Mikoyan-Gurevich I-250

The Mikoyan-Gurevich I-250 (Samolet N), aka MiG-13, was a Soviet fighter aircraft developed as part of a crash program in 1944 to develop a high-performance fighter to counter German turbojet-powered aircraft such as the Messerschmitt Me 262. The Mikoyan-Gurevich design bureau decided to focus on a design that used something more mature than the jet engine, which was still at an experimental stage in the Soviet Union, and chose a mixed-power solution with the VRDK (Vozdushno-Reaktivny Dvigatel Kompressornyi – air reaction compressor jet) motorjet powered by the Klimov VK-107 V12 engine. While quite successful when it worked, with a maximum speed of being reached during trials, production problems with the VRDK fatally delayed the program and it was canceled in 1948 as obsolete.

Design and development

By January 1944 the Soviets were aware of successful British and American jet aircraft projects and that the Germans were about to deploy jet and rocket-propelled aircraft of their own. The GKO ordered on 18 February that the NKAP (People's Commissariat for Aviation Industry) centralize jet research under its control and that the NKAP was to present proposals to alleviate the situation within a month. As a result of this meeting the NKAP ordered the Lavochkin, Sukhoi, Yakovlev and Mikoyan-Gurevich design bureaus (OKBs) to develop and build jet aircraft with the utmost dispatch. Aware of earlier problems encountered with other novel propulsion systems such as ramjets both Sukhoi and Mikoyan-Gurevich chose to use the VRDK booster engine that had been under development since 1942.

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 Klimov VK-107R V12 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 at . 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.

thumb|250px|Diagram of the I-250's engine installation

The I-250 was a low-wing, all-metal aircraft with a monocoque fuselage. Other than the VRDK the aircraft was largely conventional in layout, although the cockpit was set very far back in the fuselage, almost to the base of the vertical tail. The two-spar wings had a thickness of 10% to preserve aileron control and avoid tip stall. They were fitted with Frise ailerons and slotted flaps. Fuel was contained in a fuselage tank and a tank in each wing. The conventional landing gear mainwheels had a levered suspension and retracted inwards. The tail wheel retracted aft into the very small ventral fin. The VK-107A engine initially drove a VISh-105SV propeller.

The NKAP approved the preliminary specifications of the I-250 on 19 September 1944 which included a maximum speed of a take-off weight of and a time to of 3.9 minutes. A mock-up was inspected on 26 October and rejected because of the poor cockpit layout, although this decision was reversed because the fuselage of the first prototype was too far along for major changes that would have significantly delayed the program. That same month the combustion chamber was sent to TsIAM for testing which revealed it to be too weak. The complete powerplant was tested in December, but the drive shaft connecting them failed several times.

The first prototype was completed on 26 February 1945, although the VRDK was not yet ready. It was fitted with a new AV-10P-60 propeller before making its first flight on 4 April according to Gordon and Komissarov, although Belyakov and Marmain say 3 March. It was armed with three 20 mm Berezin B-20 cannon with 160 rounds each. One gun was fitted on each side of the nose and the third fired through the hollow propeller shaft.

Joseph Stalin convened a meeting on 29 November 1946 to settle the future policy on jet fighters for the VVS and PVO. The Mikoyan-Gurevich MiG-9 was selected as the primary fighter with the Yakovlev Yak-15 relegated to conversion training and familiarization. The imminent availability of British jet engines and access to German jet technology and engineers meant that there was no longer any need for mixed-power aircraft, but Stalin insisted that the I-250 finish its trials and that the tooling and drawings should be retained along with unassembled parts even though the program was cancelled.

Completion of the trials would be delayed until May 1947 when the trials of the VRDK itself would be completed and Mikoyan tried to sell the aircraft to Naval Aviation for use as escorts for torpedo bombers. Additional fuel tanks were fitted in the wings and fuselage of 3810102 to carry an additional of fuel and the capacity of the oil tank was increased to . These changes increased its empty weight to and its take-off weight to . A new round of State trials began on 9 October 1947, but these were delayed by bad weather and reliability issues. Only six flights totaling two hours and twenty-five minutes had been completed by 21 January 1948 and the VRDK had only been run for a minute and half during a ground test. The I-250 was unsurprisingly declared to have failed the trials on 3 April 1948. OKB MiG

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General characteristics

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|empty weight kg=2797

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|gross weight kg=3650

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