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NASA picks shuttle repair techniques for space tests
Posted: February 10, 2005

After weeks of internal debate, testing and analyses, NASA managers today selected four rudimentary tile and wing leading edge repair techniques to demonstrate during the first post-Columbia shuttle mission.

Spacewalking astronauts will test tile and RCC repair concepts during STS-114. Photo: NASA
Two of the repair procedures will be carried out inside the shuttle Discovery's crew cabin, a so-called "overlay" technique that could prove useful for fixing damage to heat-shield tiles and a "plug" procedure for repairing larger holes in wing leading edge panels. A less sophisticated tile repair technique, one intended for minor damage, will be demonstrated during a spacewalk, along with a technique for repairing small cracks in leading edge panels.

A tile repair technique that had long been considered a front runner was ruled out, however, and while two large backpacks and high-tech caulk guns will be carried aloft aboard Discovery, they will not be tested in flight, according to NASA spokesman Kyle Herring.

Instead, engineers will continue work to perfect the technique and if all goes well it may be tested on the second post-Columbia mission.

Today's Program Requirements Control Board was chaired by shuttle program manager William Parsons. His decision will be presented to NASA's Spaceflight Leadership Council a week from Friday for formal approval.

Discovery is scheduled for launch on the first post-Columbia shuttle mission around May 15. Three spacewalks are planned, two devoted to space station repair and servicing and one to demonstrate repair procedures. But technical questions and concerns have held up a decision on which repair techniques to demonstrate and with time running out to complete crew training and equipment testing, Parsons, deputy program manager Wayne Hale and other senior managers heard final presentations today.

Three options were on the table.

The first option called for spacewalkers Stephen Robinson and Soichi Noguchi to use the cure in-place-ablator applicator - CIPAA - backpacks, loaded with a tile repair material known as STA-54, to fill in deliberately damaged tiles in Discovery's cargo bay.

Questions about the reliability of the procedure surfaced last year when engineers noticed the formation of air bubbles in the viscous STA-54 material as the two compounds that made it up were mixed together in the backpack. After extensive troubleshooting, engineers were able to reduce the bubbling but they have not yet eliminated it. The concern is that bubbles could migrate in weightlessness and form large voids as the material cures. Those voids could weaken the patch and its ability to shield against re-entry heating.

Chief astronaut Kent Rominger told CBS News last week that his office opposed in-flight testing aboard Discovery's flight and sources said later that Discovery commander Eileen Collins agreed with that position.

A second option debated today called for eliminating a repair demonstration spacewalk altogether. Instead, the crew would demonstrate the overlay tile repair procedure in the shuttle's cabin, along with the plug technique for repairing small holes in leading edge panels. By eliminating the spacewalk, the crew would have more time for external tile inspections and logistics transfers to the international space station.

A third option, the one ultimately selected, was chosen because the techniques in question were the most technically mature and offered the best opportunity to collect useful in-flight data, Herring said.

Robinson and Noguchi now plan to test a tile repair technique known as "emittance wash" in Discovery's cargo bay. Using a demonstration kit with deliberately damaged tiles, the spacewalkers will paint exposed surfaces with a material that will replace damaged or eroded coating and improve heat rejection.

Columbia was destroyed two years ago by a hole in a reinforced carbon carbon wing leading edge panel that hot gas to burn its way into the wing's interior during re-entry. NASA still has no way to repair that level of damage, but Robinson and Noguchi will test a rudimentary technique in which a heat-resistant material known as NOAX will be smoothed over small cracks in RCC material.

NOAX, which stands for non-oxide adhesive experimental, will be squirted from a caulk gun-like device and then smoothed out with trowels. The original procedure required the RCC to be heated prior to NOAX application and for the patch itself to be heated for a half hour after that to cure the material. Whether a heater remains part of the new spacewalk demonstration is not yet known.

Two mechanical fixes that would not be affected by exposure to vacuum will be tested inside the shuttle's crew cabin, assuming a safety analysis determines the equipment poses no risk to the astronauts.

The so-called overlay technique for tile repair calls for the astronauts to cover a panel of damaged tiles with a thin, flexible sheet of heat-resistant carbon silicon-carbide that would be mounted atop a gasket and attached with fasteners similar to drywall bolts that would be screwed into surrounding tile.

The final repair procedure, aimed at fixing small holes in RCC panels, requires a flexible carbon silicon-carbide patch called a "plug." After fit checks and application of a sealant, a plug would be inserted into a hole and held in place from behind by expansion bolts.

Between 20 and 30 different plugs, each with slightly different geometries, would be needed in a real repair kit to ensure a good fit virtually anywhere in the curving leading edge.

See the Feb. 6 story for the first take of a detailed overview of all the shuttle in-flight repair options.