Spacewalkers test shuttle heat shield repair materials
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 12, 2006
The Discovery astronauts chalked up a third successful spacewalk today, demonstrating repair techniques that could help a future shuttle crew fix damage to a ship's wing leading edge panels. While engineers will not be able to fully assess the repair tests until samples are returned to Earth for laboratory analysis, flight controllers said they were pleased with the crew's performance.
"This might sound like a broken record, but we had a very successful flight day nine today," said lead flight director Tony Ceccacci. "We completed the last of our three planned EVAs (spacewalks) for the mission. Not only did we accomplish all our high priority objectives, but we also did get one of our get-ahead tasks completed."
Just before bidding the astronauts good night, mission control informed commander Steve Lindsey that engineers were monitoring two potential issues with the shuttle's hydraulic system.
Auxiliary power unit - APU - No. 1 has shown signs of a very slow leak in its fuel tank, or perhaps a problem with the nitrogen gas system used to pressurize it. So far, the pressure has dropped about 22 pounds per square inch since the unit was shut down following launch July 4.
In addition, thermostats in heaters used to keep APU 3 at the proper temperature acted a bit erratically today, resulting in a slightly broader range of temperatures than usual. But Lindsey was told both APUs are considered operational and available for use during re-entry next Monday.
"What this could be is a number of things," astronaut Lee Archambault in mission control said of the presumed leak in APU 1. "It could be an N2 leak, it could be a fuel decay, it could be none of the above and something completely different. Of course, the MMT (Mission Management Team) and MER (mission evaluation room) is looking at it. But for now, we are ops nominal on APU 1 and we expect to use it during entry."
The shuttle is equipped with three APUs to generate the hydraulic power needed to move the shuttle's engine nozzles during launch and its wing flaps, body flap, rudder/speedbrake and landing gear brakes during re-entry and touchdown. While a shuttle can safely land with just one operational APU, "we're keeping a couple of sets of eyeballs on it," Archambault promised Lindsey.
The astronauts then signed off for the night.
"Tomorrow, the crew is going to get a well-deserved day off," Ceccacci said. "Basically, the only thing they'll be doing at the end of the day is starting the MPLM (cargo module) closeout activities and just doing the standard taking a break, looking out the window and enjoying the day in orbit."
Tomas Gonzalez-Torres, the lead spacewalk officer in mission control, said astronauts Piers Sellers and Mike Fossum were able to complete five test panel repairs during today's spacewalk, about the same number engineers expected based on pre-flight simulations.
Using spatulas to spread on a thick sealant called NOAX, short for "non-oxide adhesive experimental," Sellers and Fossum attempted to fill in deliberate cracks and gouges similar to what might be expected from ascent impact damage.
NOAX is very sensitive to temperature, which can cause gases to bubble out of the mixture. In weightlessness, such bubbles can combine and merge inside the material, forming a void when it hardens.
"You don't want a direct path down through into the damage area," Gonzalez-Torres said. "The repair itself, you're trying to have a protective layer, a protective coating."
During re-entry, when wing leading edge temperatures can exceed 3,000 degrees, "you have some slight erosion," he said. "If you have that erosion and you get into a void and that void actually goes all the way down to the damage, you haven't enacted a good repair. So we want to minimize those voids and make sure whatever voids we do have, if we've done a good repair, they shouldn't be one big void that wouldn't be able to protect us."
Sellers and Fossum practiced applying the material in thin layers, working it with spatulas to help with "outgassing" and then smoothing it in place. Then another thin layer would be applied.
Gonzalez-Torres said the test samples will be subjected to thermal-vacuum cycling after their return to the Johnson Space Center, along with 3D laser scans, CT scans and finally, arc jet testing to see how well the repairs stand up to re-entry temperatures.
Based on video from the astronauts, it appeared the material was properly applied and that the new layer-by-layer technique resulted in fewer bubbles than astronauts experienced during tests last year.
The spacewalkers ran into one minor problem when one of six spatulas used for today's repair work floated away from Sellers unnoticed. Flight controllers said it posed no threat.
Asked to rate his concern over the lost spatula and earlier problems with safety tethers, Ceccacci said, "If that's all that happened, we're happy."
"You're kidding yourself not to expect little things like that to happen," he said. "If that's all that happens, you know, like everyone was talking about forgetting to lock the tether or unlock it, if that's all that happens, we're happy with that."