The U.S. Air Force has estimated a development cost of nearly $6.7 billion for the Adaptive Engine Transition Program (AETP) to provide more range and improved thermal management/engine performance and increased weapons load for the Lockheed Martin [LMT] F-35A, but the service has not released a breakout of such estimated costs.

Pratt & Whitney [RTX] builds the F135 engine for the three versions of the F-35.

The House Appropriations Committee, in a report on the committee’s version of the fiscal 2023 defense appropriations bill, said that the committee “is aware the power and cooling demands of continuous upgrades to all variants of the Joint Strike Fighter (JSF) will degrade the performance of the existing F135 propulsion system and ultimately require a modernized solution to meet future capability requirements.”

“The committee also is aware that AETP is one among multiple potential solutions, and that these solutions present divergent implications in terms of benefits, cost, schedule, technical risk, supply chain management, and the value of a joint program that was foundational to the JSF program’s creation,” the report said. “The F–35 Joint Program Office (JPO) currently is conducting a business case analysis (BCA) comparing multiple combinations of engines and power and thermal management systems.”

Since 2016, the Air Force has funded the 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, as the latter requires an engine that drives the lift-fan system, provides bleed air to the roll-posts, and uses a swivel exhaust duct.

General Electric [GE] has said that it began working with the F-35 JPO in the fall of last year on evaluating whether GE could alter its proposed XA100 engine for AETP to fit on the F-35B (Defense Daily, June 23). GE said that it plans to submit an interim report to the F-35 JPO before September’s meeting of the JSF Executive Steering Board (JESB) and that the company is to submit a final report by the end of the year.

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).

In June 2016, Pratt & Whitney and GE each received contracts worth more than $1 billion each for AETP.

It appears that Pratt & Whitney is looking at how it could adapt the XA101 for the F-35B, but the company referred questions on the matter to the F-35 JPO, which has not responded.

The House appropriators’ report said that the F-35 JPO plans to meet with the JESB to brief the office’s BCA on F-35A engine options “not later than the first quarter of fiscal year 2023.”

The committee wants the Pentagon Director of Cost Assessment and Program Evaluation “to conduct an independent cost estimate and comparative assessment of all propulsion solutions that are the subject of the JPO’s BCA and brief its findings to the congressional defense committees by the earlier of not later than 60 days following the submission of the president’s fiscal year 2024 budget request, or April 30, 2023.”

The Air Force has told lawmakers that the service plans to award a contract for AETP engineering and manufacturing development by the end of fiscal 2024, if DoD decides to pursue AETP. The Air Force may begin retrofitting F-35As with AETP in fiscal 2027 or earlier.

The F-35 program has said that the plane will need improved engine capabilities after DoD fields the third increment of Block 4, which is to include dozens of new features, including the ability to carry 16 new weapons.

In May, Air Force Secretary Frank Kendall told the Senate Armed Services Committee (SASC) that “there are a number of options to improve the performance of the F-35 engine” and that AETP thus far appears to feature “the most impressive technology” in terms of fuel efficiency, range extension and other factors (Defense Daily, May 17). Kendall said that DoD will have to make a decision on the future of F-35 engine modernization–a decision that is likely in the coming months before the Biden administration releases its fiscal 2024 budget request.

“We will have the largest [F-35] fleet in the Air Force so we have the greatest interest in this [AETP], I think, but there’s a significant cost associated with the development of a brand new engine,” Kendall told SASC. “It’s less expensive to do an upgrade, perhaps, and get increased performance, and there are other options in between.”

F135 sustainment has been a significant problem and has decreased F-35 mission capable rates.