In 1929 and 1930 the Soviet Union purchased several experimental convertible drive tanks from American engineer Walther Christie. When the prototypes were tested in the Soviet Union, it was decided to mass-produce these tanks for the RKKA. Copying Christie's tank made no sense, since it was quite shoddy and unreliable. Domestic engineers had to do some work, and produced the first fast tank for the Red Army: the BT-2.
In 1933, a project was launched to make a more polished fast tank, the BT-5. The most advanced tank in this series was the BT-7, mass produced since 1935. By 1937, the tank received a new conical turret, new tracks with smaller links, and an improved transmission and suspension. For its time, the BT-7 was a very effective tank, and its ability to accelerate up to 72 kph on wheels remains respectable even in our time. The ABTU of the RKKA had problems with the tank's protection: 22 mm of front armour was questionable at best, especially once information about a new French tank with 45 mm of armour was obtained.
In 1937, a group of engineers from Mariuopol factory #48, led by Nikolai Federovich Tsyganov, started designing a new tank based on the BT-7, with increased armour protection.
There is a possibility that Tsyganov may have been inspired by the French FCM 36. Its armour plates were sloped, to increase chance of ricochet. Due to this, the tank looked very unusual for the time. This theory is backed up by a letter from S. Ginzburg to the head of the ABTU, G. Bokis, in which the FCM 36 is mentioned by name. It is also quite possible that Ginzburg was pointing out mistakes with the FCM 36's design. He mentioned that the FCM 36 is very complex to produce, does not have its internal components logically arranged, and does not allow to access tracks for repairs easily.
The factory #48 design bureau's project was designated BT-SV (fast tank Stalin-Voroshilov). The hull of the new tank was assembled from armour plates, arranged at very sharp angles: 55 degrees for upper side, 53 degrees for upper front, 58 degrees for lower front. The engineers came up with two versions of the hull with different ways to connect the plates and different armour thicknesses, depending on if the armour was going to be cemented or homogeneous. The tank's hull shape led to its nickname: "Turtle". It did end up resembling one. Due to its unusual shape, gas tanks had to be relocated to the sides of the hull. The rear gas tank was entirely removed, since only the engine fit under the sloped rear armour. If you looked at the BT-7 from the top, you could see that the tank became narrower in the front. The BT-SV did not have this hull shape, remaining rectangular. This led to the front wheels only being able to turn 12 degrees, and reduced manoeuvrability on wheels compared to the BT-7. The suspension was not very much different from the BT-7. Replacing the tracks on the Turtle was even harder than on the FCM 36, since the armour partially concealed the wheels.
A prototype was constructed in late 1937 to test the design. It was made from regular steel, with armour thickness of 10-12 mm, which was significantly lower than the designed 40 mm of homogeneous or 20-25 mm cemented armour. The tank was tested over a 2000 km march.
After trials in the beginning of 1938, the BT-SV was modified. The armour layout was approved, but the lower part of the tank had to be modified. The engine compartment was redesigned, the turret gained additional observation devices and a PTK commander periscope. The new tank received the designation BT-SV-2.
Trials brought up a problem, which would have come up as soon as the tank was built with proper armour. The tank with normal armour would weigh 24-25 tons, nearly twice as much as a BT-7. The suspension would barely be able to deal with that kind of load. In order to evaluate the effect of the armour on the existing suspension, a prototype of the BT-SV-2 was planned, but never built.
In the beginning of 1938, Tsyganov was arrested, and work on the tank ceased.
Original article available here.