Ever since Columbia went down, NASA and contractor engineers have been struggling to come up with reliable techniques for repairing minor damage to the shuttle's thermal protection system - TPS - tiles and the reinforced carbon carbon panels protecting the nose and wing leading edges from the extreme heat of re-entry. Repairing Columbia-class damage is not considered possible.

During Discovery's last flight, the astronauts tested modest techniques for repairing tile damage and while those procedures are not yet officially certified for use, the equipment and repair material will be on board again in case it's actually needed.

This time around, assuming a mission extension is granted, the crew will focus on techniques for repairing small cracks and damage to wing leading edge panels and nose cap material mounted in a special carrier at the rear of Discovery's cargo bay. The spacewalk would be inserted into the flight plan two days after the crew's second spacewalk. All subsequent activity would simply shift one day to the right.

"The main focus is to evaluate repair techniques for cracks in the carbon carbon material that makes up the leading edge of the shuttle," Sellers said in a NASA interview. "This is hard, black material; it's laid down a bit like fiberglass layers of carbon matting with resin, then matting, then resin built up in layers. This was the material that got damaged during Columbia's launch. So obviously we're very interested in trying to find a fix for it.

"We're going to take up some materials and plugs and test different techniques for repairing these on a special set of samples in the back of the payload bay of the shuttle. That's going to be the bulk of the content of that EVA. But we're also testing an infrared camera which, basically, sees heat or the disturbing of heat flows in materials. We can see how well that does at picking up cracks in the carbon carbon.

Small cracks in the RCC material might not be visible to the human eye, Sellers said, "but the heat will have to flow around the crack, it can't jump across the crack. So, the surface temperature is distorted. And, hopefully this infrared camera that detects heat will be able to see some changes in heat flow around crack fields."

To repair a small crack in an RCC panel, an astronaut would use a special material dubbed "the black goo" by Fossum and Sellers.

"We have a special space caulking gun that we'll use to squirt out a small amount of this repair material," Fossum explained. "In the vacuum of space, the stuff boils. It's just amazing to see it in a vacuum. The black goo literally, it boils and sputters. We work it with a putty knife until it settles down and becomes more workable. We'll then use that to make repairs, filling small cracks and holes in the sample tiles."

Another potential repair technique, one that would be useful for larger holes in an RCC panel, calls for the astronauts to bolt RCC plugs down over an impact site much like band aid.

"We've got some plugs that have been designed to go into the hole and then there's a surrounding kind of a cap area on this, and it's literally a toggle bolt kind of setup just like you might install a towel rack in your bathroom with," Fossum said. "We'll use a space drill to tighten this thing up. The toggle part opens up on the backside to hold it flush. We'll use feeler gauges to make sure that the gap is right, and then we'll use some of the same black goo to kind of seal up the edges."

NASA manaqgers eventually hope to certify at least some of the techniques for possible use repairing minor damage by the end of the year.

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