Bolkhovitinov S  The Bolkhovitinov S ("Sparka" – twin/joined (engine)) was a high speed bomber aircraft designed and built in the USSR from early in . Other designations used at various times: BBS, BB, LB-S or SSS. The plane was conceived as a fast light bomber and reconnaissance plane. To obtain high speed and good aerodynamics, it was powered by two M-103 inline engines in tandem in a fuselage nose, driving two contra-rotating coaxial three-blade propellers. The design had quite unusual look, with long fuselage nose and a crew cab moved rearwards, towards double tailfin. Wings had a fighter-like form and their span was rather short. The VVIA under Bolkhovitinov was enlarged to tackle the expected problems in the design of the Bolkhovitnov S. The rear engine was geared to two high-speed shafts which passed either side of the forward engine to drive the rear propeller gearbox, whilst the forward engine and gearbox drove the forward propeller via shaft through the middle of the rear propeller gearbox. The structure of the S was predominantly light alloy stressed skin, the wing having two spars with heavy flush-rivetted upper and lower skins, and the fuselage built up from pre-formed upper, lower, and side panels attached to four longerons to give a strong but relatively simple structure to build. A total of 29 electrical actuators drove the Fowler Flaps, undercarriage, (rotating 90deg to lie flat in the wing), exit flap of the large common radiator duct, and many other services. The tail unit consisted of twin fins with rudders attached to the tips of the tailplane, variable geared elevators and separate servo and trim tabs on elevators, and rudders. Detail design of the aircraft began in 1937 and construction in July . The first prototype S-1 was built in 1939, but it was made in order to evaluate an airframe only and therefore was powered by only one engine M-103. It was unarmed and fitted with a fixed skid landing gear. It first flew on January 29, 1940, with B.N. Kudrin at the controls. A maximum speed was 400 km/h. The second prototype S-2 was powered by two engines and was armed with a single machinegun and bombs. Four 100 kg bombs were carried vertically in a bay between pilot and a rear gunner-navigator, under a common canopy.