Lockheed Martin has rolled out the first supersonic STOVL F-35B Joint Strike Fighter. Think about those words – “supersonic” and “STOVL”. I worked in future projects at Hawker Siddeley in the late 1970s, so I have an idea of how hard it is to get those two characteristics together in one aircraft. Most of the designs I worked on used Harrier-style vectored thrust, which put a big fat engine right in the middle of the airframe – not exactly what you want for supersonic fineness. Remember Boeing’s X-32?
But the F-35B looks quite sleek. The secret is its shaft-driven lift fan, installed behind the cockpit under a massive rear-hinged door that engineers have dubbed “the 56 Chevy hood”. The lift system allows the engine to produce about 40,000lb of vertical thrust without needing reheat. About half that comes from the lift fan and the rest from the roll posts and rear swivelling nozzle. The system worked extremely well on Lockheed’s X-35B concept demonstrator.
The potential disadvantage could be its complexity. To transition from wingborne to jetborne flight, doors have to open, clutch engage, gearbox and two-stage contra-rotating fan spool up, vanes move and nozzle swivel. The process is automated to make it simple for the pilot, but there are still a lot of moving parts. As F-35B ground and flight testing gets under way in 2008, the reliability of the system will be a critical factor.
A closer look at the first F-35B shows the changes from the CTOL F-35A. The lift fan is visible behind the truncated canopy under the sloping door, which helps direct air in the fan in forward flight. The lip of the lift-fan duct, called the “horsecollar”, is a revised design that smoothes the airflow into the duct and which has increased the fan’s performance in engine ground tests. Behind the lift fan are the open doors of the auxilary inlet that provides extra airflow to the engine at low speed. Below the fan is the open door that covers the variable-area vane box nozzle, used to vector lift-fan thrust for hover flight control.
A couple of other changes are notable – the retractable refuelling probe fitted to the STOVL F-35B and F-35C carrier version, but not the US Air Force-standard F-35A. Also the two-piece nosegear doors, which replace the single-piece “barn door” that catches a little too much crosswind on the first F-35, aircraft AA-1.