If Congress decides to continue the U.S. Air Force Advanced Engine Transition Program (AETP), integration of an AETP engine on the Lockheed Martin [LMT] F-35 would require further analysis before proceeding, the F-35 Joint Program Office (JPO) suggested.

The Air Force’s fiscal 2024 budget request cancels the AETP leap-ahead engine effort and provides $245 million for the Raytheon Technologies‘ [RTX] Pratt & Whitney Engine Core Upgrade (ECU) for the company’s F135 engine.

The weight of the F135 for the F-35A and C is 6,400 pounds, while the F-35B has a lift fan system, which means the engine weight for that variant is about 10,400 pounds. The AETP requirement weight is 6,600 pounds.

“Size, weight, power and cooling (SWaP-C) are fundamental considerations for aircraft performance,” the F-35 program said in response to written questions on AETP. “It is understood that an increase in the weight of the aircraft will have a negative impact on its performance. AETP with its higher thrust, higher weight and further aft center of gravity present additional integration risk compared to ECU, and further structural analysis is required to quantify short-term and long-term impacts on the airframe and overall performance.”

Air Force Secretary Frank Kendall has said that AETP promised significant performance improvements for the F-35 and that the service may have decided to continue AETP, “if the cost had been lower.”

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. In June 2016, Pratt & Whitney and General Electric [GE] each received contracts worth more than $1 billion for AETP–GE for its XA100 and Pratt & Whitney for its XA101.

“From the start of the AETP, GE Aerospace developed the XA100 to meet F-35 performance, reliability and single-engine safety needs,” David Tweedie, vice president and general manager for advanced products at GE Edison Works, wrote in an email answer to questions. “For more than a decade, GE Aerospace has been working with Lockheed Martin in concert with the U.S. Air Force to assess the engine’s performance and integration, understand the challenges, and de-risk putting an adaptive engine in the F-35, efforts that are continuing during the Design and Manufacturing Advancement.”

The XA100 has been above the 6,600-pound AETP specification, but GE said that its engineers have reduced weight, and the company believes the XA100 will meet the weight specification.

GE said that the additional thrust provided by the XA100 would offset additional engine weight and still provide more than a 10 percent increase in overall thrust across all parts of the flight envelope.

500 GE Aerospace employees, including 400 engineers, continue their work on the company’s XA100 in Evendale, Ohio, GE has said.

The F-35 JPO said that it considered ECU, the XA100, and the XA101 adaptive cycle engine offerings in a business case analysis (BCA) that informed the Air Force’s decision on AETP and ECU.

Lockheed Martin declined to answer whether ECU will be sufficient to handle the higher cooling and power requirements for the Block 4 and 5 F-35 upgrades, which are to accomodate new weapons and mission systems.

“An engine upgrade program that provides more power and enables greater cooling capability is required as we modernize the F-35 to continue to deter and defeat advanced global threats,” the company said. “We stand ready to support and continue to work with the U.S. government on capability and performance upgrades at the speed of technology to ensure the F-35 remains ahead of ready.”

The Government Accountability Office (GAO) has questioned the BCA’s consideration of engine and power and thermal management system options and said that the analysis “did not follow any particular DoD guidance related to business case analysis [BCA] or analysis of alternatives” (Defense Daily, March 29).

GAO’s comments may lend support to lawmakers who want to keep the AETP option open for the F-35.

GE has said that the Air Force has spent more than $4 billion on adaptive cycle engine development.

The Air Force has estimated a development cost of nearly $6.7 billion for AETP but has not released a breakout of such estimated costs (Defense Daily, June 30, 2022).

GE said that Block 5 F-35As, which may field around 2030, will need the thrust, range, fuel efficiency, and cooling afforded by the XA100 engine. GE has said that “only XA100 can meet those demands by 2028 and for decades to come.”

GE has said that it wants the XA100 to have 30 percent more range than the F135, 20 percent more acceleration, and twice the thermal management. As an adaptive cycle engine, the XA100 is to switch automatically, as needed, between high thrust mode for maximum power during basic fighter maneuevers, for example, and high efficiency mode to extend aircraft range.

The XA100 adds a third airstream for engine cooling. In addition, ceramic matrix composites (CMCs)—an innovation started in the military realm and migrated and matured for the A320 and 737 commercial airliners–replace a significant number of metal parts in the XA100 and allow 500 degrees more heat resistance at one-third the weight of metal components, GE said.

Pratt & Whitney has said that ECU is a core upgrade that will have 70 percent materiel commonality with the F135 and that the company will rely on its existing F135 supplier base for ECU. Pratt & Whitney has also predicted that testing and ramping up a new engine for the F-35 will take years more than a core upgrade.