NASA plans tile improvement

Glued-on heat-resistant tiles, the focus of investigations into the cause of the Columbia shuttle disaster, could be replaced by tougher ceramics in the next generation of reusable launch vehicles.

Glued-on heat-resistant tiles, the focus of investigations into the cause of the Columbia shuttle disaster, could be replaced by tougher ceramics in the next generation of reusable launch vehicles, in research being supported by NASA.

Experts have suggested that damage to ceramic tiles on the underside of one of Columbia’s wings allowed heat to penetrate to the aluminium structure beneath the tiles, causing the spacecraft to break up as it re-entered Earth’s atmosphere. If heat-resistant materials could be used to build a shuttle’s body, a similar situation could be avoided.

Now NASA’s Ames research centre in California, together with Boeing and the US Air Force Office of Scientific Research, is funding research at the University of Missouri-Rolla to develop fabrication techniques capable of creating tiles able to withstand 50 per cent more heat than current silica-based structures.

The Missouri-Rolla team says that as the materials are stronger and harder than silica, they could form part of the shuttle’s structure rather than being attached afterwards.

Ceramics specialists believe that renewed interest in heat-resistant systems following the Columbia disaster could bring in significant new funding for the development of safer materials. ‘Owing to its limited budget, NASA funds only a small fraction of the research,’ said Missouri-Rolla assistant professor of ceramics Dr William Fahrenholtz. ‘If anything I would expect a greater emphasis on research in this area owing to the attention it has received.’

Improved heat resistance would allow the spacecraft to land at a steeper angle, increasing the number of runways that could be used. It could also have sharper leading and trail edges on its wings and a sharper nose, allowing astronauts to fly it like a plane. Sharper edges suffer from higher heat loads on re-entry, but would be protected using the improved ceramics.

‘Ceramics such as zirconium diboride, hafnium diboride, zirconium carbide and hafnium carbide have melting temperatures in excess of 3,400 degrees C, making them likely candidates for applications which require extreme temperatures,’ said fellow researcher Dr Greg Hilmas. ‘As a comparison the current materials on the bottom of the space shuttle orbiter have a melting temperature that is closer to 2,200 degrees C. The aluminium being protected melts at 660 degrees C.’

To make sure the tiles do not deteriorate during long periods of stress, re-entry flight plans are arranged so that they are exposed to 1,000-1,200 degrees C at most. But the new materials could increase this by 1,000 degrees C.

‘The current shuttles are more like a brick with plates for wings than aircraft,’ said Hilmas. ‘NASA knows that having a reusable launch vehicle (RLV) with a sharper design will actually allow them to fly the vehicle, opening up a new realm of possibilities both for capability and safety. The current shuttle has to hit a very narrow window or possibly two small windows at the Earth’s outer atmosphere in order to fall through and make a safe landing at either White Sands or Kennedy Space Centre.

‘If it had a sharper design they could fly it in from almost anywhere and land it almost anywhere. During take-off you could potentially abort the mission during a longer period of the launch and the crew could fly the RLV back to a safe landing.’

Hilmas added that NASA had much to gain from the development of an RLV that used thermal protection materials in its structure rather than as a coating. ‘There has been some discussion at NASA over the past decade about making the thermal protection materials a structural member of the next-generation RLV,’ he said. ‘If they are going to be an integral part they are going to have to impart significantly more strength to the design than the current tile system which is not a structural member of the shuttle at all.’

Though the improved materials are up to three times as dense as current tiles and may be designed to be pore-free, making them still denser, the researchers say their heat resistance will allow the insulating layers to be thinner.

‘If components are incorporated in the structure it would allow for the reduction of the size and weight of other components,’ said Fahreholtz. ‘This may offset some of the weight gain.’