Joint Strike Fighter Dual-Cycle Propulsion System

Abstract

The Joint Strike Fighter (JSF) is a family of aircraft that will be built in conventional, naval, and short-takeoffand-vertical-landing (STOVL) variants. The key to the development of this family of aircraft is a new dual-cycle propulsion system, which is used to convert some of the jet thrust to shaft horsepower in order to power a lift fan in the STOVL variant. The theoretical basis for the dual-cycle operation of this engine will be presented. Some results of the engine test and development program conducted by Pratt and Whitney and Rolls Royce and the JSF STOVL flight-test program will also be discussed. Potential future applications of this dual-cycle propulsion concept to STOVL transport aircraft, compound rotorcraft, and for takeoff noise reduction will be described. I. Introduction T HE F-35 Joint Strike Fighter will combine the supersonic performance of the F-16C Falcon with the short-takeoff-andvertical-landing (STOVL) capabilities of the AV-8B Harrier, while providing greater range and increased survivability. There will be three variants: a conventional takeoff-and-landing variant for the U.S. Air Force, a short-takeoff-and-vertical-landing variant for the U.S. Marine Corps and United Kingdom, and a carrier-based variant for the U.S. Navy. These three variants are shown in Fig. 1. The naval variant has a somewhat larger wing, in order to reduce landing speeds for carrier operations. This also gives it somewhat greater range, both by reducing the induced drag and by providing additional volume for fuel. The STOVL variant has a shorter canopy and a slight bulge behind the cockpit. These accommodate a lift fan installed in a bay between the inlet ducts. As shown in Fig. 2, the lift fan is driven by a drive shaft extending from the front of the cruise engine. This lift fan provides 18,000 lbs of thrust, almost half of the total lift in hover. A thrust-vectoring nozzle at the rear of the aircraft deflects the core thrust of the cruise engine, which provides another 17,000 lbs of thrust. A roll control nozzle in each of the wings is fed by fan air diverted from the cruise engine. These provide approximately 2500 lbs of thrust apiece. The two main lift nozzles and the two auxiliary roll control nozzles constitute a two plus two hover lift and control system. The cruise engine is a conventional mixed-flow turbofan, providing more than 25,000 lbs of dry thrust. The lift fan is not connected to the engine during cruise. For STOVL operations, the engine operating point is changed so that its turbine section extracts additional energy from the exhaust jet and converts it to shaft horsepower. This power is delivered to the lift fan by engaging a clutch at the end of the drive shaft extending from the front of the engine. The airplane is controlled in pitch by shifting power between the lift fan and the cruise engine nozzle. The cruise engine nozzle rotates from side to side in order to control the aircraft in yaw. To control the aircraft in roll, the area of the auxiliary nozzle on one side is opened, and the nozzle on the other side is closed. The total thrust remains constant while the thrust of the nozzle pairs are varied to provide control forces.