The Joint Strike Fighter (JSF) was to be a fifth generation aircraft incorporating advanced stealth technologies, with the STOVL version fly-away cost of $30 - 35 million dollars. Hundreds of cost-performance trade studies were carried out, the propulsion configuration being particularly difficult. The baseline engine was the 24,000 lbs thrust Pratt & Whitney F119 engine in production at that time. Supersonic performance and agility dictated a rear engine and stealth required a long, curved intake duct and internally carried weapons. The aircraft empty weight was set at 24,000 lbs but it was required that it could land with 5,000 lbs of unexpended expensive weapons which drove the thrust required for STOVL to 40,000 lbs, making vertical thrust augmentation necessary.
    Propulsion studies covered direct lift with plenum chamber burning like Hawker’s cancelled P.1154 of the 1960s and their 1980s P.1216 project; a remote forward nozzle augmentation system fed with compressed air ducted from the engine; an additional forward lift engine; and a tandem shaft driven front fan exhausting through rotatable forward nozzles. These studies led to a vertically mounted front fan, shaft driven from the main engine, the layout chosen by the Lockheed-Martin consortium, which after a flight demonstration programme with the X-35, won the JSF competition. BAe was then invited to join Lockheed-Martin after their McDonnell-Douglas partner’s failure in the competition. The lift system, with the front lift fan developed by Rolls-Royce, comprised a lift fan driven via a clutch and gearbox from the engine shaft, a three bearing swivelling rear nozzle on the engine jet pipe, and roll reaction controls on engine outriggers feeding high pressure air tapped from the engine compressor casing. Pitch control was by differential thrust from the lift fan and the rear nozzle; yaw control by swinging the rear nozzle. All this and the engine was controlled through an integrated flight and propulsion control system based on techniques developed in the UK by the RAE/DERA./QinetiQ using the ‘VAAC’ two-seat Harrier.
    The P&W 28,000 lb thrust F-135 engine for the F-22 is the current baseline engine. For the STOVL F-35B in VTOL mode the vertical thrust is distributed as follows: 18,000 lb from the rear nozzle, 18,000 lb from the front fan and 4,000 lb from the two roll reaction control nozzles; a total thrust of 40,000 lb.
    Other important technologies in the F-35 include advanced aerodynamics and propulsion integration, integrated radar, electronic warfare and electronic countermeasures systems and maintainable, multi-spectrum stealth. For the latter, vehicle shaping is fundamental in minimising radar return spikes (from engine intakes, leading edges, the forward fuselage and canopy) by aligning edges, collecting the spikes and absorbing them using radar absorbent materials (RAM - putty and ferrite) in critical areas. Engine fan radar returns are addressed by having long, curved intake ducts to shield the fan and cause internal reflection into RAM. High airframe build accuracy, surface finish and lack of gaps is essential which led to building the airframe, using improved techniques developed by BAE Systems, from the outside skins and then adding the internal structure. The outcome is low radar signature head-on with reduced all-round signature providing additional survivability in manoeuvring flight. Infrared and optical stealth are also addressed.
    The vote of thanks for this engrossing talk was given by our President, Colin Wilson.
Editor’s note - As at mid 2019 some 400 F-35s of all models have been delivered; 283 conventional land based F-35As, 87 ASTOVL F-35Bs and 30 carrier capable F-35Cs.