The US Air Force (USAF) supports upgrading the propulsion system on its existing Lockheed Martin F-35A stealth fighters, rather than purchasing an entirely new adaptive engine for the jets.

The decision, revealed by USAF secretary Frank Kendall on 13 March, comes as the Biden administration released its proposal for military spending in fiscal year 2024.


Source: Lockheed Martin

Lockheed Martin’s F-35 production line in Fort Worth, Texas

That request notably does not include funds for the Adaptive Engine Transition Program (AETP), which produced two entirely new, modernised engines designs for the F-35 airframe.

“We were not able to fund the AETP programme,” says Kendall during a budget briefing on 13 March. The USAF confirms that decision in a statement to FlightGlobal.

”[AETP] is not transitioning to a programme of record,” the service says. 

Although funding for AETP is zeroed out in the Department of Defense FY24 budget request, elected members of Congress will have the ultimate say in what programmes are funded.

Under AETP, the USAF in 2016 began funding development of a new F-35 engine, centred on an adaptive-cycle design, to boost electric-power generation and thrust.

The service spent some $4 billion on the effort, with both current F-35 engine supplier Pratt & Whitney (P&W) and GE Aerospace producing prototypes.

The programme was prompted by steady growth in demand for electrical power and cooling capacity to operate the F-35’s onboard sensors. As more advanced systems were added to the jet over subsequent years, power requirements began outstripping the design capacity of the single-engined fighter’s current P&W F135 power plant.

While GE Aerospace pushed for the USAF to adopt its XA100 adaptive engine, P&W ultimately threw its support behind a third option: an upgrade to the existing F135 propulsion system, known as the Engine Core Upgrade (ECU).

The USAF now makes clear it prefers that package, rejecting a fleet-wide retrofit with new adaptive engines. A major factor appears to have been the AETP engines’ lack of suitability for all F-35 types.

”Although the results of analyses determined AETP provided the best overall F-35A operational performance, the F135 Engine Core Upgrade will restore engine life and prevent degradation for all three F-35 variants at the lowest cost,” the service tells FlightGlobal. 

While GE Aerospace says its adaptive-propulsion system is compatible with the USAF’s F-35As and the US Navy’s carrier-launched F-35Cs, the XA100 is incompatible with the short take-off and vertical landing F-35B operated by the US Marine Corps.

That lack of cross-variant compatibility left the USAF facing the prospect of funding an engine replacement programme without support from other services.

“We needed something that was affordable, and that would support all the [F-35] variants,” Kendall says.

F-35 manufacturer Lockheed has remained neutral on the matter.

Speaking at the Avalon air show near Melbourne, Australia on 27 February, Lockheed director of international business development Steve Over told FlightGlobal Lockheed has provided technical feedback on the engine options to USAF policy makers, but has not been directly involved in the decision.

“We do not have a preference,” says Over. “We’re supporting them in their studies… international countries are very interested.”

The F-35 propulsion battle created a major showdown in the US defence industry, with Raytheon-subsidiary P&W and GE Aerospace both lobbying heavily to provide the next generation of propulsion for the USAF’s fleet of the stealthy, fifth-generation fighters.

With the USAF favouring the F135 core update, P&W is quick to tout the package’s cost savings.

“We can deliver upgraded engines starting in 2028,” says Jill Albertelli, president of P&W’s military engines division. “The F135 ECU saves billions, which ensures a record quantity of F-35s can be procured.”

But GE Aerospace criticises the USAF’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,” GE Aerospace says on 13 March. “The XA100 engine is ready to power US war fighters today and in the future.”

GE Aerospace has long maintained that adoption of the XA100 would produce some $10 billion in maintenance and sustainment cost savings over the F-35’s lifespan. P&W claims ECU will save $40 billion in such “life cycle” costs.

The USAF’s preference for the F135 engine update is no guarantee. Lawmakers – many of whom represent regions with financial and economic interests tied to various procurement programmes – will have the final say. In years past, Congress has overruled presidential budget requests in favour of other spending priorities.

“Nearly 50 bipartisan members of Congress wrote in support of advanced engine programmes like ours because they recognise these needs, in addition to the role competition can play in reducing past cost overruns,” GE Aerospace says.

The USAF is already funding other advanced engine development initiatives, including the nearly $5 billion Next-Generation Adaptive Propulsion (NGAP) programme to produce an engine for the service’s planned sixth-generation fighter.

Both GE Aerospace and P&W were awarded NGAP research and development contracts in August 2022, along with Boeing, Lockheed and Northrop Grumman. Propulsion advancements developed under AETP will feed into NGAP designs, according to the USAF.

”Findings and results from AETP prototype engine testing are informing ongoing design activities for the [NGAP] programme,” the service says.

The USAF expects NGAP design and prototyping to be completed by 2032.