Plenum chamber burning progress

In the UK both the RAF and the RN were encouraged by the Government to develop versions of the P1154 tailored to suit their individual needs.

The RAF’s requirement was for a single-seat V/STOL close support/strike fighter with a secondary interception/air superiority role. Part of its advanced weapons system eventually appeared in today’s transonic Harrier (the inertial navigation and the Head-Up Display sub-system).

By 1964 the RN had withdrawn its interest in favour of the F4 Phantom. Drawings for the definitive RAF P1154 were released for manufacture and, by the time the project was cancelled in February 1965, the first aircraft was 30 per cent complete. Its Bristol-Siddley (later Rolls-Royce) BS 100 engine had produced over 30,000lb of test-bed thrust before being put into store.

The BS 100 was derived from the Pegasus engine, which at that time powered the subsonic Hawker P1127 prototypes. It was, however, of a lower bypass ratio than the Pegasus and there were changes to the thermodynamic cycle and improved materials to permit operation up to Mach 2 at the tropopause. To provide the thrust needed for supersonic flight fuel was burned in the engine’s plenum chamber, the duct which supplies air to the front “col” nozzles. This technique of boosting thrust is known as Plenum Chamber Burning (PCB) and is particularly effective due to the greater overall temperature rise available to air entering the fan compared to a normal reheat system.

The potential boost to the front nozzle thrust at take-off is in the region of 100 per cent, with even greater benefits at high forward speed.

In December 1962, a modified Pegasus 2 engine complete with a pair of PCB front nozzles was tested, proving the reliability of ignition and the efficiency of the system. The Pegasus 2 was initially configured to the shape of the standard engine at that time. The plenum chamber was later modified to represent the “droop and trail” shape which eventually appeared on the BS 100.

By 1965 PCB testing amounted to some 650 hours, including a 25-hour endurance test on the BS 100, when the programme was curtailed with the demise of the P1154.

Small-scale development tests continued both at HSA Kingston and RR Bristol to determine the effects of operating PCB on hot gas recirculation into the engine intake with the aircraft hovering, though even this research effort dwindled in recent years.

Having successfully completed a short test programme to demonstrate the ability of the system to tolerate intake flow distortion, and water ingestion, and to achieve cross-lighting (relighting one nozzle from the other’s flame), the rig is now being transferred to the National Gas Turbine Establishment at Pyestock for testing in the Altitude Test Facility there. Plans are currently being examined to modify a damaged Harrier airframe and install the PCB Pegasus 2 with a view to evaluating PCB operation at various heights off the ground.

Where then is this activity leading? . . . a V/STOL tactical fighter with supersonic capability is the aim; combining the V/STOL advantages of the Harrier with the high speeds and energy manoeuvrability necessary to perform successfully in tomorrow’s air combat theatre over the battlefield.