Adaptive cycle engines are specially designed to alternate between efficiency and raw power—extending an aircraft’s range without compromising on speed during engagements.

The modern fighter jet is built around its engine, which has a disproportionate influence on the plane’s range, survivability, payload, and maneuverability. In the past, engine design has forced engineers to select a tradeoff—speed and thrust versus efficiency and range.

However, emerging adaptive cycle engines have the potential to relax the tradeoff, offering fighter jets variable performance profiles based upon the specifics of the circumstances. Increasingly central to NGAD discussions, and to fifth-generation fighter upgrade discussions, adaptive cycle engines seem poised to be a fixture of the near-future of air combat.

What Are Adaptive Cycle Engines?

Adaptive cycle engines are variable-bypass turbofans that can shift between high-thrust, low-bypass mode and efficient, high-bypass mode. Often described as “three-stream” engines, the adaptive cycle allows for better thermal management and improved fuel efficiency. Unlike fixed-cycle engines, adaptive cycle performance adapts to the mission phase. Essentially, the idea is to have one engine that can operate within multiple optimized regimes. 

The concept has been explored since the 1960s and 1970s. But early barriers in materials, control systems, and reliability limited their application. Digital engine controls (FADEC) improved the technology’s viability over time, as did composites and high-temperature materials. Renewed strategic urgencies for longer ranges and higher electrical loads and hypersonic-adjacent performance have also served to push adaptive cycle engines to the forefront of fighter jet innovation in recent years.

How Do Adaptive Cycle Engines Actually Work?

The key components of an adaptive cycle engine are a variable geometry fan system and adaptive flow paths. These components allow an engine to route air for thrust—or divert the air for cooling and efficiency—as needed, depending upon the circumstances. The benefits here include reduced fuel burn in cruise, but also increased thrust during combat—the best of both worlds, so to speak.

Thermal management is critical in such an engine, which has immense control complexity, requiring software to oversee the engine’s moment-to-moment operations. Adaptive cycle engine integration is highly coupled with the airframe and its power systems. Though complex, the upside is significant; adaptive cycle engines permit greater combat radius while allowing more electrical power to be used for sensors and electronic warfare operations. Adaptive cycle has better performance in hot-and-high conditions and can enable new mission concepts, including longer-range penetration and extended loiter in contested airspace. Of course, all of these upsides are especially valuable in a geographically expansive theater like the Indo-Pacific—which is a large part of why adaptive cycle engines have been getting so much attention lately.

The Downsides of Adaptive Cycle Engines

The technology does have tradeoffs. The engines are extremely complex, which increases cost and maintenance burdens. More moving parts and more oversight software necessarily correlates with reliability problems. And retrofitting will be a challenge, as existing airframes are not designed for adaptive cycle engines. Questions linger about whether such a heavy investment in new engine technology will be worth the performance gains. 

Still, expect adaptive cycle engines to be a serious candidate for use in the upcoming NGAD fighter, entering service in the 2030s. Adaptive engines would not serve as a panacea to the strategic complications of operating in the Indo-Pacific, but the technology could extend reach and permit flexibility with respect to force structure and basing options. 

About the Author: Harrison Kass

Harrison Kass is a senior defense and national security writer at The National Interest. Kass is an attorney and former political candidate who joined the US Air Force as a pilot trainee before being medically discharged. He focuses on military strategy, aerospace, and global security affairs. He holds a JD from the University of Oregon and a master’s in Global Journalism and International Relations from NYU.

Image: Shutterstock / Minh K Tran.

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