Testifying before the House Armed Services Committee on Tuesday, JSF program executive officer Adm David Venlet said afterburner screech on the F135, which prevents the engine from sustaining full thrust, 'caused us to avoid certain portions of the flight envelope.' Instead, F-35s have flown to other points in the envelope to keep flight-test going. Kits are being installed to overcome the problem, he said.
So what is screech and what's the fix?
Pratt says screech is a phenomenon caused by pressure pulsations in the afterburner at low altitude and high speed. The problem was discovered during development testing around March 2009, having previously been encountered - and solved - in the F-22's F119 engine, from which the F135 is derived.
Pratt points out that the F119 and F135 are the only production engines with stealthy augmentors. Their design eliminates conventional spray bars and flame holders and integrates multi-zone reheat fuel injection into curved vanes that block the line-of-sight to the turbine.
Building on its experience with the F119, the fix for the F135 includes 'minor hardware changes to the fuel system, reduced aerodynamic leakages and upgraded software,' says Pratt, adding that the modified engine 'now provides full max augmented thrust throughout the flight envelope.'
A kit has been developed for flight-test engines, and two have been modified. The production configuration will be validated this year in both the CTOL/CV and STOVL variants of the F135, Pratt says.
I have asked GE/Rolls whether their F136 has a screech-free stealthy augmentor. Watch this space for their answer.
And here it is:
GE/Rolls says the F136 has 'no known' screech problem, but acknowledges it is four years behind Pratt in development. An augmented F136 was demonstrated recently to the JPO on its Evendale test stand without experiencing screech, but the engine has not flown yet.
GE/Rolls also points out that afterburner screech was a problem of fighters long before stealth came along...
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