Monday 21 March 2022

Bulldog's Ancestor

The Americans finished WW2 in a very strange position. On one hand they achieved great things, on the other hand they had fallen behind in a few areas. One of the aspects in which the Americans were in the lead was light tanks. Unlike other nations, which abandoned development of these vehicles by the middle of the war, the Americans rightfully kept going. The result was not just the best light tank of its time, but a whole family of SPGs. Production began to wind down in the summer of 1945, but a foundation for further work was already established. As a result, the first truly original tank built to post-war specifications was a light one. This was the Light Tank T37, an experimental vehicle that became the predecessor for the Light Tank M41.

Building on past experience

Even though the Light Tank M24 was the best light tank of WW2, it was not without issues. This vehicle was the result of compromises. It was built around components inherited from the Light Tank M5. There were several reasons for this. One was that reuse of existing components was necessary to keep production numbers up. The second was that the components, especially the engine, were already produced by the same company that made the tanks: Cadillac. There was also no good alternative to the pair of Cadillac engines. The Continental R-975-C4 used on the GMC M18 had a number of advantages, but also some drawbacks, including a short lifespan, large size, and low production volumes. It was already used on a number of fighting vehicles and Continental would not be able to produce more of them. An attempt to install the R-975-C4 on the Light Tank M24 was made anyway, but it was unsuccessful. Since the pair of Cadillac Series 44T24 engines put out only 296 hp at the highest throttle, it was impossible to increase the tank's weight any higher. The Light Tank M24 ended up with a decent power to weight ratio of 16 hp/ton, but the chassis had no room to grow. 

The Light Tank M24 was the best light tank of WW2. However, the idea for its potential replacement arrived by early 1945.

The armament was another problem. The 75 mm M6 gun was a powerful weapon for its class, but the Americans themselves considered it insufficient by the end of the war. The gun was harshly criticized after trials in the USSR, but it's hard to say that the harshness was very excessive. The gun could effectively penetrate only the side armour of German tanks. Of course, a light tank is meant for reconnaissance and not fighting enemy medium and heavy tanks, but the gun was weak nevertheless. More powerful options existed, and indeed later on the Light Tank M24 received the 90 mm D/926 gun (in the Norwegian variant known as the NM-116), but the larger gun required a larger turret, which again bumped up against the capacity of the chassis.

Power pack with a 500 hp Continental AOS-895 engine. The full power pack was 1905 mm long, less than the Ford GAF engine with the same power without its transmission.

The first work on a replacement for the Light Tank M24 began in early 1945. Army Ground Forces command initiated work on finding potential replacements for existing tanks on January 2nd, 1945. A group from the Armored Medical Research Laboratory led by Frederick C. Brecket prepared its report by June 2nd, 1945. According the the commission, the army needed a 22.7 ton tank armed with a 76 mm gun capable of penetrating 127 mm armour plate installed at 30 degrees from 1000 yards. The mobility of the tank also had to be increased, although Brecket's team did not give an answer on how to do it.

Another commission had a very different experience. The War Department Equipment Board, better known as Stillwell's Council after its leader Joseph "Vinegar Joe" Stilwell, was formed on November 1st, 1945. This commission set the path for the future of American tank building. The board submitted its report on January 16th, 1946. Its prospective light tank was not much different from the previous concept. However, they didn't start with asking for a new tank to be built. The first step was to develop the components that would go into it. Harry Knox's chassis had finally died by then, and the solutions developed into the Heavy Tank T26E3 seemed much more promising. This was especially true for the vehicle's layout. Experience showed that a rear transmission made the tank slightly longer, but resulted in many advantages. It was now possible to build a power pack, uniting the engine and transmission in one component. Servicing the transmission was much easier, and there were no longer any vulnerable hatches in the front of the hull.

Comparison of power pack dimensions. Top: Continental AOS-895. Bottom: Ford GAF.

One of the requirements of Stillwell's Council was that special tank engines had to be developed. Up to that point, American tank engines had either aircraft or automobile roots. Either ancestry led to a number of issues, which had to be eliminated. The development of these engines was entrusted to Continental, which succeeded in its task. Nevertheless, they needed time, and so development of the new light tank was postponed. Officially, the tank was named Light Tank T37 on September 27th, 1946. The Detroit Arsenal began working on the tank in July of 1946.

The work entered the practical stage in 1949. The result was worth the wait. Without a new "heart", the tank was doomed to failure. Attempts to shove the Medium Tank M26 engine into a light tank would have failed. Continental developed a quite interesting engine: the six cylinder air cooled AOS-895 (A - air cooled, O - opposite, S - supercharged). 895 indicated the volume: 895 cubic inches or 14.7 L. The engine put out 390 hp in normal operation or a maximum of 500 hp. The engine was 1188 mm long, less than the Ford GAF, and also lower due to the opposite layout. The engine worked with the Alisson CD-500 transmission developed with the use of experience from heavy tank development. The Light Tank T37's power pack was 1905 mm long, much shorter than the Medium Tank M26's engine and transmission, which were 2590 mm long.

An intermediate result

As the Light Tank T37's engine was under development, the tank itself began to take shape. Since the transmission migrated to the rear, the length of the hull increased by 559 mm compared to the Light Tank M24. This resulted in a weight increase, but the weight limit was set at 22.7 tons anyway. The running gear was based on that of its predecessor, but was seriously reworked, especially when it came to the design of the track links. The new T91 metallic track link was developed by Harry Knox. Since the pitch increased to 152 mm, the amount of links per track remained the same: 75. The track links had chevron grousers and removable rubber pads. The road wheel design was the same, but an extra small wheel was added between the last large road wheel and drive sprocket.

Model of the Light Tank T37 with the first variant of the turret. This was the only turret built for this tank.

The drive's compartment layout changed significantly. Previously, it was common for American tanks to put two crewmen here: the driver and his assistant. The Light Tank M24 even had a second set of controls for him. However, it was clear that the assistant driver's presence creates more issues, especially as the caliber of guns increased but the inner volume of a tank did not. As a result, the assistant driver on the Light Tank T37 was replaced with an ammunition rack. The tank now carried 60 76 mm rounds. The designers also revised the driver's workspace. His vision improved greatly and his position in travel mode improved considerably.

A comparison of the Light Tank M24 and Light Tank T37.

The turret changed in more interesting ways. As with its predecessor, it was designed from scratch. It had nothing in common with the designs used earlier. The turret was much larger than the turret of the M24 and had a large turret bustle. Usually these bustles are used as ready racks, but not in this case. The bustle served as a counterweight to the larger and longer gun. Additionally, it housed specialized equipment that will be described later. Another new feature was the method of manufacture. The M24's turret was cast, but the T37's turret was all welded. Finally, the turret ring diameter increased drastically to 1753 mm.

The third variant of the turret had an autoloader mechanism designed for it.

This enlarged turret was connected not just with the main armament, but also other systems. One of them was the T37 stereoscopic rangefinder placed in the front of the turret. It was smaller than the rangefinders used on the Medium Tank T25E1 and Heavy Tank T29E3. Thanks to its compact design, the rangefinder did not extend past the sides of the turret, but still gave it a distinctive look. The gunner who used the rangefinder lost his telescopic sight in the tradeoff. The installation of machine guns was even more interesting. The main gun had a coaxial Browning M2HB machine gun. Two more Browning M1919A4 machine guns were placed on the sides of the turret. This compensated for the lack of assistant driver and his hull gun. The machine guns were installed in special pods with a mechanical linkage that allowed them to be aimed vertically from -9 to +20 degrees. The machine guns were also controlled by the gunner and could be aimed through the T32 periscope in the turret roof.

Light Tank T37 prototype, Aberdeen Proving Grounds, June 3rd, 1949.

This turret was developed for what was called Phase One. At this stage, the tank received a 76 mm T94 gun similar in characteristics to the 76 mm M1 family of guns. However, this gun was more advanced. It had a fume extractor that considerably lowered the amount of gun fumes in the fighting compartment after firing. The gun also received a single baffle muzzle brake, similar to the one used on the 75 mm T21 gun. The Americans applied the same solutions to a number of guns, including the 90 mm M3A1. This gun was a temporary solution as work proceeded on the more powerful T91 gun. It looked similar to the T94, but had a longer barrel (4856 mm) and higher muzzle velocity (975 m/s vs 792). This gun was going to be installed on the Phase Two turret alongside a Vickers fire control system.

The stereoscopic rangefinder gave the tank a strange look.

The Phase Three variant was the most interesting. It was also going to have the 76 mm T91 gun, but also a loading mechanism similar to the one used on the Medium Tank T22E1. This mechanism was developed by the Rheem Manufacturing Company, most famous for water heaters. This company's mechanism was more compact than the one used on the T22E1, but the magazine capacity was smaller: 13 shots. The system allowed for a rate of fire of up to 18 RPM. The Phase Three variant also used a stereoscopic rangefinder and IBM stabilizer. This was the most advanced variant and it was delayed considerably.

Machine gun pods can be seen on the turret. The machine guns could be aimed in the vertical plane.

The Light Tank T37 never received a second turret. A tank development conference was held in Detroit in October of 1948. This conference was very important and set the further direction of American tank building. Several key decisions were made on December 1st-2nd. The Phase Two T37 tank was renamed to Light Tank T41 (this vehicle also had a different chassis, so it will be discussed in a separate article). The Light Tank T37 chassis as the most advanced American tank chassis at the time was used to develop the Medium Tank T42. A new heavy tank, the Heavy Tank T43, was also approved. The Light Tank T37 became an intermediate step before it was even built. It had no chance at mass production, especially since its gun was already weaker than its successor's.

The Light Tank T37 had a large turret bustle.

This situation didn't mean that building a Light Tank T37 was pointless. This tank was necessary as a test lab, especially since no tank with the new power pack had been built yet. The tanks also had principally different fire control systems. As a result, the first Light Tanks T37 and T41 appeared at the same time in May of 1949. Trials showed that the new tanks were considerably more agile than their predecessor. At maximum power the tank could accelerate to 66 kph, although the power pack was buggy at first. Trials also showed that the turret needs a secondary power source.

The commander and loader's hatches were protected with complex splash guards.

The gunnery trials were the most interesting part. It turned out that the desire to improve the machine gun armament only resulted in more issues. The machine gun pods only caused more headaches. The coaxial heavy machine gun was also excessive. The rangefinder fared much better, to the point where it was removed from the T37 and installed on the T41. Another interesting note was that as a result of trials a recommendation was made to move the gunner and commander to the right side of the turret, as was done on American medium tanks starting with the M4. Trials of the tank still gave a lot of interesting information.

It was already clear that the Light Tank T37 isn't going anywhere by the time the trials began, but it gave a lot of information useful for the development of another vehicle: the Light Tank T41.

Even though only one Light Tank T37 was built instead of three, and even this one did not survive to this day, this was an important step for American tank building. This was the first American post-war tank built in metal. It carried many technical solutions developed in the second half of the 1940s. It's hard to call this tank a dead end. Production of its direct successor, the Light Tank T41E1, began two years after the T37 went through trials. On May 29th, 1953, this tank was standardized as the Light Tank M41. These tanks are still in service with some armies.

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