The ‘D’ was designed as a mixed power fighter with a piston engine and Merkulin ramjet booster operating in the same duct. A 1,500 kW (2,000 hp) Shvetsov M-71 engine was intended to be the main powerplant but it is unclear how the thermodynamic cycles of the two engines were to be linked. Similar aircraft were built later in the German-Soviet War using the main engine to drive a propeller and a compressor to supply air to a ramjet/afterburner booster, both the Su-5 and MiG-13 were produced in limited numbers but the performance gains were limited and soon eclipsed by turbo-jet engines. The ‘D’ was to have been a gull- winged monoplane with high set wing, of stressed skin construction with exceptionally smooth skin stabilised by underlying corrugated structure. A heavy armament of two 37 mm (1.457 in) Nudelman-Suranov NS-37 cannon and two 20 mm (0.787 in) ShVAK cannon was included, but all work was abandoned with the German invasion of 1941.
The unit considered most suitable for the Su-7 was the RD-l , delivering 661lb (300kg) of thrust. In addition, metal plate on the wooden section of fuselage was lengthened to protect the structure from flames emitted by the turbosupercharger. During the flight tests, which began in late 1944, 84 RD-l engine starts were performed on the ground and in flight. From 31st January to 15th February 1945 18 engine test starts were made on the ground using an ether/air starting system, and from 28th Au- gust to 19th December that year the investigations were continued using the RD-l KhZ rocket motor.
Frequent failures of the RD-l prolonged the tests, but finally, in late 1945, flights conducted by test pilot Komarov showed that when the rocket was started at 20,600ft (6,300m) it increased maximum speed by 56mph (91km/h). However, the RD-l was underdeveloped and often failed, and after five changes of the liquid-propellant booster the designers decided to abandon it altogether. Nevertheless, the development and testing of mixed-powerplant prototypes was an important stage in the development of high speed jet aircraft.
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By the end of the Second World War the USA and Great Britain had developed practical jet fighters, even if they came a bit too late to take part in the action. Germany achieved even greater success, using jet fighters operationally in the closing stages of the war. The Soviet Union had fallen behind in this area, which is due to the Soviet government's scant attention to jet aircraft development and the lack of indigenous jet engines. True, as early as 1939 the design bureau led by Nikolay N. Polikarpov (OKB-51) had begun working on mixed-power fighters; the lead was quickly followed by other design bureaux headed by Aleksandr S. Yakovlev (OKB-115), Semyon A. Lavochkin (OKB-301), Artyom I. Mikoyan (OKB-155), Pavel O. Sukhoi (OKB-134) and Semyon M. Alekseyev (OKB-21). (OKB = opytno-konstrooktorskoye byuro - experi- mental design bureau; the number is a code allocated for security reasons.) These fighters employed ramjets or liquid-fuel rocket motors to give them a performance boost as required, but for various reasons none of them achieved production and service.
Several rocket-powered fighters were brought out as well, including the BI developed by A. Ya. Bereznyak and A. M. Isayev, the '302' designed by A. G. Kostikov and the Mikoyan 1-270 (aka izdeliye Zh). (Izdeliye (product) such and such was a common way of coding Soviet military hardware). However, the dangers associated with the rocket motor running on corrosive and/or toxic fuels and oxidisers, coupled with the motor's limited operation time, meant this was not a viable powerplant for a high-speed aircraft. Early research and development work on turbojet engines in the USSR dates back to the late 1930s. In 1938 Arkhip M. Lyul'ka and a group of engineers who shared his ideas came up with the project of the RTD-1 turbo- jet rated at 400 kgp (881 Ib st). Approving the project, the People's Commissariat of Aircraft Industry (NKAP - Narodnyy komissariaht aviatsionnoy promyshlennosti) allocated funds for manufacturing a prototype engine; mean- while, Luyl'ka was transferred to Leningrad to continue his work on jet engines at SKB-1 (Special Design Bureau - spetsiahl'noye konstrooktorskoye byuro). The RTD-1 evolved into the RD-1 (reaktivnyy dVigatel' - jet engine) delivering 500 kgp (1,102 Ib st) In early 1942 the Council of People's Commissars, one of the Soviet Union's highest government bodies, considered several jet fighter projects, including Mikhail I. Goodkov's proposal to re-engine the LaGG-3 fighter with an RD-1 turbojet. Concurrently the jet engine development programme was dusted off in accordance with Iosif V. Stalin's personal orders. In reality, however, the work really got underway in 1944 when Lyul'ka was put in charge of the gas turbine engine R&D section at a newly-established institute specialising in propulsion research.
Jet aircraft and jet engine development became a priority task for the Soviet aircraft industry after the war. Huge resources were committed to this task; still, all the money in the world can't buy you time, and the research and development effort was certainly going to be a lengthy one, which meant the service entry of the first Soviet jets would occur rather later than desired. Therefore, to speed up the work the Soviet government chose to make use of Germany's experience in this field.
In order to augment the fighter's speed when necessary, the designers decided to equip it with one of Valentin Glushko's liquid- propellant auxiliary rocket motors, including 0 the RD-l, RD-l KhZ, RD-2 and RD-3 with nitric acid and kerosene pump supply.
The unit considered most suitable for the Su-7 was the RD-l , delivering 661lb (300kg) of thrust. In addition, metal plate on the wooden section of fuselage was lengthened to protect the structure from flames emitted by the turbosupercharger. During the flight tests, which began in late 1944, 84 RD-l engine starts were performed on the ground and in flight. From 31st January to 15th February 1945 18 engine test starts were made on the ground using an ether/air starting system, and from 28th Au- gust to 19th December that year the investigations were continued using the RD-l KhZ rocket motor.
Frequent failures of the RD-l prolonged the tests, but finally, in late 1945, flights conducted by test pilot Komarov showed that when the rocket was started at 20,600ft (6,300m) it increased maximum speed by 56mph (91km/h). However, the RD-l was underdeveloped and often failed, and after five changes of the liquid-propellant booster the designers decided to abandon it altogether. Nevertheless, the development and testing of mixed-powerplant prototypes was an important stage in the development of high speed jet aircraft.
