A MATERIAL designed to patch cracks in the leading edge of a space shuttle’s wing, while it is in orbit, has passed its first tests. It is a significant milestone in NASA’s bid to return the shuttle to flight following the Columbia disaster last year.
The leading edges of the shuttle’s wings are protected from the heat of re-entry to the Earth’s atmosphere by panels of reinforced carbon-carbon (RCC) composite. The material can withstand temperatures of more than 1600 °C for the 15 minutes of re-entry. But the panels are thin and brittle, and if they become damaged they are much more difficult to patch than the thermal protection tiles that cover the rest of the shuttle.
Columbia disintegrated after a briefcase-sized chunk of foam detached from its external fuel tank at launch, knocking a hole in an RCC wing panel. During re-entry, the hole allowed hot gases into the wing, destroying it and the spacecraft. NASA’s Columbia Accident Investigation Board ordered the shuttle to be grounded until astronauts are able to inspect and repair the heat-shielding tiles and the RCC panels in space.
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To plug any gaps in heat-shielding tiles NASA will use a silicone-based paste (91av, 15 November 2003, p 28). This should be ready by the next shuttle launch, scheduled for March 2005. But a different approach is needed to deal with damage to RCC panels, as the paste does not bond well to them under intense heat.
For this task, engineers at NASA’s Glenn Research Center in Cleveland, Ohio, have developed a sticky black material they call the Glenn Refractory Adhesive for Bonding and Exterior Repair. GRABER is a putty-like material based on silicon carbide that cures in sunlight to a substance similar to the RCC composite. It is designed to be applied in zero gravity using a special gun.
Following the Columbia disaster, the fuel tank’s insulation has been redesigned so that it should not shed foam pieces heavier than 15 grams. However, if a fragment this size strikes an RCC panel it might still open cracks some millimetres wide. To simulate such damage, Glenn engineers took six RCC samples and scored slots 0.9 and 1.6 millimetres wide in them. When they then plugged the hole with GRABER, they found it withstood 15 minutes of exposure to air heated to simulate the incandescent plasma of re-entry. “All six samples survived without burn-through,” NASA says.
NASA is working on two other approaches to fix larger scale damage to RCC panels. Holes up to 10 centimetres across could be covered with a plug of RCC held in place by a bolt made of molybdenum, which melts at around 2600 °C. Another idea is to wrap badly damaged panels with a sheet of RCC material. Initially, NASA hopes each mission would carry a rigid wrap shaped to cover the most vulnerable panels, but the ultimate aim is to develop a flexible version of the composite (see Graphic) that can be fashioned to fit any of the RCC segments on the wing.
