US Air Force officials have taken the first concrete step towards defining a new class of adaptive jet engines to power the next generation of combat aircraft that come after the Lockheed Martin F-35.

A $437 million contract modification awarded to GE Aviation on 29 June also draws the first sharp line between an ongoing effort to develop a 45,000lb-thrust adaptive engine replacement for the F-35 fleet and a follow-on series of engines designed for the still-undefined aircraft that will replace the Lockheed F-22.

Pratt & Whitney, the powerplant supplier for the F-35 and F-22, also is expected to receive a similarly sized contract modification to develop a competing engine design for a future air superiority aircraft.

Both GE and P&W are already working on a related but separate development effort called the Adaptive Engine Transition Programme (AETP). The AETP was described when it was announced in 2016 as an effort to develop and test adaptive engines for a sixth-generation fighter propulsion system, with the possibility of re-engining the F-35 with a more powerful and fuel efficient alternative to the P&W F135.

But the new award clarifies that the competing AETP engines — embodied by GE’s XA100 and P&W’s XA101 demonstrators — are focused on a potential bid to re-engine the F-35 in the mid-2020s.

The new contract modification for GE, meanwhile, funds “next generation adaptive propulsion risk reduction for air superiority applications”, the Department of Defense states in the 29 June contract award.

In an interview with FlightGlobal, Dan McCormick, GE’s general manager for the Advanced Combat Engine Programme, agrees that the AETP demonstrators are “F-35 design-centric”. The new programme awarded in June is aimed at the next generation of aircraft, he says.

In keeping with the USAF’s secretive approach to defining the next air superiority fighter, critical details of the new programme — including its work scope and name — are not released.

“There is a significant amount of design work planned in the programme,” McCormick offers. “Because of its classification, I can’t talk about detailed content.”

It is clear that the unnamed programme features adaptive engine technology. In this context, that means an engine that can vary the volume of air flow that bypasses the core. By opening a “third stream” of air flow in cruise mode, the USAF believes such an architecture can improve specific fuel consumption of the engine by 25%, increasing range and reducing in-flight refueling requirements.

The USAF and the US Navy have been pursuing adaptive engine technology since the launch of the Adaptive Versatile Engine Technology (ADVENT) programme in 2007. The follow-on adaptive engine technology demonstrator (AETD) programme started in 2012. Four years later, GE and P&W started to work on the AETP demonstrators.

GE plans to deliver the XA100 demonstrator’s first engine to test next year under the AETP programme, McCormick says. In addition to adaptive bypass airflow, the XA100 will feature ceramic matrix composites (CMCs) in the rotating high pressure turbine blades, allowing GE to use higher temperatures or reduce cooling loads in the engine design.