BOK-7, K-17

Purpose: To continue stratospheric-flight research with an aircraft superior to BOK-1. Design Bureau: Bureau of Special Design, Smolensk. Chief designer Chizhevskii.

Design of this aircraft began in 1936. The Tupolev RD was again used as the starting point, but with features intended to enable greater heights to be reached. The test pilots were Petrov and Stefanovskii. According to Shavrov the BOK-7 was first flown in 1938, and ‘showed the same characteristics as the BOK-1’. Several two-man crews, including such important long-distance pilots as Gro­mov, Yumashev, Danilin, Spirin, Baidukov, Belyakov and others, spent periods of several days sealed in the GK checking all aspects of human life in preparation for proposed high- altitude long-distance flights in the BOK-15. According to some historians the ultimate ob­jective was a high-altitude circumnavigation, and that the by-function designation of this aircraft was K-17, from Krugosvetnyi (round the world). Photographs originally thought to be of the BOK-7 are now known to show the BOK-11.

The BOK-7 had the full-span wing of the RD, and aft-retracting landing gears, but com­pared with the RD the legs were redesigned for much lighter gross weight, and fitted with single wheels. Attention was concentrated on the fuselage, which unlike the BOK-1 had the GK (pressure cabin) integral with the air­frame, the centre fuselage being a slim cylin­der sealed by gaskets and adhesives, and with grommets fitting round the control wires and other services passing through apertures in the wall. The normal oxygen supply to the pilot and pilot/observer ‘compensated for the insignificant amount of air escaping’. The sealed drum was fitted with two hemispheri­cal domes, the front with eight and the rear with six transparent portholes so that the oc­cupants could see out, with a better view than from the BOK-1. The GK was kept at pressure by a tapping from a centrifugal PTsN (super­charger) blower driven by step-up gears from the engine. The engine was an 890hp M-34FRN fitted with two TK (turbosuper­chargers). It is probable that these delivered compressed air to the PTsN which then fed the engine, the cabin supply being taken off a small bleed pipe. Shavrov states that ‘all sys­tems worked well’, and that the experiments were ‘very interesting’.

According to Shavrov this aircraft had ‘the first GK of the combined type’ with both a sealed compartment kept under pressure and an oxygen supply. Some accounts state that AI Filin at the NIl-WS worked out details of the proposed circumnavigation, in 100- hour stages, but that the project was aban­doned after he was arrested in 1939 and executed in Stalin’s Terror of 1940. This air­craft led to the BOK-8, BOK-11 and BOK-15, but it appears that no illustrations of it have been discovered.