Air National Guard SSgt. Travis Laverne checks out an F108 engine on a KC-135 at JB McGuire-Dix-Lakehurst, N.J. Photos: A1C Cody R. Miller; MSgt. Matt Hecht/ANG
Truly game-changing breakthroughs in US fighter engines are nearly in hand. After more than a decade of labor by Air Force Research Laboratory and engine-makers Pratt & Whitney and General Electric Aviation, increases in speed and range, reduced dependency on tankers, and a menu of new tactics are just some of the advantages coming in the next few years.
By 2021, engineers are expected to have built and tested flightworthy engines that could, for example, give new fighters 30 percent more range than they have today, produce enough spare power to fire directed energy weapons, or run cool enough to improve stealth. Besides those advantages, new engines could provide great benefit to the F-35 strike fighter, allowing it to sustain high-speed flight at treetop altitudes, something it can’t do today. The work is advanced enough that, given a green light, a new development program with a short execution time line could be launched and start producing new power plants by the early 2020s.
So significant are the improvements that the new engine technology effort has been exempt from recent budget cuts, to quicken the pace that the new power plants can be deployed in the inventory.
“We’ve gained tremendous insight from our experience designing engines for the F-22 and the F-35, which are truly a generation ahead,” said Pratt & Whitney’s James Kenyon, senior director of advanced programs and technology, in a 2016 news release. Subsequent development—funded by the Air Force, Navy, and in-house—have yielded “tremendous progress” since 2012 and “we’re eager to move into the next phase of adaptive engine development,” he said.
That next phase is the Adaptive Engine Transition Program, or AETP, a five-year project that began last summer with $1 billion contracts each to Pratt & Whitney and General Electric Aviation. It will refine and mature technologies developed in the Adaptive Engine Technology Development program, launched in 2012 and concluding this year.
The term “adaptive” refers to an engine that can change its internal geometry to be efficient in a variety of missions and flight conditions.
Aircraft jet engines operate on what’s called the Brayton Cycle, named after the Englishman George Brayton, who commercialized this thermodynamic cycle in the 19th century. It describes the gas turbine process of compressing air, mixing it with fuel, causing combustion, expansion of the resulting gases, and vectoring them through an exhaust to produce thrust.
By a decade ago, refinements of the Brayton Cycle had nearly run their course. Engineers, at the cost of millions of dollars, worked to find even one or two percent improvements in fuel efficiency.
These refinements focused on managing the trade-offs between higher thrust and fuel efficiency. In the 1990s, those compromises led to the introduction of commercial “big fan” engines—typically seen slung under the wings of airliners—designed to move large masses of air with the least fuel possible. The bigger the fan, the more air that could move and thus the more thrust that could be generated per input of fuel. Typically, more than three-quarters of the air “bypassed” the core engine.
In the tactical arena, though, a 12-foot fan would never work on a needle-nosed fighter and would be incapable of developing the kind of high-thrust performance a fighter must have.
Then came ADVENT, shorthand for the Air Force’s ADaptive Versatile ENgine Technology. Started in 2007, it coalesced ideas on adaptive designs and variable cycles that engineers had been talking about for years. In ADVENT, the Air Force funded GE Aviation and Rolls-Royce North America—the two engine-makers not then involved in the F135 engine program for the F-35 strike fighter—to work on very high-pressure ratio engine cores and adaptive, multistage fans and low-pressure sections.
Higher pressure ratio compressors can drive more core engine airflow, producing higher thrust. With computer controls, fan pressure ratios could be adjusted as needed. That meant obtaining the fuel-sipping benefits of big commercial airliner-style turbofans while being able to reconfigure an engine on the fly to create higher fan-pressure ratios needed for fighter-like maneuvers.
ADVENT and the Adaptive Engine Technology Development (AETD) program have produced impressive gains. Engine-makers have run core engines at high temperatures never before seen in turbomachinery. Meanwhile, efficiencies have been measured at record levels.
Materials such as ceramic-matrix composites (CMCs)—lightweight, nonmetal materials able to withstand immense heat—have been matured to where they can take up a larger share of an engine’s components. That paves the way for engines to run hotter, making them more efficient. CMCs can be as much as a third lighter than nickel alloys, and components made of them can run at temperatures that would melt metals. They also need less air to be diverted for cooling, which reduces efficiency.
Most importantly, the technology programs have proved the feasibility of a true “three-stream” jet engine, where the core engine airstream and fan-bypass stream is joined by a third bypass stream that flows around the outside area of the engine case. According to F135 maker Pratt & Whitney, this third stream of airflow can be put to work, improving propulsion efficiency and lowering fuel burn, or delivering additional airflow through the core for higher thrust and cooling air.
Dan McCormick, general manager of advanced combat engine programs at GE Aviation in Evendale, Ohio, explains the challenge: “How do we create thrust in a much more fuel-efficient manner in a combat environment?” Adaptive technology, variable-cycle, and high-temperature materials hold the keys to the answer, he said.
While “the basic physics of the cycle haven’t changed, ... changing the architecture of the engine allows us to move air around the engine in a different way than we did before,” McCormick said. This “step-change in fuel-efficient thrust generation” is creating an engine as different from today’s frontline fighter engines as those engines are from Frank Whittle’s first crude turbojets in the early 1940s.
The engine configuration that emerged from GE’s AETD work includes a three-stage, variable-geometry adaptive fan, a large annular duct to accommodate the third stream, and a very high-pressure ratio compressor. This last element was borrowed from the commercial CFM International’s Leap engine that will power the Airbus A320neo and Boeing 737 MAX airliners.
General Electric is a 50/50 partner in CFM with France’s Safran Aircraft Engines.
“What we’re seeing now is that many, if not most, of the technologies that have to do with generating more thrust more efficiently are really being matured in our commercial product lines,” McCormick said. Those commercial centerline engines allow him to “reverse history and pull those technologies into the engines we’re doing for the military.”
It’s truly an example of technology coming full circle: The CFM56 product line—the world’s most successful commercial jet engine, with 30,000 delivered to date—was built on the core of the F101 turbofan, developed to power the B-1B Lancer bomber. The CFM56 itself found some of its first work re-engining KC-135 tankers.GE has worked “very hard to be able to exploit any technology that we have for our commercial business as well as applying those technologies on the military side of our business,” McCormick added.
It’s not just about bringing fuel efficiency to the world of fighter engines, however. GE reports adaptive-geometry variable-cycle engines can deliver a 25 percent improvement in specific fuel consumption. That can save billions of gallons of fuel outright, but fighters equipped with new generation engines would also enjoy a 25 to 30 percent increased combat radius, as much as 40 percent more persistence in the target area, or the ability to reach 36 percent more targets. That would translate to as much as 74 percent less dependency on aerial tanker support.
These enhancements become even more crucial when considered in the context of the Air Force’s sharply reduced capacity. The service has 59 percent fewer combat-coded fighter squadrons and 37 percent fewer aircraft overall than it did during the 1991 Gulf War. Making each aircraft more effective and productive can help close the capacity gap.
“The adaptive feature of the engine and its fuel efficiency plays most advantageously in a platform that has a very diverse mission,” McCormick observed. For fighters, getting into the target area, persisting there, and getting out are phases of the mission where “you would like all of that to be extremely fuel efficient.” When engaging an enemy, that’s when “you want that combat capability” of high thrust and performance.
Another advantage is in cooling. In an F135-sized engine, that third airstream could be a significant source of cooling, drawing off heat generated by sophisticated fighter aircraft equipment.
“We’ve created great, insulated fuselages made of composites and packed them with electronics that are good at making heat,” McCormick noted. “That third stream of air provides a significant opportunity” with thermal management. “It really is a game-changer; it’s not just an incremental improvement,” he asserted.
Better thermal management should make it possible for the F-35 to spend a lot of time flying at near supersonic speeds at altitudes as low as 500 feet—a feat the F-35 today is restricted from sustaining because of thermal management concerns.
In a different non-F135 engine configuration, the third stream could also do double duty. Besides helping to cool all the heat-generating onboard electronics, it could potentially create a source for as much as a megawatt of onboard power for next generation weapons and systems, such as electric lasers and high-powered microwaves.
The Air Force is showing increased interest, now that such capabilities are tantalizingly close. In Fiscal 2016, the service acclerated funding to the AETP. USAF sees this as a low-risk way to start refining technologies for a next generation engine development program.
ADVENT began as a pure science and technology program. It was to solve the basic engineering and physics problems of propulsion improvement without worrying about creating flight-rated hardware or addressing installation challenges. The Air Force created the Adaptive Engine Technology Development program as a bridge, with component development and rig testing intended to lower the risk associated with eventually maturing the technologies.
Pratt & Whitney demonstrated a three-stream fan in a rig in 2013 as part of the AETD agenda. Earlier this year, the company was scheduled to demonstrate the three-stream technology in an actual engine environment.For its part, GE finished its compressor rig testing in fall 2016 at Wright-Patterson AFB, Ohio. In May, a fan rig and core engine were being tested as well. GE expected to finish its AETD fan rig tests near the end of May, and core engine tests were to finish in late May or early June.
In the follow-on, five-year AETP, both engine-makers will design, develop, build, and test full-scale adaptive engines in the 45,000-pound thrust class. Pratt sees it as the logical extension and maturation of the next generation F135 engine it has been producing for the F-35 fighter.
For GE Aviation, AETP represents a chance to get back in the frontline fighter engine game. GE wasn’t chosen to be the primary engine-maker for either the F-22 or F-35. Then, a planned, interactive competition with Pratt & Whitney for F-35 engines was nixed in 2011 by Defense Secretary Robert M. Gates as a short-term cost-saving measure.
The AETP could be GE’s ticket to reprise its role in the “Great Engine War” of the 1980s when the Air Force competed construction of power plants for the F-15 and F-16.
The Air Force has broken the AETP out of the broader Propulsion Technology Transition research, development, test, and evaluation program for “greater transparency,” the service said.
Despite USAF’s belt-tightened Fiscal 2017 budget request, AETP was given priority status and the five-year proposed spending profile is a robust $2.4 billion. It steadily grows from $285 million in Fiscal 2017 to $603.2 million by Fiscal 2021.
Pratt & Whitney said in a 2016 news release that its AETP engine will “benefit fully” from technologies refined and matured in AETD, noting its successful “demonstration of advanced turbine blade cooling technologies that allowed the company to achieve the highest ever turbine temperature in a production-based fighter engine.”Although the AETP engine will be nominally aimed at an F-35 class power plant, the Air Force is treating it as a more generic engine development effort—at least for now. There also seems to be some room for additional competition, with budget documents hinting that the engines that result from AETP could become the basis for a family of engines with multiple applications.
“The program will leverage adaptive turbine engine science and technology demonstrations to develop a multiplatform common adaptive engine built around a commercially derived core,” the Air Force said in Fiscal 2017 budget documents. This suggests this engineering and manufacturing development work will set the stage for a comprehensive roadmap for powering combat aircraft. AETP will mature component technologies and reduce risk to prepare for “next generation propulsion system development for multiple combat aircraft applications,” according to the DOD contract listing.
Lt. Gen. Arnold W. Bunch Jr., USAF’s top uniformed acquisition official, told Air Force Magazine there may be spin-offs from the new technology that could benefit the F-15 and F-16. Though the engines being looked at are designed to fit the power plant space in the F-22 and F-35—larger than the space in the F-15 and F-16, carrying an older and slimmer engine—Bunch said one aim of the program is to see if it can be “scaleable” to a smaller engine.
“We’re keeping a focus on keeping our options open for whatever we need to do in the future,” Bunch said. tough choices
Does the Air Force’s planned $2.4 billion investment signal a return to the Great Engine War?
The service, “at least through this point in the program” sees “some advantages in keeping us both involved,” McCormick said. Although USAF is keeping mum on the details of its thinking, “the behavior we see today shows they’re certainly very much interested in retaining competition.”
With the possibility of many different combat aircraft applications on the table, Pratt & Whitney and GE could each supply adaptive engine technology for different combat platforms and avoid splitting the fighter-engine buy as was done in the 1980s. The reality is that it costs a lot of money to certify two engine-makers to compete year after year in a budget environment that supports two-thirds fewer combat squadrons than 25 years ago.On the other hand, each engine company is bringing unique technologies to bear and until very recently, it appeared that USAF was willing to spend money on both companies as an investment in combat capability. It may well now boil down to a tough choice for the Air Force: financially unaffordable competition or strategically unaffordable sole-source next generation engines.
_____Jim Mathews became CEO of a national transportation nonprofit organization after 26 years as a reporter, editor, and executive at Aviation Week. His most recent article for Air Force Magazine was “CAP Joins The Total Force” in the January issue.
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