Saturday 8 April 2017

Light Tanks T1E4 and T2E1: Experiments on an Ideal Platform

The idea of a light tank with a front engine that the American Ordnance Department insisted on was at a dead end by 1932. Trials of the Light Tank T1 family and later the Medium Tank T2 showed that the idea was unacceptable. Poor visibility, excessive mass, bad crew conditions, and, most importantly, the limits of further development, put an end to such tanks. Designers moved on to working on other tanks with different layouts. Harry Knox, the father of the front engine American tanks, did not abandon his idea, and kept looking for a place for his idea. Stooping down to plagiarism, he crossed his Light Tank T1E1 with the Vickers Mk.E, its overseas competitor. The resulting "hybrid" Light Tank T2E1 was not that bad.

Anthology of Inspirations

The main counterpart of the Light Tank T1 and Medium Tank T2 at the time was the Christie tank, which shattered the dreams of the Ordnance Department to sell them to the American army. Compared to it, all prior American tank designs looked like toys. Meanwhile, another tank appeared on the other side of the Atlantic that contributed a few nails to their coffin: the Vickers Mk.E, designed by John Carden and Vivian Loyd.

For a number of reasons, the British army had no place for this tank, but it had a huge export potential. The Vickers Mk.E and its variants became the most popular tank of the 1930s. It was bought by Bulgaria, Bolivia, Greece, Poland, China, Thailand, Finland, Portugal. This tank was picked by the Soviet purchasing commission headed by I.A. Khalepskiy as an infantry support tank, purchasing a license for its production in the USSR under the name T-26. Poland also designed the 7TP tank on its chassis. The combat career of the Vickers Mk.E began in the fall of 1932 in South America, and concluded a quarter of a century later in 1959.

The secret of this tank was that its creators catered to the most in-demand market segment. With its low mass, the tank had reliable bulletproof armour, high mobility, and good firepower for an infantry support tank. The Vickers Mk.E was reliable and had good potential for modernization. It's enough to say that while the first British tanks weighed about 7 tons, the latest Soviet T-26es weighed close to 11 tons. Thanks to the rear engine and front transmission, the tank was very compact. The suspension was also very good, even though the British military had their doubts about it.

Vickers Mk.E Type A at the Aberdeen Proving Grounds, June 15th,1931

Thanks to the close ties with the British, the American military managed to get a Vickers Mk.E Type A. Trials held in June of 1931 compromised the Light Tank T1 even further. Its only advantage over the British tank was its armament, and even then, the gun was experimental. The mass produced T1 gun mount with the short 37 mm M1916 had few advantages over the two machineguns on the Vickers Mk.E. In addition, there was also the single turret version, the Vickers Mk.E Type B, which carried a much more powerful 47 mm gun in a two-man turret. The British tank was lighter and 25% faster, had identical armour, and better crew conditions.

Harry Knox patented the tank and its suspension in March of 1934.

After such a crushing defeat, the Ordnance Department had to stop and think. On one hand, there was the Christie tank. On the one hand, a decent foreign tank that was actively exported. The result of this deliberation was a stunningly simple decision: Knox and his team didn't think of anything better to do than to copy the progressive solutions of the Vickers Mk.E.

The USSR is often accused of stealing technologies. In truth, before the war, foreign technology was usually licensed. In this case, Harry Knox resorted to outright plagiarism. In addition, he even had the gall to patent his stolen solutions!

In late 1931, one Light Tank T1E1 was sent to the Rock Island Arsenal in Illinois. The tank was radically altered to make it more like a Vickers Mk.E. The result of this conversion, which was completed in March of 1932, left little of the initial tank. The hull was partially retained, at least its front, which became the rear after the conversion. The engine remained in place, but it was now separated from the fighting compartment. The external fuel tanks also remained. Otherwise, this was a completely different tank, much more modern than the Light Tank T1E1.

Light Tank T1E4 at the Rock Island Arsenal, March of 1932.

Like the Vickers Mk.E, the new tank, designated Light Tank T1E4, had a front transmission, designed completely anew. The central front plate was now cast. The driver's compartment was so roomy that it now fit two people, with the addition of the assistant driver. This idea was well accepted by the American military. In the future, the assistant driver received a machinegun so he did not simply take up space.

Thanks to the migration of the transmission, the engine compartment was not changed. However, the length of the hull grew from 3860 mm to 4600 mm. The width of the tank also increased from 1786 mm to 2200 mm, but the height decreased from 2200 mm to 2000 mm. The turret retained its shape, but the diameter increased. The 37 mm M1924 gun was installed in it, with a long counterweight.

The same tank in September of 1932, after receiving a more powerful engine.

The suspension was borrowed from the Vickers Mk.E, with some changes. The idler also looked similar to the British inspiration, but the drive sprocket was new. It's hard to say that the new design was better. If the teeth of the drive sprocket on the Vickers Mk.E broke, one only had to change the crown. Here, the whole sprocket had to be replaced. The track links were also designed anew by Knox. Thanks to their width of 337 mm, the tank could drive comfortably on soft ground.

The results of trials in the summer of 1932 were mixed. The tank was heavier than the Light Tank T1E1, but had the same engine. Its agility and mobility decreased. One solution was the installation of a Cunningham 140 hp engine. The top speed grew to 32 kph, but that was still less than the top speed of the Vickers Mk.E.

Light Tank T1E6. The biggest difference compared to the T1E4 is in the rear.

The issue of insufficient power was resolved in late 1932. The tank received a 12 cylinder American LaFrance engine that could reach a maximum output of 244 hp. The more powerful engine required serious changes to the engine compartment. Its height was equal to that of the turret platform. The lower part also changed. Large air intake "ears" appeared along the sides. The changes were serious enough to give the tank an new name, Light Tank T1E6.

The American LaFrance engine required a larger compartment.

The changes increased the tank's mass to 9030 kg. However, the more powerful motor promised improved mobility. The power to weight ratio was 24.5 hp/ton, almost as high as on the Christie Medium Tank T3! The tank should have been able to fly with that kind of power, but trials showed that the top speed was... 32 kph. The same as on the Light Tank T1E4 with a 140 hp engine.

Unlike the speed, which did not increase, the amount of technical problems did. The patience of the Secretary of War was not infinite, and a decree passed in the spring of 1933 limited the mass of light tanks to 7.5 tons. Work on the Light Tank T1E6 continued until 1934, but it more like death throes. The joint Ordnance Department and Cunningham project had no future.

Smaller, Lighter, Faster

The failure of the Light Tank T1 did not mean that Harry Knox was done for. On the other hand, Christie's tanks were out of the game by mid-1933. The military stopped ordering tanks from Christie in November of 1932, preferring to give the contract to American LaFrance. Around the same time, the cavalry ordered the Combat Car T4, which was also based on the Christie design. Gladeon Barnes worked on this tank. His convertible drive tanks turned out too heavy and too expensive. As a result, Harry Knox and the Ordnance Department held the monopoly on tanks once more.

Further development of American light tanks happened within the cavalry's orders. The resulting vehicle was called Combat Car T5. Its development began on June 3rd, 1933. According to the specifications sent to the Ordnance Committee on July 10th, 1933, the combat car had to weigh 14,000 lbs (6,300 kg), reach the top speed of 30 mph (48 kph), have 100 miles of range (160 km). Its armament had to consist of one 12.7 mm machinegun and two 7.62 mm machineguns, and its armour had to protect from rifle caliber bullets. The requirements for a 7.5 ton tank were approved on August 9th, 1933. 

Light Tank T2, Aberdeen Proving Grounds, May of 1934

A light tank for infantry, called Light Tank T2, was developed in parallel with the Combat Car T5. Both tanks were built at the Rock Island Arsenal bye early April of 1934. These two tanks were very similar in design. The strict speed and weight requirements resulted in a tank that was a further development of the Light Tank T1E4 (and the Vickers Mk.E), but was also original in many ways.

Since the 7.5 ton weight limit did not allow the tank to be big, and no suitable automotive engine was found, the Continental R-670 aircraft engine was chosen. This air cooled 7 cylinder engine was used on the Combat Car T4, but it came in one unit with the transmission. The Light Tank T2 had its transmission in the front. Thanks to the star-shaped engine design, the engine compartment was very compact, and the tank was only 4077 mm long. On the other hand, the size of the engine forced the hull to be rather tall. The overall height was 2057 mm, which was not that much, but the hull itself was over 1.5 meters tall. The high center of the engine also raised the driveshaft.

The same tank, seen from the rear.

The Light Tank T2 and Combat Car T5 were opposites in turret and suspension. The infantry tank kept the Vickers Mk.E style suspension with 4-wheel bogeys, albeit with some changes. Its tracks were a further development of the tracks used on the Combat Car T4. The track link was metallic, but the joints were of a rubber-metallic design. Due to the raised driveshaft, the Combat Car T5 had two turrets. The Light Tank T2 had one turret, with two men. A bulge in front of the turret housed the Browning M1919 and Browning M2HB (with a shortened barrel) machineguns. Another machinegun was in the hull, used by the assistant driver.

On the Way to the Classics

Both tanks arrived at the Aberdeen Proving Grounds on April 13th, 1934. Intensive trials began. The Light Tank T2 finished trials on May 17th. Its mobility and crew comfort were tested.

Light Tank T2 on trials, April of 1934.

The design of the tank, especially the hull, was deemed good. However, the testers criticized the position of the driveshaft from the engine to the transmission. It was difficult to leap over it in battle. Unfortunately, there was nowhere else to put it, and the raised driveshaft became a staple of American tank design. The visibility was deemed good, but the installation of side mirror was recommended. The mirrors finally appeared 10 years later on the M4A3E8 medium tank.

A high driveshaft is one of the issues with a rear star-shaped engine.

Defects were discovered after 115 km of trials, and the tank was sent back to Rock Island. Nevertheless, enough data was gathered for preliminary conclusions. The top speed achieved during trials was 43.4 kph. Not quite the 48 kph that was required, but it was still very good. However, the leftovers from the Light Tank T1E4 let themselves be known. The smoothness of the suspension was deemed insufficient, especially off-road. In addition, the short hull made the tank drift at high speed. The tank was very loud inside, which prevented the crew from talking. The track link design was also deficient.

An illustration from the Vertical Volute Spring Suspension patent.

The list of defects made during the trials were considered during modernization. In October of 1934, the improved tank, indexed Light Tank T2E1, entered trials. The biggest change was the suspension. Instead of the admittedly poor Vickers Mk.E style bogey suspension, the much simpler and more elegant suspension that Harry Knox worked on since 1933 was used. It used two-wheel bogeys with a vertical spring. This system was much more compact and had larger travel. The Vertical Volute Spring Suspension (VVSS) was first used on the Combat Car T5, and the infantry tank received an improved version.

Light Tank T2E1 on trials at Fort Benning, January of 1935.

The tank received new track links in addition to a new suspension. Knox worked on rubber-metallic joints since the early 1930s, but he went even further in his experiments with rubber in 1934. He patented the rubber-metallic track link on March 6th, 1934, an invention that was as revolutionary for American tank building as the Christie suspension. The resulting design was very good, and its pair with the VVSS suspension became the distinguishing mark of American tanks for a decade. The new track links were tested on the Light Tank T2E1. The track was improved based on the results of the October 1934 trials.

Demonstration of the VVSS suspension.

On January 11th, 1935, the tank was sent to Fort Benning for military trials. Tankers from the 67th Infantry Company (Medium Tanks) tested the Light Tank T2E1 in the most difficult conditions. The trials concluded on February 14th, and a list of desired improvements was composed. They included the installation of a commander's cupola, movement of the mufflers out of the engine compartment, and reinforcement of the suspension. There was not a single serious breakdown during the trials. Overall, the infantry deemed the Light Tank T2E1 to be acceptable as a light tank. 9 years later, the program to replace the light 6-ton M1917 tanks finally birthed a worthy successor. The concepts used to build the Light Tank T2E1 were used in American tank design until the end of 1942.

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