F-35 Engine Upgrade Clears Key Hurdle, Pratt & Whitney Says

A key design review moves the F135 core upgrade closer to production as the F-35’s Block 4 weapons and sensors push the fighter’s engine past its original limits.

An F-35 engine upgrade designed to power and cool the fighter’s next generation of weapons and sensors has passed a key design review, moving it closer to production, Pratt & Whitney said Tuesday.

The engine maker, an RTX business, said the F135 Engine Core Upgrade, or ECU, completed its Risk Reduction Design Review, or RRDR. The review examined detailed design elements, propulsion technologies, aircraft integration considerations and the risk-reduction work needed to deliver full engine life.

The milestone matters because the F-35’s current engine is being asked to do more than it was built to handle. The core upgrade is meant to supply the extra power and thermal management capacity the Block 4 modernization requires across all three F-35 Lightning II variants.

The jet’s Power and Thermal Management System is overtasked, forcing the F135 to operate beyond its original design parameters to generate enough bleed air for cooling. Running the engine hotter than expected increases wear, shortens its service life and is adding $38 billion in maintenance costs, the Government Accountability Office said in a May 2023 report.

Officials say F135 upgrade program remains on track

“Completing RRDR is an important milestone that demonstrates the maturity of the F135 Engine Core Upgrade and the progress our team is making to deliver this capability to the warfighter,” said Jill Albertelli, president of Pratt & Whitney Military Engines.

Navy Capt. Mitch Grant, the F-35 propulsion program manager, credited the partnership between the contractor and the government.

“The Risk Reduction Design Review highlighted strong collaboration among our industry and government team, demonstrating a mature ECU design,” Grant said.

“The ECU program remains on track to deliver substantially improved readiness and to support F-35 modernization to outpace our adversaries,” he said.

Pratt & Whitney designed the upgrade to keep high commonality with the existing engine, making it a drop-in replacement for both new-production jets and aircraft already in the fleet. The company is funded this year to begin delivering long-lead materials for future ECU ground test engines, according to military budget justification documents.

“As the selected propulsion modernization solution for the F-35, ECU builds on the proven F135 architecture to enable the power and thermal management needed for future capabilities, while leveraging an established global production and sustainment enterprise. This approach strengthens fleet readiness and provides operators with a proven, cost-effective path to sustain and modernize the F-35 for decades to come,” Albertelli said.

Paired with a separate upgrade to the jet’s Power and Thermal Management System, the engine improvements are expected to bring a 7% increase in range and thrust while doubling the F135’s cooling capacity, though that projection has not been independently confirmed. Marine Corps Lt. Gen. Gregory Masiello, who heads the F-35 Joint Program Office, told senators in June that the upgraded engine core will offer a “marginal increase” in cooling, with further cooling-system upgrades coming a few years later. Pratt & Whitney said in 2017 that testing of earlier core technologies showed a 6% to 10% thrust increase and a 5% to 6% fuel burn improvement.

The F135-PW-100 and F135-PW-400 currently produce up to 43,000 pounds of thrust with afterburner and 28,000 pounds of dry thrust at military power.

The F-35 Joint Program Office named Lockheed Martin, the jet’s prime contractor, as lead systems integrator for the Power Thermal Management Upgrade, a complementary effort focused on the aircraft’s cooling system.

Pentagon passed on an all-new engine

The Pentagon, in its fiscal 2024 budget request, rejected an all-new adaptive cycle engine developed under the Adaptive Engine Transition Program because of high expected costs, technical hurdles, and the inability to fit such an engine into the Marine Corps’ vertical-landing F-35B without a major redesign.

In June 2023, months after the Pentagon chose the core upgrade, Lockheed Martin publicly advocated an adaptive engine for the F-35, arguing the upgrade would not provide the same margin for future growth. A Lockheed spokesperson said the adaptive engine technologies offer more power and greater cooling, which the company said the F-35 will need as it is modernized beyond Block 4.

Pratt & Whitney executives sharply criticized that push. Jeff Shockey, RTX’s senior vice president of global government relations, accused Lockheed of putting its bottom line ahead of taxpayers and said placing an unproven adaptive engine on the F-35 would delay critical capabilities.

Jen Latka, then Pratt’s F135 program chief, argued the core upgrade, paired with an upgraded cooling system, would leave enough margin that the program would never need another engine upgrade for power and cooling. She said it would also avoid the tens of billions of dollars needed to duplicate the global sustainment network for a second engine type.

Richard Aboulafia, an aerospace analyst with AeroDynamic Advisory, said he was surprised by the public rift between the prime contractor and the engine maker, and described airing the disagreement publicly as ill-advised for program stability.

Engine work tied to wider Block 4 challenges

The engine upgrade is one piece of a modernization effort that has run into friction on software, radar and thermal management at the same time.

The program built Technology Refresh 3, known as TR-3, to replace the F-35’s legacy computing hardware with an integrated core processor offering 25 times more computing power. Block 4 and future upgrades will require more power and cooling than the program originally projected, the Government Accountability Office has said.

Software instability tied to TR-3 led the Defense Department to stop accepting newly built F-35s from July 2023 until July 2024. To clear a backlog of more than 100 undelivered jets parked at Lockheed’s Fort Worth facility, the Joint Program Office approved acceptance of aircraft running an interim version of the software, designated 40P01, which supports combat training but lacks full combat capability.

Releasing that inventory helped Lockheed deliver a record 191 F-35s in 2025, a total helped by the cleared backlog; Lockheed has said it can build 156 F-35s a year.

The AN/APG-85 radar, a core Block 4 component, has created its own mismatch. The new radar requires a redesigned nose bulkhead and mounts that cannot accept the older AN/APG-81. F-35s will eventually need cooling for systems drawing 62 to 80 kilowatts, more than double the 32-kilowatt limit of the current cooling system, Masiello told senators.

Lockheed began building the new bulkheads into Marine Corps jets with Lot 17, but the first production APG-85 radars are not scheduled for delivery until April 2028. As a result, the military has accepted new F-35s without radars installed.

Marine Corps Lt. Gen. Gregory Masiello, who heads the F-35 Joint Program Office, told the Senate Armed Services Committee on June 23 that the Marine Corps had accepted six such F-35Bs. The jets carry ballast in the nose and can be used for basic flight training but not for combat training or missions until radars are installed.

Flight testing of the Engine Core Upgrade is expected to begin around 2030, an Air Force propulsion official said in July. The upgraded engines, originally meant to enter service in 2029, are now expected in 2031, Masiello told senators in June. The Government Accountability Office said in September 2025 that a scaled-back Block 4 modernization effort is expected to finish in 2031.

Engine fleet, depots and congressional backing

Pratt & Whitney has delivered more than 1,500 F135 production engines to customers in 20 allied nations.

The upgrade is designed to fit into the existing sustainment network without new facilities. That network is anchored in the U.S. by the Heavy Maintenance Center at Tinker Air Force Base in Oklahoma and Fleet Readiness Center Southeast in Jacksonville, Florida.

Process changes at Tinker cut heavy maintenance turnaround time for F-35 engines from an average of 244 days to 106 days between early 2021 and mid-2022, according to Air Force officials. The Jacksonville center inducted its first F135 power module for regular depot maintenance and its first F-35B airframe in August 2024.

In September 2024, the Navy awarded Pratt & Whitney a cost-plus-incentive-fee contract valued at up to $1.3 billion to keep maturing the upgrade. In March, the Pentagon awarded Pratt & Whitney a $3.8 billion contract modification for Lot 18 and 19 F135 engine production, bringing that contract’s total value to $6.6 billion.

The F135 program supports more than 66,000 jobs across 47 states and territories and generated more than $9 billion in U.S. economic impact in 2025, according to Pratt & Whitney. Suppliers include Michigan-based Jedco Inc., which relies on the F-35 program for half its business, and BMT Aerospace, which is developing updated part configurations for the new core.

Senior appropriators including Rep. Rosa DeLauro, D-Conn., and Sen. Susan Collins, R-Maine, have backed the engine modernization, and RTX has credited them with steadfast support for the F135.

F-35 acquisition costs had grown to more than $485 billion as of December 2023, and the program office estimates it will cost at least $1.58 trillion to operate and sustain the fleet over its 77-year life cycle, according to a September 2025 Government Accountability Office report.