BT-SV-2 (SV - "Stalin-Voroshilov")
At the end of 1937, a new machine was produced, which was called the BT-SV-2 (SV - "Stalin-Voroshilov") "Cherepakha" ["Turtle"]. The letters "SV" after the name of a series of BT tanks (high-speed tank) meant "Stalin-Voroshilov", which quite fit into the mainstream of the traditions that were formed at that time to give the names of the leaders to everything that is possible.
The main thing with which this tank was different from all the others was the design of its armored hull, the sheets of which were located at very large tilt angles - 15-58 degrees. Two versions were developed – one with light protection up to 25 mm thick, and another using 45, 50, and 55 mm armor plates each. However, in early 1938, N.F.Tsyganov and other engineers from his group were arrested, and work on the BT-SV-2 was stopped.
Continuing the work on enhancing the armor of serially produced BT tanks, the design team headed by N.F.Tsiganov created an almost new tank, in the design of which the BT-7 tank units were used to the maximum.
The principal difference between the new project was a completely new body, distinguished by large angles of inclination of installed armor plates. For example: the upper front sheet was set at an angle of 53 degrees, the upper aft and lower fore - at an angle of 58 degrees, the lower aft - 48 degrees, side sheets (upper and lower), respectively, at angles of 55 and 15 degrees. This solution allowed, while maintaining the mass of the tank, to significantly increase its security. The car received an emergency landing hatch in the bottom of the control compartment.
The nose section had the same width as the whole body. The suspension of the front track rollers was not changed, except for the fact that its spring springs tilted back at an angle of 38 degrees. The BT-SV-2 case did not have protruding parts, with the exception of spring suspension caps. All armor plates were removable and bolted to the hull. For the purpose of greater rigidity of the armor cover of the tracks, special jumpers were provided in its lower part, 3 on each side. Aft fuel tank BT-7 was removed from the tank.
The design of the new tank abandoned the pipe bracket, which was installed on the serial BT for the wheels of the guides. This became possible due to the fact that the width of the bow of the tank hull was made equal to the width of the hull itself. This technical solution reduced the agility of the machine, due to the reduction of the angles of rotation of the front rollers support to 12 degrees. The height of the fighting compartment of 1400 mm did not allow the loader to work while standing. The turret housed only 5 shots due to its small size. The rest of the ammunition for the gun was placed in special packings on the sides and on the floor of the fighting compartment. For shooting at night, a pair of searchlights was installed on the tank (on the gun mask).
The new machine had so streamlined shape that it had practically no protruding parts. The only exceptions were exhaust pipes and suspension springs. The side plates, both lower and upper, were removable structures mounted on bolts. Because of this design feature, the car received the name "turtle". In order for the chassis armor protection to be more rigid, three special jumpers were installed on each side between the inner walls of the hull and the lower edges of the sheets. The fuel tank was removed from the stern of the tank, replacing it with two placed in the cavities of the tank sides.
The tank received a new conical turret, the top sheet of which (the roof) was also attached at an angle of 5 degrees to the horizontal. The turret had a common hatch for the crew. From aft niche turret refused. In this regard, there was a need to transfer the location of the radio station in the bow of the body. One more member was added to the crew - a radio mechanic. The engine of the tank received a new cooling system, which worked in two modes: marching and combat.
The first BT-SV prototype machine was made of their usual “3” grade structural steel (sheet thickness ranged from 6 to 12.5 mm) at the auto-armored repair base No. 12 of the Kharkov Military District (CVT) in 1937 (the exact date is unknown). The body of the new tank was made of 12mm armor plates.
The project provided two armor options. According to one of their booking schemes, the tank received bulletproof armor with armor plates IZ with a thickness of 20 to 25 mm. According to the second variant, the car turned out to be heavier, but it was equipped with counter-spun armor (FD steel armored steel with sheet thickness from 50 to 55 mm) and could withstand hits of 37-45 mm projectiles.
At the end of 1937 the tank passed factory and field tests. According to their results in early 1938 the car was finalized. The second version of the car was designated BT-SV-2 and prepared for testing at the beginning of 1938. The crew of the tank received improved observation of the battlefield. This was reflected in the fact that triplexes and a periscope sight were mounted on the tank in the sides of the turret and in the building of the department of management and installed a commander's panorama on the roof of the turret.
According to the decision of the selection committee, in real booking according to the second option, the security of the car was found to be satisfactory. However, the standard undercarriage of the BT-7 was insufficient for a tank of similar mass (24-25 tons). Therefore, it was recommended to make a full-tank prototype and repeat the tests in order to refine the chassis of the machine.
However, the chief designer was arrested. And the project was closed. Even earlier, in March 1937, a large group of designers at the KhPZ was arrested, in particular, the head of the tank design bureau A. Firsov. Technical solutions for armor were used later in the design of the T-34 tank.
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