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Pratt & Whitney F135-PW-100 Augmented Turbofan
While the General Electric GE90 sets the standard for gas turbine engines in the field of commercial airliners, the Pratt & Whitney F135 Augmented Turbofan engine is the benchmark fighter engine of today. The F135 is a development of the P&W F119-PW-100, which was developed by Pratt & Whitney for the twin engine Boeing/Lockheed F/A-22 Raptor. For the military Joint Strike Fighter program, an even more powerful and reliable version of the F119 was developed for what is now going into production as the lower cost F35 Joint Strike Fighter.
The F135 is a typical modern augmented turbofan engine. It has a very low bypass ratio of approximately 0.2:1, which means that for every part of air that goes into the core, 0.2 parts enters the bypass duct, much less than on a commercial turbofan. In this way, these engines are closer to turbojets than commercial engines, with all of a turbojet's attendant benefits, like smaller frontal area, and better high speed efficiency. While the General Electric GE90 commercial turbofan makes nearly 3 times as much thrust as this engine, when you compare power to weight ratios, and more importantly, power to engine frontal area ratios, you will see the benefits of an engine like this in a fighter.
Air is drawn in at the front of the engine, past the variable geometry inlet guide vanes, by the three stage fan, which is also considered to be a low pressure compressor. The titanium, hollow bladed 3 stage fan is driven by a two stage low pressure turbine at the rear of the engine. Fan discharge air is sent to the engine core and the bypass duct. Core air is further compressed by the counter-rotating six stage axial compressor, increasing pressure ratio to approximately 30:1. The compressor blisks are made of a high strength titanium alloy developed by Pratt & Whitney.
Compressed air is fed to the annular combustor, which features Pratt & Whitney developed "Floatwall" construction. The Floatwall combustor features a number of thermally isolated panels surrounded by compressor discharge air to protect them from the incredible temperatures of combustion. The combustor panels themselves are fabricated of high cobalt alloys.
Combustion gases are fed to the single stage, cooled, axial high pressure turbine and then to the counter-rotating, dual stage cooled axial low pressure turbine. The high presssure turbine drives the compressor and the accessory drive, while the low pressure turbine drives the engine fan, as well as an external driveshaft on the F35C Marine Corps version, which is used to drive an external lift fan, for STOVL operation. The counter-rating turbines allow for the exclusion of low pressure nozzle vanes, which simplifies construction and makes the engine shorter and lighter.
After passing through the turbines, the hot combustion gases are collected in the jet pipe/afterburner, where they are mixed with the bypass air which has passed through the bypass duct. The bypass air serves to cool the engine core as it passes around it, and it also reduces the temperature of the gases in the jet pipe, which reduces the aircraft's infra-red signature and engine noise somewhat. Another advantage of the bypass air is that it greatly adds to the oxygen content in the jet pipe, which provides a distinct advantage over a basic turbojet: when the afterburner (or augmeter) is introduced, the fuel has a lot more oxygen to mix with which means that the afterburner can provide an even more substantial thrust boost. The gases in the jet pipe are accelerated and then expanded through the convergent/divergent jet nozzle, which ultimately ejects the exhaust gas in a high velocity jet which provides as much as 40,000 lbs. of thrust under full power/full augmentation. The jet nozzle on this engine is fairly unique. Not only is it a variable area nozzle, which automatically varies the exhaust outlet area to match different flight regimes, but it also features a 3 bearing swivel arrangement which allows the nozzle to be swivelled downward, redirecting the jet gases downward to provide vertical lift in the STOVL configuration.
On the Rolls Royce Pegasus engine page, we went into the short takeoff, vertical landing system as implemented on the British Aerospace AV-8B Harrier fighter aircraft. While the Harrier is a capable aircraft, its STOVL feature imposes many limitations on the aircraft. Its biggest disadvantage is that, despite the tremendous power output of its engine, it is a subsonic aircraft. It is subsonic for three reasons, all related to its STOVL capabilities. The first reason is because of the unique arrangement of its exhaust nozzles, it is exceedingly difficult to implement afterburning, which in most cases provides the necessary increase in thrust and exhaust gas velocity to accelerate an aircraft beyond the speed of sound. The second reason is due to the large frontal area of the engine, which is required to ensure that the fan is producing commensurate levels of thrust as the engine core, to ensure that the aircraft remains balanced in the hover. The large frontal area creates drag which slows the aircraft down. Finally, the amidships engine location on the Harrier means that there wouldn't be sufficient rear weight distribution in the aircraft to counteract the nose-down pitching effect caused by the increase in pressure at the nose of the aircraft during supersonic flight.
When designing the F35 for the Joint Strike Fighter Program, Lockheed Martin had to design an aircraft with the same STOVL capabilities as the Harrier, while maintaining mission requirements for supersonic flight, low observability, manueverability, and range. Lockheed has accomplished this by using a front mounted vertical lift fan nestled in the fuselage directly behind the cockpit. The front mounted lift fan would provide vertical lift at the front of the aircraft to balance out the vertical lift at the rear which is accomplished by swivelling the exhaust nozzle downward. The two stage counter-rotating lift fan would be mechanically driven by a shaft connected to the front of the main thrust engine, driven by the engine low pressure spool. When the aircraft is in normal flight mode, the lift fan is inactive and totally concealed in the fuselage. The F35 is essentially just a simple, lightweight, single engine fighter. However, when the lift fan clutch is engaged, the Pratt & Whitney F135 engine acts as part turbofan and part turboshaft. 17,000 horsepower is extracted from the front of the engine off the low pressure spool to drive the lift fan. The lift fan draws air in through a dorsal intake just behind the cockpit, and accelerates it through a D-ring nozzle which can be pivoted through 90 degrees to vector fan thrust. Variable guide vanes at the lift fan inlet control how much airflow goes through the fan. At the rear of the aircraft, the engine nozzle can be swivelled downward to vector thrust downward and balance the lift forces. When the lift fan clutch is engaged, the engine nozzle area is automatically increased to allow the fan turbine to extract more energy from the combustion gases to drive the lift fan. When in vertical flight, the aircraft controls the exhaust nozzle area and the lift fan inlet guide vanes to provide pitch control. Roll control is provided by two wing mounted ducts which tap air off of the engine bypass duct to roll and stabilize the aircraft. Yaw is controlled by moving the engine exhaust nozzle slightly from side to side. Using the engine driven lift fan concept, the F35 is able to fly supersonically and perform like a true jet fighter, while still having effective STOVL capability. In fact, an F35 prototype was the first aircraft to perform what is called a "hat trick;" that is, perform a short takeoff, break the sound barrier in level flight, and then perform a vertical landing all in the same flight.
F119-PW-100, essentially identical to the F135 engine
Pratt & Whitney F135-PW-100 Augmented Turbofan
JSF Shaft Driven Lift Fan and Engine
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