For the Soviets prior to World War 2 (1939-1945), there were a bevy of experiments undertaken concerning light-class combat tanks. One of the focuses became the BT series of "fast tanks" prized in Soviet armored doctrine for their inherent speed and agility. The "BT-7" became the pinnacle design of the family line that began with the "BT-2". The BT-7 was taken into service in 1935 and over 5,000 examples were produced before the end - many fought on into the final year of the war in 1945.
This same tank was to form the framework for a new, evolved model which followed a failed attempt tp breath more life out of the BT-5 through the abandoned "BT-IS" venture. In 1937, work began on the "BT-SV" ("Stalin-Woroshilov") utilizing the same running gear as the BT-7 but incorporating an all-new (and rather revolutionary) hull superstructure and turret design. The armor of this "fast battle tank" was of particular note for it presented its attackers with many well-angled sides for inherently strong ballistics protection - giving the vehicle an almost pyramidal appearance (with the top of the pyramid being cut off). Additionally, this armor protection was draped down over the sides of the hull to give the more vulnerable faces of the tank and track system additional protection (these "skirts" being wholly removable for maintenance and repair of the vehicle components underneath). This approach to armor protection would be enhanced in the upcoming, war-winning T-34 Medium Tank (detailed elsewhere on this site).
The hull roof line was appropriately flat to take on a single rotating turret housing two crew and the main armament. Internal space at the bow allowed for the driver and radioman to be seated side-by-side while the gunner and commander resided in the turret. Armor protection reached up to 30mm thick at the critical frontal facing. Primary armament was the 45mm 20-K L/46 main gun seated in the frontal face of the turret and to this was added a coaxial 7.62mm DT machine gun as an anti-infantry measure.
The finalized 25-ton vehicle sported dimensions that included a running length of 5.6 meters, a width of 2.8 meters and a height of 2.2 meters.
Drive power was from a single M-17T 12-cylinder gasoline-fueled engine of 500 horsepower driving a conventional track-and-wheel arrangement. This arrangement involved four roadwheels to a hull side and the drive sprocket was fitted to the rear and the track idler was positioned at the front. Performance showcased a road speed of 62 kilometers-per-hour with a range out to 130 kilometers.
The pilot form of the BT-SV was made ready before the end of 1937 and two prototypes would be built (by Locomotive Works Kharkov) for evaluating the design. Testing ensued the following year and ultimately proved the vehicle inherently sound. However, its development was severely derailed as one of its key designers, N.F. Tsyganowich, fell victim to Stalin's "Great Purge". As such, engineers pressed on with work on what would become the classic T-34 and the BT-SV was itself abandoned.