Rolls-Royce LiftSystem

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:[3]

• LiftFan
• Engine to fan driveshaft [12]
• Three-bearing swivel module (3BSM)
• Roll posts (two)

The three-bearing swivel module (3BSM) is a thrust vectoring nozzle at the rear of the aircraft which directs engine exhaust to pass either straight through with reheat capability for forward flight, or to be deflected downward to provide lift.[13]

For vertical flight, 29,000 hp[14][15][16] is transferred by an extension shaft on the engine fan using a clutch[17] and bevel-gearbox to a contra-rotating lift-fan located forward of the engine. The fan airflow (low-velocity unheated air) leaves through thrust-vectoring vanes on the underside of the aircraft, and balances the lift from the rear nozzle. For lateral stability and roll control, bypass air from the engine is used in a roll-post nozzle in each wing.[18] For pitch control, the areas of exhaust nozzle and LiftFan inlet are varied while keeping the total lift constant. Yaw control is achieved by yawing the 3BSM.[16] Forward, and also backward, motion is controlled by tilting the 3BSM and vanes in the LiftFan variable area vane box nozzle.[5]

The following are the component thrust values of the system in lift mode:[3]

In comparison, the maximum thrust of the Rolls-Royce Pegasus 11-61/F402-RR-408, the most powerful version which is used in the AV-8B, is 23,800 pounds-force (106 kN).[19] The weight of the AV-8B is about 46% of the weight of the F-35B.

Like lift engines, the added LiftSystem components are dead weight during flight, but the advantage of employing the LiftSystem is that its greater lift thrust increases takeoff payload by an even larger amount.

While developing the LiftSystem many engineering difficulties had to be overcome, and new technologies exploited.[20]

The LiftFan uses 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).[21] Organic matrix composites are used for the interstage vanes. The LiftFan is cleared[22] for flight up to 250 knots (130 m/s) This condition appears as a crosswind to the horizontal intake and occurs when the aircraft transitions between forward flight and hover.[23]

The clutch mechanism uses dry plate carbon–carbon technology originally derived from aircraft brakes. Friction is only used to engage the lift fan at low engine speeds. A mechanical lock-up is engaged before increasing to full power.[24]

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. The "fueldraulic" actuators for the 3BSM use fuel pressurised to 3,500 pounds-force per square inch (24,000 kPa; 250 kgf/cm²), 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,[25] 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.[4] 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.[26]

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.[27]

In 2001, the LiftSystem propulsion system was awarded the Collier Trophy,[28] 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."[4]