Powering the future fight: inside sixth-generation fighter engines

Now, with the United States Air Force pursuing a sixth-generation combat aircraft under the Next Generation Air Dominance (NGAD) program, propulsion is once again at the centre of strategic and technological competition.

The Next Generation Adaptive Propulsion (NGAP) program represents the culmination of more than a decade of research into adaptive cycle engines, a revolutionary approach designed to overcome the traditional compromises between range, thrust and thermal performance.

NGAP is intended to power future air superiority platforms and enable capabilities that will define air warfare well into the mid-21st century.

From adaptive engine transition to NGAP

NGAP’s origins lie in earlier experimental efforts to fundamentally redesign fighter propulsion.

Traditionally, jet engines operate with a fixed bypass ratio, the proportion of air routed around the engine core versus through it. This forces a compromise: engines optimised for high thrust sacrifice efficiency, while those optimised for fuel efficiency lack peak performance.

Adaptive cycle engines resolve this dilemma by incorporating a third airflow stream, allowing the engine to dynamically shift between operating modes.

In high-efficiency mode, the engine behaves more like a turbofan, maximising fuel economy and extending range. In high-thrust mode, it behaves more like a turbojet, maximising acceleration and combat performance.

This concept was matured under the Adaptive Engine Transition Program (AETP), which produced two demonstrator engines: General Electric’s XA100 and Pratt & Whitney’s XA101.

These engines demonstrated transformative improvements, including approximately 25–30 per cent improved fuel efficiency, significantly increased thrust, and dramatically improved thermal management capacity.

Originally intended to replace the F135 engine in the F-35 Lightning II, the AETP engines ultimately proved too large and complex to integrate across all F-35 variants, particularly the short take-off and vertical-landing F-35B and carrier-capable F-35C. Retrofitting the entire fleet would have imposed substantial costs and logistical complexity.

Instead, the Air Force and Pentagon elected to pursue a more incremental upgrade to the F-35’s existing engine while redirecting adaptive engine development toward NGAP, where the technology could be fully integrated into a new aircraft designed around its capabilities.

NGAP’s role in the sixth generation of air combat

NGAP is fundamentally tied to the NGAD fighter program, which is intended to replace the F-22 Raptor as the cornerstone of US air superiority. Unlike previous fighters, NGAD is being developed as part of a broader “family of systems” that integrates crewed aircraft, autonomous platforms, advanced sensors, and networked weapons.

In this environment, propulsion becomes far more than a source of thrust. It becomes a central enabler of the aircraft’s entire combat system. Future combat aircraft will require unprecedented electrical power to operate advanced radar systems, electronic warfare suites, secure communications, and, potentially, directed-energy weapons.

These systems generate significant heat, which must be managed without compromising stealth or performance. Adaptive engines provide the thermal management capacity necessary to support these systems.

By diverting airflow to cooling functions when needed, they allow aircraft to operate more powerful electronics while maintaining stealth and survivability.

Equally important is range. The vast distances of the Indo-Pacific theatre place enormous demands on combat aircraft endurance.

Improved fuel efficiency directly translates into increased combat radius, allowing fighters to operate further from vulnerable forward bases.

Industrial competition – GE versus Pratt & Whitney

NGAP is being developed through a competition between two of America’s premier propulsion companies: GE Aerospace and Pratt & Whitney.

Both companies received contracts to design, build and test prototype engines, designated XA102 and XA103, respectively.

These engines build directly upon the lessons and technologies developed under AETP, incorporating advanced materials, digital design processes and adaptive cycle architectures.

Digital engineering has been particularly important in accelerating development. Engineers now use advanced modelling and simulation tools to design and validate engines before physical prototypes are built, reducing risk and shortening development timelines.

Both companies have completed major design milestones, and prototype construction and testing are expected later this decade.

Maintaining competition between the two firms is a deliberate policy choice. The Air Force and Congress view competition as essential to controlling costs, encouraging innovation and ensuring industrial resilience.

Political and budgetary uncertainty

Despite its technological promise, NGAP faces significant political and budgetary uncertainty.

The NGAD aircraft program itself had been temporarily paused for review until the recent contract award to Boeing amid concerns about cost and changing strategic priorities. Estimates suggest that sixth-generation fighters could cost significantly more than current fifth-generation aircraft, raising questions about affordability even for the US.

Some policymakers and defence analysts have argued that increasing reliance on autonomous systems and collaborative combat aircraft may reduce the need for expensive crewed fighters.

This uncertainty has implications for NGAP, which was originally conceived specifically to power NGAD. Without a defined aircraft platform, the engine program risks losing its primary application.

However, the Air Force and Congress have continued to fund NGAP development, recognising its importance to future combat capability and technological leadership.

Propulsion technology is widely regarded as a strategic capability that must be preserved, regardless of specific aircraft programs.

Maintaining expertise, industrial capacity and technological momentum is seen as essential in an era of renewed great-power competition.

Implications for fifth-generation aircraft, Raider and next-gen long-range strike

While NGAP is primarily intended for sixth-generation aircraft, its technological influence will also extend to existing fifth-generation platforms.

The F-35 Lightning II is undergoing a major engine upgrade to improve power generation and thermal management, enabling it to support new sensors, weapons and electronic warfare systems.

Although this upgrade does not incorporate a full adaptive cycle engine, it leverages technologies developed through adaptive engine research, including improved core efficiency and enhanced cooling capacity.

These improvements will allow the F-35 to remain effective in increasingly contested environments, extending its operational relevance for decades.

Adaptive engine technologies may also influence future upgrades to other aircraft, including bombers and unmanned systems.

Meanwhile, the B-21 Raider stealth bomber represents another potential application for adaptive propulsion technologies.

Although the B-21 is currently powered by conventional engines, future variants or upgrades could incorporate adaptive engine technologies to improve range, survivability and power generation.

This would enhance the bomber’s ability to operate deep within contested airspace while supporting advanced sensors and electronic warfare systems.

Adaptive propulsion could also benefit unmanned systems, enabling longer endurance and improved performance for collaborative combat aircraft and other autonomous platforms.

Strategic importance in great power competition

The development of adaptive engines is taking place amid intensifying technological competition between the US and its strategic rivals.

China, in particular, has invested heavily in jet engine development as part of its broader military modernisation effort. Historically dependent on foreign engine technology, China has made significant progress in developing indigenous engines for its latest combat aircraft.

Maintaining a technological advantage in propulsion is therefore seen as critical to preserving US air superiority.

Adaptive engines offer a combination of range, performance and survivability that could provide a decisive operational advantage in future conflicts.

Transforming combat aircraft design

Perhaps the most significant impact of NGAP will be its influence on aircraft design itself.

Historically, aircraft have been designed around the limitations of their engines. Adaptive engines reverse this relationship, enabling designers to create aircraft with unprecedented performance characteristics.

An increased range reduces dependence on vulnerable forward bases and on tanker aircraft. Improved thermal management enables more powerful sensors and electronic warfare systems. Greater thrust improves manoeuvrability and survivability.

These capabilities collectively enhance the effectiveness and survivability of combat aircraft in highly contested environments.

Toward the future

The NGAP program remains in development, with prototype testing expected later this decade and operational deployment likely in the 2030s.

Its ultimate success will depend on continued funding, political support and the successful development of sixth-generation aircraft platforms.

Even if NGAD evolves, the technologies developed under NGAP will almost certainly influence future aircraft.

Adaptive propulsion represents a fundamental shift in engine design, comparable to the transition from turbojets to turbofans in the mid-20th century.

Undeniably, the Next Generation Adaptive Propulsion program represents one of the most important and consequential developments in modern military aviation.

By enabling unprecedented combinations of range, power, efficiency and thermal management, adaptive engines will play a central role in defining the capabilities of future combat aircraft.

While political and budgetary uncertainty remain, the strategic logic behind NGAP is clear. In an era of renewed great-power competition, propulsion technology will remain a key determinant of air superiority.

Whether powering sixth-generation fighters, advanced bombers or future autonomous systems, adaptive engines will shape the future of air combat.

In the contest for technological dominance in the skies, propulsion remains the ultimate enabler, and NGAP represents the next decisive step forward.