| LiftSystem | |
|---|---|
| The Rolls-Royce LiftSystem coupled to an F135 turbofan at the Paris Air Show in 2007 | |
| Type | STOVL Lift system |
| Manufacturer | Rolls-Royce plc |
| Major applications | F-35 Lightning II |
The Rolls-Royce LiftSystem is an innovative propulsion system designed for use in the STOVL variant of the F-35 Lightning II developed during the Joint Strike Fighter Program. The system was awarded the Collier Trophy in 2001.[1]
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The F-35B STOVL variant of the Joint Strike Fighter (JSF) aircraft is intended to replace the vertical flight Harrier, which was the world's first operational short-takeoff / vertical-landing fighter. The Royal Navy originally intended to use this aircraft to replace its Sea Harrier FA2s, and the RAF its GR9s however after the 2010 Strategic Defence and Security review the decision was made to instead order the F-35C CV (Carrier Variant) version. The U.S. Marine Corps will use the F-35B to replace both its AV-8B Harriers and F/A-18 Hornet fighters with a design similar in size to the Air Force F-35A, trading fuel volume for vertical flight systems.[2]
A requirement of the JSF is that it can attain supersonic flight, and a suitable vertical lift system that would not compromise this capability was needed for the STOVL variant. The solution came in the form of the Rolls-Royce LiftSystem, developed through a $1.3 billion System Development and Demonstration (SDD) contract from Pratt & Whitney.[3] This requirement was met on 20 July 2001.[1][4]
Instead of employing lift engines or rotating nozzles on the engine fan like the Harrier, the "LiftSystem" employs a shaft-driven LiftFan, patented by Lockheed Martin and developed by Rolls-Royce,[3] and a thrust vectoring nozzle for the main engine exhaust that provides lift and can also withstand reheat in conventional flight to achieve supersonic speeds.[1] The system has more similarities to the Russian Yakovlev Yak-141 and German EWR VJ 101D/E than the preceding generation of STOVL designs to which the Harrier belongs.
The entire team responsible for developing the propulsion system includes Lockheed Martin, Northrop Grumman, BAE Systems, Pratt & Whitney and Rolls-Royce, under the leadership of the United States Department of Defense Joint Strike Fighter Program Office. Paul Bevilaqua,[5] Chief Engineer of Lockheed Martin Advanced Development Projects at their Skunk Works, is credited[6] the lift fan for the Joint Strike Fighter F-35B with being responsible for the initial development of the LiftSystem although the concept of a shaft-driven lift-fan dates back to the mid-1950s.[7] Both the US[8] and the UK[9] investigated lift fan designs.
While the F-35B's primary powerplant is the Pratt & Whitney F135, specifically derived from the F119-PW-100, the U.S. Department of Defense (DOD) awarded General Electric and Rolls-Royce a $2.1 billion contract to jointly develop the F136 engine as an alternative, and the LiftSystem has been designed for interchangeability between the two engines.[3] However, it is expected that a further $1.3 billion would be needed to complete the development of the F136 and there is some doubt about its future: the DOD proposals for terminating the F136 in its FY2007 and FY2008 budgets were rejected by Congress on both occasions, but DOD has again requested termination in its FY2009 budget proposal.[10]
The Rolls-Royce LiftSystem comprises four major components:[3]
The configuration of the propulsion system is somewhat like a vertical ducted turboprop embedded into the centre of the aircraft's fuselage. The three-bearing swivel module (3BSM) is a thrust vectoring nozzle at the tail of the aircraft which allows the main turbofan cruise engine exhaust to pass either straight through with reheat capability for forward propulsion in conventional flight, or to be deflected downward to provide aft vertical lift.
In "lift" mode for assisted vertical manoeuvers, 28,000 hp of power[11] is diverted forward through a driveshaft from the engine's low-pressure (LP) turbine via a clutch and bevel-gearbox to a vertically-mounted, contra-rotating lift fan located forward of the main engine. The fan efflux (low-velocity unheated air) discharges through a thrust vectoring nozzle on the underside of the aircraft, thus balancing the aft lift generated by the 3BSM. Owing to the significant increase in LP turbine expansion ratio, implied by the large power off-take, the exhaust of the turbofan is switched from a mixed to unmixed configuration. For lateral stability and roll control, bypass air from the engine goes out through a pair of roll-post nozzles in the wings on either side of the fuselage. For pitch control, the areas of exhaust nozzle and LiftFan inlet are varied conversely to change the balance between them while maintaining their sum, and with constant turbine speed. Yaw control is achieved by yawing the 3BSM.[11] Forward, and even backward, motion is controlled by tilting the 3BSM and LiftFan outlet.[4]
The following indicates the component thrust values of the system in lift mode:[3]
| 3BSM (dry thrust) | LiftFan | Roll posts (combined) | Total |
|---|---|---|---|
| 18,000 lbf (80 kN) | 20,000 lbf (89 kN) | 3,900 lbf (17 kN) | 41,900 lbf (186 kN) |
In comparison, the maximum thrust of the Rolls-Royce Pegasus 11-61/F402-RR-408, the most powerful version of the Harrier's engine is 23,800 pounds-force (106 kN).[12]
Like lift engines, the added LiftSystem components are dead weight during flight, but the advantage of employing the LiftSystem is that its greater lifting power increases takeoff payload by an even larger amount. Also, the fan's cool efflux reduces the harmful effects of hot, high-velocity air which can harm runway pavement or an aircraft carrier deck.
While developing the LiftSystem many engineering difficulties had to be overcome, and new technologies exploited.[13]
The LiftFan utilises hollow-bladed titanium blisks (a bladed disk or "blisk" achieved by super-plastic forming of the blades and linear friction welding to the blisk hub).[14] Organic matrix composites are used for the interstage vanes. The LiftFan must safely function in a 250 knots (130 m/s) crosswind, which occurs at its intake when the aircraft transitions between forward flight and hover.
The clutch mechanism, which uses dry plate carbon–carbon technology originally derived from aircraft brakes, needs the ability to transfer 29,000 shaft horsepower (22,000 kW) without chatter or wear while ensuring high life.
The gearbox has to be able to operate with interruptions to its oil supply of up to a minute while transferring full power through 90 degrees to the LiftFan.
The Three-Bearing Swivel Module has to both support the final hot thrust vectoring nozzle and transmit its thrust loads back to the engine mounts. A new word, "fueldraulics", was coined by the engineers to define the power system of the 3BSM's actuators, which uses fuel pressurised to 3,500 lbf/in2 rather than hydraulic fluid to reduce weight and complexity. One actuator travels with the swivel nozzle, moving through 95 degrees while subject to intense heat and vibration.
During concept definition of the Joint Strike Fighter, two Lockheed airframes were flight-tested: the Lockheed X-35A (which was later converted into the X-35B), and the larger-winged X-35C,[15] with the STOVL variant incorporating the Rolls-Royce LiftFan module.
LiftSystem flight testing commenced in June 2001, and on 20 July that year the X-35B became the first aircraft in history to perform a short takeoff, a level supersonic dash and vertical landing in a single flight. By the time testing had been completed in August, the aircraft had achieved 17 vertical takeoffs, 14 short takeoffs, 27 vertical landings and five supersonic flights.[1] During the final qualifying Joint Strike Fighter flight trials, the X-35B took off in less than 500 feet (150 m), transitioned to supersonic flight, then landed vertically.[16]
Ground tests of the F136/LiftSystem combination were carried out at the General Electric facility in Peebles, Ohio in July 2008. On 18 March 2010, a STOVL equipped F-35B performed a vertical hover and landing demonstration at Patuxent River Naval Air Station in Lexington Park, MD.[17]
In 2001, the LiftSystem propulsion system was awarded the prestigious Collier Trophy,[18] in recognition of "the greatest achievement in aeronautics or astronautics in America", specifically for "improving the performance, efficiency and safety of air or space vehicles, the value of which has been thoroughly demonstrated by actual use during the preceding year."[1]
Components:[3]
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