“In all the years that I’ve worked on airplanes, there has never been a case where the engine has been so highly integrated with the aircraft,” says Ed O’Donnell.
The vice-president of international business development, military engines, for Pratt & Whitney is on a conference call with Canadian media to answer questions about the F-35 Joint Strike Fighter. The effort is part of a larger Lockheed Martin campaign in recent months to counter negative press and other reports and assure government customers that the fifth generation fighter is more than a paper airplane.
While much of the focus in Canada has been on characteristics of the aircraft – stealth, sensor fusion and, of course, cost ¬– it has rarely turned to the massive single Pratt & Whitney F-135 engine that powers the aircraft. And then, the debate has revolved around a primarily Canadian conundrum – do we require one or two engines for those long flights over the barren Arctic? Far less attention has been paid to the engine itself.
The F-135 has not been without problems. As recently as February, the Pentagon temporarily grounded all F-35 aircraft after a routine check at Edwards Air Force Base found a crack in a low pressure turbine blade in the F-35A variant. Issues with turbine blades were revealed in 2007 and 2008, and again in 2009, and the entire fleet has been grounded at least once per year since 2008 to resolve systems issues, several related to the engine.
Though the recent grounding was a concern, problems with the engine have been addressed and resolved throughout its development phase. Some of that is to be expected, O’Donnell explains, because the F-135 represents a significant technological leap forward.
“From the beginning of the design all the way to the installation, and the flight activities today, it is almost as if it is a single system…that talks [between the airplane and the engine],” he said. “The airplane carries a number of propulsion system components and the engine actually carries a number of aircraft components, and in the past we’ve never seen anything like that.”
Engines are often built with more than one aircraft in mind, but the F-135 propulsion system is unique to the Joint Strike Fighter. And because of its integration with the rest of the aircraft “we did some things that are unique to this engine,” says Stan Stevens, Pratt & Whitney’s site lead at Eglin Air Force Base, where the F-35 is undergoing pilot and maintenance crew training.
“It is the biggest fighter engine that has ever been developed, certainly the biggest engine that Pratt & Whitney has developed for a fighter application.”
Stealth, one of the F-35’s key selling features, also applies to the engine, which uses a variety of techniques to manage and mask heat generation, including the engine inlets. “It’s very important that you are not able to see the front of the engine because it is a great radar reflector,” said Dave Scott, Lockheed Martin’s director of F-35 international business development in a briefing to Canadian media at the company’s production facility in Forth Worth, Texas. “If you can see the engine front face – the compressor blades – you can bounce a radar beam off it and get a very good radar reflection.”
Like O’Donnell, though, Stevens says that what distinguishes the F-135 from many of its predecessors is the constant communication between the engine and its two full authority digital engine controllers.
“Those computers not only talk to over 400 sensors on the engine and the main fuel transfer valve and other things, but they are also interfacing with the airplane,” he says. “They are talking to the airplane all the time. About every 20-30 milliseconds, those computers sweep across [those] 400 sensors to determine the health and the performance of the engine.
“This weapon system is more integrated than any other weapon system, particularly where the propulsion system is concerned. We are more integrated and interfaced with the aircraft systems than what you would see on any of the legacy jets.”
And that data can be downloaded by ground crews through a permanent memory device when the aircraft lands to assess all the performance parameters for that flight.
At its core, the F-135 consists of two counter rotating spools that produce 40,000 pounds of thrust. The engine is designed in five basic modules that can be removed and replaced individually. The significance of the modular design is apparent as Stevens describes engine maintenance. Where legacy concepts typically have a three-stage process, starting at the flight line and moving back through a repair shop to a depot, the F-135 removes that intermediate layer.
“If we introduced foreign material and damaged the fan,” Stevens says by way of example, “we would remove the engine from aircraft, separate this fan, put it in a shipping container and send it to the depot; the depot would have sent us a fan (which had already been tested), we’d put it on the engine, roll the engine back into the aircraft, run the engine with some built in tests, and then determine if the airplane is serviceable. The same is true for all the rest of the modules. By eliminating that one level of maintenance we can help control sustainment costs for the future.”
Costs, of course, are a major consideration with the Joint Strike Fighter. The F135 engine, which is still in low rate production, is estimated at about $10 million dollars today, but Pratt & Whitney has implemented an aggressive cost reduction program, says Stevens, and is “meeting all the goals that have been established by the Joint Program Office. The cost continues to come down.”
Commonality across the three engine variants – conventional take off and landing, short take off and vertical landing (STOVL), and carrier variant – has also helped contain costs, O’Donnell said. So have changes to production processes as the engine transitions from development to full production. The company recently signed a low rate initial production (LRIP) 5 agreement that is six percent lower than its LRIP 4 contract, he added.
“Last year we produced 48 engines and have produced 87 to date. [In 2013] we will produce more F135 engines than all of our other military engines combined. I think it is reflective of the success we have had on the program. Each year we’ve demonstrated the ability to double production on the production line,” which is being matched by improvements to the supply chain.
One of the companies in that chain is Ottawa-based GasTOPS, which designs and manufactures sensors that provide engine bearing and blade health data on all three variants to improve maintenance task management. The company is also developing a blade health sensor for the lift fan on the STOVL aircraft.
In a questionnaire earlier this year to the companies contending for Canada’s next-generation fighter program, the National Fighter Procurement Secretariat asked specially about engine performance under a variety of circumstances and mission configurations. During its briefing in Forth Worth, Lockheed Martin highlighted comparisons with its F-16 Fighting Falcon, including internal fuel carrying capacity (7000 lbs versus 18,000 lbs on the F-35) and engine thrust (29,000 lbs versus 40,000 lbs, though whether the F-35 burns fuel more rapidly to gain that thrust was unclear).
Despite the temporary grounding in February, it’s unlikely questions about the engine – from media and government – will fazed either Lockheed Martin or Pratt & Whitney.
“This engine is probably more tested than any engine we’ve done before,” Stevens says with some pride as the roar of a jet from overhead rumbles into the hanger. “We’ve got 25,000 engine operating hours just in the ground testing environment alone. We’ve done everything imaginable to this engine to get it to fail.”