The Rolls-Royce jet engine that powers the Harrier jump jet is the Pegasus. Its defining feature is vectored thrust: four swivelling nozzles turn the engine’s airflow downward for vertical or short takeoffs and landings.
The engine began as the Bristol Siddeley Pegasus and first ran in September 1959. After Rolls-Royce acquired Bristol Siddeley, the company continued developing and manufacturing it. The Pegasus became the powerplant for the Harrier family, including the Hawker Siddeley Harrier, Sea Harrier and McDonnell Douglas AV-8B Harrier II.
The engine is a two-shaft turbofan with separate low- and high-pressure compressor systems. Its unusual nozzle arrangement sends cooler compressor air through the forward pair and hot exhaust through the rear pair. In U.S. service, Pegasus engines received the designation F402. “Merlin” is a Rolls-Royce piston engine, while “Trent” and “Olympus” are unrelated gas-turbine engine families, making them plausible but incorrect alternatives.