Saturday, 7 May 2016

Halfway to the Hellcat

In late 1941, design of a new light tank destroyer on the Light Tank M3 chassis began in the USA. Work on equipping the T56/T57 GMC with a 76 mm gun game to a dead end. It turned out that the M3's light chassis is unsuitable for such vehicles. By the time the T56 arrived at Aberdeen, the T49 was already undergoing trials. Even though it also never made it to production, further development of its design resulted in one of the best known American tank destroyers of WWII.

Upward Trend

In May of 1941, Marmon-Herrington Company Inc. received an order for an airborne tank. In August, a full sized model of the newly indexed Light Tank T9 was ready. Further development of this project resulted in the M22 airborne tank, referred to as "Locust" by the British. This was the only airborned tank used for its intended purpose in WWII.

Light, low, and fairly long, the Light Tank T9 interested the military as a chassis for a tank destroyer. Unlike the Light Tank T3 chassis, the goal here was not to include a large caliber. The year was 1941, and the 37 mm gun was sufficient to fight most enemy tanks. The American military did not know about the Soviet T-34 and KV-1 or that the Germans were hard at work on tanks with thick, anti-shell armour.

In the fall of 1941, the 37 mm Gun Motor Carriage T42 project was launched. The draft project was ready by October 27th. The initial concept of the vehicle differed little from the Light Tank T9. The biggest difference was in the roomier turret with an open to. This turret contained the same 37 mm M5 gun and a Browning M1919 machinegun. Another machinegun was installed in the hull.

This concept did not stick around for long. Calculations showed that this design would not work. The new turret would need a longer hull, and the suspension had to be lengthened to make the vehicle more stable. On November 7th, 1941, a document was prepared listing improvements in the T42's design. The hull and tracks were to be lengthened by 30 cm and the clearance increased from 28 cm to 35.5 cm. The suspension would also be redesigned.

37 mm Gun Motor Carriage T42 as of December 1941

The redesigned T42 Gun Motor Carriage was ready on December 11th, 1941. Its hull grew to 3917 mm in length, and each road wheel received its own individual spring suspension. The turret was taken from the wheeled T22 Gun Motor Carriage, which would later become the M8 armoured car. Even with these changes, the military did not see the T42 GMC as a stable platform for an anti-tank gun. The recommendations prepared after studying this new design changed it even more.

The final design of the T42 GMC differed noticeably from the initial project.

The first draft of the modified T42 GMC was ready by December 29th, 1941, and Marmon-Herrington presented an improved version on January 5th. The front and rear of the hull were not changed, but the overall length grew to 4715 mm. The hull of the new tank destroyer was now longer than that of the Light Tank M3, and thus a more stable platform. The turret was also changed slightly, but overall it was analogous to what was installed on the T22 GMC. 

The suspension was redesigned much more radically. Gladeon Barnes had his revenge on Harry Knox and his design deservedly won out. Some sources say that the vehicle used a Christie suspension, but that is not the case. Barnes used the suspension that he developed in 1933 for the Combat Car T4 and Convertible Medium Tank T4. It was based on the Christie suspension, but it was more compact and more effective. The T42 GMC had 4 road wheels with a diameter of almost 840 mm per side. The drive sprockets were also changed. Very little was left of the Light Tank T9 in this vehicle, only parts of the hull and the engine.

According to calculations, the combat mass of the T42 GMC would be about 6.5 tons. Its armour would be 33 mm thick in the front of the hull and the turret, and the sides and rear were 9.5 mm thick. This armour was the cost of high maneuverability.

All Anew

These radical changes to the initial 37 mm Gun Motor Carriage T42 cast doubt on the assignment of the project to the Marmon-Herrington Company. The small company was busy with work on the Light Tank T9, which had to be seriously changed in accordance with the military's requirements. A logical solution was to transfer the project to another contractor: Buick, one of the divisions of the automotive giant General Motors. By that time, Buick completely ceased producgion of cars and focused on military needs. Its main product was aircraft engines, but the conveyor belts could be filled with something else.

76 mm Gun Motor Carriage T50, on the T49 chassis. This was the initial T49 hull as developed by Buick.

On April 1st, when Buick's design bureau had just started work on their own version of the T42 GMC, the Ordnance Committee made another radical change to the requirements. The 37 mm M5 gun was no longer considered sufficient for a light tank destroyer. This conclusion was made after analysing the British experience of using American tanks in North Africa and information on new tanks obtained by intelligence. The proposed solution to this problem was the use of the British 6-pounder (57 mm) QF Mk.III gun. The mount for such a gun was already being worked on since winter of 1942 for the Light Tank T7E2.

Full size model of the 57 mm Gun Motor Carriage T49, June 1942.

The new gun, approved by Barnes, made the T42 unacceptable in its current form. The use of a more powerful gun meant a significant increase in weight, which meant that the chassis had to be strengthened and a bigger engine had to be used. The T42 GCM vanished into the aether. Instead, Buick received an order to develop and produce two prototypes of the T49 Gun Motor Carriage T49. By the way, this vehicle was sometimes called Christie GMC in documents, which could confuse researchers.

Assembly of a T49 GMC experimental prototype at Buick. Captain Allerton Cushman, supervisor of tank destroyer work, is standing in the vehicle.

In May-June of 1942, an alternative 76 mm Gun Motor Carriage T50 was discussed, which would use elements of the T49 GMC design. The turret was removed, and instead, a 76 mm AA gun with a 15 degree horizontal traverse to each side was added. Using this project, one can trace the evolution of the T49 GMC.

While the suspension remained a copy of the T42, the hull changed radically. It is not listed in the documents, but the engine was likely changed. Examining the project, the Ordnance Committee rejected it. In its current state, the center of mass was too high, which would have negatively impacted the stability of the platform. It was also unsatisfactory that the gun had no shield. The limited traverse also negatively impacted the maneuverability of fire.

An experimental prototype of the T49 GMC after installation of the gun.

Significant changes made to the design meant that the model presented by Buick in June of 1942 no longer had anything to do with the T49 GMC. The hull was lengthened to 5280 mm, increasing the amount of road wheels to 5. The turret was designed anew, and the hull was also a new design. The crew was increased to five men. Even the suspension was different: while it was still based on the Christie design, the springs were now outside. This solved one problem of the Christie suspension: taking up a large amount of space in the hull.

Initially, the military met the new Buick design coolly. A list of necessary changes was made. It was proposed that the driver's compartment be changed to be more like the M8 armored car, the hull machinegun be removed, the communications system changed, the turret be made open, and the crew reduced to 4. Nevertheless, in the middle of June of 1942, approval was given to produce the T49 GMC in effectively the same configuration as was presented.

A telling comparison. The T49 GMC was not only lower than the Medium Tank M4, but lower than medium tank destroyers.

The experimental 57 mm GMC T49 was ready in July of 1942. Rock Island Arsenal was developing the gun, and it was not yet ready when the experimental vehicle was built. The GMC with registration number USA 6029910 began its trials without a gun.

The mass of the vehicle was 14.4 tons, more than twice as much as the T42 GMC. To compensate, the vehicle used two Buick Series 60 engines. Each of these 8-cylinder engines had a volume of 5.24 L and output 165 hp. Earlier, these engines were used in passenger cars. Due to their automotive origins, launching the T49 GMC into production would not have been a problem.

T49 during trials

In theory, the pair of motors would have been enough to accelerate to 55-60 mph (88-96 kph). but in practice achieved only 53 mph (84.8 kph), which was still an impressive result. At the time, no tracked tank was capable of such speed. Loss of power in the hydraulic torque converter further reduced that to 38 mph (60.8 kph). It was proposed that the problem could be resolved by using a hydraulic transmission.

Engine compartment of the T49 and T67 tank destroyers.

Despite all of its problems, the vehicle turned out far superior to the T56 GMC. The use of a fully rotating turret gave the vehicle high maneuverability of fire. The modified Christie suspension proved itself, again showing the preference for an allegedly bad coiled spring suspension. The report on T49 GMC carriage trials says that the suspension worked better than Knox's alternative.

Mobility trials showed that the T49 starts off slower than the Light Tank M5, but then breaks out ahead, noticeably surpassing the M5 at the finish line. In other words, the vehicle had potential.

Almost Accepted

While the T49 was undergoing trials, Tank Destroyer Command was thinking about how useful the M1 57 mm gun was. The difference in penetration was not that high. The result was the decision to build a second prototype of the T49 with a 75 mm M2A3 gun. According to the initial decision, the tank destroyer was built with a the same turret, since the gun could fit. Later, this idea was rejected, since the closed turret had problems extracting gases during shooting.

The final decision regarding the conversion of the T49 GMC was made by the Ordnance Committee on October 10th, 1942. The 75 mm M2A3 gun was installed in the open top turret borrowed from the T66 GMC wheeled tank destroyer. The second vehicle was build from scratch, but the first was built on the chassis of the T49 from Aberdeen, sent back to Buick. During the conversion, the hull machinegun was removed, as requested in the June 1942 requirements. The new vehicle was indexed 75 mm Gun Motor Carriage T67, and was finished in November of 1942. Technically, both vehicles were identical. Sadly, no photos remain of them in the original configuration.

76 mm Gun Motor Carriage T67 at Aberdeen Proving Grounds.

The T67 GMC arrived at the Aberdeen Proving Grounds in November of 1942. Despite its increased mass, its characteristics remained about at the same level as the T49 GMC. Gunnery trials showed that the direction given by Tank Destroyer Command was correct. The chassis had enough wiggle room to put on a more powerful gun with satisfactory precision of fire.

The successful gunnery trials showed that the T67 GMC could be used for a more powerful gun, like the M1 76 mm tank gun with AA ballistics. It was installed in late November of 1942, and surviving photos of the vehicle show this configuration. To compensate for the increased mass, the suspension was reinforced. Trials showed that the use of a more powerful gun had little impact on the precision of fire and mobility. These results were a death sentence for the T56/T57 GMC, which was a nightmare for its crew. Compared to that vehicle, the T67 GMC provided satisfactory comfort to its 5 man crew and did not raise any questions about its role on the battlefield.

Barring the lack of the hull machinegun, the T49 and T67 were almost identical.

All of the above indicates that the T67 GMC with a 76 mm gun could have gone into production with minimal changes. Nevertheless, the story of this tank destroyer does not have a happy ending. The official version of events states that the transmission had to be redesigned due to problems, and the engine changed as well. In reality, the T70 GMC accepted into service as M18 GMC, also known as Hellcat, was a completely different vehicle. Conceptually a successor to the T67 GMC, it had a different hull, turret, suspension, and engine, and the new transmission was at the front. It's hard to agree that only a transmission redesign makes all those changes necessary.

The Christie springs as seen clearly on this photo. The T67 GMC was the last American vehicle built in metal to use this type of suspension.

It's possible that one of the goals of the modernization was the unification of the future T70's engine with that of the M4 Medium Tank. Both used the Continental R-975 engine, but slightly different modifications of it. The new engine was about 25% more powerful than the Buick Series 60 pair, and allowed the fighting compartment to be larger, with the turret shifted backwards. On the other hand, the cost of all these changes was about half a year spent on the design and trials. There was no major conceptual difference between the T67 and T70. In the conditions of war where the enemy is constantly upgrading his vehicles, half a year is a long time.

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