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Could water replace heat
shield tiles? |
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New Scientist vol 177 issue 2388 -
29 March 2003, page 21 |
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SPACE scientists looking for
new ways to cool spacecraft on re-entry into Earth's atmosphere are turning
to one of the world's oldest coolants: water. Existing heat-shield
technology leaves a lot to be desired. In the 1960s Apollo rockets used a
heat shield that burnt off slowly - but this is no good for reusable
spacecraft like NASA's fleet of space shuttles. And the silica tiles the
shuttle uses are fragile and prone to damage. So space-flight engineers
from the Delft University of Technology in the Netherlands are playing with
the idea of using water. They believe water boiling off and evaporating just
beneath a spacecraft's nose cone will soak up and disperse the heat of
re-entry. They have developed a
cone-shaped re-entry vehicle, called Dart, which they hope the European Space
Agency will launch on a redundant Soviet ballistic missile. Delft's Dart will
be made of an alloy called PM1000 which is currently used in heat-resistant
jet engine turbine blades and is designed to withstand temperatures of up to
1200 °C. But Dart's nose would have to
withstand temperatures of up to 1650 °C on re-entry, which is where the water
cooler comes in. A ball-shaped water tank inside the tip will store 10 litres
of water (see Graphic). On the way
down to Earth, the heat of re-entry boils the water in the tank, slowly
drawing heat from the spacecraft's surface. Some of the superheated steam is
then vented through an exhaust. The pressure in the tank will be kept at five
times atmospheric pressure, which raises the boiling point of water to 150 °C
to make heat evaporation even more efficient, says Tom van Baten, leader of
the Dart research group. The flanks of the capsule will
reach a mere 1400 °C, and don't need so much cooling. They are lined with a
highly porous aluminium oxide layer, which is soaked in water, at a small
distance from the outer wall. Heat should be absorbed all along the
water-soaked layer, keeping the craft's electronics below 50 °C. "In principle, this
water cooling will work," says Markus Fertig, a heat-shield specialist
at the University of Stuttgart in Germany. But the weight of all that water
on a real mission may well swell launch costs. Fertig says the success of
missions will depend on the tank remaining pressurised to stop the water
evaporating, or freezing, before re-entry. Van Baten agrees that freezing is
a "potential problem". And a water leak during a mission would
obviously be catastrophic. Dart has so far been tested only in wind tunnels. The Delft group is working with ESA on a mission called Expert, which will test the durability of PM1000 and a patch of the porous cooling layer sometime in 2005. Van Baten hopes a full test of Dart will follow later. |
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