USAF Chief: New F-35 Engine Development Needed, Even Without Commitment To Buy

Pratt & Whitney F135 engine
Credit: U.S. Air Force

The continued research and development for a possible F-35 engine replacement is needed, even without a commitment to replace the current F135 powerplant, so the U.S. Air Force can keep the option open and the technology evolving, the top service general said Sept. 8.

Chief of Staff Gen. Charles Q. Brown Jr., speaking during a Defense News virtual conference, said eventually there will be a “fork in the road” when the service needs to decide if there will be a second engine for its F-35 fleet, “but the value of actually continuing the research and development is important so we do have options in the future.”

Since 2016, the Air Force Life Cycle Management Center has been conducting the Adaptive Engine Transition Program (AETP) to demonstrate three-stream engines. The competition is between Pratt & Whitney’s XA101 and GE Aviation’s XA100 designs, which could be added to F-35s or the future Next Generation Air Dominance system.

Brown and new Air Force Secretary Frank Kendall have reviewed the ongoing effort and decided “if we stopped the R&D … we basically shut ourselves off from having an option to go forward, and so this is why it’s important for us to continue research and development on this particular capability.”

The House Armed Services Committee (HASC), in its markup of the 2022 defense policy bill approved Sept. 2, calls on the Pentagon to approve the use of the AETP on the F-35A. The bill specifically calls on the Defense Department to outline an acquisition strategy for development, integration, and operational fielding of the new propulsion system on to the Air Force’s F-35A fleet starting in fiscal 2027.

The AETP engine is not a requirement for the F-35 yet, but the Joint Program Office has said it is following the development for possible Joint Strike Fighter use.

Meanwhile, Pratt is also proposing a series of upgrades to its F135 engine to improve fuel efficiency while also bettering thrust.

Kendall, in a recent interview with Air Force Magazine, said the service is following “some technologies that could go into future upgrades that could reduce some of the operational costs, such as fuel, significantly,” but added he was not ready to commit to those.

The same HASC bill also outlines a plan to link the total number of F-35s in the Pentagon’s fleet to sustainment costs, cutting tails from the overall force size if the jets remain expensive to operate. Brown said what Congress is trying to do is in line with Air Force plans.

“We are all committed to the same thing—to make it affordable and to make the sustainment costs more reasonable, and so that is a focus for us,” Brown said. “The language from Congress is really in line with what we’re trying to get done.”

Brian Everstine

Brian Everstine is the Pentagon Editor for Aviation Week, based in Washington, D.C. Before joining Aviation Week in August 2021, he covered the Pentagon for Air Force Magazine. Brian began covering defense aviation in 2011 as a reporter for Military Times.


I can´'t understand why they stopped RRs development of an alternate engine for F35, if they had not stopped that they would now have two competing engines.
M Sandberg
So we're restarting an effort that will now require a full new contract with all the costs associated ... great use of the taxpayers dollars *eyeroll*
Walking away from continuous improvements would be highly damaging to the US' reputation as a reliable supplier of complex systems...
Reengining a fighter is not like dropping a bigger engine in a race car.
Serious effort is required by aircraft and engine companies, to work cooperatively together on an advanced engine design concept (like variable cycle and/or three stream engines). This overall systems need recognizes the fact that the aircraft inlet and exhaust system are integral parts of the air handling components of a complete propulsion system. Like a turbine engine compressor, combustor, turbine require 'matching' so does the complete propulsion system (inlet-engine-exhaust). Engineering design and test approaches that appreciate the needs to properly match the entire propulsion system also recognize the need for both inlet and exhaust component testing as part of the total matching of inlet, engine and exhaust flows with an eye toward minimizing installed propulsion system losses and maximizing net propulsive effort. Simply put, engine and aircraft company advanced design teams need to work the 'whole enchilada' in a cooperative and data sharing arrangement. Simply designing and engine to vary airflow, bypass streams, etc. in isolation is not enough. Doing the 'whole enchilada' is also needed in order to evaluate and quantify the installed effects/benefits of any engine with variable cycle capabilities. This includes defining both installed propulsion systems performances as well as overall aircraft mission capabilities and range....something one might observe did not happen with the last variable cycle engine considered years ago for the ATF. If the 'life cycle management team' is focused primarily on 'flange to flange' engine performance and variability' and not supporting strong aircraft company participation in efforts to integrate the engine into a propulsion system, then more time will pass until the of the system and mission effects of the various Variable Cycle engine concepts can be validated along with a proper risk assessments to systems development time, aircraft performance and costs.