Kholshchevnikov VRDK

Design work on VRDK technology started in 1941 at the Central Aerohydrodynamic Institute with the goal of developing a motorjet where a piston engine would be combined with an axial flow compressor. From 1943 work was handed over to the Central Institute of Aviation Motors where design studies separated into two branches. One branch of research headed by A I Tolstov focused on diesel engines the other headed by Professor K.V Kholshchevnikov focused on spark ignition engines.[2]

In May 1944 the GKO (state committee for defence) ordered the major aircraft design bureaus to start developing aircraft that used a piston engine as their primary powerplant augmented with a small rocket or jet engine booster for short bursts of speed. The Mikoyan-Gurevich and Sukhoi design bureaus opted to use the VRDK developed by Kholshchevnikov's team[2][1] which was informally known as the Kholshchevnikov accelerator.[2][3]

A diagram of the VRDK as installed in the I-250

Both the Sukhoi Su-5 and Mikoyan-Gurevich I-250 used a single 1,230 kW (1,650 hp) Klimov VK-107 engine to drive a propeller and the VRDK’s single stage axial flow compressor. The VRDK was driven from a step-up gearbox and could be engaged/disengaged by the pilot via a clutch mechanisim which connected it to the piston engine via a shaft. Air supplied by a duct running from the front of the aircraft was mixed with engine exhaust then compressed and fed to an afterburner before exiting via a nozzle at the back of the aircraft. Part of the compressor exhaust was also used to supercharge the engine.[2] To protect the pilot and airframe, the VRDK was cooled during operation by water from a 78 L (20.6 US gal) tank mounted in the fuselage.[1]

When tested in the I-250, the thrust from the VRDK was 344 kg (758 lb) and combined output from the piston engine + VRDK was calculated to be the equivalent of 1,976 kW (2,650 hp).[2]

The VRDK was designed to operate for 10 minutes continuously and provide a speed increase of 90 km/h (56 mph).[4] Both the Su-5 and the I-250 had top speeds faster than contemporary piston engined fighters but slower than early jet fighters such as the Gloster Meteor and Messerschmitt Me 262.[1][4]

Only small numbers of Soviet motorjet aircraft were built and none saw service during World War II. After the war interest in motorjets quickly declined as the Soviets acquired German and British turbojet engines. Research, development and flight testing of Soviet motorjet aircraft had finished by 1950.[2][3]

• Mikoyan-Gurevich I-250
• Sukhoi Su-5

Data from .[2] Performance figures are for the VRDK only. In practice the VRDK could not be operated without being coupled to an external driver.

• Ishikawajima Tsu-11
• Caproni Campini N.1