F-35 Propulsion Upgrade Moves Forward Despite Uncertainty

Steve Trimble

Stabilizing the production system and securing a funded, long-term upgrade plan are now the main objectives for Pratt & Whitney's F135 propulsion system for the Lockheed Martin F-35.

Although first delivered for ground--testing 17 years ago, the F135 remains a lifeline in Pratt's combat aircraft engines portfolio for new-development funding. The U.S. military engines market is entering an era of transition with great uncertainty for the timing of the next major combat aircraft program.

• Enhancement Package replaces “Growth Option”

• New F-35 propulsion road map due in six months

The transition era begins with the likely pending delivery of Pratt's most secretive development project. In 2016, the U.S. Air Force named Pratt as one of seven major suppliers for the Northrop Grumman B-21 bomber. The Air Force also has set the first flight of the B-21 for around December 2021. That timing means Pratt is likely to have delivered the first engine for ground-testing. At some point within the next year, Pratt should be planning to deliver the first flight-worthy engine to Northrop's final assembly line in Palmdale, California, to support the Air Force's first B-21 flight schedule.

As the bomber engine development project winds down, the propulsion system for the next fighter aircraft continues to be developed, but without a clear schedule for transitioning to an operational system.

The Air Force Research Laboratory's Adaptive Engine Transition Program (AETP) is sponsoring a competition to develop an adaptive engine that can modulate the airflow into and around the core to improve fuel efficiency and increase range. The AETP competition is between Pratt's XA101 and GE's XA100 designs, with the first engines set to be delivered for ground-testing by the end of this year or early next year.

As 45,000-lb.-thrust-class engines, the first AETP designs are optimized for repowering the single-engine F-35, but the F-35 Joint Program Office (JPO) has established no requirement to replace the F135 for at least another five years. A follow-on effort within the AETP is developing a similar engine for a next-generation fighter, but neither the Air Force nor the Navy have committed to a schedule for transitioning the technology into an aircraft-development program. That leaves Pratt's F135 as the only feasible application for inserting new propulsion technology for a decade more.

After spending the last decade focused on completing development of the F-35 and upgrading the software, electronics and mission systems, the JPO is developing a road map to improve the propulsion system through 2035.

As the road map is being developed, program officials also are seeking to stabilize the engine production system. Pratt delivered about 600 F135s to Lockheed through the end of last year, including 150—or about 25%—in 2019 alone. The JPO signed a $7.3 billion contract with Pratt last year to deliver another 509 engines in 2020-22, or about 170 a year.

Although Pratt exceeded the delivery goal in 2019 by three engines, each shipment came an average of 10-15 days behind the schedule in the contract. The fan, low-pressure turbine and nozzle hardware drove the delivery delays, according to the Defense Department's latest annual Selected Acquisition Report on the F-35. Lockheed's production schedule allows more than two weeks before the engine is needed for the final assembly line, so Pratt's late deliveries did not hold up the overall F-35 schedule, says Matthew Bromberg, president of Pratt's Military Engines business.

F135 deliveries finally caught up to the contract delivery dates in the first quarter of this year, but the supply chain and productivity disruptions caused by the COVID-19 pandemic have set the program back. About five engines scheduled for delivery in the second quarter fell behind the contractual delivery date, Bromberg says. The pressure will grow as a loaded delivery schedule in the second half of the year adds pressure on deliveries, but Pratt's supply chain managers expect to be back within the contract dates in the first quarter of next year, he says.

The F-35 program's political nature also has caused program disruptions. The Defense Department's expulsion of Turkey from the F-35 program last year also banished the country's supply chain, which contributed 188 parts to the F135. In particular, Alp Aviation produces the Stage 2, 3, 4 and 5 integrally bladed rotors (IBR) for the F135.

As of early July, about 128 parts now made in Turkey are ready to transition to other suppliers, of which about 80% are based in the U.S., according to Bromberg. The new suppliers should be requalified to produce those parts in the first quarter of 2021 and ready to meet production rate targets for Lot 15 aircraft, which will begin deliveries in 2023.

“The overriding objective was to move with speed and diligence along the transition plan and ensure we are ready to be fully out of Turkey by about Lot 15,” Bromberg explains. “And we are on track for that.”

As Pratt transfers suppliers, the company also has to manage the effect on potential upgrade options. Alp Aviation, for example, had announced a research and development program to convert the finished titanium IBRs to a more resilient nickel material.

For several years, Pratt has sought to improve the performance of the F135 above the baseline level. In 2017, the company unveiled the Growth Option 1.0 upgrade, which is aimed at delivering modular improvements that would lead to a 5% or 6% fuel-burn improvement and a 6-10% increase in thrust across the flight envelope. The Marine Corps, in particular, was seeking additional thrust to increase payload mass for a vertical landing, but the proposed package did not go far enough to attract the JPO's interest.

“It missed the mark because we didn't focus our technologies on power and thermal management,” Bromberg says.

A year later, Pratt unveiled the Growth Option 2.0. In addition to providing more thrust at less fuel burn, the new package offered to generate more electrical power to support planned advances in the aircraft's electronics and sensors, with the ability to manage the additional heat without compromising the F-35's signature in the infrared spectrum.

Last fall, the JPO's propulsion management office teamed up with the Advanced Design Group at Naval Air Systems Command to analyze how planned F-35 mission systems upgrades will increase the load on the engine's thrust levels and power generation and thermal management capacity. In May, the JPO commissioned studies by Lockheed and Pratt to inform a 15-year technology-insertion road map for the propulsion system. The road map is due later this year or in early 2021, with the goal of informing the spending plan submitted with the Pentagon's fiscal 2023 budget request.

As the studies continue, a name change to Pratt's upgrade proposals reveals a fundamental shift in philosophy. Pratt's earlier “Growth Option” terminology is gone. The proposals are now called Engine Enhancement Packages (EEP). The goal of the rebranding is to show the upgrades no longer are optional for F-35 customers.

“As the engine provider and the [sustainment] provider, I'm very interested in keeping everything common,” Bromberg says. “The idea behind the Engine Enhancement Packages is they will migrate into the engines or upgrade over time. We don't have to do them all at once. The [digital engine controls] will understand which configuration. That allows us again to be seamless in production, where I would presumably cut over entirely, but also to upgrade fleets at regularly scheduled maintenance visits.”

Pratt has divided the capabilities from Growth Options 1 and 2 into a series of EEPs, with new capabilities packaged in increments of two years from 2025 to 2029.

“If you go all the way to the right, you get all the benefits of Growth Option 2, plus some that we've been able to create,” Bromberg says. “But if you need less than that and you're shorter on time or money, then you can take a subset of it.”

Meanwhile, the Air Force continues to fund AETP development as a potential F135 replacement. As the propulsion road map is finalized, the JPO will decide whether Pratt's F135 upgrade proposals support the requirement or if a new engine core is needed to support the F-35's thrust and power-generation needs over the long term.

Previously, Bromberg questioned the business case for reengining the F-35 by pointing out that a split fleet of F135- and AETP-powered jets erodes commonality and increases sustainment costs. Bromberg also noted it is not clear the third-stream technology required for the AETP can be accommodated within the roughly 4-ft.-dia. engine bay of the F-35B.

Now Bromberg says he is willing to support the JPO's decision if the road map determines a reengining is necessary. “If the road map indicates that they need significantly more out of the engine than the Engine Enhancement Packages can provide, we would be the first to say an AETP motor would be required,” Bromberg says. “But we think a lot of the AETP technologies will make those Engines Enhancement Packages viable.”

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