Newsletter 7
Autumn 2004
Updated on 12Oct2004

Published by the Hawker Association for the Members.
Contents © Hawker Association

Pegasus: The First Years
On 14th July Gordon Lewis gave a detailed and fluent talk on the origins and development of the Pegasus. It was he who took Wibault's swivelling nozzle concept and developed it into the vectored thrust Pegasus around which Ralph Hooper designed the P.1127. The following paragraphs are condensed from Gordon's paper which was illustrated with a number slides showing the various stages in the evolution of the Pegasus.

The first response from Bristol to the Wibault brochure was a memo from Gordon to Stanley Hooker dated 2 August 1956. A sketch showed Wibault's four centrifugal compressors replaced by one axial compressor (fan), thrust orientation being achieved by means of a rotating nozzle on each side of this axial fan casing. Advantages claimed included reduced weight and complication, use of an existing compressor (fan) and a more straightforward installation. Wibault's use of an Orion was retained to drive the fan and some sort of thrust diverter was assumed to vector the residual Orion thrust at the rear of the aircraft.
The proposed axial fan had approximately the same performance as Wibault's four centrifugals so it was reasonable to state that the overall performance would be close to that defined in Wibault's brochure. This proposal was Bristols' response to the MWDP who asked them to obtain Wibault's reaction. (MWDP - Mutual Weapons Defense Program. Set up by the USA to fund the development of European defence projects. At this time they were looking for a light-weight V/STOL strike fighter for NATO countries.)

Wibault accepted this proposal with enthusiasm and a scheme for a 'Gyropter' ground attack fighter using this BE 48 powerplant resulted. The BE 48 consisted of a Bristol Orion turboprop driving the first two stages of the Olympus low pressure compressor through an epicyclic reduction gear, the fan and engine each
having a separate air intake to reflect the thermodynamic cycle defined by Wibault. The fan air was exhausted through two rotating nozzles whilst the residual turboprop exhaust thrust was diverted by cascades at the rear of the aircraft fuselage.

It was soon realised that the reduction gear was undesirable incurring high weight, cost and development risk as it had to transmit about 11,000 hp, at high aircraft speeds, resulting in problems of heat dissipation and oil cooling. The solution was to use the Orpheus jet engine as the power source. This single spool turbojet's compressor was the same as the Orion LP spool so the power potential was similar. The overall pressure ratio was lower resulting in higher fuel consumption but this was offset by much reduced weight. This engine was designated BE 52.

Parametric studies were carried out featuring alternative combinations of Olympus based fans and versions of the Orpheus, using major components already in development as it was felt that a possible way forward was via a demonstrator engine. It was also assumed that the residual engine exhaust thrust would appear at the rear of the aircraft so to achieve balance in the hover most of the thrust needed to be provided by the front fan jets.

At the end of 1956 a joint Wibault/Lewis patent was filed covering the rotating fan nozzles and an option of similar nozzles at the end of the jet pipe. The value of contra-rotation of the fan and engine spools to minimise gyroscopic moments was recognised although this was not an immediate option using existing components. It was also recognised that higher performance could be achieved by passing a proportion of fan air into the engine but this had the effect of increasing the exhaust thrust and moving the thrust centre rearwards posing apparent problems of balance in the aircraft layouts assumed at the time.

The study work was recorded in a project brochure which identified the BE 53 as having three stages of the Olympus LP compressor driven by an uprated Orpheus, with separate air intakes, giving a thrust of about 11,000 lbs.

At this stage Short Brothers and Harland, already building their lift engine powered SC 1, were approached. Shorts produced a scheme based on the BE 53 for a joint meeting with the MWDP but used the opportunity to promote the SC 1 and showed no enthusiasm for the single engine vectored thrust solution. This setback resulted in reduced activity at Bristol until May 1957 when Stanley Hooker received a letter from Sydney Camm expressing interest in VTOL and doubting the use of multiple lift engines. Hooker's reply enclosed the BE 53 brochure and started the close collaboration between the aircraft and engine design teams that resulted in the Pegasus and the Harrier.

Ralph Hooper picked up the Bristol brochure and schemed a tentative proposal for a STOL reconnaissance vehicle, based on the BE 53 without deflection of the rear jet. Seeking hover capability Hooper proposed the vital feature of the Pegasus-Harrier combination; the use of two rotating nozzles for the engine exhaust but moved close to the final turbine stage. This originated the "four poster" arrangement which led to a realistic single engined VSTOL project. This bifurcated rear exhaust system so close to the turbine was initially seen by Bristols to present hazards in the form of blade vibration and duct integrity but these were soon accepted when the advantages became apparent. The forseen problems did arise but were solved during early engine development. Ralph Hooper's July 1957 drawing demonstrated the potential for a winning formula and inspired a period of rapid design innovation by close collaboration between the two design teams.

The first consequence of the close coupled rear nozzles was to move the vertical thrust centre forward thereby allowing increased rear thrust. The previously studied by-pass cycle, in which the engine was in effect supercharged by the fan, became viable, increasing the total thrust potential. With a realistic aircraft project in prospect the restraint of using existing fan components was abandoned and a larger two-stage fan was defined, enabling contrarotation to be adopted, as forseen in the early studies and strongly requested by Hawkers.

Stabilisation of the aircraft in the hover was to have been by jets fed by fan bleed air but the large diameter ducts required were very difficult to accommodate. The use of high pressure engine compressor bleed air solved the installation problem but had inherent and severe performance penalties. The solution was to take advantage of thermal lag to allow a significant increase in turbine inlet temperature during brief periods of maximum bleed without exceeding acceptable metal temperatures in the turbine blades.

The nozzle rotation system using an air motor, shafting and final chain drives was evolved as was the Hawker proposal to mount the accessory gearbox on top of the engine where it could be enclosed ahead of the wing structural box. The short intake and the requirement for minimal thrust loss in the static condition was seen as a major problem and became the subject of very close collaboration between specialists in the two companies.

Thus the Pegasus 1 vectored thrust engine was defined, embodying the basic features that have carried through to the engines currently in service.

After a lengthy question time the vote of thanks