Wednesday 6 January 2016

How the LTTB Was Born

Light tanks made during the Great Patriotic War were cheap and simple vehicles, capable of being put into mass production in the most difficult conditions. By 1943, Soviet industry was restored, but the need of a good light tank did not go anywhere. Design bureaus got to work, and by 1944 they had a tank that was difficult to call anything but "light tank with heavy armour". This is the name that the tank was given when it was modelled for World of Tanks.

Return to the T-50

In the spring of 1943, it was obvious to GABTU that the T-70 is running out of room for modernization. One last leap was the T-80, a tank with a two-man turret and turbocharged engine, but even it was obsolete by the time trials were finished. Even the installation of the VT-43 gun, which was never put in mass production, left much to be desired. The situation with armour was even worse. The T-70 was designed to withstand the German 37 mm AT gun,and by 1943, the main anti-tank weapon of the Wehrmacht was the 75 mm Pak 40, which was replacing the 50 mm Pak 38, and even the Pak 38 was enough to penetrate Soviet light tanks.

The problem of up-armouring the T-80 proved unsolvable, but how much further could you really modernize what was effectively a long-running redesign of the T-40 amphibious tank? The T-60 was supposed to be a "budget" companion of the T-50, which weighed almost 3 times as much and had plenty of modernization resources. Unfortunately, a series of problems, mostly with the engine, did not allow the T-50 to enter production, resulting in the T-70's arrival in the winter of 1942. Astrov's design was inferior to the T-50 in almost every way, but it was also simpler, did not use any deficit materials, and starting production at factories which already produced the T-60 was a simple task.

Many think that the end of the T-80 was the end of light tanks in the Soviet Union. Some historians recall projects proposed by factories that produced the SU-76. In reality, work never stopped. However, nobody bet on GAZ and other factories that already made light tanks. The T-70 chassis had a limit of 10-12 tons, and could not fit an appropriate engine. The development would have to start from scratch.

The T-50 was the most advanced light tank at the time it was built. Fate was not kind to it, but it was in demand again 1.5 years after the end of production.

The first signs of changing times came in April 1943. At this time, results of T-50 trials were extracted from the archives. The tank, built according to specifications from the start of 1940, still surpassed the T-80 in every way. In order to upgrade it for 1943, only three things were needed:
  • Replace the 45 mm gun with a more powerful weapon.
  • Resolve the engine issue.
  • Decide on a production base and a design bureau.
Item 1 was the easiest. The size of the turret allowed the installation of a 76 mm F-34 gun or 57 mm ZiS-4, at the cost of removing the commander. This problem could also be solved with a new turret design. The engine issue could also be resolved. YaAZ was preparing to begin production of licensed American GMC-4-71 diesels, and a pair of these engines could provide the necessary power.

Even the issue of a factory and a design bureau seemed solvable. In the summer-fall of 1941, factory #174, the producer of the T-50, was evacuated to Chkalov (today Orenburg), them to Omsk. In 1942, after the cancellation of the T-50, the factory began producing T-34s. Nevertheless, the staff with significant experience in light tank production remained, and so did their desire to not build someone else's tank. Omsk also housed designers from factory #185, the main incubator of new ideas in Soviet tank building before the war. In other words, the conditions for a modernized T-50 were there.

Clearer and clearer

The first mention of a reworked T-50 shows up in a report of BTU Chief Engineer-Colonel S.A. Afonin in July of 1943. The report was composed immediately after the battle of Kursk where the Germans used many new tanks and SPGs. According to the report, the T-80 should enter production, but it was also described as inadequate for the modern battlefield. Afonin called to assemble a team to develop a more modern light tank.

57 mm ZIS-4 gun model 1943. After a Tiger tank was shot up, this gun was chosen as one of the means of fighting the new German tank.

The report contained requirements for this tank, miraculously similar to the T-50. The new tank would weigh 15 tons with 45 mm thick armour. The crew, same as the T-50, would consist of 4 tankers. The engine would be a pair of GMC diesels with a total power of 220 hp. According to calculations, this would allow the tank to reach a speed of 45 kph. The transmission, however, was moved from the rear to the front of the tank.

Significant changes were also made to the turret and armament. The gun would be a 57 mm ZiS-4, the production of which briefly resumed in 1943 at factory #92. An alternative was the F-34, with which the ZiS-4 shared many components. The tank was to have a three man turret with a 1600 mm turret ring, which could fit a more powerful gun without removing crew members. Instead of a DT machinegun, a GVG (SG-43) machinegun would be used, one coaxial and one in the hull next to the driver.

Work on this first variant of the tank did not move past the composition of requirements, but only the first, as this tank changed several times over the next two years. The first serious change was at the end of December of 1943. At this time, the ZiS-4 disappeared completely from future plans, and there was a problem with GMC engines. Bombings prevented production from taking off, and the amount of engines received through Lend-Lease was barely enough for Ya-12 tractors. The mass of the tank also grew to 20 tons, so two 110 hp engines were no longer enough.

According to new requirements, the light tank would be equipped with a 300 hp engine. An engine that matched these requirements existed, but the last attempt to produce the V-4 ended in 1941. The inability to mass produce the V-4 and the shortage of V-2 engines was the main reason for removing the T-50 from production. It was not known where the V-4 would be produced this time around. The engine did eventually enter production under the name V-6, for PT-76s and BTR-50s, but this happened after the war.

The 76 mm S-54 gun was also proposed as a potential weapon for the new light tank.

The increase in mass was for a good reason. According to new requirements, the thickness of the front plate grew to 75 mm, and the turret to 60 mm. Just in case, let me remind you: we're talking about a light tank, and not the T-43. The armament was also upgraded to the 76 mm S-54 gun with 3-K ballistics. This gun was explored in 1943 as an alternative to the F-34, and it was being tested in the T-34 and SU-76BM. The S-54 eventually lost to the more promising D-5T 85 mm gun.

The project was taken over by the design bureau of factory #174, supervised by G.V. Gudkov, the chief designer of the T-50. I.S. Bushnev from factory #185, another notable contributor to the T-50, was also working on this tank. Correspondence was exchanged between the design bureau and GABTU. According to a letter from January 26th, 1944, the placement of the transmission was up to the design group (it was immediately returned to the rear). It was also clarified that the 60 mm thickness requirement only concerned rolled welded turrets. In case of a cast turret, the required thickness increased to 75 mm. The engine requirements changed once again. According to this letter, SKB-75 (Chelyabinsk) began work on an 8-cylinder V-shaped V-20 engine, which was essentially a shorter V-2. The factory #174 design bureau got to work.

On March 20th, 1944, the design bureau presented their work on a potential light tank. The hull and, in theory, the suspension were ready. The hull was 5450 mm long (half a meter longer than the T-50), the width was only 10 cm greater, and the height a mere 3 cm. In order to install a wide turret ring (1660 mm, bigger than on the T-34-85), the sides were made from two pieces. Despite significant differences from the T-50, its influence was easy to spot. As for the suspension, factory #185 showed its mark here. Instead of individual torsion bars, the tank would use bogeys of two road wheels each, mounted on a single torsion bar. A similar layout, except on springs, was initially planned for the heavy T-100 tank that was developed at factory #185.

A draft blueprint of the light tank hull designed at factory #174's design bureau. The T-50's influence is obvious, despite a number of differences.

The tank would either use the V-4 or the prospective V-20 engine. According to the design, it would generate 300 hp of power at 1750 RPM. A turbocharged variant was also proposed, with a power of 400-450 hp. There was also a choice of transmissions: a regular mechanical one with six gears forward and two back, or an alternative six-speed planetary gearbox with electromagnetic controls. The gearbox was to be grouped with the turning mechanism, like on the IS. Two variants of a planetary turning mechanism were also planned. Two experimental prototypes would be built, one with each type of transmission.

Light tank, heavy armour

By the time the hull and suspension designs were finished, GABTU's appetite increased. The 75 mm front plate was no longer enough. According to corrections, the new light tank should have 90 mm of front armour, and 90 mm on its upper sides. The turret was initially also 90 mm thick, but further corrections reached a whopping 200 mm. The sides of the turret thickened from 60 mm to 90 mm. The mass of the tank increased to 22 tons.

The armament was also unsatisfactory, since the S-54 was to be replaced with the D-5T. The tank also received a GVG AA machinegun, later upgraded to a DShK. The result in March of 1944 was a project that could be called nothing else but a light tank with heavy armour. To compare, a 90 mm rolled plate was meant for the IS-2 heavy tank, and could not be penetrated by the 88 mm Pak 43 gun. This was the name that the vehicle was given for World of Tanks, as it never received an index.

Factory #174's new light tank could have looked like this after all the changes requested by the GABTU were implemented. The reconstruction was performed by Vsevolod Martynenko and later used as the basis for the model in World of Tanks.

After the memo on March 20th, 1944, no news came from factory #174. This was partially caused by the fact that work on the light tank had the lowest priority, and there was no time for it in the spring of 1944: the design bureau was focused on the T-34-85. Additionally, SKB-75 nearly abandoned all work on the V-20 by April of 1945. GABTU attempted to find another developer, but fruitlessly. An attempt to produce the V-4 also failed. No engine, no tank.

One could stop here, but it's not so simple. In July of 1945, with GABTU's approval, work was performed to evaluate the potential to increase the armour on the T-50. Crude estimates showed that the tank could receive armour to rival the IS-2 at 24 tons. The turret, according to the description, would be similar to the IS-3 turret. This is where things get interesting. A light tank called T-64 appears in documents. There are no images of it, but the description boggles the mind. The 26 ton vehicle would have a 45 mm upper front plate at 8 degrees from horizontal, and the cast front section was a whopping 200 mm thick. The sides of the hull were 150 mm thick, the rear 75 mm. The turret would be 220 mm thick. Here we have a real super-heavy light tank.

One can assume that this was a crude estimate and no work was performed in this direction, but that is not so. It was proposed that SPGs be built on this chassis. These same SPGs appear in RKKA armament proposals in October of 1945. The first of these vehicles was a tank destroyer wit a 100 mm gun and armour that guaranteed immunity to the German 75 mm gun at any distance. The gun mentioned here is the 7.5 cm KwK 42 L/70 used on the Panther and Panzer IV/70 tank destroyer. One could assume that the tank described here was the SU-101 (Uralmash-1), but the combat mass was only 25 tons, or 10 tons lighter. The second SPG would have a 122 mm howitzer and weigh 20 tons. Factory #40 was designated responsible for these SPGs, and the first 50 were expected to be built in 1947.

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