Pentagon NGAD Uncertainty Pressures Adaptive Engine-Makers To ‘Adapt’

After decades of development and billions of dollars of investment, the destiny of U.S. adaptive combat engine technology hangs in the balance as the U.S. Air Force reconsiders requirements for its intended initial application, the Next-Generation Air Dominance platform.

Offering a step change in performance over conventional fixed-cycle designs, the adaptive engine’s ability to transition automatically between fuel-efficient and high-thrust settings has been heralded as the biggest game changer in military propulsion since the invention of the turbofan. Adaptive engines achieve this by continually varying the two main propulsion parameters that affect thrust and specific fuel consumption: bypass ratio and fan pressure ratio.

Before mid-June, the Next-Generation Air Dominance (NGAD) program and the propulsion systems being groomed to power it appeared to be on a firm path toward fielding at the end of the decade. But as Boeing or Lockheed Martin came within weeks of being awarded a full-scale airframe development contract, Air Force leaders suddenly expressed second thoughts and pressed the pause button.

After reconsidering undisclosed threats and requirements for the NGAD platform, the Air Force put plans for a contract award on indefinite hold in late July. As a result, uncertainty also hovers over the Air Force’s parallel Next-Generation Adaptive Propulsion (NGAP) program, which funds development of the GE Aerospace XA102 adaptive turbofan and the competing Pratt & Whitney XA103.

Amid the NGAD turmoil, Air Force Secretary Frank Kendall has indicated that the issue is partly due to the cost and complexity of NGAP engines. He has strongly suggested that the engine-makers study potential alternatives that are simpler and smaller.

Other insiders, meanwhile, warn that the nearly unrivaled position of the U.S. in high-performance military engines could be eroded if initiatives such as NGAD and NGAP stall. “The U.S. Air Force is at a key inflection point when it comes to stewarding this technology,” says Douglas Birkey, executive director of the Mitchell Institute for Aerospace Studies. “Sustaining existing designs is not the same as developing, fielding and producing new technology. The Air Force knows this, and that is why it has invested in multiple programs to pursue new engines that deliver enhanced performance, reliability and efficiency attributes necessary to meet current and future mission requirements.

“However, R&D investment must eventually transition to production in volume for tangible results to positively shape the operational realm,” Birkey continues. “Far too many propulsion programs and their related efforts, like NGAD, have been canceled and or curtailed. This is a competitive realm, and adversaries are pressing hard. It’s crucial that the Air Force, [Pentagon] and Congress take this into account as they consider the future of these programs.”

The engine-makers have already developed and tested variable-cycle engines (VCE) for two thrust requirements—a 45,000-lb. powerplant suited for the Lockheed Martin F-35 and a lower-thrust variant for NGAD—and say they are prepared to be adaptable.

GE, the main driving force behind the service’s two-decade investment in adaptive propulsion, has emphasized the inherent flexibility of the VCE three-stream engine technology as NGAD platform requirements evolve. “We’re as anxious as anyone to see which way they go,” GE Defense and Systems CEO Amy Gowder tells Aviation Week. “We stand ready for the government to figure out their form factor.”

Despite the platform-level uncertainty, Gowder asserts that a portfolio with two adaptive turbofans of different sizes offers an advantage as the Air Force determines final requirements for the NGAD platform. Having two options redefines the term “adaptive” to apply to sizing the engine for thrust, too.

GE’s XA100 “is actually a much larger engine, but it’s certainly available,” she says. “The XA102 kind of scales down. Whichever way they land on their requirements, we believe we have technology that’s relevant.”

The XA102 and Pratt’s XA103 follow a $6 billion technology investment plan the Air Force launched in 2006. That path most recently produced the 45,000-lb.-thrust GE XA100 and Pratt XA101 under the Adaptive Engine Technology Development (AETD) program. Both engines entered development in 2016 to become candidates to reengine the F-35. But the Air Force ultimately decided to upgrade the core of the incumbent Pratt F135. GE completed a fourth series of ground tests on the XA100 last year.

“We are hitting the max of the fourth-generation or even, you could argue, the fifth-gen engines,” Gowder says. “You can keep doing some materials improvements, but this architecture—the ability to cruise like an airliner, and without pilot intervention other than pulling the throttle, you’re able to fly like a fighter—I think that technology will ultimately make its way into the solution and ideally sooner than later to keep our asymmetric advantage.”

Overall, GE remains optimistic. “The Department of Defense needs advanced engine capabilities, regardless of the platforms they power, and we are delivering on that front,” GE said in a statement. “We continue to see bipartisan support in both the House and Senate for the development and testing of adaptive-cycle engines to ensure our ability to maintain U.S. air superiority.”

Pratt & Whitney completed a critical design review of the XA103 with the Air Force in the first quarter and is considering options as it awaits direction from the government. “We have 500-plus people working on NGAP today,” says Jill Albertelli, president of Pratt & Whitney Military Engines. “We will support the timeline that our customer needs with all of that. So at this point in time, it is something that we are continuing to carry forward until we are told something different.”

In terms of new engine designs that could emerge from the ongoing Air Force deliberations, Albertelli says: “As far as shape and size, we always base things on requirements. We’re working to the set of requirements that we have today, and we’ll wait and see if something changes, and we’ll adapt—no pun intended—accordingly.”

Pratt is “working closely with the U.S. Air Force on exactly what they want to do from a hardware standpoint,” she says, referencing the disposition of test assets, such as the XA101. “But we have all that knowledge and learning and have completely accomplished our objectives for that program, which is phenomenal, to tell you the truth.”

Several advances gained through NGAP—particularly in hot-section technologies, such as materials, coatings, cooling and systemwide thermal management—also are transferable to the F135 engine core upgrade (ECU) program. “Some of the technology will go right into the ECU, and the balance of it goes into NGAP for our sixth-generation propulsion system,” Albertelli says. “A lot of experts over the years have worked on the development of these. As we work the NGAP program, those experts are able to transition from the success of the current program over to these other areas.”

A key element that Albertelli highlights is NGAP’s focus on the advanced use of digital tools. “We’re demonstrating it every single day,” she says. “It might be certainly for design, but for sixth-generation, it should be able to help you prioritize what your testing will be. Some things I would even say, from a design standpoint as a former engineer, you can just do it faster. So that’s very key in this entire approach. We’ve received very positive feedback from the Air Force on how we’re engaging and really working digital modeling throughout.”