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Aviation History
1972
1972 - 1422.PDF
814 FLIGHT International, I lune 1972 •:: mm sfeli US/UK Advanced Lift Engine Programme f=^ Phase 1 *& US/UK govt MofU Commercial agreement R.R. & DD Allison 69 70 Supporting technology rig testing Pre definition study ' | and component test Engine definition Successful contract completion J Design and procurement US/FRG concept _ definition support I ~T IE Aeromechanical rig testing zc IT Demonstration engine testing US/FRG cancellation Relative Merits of Thrust/Weight & Thrust/Volume ^) 300 Radius of action nautical miles 200 100 20 30 40 THRUST/WEIGHT 50 /FZ> Direct lift engines-Future development @J NON-BLEED ENGINES Thrust class lb 7W Ratio %% Ratio lb/in1 1 • 1 XJ99 I A • 2725 > 18 9000 20 10 PROJECT 'A' A -1-r 41-8 11000 22 12 I PROJECT If • 2725 > 12000 26 16 XJ99: engine in search of an airframe One significant powerplant which has received less than its fair measure of recognition, because of the dictates of military security, is the advanced lift-jet designed jointly by Rolls-Royce and Alli son for the remarkable US-FRG VTO fighter. This ambitious aircraft was cancelled in 1968, at a very early stage, for a more conventional successor, NKF, which itself gave way subsequently to MRCA. But the XJ99 powerplant was considered sufficiently promising to warrant development up to the demonstrator stage. This phase was completed last year and the project will not be resumed until definite applications have been identified. The following extract from a joint paper by the two engine firms discusses the origins of the engine and possibilities for the future. IN 1965 the American and British Governments recognised jointly that there was a requirement for an advanced lift engine for military use. A Memorandum of Under standing was signed between them, covering collaboration between the Derby Engine Division of Rolls-Royce and the Detroit Diesel Allison Division of General Motors for the design and development of such a powerplant, to be designated the RA-XJ99. At the time of the memorandum, Rolls-Royce had com pleted feasibility studies, including component testing, on an advanced lift engine designated the RB.189. This project was a third-generation powerplant and was based on the unique experience built up by the company on the RB.108 and RB.162 series of lift engines. The XJ99 grew out of this work. The function of the lift engine is to support the aircraft in vertical flight and transition, either alone or by supple menting vectored thrust from the main propulsion engines. It follows that lift engines are required only to operate intermittently and at low altitudes and that the fuel con sumption is relatively less important than for propulsion engines. The cycle can therefore be simple and it becomes possible, by the use of high-duty components, to design engines which have a very low weight and volume relative to the thrust generated. Low cost, both of engine and control system, toleration of intake air-flow distortion, rapid response and minimum gyroscopic couple are also major design requirements. In addition, some applications require a large continuous air bleed to stabilise the aircraft in vertical flight. The XJ99 offers improvements in thrust relative to weight and, in particular, to volume when compared with the RB.162. Studies have shown that, for a Vtol strike aircraft, improving the thrust-to-weight ratio offers a diminishing return, but that considerable advantages remain in improving the thrust-to-volume ratio. The principal benefit of minimising engine volume is a reduc tion in the size of the fuselage, a particularly attractive factor in supersonic applications. The XJ99 is a two-shaft turbojet with a thrust in the 9,0001b class. The compressors, which have two stages in the low-pressure system, and four stages in the high- pressure spool, are both axial-flow. The two systems An impression (top) of the MS (Advanced Vertical Strike) fighter projected some years ago for the Luftwaffe. It was to have four swing- out XJ99 lift-jets, two P&W JTFI6 vectored-thrust cruise engines of 13,0001b thrust, and a variable-geometry wing. Estimated weight was 45,0001b. It was cancelled in 1968, not unexpectedly
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