Magazine section: Frontiers



Could water replace heat shield tiles?

New Scientist vol 177 issue 2388 - 29 March 2003, page 21


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.



Bruno van Wayenburg