DARPA is Returning to Turbine-based Combined Cycle Propulsion System for Routine Hypersonic Flight

A turbine-based combined cycle (TBCC) propulsion system to enable routine hypersonic flight by a vehicle that can take-off and land from a runway is back on the agenda at the Defense Advanced Research Projects Agency (DARPA) after a five-year hiatus.

The experimenting agency has set a “proposers day” on 13-14 July for potential bidders of the Advanced Full Range Engine (AFRE) programme, which is scheduled to launch as a new-start effort in Fiscal 2017.

Combining a turbine engine with a ramjet in the same vehicle has been a dream for the aerospace industry since the early 1950s, when the US Air Force proposed adapting Republic’s concept for the XF-103 fighter with a ramjet to intercept Soviet bombers at speeds up to Mach 5.

But TBCC concepts are limited by a propulsion gap between the Mach 2.5 top speed of a turbine engine and the Mach 3-3.5 minimum speed for a ramjet engine.

In 2009, DARPA attempted to bridge that gap with a high-speed turbine and a low-speed ramjet under the Mode Transition (MoTr) programme, but the project was cancelled two years later. By 2013, Lockheed Martin’s Skunk Works organisation appeared to lobby for a revival of the research effort by releasing a concept for a Mach 6.0-capable SR-72 for high-speed surveillance missions, which was based on a similar TBCC propulsion system.

The AFRE programme now seeks to pick up where MoTr left off, leading to a ground demonstration of a fully integrated propulsion system capable of taking-off from a runway and accelerating beyond Mach 5. The system will include an off-the-shelf turbine engine and a dual mode ramjet/scramjet capable of operating with subsonic or supersonic airflows. Both engines share a common inlet and exhaust nozzle, but transition from turbine to ramjet power at a certain speed over Mach 2.5.

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