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Heat shields tested for shuttle replacement
17:59 16 May 2006
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The heat shield sample is glowing bright yellow to the right of the arc jet, having been moved aside on completion of a test (Image: NASA/Ames)
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NASA has finished tests of five heat shield candidates for the shuttle's successor, the Crew Exploration Vehicle.
Several of these have been used in space before – in the Apollo moon landings and space science missions like Genesis and Stardust – but the planned CEV capsule is much larger than any of these craft. It will measure 5 metres across, posing a new set of heat-shield design challenges.
The first phase of thermal testing was completed at the arc jet facility at Ames Research Center in Mountain View, California, US, on 8 May. So far they have only tested objects the size of hockey pucks. Despite the large size of the arc jet, sometimes referred to as a "room-sized blow torch", it could not heat up a 5-metre-wide sample enough to simulate the conditions of atmospheric re-entry.
Unlike the shuttle's tiles and thermal blankets, which can be reused, the CEV will have an ablative underside, meaning it will deal with the heat of re-entry by burning successive areas away.
NASA hopes to avoid some of the mistakes of the past. When the Apollo heat shield was built, technicians spent a lot of time doing repairs because the material had bubbles in it.
"Now we're talking about something that's much, much bigger, with a much larger surface area," says George Sarver, who leads the Ames CEV support office. "We need to make sure we have a heat shield design that we can repeatedly build in a fast and efficient manner. If the heat shield fails, that's it."
The lunar heat shields have to be better than those for flights returning from low-Earth orbit and the International Space station because Moon capsules enter the atmosphere at a higher speed – 11 kilometres per second compared to 7.5 kilometres per second. That difference in speed translates into a temperature increase of about 3000°C.
And for future missions to Mars and back, a spacecraft would be entering Earth's atmosphere even faster – 12.5 to 14 kilometres per second. "It's not a huge bump up, but in terms of heating it will be substantial," says James Reuther, the project manager for the CEV thermal protection system.
So NASA is considering making the CEV's heat shield for Mars missions either even thicker or slightly asymmetric, which would give the capsule a little lift when it entered the atmosphere, slowing its descent.
The five materials that were tested are listed below. They are often made with silica or carbon fibres and sometimes impregnated with a resin.
* Avcoat – This was the heat shield of the Apollo capsules, but like the other materials, has never been used in a capsule 5 metres across. The Apollo capsules were 3.9 metres in diameter.
* Pica – This material was used on the Stardust mission that brought back samples from Comet Wild 2 in January 2006. But that capsule was only 1 metre across.
* Advanced Carbon Carbon – This material was used in the heat shield of the Genesis probe return capsule. That spacecraft crashed in the Utah desert because its parachute did not properly deploy, but Reuther says it appears the Genesis heat shield worked well.
* 3DQP – This has been used as a covering on the sides of atmospheric ballistic missiles for the US Department of Defense. As a covering, it allows antennas in the vehicle to communicate through the shock layer created around the vehicle as it zooms through the atmosphere. It can withstand high temperatures for a short period of time.
* Phencarb 28 – This material is developed by Applied Research Associates and has been proposed for use on a mission to the outer planets.
Of these five, three will be selected for a battery of additional tests in January or February 2007.
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