Cold water treatment

Maintaining traditional ceramic heatshields is a costly and time-consuming business, but water-cooled alloy could create a cheaper re-usable alternative. Stuart Nathan reports.

Water-cooling could be the key to the safe re-entry of the next generation of reusable spacecraft, according to aerospace engineer Jeroen Buursink of the DelftTechnicalUniversity.

Using the combined properties of a self-repairing alloy and a highly-porous ceramic, Buursink has shown that the water-cooled system can absorb heat loads equivalent to a ceramic heatshield material similar to the Space Shuttle’s — but for a fraction of the cost.

Currently, metal heatshields are only practical on single-use spacecraft, because they work by ablation — as the spacecraft re-enters the atmosphere, the shield begins to burn away. The Shuttle’s ceramic shield absorbs heat and is lighter, but the material is expensive and needs a vast amount of maintenance; checking every individual tile on the craft takes around 40,000 man-hours for each flight.

Buursink’s shield could help solve this problem, by creating a cheaper alternative heatshield for small re-usable craft. The system makes use of water’s almost unsurpassed capacity to absorb heat as it boils, combined with the properties of the metal from which the shield is made. Known as PM1000, this is a nickel-chromium alloy containing small quantities of iron, titanium, aluminium and yttrium oxide. It is made by mixing metal powders under high pressure and temperature in an oxygen atmosphere, which creates a corrosion-resistant chromium oxide layer on its surface.

If this is damaged by a meteorite, for example, the chromium beneath it immediately re-oxidises, using the oxygen within the alloy. Moreover, the alloy radiates heat very efficiently.

Beneath the alloy shield is a layer of high-porosity aluminium oxide, which can hold up to 70 per cent water by volume. Capillary action transfers the water to the hot inner surface of the shield, where it evaporates, removing thermal energy. This, combined with the alloy’s heat-radiating ability, means that the heat can radiate from both the outside and the inside of the shield, keeping the temperature low — around 250°C.

The system has now been taken up by ESA, which plans to test it on a spacecraft in 2007. But Buursink is careful to point out the system’s limitations. ‘The idea is mainly suited to small spacecraft and short missions, or for small parts of a craft,’ he said. ‘We must not give the impression that any successor to the Space Shuttle will be clad entirely in a cooled metal heatshield — at most, it will cover critical points. For 80–90 per cent of the surface, an uncooled metal shield will suffice.’