 The VRDK was a motorjet , a rudimentary type of jet engine where an external power source drove the engine 's compressor . It forced air into the stainless steel combustion chamber where fuel was sprayed from seven nozzles and ignited to exhaust out the variable rear nozzle . In the I @-@ 250 a 1 @,@ 650 @-@ horsepower ( 1 @,@ 230 kW ) Klimov VK @-@ <unk> V @-@ 12 piston engine was used as the primary powerplant . After takeoff a clutch at the end of the crankshaft could be engaged which drove a step up gearbox with a ratio of 13 : 21 to an extension shaft that powered the compressor of the VRDK . The air for the compressor was fed through a long duct that ran from the inlet underneath the propeller spinner , thence under the engine and through the belly of the aircraft . This duct also fed air to the oil cooler near the engine , but the water radiator was positioned behind the compressor to maximize airflow over it . A secondary duct led from the main duct to the VK @-@ 107 's supercharger ; when the VRDK was running the secondary duct diverted some of that additional air to the supercharger which boosted the engine 's output to 2 @,@ 500 @-@ horsepower ( 1 @,@ 900 kW ) at 7 @,@ 000 m ( 22 @,@ 966 ft ) . The increased airflow over the engine radiator helped to dump the engine 's excess heat into the exhaust stream . However the VRDK was limited to only ten minutes ' operating time per sortie , which meant that it was useless weight during the rest of the flight . 
