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Pratt Ripostes GE's Thrust Parry


Stand on the edge of the Everglades with the August sun beating down on your head, the summer humidity wrapping your skin and the thunderous beat of 43,000 pounds of power causing your chest and the very ground beneath you to thrum. I waggled my jaw a couple of times so the seal on the hearing protection broke and my ears absorbed the awesome roar of the test engine firing about 100 feet away,  hung high in the air.

That's what it's like to experience testing of Pratt & Whitney's F135 STOVL engine for the Joint Strike Fighter. It was my first military jet engine test and you can take it to the bank that I was impressed with the technology and with the brute power unleashed and then channeled.

That was one of the last STOVL tests Pratt expects to run before processing the data and preparing reports to win Initial Service Release for the STOVL version. “We will write 3,000 verification reports to the JSF Program Office, and late in the fall the system program office will stamp off the ISR for the F-35B engine,” Pratt's president of military engines, Warren Boley, told us. Official and final ISR clearance is expected in early December.

Pratt clearly put on the trip to see the STOVL engine to ramp up pressure on GE and Rolls Royce as Congress prepares to come back from the summer recess. There were two messages: all our engines are fully tested and will soon be in production, while you haven't got a single engine variant into production yet; we can put out 20 percent more thrust than the Pentagon requires.

GE and Rolls Royce have touted for months their engine's ability to deliver 5 percent greater thrust using a software upgrade. They can supply up to15 percent greater thrust than the specifications require, but that would necessitate changes to the nozzle. GE has long argued that its larger core allows its power plant to deliver more thrust at cooler temperatures, thus giving their engine longer life and more time between required service intervals.

Pratt took this argument on head-on, with Boley and his team arguing during our visit that the GE/RR core weighs more because it requires more metal. And larger cores, they said, reflect older technology. Smaller cores are lighter.

Boley also noted that any increased thrust above 5 percent might require serious changes to the STOVL version of the F-35. If the main power plant provides too much thrust, it could overpower the lift fan in the front of the plan, flipping the aircraft over.  Of course, that would apply to both the Pratt and the GE/RR engines since physics applies equally no matter who votes for you in Congress.

In addition to all that, Boley made clear that F-35s will not get the kinds of upgrades that F-16s have over the years. The requirements for stealth mean that fifth generation fighters' requirements are pretty set at the beginning of the program. Since you can't hang all sorts of new gear on the wings of a fifth generation fighter you are unlikely to increase its weight nearly as much as you would on a traditional fighter like the F-16. That means you are unlikely to need great increases in thrust over the life of the aircraft, except for the simple need for speed. Most fifth-gen aircraft upgrades will be to and through software, Boley and his team said, and to the capabilities of the weapons they carry.

All that led to me to wonder just why being able to deliver significantly greater thrust than the other guy mattered and I remembered I was dealing with engineers. And, of course, being able to deliver greater thrust will matter down the road operationally. And it shows that the engines can exceed their requirements which means they will probably last longer and may require servicing less often than planned.

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