The Pentagon’s Defense Advanced Research Projects Agency (DARPA) has launched its Advanced Full Range Engine (AFRE) programme aimed at developing and demonstrating a new aircraft propulsion system that could operate over the full range of speeds required from low-speed takeoff through hypersonic flight.

The ultimate goal is a reliable, affordable system combining turbine and hypersonic engine technologies for seamless transition from low-speed takeoff to Mach 5+ and back.

Engineers have been grappling with two intertwined, seemingly intractable challenges: The top speed of traditional jet-turbine engines maxes out at roughly Mach 2.5, while hypersonic engines such as scramjets cannot provide effective thrust at speeds much below Mach 3.5. This gap in capability means that any air-breathing hypersonic vehicles developed today would use disposable rockets for one-time boosts up to operating speed, limiting the vehicles’ usefulness.

AFRE aims to explore a turbine-based combined cycle (TBCC) engine concept, which would use a turbine engine for low-speed operations and a dual-mode ramjet—which would work efficiently whether the air flowing through it is subsonic (as in a ramjet) or supersonic (as in a scramjet)—for high-speed operations. The two components of the hybrid engine would share a common forward-facing air intake and rear-facing exhaust nozzle to release thrust.

AFRE aims to develop critical technologies and culminate in ground-based testing of a full-scale, integrated technology demonstration system. If that testing is successful, further development of the AFRE technology would require flight testing in a potential follow-on demonstration programme.