Adaptive-cycle fan engines

TECHNOLOGY

Chris Kjelgaard spoke with GE Aviation about its adaptive-cycle fan technologies for future fighter aircraft turbofan engines

ADAPTIVE CYCLE ENGINES

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Captain Andrew Olson, F-35 Heritage Flight Team commander and pilot pulls a tactical pitch in an F-35A Lightning II using the immense power of the Pratt & Whitney F135 engine. Airman 1st Class Alexander Cook/US Air Force

New $437 million contracts awarded by the US Air Force Life Cycle Management Center (AFLCMC) this summer to GE Aviation and Pratt & Whitney allow the two companies for the first time to design and develop adaptive-cycle fan technologies for turbofan engines for future US Air Force air superiority aircraft beyond the F-22 and the F-35.

Announced by the AFLCMC as contract modifications “for next-generation adaptive propulsion risk reduction for potential air superiority applications”, the scope of the two new awards goes beyond previous $1 billion adaptive-cycle fan R&D contracts GE and P&W were awarded in June 2016, according to Dan McCormick, GE Aviation’s general manager for advanced combat engine programs.

Those 2016 contracts were offcially awarded under the US Air Force Research Laboratory’s (AFRL’s) Adaptive Engine Transition Program (AETP) to develop adaptive-cycle fan engines for unspecified sixth-generation US Air Force fighter aircraft – but in fact they very much had in mind a potential replacement for the P&W F135 engine which today powers all Lockheed Martin F-35s.

The 2016 $1 billion AETP contracts – due to run until 2021 – followed on from the adaptive-cycle fan R&D work GE began in 2007 under the US Air Force’s Adaptive Versatile Engine Technology (ADVENT) programme and the subsequent additional adaptive-fan development contracts GE and P&W were awarded in 2012 under the AFRL’s Advanced Engine Technology Development (AETD) programme, which ran until 2017.

All of that work (and the R&D work performed under the follow-on AETP programme) has focused on designing turbofan engines which create not just two airstreams – the core airstream flowing through the engine and the bypass airstream flowing around it – but also a third engine airstream. The basic idea behind the adaptivecycle fan concept is to make flexible – “adaptive” – use of this third airstream to improve the engine’s efficiency during every phase of flight, either by boosting its thrust when more airspeed is required from the aircraft or by increasing its fuel-efficiency when more range is required.

A potential F-35 replacement engine

GE and P&W originally said the AETD and AETP contracts specified purely as an initial design-reference point that the adaptivecycle fan engines developed under AETD and AETP should fit into the same space in the F-35 as its existing 45,000lb-thrust F135. However, while P&W declined to grant AIR International an interview for this article, GE’s McCormick confirmed that the 2016 contracts specifically envisaged GE and P&W developing competing designs – respectively designated the XA100 and XA101 – for an engine potentially to replace the F135 in the F-35 from the mid-2020s.

Both companies continue to perform technology-maturation work on that replacement engine under their respective 2016 AETP contracts. According to McCormick, in GE Aviation’s case the work has progressed to the point where GE has already sent manufacturing orders to its suppliers for parts for its final AETP design. At an unspecified time in the next three years GE’s AETP engine design will be the subject of a detailed design review (DDR) by the AFLCMC. However, the “additional [$437 million AETP] programme extends a little further into the future” than the duration of the initial AETP contract and is not focused on the F-35, said McCormick.

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“The diff erence between this [new $437 million award] and the previous [$1 billion AETP] contract in summer 2016 is that we’re looking at these technologies for potential use on other platforms,” said McCormick. “We are validating technologies so they could be applicable to other aircraft.” While one potential application could theoretically be “an engine change for the F-35 … we’re more focused on looking at these technologies for future air superiority” requirements, he said. “We’re burning down [technological] risk on engine sizes in other aircraft. With the new contract, we are looking at air superiority-type aircraft [which are] specific to the current [US] Air Force in looking at what the future mission-capability requirements might be.”

GE’s progress under AETP phase one

As they begin work under the new phase two of their AETP contracts, GE and its competitor P&W continue to refine their F-35-sized adaptive-cycle fan designs under the first phase of the programme. From GE’s viewpoint, “the news is that [the company has developed] a whole sequence of successful module designs over the last couple of years, based on testing, not just model analysis,” said McCormick. “It was a big accomplishment of the team in getting those extremely successful tests run.”

Under its original 2007-2012 ADVENT contract (the AFRL did not contract P&W), GE ran a rig test of an adaptive-cycle fan module it had designed, after which it tested an engine-core design and then ended the programme by completing a successful test of an entire adaptive, third-airstream engine. Its subsequent 2012-2017 AETD contract saw GE completing rig tests of an F-35-sized adaptive-cycle fan module and a compressor module, after which it tested an adaptivecycle F-35 engine core in the summer of 2017. All three tests were “highly successful” and allowed GE to pass with flying colours the AFRL’s preliminary design review of its AETD engine design, said McCormick, adding that its AETD R&D now “is informing the [company’s] work for AETP.”

In a “very robust test programme” conducted for AETP phase one, GE Aviation has already successfully completed rig tests of all the major modules of its F-35- sized adaptive-cycle fan engine design, according to McCormick. Modules already tested include the engine’s fan module (which includes its “booster” low-pressure compressor stages), its combustor, its turbine module (which includes its high-pressure and low-pressure turbine stages) and the engine’s exhaust module, which includes the augmenter and exhaust nozzle. GE is also now “doing some regression testing [of modules] where we want to modify and retest to get the change in performance we’re looking for”, he said.

Full-engine tests

After that, “over the next couple of years,” GE Aviation will build and run three entire F-35-sized adaptive-cycle fan engines, said McCormick. It will number them as adaptivecycle fan engines two, three and four, because GE long ago successfully designed and ran its first full adaptive-cycle fan engine design under its 2007-2012 ADVENT contract.

As with most test programmes for new turbine engines, the company will focus on three major test areas in its AETP full- engine testing: system-validation testing, performance testing and endurance testing. Each of the three engines will primarily be dedicated to one of the three major areas of testing, according to McCormick, though “a mix” of engines will be used to conduct some tests because each one will contain common instrumentation required for certain of the major test areas.

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GE Aviation’s Adaptive Engine Technology Demonstrator core engine at its rollout. GE Aviation
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Captain Andrew Olson, F-35 Heritage Flight Team pilot and commander, performs a high-speed pass with the afterburner of the Pratt & Whitney F135 engine engaged. The vapour cone shrouding the aircraft is a cloud of condensed water which forms around the aircraft when flying at transonic speed through moist air. Airman 1st Class Alexander Cook/US Air Force

The systems-validation testing will include assessment of the adaptive-cycle engine’s basic turbo-mechanical design; how well its oil system works; analysis of its aeromechanical/aerodynamic properties; and functional checks of the engine’s computerised control system. Performancetesting of the design will involve “very detailed performance testing on thrust and fuel-efficiency”, while the endurance testing “will give us a first look” at how well the engine performs during long runs, according to McCormick.

Assuring the supply chain

While the likely date of the AFRL’s detailed design review (DDR) of GE Aviation’s potential F135-replacement adaptivecycle fan engine remains classified, it is “fair to say we have made significant, substantial detailed-design progress”, said McCormick. In fact, by September GE Aviation had made enough progress on its AETP design to release “a significant number of parts … into the supply chain” for fabrication and manufacturing.

As might be expected from a company which has already demonstrated very considerable expertise in designing and manufacturing ceramic matrix composite (CMC) parts for the hot sections of military and civil turbofan engines, GE Aviation is using CMC parts in its AETP engine design. McCormick also confirmed that the company is making use in its AETP design of parts made using additive-manufacturing techniques, another area of advanced manufacturing technology in which GE has already demonstrated strong competence. “There is a significant amount of both [types of part] in this design,” he said.

“The [US] Air Force has been highly interested in CMCs and additive manufacturing. Proving manufacturing technologies is a big part of the riskreduction programme,” McCormick added. “There is a whole sequence of a couple of dozen areas where the Air Force is working with us on manufacturing readiness. It’s not unusual – they’re just assuring the supply chain. The Air Force has been working with us not only to understand the technology maturation but also on manufacturing readiness, on assessing our capabilities over time to build a larger volume of parts in repeatable fashion.”

Thermal management capabilities critical

The US Air Force’s original AETD and AETP design criteria called for an adaptive-cycle fan engine which would offer 10% to 20% more thrust than today’s F135 engine and provide the F-35 with a range increase of more than 30%. However, the ability to remove from an F-35 or other future US Air Force airsuperiority aircraft the massive amounts of heat generated by its avionics and weapon systems has become just as important a performance factor for any new adaptivecycle fan engine as thrust and fuel-burn, according to McCormick.

“Thermal management is an extremely important part of the programme – from my perspective, it has become at least an equal to us as fuel-efficiency,” said McCormick. “Even in current aircraft – the F-22 and the F-35 – the amount of avionics creates a very large amount of heat. That is difficult to get off the aircraft.” For performance reasons, modern air-superiority aircraft can’t be designed to have big air scoops or other mechanisms protruding from their fuselages or wings to cool down the aircraft, “so having an avenue to get the heat off the aircraft is becoming very, very critical,” he said.

But “how do you do that?” asked McCormick. “This is the new science of engine architecture. You can’t just take [compressor] bleed air and run it through a heat exchanger. How do you accomplish thermal management without [incurring] measurable penalties for [thrust] performance and fuel burn?” Historically, fighter engine designers have only had to maximise an engine’s thrust and then perhaps consider its fuel-efficiency, he said: “For the F-16, it was thrust, and fuel-efficiency was secondary, [but] range is now as important as thrust [and] thermal management is becoming an equal third player. You have to solve for three equations and those are the drivers of the [engine’s] architecture and controls.”

What’s next?

GE Aviation and Pratt & Whitney are still maturing their technologies for the F-35- size AETP engine and as yet “the [US] Air Force has not made any decisions regarding an EMD (engineering, manufacturing and development) phase” for an adaptivecycle fan engine for the F-35 or any other future air-superiority aircraft, noted McCormick. “The F-35 is the obvious near-term opportunity. The first decision will be, will an adaptive engine go into the F-35? If yes, then the Air Force will need to define an acquisition strategy and [decide] if it will down-select” to contract just one manufacturer to provide that engine.

If the US Air Force were to choose GE Aviation to supply a production adaptivecycle replacement engine for its F-35s, GE would be in a position to provide it very quickly, said McCormick. Because the F -35 is now well into operational service after going through a very exhaustive test and evaluation programme, during which its strengths and weaknesses became well understood, “We’re really in a different phase” now regarding the design and development of a potential production adaptive-cycle fan engine for the F-35.

“The definition of the aircraft is evolving over time, so we’re going through design iterations with contractors and the [US] Air Force to evolve the requirements” for the engine, said McCormick. “We have been working with the F-35 design since 2012 and [GE’s adaptive-cycle F-35 engine design] has moved forward rather rapidly. We have had a highly integrated relationship with Lockheed Martin as we have gone through AETD and AETP [and] all of the F-35 operational lessons learned are being rolled into our programme to enhance that capability.”

In the F-35, “thermal management has been a challenge,” McCormick acknowledged. “We believe our new architecture can take those thermal-management challenges away and give [F-35 operators] growth capability.”

GE’s adaptive-cycle turbine research

For the US Air Force, beyond even an adaptive-cycle fan engine for the F-35 or any future air superiority fighter there lies an even more enticing prospect – that adaptivecycle fan technologies might be combined in a future engine with potential adaptivecycle turbine technologies to produce even greater thrust, fuel-efficiency and thermalmanagement benefits.

In the past year or two Pratt & Whitney has been willing to admit publicly – but not discuss in any great detail – that it has been working on adaptive-cycle turbine technologies and that it thinks those technologies could offer future fighter engines important performance benefits once they are mature enough to incorporate in production engines.

McCormick was quick to confirm that no adaptive-cycle turbine technologies are involved in GE’s 2021 AETP programme and that none of the three full adaptive-cycle fan engines it has now begun manufacturing won’t include any adaptive-cycle turbine features. But he was also willing to confirm to AIR International that GE Aviation is working on “adaptive-core” programmes that include such technologies. It is very likely that GE’s adaptive-cycle turbine R&D is still at an early developmental stage: McCormick declined to say how long the company has been working on those programmes and who is funding them. But for the US Air Force and other US and allied foreign air arms, the knowledge they exist must be alluring.