Chizhevski BOK-1
The BOK-1, where "BOK" stands for Byuro Osobyh Konstrukcij meaning "Bureau of Special Designs", also known as "SS" — Stratosfernyj Samolyot meaning "Stratospheric Plane", was a 2-seat, single-engine, low-wing monoplane, designed as an experimental high-altitude aircraft to study the feasibility of performing flights at altitudes above. It was the first Soviet plane to implement a pressurized cabin.
Development
The BOK was formed as part of TsAGI on the orders of the Soviet Revolutionary Military Council in Dec 1930. One of the first tasks of BOK was to design and build the hermetically sealed gondola of the high-altitude balloon. BOK engineers were sent to the Junkers factory at Dessau to study the Junkers Ju 49, which was the first aircraft designed from scratch to be equipped with a pressurized cabin for high-altitude flight. The gondola for the SSSR-1 was designed, built and flown successfully to an altitude of on September 30, 1933. BOK was then assigned the task of applying the 'hermetic' cabin to an aircraft with the Tupolev RD chosen as the basis of the BOK-1. A sealed 1.8 m3 cylindrical cabin constructed from D1 light alloy was fitted to a modified RD airframe.The cabin had convex bulkheads front and rear with the main entry hatch in the roof and emergency exit through a porthole at the rear. Seven portholes gave sufficient vision with five for the pilot and two for the radio operator/observer. A maximum design pressure differential of 0.22 kg/cm2 held the cabin altitude at up to the aircraft ceiling of. With the cabin structure sealed, cabin air was dumped overboard at a controlled rate through a dump valve and replaced with oxygen, from storage bottles, maintaining oxygen levels roughly constant. Heating was supplied through radiators in the engine cooling circuit, to keep the cabin between 15 and 18 °C.
The RD airframe was modified with reduced span wings, restressed structure for lower gross weight, fixed spatted single mainwheels, an M-34RN engine and three-bladed propeller, and later an M-34RNV engine and four-bladed propeller. Built at GAZ-35 the BOK-1 was first flown by I.F. Petrov in the summer of 1936 and reached a maximum altitude of 14,000 m in 1938.