Jet engines had only been powering aeroplanes for 14 years when the Bristol Engine Company took up the challenge to add thrust vectoring to their single spool Orpheus engine.  As per a normal jet engine, the pegasus uses a compressor to compress air for efficient burning to turn the turbine, thus turning the compressor.  In a normal turbofan jet engine, the air is fed directly rearwards; a proportion through the core of the engine, and a proportion of unburned air our of the fan bypassing the engine.  In the pegasus these two airstreams are fed into vectoring nozzles to enable the air to be directed downwards as well as backwards.

PCB

For many years, the pegasus has been one of the most powerful jet engines in production, however inefficiencies of vectoring demanded even more power to give supersonic performance.   The solution to supersonic flight was Plenum Chamber Burning (PCB).  Whilst McDonnell Douglas evolved their AV8A into an AV8B, Hawker opted for a more revolutionary step to re-invent the PCB pegasus cancelled with the cancellation of the supersonic P1154.  PCB involved reheating the air (afterburner) with the unburned front air, not the normal afterburner approach of the burned exhaust air.

Jet Efflux

One of the main problems of the Harrier is due to history.  The P1127 was designed to be the most basic airframe to wrap around the Pegasus engine.  The jet exhausts were therefore sticking out of the side of the fuselage, and this airflow made the rear control surface difficult to control, and wear out quickly.  The solution designed for the P1185 and carried forward through later variants was to move the pegasus’ nozzles to the bottom of the engine, as opposed to the sides.  Together with positioning the engine lower in the airframe resolved the jet efflux problem.

Single Rear Opening

The lower positioning of the engine created as big a problem as it solved.  Moving the centre of gravity (mass of the engine) and centre of thrust created pitching problems with the design of aircraft such as the P1205 and P1208.  These problems were resolved in the P1214 and P1216 by using an alternative approach to jet efflux - remove the fuselage behind the engine.  In addition, the opportunity was taken with these designs to combine the rear nozzle into a single unit that gave a greater amount of thrust.  Hawker did not take the engine the stage further to abandon fore nozzle PCB and add afterburning to the rear nozzle,but this approach would be adopted in the X-32.