Fei Tang and his colleagues at Tsinghua University have etched water thrusters from two silicon wafers, each less than a centimetre square. One wafer has a water inlet channel and a vaporisation chamber. The other has an outlet nozzle. The two wafers were then joined together.

To fire the steam rocket, droplets of water are pumped into the vaporisation chamber. Pulses of current from a battery or solar panel rapidly heat a titanium resistor, which vaporises the water, forcing it out through the nozzle. The thrusters can eject tiny blasts of steam travelling at 28 metres per second.

Tang designed the thrusters for satellites weighing no more than 1 kilogram. "Other systems are either too heavy or too large for use on such tiny satellites," he says. To generate more thrust, Tang says the water could be replaced with ammonia or hydrazine, a common rocket fuel.

But water has its advantages, says Dave Gibbon, chief propulsion engineer at Surrey Satellite Technology in Guildford. "Water's a good propellant to use -- it's very low cost and very safe," he says. "Even if you use a thimbleful of hydrazine, it's still very toxic and flammable -- there are still a lot of safety issues."

Although the thrust produced by the steam rocket is meagre, that needn't be a problem if the spacecraft is small enough, says Gibbon. "People are now talking about making satellites on chips: you could have a tiny camera and a little battery and a water system like this," he says. "You could take them up on the back of another spacecraft and if you have a problem, you just let one ping off and nip around on an inspection mission."

But Gibbon predicts that connecting the system to a water supply won't be easy. "People trying to make valves for these things are having a horrendous time making them leak-proof," he warns. "Something this size could lose all its propellant in days."

This article was published in the March 24, 2001 issue of New Scientist

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