In our third special preview of the upcoming space shuttle flight, New Scientist examines how astronauts will test techniques to repair the orbiter's heat shield in space
Following the Columbia accident, NASA has taken great care to ensure future shuttle crews are not stranded in space without a puncture repair kit.
Columbia was destroyed after a piece of foam fell from its external fuel tank during lift-off, punching a hole in the panels on its left wing. This allowed super-heated gas to breach the orbiter's heat resistant skin during re-entry, causing the orbiter to disintegrate in seconds.
NASA says nothing could have been done to save the shuttle and its crew. Even if they had spotted the hole in its wing, there was no means of repairing the craft in space.
So the space agency developed five experimental methods to patch up a shuttle's heat shield. All of them will travel aboard Discovery, which is scheduled for launch on 13 July.
None of the techniques have been rated fully safe for use in an actual emergency on the shuttle, but NASA hopes to test several of them in space for the first time during Discovery's flight.
After the mission, the repairs will undergo further testing back on Earth in the superheated conditions inside an arc jet facility. This will measure how well the fixes would withstand the trauma of atmospheric re-entry.
The shuttle's heat shield includes two key pieces of armour - 20,000 heat-resistant silica fibre tiles fixed to the craft's underbelly, and a series of larger, reinforced carbon-carbon (RCC) panels covering the orbiter's nose and the leading edges of its wings. The tiles must endure temperatures of 1260°C while the RCC panels are exposed to scorching winds of 1600°C.
Experiments have revealed the RCC panels to be particularly sensitive to foam damage. "Our testing has now shown us that even a crack, much less a hole, can be catastrophic," says deputy shuttle manager Wayne Hale.
The agency has devised two ways of repairing damage to these panels. The first is combination of a pre-ceramic polymer sealant and carbon-silicon carbide powder, known as Non-Oxide Adhesive eXperimental (NOAX). This is capable of repairing cracks up to 10 centimetres long. Astronaut Steve Robinson will test NOAX on pre-damaged pieces of RCC during a spacewalk conducted within the shuttle's opened cargo bay.
The second method employs a "plug" and a T-shaped bracket that can be tightened over a damaged panel. Discovery will fly with 13 covers and four brackets and these, too, will be tested in micro-gravity during the mission.
NASA has struggled to find a reliable way of repairing damage to the silica tiles on the harder-to-reach underbelly of the shuttle since the 1970s. "The tile-repair situation is a little tougher [than the RCC situation]," says Hale. As a result, NASA managers will try to avoid a repair if a tile becomes damaged on Discovery. "We would have to do some more soul-searching to commit to a repair in those areas," Hale says.
Discovery will carry three tile-repair kits into space. The first is a grey paint-like mixture known as Emittance Wash - consisting of silicon carbide granules and room-temperature vulcanizing agent - that can be painted onto a tile that has suffered shallow gouges. Astronaut Soichi Noguchi will apply the wash to pre-damaged tiles during his spacewalk on Discovery.
The second is an insulating material resembling loosely packed cotton balls that can be stuffed into a hole and covered with an overlay, which must be screwed into place.
The third is a hand-held goo-gun known as the Cure-In-Place-Ablator Applicator, connected to a bulky astronaut backpack. NASA has delayed its in-space testing of this tool until the Atlantis shuttle flight, currently scheduled for September 2005. One concern over this method is how bubbles in the goo might form in microgravity. "Is it all going to coalesce into a larger hole or is it going to stay separated into what might look more like a sponge?” wonders Steve Poulos of NASA's orbiter project office.
NEW SCIENTIST’S SPECIAL REPORT: Follow the countdown as the NASA’s shuttle returns to flight. Read all the latest news, plus our Expert Guide and more…
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