The Pentagon’s fiscal 2024 budget may lay out a future path for engines for the Lockheed Martin [LMT] F-35 fighter–a course of action that may upgrade the existing Pratt & Whitney [RTX] F135 engines or replace them with more fuel efficient ones that would give the fighter more range.

At a May 17 hearing of the Senate Appropriations Committee’s defense subcommittee on the U.S. Air Force’s fiscal 2023 budget request, Sen. Susan Collins (R-Maine) asked Air Force Secretary Frank Kendall whether the service had studied the comparative life cycle costs of replacing the F135 or modernizing it.

Since 2016, the Air Force has funded the Advanced Engine Transition Program (AETP), but thus far it appears that an AETP engine will fit on the F-35A and possibly the Navy F-35C, not on the Marine Corps’ F-35B short takeoff and vertical landing variant.

“There are a number of options to improve the performance of the F-35 engine,” Kendall told Collins on May 17. “The problem we have is that it’s a fairly complicated situation…The program [AETP] that we’ve been funding for a long time, which has the most impressive technology at this point, in terms of fuel efficiency, range extension, and so on, won’t go into all three of those aircraft. At best, it will go into two of them.”

“So, the Department of Defense has to make a decision overall about engine modifications and upgrades for the F-35,” he said. “I expect that process to take place over the next few months, as we build the ’24 budget. What the Air Force has funded is continuing the AETP technology development. We’re going to need to have a decision at a higher level about the overall program for F-35 engine modifications and upgrades.”

Kendall, in response to a follow-up question from Collins, said that the Navy, Marine Corps, and foreign partners on the F-35 program have not agreed to the concept of replacing the F135 engine with AETP. “Each of these services and each of our partners is in a different situation with regard to the economics of doing that and the performance requirements,” he said. “We will have the largest [F-35] fleet in the Air Force so we have the greatest interest in this, I think, but there’s a significant cost associated with the development of a brand new engine. It’s less expensive to do an upgrade, perhaps, and get increased performance, and there are other options in between.”

The Air Force has said that development of AETP may cost $6 billion, yet service officials have also said that F135 sustainment has been a significant problem and has decreased F-35 mission capable rates.

General Electric [GE] said that it had begun Phase 2 testing of GE’s second XA100 adaptive cycle engine for AETP at the Air Force’s Arnold Engineering Development Complex on March 25 after wrapping up Phase 1 testing at GE’s Evendale, Ohio altitude test plant in December.

The Fiscal 2022 Consolidated Appropriations Act funded AETP at $473 million–$460 million more than the Air Force requested.

The Air Force had requested just $13.5 million in fiscal 2022 for AETP, down from more than $214 million appropriated by Congress in fiscal 2021 and nearly $527 million in fiscal 2020.

Launched in 2016, AETP followed Air Force engine developments in the Adaptive Versatile Engine Technology (ADVENT) program, begun in 2007, and the Adaptive Engine Technology Development (AETD) program, started in 2012. Pratt & Whitney and GE each received $1 billion contracts for AETP in 2016.

The Air Force has said that its focus for AETP has been on the Next Generation Air Dominance (NGAD) fighter, not the F-35A, due to what the service has said are significant costs for retrofitting AETP engines on the F-35A.

In fiscal 2018, the Air Force exercised options in the AETP contract for the Next Generation Adaptive Propulsion (NGAP) risk reduction effort for NGAD engine components. The Air Force said in its fiscal 2023 budget request that the service had broken out NGAP to reveal funding levels. In fiscal 2023, the Air Force requests nearly $68 million for NGAP and about $286 million for AETP.

Pratt & Whitney has participated in AETP with the company’s XA101 engine. The company said it has also been investing internal funds in an Enhanced Engine Package for the company’s F135 engine for all three F-35 variants–a package that Pratt & Whitney said could save tens of billions of dollars in engine life cycle costs and provide the cooling needed for advanced weapons on the fighter by 2027-28 (Defense Daily, Nov. 8, 2021).