Lockheed Propulsion System

Lockheed Martin is developing a STOVL lift system that uses a vertically oriented Lift Fan. As with the Boeing system, both P&W and RR are developing the X-35 propulsion system. The responsibilities of P&W, RR and Lockheed are also similar. A two-stage low-pressure turbine on the P&W SE611 engine delivers the horsepower to drive a new, larger fan than the one on the F119 and also powers the STOVL Lift Fan. The Lift Fan provides up to 18,500 lb of thrust, using variable inlet guide vanes to modulate the airflow and therefore the thrust. The Lift Fan has a clutch that engages for X-35C STOVL operations and a telescoping "D"-shaped nozzle to provide thrust deflection; the D nozzle consists of four sections with the final part containing fixed vanes.

Lockheed Martin X-35A JSF in late February 2000. The large dark spot behind the canopy displays the size of the Lift Fan cavity, aft of that are the two auxiliary air intakes.

The Allison Lift Fan is located behind the cockpit in a bay with upper and lower clamshell doors. When operating at normal speeds, the Lift Fan is capable of supporting nearly half of the weight of the X-35. Another STOVL-unique feature on the X-35 is the auxiliary inlet for the main engine, located above the fuselage and behind the lift fan; this is used for the high air flow demands of hover.

The engine exhausts through a three-bearing swivel nozzle (3BSN) that can deflect the thrust from horizontal to just forward of vertical. Two roll ducts supplied by engine fan air provide roll control. Yaw control is through swivel nozzle yaw. Pitch control is effected via Lift Fan/engine thrust split.

Lockheed Martin STOVL propulsion system

For conversion to short take-off mode, the Lift Fan inlet and exhaust doors open, the inlet guide vanes are closed down to minimize air flow, and the clutch is engaged. As the clutch plates synchronize, the Lift Fan gear drive accelerates and is brought up to the input shaft speed. A mechanical lock-up device then assures that the clutch does not slip once the Lift Fan is fully engaged. The inlet guide vanes are then opened to bring the Lift Fan up to speed and the D nozzle is rotated down to vector the Lift Fan thrust aft; with the main engine thrust, this helps accelerate the aircraft forward and upward. After transitioning to wing-borne flight, the inlet guide vanes are again closed down to reduce the air flow through the Lift Fan, the clutch is disengaged, the nozzle is retracted, and the inlet and exhaust doors are closed.

For the conversion to vertical landing mode, the aircraft decelerates and the Lift Fan inlet and exhaust doors open. The Lift Fan is then brought up to speed as described above, but the D nozzle is left retracted to its fully vertical position.

The clutch is designed to engage in 3-7 seconds. With the variable geometry vanes closed and the engine speed reduced to 80-85%, horsepower during engagement is reduced to about 4,000 hp. After engagement, it transmits approximately 28,000 hp at 8,500 rpm. The clutch plates absorb energy during engagement and then dissipate it before the next engagement via cooling air.

Simple configuration changes enable the conversion of the SE611 from a CTOL/CV to a STOVL engine. Engine controls and software will differ among the various configurations. For the STOVL variant, the fan duct incorporates a bypass offtake system for aircraft roll control. A shaft is attached to the engine's low-pressure rotor. The axisymmetric nozzle is replaced with the 3BSN.

Lockheed Martin STOVL JSF

The 3BSN nozzle, developed by Rolls-Royce, was patterned along the lines of the exhaust system on the Yakovlev Yak-141 STOVL prototype that last flew at the 1992 Farnborough air show. A US Navy program also developed swivel nozzles in the late 1960s and was proposed for a supersonic STOVL design by Convair (one of the Lockheed Martin heritage companies) in the early 1970s.

Lockheed Martin three bearing swivel nozzle at 0°, 40° and 105° of rotation

The roll control ducts are located on either side of the SE611 engine and are also produced by Rolls-Royce. These roll control ducts extend out to the point of the wing fold and are supplied with their thrust with the air from the engine fan. The ducts on the end of the post open and close differentially for roll control.

The Shaft Driven Lift Fan (SDLF) concept, patented by Lockheed Martin, was successfully demonstrated through their Large Scale Powered Model (LSPM) in 1995-96. According to Lockheed Martin, they selected the SDLF propulsion system for three primary reasons: the STOVL Lift Fan thrust can be de-coupled from the P&W cruise engine, thereby enabling the cruise engine to be appropriately sized for conventional flight; the significant amount of thrust augmentation obtained from the Lift Fan greatly exceeds the additional weight incurred; and the lower exhaust jet temperature and pressures result in a more benign ground environment during hover than that produced by direct lift.

Rolls-Royce Allison Lift Fan as it will be situated when installed in the X-35

During the summer of 1997, Allison conducted testing of a model of the Lift Fan nozzle at the NASA Lewis Powered Lift Facility in Ohio. The test results validated the computational fluid dynamics predictions of exhaust nozzle performance. B.F. Goodrich conducted testing of the Lift Fan clutch being developed under a subcontract to Allison. Testing demonstrated high-speed clutch engagements that were representative of the X-35 STOVL operating conditions. A favorable clutch plate wear rate translated into a clutch plate life of over four times the X-35 flight demonstration requirement.

The SE611 STOVL engine first drove the Lift Fan on 10 November 1998 and it operated at 100% power for the first time on 22 November. By the end of 1998, the Lockheed Martin SE611 STOVL engine had operated with the Rolls-Royce Allison Lift Fan both engaged and disengaged with the three bearing swivel nozzle actuated from 0-90° at full STOVL power.

A Lift Fan test rig, illustrating the diameter of the Lift Fan

P&W delivered a flight engine to Lockheed in December 1999 for a fit check. On 9 December, Lockheed successfully installed the JSF119-611 flight engine on the X-35A demonstrator aircraft at Lockheed Martin Skunk Works, Palmdale, Calif. Installation was completed in only three hours, including the time spent confirming procedures and documenting interfaces. Following the check, the engine was returned to P&W. Following acceptance testing, the flight test engines were shipped to Lockheed for CTOL flight tests, Lockheed took possession of the CTOL flight test engine on 13 February 2000.

On 28 January 2000 the FX662 STOVL engine exceeded its vertical lift operational thrust requirements. However, on 8 February a bracket supporting the No. 3 bearing inside the Lift Fan gearbox failed; the failure was not catastrophic and was limited to the bearing housing. Prior to its failure, this Lift Fan had run for 67 hours, which is equivalent to more than 18 months of operational service. Testing is scheduled to resume following an investigation of the failure. Lockheed Martin is currently estimating the first flight of their STOVL JSF to take place during the fourth quarter of 2000.