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Bristol BE53 Pegasus (VTOL)
Rolls-Royce Pegasus / F402 (VTOL)


Michel Wibault, the French aircraft designer, had the idea to use vectored thrust for vertical take-off aircraft. This thrust came from four centrifugal compressors driven by a Bristol Orion turboprop, the exhaust from each could be directed by rotating the outlets. Gordon Lewis initially planned an engine with two thrust vectors, driven by the compressor, with forward thrust from a conventional rear exhaust in his initial BE.52 design. The BE.52 design was built around a Bristol Siddeley Orpheus which through a shaft drove the first three stages of a Bristol Olympus engine which had inlet and outlets separate of those of the Orpheus. Work was overseen by Bristol Siddeley's Technical Director Stanley Hooker.

The Bristol Engine Company began work on the BE.53 Pegasus in 1957. While the BE.52 was a self-contained powerplant and lighter compared to Wibault's concept, the BE.52 was still complicated and heavy. In the BE.53 the Olympus stages were fitted close to the Orpheus stage; this simplified the inlet ducting and the Olympus stages now supercharged the Orpheus improving the compression ratio.

The engine was designed in isolation for a year, then it was helped greatly by understanding what type of aircraft it was designed for. The team received a supportive letter from Sydney Camm of Hawker in May 1957. Hawker were looking for a Hawker Hunter replacement. The aircraft designer, Ralph Hooper, suggested having the four thrust vectors (originally suggested by Lewis), with hot gases from the rear two. Two thrust vectors did not provide enough lift. The 1957 Defence White Paper, which focused on missiles, and not aircraft, also was not good news.

The further development of the engine then proceeded in tandem with the airframe the Hawker P.1127, which first flew in 1960. The next stage of design and development was then flown in the Kestrel, of which nine were built. This was then developed into the Harrier combat aircraft. The engine was partially supported financially from the Mutual Weapons Development Programme.




The flight testing and engine development received no government funding; the plane's funding came entirely from Hawker. There was only enough thrust in the first engines to barely lift the plane off the ground because of weight growth problems. Flight tests were conducted when the aircraft was tethered. The first free hover was achieved on 19 November 1960. The first, and difficult, transition from static hover to conventional flight was achieved on 8 September 1961. 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 ever higher thrusts of the Pegasus was continued by Bristol engines beyond 1966, which was 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.

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. 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. This unique design highly contributes to the sustainability and success of its carrier, the Hawker Harrier. Many other designs faced flaws as the lift and forward propulsion functions were split using separate engines. In such a concept once the transition from lift to forward or vice versa is completed, any such engine is reduced to dead weight. I.e. in forward flight lift engines do not need to provide thrust anymore, but still need to be carried by the airframe, consuming valuable fuel, increasing weight, decreasing range and occupying costly space that cannot be used for other purposes anymore. Like fuel, avionics or additional ordnance.

The front two nozzles are fed with air from the LP compressor, the rear with hot (650 °C) jet exhaust. The airflow split is about 60/40 front back. It was critical that the nozzles rotate together. This was achieved by using a pair of air motors fed from the HP (high pressure) compressor, in a fail over configuration, pairs of nozzles connected with, surprisingly, motor-cycle chains.

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 such it is necessary to remove the wing to change the powerplant having already sat the fuselage on trestles; the whole change took a minimum of eight hours.

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 35gpm (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.

Over 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.

The unique Pegasus engine powers all versions of the Harrier family of multi-role military aircraft. In US service, the engine is designated F402. 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.


Pegasus 2
Otherwise known as the BE53-3, used in the P.1127, 11,500 lbf (51 kN)

Pegasus 3
Used on the P.1127 prototypes, 13,500 lbf (60 kN)

Pegasus 5
Or BS.53-5 (Bristol-Siddeley 53-3). Used for the Hawker Siddeley Kestrel evaluation aircraft. 15,000 lbf (67 kN)

Pegasus 6 (Mark 101)
For first Harriers. 19,000 lbf (85 kN), first flown in 1966 and entered service 1969

Pegasus 10 (Mark 102)
For updating first Harriers with more power and used for the AV-8A, 20,500 lbf (91 kN), entering service in 1971.

Pegasus 11 (Mark 103)
The Pegasus 11 powered the first generation Harriers, the RAF's Hawker Siddeley Harrier GR.3, the USMC AV-8A and later the Royal Navy's Sea Harrier. The Pegasus 11 produced 21,000 lbf (93 kN) and entered service in 1974.

Pegasus 14 (Mark 104)
Navalised version of the Pegasus 11 for the Sea Harrier, same as the 11 but some engine components and castings made from corrosion-resistant materials.

Pegasus 11-21/Mk.105/Mk.106
The 11-21 was developed for the second generation Harriers, the USMC AV-8B Harrier II and the BAE Harrier IIs. The original model provided an extra 450 lbf (2.0 kN). The RAF Harriers entered service with the 11-21 Mk.105, the AV-8Bs with F402-RR-406. Depending on time constraints and water injection, between 14,450 lbf (64.3 kN) (max. continuous at 91% RPM) and 21,550 lbf (95.9 kN) (15 s wet at 107% RPM) of lift is available at sea level (including splay loss at 90°).
The Mk.106 development was produced for the Sea Harrier FA2 upgrade and generates 21,750 lbf (96.7 kN).

Pegasus 11-61/Mk.107
The 11-61 (aka -408) provides 23,800 lbf (106 kN). This equates to up to 15 percent more thrust at high ambient temperatures, allowing upgraded Harriers to return to an aircraft carrier without having to dump any unused weapons which along with the reduced maintenance reduces total cost of engine use.
The RAF/RN upgraded its GR7 fleet to GR9 standard, and part of this process is the upgrade of the Mk.105 engines to Mk.107 standard.


AV-8B Harrier II
BAE Sea Harrier
BAE Harrier II
Dornier Do 31
Hawker Siddeley Harrier
Hawker Siddeley P.1127
Intended application -
Armstrong Whitworth AW.681


Pegasus 11-61
Type: Twin-spool turbofan
Length: 137 in (3.480 m)
Diameter: 48 in (1.219 m)
Dry weight: 3,960 lb (1,796 kg)
Compressor: 3-stage low pressure, 8-stage high pressure axial flow
Combustors: Annular
Turbine: 2-stage high pressure, 2-stage low pressure
Maximum thrust: 23,800 lbf (106 kN)
Overall pressure ratio: 16.3:1
Specific fuel consumption: 0.76 lb/lbf-hr
Thrust-to-weight ratio: 6:1



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