Tests on GE Aviation's XA100 adaptive cycle engine have shown that the technology has matured enough and that the time has come to move the program forward.
In March the second of two prototypes of the XA100 began testing with the U.S. Air Force’s Arnold Engineering Development Complex in Tullahoma, Tennessee. The engine was developed as part of the USAF’s Adaptive Engine Transition Program (AETP) and is aimed at providing a new powerplant for the F-35A (and the Navy’s F-35C) around the end of the decade.
Results from the ongoing Arnold tests, and those conducted by the engine-maker at its Evendale, Ohio, plant—which now total several hundred hours of run-time.
“We believe that we have reached a critical decision point,” said David Tweedie, v-p and general manager advanced projects at GE’s Edison Works, the company’s research and development business unit for military solutions. “Everyone agrees that something needs to be done. The technology is ready, and the needs are greater than they were. Now it’s time to proceed.”
Tweedie describes the XA100 as a transformational engine, marking a step-change between the current twin-stream turbofan and a new three-stream approach to combat aircraft powerplants. The change, he says, is as momentous as the transitions from piston to turbojet power, and then from turbojet to turbofan. Twin-stream turbofan technology has essentially reached the limit of its potential, requiring a new concept that also takes into account the rapidly rising requirement for thermal management as more power-hungry and heat-producing mission systems go into combat aircraft.
Exactly how the adaptive cycle XA100 works remains proprietary information, but in essence the engine combines the fuel efficiency of the high bypass ratio turbofans that power commercial airliners with the high thrust offered by low bypass ratio turbofans in today's fighters. The adaptive cycle engine automatically reconfigures itself in flight to match the particular requirements of the moment, be that economical cruise or high thrust. For the pilot, there is no difference insofar as they still control the engine power setting through a single thrust lever.
The results are impressive, offering a range increase of around 30 percent, and 10 to 20 percent more thrust. Through having a third airflow stream and advanced heat exchangers the thermal management capability doubles. GE has estimated that implementation of the AETP could save the Air Force as much as $10 billion over the F-35A’s life-cycle through better fuel efficiency and reduced maintenance requirements.
Development of the XA100 has its roots in the Advent science and technology project that ran from 2007 to 2014 and, among other areas, explored engine operations at very high internal temperatures. A subsequent Air Force project got underway in 2012, during which some components of an adaptive cycle powerplant underwent testing before the launch of a full-scale AETP prototype in 2016. The first of two XA100 prototypes ran for the first time at Evendale in December 2020, with the second following in August 2021 before its move to the Arnold EDC, where it is now being tested across the full flight envelope in a campaign that has neared completion.
The F-35A was selected as the target vehicle for the new engine as, according to Tweedie, “it is the fastest path to get this technology into the air,” with a targeted initial service release in fiscal year 2028. The XA100 is designed to fit within the F-35A’s engine bay with no modifications required to the aircraft. Although the XA100 is slightly heavier than the incumbent F135 engine, the use of technologies cascaded from the company’s commercial engine programs, such as advanced additive manufacturing and the use of lightweight ceramic matrix composites, has mitigated the weight gain. The weight is comfortably within the capacity of the existing aircraft structure.
With the Air Force as the prime driver, the F-35A represents the primary focus of the XA100, but GE also has considered the additional requirements of the Navy’s F-35C and is proposing a common engine for both versions. The STOVL engine/lift fan powerplant of the Marine Corps’ F-35B offers more challenges, but GE is already in discussion with the F-35 Joint Program Office regarding the potential adaptation of the XA100 to the F-35B.