun it now had the M3 with a longer barrel.

Work on the production Medium Tank M4 began in November of 1941. This designation did not last for long in its initial form. The index M4 applied only to the tank with a welded hull, the one with cast hull was designated M4A1. The first production tanks entered trials soon after, but that did not mean that the Medium Tank T6 would be retired. It was still used as a driving test bench. For instance, the tank received the 75 mm M3 gun in early 1942. By this point the turret received lifting eyes, which made removal and installation much easier. In March of 1942 the tank received a new tougher cast transmission cover. The Medium Tank T6 also used new types of track links. The production Medium Tank M4 weighed 30 tons and needed wider tracks. The 420 mm wide metal-rubber T48 and all-metal T49 were tested on this tank.

Medium Tank T6 at the end of its life, 1947.

The prototype was always tested at the Aberdeen Proving Grounds. After the war it was moved to a demonstration area next to the proving grounds which doubled as a museum. The tank did not last here for long. A large scale scrapping drive took place in the early 1950s, and the Medium Tank T6 was melted down. Some researchers hope that it had a different fate and it became a shooting range target somewhere close to the museum.

Original article by Yuri Pasholok.

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