Pegasus Vectored-thrust Turbofan Engine

Matador Harrier

Sea Harrier AV-8A

I MECH E

InternationalHistoric Mechanical EngineeringLandmark

24 July 1993

International Air Tattoo '93RAF Fairford

The American Society ofMechanical Engineers

IN T E R N A T I O N A L H ISTORIC M E C H A N I C A L E N G I N E E R I N G L A N D M A R K

PEGASUS

VECTORED - THRUST TURBOFAN ENGINE

1960

THE BRISTOL AERO-ENGINES ( ROLLS-ROYCE) 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. PE G A S U S

ENGINE REMAIN IN PRODUCTION FOR THE H ARRIER II A I R C R A F T.

THE AMERICAN SOCIETY OF M ECHANICAL ENGINEERS

THE INSTITUTION OF M ECHANICAL ENGINEERS

1993

Evolution of the Pegasus Vectored-thrust Engine

IntroductionThe Pegasus vectored

thrust engine provides the powerfor the first operational verticaland short takeoff and landing jetaircraft. The Harrier entered ser-vice with the Royal Air Force (RAF)in 1969, followed by the similarAV-8A with the United States Ma-rine Corps in 1971. Both serviceshave continued to operate devel-oped versions of the basic aircraft,and it has been adopted by theRoyal Navy and the Spanish, In-dian, and Italian navies. More ad-vanced versions of the engine andaircraft are still under development,to extend the capability of theweapon system and ensure con-tinuing operation well into the nextcentury.

The engine has the specialcharacteristic of thrust vectoring,enabling the thrust to be directedrearward for propulsion, downwardfor lift, and forward for braking.The concept, which has remainedbasically unchanged since the firstprototype P.1127 flew in 1960, en-abled the goal of a single- enginedVSTOL (very short takeoff andlanding) jet aircraft to be realized.

The aircraft deliveries pro-grammed for the present decadewill ensure that the Pegasus en-gine, incorporating progressive de-velopment and refinement, willcontinue in service more than halfa century after the first drawingswere made in 1956.

BackgroundIn the late 1940s immedi-

ately following World War II, thedevelopment of ballistic missilesand sophisticated munitions led toconcern in the Western Allianceregarding the vulnerability ofNorth Atlantic Treaty Organiza-tion (NATO) airfields. This con-

cern resulted in a perceived needfor combat runways for takeoff andlanding, and which could, if re-quired, be dispersed for operationfrom unprepared and concealedsites. Naval interest focused on asimilar objective to enable ship-borne combat aircraft to operatefrom helicopter-size platforms andsmall ships, because of the highcost and expected vulnerability oflarge aircraft carriers.

During the 1950s, numer-ous projects and research programswere initiated in the United Statesand Western Europe to study andvalidate alternative means ofachieving the required short or ver-tical takeoff (VTO) and landingcharacteristics. The advancingtechnology of the gas turbine of-fered steadily increasing values ofengine thrust-to-weight ratio,which encouraged the study of di-rect jet lift systems and of the con-

trol and stability problems associ-ated with the transition from hoverto wing-borne flight.

The concepts examinedand pursued to full-flight demon-stration included "tail sitting" typesexemplified by the Convair XFY-1and mounted jet engines, while oth-ers used jet augmentation by meansof lifting fans and ejectors.

In the United Kingdom, ef-fort was concentrated on the devel-opment and application of very highthrust-to-weight lift engines fol-lowing the ideas put forward by Dr.A. A. Griffith. This work resulted,in 1954, in the launch of the ShortSC1, a small VTO flat-rising re-search aircraft able to take off ver-tically with the thrust of four Rolls-Royce RB108 vertically mountedlift engines. The achieved enginethrust-to-weight ratio was 8:1, anda further similar engine was in-stalled with a horizontal jet efflux

Hawker P.1127

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for propulsion.The two UK companies to

become responsible for Pegasus andHarrier development were, at thistime, not active in the field of VTO.The Bristol Aeroplane CompanyEngine Division, later to be ab-sorbed into Bristol Siddeley En-gines and then into Rolls-Royce,had responded to a requirementfor a light-weight engine to powera NATO strike fighter, conven-tional in configuration but able tooperate from relatively unpreparedrunways. This engine, the Orpheusturbojet, was selected for the FiatG91 aircraft to meet the NATOrequirement. The developmentprogram was managed and fundedby the Mutual Weapons Develop-ment Program (MWDP), a UnitedStates agency with an office in Parishaving the objective of supportingprojects of potential value to theNATO forces. The Fiat G91, whichlater entered service with the Ger-man and Italian air forces, was tobe followed in a second-phase pro-gram by an aircraft with enhancedperformance and in a third phaseby a strike fighter with short take-off and vertical-landing capability.

The Evolution of Vectored ThrustIn March 1956, when

MWDP was turning its attentionto the third-phase NATO require-ment, a proposal was submitted tothe Paris office by Michel Wibault.Wibault was well known in avia-tion in the pre-war period, the com-pany of that name having beenresponsible for a range of Frenchtransport and fighter aircraft. Post-war Wibault had been working onnovel aviation projects and hadproduced schemes for a VSTOLstrike fighter, which were entitled“Ground Attack Gyropter,” the sub-ject of the March proposal.

This proposal introducedthe concept of thrust vectoring anddescribed the basic principles ofoperation that were subsequently

Orpheus Turbojet

Fiat G91

incorporated into the Pegasus en-gine and the Harrier airframe. Thesystem proposed consisted of a tur-boprop engine, the Bristol BE25Orion of about 8,000 horsepower,driving four large centrifugal com-pressors, arranged like wheels atthe sides of the fuselage. The blowercasings could be rotated to directthe compressed air, and hence thethrust, through over 90 degrees.The air was directed downwardsfor vertical takeoff and landing,obliquely for climbing or transi-tion, and horizontally for levelflight. The exhaust gas from theturboprop was also used for verti-cal or horizontal thrust using a gasdeviator mechanically connectedwith the rotation of the four blowercasings. The proposed aircraft was

stabilized in hovering and low-speed flight by means of air bledfrom the compressors and ejectedfrom the wing tips and nose andtail of the airframe.

Col. John Driscoll, at thetime head of the MWDP Paris of-fice, sent the proposal to Dr. StanleyHooker (later Sir Stanley) at BristolEngines. It was studied by GordonLewis, who was responsible for newprojects. Lewis recognized the im-portance of the thrust-vectoringprinciple but considered theWibault mechanical arrangementof four compressors driven by shaftsand bevel gear boxes to be complexand heavy. He proposed an alter-native lighter and simpler arrange-ment, substituting a two-stage axialflow fan for the four centrifugal

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Michel Wibault’s Ground Attack Gyropter,March 1956

Original sketch of BE 48, August 1956

Wibault-Lewis patent

Wibault Gyropter scheme using BE 48

blowers and vectoring thethrust through two rotatingnozzles, one on each side ofthe engine.

This layout, desig-nated BE 48, retained theOrion engine and a reduc-tion gear to drive the fanFurther weight reduction re-sulted from using a powerturbine running at the ap-propriate speed, dispensingwith the reduction gear, andreplacing the turboprop withthe simpler and lighterOrpheus turbojet. The re-sulting design, the BE 52,was further evolved into theBE 53 with a three-stage fan.

Wibault quickly ac-cepted the changes to his me-chanical design and produceda scheme of a strike fighterusing the new proposed en-gine. This was presented toMWDP, where Col. WillisChapman, later BrigadierGeneral, had replaced Col.Driscoll. Chapman encour-aged Bristol to proceed withthe design, and a joint patentwas registered at the end of1956 in the names of Wibaultand Lewis. This patent iden-tified the main feature of ro-tating nozzles, including apair at the rear of the air-craft to deflect the turbineexhaust gas, and contra-ro-tation of the engine spools tominimize the effect of gyro-scopic couples on hoveringstability. By this time thebenefit of using a proportionof the fan delivery air to su-percharge the engine com-pressor had been realized,resulting in the necessity foronly a single air intake. Thisaspect was covered in thepatent, and the engineemerged as a turbofan, orbypass engine, an arrange-ment used extensively in

later commercial engines.In May 1957 Sir Sydney

Camm of Hawker Aircraft con-tacted Sir Stanley Hooker to dis-cuss possible VSTOL projects. TheBE 53 schemes were presented,and Ralph Hooper of Hawkers car-ried out a series of project designsaiming at a VSTOL fighter withreal military capability. His workcontributed much to the refine-ment of the engine design, includ-ing the major feature of rotatingrear nozzles, exiting on each side ofthe fuselage and closely integratedwith the engine. The nozzle rota-tion mechanism had received muchattention by F. C. Marchant, incharge of the Bristol design office,and he evolved the scheme of usinglarge diameter bearings to supportthe nozzles with air motor drivenchains round the periphery of thebearings to affect rotation.

By the autumn of 1957, theHawker designers had prepared theinitial schemes of the prototypeP.1127, and the engine design,named Pegasus, had become a com-pact and light thrust vectoring en-gine, with the thrust line passingthrough the center of gravity in allangles of deflection. The engineconfiguration at the commence-ment of joint MWDP and company-funded development in 1958 com-prised a two-stage fan, seven-stagecompressor, and three stages ofturbine.

The program moved for-ward rapidly when Hawkerlaunched the P.1127 prototype withUK funding, and in September1959, the first engine ran on thetest bed, giving 9,000 lbs of thrust.The Hawker Chief Test Pilot, BillBedford, flew the P.1127 for thefirst time in the hovering mode inOctober 1960, by which time theengine thrust had been increasedto 12,000 lbs.

John Dale, who had beenresponsible for development of theOrpheus engine in the Fiat G91,

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took charge of Pegasus develop-ment engineering and led the teamthat progressively raised the en-gine thrust to over 21,000 lbs forthe RAF’ Harriers and US MarineCorps AV-8As.

In the process of thrustenhancement, the engine passedthrough several phases of evolu-tion to the present-day configura-tion of three fan stages, eight-stagecompressor, and four stages of tur-bine with an annular combustor.Special features of the engine in-clude contra-rotation, provision forair bleed for the aircraft stabiliza-tion nozzles, and a transonic fanwith no inlet guide vanes. Thislatter type of fan was subsequentlyadopted for large turbofan com-mercial engines.

Development ProblemsThe Pegasus Harrier com-

bination has been very successful,but nevertheless there have beendesign and development problemsto be solved during its career. Theearly problems emerged under thefollowing headings:

Intake ducting and fanoutlet system. Because the enginehas to be located at the aircraftcenter of gravity, the intake duct isonly about one diameter long. Con-necting the two large fuselage sideintakes with the engine face re-quired short high curvature duct-ing, which introduced air flow dis-tortion at the fan inlet and led toexcessive blade vibration. To keepthe vibration frequencies of theblades outside the revolutions-per-minute (RPM) range of the engine,t h e y w e r e m a c h i n e d w i t hinterblade supports and made intitanium for added strength. Thisalso solved the problem of furtherair distortion caused by the fanoutlet ducting. The early fairedducts to the nozzles were replacedby a plenum chamber to insulatethe fan from the upstream effect ofthe two nozzles.

Early Hawker P.1127 configuration

Pegasus 2

P.1127 first flight October 1960

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Exhaust duct. The task ofthe exhaust duct is similar to thatof the plenum chamber, that is, tosplit the flow of gas from the tur-bine into two nozzles. This in-duced vibration in the final-stageturbine blades, and the develop-ment engineers introduced wirelacing as used in steam turbines torestrain the blade movements.Later in the program, the turbinewas redesigned with shroudedblades, also similar to steam tur-bine practice.

Ground erosion. Withnozzles down, the jets impinge onthe ground and can cause erosionof the surface and throw up debriswith damaging effects. The tech-nique of rolling takeoff, makinguse of the thrust vectoring prin-ciple, enabled this issue to bebrought under control by acceler-ating the aircraft forward beforedirecting the nozzles downward.

Hot gas recirculation. Dur-

ing jet-borne flight close to theground, the exhaust gases recircu-late round the aircraft and can en-ter the engine intake and producetemperature rise and flow distor-tion effects, which reduce enginepower. These effects are causedmainly by “fountains” of exhaustgas that form between pairs of jetstreams. The Pegasus, with fournozzles, produces two fountains,the one nearest the intake com-prising relatively cool air from thefan, which effectively forms a bar-rier preventing hot exhaust gasfrom reaching the intake. Prob-lems of this nature are intrinsicallyassociated with a VTO system, andthe Pegasus configuration has beendemonstrated to have particularadvantages over most alternatives.The engine and airframe compa-nies have worked closely togetherthroughout the program to solvesuch problems associated with en-gine installation. This close col-

laboration has been a special fea-ture of the Pegasus Harrier pro-gram.

Thrust GrowthThe Hawker designers con-

tinually demanded increases inthrust from the engine to maturethe weapon-carryingcapability andrange of the aircraft. The Pegasus5 was designed for the Kestrel, adevelopment of the P.1127 forwhich a thrust of 15,500 lbs wasrequired. In 1963 the UnitedStates, United Kingdom, and WestGermany signed the TripartiteAgreement to fund a squadron ofnine Kestrels, which carried outtrials in 1965 to evaluate the feasi-bility of dispersed operations andestablish practical procedures. Toachieve the necessary thrust,Bristol introduced most of the de-sign features that were to be incor-porated in the later production en-gines.

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The Pegasus Mk101 wasthe first full-production Pegasusand entered RAF service in theHawker Siddeley Harrier in April1969 with a thrust of 19,000 lbs.The Pegasus Mk102 was an in-terim production standard pend-

ing introduc-t i o n o f t h eMk103, whichpowered a l lHarriers of theRAF and thecorrespondingAV-8A of theU S M a r i n eCorps . Thise n g i n e w a salso acquiredfor the Mata-dor, the namegiven to theaircraft pur-chased by theSpanish Navy.Thrust of theMk103 was in-c r e a s e d t o

21,500 lbs using an increased airflow fan and improved turbine cool-ing. The later Mk104 had somechanges to resist salt-water corro-sion for the Royal Navy version ofthe aircraft, the Sea Harrier.

The McDonnell DouglasCompany, which had formed a col-laboration with Hawker Siddeleyto support the AV-8A program, car-ried out an extensive redesign ofthe aircraft to meet the require-ments set for the AV-8B. Thisnecessitated further thrust in-crease and detailed engine changesto improve reliability and enginelife and modified exhaust nozzlesto integrate with the aerodynamicchanges that greatly increased thetakeoff weight capability. Largeorders were placed for the AV-8B,and parallel developments in theUnited Kingdom resulted in theHarrier Mk2 for the Royal AirForce.

An engine demonstrator

Tripartite Squadron Kestrel

program established the basis formore thrust increase, enablingfur-ther developments of the AV-8Band Harrier Mk2 to be introducedfor ground-based and naval appli-cations. The engine is now equippedwith digital electronic control, re-placing the original hydromechani-cal system.

Operating ExperienceThe deployment of RAF

Harriers to dispersed sites, the flex-ible operation of the US MarineCorps AV-8A and AV-8B, and thenaval operations from small air-craft carriers have all vindicatedthe claims made for vectored thrustVSTOL in the original Wibault pro-posal and detailed in the submis-sions made for funding of theP. 1127 and early Pegasus engines.

In 1982 in the South At-lantic and 1992 in the Persian Gulf,these characteristics have beendemonstrated in action.

Pegasus 11 (Mk 103)

Pegasus Engine thrust progression

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Why Vectored Thrust SucceededDuring the 1960s and into

the 197Os, many extensive pro-grams were committed in theUnited States and Europe to evolveVSTOL systems using a variety ofconcepts. These included liftingfans installed horizontally in thewing section, ejector augmentorsystems, multiple lift engines, androtating engine nacelles. None ofthese was developed to operationalstatus.

One powerful reason whythe single-engine solution embod-ied in the Harrier was pursued wasthe essential simplicity of the con-cept. The basic principle of vec-tored thrust is that all thepowerplant thrust can be orien-tated in any direction between hori-zontal and vertical, so that it pro-vides lift as well as propulsion.Given sufficient thrust to lift offvertically, the aircraft can make asmooth transition from hover toforward flight. Vectored thrust, asrepresented by the Pegasus-Har-rier combination, has the followingcharacteristics and advantages:� A single engine located near theaircraft center of gravity, with arotating nozzle system producing athrust resultant that can be vec-tored between horizontal and ver-tical.� Engine and nozzles together form-ing a compact, self-contained powerunit,� Rapid nozzle vectoring (over 90°/sec) actuated by a powerful air mo-tor drive system using engine-sup-plied air.� Short takeoff, at weights substan-tially greater than those that wouldbe possible for VTO, is effectiveand easy, since all the thrust isavailable for ground acceleration,lift-off and transition. Also, rollingvertical takeoff and short takeofftechniques minimize the impact ofjets on the ground or ship’s deck.� The pilot has only one extra leverin the cockpit, to affect nozzle ro-

AV-8B

tation. Since the engine spoolsrotate in opposite directions, thereis virtually no gyroscopic effect,and control of theaircraf t dur inghover and transi-tion is not totallydependent on elec-tronic controls andauto-stabilization.• Thrust vectoringin forward flightcan be used to in-crease maneuver-ability in combat.

The MacRobertAward

In 1969 thefirst MacRobertAward was sharedby a team of fivepersons associatedwith the early designand development ofthe Pegasus engine.This award was setup in 1968 by thet r u s t e e s o f t h eMacRobert Trusts,to be allocated in rec-ognition of engineer-ing innovation lead-ing to the benefit ofthe United King-dom. The Council ofEngineering Institu-tions initiated theprocess, and theaward is now presented annuallyby the Royal Academy of Engi-neering.

Leading the team was Dr.Stanley Hooker, in charge of engi-neeringat Bristol in the 1950s. Hehad been responsible at Rolls-Royce for the development of theWhittle jet engine to productionstatus. Sir Stanely, together withSir Sydney Camm at Hawker Air-craft, placed his considerable repu-tation behind the Pegasus conceptand laid the foundations for subse-quent success.

Vectored Thrust Principle

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Left to right: Frank Marchant,Gordon Lewis, John Dale, SirStanley Hooker, and Neville Quinn

Gordon M. Lewis was responsiblefor the conceptual design of thePegasus, emerging from the origi-nal ideas of Michael Wibault. Hewas originally a specialist in axialflow compressor design and latermanaged major engine programsbefore retiring as Technical Direc-tor

N. R. Quinn, having beenin charge of supercharger work atBristol in the piston-engine era,was heading the performance de-partment in 1956. Quinn’s percep-tion of the merit of the Pegasusconcept and execution of many per-formance studies made a major con-tribution to the engine definition.

F. C. Marchant was in 1956Chief Designer at Bristol Engines,and he carried out the mechanicaldesign of the Pegasus to achievethe high thrust-to-weight rationeeded for VSTOL. He introducedinnovative design features thatwere later widely adopted in theaero gas-turbine industry.

John H. Dale had been re-

sponsible for the Orpheus jet en-gine at Bristol and took charge ofPegasus development from thestart of the program. He carriedPegasus engineering onward forabout 20 years, earning great re-spect from operators for engineer-ing integrity and attention to flightsafety.

The above were nominatedin 1969 as having particular in-volvement in Pegasus. However, amajor engine development programrequires a large team of engineersover several decades to mature andimprove the product. Many of theengineers involved made signifi-cant contributions to the designand took part in the solution ofproblems as they arose. It is notpossible to do justice to all thesepeople in this brochure.

It should be emphasizedagain that the close partnershipbetween the engine and airframeengineers has always been and stillis a very special feature of the Pe-gasus Harrier program.-- Dr. G. M. Lewis, CBE, MA, F.Eng.,F.R.Ae.S., F.I.Mech.E.

Bibliography• Not Much of An Engineer, SirStanley Hooker, Airlife Pub-lishing Ltd.

• Harrier and Sea Harrier, RoyBraybrook, Osprey PublishingLtd.

• Harrier, Francis K. Mason,Naval Institute Press

• Harrier, Bill Gunston, IanAllan Ltd.

• V/STOL from the PropulsionViewpoint, G. M. Lewis, RoyalAeronautical Society, SirSydney Camm Memorial Lec-ture 1983

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•

The History and Heritage Program of the ASMEThe ASME History and Heritage Recognition Program began in

September 1971. To implement and achieve its goals, ASME formed aHistory and Heritage Committee, initially composed of mechanical engi-neers, historians of technology, and curator (emeritus) of mechanicalengineering at the Smithsonian Institution. The Committee provides apublic service by examining, noting, recording, and acknowledging me-chanical engineering achievements of particular significance. The Historyand Heritage Committee is part of the ASME Council on Public Affairs andBoard on Public Information. For further information please contact PublicInformation, American Society of Mechanical Engineers, 345 East 47Street, New York, NY 10017-2392, (0101) 212-705-7740.

DesignationThe BS 916 Pegasus Engine is the 38th ASME International

Historic Mechanical Engineering Landmark to be designated. Since theASME Historic Mechanical Engineering Recognition Program began in1971, 156 Historic Mechanical Engineering Landmarks, 6 MechanicalEngineering Heritage Sites, and 4 Mechanical Engineering Heritage Collec-tions have been recognized. Each reflects its influence on society, either inits immediate locale, nationwide, or throughout the world.

It is the sixth international landmark designated in conjunctionwith IMechE: SS Great Britain in Bristol, the Newcomen Steam Atmo-spheric Engine in Devon, and the Steam Turbine Yacht Turbinia inNewcastle-upon-Tyne as well as the Cruquius Steam Drainage PumpingStation in Haarlemmermeer, The Netherlands, and the Boulton and WattRotative Steam Engine in Sydney, Australia.

A landmark represents a progressive step in the evolution ofmechanical engineering. Site designations note an event or development ofclear historical importance to mechanical engineers. Collections mark thecontributions of a number of objects with special significance to thehistorical development of mechanical engineering.

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•

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