Elements of the X-35 design were pioneered by the
F-22 Raptor, and portions of the VTOL
exhaust duct design were previously used by the
Convair Model 200, a 1972 supersonic VTOL fighter requirement for the
Sea Control Ship; in particular, the three-bearing swivel nozzle used in the X-35B was pioneered by the Convair design. Additionally, Lockheed purchased technical data from the canceled
Yakovlev Yak-141 in 1991 for examination and analysis of its swivel nozzle. Although
helmet-mounted display systems have already been integrated into some
fourth-generation fighters such as the
JAS 39 Gripen, the F-35 will be the first modern combat aircraft in which helmet-mounted displays will replace a
head-up display altogether. During concept definition, two demonstrator airframes for each contractor team would be flight-tested. Lockheed Martin's demonstrator aircraft consisted of the X-35A (which was later converted into the X-35B), and the larger-winged X-35C. Both the
X-32 and X-35 power plants were derived from Pratt & Whitney's
F119, with the STOVL variant of the latter incorporating a Rolls-Royce Lift Fan module. Because these were proof of concept demonstrators for STOVL risk reduction, the demonstrator aircraft did not need to have the internal structure or most subsystems of the final aircraft as a weapon system.
Shaft-driven lift fan Instead of lift engines or using a direct lift engine like the
Rolls-Royce Pegasus in the
Harrier jump jets, the X-35B was powered by the F119-PW-611 which used the new shaft-driven
lift fan system, patented by Lockheed Martin engineer
Paul Bevilaqua, and developed by Rolls-Royce. In normal wing-borne flight, the F119-PW-611 was configured as a normal medium-bypass
reheated turbofan. The turbofan acted somewhat like a
turboshaft engine embedded into the fuselage (but with a much smaller percentage of total heat energy being extracted by the turbine stage). A portion of engine power was extracted via a turbine, and used to drive a shaft running forward via a clutch-and-bevel gearbox to a vertically mounted,
contra-rotating lift fan. This was located forward of the main engine in the center of the aircraft (this can also be viewed the same as a
high-bypass turbofan but with the low-pressure fan stages mounted remotely from the engine core on an extended, clutched shaft, and creating thrust downwards rather than back around the engine core as in a conventional turbofan). Bypass air from the cruise engine medium-bypass turbofan compressor stages exhausted through a pair of roll-post nozzles in the wings on either side of the fuselage, while the thrust from the lift fan balanced the thrust of the hot core stream exhausting through vectored cruise nozzle at the tail. The X-35B powerplant effectively acted as a flow multiplier, much as a
turbofan achieves efficiencies by moving unburned air at a lower velocity, and getting the same effect as the Harrier's huge, but supersonically impractical main fan. Like lift engines, this added machinery was dead weight during flight, but the increased lift thrust enhanced take-off payload by even more. The cool fan also reduced the harmful effects of hot, high-velocity air which could harm runway pavement or an aircraft carrier deck. Though risky and complicated, it was made to work to the satisfaction of DoD officials, and flight testing of the X-35 demonstrators reduced risk to
Technology Readiness Level 6. == Operational history ==