|Pegasus / F402|
|Rolls-Royce Pegasus on display at the Royal Air Force Museum London|
|National origin||United Kingdom|
|First run||September 1959|
|Major applications||Hawker Siddeley Harrier |
BAE Sea Harrier
McDonnell Douglas AV-8B Harrier II
|Number built||Over 1,200 (through 2008)|
|Developed from||Bristol Siddeley Orpheus|
The Rolls-Royce Pegasus, formerly the Bristol Siddeley Pegasus, is a turbofan engine originally designed by Bristol Siddeley. It was manufactured by Rolls-Royce plc. The engine is not only able to power a jet aircraft forward, but also to direct thrust downwards via swivelling nozzles. Lightly loaded aircraft equipped with this engine can manoeuvre like a helicopter. In particular, they can perform vertical takeoffs and landings. In US service, the engine is designated F402.
The unique Pegasus engine powers all versions of the Harrier family of multi-role military aircraft. Rolls-Royce licensed Pratt & Whitney to build the Pegasus for US built versions. However Pratt & Whitney never completed any engines, with all new build being manufactured by Rolls-Royce in Bristol, England. The Pegasus was also the planned engine for a number of aircraft projects, among which were the prototypes of the German Dornier Do 31 VSTOL military transport project.
Michel Wibault, the French aircraft designer, had the idea to use vectored thrust for vertical take-off aircraft. This thrust would come from four centrifugal blowers shaft driven by a Bristol Orion turboprop, the exhaust from each blower being vectored by rotating the blower scrolls. Although the idea of vectoring the thrust was quite novel, the engine proposed was considered to be far too heavy.
As a result, an engineer at Bristol Engine Company, Gordon Lewis, began in 1956 to study alternative engine concepts, using, where possible, existing engine components from the Orpheus and Olympus engine series. The work was overseen by the Technical Director Stanley Hooker. One concept which looked promising was the BE52, which initially used the Orpheus 3 as the engine core and, on a separate coaxial shaft, the first two stages of an Olympus 21 LP compressor, which acted as a fan, delivering compressed air to two thrust vectoring nozzles at the front of engine. At this point in the design exercise, the exhaust from the LP turbine discharged through a conventional rear nozzle. There were separate intakes for the fan and core compressor because the fan did not supercharge the core compressor.
Although the BE.52 was a self-contained power plant and lighter than Wibault's concept, the BE.52 was still complicated and heavy. As a result, work on the BE.53 concept started in February 1957. In the BE.53 the Olympus stages were fitted close to the Orpheus stages; thus simplifying the inlet ducting. The Olympus stages now supercharged the Orpheus core, improving the overall pressure ratio, creating what is now considered a conventional turbofan configuration.
For a year Bristol designed the engine in isolation, with little feedback from the various airframe manufacturers furnished with data. However, in May 1957 the team received a supportive letter from Sydney Camm of Hawker Aviation They were looking for a Hawker Hunter replacement. The aircraft designer, Ralph Hooper, suggested having the four thrust vectoring nozzles (originally suggested by Lewis), with hot gases from the rear two. Further joint discussions helped to refine the engine design.
The 1957 Defence White Paper, which focused on missiles, and not manned aircraft – which were declared 'obsolete' - was not good news, because it precluded any future government financial support for development of not already extant manned combat aircraft. This prevented any official financial support for the engine or aircraft from the Ministry of Defence. Fortunately, engine development was financially supported to the tune of 75% from the Mutual Weapons Development Program, Verdon Smith of Bristol Siddeley Engines Limited (BSEL), which Bristol Engines had by then become on its merger with Armstrong Siddeley, quickly agreeing to pay the remainder.
The first prototype engine (one of two BE53/2s built), ran on 2 September 1959 and featured a 2-stage fan and used the Orpheus 6 core. Although the fan was overhung, inlet guide vanes were still incorporated. The HP spool comprised a 7-stage compressor driven by a single stage turbine. A 2-stage LP turbine drove the fan. There was no plenum at fan exit, but 4 thrust vectoring nozzles were fitted.
Further development of the engine then proceeded in tandem with the aircraft, the Hawker P.1127. The aircraft first flew (tethered hover) on 21 October 1960, powered by the BE53/3 (Pegasus 2). Free hover was achieved on 19 November of the same year. Transition to wing-borne flight occurred in 1961. Later versions of the P.1127 were fitted with the Pegasus 3 and eventually the Pegasus 5.
The Pegasus 5 was also used in the Kestrel, a refinement of the P.1127, of which nine were built for a Tripartite evaluation exercise. The Kestrel was subsequently developed into the Harrier combat aircraft. By the time the Pegasus 5/2 was built, both the fan and HP compressor had been zero-staged and 2nd stage added to the HP turbine.
The flight testing and engine development received no government funding; the plane's funding came entirely from Hawker.
The first engines had barely enough thrust to lift the plane off the ground due to weight growth problems. Flight tests were initially conducted with the aircraft tethered, with the first free hover achieved on 19 November 1960. The first transition from static hover to conventional flight was achieved on 8 September 1961. It was originally feared that the aircraft would have difficulty transitioning between level and vertical flight, but during testing it was found to be extremely simple. Testing showed that because of the extreme power to weight ratio it only took a few degrees of nozzle movement to get the aircraft moving forward quickly enough to produce lift from the wing, and that even at a 15 degree angle the aircraft accelerated very well. The pilot simply had to move the nozzle control forward slowly. During transition from horizontal back to vertical the pilot would simply slow to roughly 200 knots and turn the nozzles downward, allowing the engine thrust to take over as the aircraft slowed and the wings stopped producing lift.
The RAF was not much of a convert to the VTOL idea, and described the whole project as a toy and a crowd pleaser. The first prototype P1127 made a very heavy landing at the Paris Air Show in 1963.
Series manufacture and design and development improvement to the Pegasus to produce ever higher thrusts were continued by Bristol engines beyond 1966, when Rolls-Royce Ltd bought the Company. A related engine design, the 39,500 lbf (with reheat) Bristol Siddeley BS100 for a supersonic VTOL fighter (the Hawker Siddeley P.1154) was not developed to production as the aircraft project was cancelled in 1965.
To date,[when?] 1,347 engines have been produced and two million operating hours have been logged with the Harriers of the Royal Air Force (RAF), Royal Navy, U.S. Marine Corps and the navies of India, Italy, Spain and Thailand.
A non-vectored 26,000 lb thrust derivative of the Pegasus running on liquid hydrogen, the RB.420, was designed and offered in 1970 in response to a NASA requirement for an engine to power the projected Space Shuttle on its return flight through the atmosphere. In the event, NASA chose a shuttle design using a non-powered gliding return. 
The Pegasus vectored-thrust turbofan is a two-shaft design featuring three low pressure (LP) and eight high pressure (HP) compressor stages driven by two LP and two HP turbine stages respectively. Unusually the LP and HP spools rotate in opposite directions to greatly reduce the gyroscopic effects which would otherwise hamper low speed handling. LP and HP fan blading is titanium, the LP fan blades operating in the partly supersonic region, and airflow is 432 lb/s. The engine employs a simple thrust vectoring system that uses four swiveling nozzles, giving the Harrier thrust both for lift and forward propulsion, allowing for STOVL flight.
The front two nozzles, which are made of steel, are fed with air from the LP compressor, the rear nozzles, which are of Nimonic with hot (650 °C) jet exhaust. The airflow split is about 60/40 front back. It is critical that the nozzles rotate together. This is achieved by using a pair of air motors fed from the HP (high pressure) compressor, in a fail over configuration,[clarification needed] pairs of nozzles connected with motorcycle chains. The nozzles rotate over an angular range of 98.5 degrees.
The Pegasus was also the first turbofan engine to have the initial compressor fan, the zero stage, ahead of the front bearing. This eliminated radial struts and the icing hazard they represent.
The engine is mounted in the centre of the Harrier and as a result it was necessary to remove the wing to change the powerplant after mounting the fuselage on trestles. The change took a minimum of eight hours, although using the proper tools and lifting equipment this could be accomplished in less than four.
The maximum take-off thrust available from the Pegasus engine is limited, particularly at the higher ambient temperatures, by the turbine blade temperature. As this temperature cannot reliably be measured, the operating limits are determined by jet pipe temperature. To enable the engine speed and hence thrust to be increased for take-off, water is sprayed into the combustion chamber and turbine to keep the blade temperature down to an acceptable level.
Water for the injection system is contained in a tank located between the bifurcated section of the rear (hot) exhaust duct. The tank contains up to 500 lb (227 kg, 50 imperial gallons) of distilled water. Water flow rate for the required turbine temperature reduction is approximately 35 gpm (imperial gallons per minute) for a maximum duration of approximately 90 seconds. The quantity of water carried is sufficient for and appropriate to the particular operational role of the aircraft.
Selection of water injection engine ratings (Lift Wet/Short Lift Wet) results in an increase in the engine speed and jet pipe temperature limits beyond the respective dry (non-injected) ratings (Lift Dry/Short Lift Dry). Upon exhausting the available water supply in the tank, the limits are reset to the 'dry' levels. A warning light in the cockpit provides advance warning of water depletion to the pilot.
Pegasus engines are on public display at the following museums:
Data from 
|Wikimedia Commons has media related to Rolls-Royce Pegasus.|