Rolls-Royce LiftSystem

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LiftSystem
Engine of F-35.jpg
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 aircraft 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]

Requirement[edit]

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. 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.[2] This requirement was met on 20 July 2001.[1][3]

Design and development[edit]

Instead of employing lift engines or rotating nozzles on the engine fan like the Harrier, the "LiftSystem" employs a shaft-driven LiftFan, designed by Lockheed Martin and developed by Rolls-Royce,[2] and a thrust vectoring nozzle for the main engine exhaust that provides lift and can also withstand the use of afterburners 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,[4] Chief Engineer of Lockheed Martin Advanced Development Projects (Skunk Works) is credited[5] as being responsible for the initial development of lift fan for the LiftSystem on the F-35B Joint Strike Fighter. The concept of a shaft-driven lift-fan does date back to the mid-1950s.[6] The lift fan was demonstrated by the Allison Engine Company in 1995-97.[7]

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.[2] 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.[8]

Rolls-Royce is managing the overall development and integration programme from its site in Bristol, UK, which is also responsible for the LiftFan turbomachinery, 3BSM and Roll Post designs. The team in Indianapolis, US, will provide the system’s gearbox, clutch, driveshaft and nozzle and will conduct the build and verification testing of the LiftFan.

Operation[edit]

Diagram of LiftSystem components and airflow
Diagram of turbojet energy for LiftSystem prototype
Diagram of powered lift aircraft

The Rolls-Royce LiftSystem comprises four major components:[2]

  • LiftFan
  • Engine to fan driveshaft [9]
  • Three-bearing swivel module (3BSM)
  • Roll posts (two)

The configuration of the propulsion system is somewhat like a vertical ducted turboprop embedded into the center 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.[citation needed][10]

In "lift" mode for assisted vertical maneuvers, 29,000 hp[11][12][13] is diverted forward through a driveshaft from the engine's low-pressure (LP) turbine via a clutch[14] 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.[15] 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.[13] Forward, and even backward, motion is controlled by tilting the 3BSM and LiftFan outlet.[3]

The following indicates the component thrust values of the system in lift mode:[2]

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).[16]

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.[citation needed] 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.[citation needed]

Engineering challenges[edit]

While developing the LiftSystem many engineering difficulties had to be overcome, and new technologies exploited.[17]

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).[18] Organic matrix composites are used for the interstage vanes. The LiftFan must safely function[19] in a 250 knots (130 m/s) crosswind, which occurs at its intake when the aircraft transitions between forward flight and hover.[citation needed]

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.[citation needed]

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.[citation needed]

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.[citation needed]

Testing[edit]

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,[20] 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.[21]

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.[22]

Collier Trophy award[edit]

In 2001, the LiftSystem propulsion system was awarded the prestigious Collier Trophy,[23] 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]

Specifications (LiftSystem)[edit]

Main engine
Two options:
Pratt & Whitney F135
17,600 pounds-force (78 kN) dry thrust
Fighter Engine Team General Electric/Rolls-Royce F136
18,000 pounds-force (80 kN) dry thrust

Components:[2]

LiftFan
Two-stage contra-rotating hollow titanium blisk fan of 50 inches (1.3 m) diameter. Uppermost fan fitted with variable inlet guide vanes. Capable of generating more than 20,000 pounds-force (89 kN) cold thrust[18]
Three-bearing swivel module
Able to rotate through 95 degrees in 2.5 seconds and vector 18,000 pounds-force (80 kN) dry thrust in lift mode, with reheat capability in normal horizontal attitude
Roll posts
Two: hydraulically actuated

Gallery[edit]

See also[edit]

Related lists

References[edit]

  1. ^ a b c d e Propulsion System in Lockheed Martin Joint Strike Fighter wins Collier Trophy. Lockheed Martin press release, 28 February 2003. Retrieved: 3 November 2008
  2. ^ a b c d e f LiftSystem Rolls-Royce website. Retrieved: December 2009
  3. ^ a b From Supersonic to Hover: How the F-35 Flies By Chris Kjelgaard Senior Edito posted: 21 December 2007
  4. ^ Undated Lockheed Martin video. Retrieved December 2009
  5. ^ "Propulsion system for a vertical and short takeoff and landing aircraft", United States Patent 5209428
  6. ^ Rolls-Royce LiftSystem (United States), AERO-ENGINES - LIFTFAN Jane's Aero-Engines. Retrieved: 4 November 2008[dead link]
  7. ^ "-as Allison begins JSF lift-fan tests" Flight International, 21 May 1997. Retrieved: 19 September 2010.
  8. ^ Proposed Termination of Joint Strike Fighter F136 Alternate Engine Congressional Research Reports at opencrs.com. Retrieved: 10 November 2008
  9. ^ Warwick, Graham. "F-35B - Driveshaft" Aviation Week & Space Technology, December 09, 2011. Accessed: April 10, 2014.
  10. ^ Warwick, Graham. "F-35B - Swivel Nozzle" Aviation Week & Space Technology, December 09, 2011. Accessed: April 10, 2014.
  11. ^ Warwick, Graham. "F-35B - The STOVL Challenges" Aviation Week & Space Technology, December 09, 2011. Accessed: April 10, 2014.
  12. ^ Warwick, Graham. "F-35B - Lift Fan" Aviation Week & Space Technology, December 09, 2011. Accessed: April 10, 2014.
  13. ^ a b Lockheed Propulsion System VTOL.org. Retrieved: 19 September 2010.
  14. ^ Warwick, Graham. "F-35B - Clutch" Aviation Week & Space Technology, December 09, 2011. Accessed: April 10, 2014.
  15. ^ Warwick, Graham. "F-35B - Roll posts" Aviation Week & Space Technology, December 09, 2011. Accessed: April 10, 2014.
  16. ^ STOVL pedigree gives Rolls-Royce key technology edge. Rolls-Royce: Defence Aerospace. Retrieved: 5 November 2008
  17. ^ Going vertical – developing a short take-off, vertical landing system. Ingenia Online (PDF) August 2004. Retrieved: December 2009. Raw text: http://www.ingenia.org.uk/ingenia/articles.aspx?Index=271
  18. ^ a b "Rolls-Royce's LiftSystem for the Joint Strike Fighter" By Ellie Zolfagharifard, The Engineer 28 March 2011
  19. ^ Warwick, Graham. "F-35B - Doors 1 Doors 2" Aviation Week & Space Technology, December 09, 2011. Accessed: April 10, 2014.
  20. ^ Joint Strike Fighter official site - History page
  21. ^ PBS: Nova transcript "X-planes"
  22. ^ Lockheed Martin press release Retrieved: 18 March 2010
  23. ^ Collier 2000–2007 Winners National Aeronautic Association. Retrieved: 10 November 2008

External links[edit]