Propulsion
The key to the Harrier's unique abilities lies in its Pegasus engine. Like the airframe, this has developed considerably since it first ran in 1959, but the fundamentals have remained unchanged.

Air enters the engine via the two intakes and first passes through the low-pressure compressor. Upon exiting the LP compressor around 58% percent of the airflow enters a plenum chamber. On either side of this chamber are the two forward vectoring nozzles through which this cold (100 C) air is expelled to provide thrust. The remaining 42% of the airflow passes from the LP compressor to the high-pressure compressor. On leaving the HP compressor it enters the combustion chambers, is heated by burning fuel in the air stream and then passes over the HP and LP turbines, which drive their respective compressors. Once the heated air leaves the turbines it passes into a bifurcated duct which has a further pair of lateral vectoring nozzles. These nozzles allow the hot (650 C) air to exit the aircraft and balance the thrust from the forward nozzles, the two sets of nozzles being set about the aircraft's centre of gravity. In order to eliminate gyroscopic precessional effects when manoeuvering in the hover, the LP and HP spools of the engine contra-rotate, their respective gyroscopic forces cancelling each other out.

From this brief description it can be seen that the Pegasus is essentially a conventional turbofan engine. The only exceptions are the four nozzles that are required to vector the engine's thrust. In fact it is the control of these nozzles that represents the Harrier's only marked departure from a conventional aircraft. In the cockpit, next to the throttle, the pilot is provided with an additional lever that controls the angle of the nozzles and therefore the amount of jet lift imparted. By the judicious selection of throttle and nozzle angle it is possible to fly the aircraft from 50 knots backwards to 600+ knots forward, including many low speeds where the aircraft is supported on a mixture of jet and wing lift.

It is important that all four nozzles move at the same time to ensure the stability of the aircraft. To this end they are linked by a system of shafts and chains that are driven by an air motor using air bled from the engine. The engine also provides power for the electrical, hydraulic and conditioning systems via a number of generators, pumps and air bleeds.