Riding the wave

By Charles Seife in Washington DC MICROWAVE ovens have inspired the development of an entirely new kind of spacecraft engine. The inventor hopes that his microwave-based thrusters will become the standard propulsion system for small, manoeuvrable satellites. The principle behind all rockets is to throw material out of the back at high speed, driving the craft forward. Chemical rockets burn fuel, creating hot gas that is ejected from the rear of the craft. Newer types of thruster such as ion engines and Hall thrusters (New Scientist, 21 November 1998, p 22) use electric and magnetic fields respectively to propel charged particles out of the engine, providing a very small but energy-efficient push. Now Michael Micci, an aerospace engineer at Penn State University, is working on engines that use electromagnetic radiation in the microwave band as a heat source, just as a microwave oven does. “It’s just like a chemical rocket, but the thermal energy comes from microwaves instead of a chemical reaction,” he says. “The advantage is, you get much higher propellant temperatures, so you get much more thrust per mass flow.” Unlike chemical rockets, a microwave-driven engine could use just about any gas as the propellant. “We’ve used helium, nitrogen, ammonia—it would also work with water vapour, hydrogen or pretty much anything,” says Micci. Better still, unlike ion engines and Hall thrusters, the microwave rocket can be scaled down without losing most of its performance. Micci has already got a thruster working efficiently with microwave power as low as 80 watts—about the power of a light bulb. “It’s geared towards mini and micro-satellites,” he says. The microwave thruster uses much less fuel than a chemical rocket, but it produces about three to five times as much thrust as a ion drive at the same power levels, allowing operators to manoeuvre satellites faster than is possible with existing drives. The US Air Force has already expressed an interest in Micci’s work,
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