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GENTLE puffs of steam could one day propel tiny spacecraft round the cosmos,
say space scientists in Beijing. Steam power would provide a green alternative
to toxic fuels in miniature rocket motors.

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.
鈥淥ther systems are either too heavy or too large for use on such tiny
satellites,鈥� 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. 鈥淲ater鈥檚 a good propellant to use鈥攊t鈥檚 very low
cost and very safe,鈥� he says. 鈥淓ven if you use a thimbleful of hydrazine, it鈥檚
still very toxic and flammable鈥攖here are still a lot of safety
颈蝉蝉耻别蝉.鈥�

Although the thrust produced by the steam rocket is meagre, that needn鈥檛 be a
problem if the spacecraft is small enough, says Gibbon. 鈥淧eople are now talking
about making satellites on chips: you could have a tiny camera and a little
battery and a water system like this,鈥� he says. 鈥淵ou 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.鈥�

But Gibbon predicts that connecting the system to a water supply won鈥檛 be
easy. 鈥淧eople trying to make valves for these things are having a horrendous
time making them leak-proof,鈥� he warns. 鈥淪omething this size could lose all its
propellant in days.鈥�