It wasn't just the British, Americans, Germans, and French that undertook special aircraft projects prior to World War 2 (1937-1945) for Soviet engineers were also hard at work on perfecting various types. The Bolkhovitinov "S" - or "Sparka" - was one such entry into the Soviet stable, originating in the pre-war period of 1937 and recorded a first-flight in 1940. Designed as a high-speed / light-class bomber, just two of the kind were built for the program.
One of the unique characteristics of the Sparka was its conjoined engine fit, coupling a pair of Klimov M-103 inline piston engines (960 horsepower output each) and these used to drive a pair of three-bladed contra-rotating propeller units at the nose. The inline engines allowed for a streamlined shape to be achieved and this produced a sleek airframe with the cockpit / crew area set over midships and aft. This section was noted for its long-running, greenhouse-style canopy (housing a standard operating crew of two). The wing mainplanes were set at midships as well and these were straight appendages mounted low with swept forward trailing edges. The tail unit was another unique characteristic of the design as a twin-rudder arrangement was used with horizontal planes set between the two fins. The tail planes were mounted atop the fuselage spine. The undercarriage was of a typical tail-dragger arrangement consistent with developments of the period. Light alloys were used throughout the aircraft's construction. Dimensions of the aircraft included a length of 43.3 feet and a wingspan of 45.2 feet. Gross weight was 12,460lb.
Proposed armament was 1 x 7.62mm ShKAS machine gun for the rear crewman (the gun positioned on a trainable mounting). This armament was revised at a later date to become 2 x 12.7mm UBT series machine guns for a heavier defensive "punch". For the light bomber role, up to 882lb of internal stores could be carried. A bomb bay separated the two cockpits and the ordnance was held vertically to save space.
Design work on the Sparka was begun in 1937 and this led to construction of the first prototype the following year. By 1939, the first flyable form was readied as the "S-1" but the example lacked the proposed armament, wheeled undercarriage (skids were fitted instead), and installed just one of the two intended powerplants. A first-flight was recorded on January 29th, 1940. S-1 was then followed by "S-2" and this included all of the proposed components the S-1 lacked. In testing, the pair performed adequately - handling was a strong point - but was not as fast as expected and proved tricky when attempting take-off and landing actions.
The design was evolved into the ultimately-abandoned Bolkhovitinov "I" - with twin M-107 engines and other slight changes - and the Bolkhovitinov "D", a heavy bomber form which never saw the light of day. Other proposed forms related to the Sparka project included a dedicated attacker (with downward-firing heavy machine guns) and a dedicated interceptor (with upward-firing 37mm automatic cannon).
As tested, the Sparka managed a maximum speed of 354 miles per hour and ranged out to 435 miles. Nothing more came out of the project beyond the two aforementioned prototypes.
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