Elected lawmakers in US Congress plan to continue funding a research programme that delivered two experimental jet engines for the Lockheed Martin F-35 stealth fighter.

Overruling opinions at the Pentagon, lawmakers proposed to spend $280 million on the Adaptive Engine Transition Program (AETP) in the 2024 National Defense Authorization Act (NDAA). That legislation was passed by Congress on 14 December.

The move comes despite a decision in 2023 by senior defence officials not to adopt either of the AETP powerplants.

Under the propulsion research initiative, Washington provided some $4 billion in funding to manufacturers Pratt & Whitney and GE Aerospace, with the goal of developing adaptive cycle engines for the F-35.

The concept represents a generational leap in propulsion technology, combining the thrust performance of a fighter aircraft with the fuel efficiency of a commercial jet.

GE’s First XA100 Adaptive Cycle Engine c GE Aviatoin

Source: GE Aerospace

GE Aerospace claims its XA100 adaptive cycle engine can deliver 20% increased thrust and 30% greater range to the F-35

Both GE Aerospace and P&W were successful, delivering prototype AETP engines to the US Air Force (USAF) for testing and evaluation in 2022.

However, concerns over the cost of retrofitting the entire F-35 fleet with a new engine, and a lack of compatibility with all variants of the multi-service stealth jet, led the Pentagon to pursue an upgrade of the existing P&W F135 powerplant.

The USAF had initially supported integrating an AETP engine into its F-35As. But a lack of support from the US Navy and Marine Corps, combined with concerns about lack of commonality with allied F-35 operators, pushed the USAF to shift support to an F135 core upgrade.

The service subsequently launched a separate effort – dubbed Next-Generation Adaptive Propulsion (NGAP) – to produce an adaptive cycle engine for a sixth-generation fighter. The USAF says knowledge gained from AETP will support work under NGAP.

As a result, the Pentagon did not request any funding for AETP in 2024.

However, lawmakers in Congress appear to have overruled the generals, inserting a fresh round of AETP funds into the 2024 NDAA.

GE Aerospace praised the move, saying it “stands ready” to deliver new technologies and capabilities.

“This bipartisan legislation sends a clear signal that advanced engines are needed to ensure US air superiority for years to come,” the company tells FlightGlobal. “More range, greater thrust and better thermal management.”

GE Aerospace claims its XA100 engine can deliver at least 20% more thrust and 30% greater range for the F-35.

P&W also welcomed the latest round of funding, saying it will support propulsion development for sixth-generation aircraft.

“The USAF investment in AETP has been key in developing technologies and architectures that are flowing into the Next Generation Adaptive Propulsion programme,” P&W says. “That will deliver the advanced survivability, fuel efficiency and robust power and thermal management required for next-generation platforms.”

The RTX subsidiary adds continuing the AETP effort will also support its Engine Core Upgrade programme to overhaul and improve capacity of the F135 engine.