http://www.defenseindustrydaily.com/DARPAs-Hypersonic-Vulcan-Engine-Meld-05466/(Credit: darpa)

Logical progression

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It might not be a Vulcan mind-meld, but it’s pretty close. The Department of Defense’s technology brain trust, the Defense Advanced Research Projects Agency (DARPA), has given 4 contractors the go-ahead to develop the advanced Vulcan combination engine system for hypersonic flight. The 8-month first phase features awards to: Alliant TechSystems, General Electric, Rolls Royce, and United Technologies.

The Vulcan engine will integrate a traditional jet turbine engine that performs well at low speeds, with a constant volume combustion (CVC) engine that performs well at higher speeds. The combination will help the vehicles go from standing starts to Mach 4 or so, where hypersonic engines can take over. DARPA’s ultimate goal is to design, build, and fly Mach 6+ re-usable, air-breathing, turbine-based hypersonic vehicles.

What current engines will the Vulcan program modify? What are the program’s goals? What is its structure? DID has answers…

Brayton Cycle engines

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Contractors will use an in-production turbine engine that is capable of operating at or above Mach 2, such as United Technologies’ subsidiary Pratt & Whitney’s F100-229 (in F-16/F-15 fighters) and F119 (F-22A) engines, and GE’s F110-129 (F-16/F-15) and F414 (F/A-18E/F, JAS-39NG) engines. The hope is that picking a proven high-Mach conventional engine will save a lot of money and time, as opposed to past efforts that have tried to develop an entirely new conventional engine for use in hypersonic mixed-cycle systems.

The winning contractors may choose from a number of engine architectures for the CVC portion, such as pulsed detonation engines, continuous detonation engines or other “unsteady” CVC engine architectures. In simplified terms, those “Otto Cycle” CVC architectures burn fuel at a constant volume, with thrust coming from increases in pressure. In contrast, conventional “Brayton Cycle” jet engines burn fuel at a constant pressure, and use the rapidly expanding volume to provide thrust.

DARPA believes that it may be possible to take advantage of each cycle type’s performance strengths, achieving full-spectrum performance and 30-35% gains in fuel efficiency, using a melded “Humphrey Cycle” approach. The Vulcan program envisions a number of possible layouts from side-by-side “dual flow path” turbine and CVC engines, to “annular” systems that share a common fixed air inlet and exhaust nozzle.

Scramjets explained

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The idea is to use conventional jet engines for takeoff, use the combined engines to get up to speed, then switch to CVC/scramjet-only power during hypersonic flight above Mach 4. At those speeds, the conventional turbine will need to be “cocooned,” in order to protect its components from the high heat and pressures generated by hypersonic speeds in the atmosphere. Full low-altitude spaceflight would add an additional set of complications, including the need for cryogenic fuels, for materials that can withstand a broader range of extreme environments during flight and re-entry, and for engines with even broader performance ranges.

Vulcan’s Phase 1 work involves concept definition. Its most critical element is the firms’ technology development plans, designed to reduce the considerable technical risks inherent in any DARPA project. Key technical challenges include efficient start, low total pressure loss detonation initiation devices, low total pressure loss air valves, thermal management systems, efficient nozzles, and control systems, among others. Despite these difficulties, DARPA is encouraged by recent advances in liquid hydrocarbon/air detonation ignition, low total pressure loss detonation transition devices, air valve and nozzle demonstrations, and computer modeling, measurement techniques.

Vulcan tech

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DARPA will use the results of the first phase of the Vulcan program to make decisions regarding future phases.

These could include an 18-month 2nd phase for risk reduction testing of full-scale components, which would conclude with a preliminary design review of the CVC demonstration engine. A 3rd phase would involve 18 additional months for detailed design, fabrication and demonstration of the CVC engine system, with a final 18-month 4th phase to design, build and demonstrate the full Vulcan system.

DARPA’s ultimate hope is to integrate the Vulcan engine into the HTV-3X FALCON/Blackwing vehicle (a.k.a. “SR-72”), and leverage either the engine design or its key concepts and lessons in other full-size hypersonic cruise vehicles.