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DARPA revives turbine-ramjet concept for hypersonics

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.

“This won’t be the first time that ambitious engineers will attempt to combine turbine and ramjet technologies. But with recent advances in manufacturing methods, modeling, and other disciplines, we believe this potentially groundbreaking achievement may finally be within reach,” says Christopher Clay, DARPA programme manager.

The programme could benefit from other recent experiments, including the Boeing X-51 Waverider programme funded by the Air Force Research Laboratory. The X-51 completed the first flight tests of a ramjet powered by hydrocarbon fuel, which also served as a coolant. The X-51, however, required a disposable rocket — a booster stage from the Army Tactical Missile Systems (ATACMS) — to accelerate to Mach 4.0, where the ramjet took over.

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