WASHINGTON — The Pentagon is abandoning its efforts to develop a next-generation adaptive engine for the F-35 Joint Strike Fighter, according to Air Force Secretary Frank Kendall.

Instead, Kendall said during a March 10 briefing on the fiscal 2024 budget, the military will stick with and upgrade the F-35’s current engine — a major win for F135 maker Pratt & Whitney.

The decision means the military will not move forward with its Adaptive Engine Transition Program, an effort to fund research, development, prototyping and testing of a new kind of engine with enhanced thrust, power and cooling ability, The adaptive engine uses three streams of air to cool the engine and the jet, and has an adaptive cycle that allows it to adjust to the configuration that provides the most thrust and efficiency for a given situation.

Both General Electric Aviation and Pratt & Whitney had designed new engines as part of AETP, but only GE had pitched its engine — the XA100 — as a replacement for the F-35.

Military officials said Friday they determined Pratt & Whitney’s proposal to modernize the already existing F135s in a program called the Engine Core Upgrade was the most cost-efficient option that would work for all F-35s.

“We were not able to fund the AETP program,” Kendall said. He told reporters the cost of GE’s XA100 adaptive engine and doubts it would work in all versions of the F-35 led the military to stick with the F135.

“We needed something that was affordable, and would support all the variants” of the F-35, he added.

An Air Force spokesperson said Tuesday the service plans to spend $245 million on the Engine Core Upgrade in 2024.

The Air Force also said Tuesday it has obligated nearly $2.7 billion for AETP since the program launched in 2016.

In a statement provided to Defense News, a GE spokesperson expressed the company’s displeasure with the Pentagon’s decision.

“This budget fails to consider rising geopolitical tensions and the need for revolutionary capabilities that only the XA100 engine can provide by 2028,” the GE spokesperson said. “Nearly 50 bipartisan members of Congress wrote in support of advanced engine programs like ours because they recognize these needs, in addition to the role competition can play in reducing past cost overruns. The XA100 engine is ready to power U.S. warfighters today and in the future.”

The GE spokesman said in a follow-up email that the company plans to keep working on the XA100 this year as it moves into the design and manufacturing advancement phase. The Air Force awarded GE a contract Dec. 27 worth up to $203 million to keep maturing its AETP design.

Pratt & Whitney applauded the military’s decision to fund its Engine Core Upgrade in its own statement.

“All F-35 variants need fully-enabled Block 4 capabilities as soon as possible, and with this funding, we can deliver upgraded engines starting in 2028,” Jill Albertelli, president of military engines at Pratt & Whitney, said in the statement. “The F135 ECU saves billions, which ensures a record quantity of F-35s can be procured. It also ensures funding will be available to develop 6th generation propulsion for the Air Force’s Next Generation Air Dominance Platform.”

Pentagon officials have repeatedly said that as the F-35 receives more upgrades and additional capabilities — particularly with its upcoming Block 4 modernization — it will need more power and cooling than the current F135 engine can provide.

GE officials said the company’s XA100 engine would use advanced composites and new technologies such as a third stream of air to produce better fuel efficiency, thrust, speed, range and heat management.

But the XA100 would have proven costly. Kendall last year warned outfitting the F-35 with an adaptive engine could cost more than $6 billion, which he said would force the military to buy roughly 70 fewer fighters.

An F-35A Lightning II assigned to Luke Air Force Base, Ariz., prepares to be refueled by a KC-135 Stratotanker assigned to the 161st Air Refueling Wing during a local sortie, Sept. 16, 2022. (Staff Sgt. James A. Richardson Jr./Air Force)

Kendall also noted the military had serious doubts about GE’s ability to make its adaptive engine work in the F-35B, the Marine Corps variant.

“The Air Force, with the A variant, was the only service that was really seriously interested in AETP, for which it was a really good fit,” Kendall said. “There was some discussion about whether it could be made to go in the [Navy and Marine Corps’ carrier-based] C variant. But the Marine Corps variant was going to be very, very difficult, if not impossible.”

Dave Tweedie, vice president and general manager for advanced products at GE Edison Works, said in a Feb. 16 call with reporters the company had found a way to adapt its XA100 into the F-35B.

Pratt & Whitney, owned by Raytheon Technologies, has developed its own adaptive engine, the XA101. But the company’s executives have said the adaptive engine is better suited to future aircraft such as the Next Generation Air Dominance platform, and upgrading the F135 is a cheaper, more reliable way to improve the F-35′s power, thrust and cooling ability.

In a Feb. 28 call with reporters, Jen Latka, vice president of F135 programs at Pratt & Whitney, said the company has already been working on preliminary design for the Engine Core Upgrade, using $200 million in funds appropriated in 2022 and 2023. That includes $75 million in the omnibus appropriations bill Congress passed in December.

Latka told reporters in that call an upgrade of the existing engines will cost about one-third as much as developing a new engine and will work in all three F-35 variants.

This would be a “drop-in” upgrade to existing F135 engines, she said, and retrofitting could occur in existing depots.

Latka said it could be easily adopted by international partners under already-existing frameworks and would be supported by the current framework for F135 export authorizations.

“Establishing an integrated global sustainment network does not happen overnight,” she said. “The existing structure of the [maintenance, repair and overhaul] facilities and depots is more than a decade in the making. The Engine Core Upgrade is not going to change the work share with current partners domestically or globally.”

In the Feb. 16 call, Tweedie told reporters that if the Pentagon chose not to move forward with AETP for the F-35, GE could roll some of the technology it developed for the XA100 into its work on the Next Generation Adaptive Propulsion program to power NGAD.

“While the engines are different, there’s quite a bit of leverage in technology from AETP to NGAP,” Tweedie said. “We certainly in all scenarios will take the data and the lessons and the technology matured under AETP for beyond the F-35 as well.”

Kristyn Jones, the Air Force’s assistant secretary for financial management and comptroller, who is performing the duties of undersecretary of the Air Force, said in a briefing at the Pentagon Monday that although military is not planning to transition AETP to a program of record, the Pentagon will leverage several of its technologies on future engine programs.

Jones highlighted the Next Generation Adaptive Propulsion program as an example of a program that can take advantage of the work on AETP. The Air Force is requesting $595 million for NGAP in 2024, which would be an increase of $375 million over 2023.

“It was not necessarily a sunk cost,” Jones said. “We’ll be building on the lessons learned from that program.”

Stephen Losey is the air warfare reporter for Defense News. He previously covered leadership and personnel issues at Air Force Times, and the Pentagon, special operations and air warfare at Military.com. He has traveled to the Middle East to cover U.S. Air Force operations.

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