By Graham Warwick in Washington, DC
Pratt & Whitney is working to reduce temperatures in its F135 Joint Strike Fighter engine as the US Department of Defense comes under criticism for its decision to cancel development of an alternative engine for the Lockheed Martin F-35.
Lockheed says the F135 is running about 90°C (190°F) above its target temperature, which could cause thermal management issues during ground operation of the F-35 in high ambient temperatures. “We expect the F-35’s thermal management system to allow adequate ground operations to accomplish our flight test objectives,” the company says.
P&W says the engine is about 5% short of rotor inlet temperature margin desired at the end of development, adding: “We have plans in place to recover the margin before initial service release engines are built.”
Thermal management was an issue during development of previous stealthy aircraft, as infrared signature must be tightly controlled. Lockheed’s F-22 had cooling problems during testing at Edwards AFB, California. If F-35 ground and flight testing indicates additional cooling is required, Lockheed says options include using cooled fuel or keeping the aircraft under shelter.
P&W says improvements to recover temperature margin include optimising the bypass ratio, using cooling air, minimising leakage paths and increasing component efficiencies. The changes are being incorporated during development and should not affect the flight test schedule, the company says. The first F-35 is expected to fly in October.
Separately, P&W says an F135 was removed from the test stand earlier this year to investigate “very minor wear” on the trailing edge of three fifth-stage compressor blades. The engine was returned to the factory for analysis, new hardware fitted and returned to test. “The event has not affected our ability to support the flight test schedule.”
The DoD cited the relatively low risk in developing the F135, derived from the F119 powering the F-22, as one reason for canceling the General Electric/Rolls-Royce F136 competitive engine. Two Congressional committees have restored funding for F136 development, and the Government Accountability Office (GAO) has faulted the DoD decision.
The GAO says the DoD used selective elements of two previous studies to support its argument that an engine competition would not save money. Those studies, in 1998 and 2002, found cost savings to be marginal, but supported development of the alternative engine for other factors, including fleet readiness and industrial base.
See Defence P15