Pegasus Vectored-thrust Turbofan Engine Matador Harrier Sea Harrier AV-8A International Historic Mechanical Engineering Landmark 24 July 1993 International Air Tattoo '93 RAF Fairford The American Society of Mechanical Engineers I MECH E I NTERNATIONAL H ISTORIC M ECHANICAL E NGINEERING L ANDMARK PEGASUS V ECTORED-THRUST T URBOFAN ENGINE 1960 T HE B RISTOL AERO-ENGINES (ROLLS-R OYCE) PEGASUS ENGINE POWERED THE WORLD'S FIRST PRACTICAL VERTICAL/SHORT-TAKEOFF-AND-LANDING JET AIRCRAFT , THE H AWKER P. 1127 K ESTREL. USING FOUR ROTATABLE NOZZLES, ITS THRUST COULD BE DIRECTED DOWNWARD TO LIFT THE AIRCRAFT, REARWARD FOR WINGBORNE FLIGHT, OR IN BETWEEN TO ENABLE TRANSITION BETWEEN THE TWO FLIGHT REGIMES. T HIS ENGINE, SERIAL NUMBER BS 916, WAS PART OF THE DEVELOPMENT PROGRAM AND IS THE EARLIEST KNOWN SURVIVOR. PEGASUS ENGINE REMAIN IN PRODUCTION FOR THE H ARRIER II AIRCRAFT. T HE AMERICAN SOCIETY OF M ECHANICAL ENGINEERS T HE INSTITUTION OF M ECHANICAL ENGINEERS 1993 Evolution of the Pegasus Vectored-thrust Engine Introduction cern resulted in a perceived need trol and stability problems associ- The Pegasus vectored for combat runways for takeoff and ated with the transition from hover thrust engine provides the power landing, and which could, if re- to wing-borne flight. for the first operational vertical quired, be dispersed for operation The concepts examined and short takeoff and landing jet from unprepared and concealed and pursued to full-flight demon- aircraft. The Harrier entered ser- sites. Naval interest focused on a stration included "tail sitting" types vice with the Royal Air Force (RAF) similar objective to enable ship- exemplified by the Convair XFY-1 in 1969, followed by the similar borne combat aircraft to operate and mounted jet engines, while oth- AV-8A with the United States Ma- from helicopter-size platforms and ers used jet augmentation by means rine Corps in 1971. Both services small ships, because of the high of lifting fans and ejectors. have continued to operate devel- cost and expected vulnerability of In the United Kingdom, ef- oped versions of the basic aircraft, large aircraft carriers. fort was concentrated on the devel- and it has been adopted by the During the 1950s, numer- opment and application of very high Royal Navy and the Spanish, In- ous projects and research programs thrust-to-weight lift engines fol- dian, and Italian navies. More ad- were initiated in the United States lowing the ideas put forward by Dr. vanced versions of the engine and and Western Europe to study and A. A. Griffith. This work resulted, aircraft are still under development, validate alternative means of in 1954, in the launch of the Short to extend the capability of the achieving the required short or ver- SC1, a small VTO flat-rising re- weapon system and ensure con- tical takeoff (VTO) and landing search aircraft able to take off ver- tinuing operation well into the next characteristics. The advancing tically with the thrust of four Rolls- century. technology of the gas turbine of- Royce RB108 vertically mounted The engine has the special fered steadily increasing values of lift engines. The achieved engine characteristic of thrust vectoring, engine thrust-to-weight ratio, thrust-to-weight ratio was 8:1, and enabling the thrust to be directed which encouraged the study of di- a further similar engine was in- rearward for propulsion, downward rect jet lift systems and of the con- stalled with a horizontal jet efflux for lift, and forward for braking. The concept, which has remained basically unchanged since the first prototype P.1127 flew in 1960, en- abled the goal of a single- engined VSTOL (very short takeoff and landing) jet aircraft to be realized. The aircraft deliveries pro- grammed for the present decade will ensure that the Pegasus en- gine, incorporating progressive de- velopment and refinement, will continue in service more than half a century after the first drawings were made in 1956. Background Hawker P.1127 In the late 1940s immedi- ately following World War II, the development of ballistic missiles and sophisticated munitions led to concern in the Western Alliance regarding the vulnerability of North Atlantic Treaty Organiza- tion (NATO) airfields. This con- 1 for propulsion. The two UK companies to become responsible for Pegasus and Harrier development were, at this time, not active in the field of VTO. The Bristol Aeroplane Company Engine Division, later to be ab- sorbed into Bristol Siddeley En- gines and then into Rolls-Royce, had responded to a requirement for a light-weight engine to power a NATO strike fighter, conven- tional in configuration but able to operate from relatively unprepared Orpheus Turbojet runways. This engine, the Orpheus turbojet, was selected for the Fiat G91 aircraft to meet the NATO requirement. The development program was managed and funded by the Mutual Weapons Develop- ment Program (MWDP), a United States agency with an office in Paris having the objective of supporting projects of potential value to the NATO forces. The Fiat G91, which later entered service with the Ger- man and Italian air forces, was to be followed in a second-phase pro- gram by an aircraft with enhanced performance and in a third phase by a strike fighter with short take- Fiat G91 off and vertical-landing capability. The Evolution of Vectored Thrust incorporated into the Pegasus en- stabilized in hovering and low- In March 1956, when gine and the Harrier airframe. The speed flight by means of air bled MWDP was turning its attention system proposed consisted of a tur- from the compressors and ejected to the third-phase NATO require- boprop engine, the Bristol BE25 from the wing tips and nose and ment, a proposal was submitted to Orion of about 8,000 horsepower, tail of the airframe. the Paris office by Michel Wibault. driving four large centrifugal com- Col. John Driscoll, at the Wibault was well known in avia- pressors, arranged like wheels at time head of the MWDP Paris of- tion in the pre-war period, the com- the sides of the fuselage. The blower fice, sent the proposal to Dr. Stanley pany of that name having been casings could be rotated to direct Hooker (later Sir Stanley) at Bristol responsible for a range of French the compressed air, and hence the Engines. It was studied by Gordon transport and fighter aircraft. Post- thrust, through over 90 degrees. Lewis, who was responsible for new war Wibault had been working on The air was directed downwards projects. Lewis recognized the im- novel aviation projects and had for vertical takeoff and landing, portance of the thrust-vectoring produced schemes for a VSTOL obliquely for climbing or transi- principle but considered the strike fighter, which were entitled tion, and horizontally for level Wibault mechanical arrangement “Ground Attack Gyropter,” the sub- flight. The exhaust gas from the of four compressors driven by shafts ject of the March proposal. turboprop was also used for verti- and bevel gear boxes to be complex This proposal introduced cal or horizontal thrust using a gas and heavy. He proposed an alter- the concept of thrust vectoring and deviator mechanically connected native lighter and simpler arrange- described the basic principles of with the rotation of the four blower ment, substituting a two-stage axial operation that were subsequently casings. The proposed aircraft was flow fan for the four centrifugal 2 blowers and vectoring the later commercial engines. thrust through two rotating In May 1957 Sir Sydney nozzles, one on each side of Camm of Hawker Aircraft con- the engine. tacted Sir Stanley Hooker to dis- This layout, desig- cuss possible VSTOL projects. The nated BE 48, retained the BE 53 schemes were presented, Orion engine and a reduc- and Ralph Hooper of Hawkers car- tion gear to drive the fan ried out a series of project designs Further weight reduction re- aiming at a VSTOL fighter with sulted from using a power real military capability. His work turbine running at the ap- contributed much to the refine- Michel Wibault’s Ground Attack Gyropter, propriate speed, dispensing ment of the engine design, includ- March 1956 with the reduction gear, and ing the major feature of rotating replacing the turboprop with rear nozzles, exiting on each side of the simpler and lighter the fuselage and closely integrated Orpheus turbojet. The re- with the engine. The nozzle rota- sulting design, the BE 52, tion mechanism had received much was further evolved into the attention by F. C. Marchant, in BE 53 with a three-stage fan. charge of the Bristol design office, Wibault quickly ac- and he evolved the scheme of using cepted the changes to his me- large diameter bearings to support chanical design and produced the nozzles with air motor driven a scheme of a strike fighter chains round the periphery of the using the new proposed en- bearings to affect rotation. gine. This was presented to By the autumn of 1957, the Original sketch of BE 48, August 1956 MWDP, where Col. Willis Hawker designers had prepared the Chapman, later Brigadier initial schemes of the prototype General, had replaced Col. P.1127, and the engine design, Driscoll. Chapman encour- named Pegasus, had become a com- aged Bristol to proceed with pact and light thrust vectoring en- the design, and a joint patent gine, with the thrust line passing was registered at the end of through the center of gravity in all 1956 in the names of Wibault angles of deflection. The engine and Lewis. This patent iden- configuration at the commence- tified the main feature of ro- ment of joint MWDP and company- tating nozzles, including a funded development in 1958 com- pair at the rear of the air- prised a two-stage fan, seven-stage craft to deflect the turbine compressor, and three stages of exhaust gas, and contra-ro- turbine.