The Rolls-Royce Pegasus is a British turbofan engine originally designed by Bristol Siddeley.

The engine is not only able to power a jet aircraft forward, but also to direct thrust downwards via swivelling nozzles.

Originally the Bristol Siddeley Pegasus, the engine powers all versions of the Harrier family of multi-role military aircraft.

However Pratt & Whitney never completed any engines, with all new build being manufactured by Rolls-Royce in Bristol, England.

One concept which looked promising was the BE52, which initially used the Orpheus 3 as the engine core and, on a separate coaxial shaft, the first two stages of an Olympus 21 LP compressor, which acted as a fan, delivering compressed air to two thrust vectoring nozzles at the front of engine.

At this point in the design exercise, the exhaust from the LP turbine discharged through a conventional rear nozzle.

The Olympus stages now supercharged the Orpheus core, improving the overall pressure ratio,[6] creating what is now considered a conventional turbofan configuration.

For a year Bristol designed the engine in isolation, with little feedback from the various airframe manufacturers furnished with data.

[7] The first prototype engine (one of two BE53/2s built), ran on 2 September 1959 and featured a 2-stage fan and used the Orpheus 6 core.

The first engines had barely enough thrust to lift the plane off the ground due to weight growth problems.

Flight tests were initially conducted with the aircraft tethered, with the first free hover achieved on 19 November 1960.

It was originally feared that the aircraft would have difficulty transitioning between level and vertical flight, but during testing it was found to be extremely simple.

Series manufacture and design and development improvement to the Pegasus to produce ever-higher thrusts were continued by Bristol engines beyond 1966, when Rolls-Royce Ltd bought the Company.

A non-vectored 26,000 lb thrust derivative of the Pegasus running on liquid hydrogen, the RB.420, was designed and offered in 1970 in response to a NASA requirement for an engine to power the projected Space Shuttle on its return flight through the atmosphere.

To enable the engine speed and hence thrust to be increased for take-off, water is sprayed into the combustion chamber and turbine to keep the blade temperature down to an acceptable level.

Water for the injection system is contained in a tank located between the bifurcated section of the rear (hot) exhaust duct.