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News

Delft-designed heat shield a boon for spacecraft

Delft University Of Technology : 23 December, 2005  (New Product)
The ceramic heat shields used on craft like the Space Shuttle require huge amounts of maintenance. That is far less of a problem for metal heat shields. Those currently in development, however, are unable to withstand such high temperatures as their ceramic equivalents. But using water to cool the metal could change that.
The ceramic heat shields used on craft like the Space Shuttle require huge amounts of maintenance. In theory, this makes water-cooled metal shields a better option. In 2007 the European Space Agency will be testing just such a shield, developed in Delft.

Reusable launch vehicles are one way to cut the cost of space missions significantly. And they have to be fitted with a heat shield so that they can survive atmospheric re-entry. But as experience with the Space Shuttle's ceramic heat shield shows, this can make the launch vehicle itself very costly. After all, every single tile has to be checked individually for damage after each flight.

That is far less of a problem for metal heat shields. Those currently in development, however, are unable to withstand such high temperatures as their ceramic equivalents. But using water to cool the metal could change that. This idea was first raised at Delft University of Technology some years ago. The metal in question is called PM 1000, an alloy of nickel, chromium, aluminium, titanium and yttrium oxide. Its temperature is reduced by allowing it to radiate heat both outwards and inwards. The inward radiation is absorbed by a layer of porous material saturated with water. That water keeps the porous layer cool and evaporation removes a large amount of thermal energy. This means that relatively little water is needed for the actual cooling.

The porous layer consists of high-porosity aluminium oxide which can hold up to 70 per cent water by volume. The internal structure ensures that this water is retained in the material, moving towards the evaporation surface by capillary action. Theoretically, then, a cooled metal shield can cope with considerably higher temperatures. Research student Jeroen Buursink put the theory to the test in the laboratory, comparing a cooled sample compared directly with an uncooled one at identical thermal loads. And the predicted effects of cooling were observed from the very first tests.

This Delft discovery has now been taken up by the European Space Agency (ESA). If all goes well, the technology will be tested in space for the first time in 2007. A small section of an experimental capsule will be coated with the cooled metal heat shield. But whether such shields will ever be used operationally depends upon many factors, says Buursink. Not least the plans of the major space agencies. NASA's proposed successor to the Space Shuttle has been shelved for the time being, and the ESA is also hesitant. It is quite possible that it will continue to use ‘disposable’ rockets.

Buursink also points out that the cooled metal heat shield has its limitations. “The idea is mainly suited to small spacecraft and short missions,” he says, “or for small parts of a craft. We must not give the impression that any successor to the Space Shuttle will be clad entirely in a cooled metal heat shield. At most, that will cover critical points. For 80 to 90 per cent of the surface, an uncooled metal shield will suffice.”
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