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The Rolls Royce Pegasus is a unique high performance turbofan that was designed and built specifically for the British Aerospace Harrier, sometimes referred to as the "jump jet." The harrier isn't the only jet fighter capable of hovering flight, or more accurately STOVL, (short takeoff and vertical landing) but it certainly is the most well known. Because of the unique requirements of the harrier, the Pegasus had to be purpose built with swiveling nozzles to be able to vector the engine's thrust downward for takeoff and landing. The harrier has four jet nozzles; two of the nozzles are located in the front fan section of the engine, and emit just the thrust of the fan, while the two rear nozzles exhaust just the core engine exhaust. At no time is the core exhaust and the fan exhaust mixed; they are kept separate at all times. The nozzles can be swivelled in unison through approximately 110 degrees of travel. Swiveling of the nozzles is accomplished by moving a cockpit lever in the cockpit, next to the throttle lever.
Aside from the vectoring nozzles, the Pegasus is a fairly typical low to medium bypass turbofan. There is a 3 stage large diameter fan, driven by a two stage low pressure turbine. The fan feeds the bypass duct which exhausts through the front swivelling nozzles, and the fan also serves as a low pressure booster for the eight stage axial high pressure compressor, which is driven off of a two stage axial high pressure turbine. The compressor feeds high pressure air to the annular through flow combustor at a pressure of 16.3 atmospheres. The high pressure air provides intense combustion to produce the power required to drive the engine cycle. The combustion gases are then exhausted out of the rear swivelling nozzles, where the remaining combustion energy is converted into thrust.
The engine has a bypass ratio of 1.2:1 The bypass ratio and low pressure turbine were designed so that a similar amount of thrust is produced by both the front and the rear nozzles, so that the aircraft remains balanced in vertical flight and in the hover.
Aside from the ability to vector its thrust from its unique four post exhaust nozzles, the Pegasus is particularly noteworthy by the fact that it is an extremely powerful engine. The Pegasus has no afterburner; it would be too difficult to engineer an afterburning system to work with the four exhaust nozzles, and the pilot would not be able to use the afterburning in vertical flight anyway, because the high exhaust velocity and temperature would destroy the landing surface. Despite the lack of an afterburner, the Pegasus still produces approximately 24,000 lbs of thrust, which is more than many fighter engines even in full afterburner, and more than all but the most powerful fighter engines in a dry thrust configuration. The Pegasus is normally equipped with water injection, to allow it to attain its peak thrust rating even on hot days or at high altitude.
The engine features an accessory gearbox mounted on the top side of the outer casing to keep it well clear of the exhaust ducts. The gearbox is driven by the high pressure shaft on the engine, and drives all of the engine accessories, including the fuel control and pump, oil pressure and scavenge pumps, generator, and hydraulic pump. Because the aircraft often has to operate from austere landing sites and cramped ship decks, the engine uses a small self contained jet fuel starter or sometimes a cartridge starter.
Rolls Royce Pegasus 11-61
A little about STOVL (Short takeoff, vertical landing)
A powerful engine is required and the Pegasus must be able to produce more thrust than the weight of the aircraft to allow it takeoff vertically. In military operation, however, the Harrier rarely takes off vertically. With a full load of fuel and a full weapons load, the weight of the aircraft will exceed the engine thrust rating. The harrier usually performs what is referred to as a short takeoff. The harrier lines up at one end of the short ship deck, and the pilot applies the brakes and then increases engine power to maximum military while the nozzles are in the fully horizontal position. Once the engine has attained full power, he will release the brakes to begin the takeoff roll. As the aircraft approaches the end of the ship deck, the pilot will pull back on the stick and simultaneously move the nozzles into the vertical position. At approximately 60 knots, the aircraft smoothly lifts into the air, partly on its wings, and partly on the thrust of the engine. As the aircraft builds altitude the nozzles can be gradually moved rearward until the aircraft has enough forward airspeed to fly on its wings.
Landing is a bit different, and is truly vertical. The aircraft sets up its approach like any other aircraft, with flaps and gear down, and a high angle of attack for low speed. Before converting into vertical flight, the pilot must dump all of his excess fuel and weapons to make the aircraft as light as possible. Then the pilot will vector the nozzles down and even slightly forward to provide a braking effect. This transition between horizontal and vertical flight is the trickiest part of the flight regime, and has led to a number of accidents in the past. This is likely how the Harrier received the unflattering nickname "Widow Maker." Once all of the airspeed is bled off the nozzles are transitioned to straight down, and the aircraft is stabilized into a hover. In a hover, the pilot can use control the attitude of the aircraft through "puffer ducts" or controllable air jets on the wing tips and the nose. The puffer ducts use compressor bleed air to operate. Once the pilot has positioned the aircraft over the landing spot, he can reduce power slightly to allow the aircraft to settle down in true vertical fashion. Once he touches down, he can throttle the engines down to idle and taxi back to the parking spot like a conventional aircraft.
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