Joint Strike Fighter F135 Engine Burns Hotter Than Desired
David A. Fulghum and John M. Doyle, Aviation Now, 28 May 2006 CORE VALUES
The primary engine for the F-35B Joint Strike Fighter is suffering growing pains and currently “running 190 degrees above our desired temperature,” say Pratt & Whitney specialists as they work to squeeze more power out of the design in the test program.
Company officials say they have plans to cut the 190F of extra heat in the short takeoff and vertical landing (Stovl) configuration engines by “more than half this summer.” The specific area of concern is the “rotor inlet temperature margin,” William J. Gostic, vice president of F135 programs, told AW&ST. The immediate fix involves adjusting the engine’s bypass ratio to optimize air flow through the engine’s core where the most extreme temperatures are experienced. JSF engine manufacturers have the unique problem of designing a Stovl propulsion system with a variable bypass ratio that supports conventional engine operations in addition to periodically supplying thrust for the wing-mounted roll posts and the tail-mounted three-bearing swivel module.
“Currently, we do not have the level of [rotor inlet temperature] margin we desire to have at the end of the [demonstration and] development program,” which is still about two years away, Gostic says. He also points out that there is no military requirement for the margin and that the Stovl engine has already demonstrated the 40,550-lb. specification level of thrust.
Some propulsion analysts suggest that the engine’s combustor–where fuel and air are mixed and ignited–is too small because, to save money, it was derived from the F-22 engine. As a result, it doesn’t have the necessary airflow without additional bypass air. Increasing bypass air can rob an engine of efficiency.
AT THE SAME TIME, CONGRESS–backed by its investigative arm, the Government Accountability Office (GAO)–has rushed to shore up support for the alternative engine program run by General Electric and Rolls-Royce. The Pentagon’s top civilian and military leaders–but not the JSF program office–wanted to kill the program in a move to cut $400 million from the defense budget.
The GAO says, however, that the decision was made without an up-to-date analysis that would have shown a “20% savings from competition [that] would allow a break-even point to occur at about 1,700 engines–not 2,500.” Additional savings would come from reliability improvements while maintaining the industrial base, enhancing readiness, instilling contractor incentives for better performance, ensuring an alternative if the current engine develops problems and improving international participation.
Ironically, congressional authorization committees have recommended cutting JSF’s budget by $1.2 billion and reducing production to five or zero aircraft (from the 16 requested) in Fiscal 2008 by saying the risk of concurrently developing and putting the aircraft into low-rate production is too high. Pentagon and aerospace industry officials say concurrency is the only way to avoid stretching the schedule and driving up costs. Some aerospace industry officials say the move is simply part of the bargaining process to decrease production and create short-term top-line budget cuts.
John Warner (R-Va.), chairman of the Senate Armed Services Committee, says he strongly supports continuing the two engine programs for another year. “In that period, we’ll have the first test flight of the Pratt & Whitney engine and I think that’s an important milestone as we, at the end of this year, begin to review once again whether we go forward with the joint program,” Warner says. “And also it gives the Pentagon the opportunity to reexamine its own decision process. The first . . . was driven by budgetary concerns and the next process, I hope, will [be] more in the best interest to achieve competition and to affect the cost statements.” Warner also puts the blame for congressional cuts on the Pentagon.
“It was generated by their own scheduling and slippage and it just didn’t require the use of those funds,” he says. “So I think they’re better applied elsewhere. It’s all with the program office. They’re the ones that have to answer that question [about disrupting the program].”
At the same time, Sen. John McCain (R-Ariz.) says not to worry about allies who have expressed worry about a schedule slip.
“We had a letter from the British minister of defense saying he strongly supported that we go slow on this until we have completed the testing,” McCain says. “The assessment of the risk [of concurrency surfaced by the GAO and other congressional watchdog groups] was that if you went ahead with production before completion of testing, history shows that can end up costing a lot more in the long run.”
Some aerospace officials contend the F135’s over-heating problems will cause it to have trouble meeting Stovl requirements and they say Pratt & Whitney has been working on the problem for some time without solving it. Others think it’s a predictable issue that comes with high-performance, stealth aircraft designs that have been made somewhat tougher by high-temperature Stovl ground ops.
BUT FOR THE STOVL ENGINE, ground operations (including hovering because it is done only for short periods) are not the most stressing part of the envelope, Gostic says. Where durability is most threatened is during “low-altitude, high Mach-number” operations, he contends.
One adjunct plan by Pratt & Whitney is to “address the remainder of the temperature overage before Initial Service Release configuration engines are produced [by late 2008],” a company e-mail says. “Design improvements include utilization of cooling air, leakage paths and component efficiencies.” The changes are to be folded into the regular development process and are not expected to affect either the overall or flight-test schedules for either conventional take off and landing (CTOL) or Stovl versions. The CTOL engine program is about a year ahead of the Stovl effort.
Company engineers have to run the engine at temperatures hotter than that of previous designs to create the extra energy, but the trick is in finding an operating level that doesn’t decrease engine life, increase maintenance and drive up the cost of operating a fleet of engines.
Gostic says that during the CTOL engine’s thrust qualification, researchers initially run engines at their maximum vibration limits and imbalance levels and at temperatures 4% hotter than specified. The additional heat equates to the current 190F overheat issue, he notes, and to a 23% higher level of thrust.
Earlier this year, a developmental F135 (CTOL) engine was removed from the test stand to investigate “minor wear on the trailing edge of three fifth-stage compressor blades,” according to a Pratt & Whitney statement. Gostic contends that minor wear is not the right description. “The corners of three compressor blades were missing” after a series of overspeed and other abusive tests that were designed “purposefully to rub the rotor into the case very hard,” he says.
Nonetheless, “The wear did not require that testing be postponed, but the decision was made that the engine would be brought to our Middletown, [Conn.], plant for analysis that is still in progress. The engine has since been returned to test and the event has not affected our ability to support the flight test schedule.”
The Pentagon is to begin its own test program for the CTOL JSF F135 engine this summer at a Fort Worth facility. “[These soon-to-begin] tests are part of the CTOL program and are not affected,” a second company statement said. “[They] are in anticipation of the CTOL first flight later this year and are on schedule.” THERE ARE HEAT CONCERNS BEYOND those associated with the F-35’s propulsion system. Excess heat is a particular problem for stealth aircraft, and for any aircraft operating either on the ground or in hot ambient temperatures such as in the Southwest U.S. and Middle East. Thermal management is less of an issue inflight due to lower air temperatures and increase flow rates of cold fuel, say airframe specialists.
“Should ground and flight testing indicate additional cooling is required [for the JSF], we have options related to our thermal management design that would alleviate the condition,” says Doug Pearson, vice president of Lockheed Martin’s F-35 integrated test force. The plans include monitoring temperatures during the test program to determine operational margins. If there are still elevated temperatures, more heat-reduction techniques would be applied to the aircraft’s design, he says.
Those in the industry who work with stealth systems expanded a bit on the problems that stealth designs encounter. They can’t dump heat into the air through ducts and vents because it increases the aircraft’s infrared signature and makes it a large target. In fact, to keep a stealth aircraft surface free of radar-reflective cavities, openings have to be minimized and heat has to be dissipated internally. The most common methods used are dumping heat from avionics and other systems into the onboard fuel which can, up to a point, hold the excess energy. Heat can then be transferred and dissipated in the engine.
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