The Rolls-Royce Pegasus is a turbofan engine originally designed by Bristol Siddeley, and was manufactured by Rolls-Royce plc. This engine is able to direct thrust downwards which can then be swivelled to power a jet aircraft forward. The BE.52 design was built around a Bristol Siddeley Orpheus which through a shaft drove the first three stages of a Bristol Olympus engine which had inlet and outlets separate of those of the Orpheus.
Work was overseen by Bristol Siddeley's Technical Director Stanley Hooker. The Bristol Engine Company began work on the BE.53 Pegasus in 1957. While the BE.52 was a self-contained powerplant and lighter compared to Wibault's concept, the BE.52 was still complicated and heavy. In the BE.53 the Olympus stages were fitted close to the Orpheus stage; this simplified the inlet ducting and the Olympus stages now supercharged the Orpheus improving the compression ratio.
The further development of the engine then proceeded in tandem with the airframe the Hawker P.1127, which first flew in 1960. The next stage of design and development was then flown in the Kestrel, of which nine were built. This was then developed into the Harrier combat aircraft. The engine was partially supported financially from the Mutual Weapons Development Programme.
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 °C) jet exhaust. The airflow split is about 60/40 front back.
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, surprisingly, motor-cycle chains. The Pegasus was also the first turbofan engine to have the initial compressor fan, the zero stage, ahead of the front bearing. This eliminated radial struts and the icing hazard they represent.