Tests Of GE XA100 Adaptive Combat Engine Exceed Performance Targets
GE Aviation has completed initial test runs of the first full-scale XA100 three-stream adaptive combat engine, marking the start of a new chapter in U.S. military turbine engine development and paving the way for a new generation of variable cycle powerplants for sixth generation fighters.
Developed by GE’s Edison Works advanced programs unit, the XA100 is one of two experimental adaptive demonstrators contracted under the U.S. Air Force’s Life Cycle Management Center’s Adaptive Engine Transition Program (AETP). The other engine, the XA101, is under development by Pratt & Whitney.
In addition to proving out the adaptive concept at full scale, both demonstrators are also designed as a 45,000 lb. thrust class engine to meet potential growth requirements for the conventional take-off and landing variants of Lockheed Martin’s F-35. For now, however, the focus is on evaluating the performance and operability of a morphing propulsion system that offers a step change in combat capability through the dynamic modulation of a third stream of air.
Engine tests mark the culmination of AETP which was launched in 2016 to develop three-stream demonstrators for full scale development from earlier research efforts. These included the Air Force Research Laboratory’s Adaptive Engine Technology Demonstration (AETD) and Adaptive Versatile Engine Technology (ADVENT) programs which proved the basics of practical variable cycle propulsion.
Results from test runs of the first XA100—which began in GE’s high-altitude test cell in Evendale, Ohio, on Dec. 22 and continued through late March—have exceeded expectations according to David Tweedie, GE Edison Works’ general manager for Advanced Combat Engines.
“We hit all of our primary test objectives,” Tweedie said. “The engine behaved right along with our pre-test predictions and was very consistent with the program goals. We were able to demonstrate the two different modes of the engine and the ability to seamlessly transition between those two modes.”
Designed to run separately to the conventional core air and lower pressure bypass flows, the additional third stream can be redirected to provide increased thrust during combat or better fuel efficiency during cruise conditions. The AETP goals aimed to improve thrust and fuel efficiency by 10% and 25% respectively compared to a 2015 state-of-the-art fighter engine. “Not only are we meeting that, we’re actually exceeding that pretty much everywhere in the flight envelope—and in a few places—up to 20% [more thrust],” Tweedie said. “We are very happy with where we are from thrust in terms of over-delivering versus the program requirement.”
“When you translate that to what it means to the platform, it’s 30% more range or 50% more loiter time depending on how you want to utilize that fuel burn improvement. It’s a significant increase in acceleration and combat capability with the increased thrust,” he said.
Another crucial parameter for the test program is the effectiveness of using heat exchangers in the third stream for thermal management—a growing challenge, particularly for low-observable, advanced combat aircraft with power-hungry sensors, systems and weapons. “We see a significant increase in capability there [with] up to two times mission systems growth enabled by the [improved] thermal management,” Tweedie said.
As well as overall performance, testing also focused on the operability of the power management system and its ability to automatically transition between modes. The system manages this as a function of the power setting on the engine. “As you get to a point where throttle demand would indicate you want to be in the higher thrust mode, that’s when the engine kicks over,” Tweedie said. “The vision here is the pilot won’t even know what mode they’re in. It should be completely transparent to them and it’s simply scheduled into the engine.”
GE’s altitude test site, which was built in the 1960s to support development of the GE4 for the U.S. civil supersonic program, has been used to clear the initial XA100 performance characteristics. “We’ve been able, within the limits of our facility, to work at multiple points of the flight envelope to get data. This has not been just sea level static testing of this engine—which is why we have the confidence we have,” Tweedie said.
Assembly of the second XA100 prototype is “well underway,” according to GE. This will be tested at the Air Force’s Arnold Engineering Development Complex (AEDC) in Tullahoma, Tennessee. “AEDC has the capabilities that we don’t have on site, which is why the second engine is scheduled to go there and finish out building the matrix to the points we weren’t able to hit,” Tweedie added.
Changes to the second engine will be limited to instrumentation, software and controls. However, the engine hardware, which includes parts made from high-temperature tolerant ceramic matrix composites (including rotating components in the low-pressure turbine) and polymer matrix composites, will be identical to the first engine.
These are the kinds of designs you get from folks who write requirements in air conditioned offices all day.
This sounds like FADEC on steroids.