Heat shield repair demo spacewalk on tap
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 12, 2006
The Discovery astronauts are set for a third spacewalk Wednesday, this one to test tools and a temperature-sensitive sealant with the consistency of peanut butter that may prove useful for repairing small cracks or other minor damage to critical wing leading edge panels.
Astronauts Piers Sellers and Mike Fossum are scheduled to begin the planned six-and-a-half-hour excursion around 7:13 a.m. Wednesday, exiting the international space station's Quest airlock module and making their way to a sample pallet at the back of Discovery's cargo bay.
Compared to two earlier spacewalks - one to test a long inspection boom as a work platform and another to repair a stalled robot arm transporter on the space station - Wednesday's outing "is more like a kind of a careful, meticulous lab experiment," Sellers said.
"Mike and I are going to be very careful, (pilot Mark Kelly is) going to coach us through this on how to get the material to the right temperature and apply it to a set of samples. It really is lab work and we're going to do the best, most careful job we can."
The repair demonstration ranked third on NASA's spacewalk priority list and was only added to the mission timeline when Discovery's flight was extended a day.
Some engineers questioned whether the spacewalk was justified, arguing laboratory testing on the ground is adequate for assessing the strengths and weaknesses of the repair materials and procedures. Given the inherent risks associated with any spacewalk, those engineers favored "real work" content more directly focused on building or servicing the international space station.
Lead flight director Tony Ceccacci disagreed, but only mildly.
"The key thing you're getting out of this is how does the material behave in zero G?" he said. "It was the lowest priority EVA, but we have the time and I think overall it's worth doing. As Mike Fossum and the crew said, we're going to get some really good data out of this. From where I stand, and since we did get that extra day, I think it's worth doing."
So does Sellers, who said in an interview that NASA is making a serious effort to develop viable repair techniques.
"It's been in parallel with the main effort, which is to stop stuff falling off the (external fuel) tank, remove the problem at its root cause," he said. "But knowing what we know about foam, you can't guarantee that nothing will fall off. And if something falls off, you can't guarantee it won't hit the orbiter. So it's prudent to try and develop some repair techniques.
"I think these repair techniques are going to be limited in capability," he said. "You cannot repair huge damage to the heat shield. i don't think it can be done. But maybe, who knows, if you see some nicks you don't like it might be a good idea to get yourself some insurance. It's nice to have that capability."
Ever since Columbia went down three years ago, 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 a year ago, 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 were on board again in case it was actually needed.
This time around, Sellers and Fossum will focus on techniques for repairing small cracks and other damage to wing leading edge panels and nose cap material mounted in a special carrier at the rear of Discovery's cargo bay.
The nose cap and wing leading edge panels experience the most extreme heating during the return to Earth - more than 3,000 degrees Fahrenheit - and developing repairs that can stand up to the extreme environment of re-entry is a daunting technological challenge.
"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 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."
Sellers plans to test the camera right off the bat, shooting stills and video from the end of the space station's robot arm as he's being moved to the back of the cargo bay for the RCC repair tests. If time is available, Fossum will shoot additional footage of test samples later in the day.
Here is a timeline of the crew's day Wednesday (in EDT and mission elapsed time):
TIME DD HH MM EVENT 01:08 AM 07 10 30 STS crew wakeup (flight day 9) 01:38 AM 07 11 00 ISS crew wakeup 02:38 AM 07 12 00 EVA-3: Preparations with ISS oxygen 05:13 AM 07 14 35 EVA-3: Spacesuit purge 05:13 AM 07 14 35 MPLM cargo module transfers resume 05:28 AM 07 14 50 EVA-3: Spacesuit prebreathe 06:28 AM 07 15 50 EVA-3: Crew airlock depressurization 07:13 AM 07 16 35 EVA-3: Airlock egress 07:28 AM 07 16 50 EVA-3: Station robot arm (SSRMS) setup 07:28 AM 07 16 50 SSRMS foot restraint installation 07:58 AM 07 17 20 ISS meals 08:03 AM 07 17 25 SSRMS moves to RCC repair demo work site 08:08 AM 07 17 30 EVA-3: Payload bay setup; IR RCC photography 08:48 AM 07 18 10 EVA-3: Heat shield repair demonstration 08:58 AM 07 18 20 ISS crew resumes MPLM transfers 11:48 AM 07 21 10 EVA-3: Payload bay cleanup 12:33 PM 07 21 55 EVA-3: SSRMS cleanup 12:48 PM 07 22 10 SSRMS moves to airlock 01:08 PM 07 22 30 SSRMS foot restraint removal 01:28 PM 07 22 50 EVA-3: Airlock ingress 01:28 PM 07 22 50 SSRMS moves to cargo module demate inspection point 01:43 PM 07 23 05 EVA-3: Airlock repressurization 01:58 PM 07 23 20 Post EVA spacesuit water recharge 05:08 PM 08 02 30 Crew sleep begins
The temperature-dependent RCC repair material, which works best between 35 and 100 degrees, is a "pre-ceramic polymer sealant impregnated with carbon-silicon carbide power," according to NASA's shuttle press kit. It is known as NOAX for short, which stands for non-oxide adhesive experimental.
Sellers and Fossum call it "the black goo."
"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."
A pallet at the back of Discovery's cargo bay carries 12 RCC samples. Eight of them have cracks or gouges or both, two have been deliberately damaged to serve as targets for the infrared camera and two are blanks that will be used by the astronauts to "work" the NOAX prior to application.
Because NOAX works best when the temperature is decreasing, Sellers and Fossum will wait for orbital sunset to begin a repair run. The work is prioritized as follows:
"Imagine the scenario that something's hit and cracked one of the wing leading edge panels," Sellers said in a NASA interview. "We have a black material called NOAX, which is non-oxidizing, non-oxidative adhesive experimental; we're going to apply this over a series of deliberately damaged RCC samples under a range of conditions. These are going to be brought home and they're going to be tested to see how well they would have stood up to an entry. We're hopeful that we will have, you know, helped develop a technique that could be used to repair a shuttle wing leading edge in space. That's the goal."
The sort of damage a future shuttle crew might face involves an impact "where you hit something hard and the impact knocks material off the back side as well as making a crack and knocking material off the front side," Sellers said.
"There are straight cracks, which is like mechanical damage, which you're trying to fill in, and there's coating loss, where the thin layer of glass over the top of the RCC has been eroded off," he said. "So we're going to try applying this material in different ways to repair all these different kinds of damage.
"Now, the 'different conditions' part is that this material doesn't seem to work very well when it's very, very cold. Likewise when it's very hot, it behaves badly. So we're going to try and catch it under the optimal conditions between extreme heat and extreme cold when the surface is cooling, and see how well we do."
The experiment pallet carries more RCC samples than Sellers and Fossum will have time to repair. They hope to complete about a half dozen. All of the samples will be returned to Earth and put through a battery of tests to find out how well the repairs might stand up to re-entry heating.
"The first thing we're getting out of this is the behavior of the NOAX material," said Ceccacci. "The second thing you're looking at is hey, for these specific damages, this stuff worked pretty good or it didn't. It'll be dependent on the arc jet testing they do post flight."
While some observers question whether NASA would ever trust a heat shield repair enough to risk a crew during re-entry, agency managers say they hope to certify at least some of the tile and RCC repair techniques for possible use repairing minor damage by the end of the year.