Pratt & Whitney [RTX] has been investing internal funds in an Enhanced Engine Package (EEP) for the company’s F135 engine for all three variants of the Lockheed Martin

[LMT] F-35 Lightning II–a package that could save tens of billions of dollars in engine life cycle costs and provide the cooling needed for advanced weapons on the fighter by 2027-28.

The EEP engine would increase F-35 range and thrust by more than 10 percent each, boost thermal management capacity by more than 50 percent, and increase powered lift for the U.S. Marine Corps’ F-35B by five percent.

Under Continuous Capability Development and Delivery (C2D2) for the F-35, the aircraft are expected to receive software updates every six months. Begun in 2018, C2D2 has included the development of Block 4 software to prepare the aircraft to counter area denial by adversaries, Technology Refresh-3 updates, and making F-35s capable of carrying conventional and nuclear ordnance. Cost estimates for C2D2, which the F-35 program is to pursue through fiscal 2025, have varied from $7 billion to more than $10 billion.

“I think there is widespread acknowledgment that we need a propulsion requirement that is commensurate with the air vehicle,” Jennifer Latka, Pratt & Whitney’s vice president of the F135 engine program, said on Nov. 8. “The C2D2 budget has never had a placeholder for propulsion, but meanwhile every time we have a capability increment coming online, it is requiring more cooling for weapons systems–cooling air off the engine. When you play out the next five plus years–Lot 20 timeframe, that’s where the rubber meets the road. When that capability increment is coming in on the air vehicle, that’s when you want to have propulsion lined up with the modernization requirement. It is driven off of power and thermal management. That’s the requirement.”

Latka said that if policy makers do not prioritize the F-35 airframe and its propulsion system equally, militaries employing the F-35 will lose capability, as they are unable to use the plane as intended, and sustainment costs “will grow tremendously.”

“If you pull more bleed air off the engine, it will perform for you, but it will run hotter, burn more fuel,” Latka said. “When the engine runs hotter, it degrades faster. It will come in for maintenance sooner.”

Asked how much engine air diversion the carriage of F-35 advanced weapons will require, Latka said that “already the engine operates outside of specification that it was designed to.”

“It’s already giving more,” she said of the F135. “With every capability increment, there are a certain number of kilowatts that they’re [the program is] looking for.”

The first F-35s are to enter scheduled maintenance late next year or early 2023 after 2,000 flight hours. Such scheduled maintenance is to occur every 2,000 flight hours up to an F-35’s scheduled life of 8,000 flight hours.

While U.S. Air Force Gen. Mark Kelly, the head of Air Combat Command, said two months ago that 46-48 of his F-35A fleet was down for engine-related needs at Oklahoma City Air Logistics Complex–a number since reduced to below 40, Latka attributed the down time to a lack of depot capacity, rather than an F135 reliability problem. The F135 has exceeded the program’s mean flight hours between unscheduled removals requirement, she said.

“The product’s reliability is really bar none,” Latka said of the F135. “The reliability has outperformed every engine we have designed and manufactured here at Pratt & Whitney. What happened in the sustainment world is we just didn’t build the depot infrastructure on time.”

Another element that may increase the availability of the F-35 for missions is an increased pool of spare engines and power modules.

“The total F-35 fleet is spared at 11 percent, including both spare engines and power mods–the high compressor through to the turbine exhaust case, the core of the engine,” Latka said. “The concern for us as the engine OEM (original equipment manufacturer) is that the fleet is spared at 11 percent. This is so far below what the legacy fighter fleets are spared at. The legacy fighter fleets, at least the ones that Pratt [& Whitney] serves, are spared at 25 to 30 percent.

“I think we all need to look at whether this design around a non-mission capable rate for propulsion–that’s what we’re designed to, not target stock levels for spares or a war readiness reserve for spares–[is appropriate],” she said. “Its objective is six percent and threshold 10 percent. That program design was determined way before anyone probably in their seat now was here. It’s our forefathers on this program a decade ago. I think that deserves to be looked at because when we think about readiness, standing up depots and spares [are] both part of the equation.”

Latka cited the Air Force’s submittal of a fiscal 2022 unfunded requirements list that included 20 F135 power modules for the F-35A.

While Pratt & Whitney pursues its Enhanced Engine Package for all three F-35 variants, the company has also participated in the Air Force’s Adaptive Engine Transition Program (AETP) for the future Next Generation Air Dominance (NGAD) family of systems and for the possible provision of longer range for the F-35A.

The Senate Appropriations Committee’s version of the fiscal 2022 defense funding bill recommends adding $460 million for AETP (Defense Daily, Oct. 19).

The House Armed Services Committee’s  (HASC) version of the fiscal 2022 defense authorization bill would provide $270 million for AETP and would require the Pentagon acquisition secretary to provide a report to Congress on the AETP acquisition strategy and starting fielding of AETP on F-35As by fiscal 2027. The HASC bill would also require the Navy to submit to Congress a report on the integration of AETP “or other advanced propulsion system into F-35B and F-35C” aircraft no later than two weeks after the Biden administration’s submittal of its fiscal 2023 budget request.

The Air Force had requested just $13.5 million in fiscal 2022 for AETP, down from more than $214 million appropriated by Congress in fiscal 2021 and nearly $527 million in fiscal 2020.

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.

The Air Force has said that its focus for AETP has been on NGAD, not the F-35A, due to what the service has said are significant costs for retrofitting AETP engines on the F-35A, which is likely the only F-35 version that will accommodate an AETP.

General Electric [GE] recently said that the company has begun testing the second prototype of the company’s XA100 adaptive cycle engine as part of AETP, and the HASC advised accelerating AETP in the committee’s version of the fiscal 2022 defense authorization bill (Defense Daily, Sept. 8).

In May, after a six-month run, GE said that it had finished testing the first prototype of the XA100, which is to provide significant fuel cost savings, performance improvements, and less wear and tear on airframes. The company said that XA100 will provide a 10 percent increase in thrust, 25 percent better fuel efficiency, and significantly enhanced heat dissipation capacity. GE said that testing on the second XA100 had begun on Aug. 26 at GE’s Evendale, Ohio, altitude test facility.

Pratt & Whitney has participated in AETP with the company’s XA101 engine.