• The Air Force has made the first big step toward new, advanced engines for the F-35.
  • The adaptive cycle engines the service is seeking would offer both superior performance and efficiency.
  • It will be some time before those engines are installed on F-35s, if they are at all, but they will likely be incorporated into future designs.

On Friday, the Air Force made the first meaningful step toward sourcing advanced new adaptive cycle engines for its primary stealth fighter, the F-35.

These new engines, while still a ways from service, would offer a significant leap in capability for the aircraft practically across the board, giving the F-35 an advantage over the growing slew of foreign 5th-generation competition.

The branch issued a “sources sought notice” for planning purposes only, which means we’re still a long way off from putting these next-generation engines in America’s top-tier fighters, but it could be well worth the wait.

These advanced engines would not fit in the short take-off, vertical landing (STOVL) iteration of the fighter (F-35B) but would fit snugly in the fuselage of the Air Force’s F-35A or the Navy’s F-35C.

What are adaptive cycle engines?

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An F-35 during a Heritage Flight at Luke Air Force Base in Arizona, March 18, 2018.

US Air Force/Staff Sgt. Tyler J. Bolken


Throughout most of aviation history, engine design has always been about compromise. Commercial, cargo, and many reconnaissance aircraft usually leverage engines that were designed to offer excellent fuel efficiency at the expense of top-end performance, while tactical jets like fighters carry engines that are designed primarily for maximum performance at the expense of fuel efficiency.

The aim of adaptive cycle (sometimes called variable cycle) engines is to eliminate this compromise and offer superior performance and efficiency in a single power plant.

The US Air Force’s Life Cycle Management Center’s Adaptive Engine Transition Program (AETP) currently has two contracts out for adaptive cycle engine technology demonstrators. GE completed its initial test runs of their own full-scale XA100 three-stream adaptive combat engine in May of last year, with Pratt and Whitney following with their XA101 in October.

“The goal for the Air Force was to develop the next-generation fighter engine architecture and technologies to provide a generational step-change in combat propulsion capability,” David Tweedie, GE Edison Works’ general manager of Advanced Combat Engines, told Sandboxx News last May.

“GE has worked hard to achieve the challenging objectives the Air Force has set out, and we believe we are delivering on what they’ve asked us to do.”

In order to do so, these engines are designed to operate in different “modes.” When the pilot needs the engine’s peak performance in combat, he or she can lean hard on the throttle and the engine’s management system will take its cue to switch into its heavy-burning high-thrust mode.

Conversely, while on a patrol, the engine would stay in its high-efficiency, low-burning mode to stretch the mileage or loiter time provided by each gallon of fuel.

“The mode transition is seamless to the pilot, and they won’t even know when it happens,” Tweedie said.

“They will control engine power using the throttle the way they always have, and the engine schedule will determine the appropriate operational mode.”

Incredibly, we’re not just talking about matching the power output of previous engines while increasing range … we’re talking about a 10% or better increase of thrust practically across the flight envelope alongside a 25% or better jump in range.

More range, more power, more capability

Navy aircraft carrier F-35 engine

US Navy sailors aboard aircraft carrier USS Carl Vinson load an F-35C engine module in a CMV-22B Osprey, February 11, 2021.

US Navy/MCS Seaman Jeffrey Kempton


While the Air Force is now exploring the idea of sticking these engines under the hoods of F-35As, that boost in range would be especially valuable to the Navy.

The F-35C has a total range of approximately 1,200 nautical miles, giving it a 600-mile combat radius in the best of times. That’s not far enough to launch sorties at China without putting America’s carriers in range of China’s hypersonic anti-ship weapons.

A 25% boost in range, say to 750 miles, isn’t enough to eliminate this capability gap, but it’s a step in the right direction.

GE tested their XA100 at their high-altitude test cell in Evendale, Ohio over the span of more than three months, starting at the tail end of 2020. According to their reports, their XA100 actually exceeded their performance targets on top of successfully demonstrating its ability to operate in both high-thrust and low-burn modes.

The engine produces a total of 45,000 pounds of thrust, slightly better than the 43,000 pounds offered by the F-35’s current Pratt & Whitney F135-PW-100 afterburning turbofan.

The aim of the program was to increase thrust by 10% and fuel efficiency by 25% over conventional designs, but in testing, the engine did even better than that.

“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 added.

While this new engine’s ability to seamlessly transition between tearing through the sky like a top fuel dragster and minding the fuel gauge like a Toyota Prius might catch the attention of aviation enthusiasts, it might be the engine’s thermal management and use of advanced component technologies that really make the XA100 a huge leap forward in fighter engines.

According to Tweedie, the XA100’s “three-stream architecture” enables a doubling of thermal management capacity, or in other words, a real reduction in the heat created by the engine’s operation.

Heat is currently a limiting factor in power production for fighters, which have to limit their output to avoid damaging the aircraft itself. That will no longer be the case with the new generation of adaptive cycle engines, meaning fighters will have more electrical power to run systems.

“We see a significant increase in capability there [with] up to two times mission systems growth enabled by the [improved] thermal management,” Tweedie said.

Adaptive cycle engines may be the future of air combat

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An F-35A at the Chicago Air and Water Show.

U.S. Air Force photo by Airman 1st Class Alexander Cook



More power means the ability to run more advanced systems too, like directed-energy weapons and even exotic new countermeasure systems like the Navy’s recent patent for “laser-induced plasma filament” holograms, effectively projecting the heat signature of another aircraft hundreds of feet away to distract inbound infrared-guided (heat-seeking) missiles.

According to GE, advanced component technologies used in the construction of their XA100 engine, including additive and Ceramic Matrix Composites, also reduce the overall weight of the engine while also increasing durability over previous designs.

The result combination of power, fuel efficiency, heat management, and resilient but lightweight construction make the XA100 the physical embodiment of a fighter engine wish-list. While any of these improvements in capability would be welcome in most fighter designs, the collection of them in a single system could well make for a power plant that is even greater than the sum of its parts.

Of course, a sources sought notice is no guarantee that these new adaptive cycle engines will find their way into America’s F-35s, but this new approach to engine design will almost certainly be incorporated into future platforms, like the Air Force’s Next Generation Air Dominance fighter.

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