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Note: This is a sub-section of Rolls-Royce
The Pegasus engine powered all versions of the Hawker Siddeley Harrier multi-role military aircraft. Rolls-Royce licensed Pratt & Whitney to build the Pegasus for US built versions. However Pratt & Whitney never completed any engines, with all new build being manufactured by Rolls-Royce in Bristol. The Pegasus has also been the planned engine for a number of aircraft projects, among which were the prototypes of the German Dornier Do 31 VSTOL military transport project.
The Pegasus vectored-thrust turbofan is a two-shaft design featuring three low pressure (LP) and eight high pressure (HP) compressor stages driven by two LP and two HP turbine stages respectively. Unusually the LP and HP spools rotate in opposite directions to greatly reduce the gyroscopic effects which would otherwise hamper low speed handling. The engine employs a simple thrust vectoring system that uses four swivelling nozzles, giving the Harrier thrust both for lift and forward propulsion, allowing for STOVL flight. The front two nozzles are fed with air from the LP compressor, the rear with hot (650 degrees C) jet exhaust. It was critical that the nozzles rotate together. This was achieved by using a pair of air motors fed from the HP (high pressure) compressor, in a fail-over configuration, pairs of nozzles connected with chains.
The Pegasus was also the first turbofan engine to have the initial compressor fan, the zeroth stage, ahead of the front bearing. This eliminated radial struts and the icing hazard they represent. Also inlet guide vanes were not used in contrast to contemporary practise; it was found that they were not beneficial as was thought.
To date, more than 1,200 engines have been produced and almost two million operating hours have been logged with the Harriers of the Royal Air Force, Royal Navy, U.S. Marine Corps and the navies of India, Italy, Spain and Thailand.
The engine is mounted in the centre of the Harrier and as such it is necessary to remove the wing to change the power-plant having already sat the fuselage on trestles; the whole change took a minimum of eight hours.
Bristol Aero-Engines began work on the BE.53 Pegasus in 1958. The engine was designed in tandem with the prototype of the Hawker Siddeley Harrier, the Hawker P.1127, which first flew in 1960. It was developed from the Bristol Orpheus, overseen by Stanley Hooker. The low pressure stages came from the Bristol Olympus engine.
Production and development of the Pegasus was continued by Rolls-Royce when it acquired Bristol in 1966. a related engine design, the 39,500 lbf (with reheat) Bristol Siddeley BS100 for a supersonic VTOl fighter (Hawker Siddeley P.1154) was not developed to production as the aircraft project was cancelled.