Monday, 28 March 2022

Intermediate Revolution

Work on the Light Tank T37 officially began on September 27th, 1946. This was the first of the radically new tanks developed after WW2. The appearance of this vehicle was an important event, since it signalled the new vision for American tanks. All that was needed was to design new components that the tank could be built around. Because of this, it took three and a half years to finish the tank. It was finally finished in 1949, by which point there was already a replacement ready for it. This was the Light Tank T41, an improved T37. This vehicle was also not mass produced, but it served as a predecessor of the Light Tank M41 that remains in service in some nations to this day.

In search of a better solution

Initial plans called for three variants of the Light Tank T37. Three pilots were ordered, which were supposed to be different from one another. The tanks were separated into phases. Phase One (essentially the only T37 that was built) called for a 76 mm T94 gun, analogous to the 76 mm M1 gun in firepower. This gun had the ballistics of an AA gun, so installing it in a light tank required introduction of a muzzle brake and a new gun mount. A fume extractor was added based on experience with the 120 mm T53 gun, which improved the situation with gun fumes. The T37 stereoscopic rangefinder was also added. The rangefinder gave the turret its distinctive "ears". There were two pairs of "ears". The second pair was made up of pods that held machine guns, which could be aimed vertically.

Light Tank T41 pilot. This vehicle remained at the Detroit Arsenal.

The rangefinder and turret machine guns were the idea of Army Ground Forces Board This organization was created in January of 1945 to determine the direction in which American armament would develop after the war. Development was assigned to the Armored Medical Research Laboratory led by Frederick C. Brecket. This seems like a strange organization to assign this work to, but the results were quite reasonable. A portion of the report prepared on June 20th, 1945, was also approved by the War Department Equipment Board. As said above, the first variant of the Light Tank T37 had a weapon similar to the 76 mm M1, but Brecket's report deemed it insufficiently powerful. According to his requirements, a gun capable of penetrating 127 mm of armour at 30 degrees from 1000 yards was needed. This gun, the T91, was to be installed on the Phase Two tank. This tank would also have a Vickers fire control system. Finally, the third tank would have an autoloader mechanism designed by the Rheem Manufacturing Company. It would also have an IBM stabilizer and stereoscopic rangefinder.

The second (first official) Light Tank T41 at the Aberdeen Proving Grounds. This tank no longer had a small road wheel in front of the drive sprocket. The exhaust system was also changed.

Some changes were made as work went on. For starters, the Ordnance Committee reduced the number of vehicles to two on May 1st, 1947. This was due to the fact that it was necessary to first refine the power pack. Experience with heavy tanks showed that work can't move forward without it. By the time that the power pack finally took shape in 1948, American tank building underwent a metamorphosis. As strange as it sounds, the Light Tank T37 concept was selected as the foundation for medium and heavy tanks. The idea of taking one tank and inflating it to make heavier types repeated itself. Seemingly experience with the Pershing taught the Americans nothing. Three projects appeared simultaneously: Light Tank T41 (essentially a renamed Light Tank T37), Medium Tank T42 (heavier and with a 90 mm gun) and Heavy Tank T43. All of these tanks resembled their common ancestor, especially when it came to the turret "ears". The Light Tank T37 was not cancelled, but only one pilot was built.

For unknown reasons narrow tracks from the Light Tank M24 were installed.

The new plans called for three prototypes of the new tank. Since the Light Tank T37 already had a Phase One turret, it was not used on the new vehicles. Only Phase Two tanks were built. It was planned that these tanks would be built at the same time as the Light Tank T37, but that was not the case. The first and only Light Tank T37 was built in February of 1949, while the first T41 was built in May. The second tank appeared in the summer and the third in December of 1949. It's not surprising that the first T37 and the first official T41 had differences in more than just the gun and controls. Factory trials at the Detroit Arsenal had a large impact on the tank's design.

Reinforcement ribs can be seen on the tank's turret. They doubled as splash protection.

The first prototype that remained at the Detroit Tank Arsenal was indeed an almost complete clone of the T37 with the exception of the gun and rangefinder "ears". The second prototype (the first that counted officially) already had a different chassis. The small road wheel in front of the drive sprocket was removed. The exhaust system was also different. For an unknown reason, the T91 533 mm wide tracks were replaced with T72 406 mm wide tracks from the Light Tank M24. Reinforcement ribs that doubled as splash protection were installed on the turret. This tank was delivered to the Aberdeen Proving Grounds.

Not bad, but needs work

The situation where Light Tank T41 prototypes were built so far apart has several explanations. One was that the creation of the new engine with the Alisson CD-500 transmission was very long and painful. This was better than the mess heavy tanks ended up in during 1946-1948, but problems with the engine and transmission elements cropped up constantly. As proof, the production Light Tank M41 had the AOS-895-3 engine and CD-500-3 transmission. They were used for a good reason. The third prototype already had the AOS-895-1A engine and CD-500-1B transmission. These letters speak volumes.

The third (second official) Light Tank T41 prototype was ready in December of 1949.

The mass of the tank grew to 23,405 kg as a result. This was a ton and a half more than the T37, but the chassis had plenty of reserves left and mobility was not significantly impacted either. The tank was very mobile and the chassis was very well put together (barring the aforementioned defects). The third (officially second) prototype was almost identical to its predecessor, at least externally.

The turret, or rather the roof, was the biggest difference.

The turret and armament were more complicated. The idea with machine gun pods proved ineffective during trials of both the Light Tank T37 and T41 at Aberdeen. Even though all three Light Tanks T41 were built with them, the result was definitely underwhelming. The pods were removed and both the Medium Tank T42 and Heavy Tank T43 already lost them at the model stage, although the medium tank concept composed by the Army Ground Forces Board had them. This idea was not unique, just recall the array of machine guns on the Object 260. The idea of creating a wall of lead to repel infantry was quite tempting. 

Vehicles were improved after trials, and so each T41 was different from its predecessor.

The fire control system was even more problematic. As envisioned on the Light Tank T37 Phase Two, the tank used a Vickers fire control system with a two plane stabilizer. It worked in conjunction with the rangefinder. The rangefinder worked well on its own, allowing for improved first shot precision at long distances. However, the Vickers system needed more work. Unfortunately, the two systems came as a pair. It was impossible to remove one but keep the other.

Like the T37, this T41 had wider T91 tracks.

The solution was quick and easy. The entire fire control system was extracted and the rangefinder was replaced with the one from the Light Tank T37. This was done during mobility trials, so there are photos where the tank is shown with empty sockets. After small modifications, the rangefinder was installed in the second T41, where it showed good results. The official story sounds nice, but later the rangefinder disappeared from the T41 completely. It would appear that the T37 rangefinder did not function as well as was hoped.

The tank kept its machine gun pods, but trials showed that they were a bad idea.

The turret itself was also not free from issues. This is why the third Light Tank T41 built in December of 1949 changed compared to the first tank. The roof was V-shaped, which improved its rigidity. Changes were made to the loader's hatch and his observation devices. These changes were almost cosmetic in nature, snice the situation that formed was quite tricky. On one hand, the tank was quite good. It had a powerful gun, the exact one that the military wanted. Its characteristics clearly surpassed those of the Light Tank M24. On the other hand, the tank collected such a large bucket of issues that it would clearly not be able to enter production as is. Nevertheless, a manufacturer for the Light Tank T41 was chosen back in 1948. The choice was made in favour of Cadillac, a reasonable choice considering that they produced the Light Tank M24. A contract was signed for 100 tanks in January of 1950, even though the tank still needed work.

This tank was the only one that survived to this day.

Some complains are only a cause for surprise and question of why it was not done this way in the first place. First of all, this has to do with the crew layout. The Light Tank T37 and T41 had the gunner and commander on the left and loader on the right. Meanwhile, the Medium Tank M4 flipped this setup, having the commander and gunner on the right and loader on the left. This was more comfortable for the crew, especially the gunner. The reason that the gunner on the light tank sat on the left originally dates back to the Light Tank M3 where he aimed the gun with a shoulder stock. After that, the position remained by inertia. Common sense triumphed, and the tradition was broken. The telescopic sight that was absent on the T37 and T41 returned.

President Truman at the Aberdeen Proving Grounds, February 17th, 1951.

There were other issues. The Light Tanks T37 and T41 had 1753 mm wide turret rings. This diameter was insufficient to work with the T91 gun. The military also requested an auxiliary motor. Such a motor existed on the Light Tank M3A1, but disappeared afterwards. Suddenly, the military remembered that an auxiliary motor was needed to drive the turret traverse motor and other systems in the turret. All of these changes resulted in a tank called Light Tank T41E1. It had to be designed quickly, as the deadline for the first 100 tanks drew ever nearer. The Ordnance Department had to fight against problems it created for itself. Even though a war broke out in 1950, one had to work to create a situation as convoluted as with the Light Tank T41. The resulting Light Tank T41E1 was a very different tank, so different that it might as well have been a whole new design.

The same tank during mobility trials. The machine gun pods are removed.

Unlike the Light Tank T37, which did not survive to today, the T41 was lucky. The third (officially second) prototype tested at the Aberdeen Proving Grounds survived. It was used in a number of trials aimed at polishing the T41E1. The second (officially first) prototype was demonstrated to President Truman in early 1951 and was later scrapped.

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