Update: This story was updated June 2 with additional information from the F-35 Joint Program Office.

WASHINGTON — The F-35 program only knows how much cooling the Joint Strike Fighter’s engine will need through 2035, government auditors said in a new report.

But that’s a few years after the F-35 program needs to finish upgrading its thermal management system to be able to support rapidly arriving new capabilities, the Government Accountability Office said in a Tuesday report.

And GAO said it’s hard to predict how long the planned upgrades to the F-35′s power and thermal management system, or PTMS, and engines will be effective, though the fifth-generation fighter is expected to fly for another 50 years. PTMS uses the “bleed air” from the engines to cool systems throughout the fighter including weapons and radars.

The finding spotlights the uncertainty the program faces as it prepares for a major upgrade to the Pratt & Whitney-made F135 engines, dubbed the Engine Core Upgrade. Just a few years after completing the effort, the F-35 program “will face a period of unknown requirements” for its future capabilities — and may be in danger of future cost overruns and other mistakes, according to the new report.

“Without defined PTMS and engine modernization requirements, the F-35 program is at greater risk of repeating prior missteps,” GAO said. “By proceeding with planning and development of future capabilities without considering the demands on the PTMS and engine, the program endorsed capabilities that neither could support. The program risks repeating a similar mismatch between PTMS and engine capability and future modernization needs if the military services select an option without first defining future requirements.”

GAO said the F-35 program can’t fully predict how much power and cooling the fighter will need until the military services flying the aircraft define their own requirements.

But already, new capabilities being added to the F-35 are stretching its cooling capabilities beyond their original design, causing the engines to wear out faster. An upgrade program known as Block 4 — a $16.5 billion project to add new sensors, more advanced weapons, and more powerful data fusion and advanced electronic warfare — will further tax the engines’ cooling capabilities.

Without an engine upgrade, GAO said, the added heat could drive up the cost of maintaining the existing engines by $38 billion.

In a statement to Defense News, the F-35 Joint Program Office said it is confident the ECU engine upgrades can “minimize” the $38 billion in costs GAO highlighted.

“The ECU will restore engine life, and the [power thermal management system improvements] will ensure that the air vehicle can support future capability growth,” JPO spokesman Russ Goemaere said in an email.

GAO recommended the Pentagon order the JPO, before moving forward with the engine modernization effort, to re-evaluate its analysis of how to upgrade the F-35′s engines after the services spell out what power and cooling capabilities they will need.

The Pentagon disagreed with that recommendation, saying the F-35 program will re-evaluate its analysis when necessary as the services’ requirements are updated.

In a Friday statement after this story’s initial publication, the JPO said, “We have a firm handle on engine and power thermal management system options that are needed for future cooling needs, based on the service’s requirements.” The JPO also said the services will provide the cooling requirements they need to support the capabilities they want for their F-35s.

Goemaere said in his original email the JPO is in the early design stages as it weighs several options for improving the fighters’ thermal management systems, which will take place alongside the F135′s core upgrades.

The upgrades to the engines’ core and thermal management system are expected to be fielded in the early 2030s, he said, though the exact schedule will depend on what design is ultimately approved.

However, that means the engine upgrades necessary to handle the added heat may come a few years later than the F-35 will receive its Block 4 upgrades, now projected to be finished in 2029.

More engine cooling — but will it be enough?

In a briefing with reporters Wednesday, Jen Latka, Pratt & Whitney’s vice president of F135 programs, said the current engines would be able to handle the Block 4 upgrades, albeit at an increased maintenance cost. An upgrade to the engine’s core alone would let the F135 more fully enable Block 4, she said. And an upgrade to the power thermal management systems would allow the F-35 to handle future upgrades beyond Block 4, Latka said.

She said the engine core and thermal system upgrades together “provides a tremendous amount of cooling margin” — more than enough to comfortably allow future F-35 capability upgrades beyond 2035, without further improvements to the engine.

“If we’re back here talking about a potential engine upgrade in another 15 years, my guess is it’s for a different reason,” Latka said. “The amount of cooling that will be feasible with the ECU and the upgraded [thermal] system … there’s a tremendous amount of design margin there.”

The Defense Department considered two main options for upgrading the F-35′s engines — Pratt & Whitney’s upgrade to the current engines, and a General Electric Aerospace-made engine it dubbed the XA100 that uses an adaptive design. The so-called adaptive engine uses three streams of air, instead of two, to cool the engine and the aircraft, and can adjust to the configuration that would give the plane the most thrust and efficiency in the moment.

But during the 2024 budget rollout in March, the Pentagon said it decided to upgrade Pratt’s engines instead of going with a new design under the Adaptive Engine Transition Program, or AETP.

Air Force Secretary Frank Kendall said at the time that while GE’s adaptive engine would provide more thrust and cooling capability, it was only certain to fit in his service’s F-35A variants. This meant the other services were not interested in an AETP engine, and the cost was too high for the Air Force to bear on its own, Kendall said.

GAO said in the report the Navy’s carrier-based F-35Cs would also be able to use an AETP engine.

But GAO said AETP engines would not fit in the Marine Corps’ F-35B, which has a unique vertical take-off capability, without a major redesign that would drive up costs and sacrifice commonality across all three variants.

In the roundtable with reporters, Latka said the GAO report supported the company’s position that an F135 upgrade was the best option for upgrading all three fighter variants’ propulsion systems at the best cost.

But in an email, GE spokesman Adam Kostecki said the report shows how important it is for the Pentagon to anticipate what the F-35 will need in the future and start investing in technology to meet those requirements. The XA100 would be able to provide considerably greater range, acceleration and cooling capacity, he said, and GE urged Congress to support its engine — effectively overruling the Pentagon’s decision — as lawmakers continue to craft the 2024 budget.

Stephen Losey is the air warfare reporter for Defense News. He previously covered leadership and personnel issues at Air Force Times, and the Pentagon, special operations and air warfare at Military.com. He has traveled to the Middle East to cover U.S. Air Force operations.

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