The age of the adaptive cycle engine as a new class of combat aircraft propulsive system in its own right is edging closer to reality with detailed design now underway of the first three-stream demonstration units under the U.S. Air Force’s Adaptive Engine Transition Program (AETP).
AETP will mature three-stream engine technology for the future U.S. Navy F/A-XX and the Air Force’s F-X sixth-generation fighters. Targeted initially at the 45,000-lb.-thrust class, the engine is also baselined to fit within the existing confines of the F-35A engine bay, making it a contender to replace the Joint Strike Fighter’s current Pratt & Whitney F135 from the mid-2020s onward. New contracts worth about $2 billion, roughly split, for the AETP demonstrators were awarded in June 2016 to General Electric and Pratt & Whitney.
To underline the importance of this step toward a new generation of engine technology, the Air Force has broken with tradition by designating the two AETP demonstrators as the XA100 and XA101. Instead of following on from the F for “fan” or “turbofan” series, which most recently saw the F135/136s developed for the F-35, the new naming convention of “A” for adaptability is a milestone on a par with the designation “J” for the turbojet series in the 1940s. The XA100 is being developed by GE, and the XA101 by Pratt.
GE and P&W defining sixth-gen fighter engine designs under Adaptive Engine Transition Program (AETP)
USAF adopts new engine-naming convention to recognize importance of adaptive cycle
GE developing XA100, Pratt developing XA101
Initial engine sized at 45,000-lb.-thrust to suit F-35
Production A100/101 derivatives targeted at F-X and F/A-XX
The third stream provides an extra source of air flow that, depending on the phase of the mission, is designed to provide either additional mass flow for increased propulsive efficiency and lower fuel burn for longer endurance, or additional core flow for higher thrust and cooling air to boost combat performance. The third stream can also be used to cool fuel, which provides a heat sink for aircraft systems. The additional flow can also be used to swallow excess air damming up around the inlet, improving flow holding and reducing spillage drag.
AETP builds on more than a decade of development starting with the Air Force Research Laboratory’s Advent (Adaptive Versatile Engine Technology) program, which examined variable-cycle architectures, three-stream flowpaths and adaptive fans. It also follows on from the Adaptive Engine Technology Development (AETD) program that proved the basic viability of the adaptive cycle. Advent is now complete, and AETD will soon culminate with a final series of demonstrator runs by the two engine-makers.
Receiving a designation validates the research, “making it more real,” says Dan McCormick, GE Aviation Advanced Combat Engine programs general manager. “It is still an ‘X’ engine, so it is still in development, but it is good that this step was taken. The Air Force debated designating it as another ‘F’ series engine before opting for ‘A.’ To me this validates a whole new architecture and a whole new line of engines. Now we have a third generation of jet engine technology, and this validates what we have believed for a long time—we are really changing the game.”
The company is progressing through the final phases of the AETD program by conducting component evaluations and running or evaluating data from three rig tests of a compressor, core engine and adaptive fan module. The compressor test, which was run at Wright-Patterson AFB, Ohio, was completed in September. “We have completed the analysis, and the data is flowing into AETP,” says McCormick. Work on the fan rig is ongoing in the same compressor research facility at Wright-Patterson. “That’s nearly complete, and we expect to finish sometime [in May],” he adds.
The third, and final, large-scale rig test is the core engine, also expected to wrap up late in May or early in June. “It is a pretty challenging program because it is very complex. With an adaptive cycle engine with the three streams and heat exchangers, when you run the core you have to simulate the rest of the engine. It is actually more complicated to run than a full-up engine test because we must adapt the facility to simulate the inputs from the rest of the engine,” McCormick explains.
Although the overall pace of testing has been slower than hoped, McCormick says: “We are still making great progress and have over 30 hr. of tests on the core.” The work is ongoing at GE’s A1 test cell at Evendale, Ohio, where the machinery has been modified to influence the flow to the inlet and outlet of the exhaust. With the imminent completion of the final rig tests, GE expects to wrap up the AETD work by year-end, he adds.
Aerodynamic and aeromechanical data from the compressor and fan rigs will aid design of the XA100 for improved operability. “All these lessons learned are very valuable,” says McCormick. “In terms of performance, we are getting the compression characteristics and stage loading we expected. And in terms of operability, it can handle the surge margin, and so on.” Meanwhile, the core engine tests provide another data set for the compressor. “And we are starting to get combustor and turbine interactions around that,” he says.
While neither GE nor Pratt have released details of their adaptive-engine design, both incorporate variable geometry devices that dynamically alter the fan pressure ratio and overall bypass ratio—the two key factors influencing specific fuel consumption and thrust. The adaptive, multistage fan boosts fan pressure ratio to fighter engine performance levels during takeoff and acceleration, and in cruise lowers it to transport aircraftlike levels for improved fuel efficiency. The third stream, which is external to both the core and standard bypass duct, is used to alter the bypass ratio.
“As we move into AETP, we continue to mature the baseline technologies as well, [deciding when to introduce] primarily material technologies into the program,” McCormick says. Although these are at lower technology readiness levels, they are in areas where GE is confident of coming maturity, including ceramic matrix composites (CMC) and polymer matrix composites (PMC).
As part of earlier AETD tests, GE ran an F414 with flaps and seals made from an oxide/oxide ceramic CMC similar to parts now used in the production version of the Passport business jet engine. An F414 was also fitted with second-stage low-pressure turbine blades made from another form of CMC. PMCs, which are composed of short or continuous fibers bound in an organic polymer matrix, are also well-known in the industry, and air splitters made from the PMC-based PMR-15 are widely used in engines. “The difference is where we are applying them, and using them in more complex parts,” says McCormick.
Although GE declines to provide a comprehensive AETP schedule, it is believed the detailed design review is slated for late 2017, followed by release of drawings for building a full engine. “We are actually starting into the supply-chain process now of getting hardware defined, so there is definitely an engagement,” says McCormick. The company is expected to build three test engines starting in 2019, with ground runs due later that year and set to run through 2020. The first engine will test the basic mechanical design of the engine, the second will assess performance and operability, and the third will assess durability.
“There is still no decision as to where this suite of technology will go with respect to a platform,” McCormick adds. “But it is designed to fit in the F-35, and we have integration work going on with Lockheed Martin. However, no solid decisions have yet been made on where this will go into the fleet, and we need to mature the technology to the point where that decision will be easier to make.”
Pratt, which has so far described very little of its AETP plan, was expected to have begun running an adaptive three-stream fan with an F135 earlier this year under the final phase of AETD. The company, which is also thought to have started test runs of a new high-efficiency core, says “adaptive engines will be a critical enabler for virtually all future combat aircraft.”
Given GE's expected end-2020 test dates, P&W should be in the 2019-2021 time-period as well. At the level of maturity for these "X" engines one could expect airframe X plane programs that could fly by the 2021-2023 time frame as well which would be in line with these programs.