Scramjet: New York to Sydney in 2 Hours at Hypersonic Speeds

Rocket engine performance from a scramjet engine? That is what the X-51A Flight Test Program project is hoping to create. DARPA and the Air Force Research Laboratory are funding the collaborative effort to achieve mach 6 speeds initially. Eventually they hope to reach mach 15.

Ordinary jets have a major limitation: They can't go faster than Mach 3 without their turbine blades melting. Rocket ships can reach Mach 25, but they have to carry tremendous amounts of liquid oxygen to burn their fuel. The space shuttle, for example, weighs only 165,000 pounds empty, but it must carry 226,000 pounds of liquid hydrogen and 1.4 million pounds of liquid oxygen to reach orbit.

An air-breathing jet engine with no moving, meltable parts, such as a scramjet, can solve these problems. A scramjet is an advanced form of a "ramjet," an engine that takes the air rushing into the engine and "rams" it into the combustion chamber, creating intense pressures that can sustain combustion at the furious rate that Mach-3-plus speeds demand. But ramjets have limits too. The air entering the engine has to be slowed to subsonic speeds for it to run efficiently. And that air is so hot that no matter what measures are taken to cool it, a ramjet-powered craft must stay under Mach 5 to keep from disintegrating.

But a scramjet—a "supersonic combustion ramjet"—changes things. A scramjet does away with the diffuser that a ramjet uses to slow down incoming air, allowing the air to move through the engine at supersonic speeds so it can fly above Mach 5. The tradeoff: A scramjet engine in flight is a delicate system. Achieving balanced combustion at those speeds is an engineering challenge often compared to keeping a match lit in a hurricane.

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The impetus for rapid development of such a craft comes from the fact that the Chinese military has its own project to develop scramjet technology.

Last July, engineers from China showed up at the American Institute of Aeronautics and Astronautics Joint Propulsion Conference in Cincinnati and revealed a growing scramjet research program of their own, including a new hypersonic wind tunnel in Beijing and work on rocket-powered combined-cycle scramjets. None of the American scramjet experts we talked to would discuss their reactions to the Chinese revelations. But Craig Covault, an editor at Aviation Week & Space Technology who reported on the conference, believes one of the main reasons the Chinese attended was to glean all available intel on Western scramjet research. "I would bet that they have a serious research program under way that has a lot more going on than just the few papers that they issued at this forum," Covault says. "The reason that they issued them was just kind of a message to the rest of the world that they are engaged in these high-tech things. It also allowed them to get the 500 or more other papers in propulsion technology of all kinds delivered at the conference."

Whether or not the Chinese are actually as far along as they say, the US is determined to proceed with the hypersonic engine development. Being able to deliver a large weapons payload, or a squad of well-equipped special forces troops anywhere in the world in under 2 hours, is a temptation the US military cannot resist.

In terms of space launch capability, using a scramjet first stage to boost a rocket powered orbital second stage might reduce launch costs to orbit by more than half. Using such a system to launch sensitive payloads such as humans into orbit, and launching non-sensitive materials by laser launch, electromagnetic launch, or other cheap and high-g launch method, may work out to a combination that gets more people into earth orbit, lunar space, and la grange orbits.