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When Luke Skywalker jumps into an X-wing fighter and flies off into space in the Star Wars movies, he's performing a feat that is impossible today. Current orbital launchers are large, multistage affairs that combine the thrust of a series of throwaway rocket stages to escape Earth's gravitational clutches and send a small payload into the void above.
How much simpler and cheaper it would be if one vehicle could accomplish the same task and return intact, again and again. Such a reusable single-stage-to-orbit (SSTO) craft might finally enable regular, truly affordable access to space. But a practical SSTO vehicle is no easy objective. For decades it has been the holy grail for the many rocket scientists and engineers who have tried—and failed—to develop one.
The main difficulty lies in designing and building a vehicle light enough and powerful enough to reach orbit with a viable payload, explains Alan Bond, one of the more persistent of the latter-day grail questers. "Existing technology can't hack it," Bond asserts, "but it's not off by much."
Almost two decades ago, Bond and two veteran research colleagues formed a small R&D firm in Oxfordshire, England. Their company, Reaction Engines, Ltd., set out to develop an SSTO space plane called Skylon. The ambitious design is based on a novel hybrid jet–rocket engine concept that just may provide the necessary boost. Today, a dozen Reaction researchers are working to perfect the engine's key technologies.
Although many aerospace observers consider the effort altogether quixotic given the daunting technical and economic barriers involved, others are cautiously hopeful. "The hybrid engine concept goes back almost a half century, and no one could make it work efficiently," says David Whalen, chair of the space studies department at the University of North Dakota (U.N.D.). "But make it work and you have a new era in access to low Earth orbit space."
Most conventional launch boosters employ weighty cryogenic propellants—liquid hydrogen and liquid oxygen—but Bond realized back in the 1980s that atmospheric oxygen might feed a jet engine for the lower-altitude part of the journey.
The fundamental problem, he says, is that conventional jets become impractical at velocities exceeding about Mach 2.7 (2.7 times the speed of sound); the inflowing air slows rapidly on entering the engine and generates more heat than most available materials can withstand.
In addition, at speeds above Mach 2.7 the incoming air is so hot that "you can't do any useful work on it," Bond says. A standard jet engine employs spinning compressor fans to pressurize inflowing air so that it releases large amounts of energy when mixed with fuel and burned. "Hot air won't do the job," he continues, "so you must precool the air before it enters the compressor, a concept that had been lurking around since the 1960s." Bond conceived the idea of using the ultracold liquid hydrogen fuel as a heat sink to take the excess heat out of the incoming air and use some of the hot air to support fuel combustion. At the fringes of space, the dual-mode power plant would switch to a conventional rocket engine, drawing on the liquid hydrogen and a small supply of liquid oxygen to propel the winged craft into orbit at a final speed of Mach 25.
The hybrid propulsion concept was attractive enough that in the mid-1980s Britain's BAE Systems and Rolls-Royce seized on it for an SSTO space plane project called HOTOL (horizontal takeoff and landing). But after HOTOL was canceled in the late 1980s, Bond, Richard Varvill and John Scott-Scott established Reaction Engines to carry on. Bond serves as the firm's managing director, Varvill is technical director and chief designer, and Scott-Scott is engineering director.
Soon the team will begin a series of milestone ground tests using a scaled-down precooler unit and a Viper turbojet engine. If the trials are successful, they could open the way for full-scale development of the air-breathing rocket engine.
As currently envisioned, Skylon would cost an estimated $10 billion to develop, but Bond claims its operational cost per kilogram of orbital payload would be one fiftieth that of current vehicles. So far, the project has consumed about $7 million in private and public funds, and this week the European Space Agency kicked in another $1.25 million. "We hope to complete demonstrating the critical technologies in three years," Varvill states. After that, Reaction Engines will seek to establish a public–private partnership to build the prototype.
Pablo de Leon, Whalen's colleague at U.N.D., shares his co-worker's views: "The concept might work. If it does, Reaction Engines will reach an SSTO vehicle before anybody else." And, he says, the engine's development seems to have advanced of late. But financing high-altitude tests will be extremely difficult, de Leon notes: "Funding to reach that level of maturity will be a challenge as difficult as the technical ones, or even greater."