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Heat shield repair demo spacewalk underway BY WILLIAM HARWOOD STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION Posted: July 12, 2006 Astronauts Piers Sellers and Mike Fossum began their first heat shield repair test run around 9 a.m., after Fossum relatched a loose safety tether. Using a high-tech caulk gun, Sellers squeezed out thick NOAX sealant on a wing leading edge sample with deliberate cracks and/or gouges. The idea is to find out how the heat-resistant material performs in weightlessness, how easy or difficult it might be to spread over cracks using putty knife applicators and how suspended air bubbles out at higher temperatures. "OK. We're starting to get goo," Sellers said as he squeezed the trigger ofthe caulk gun. "Got goo, good goo!" "You're going to want to push it into the exposed carbon carbon, swiping in multiple directions and you want to wet the surface," Kelly advised from the flight deck. "This is taking a little bit of force," Sellers commented. "Really a small, like, kid's marble's worth..." "We're gonna fill that groove pretty soon," Fossum said. "It continues to come out after I've shut the valve off," Sellers said. The NOAX did not bubble, or outgas, as much as the astronauts expected, in large part because it was applied in shadow and not in direct sunlight. Tests on the ground show the material works best when applied when the temperature is between 100 and 35 degrees and dropping. "It feels like soft putty," Sellers commented. "It's a little stiffer than expected and it's tearing a little. ... This is not like wet stuff, Mark, but sticky stuff." "Yeah, it's freezing, it's cold," Fossum agreed. "Now it's behaving, as it's cooled off more, like clay." "So it doesn't seem like it's outgassing very much, does it?" shuttle pilot Mark Kelly asked from Discovery's flight deck. "No at all," Sellers said. "It was a little bit at first, when it sets in thicker layers, you could see some bubbles rising," Fossum said. "I saw a tiny bit when it came out, but now it's doing nothing," Sellers said. "I think we want to just keep working it here until it starts outgassing some more," Kelly advised. For a NOAX repair to be effective, the material must be worked with a spatula to remove any air bubbles that could reduce its heat rejection capability. The reinforced carbon carbon nose cap and wing leading edge panels experience the most extreme heat during re-entry - more than 3,000 degrees - and any repair must be able to stand up to that hellish environment. Sellers and Fossum are filling cracks in test panels with NOAX, working the material in and then applying a "finishing" layer to smooth it out. Sellers said the NOAX pulled, or tore, as he tried to spread it but he said it wasn't a major problem. The test samples will be analyzed back on Earth and subjected to re-entry level temperatures in an arc jet facility to find out how well the material works. Views from the astronauts' helmet cams showed Sellers working the NOAX into various sample panels, working the material like caulk with a putty knife. He said it had the consistency of peanut butter. "This is quite hard work," Sellers said. "I'm getting warm, just mashing stuff in, compensating for the arm." Fossum said the NOAX "was definitely like thick putty. It is smooth in consistency, so it goes on fairly easily in that regard." In direct sunlight, the material appeared quite pliable and it outgassed as expected.
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