The Air Force had hoped to leverage the advantage of new jet technology to generate more power and cooling for the F-35, but without the support of its partners in the joint and international program, that proved unworkable. Airman 1st Class Paige Weldon
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Strategy & Policy: New F-35 Engines: You Can’t Always Get What You Want

April 27, 2023

Air Force Secretary Frank Kendall clearly regrets that the service isn’t pursuing an advanced technology engine for the F-35, a move announced with the fiscal 2024 budget. His hand was forced both by the multibillion-dollar development price tag and the recognition that, while USAF is the largest operator of the fighter, it’s only one among many F-35 users worldwide. Partners at home and abroad weren’t willing to help fund the project, and the new logistics train needed to support it.

The decision was seen as a blow to the defense industrial base at a time when Congress is becoming increasingly alarmed about its capacity to surge production in the event of war. The provision of huge numbers of munitions to Ukraine—with no quick way to replace them, due to long lead times—has highlighted the fact that Pentagon suppliers are one-deep for many critical items, with limited production capacity. 

Now, by default, the F-35 will get the Engine Core Upgrade, or ECU, a less-ambitious and less-expensive improvement of the F135 engine offered by its maker, Raytheon’s Pratt & Whitney. The decision effectively preserves Pratt’s near monopoly on fighter engine production at scale for at least half a decade, until a new powerplant is needed for USAF’s Next-Generation Air Dominance System, or NGAD. Pratt has at times been maxed out in its capacity to build F-35 engines and parts. 

The Air Force had invested some $4 billion in the Adaptive Engine Transition Program (AETP) powerplants developed by Pratt and GE Aerospace, the two main fighter engine competitors. GE developed and built the XA100, while Pratt’s version is the XA101. The engines use multiple streams of air around and through the system, employing the most efficient flow required at any given moment. The result is the most impressive boost in fighter engine performance in over 20 years: 30 percent more range or loiter time, and between 15-20 percent more thrust. The engines also offer abundant cooling for the Block 4 version of the F-35, needed to chill its hotter-running avionics. 

In its fiscal 2024 budget justifications, the Air Force explained that “adaptive cycle engine technology enables next-generation combat aircraft capabilities by combining the efficiency of high bypass turbofans used by commercial airlines with the performance demanded of military fighter engines.” The technology builds on previous USAF efforts dating back to the late 2000s, under the Adaptive Versatile Engine Technology (ADVENT) program and the Adaptive Engine Technology Demonstrator (AETD) programs.

The extra range without external fuel tanks, particularly in the Pacific theater, is “really attractive,” Kendall said in a March 10 briefing for reporters on the fiscal 2024 budget. Longer range would also have reduced the need for tanker aircraft support. 

But, “we had to make hard choices in the budget,” he said. “This was one of them. I support the decision.”  

The ECU “does not provide quite as much capability as the AETP would have, but it does meet the needs of all three services, and it provides us with the growth potential that we need, that’s been identified so far,” he said.

Kendall tried to persuade Navy Secretary Carlos Del Toro to get the Navy and Marine Corps to adopt the AETP: Spreading the funding responsibilities over two or three services might have made it a viable option for all. But it was a no-go, Kendall said, acknowledging that it would have taken “tremendous work” to get the AETP powerplants to fit the Marine Corps F-35B, which has an elaborate thrust redirection system to enable vertical flight, and the carrier-capable F-35C, the tailhook system of which would have had to be re-engineered to accommodate the AETP. 

General Electric, which saw the AETP as a way to get back into the F-35 program after its alternative F136 engine was canceled in 2012, insists that the AETP could be made to fit in the F-35B and C models. The company also claims that the fuel and maintenance savings of an AETP engine would drive $10 billion in cost-avoidance for the Air Force over the life of the program.

Pratt has countered that developing a new engine and maintaining two logistics trains—one for users of the new engine and one for the upgraded F135—would cost an extra $40 billion over the life of the program. 

Kendall estimated last year that AETP development alone would cost $6 billion.

Pratt & Whitney is “pleased to see the President’s Budget includes funding” for the ECU, a company spokesperson said.

“All F-35s need fully enabled Block 4 capabilities as soon as possible, and with this funding, we can deliver upgraded engines starting in 2028,” the spokesperson said. The savings of going with the ECU “ensures a record quantity of F-35s can be procured” and frees up enough funding “to develop sixth-generation propulsion” for the NGAD. 

GE Aerospace, though, asserted that “this budget fails to consider rising geopolitical tensions and the need for revolutionary capabilities that only the XA100 can provide by 2028.” A company spokesman noted that 50 “bipartisan members of Congress” have voiced support for AETP “because they recognize these needs, in addition to the role [industrial] competition can play in reducing past cost overruns.” The investments in new engine technologies “risks being wasted” so close to a competition, GE said. The company plans to continue testing and refining its XA-100.

GE also said Pratt’s ECU is merely “an incremental upgrade” that will still cost billions, without providing the leaps in capability offered by the AETP. It criticized Pratt’s savings claims as “cost-avoidance numbers disguising an increased baseline cost.”

Pratt has also had recent troubles with the F135. Deliveries of the engines were halted for two months between December and March due to the discovery of a “harmonic resonance” issue, which the company said only manifested after 600,000 hours of F135 fleet run time. It was discovered in the aftermath of a December 2022 crash of an F-35B in hover mode during an acceptance flight-test at Lockheed Martin’s Fort Worth, Texas, facilities. The crash and the halt on engine deliveries also caused a two-month delay in F-35 test flights and all-up ship deliveries. A fleetwide retrofit to address the harmonic resonance problem was ordered and is underway.  

Congress had previously directed that the Pentagon pursue the AETP, with the objective of having it ready for installation in new-build F-35s circa 2028. Asked in a March 28 House Appropriations defense subcommittee hearing whether that could still work, Kendall said, “I do not have a recommendation for you on whether to continue AETP.”

He explained that it would be “a several-billion-dollar bill to take it through development and get it into production. And we can’t afford everything we might like to have in the budget, under any circumstances.”

Kendall added that the decision by the Joint Program Office (JPO) “to fund the core upgrade and give us a capability that all three services can use” was “the best business case among the choices that we had.”

He voiced his continuing disappointment a week later, when he told attendees at the McAleese defense conference that the given solution is one that “I worry about a little bit” because the F-35 won’t get the performance boost USAF would like to see. 

Given a magic opportunity to revisit the budget, “I think that would be something I’d like to have another shot at,” Kendall said. But “right now, it’s unaffordable.” The Air Force, he said, is “the only service that wants the new technology,” and “we can’t afford it by ourselves.” 

Under the agreements governing the multinational F-35 program, any user that wants unique equipment on its jets has to “pay to be different,” the JPO has maintained, in order to preserve the manufacturing, parts, and systems commonality of the fighter and discourage costly customization.  

Kendall said that the billions invested in the AETP will not be lost. The program “advanced the state of the art,” he said. 

“We’re going to benefit from that indirectly,” Kendall explained, by applying what was learned on the AETP to the Next-Generation Adaptive Propulsion (NGAP) program. The NGAP is to develop a powerplant that will equip the crewed NGAD fighter, the core aircraft in a “family” of systems with stealth performance exceeding that of the F-22.

Krysten E. Jones, the Air Force’s comptroller, told reporters at a March 10 briefing that “we do plan to leverage a lot of the capabilities that were part of the AETP prototype(s) for efficiency, thrust [and] thermal management, so it was not necessarily” a waste to invest in the program. The AETP will be leveraged “as we look at the next engines” under the NGAP. 

Fiscal 2024 budget documents describe AETP activities as “foundational risk-reduction activities” for NGAP, “providing capability enabling options for Next Generation Air Dominance (NGAD)” program. Although AETP is now viewed as a pathfinder for NGAP, Congress mandated in the FY23 National Defense Authorization Act that “the Air Force … maintain separate budget lines” for AETP and NGAP. 

The Air Force has $254 million budgeted in the FY24 budget for the ECU. The NGAP is budgeted for $595 million in FY24, up from $224 million enacted for FY23. However, NGAP funding (under the line item “Advanced Engine Development”) dips to $579.8 million in fiscal 2025; $456.9 million in FY26 and $291.1 million in FY27, with no funding slotted for FY28. The program is described as “continuing” after that.    

In August of 2022, the Air Force awarded five contracts worth up to nearly $1 billion apiece to companies exploring NGAP technologies. The $975 million indefinite delivery/indefinite quantity contracts—meaning those amounts may never be exercised—went to GE Aerospace and Pratt & Whitney, but also to Boeing, Lockheed Martin and Northrop Grumman, indicating that the Air Force is looking to expand its supplier base for military engines and is potentially considering novel approaches to NGAD propulsion. 

The contracts will involve prototyping, “weapon system integration,” and “digitally transforming the propulsion industrial base,” according to USAF’s contract announcement, that suggests  it may be willing to bet that companies not historically builders of fighter engines can design and build them with new digital technologies.

The Air Force said the work is to be completed by July 11, 2032. That would seem to be late for the needs of NGAD, which Air Force leaders have said would be operational circa 2030, and may indicate that early versions of NGAD will not have the NGAP engine.