Spacewalkers Test Shuttle Heat Shield Repair Technique

Shuttle astronaut Piers Sellers (left) records heat shield repair tests from his perch on the ISS robotic arm during a July 12, 2006 spacewalk in the Discovery shuttle's payload bay. At right, spacewalker Michael Fossum shades the samples for the infrared camera video session. CREDIT: NASA TV.

HOUSTON - Working with space caulk guns and a pair of scrapers, two NASA astronauts put a black, sticky goo through its paces during a Wednesday spacewalk to test basic shuttle heat shield repair techniques.

Spacewalkers Piers Sellers and Michael Fossum spent most of their more than seven-hour excursion in the payload bay of the space shuttle Discovery, where they evaluated methods of applying an experimental heat-resistant material onto samples of the same type of panels that line the orbiters wing edges and nose cap.

The astronauts began their spacewalk at 7:13 a.m. EDT (1113 GMT) as Discovery and the International Space Station (ISS) passed over Spain, and spent seven hours and 11 minutes working outside their spacecraft.

Sellers said their primary task - testing a putty-like heat shield repair material dubbed NOAX - was more akin to a "meticulous lab experiment" rather than the heavy lifting of Monday's spacewalk to repair the space station's Mobile Transporter.

The spacewalkers squirted the heat-resistant material onto intentionally damaged sample areas using a caulk gun-like tool and smoothed it out with spatulas.

"It bubbles up nice when it comes out," Fossum said.

NOAX, which is short for non-oxide adhesive experimental, is a sticky black substance that carries the initial consistency of peanut butter before it is worked into place in orbit, NASA officials said. The space agency described the material as a pre-ceramic polymer that is impregnated with carbon silicon carbide powder.

Engineers designed NOAX as a coating, crack and gouge filler for the black panels that protect Discovery's wing leading edges and nose cap. Made of a carbon composite called reinforced carbon carbon (RCC), the panels endure the hottest temperatures when the shuttle reenters the Earth's atmosphere.

"It's easy to work with," Sellers said after applying the material into several samples of intentionally damaged samples. "It's just like anything we used in the experiments."

Today's tests were the second orbital evaluations of NOAX. The material was first used during Discovery's last flight in July 2005, when STS-114 spacewalkers spent one hour performing shuttle tile and RCC repair techniques.

In ground tests, engineers found that NOAX appears to be most effective when applied to RCC panels that are cooling from 100 to 35 degrees Fahrenheit (38 to 1.6 degrees Celsius), so Fossum and Sellers used temperature probes to pick and choose when to smear the material onto their samples. The samples will later be studied on the ground and some seared in NASA's arcjet facility to determine the repair's effectiveness

Applying the NOAX material was easy at first for the astronauts as they used spatulas to smooth the material. But applying some later coats as a finishing touch was a bit tough, they later said.

"This is quite hard work," Sellers said as he applied a finishing coat over an earlier repair. I'm getting warm just mashing this stuff in."

Discovery's STS-121 pilot Mark Kelly orchestrated today's spacewalk from inside the orbiter, while crewmates Stephanie Wilson and Lisa Nowak worked the space station's robotic arm. The spacewalk, the third and last of the STS-121 mission, was added to the spaceflight - along with a 13^th flight day - once flight controllers found the shuttle's power supply could support it, NASA said.

Heat shield repair

Developing the ability to repair at least some orbiter heat shield damage has been a critical concern for NASA since the Columbia accident. In 2003, a 1.67-pound (0.7-kilogram) chunk of shuttle fuel tank foam insulation pierced the RCC shielding along Columbia's left wing leading edge during launch, a critical wound to the spacecraft, leading to the loss of the orbiter and its seven-astronaut crew during reentry.

An investigation board later recommended that NASA develop reliable shuttle heat shield repair methods.

NASA has since redesigned shuttle fuel tanks to reduce the amount of foam they can shed during launch. The largest area shed during Discovery's STS-121 ascent weighed about 0.055 pounds - or less than an ounce, and about one-fourth the largest allowable - came off in six small pieces from a total area of 12.3 inches by 14.2 inches (31 by 36 centimeters), mission managers have said.

"Our number one goal is to never have to use this," Tony Ceccacci, NASA's lead shuttle flight director for Discovery's STS-121 mission, said of the repair method Tuesday.

Sellers and Fossum also tested a new infrared video camera during today's spacewalk. The spacewalkers used the camera to record two, 20-second videos of Discovery's wing leading edges. They then used the camera to study two of the intentionally damaged RCC samples in the shuttle's payload bay, as well as two others that had undergone a test repair.

"Their hope is that this is another sensor that we can use to detect damage," Ceccacci said of the new camera.

Ceccacci said the camera could be used by hand or be mounted to the tip of Discovery's 50-foot (15-meter) orbital boom if its images prove useful for heat shield inspections.

Sellers also used the camera to observe parts of the ISS, including its radiators and one spacewalkers - Fossum - as he crawled across the station's hull.

"It looks like a glowing person moving over a mirror," Sellers said.

Space spatula lost, SAFER taped

Wednesday's spacewalk was not without incident. At one point, one of the five spatulas Sellers carried out with him drifted away after it separated from its appointed tether.

"No sign of the spatula, guys, it is gone, gone, gone," Sellers said after an initial search around his Discovery payload bay worksite.

Flight controllers later caught the spatula on video cameras as it floated out of Discovery's bay and into open space. It is not expected to pose a debris hazard to Discovery or the ISS, NASA officials said.

"That was my favorite spatch...don't tell the other spatulas," Sellers said, then later took heart when Kelly told him it was not a debris risk. "Thanks for doing all the work on this, I'm sorry to let it go."

Unlike Monday's spacewalk, in which the latches connecting Sellers' emergency jetpack - known as the Simplified Aid for EVA Rescue (SAFER) - to his spacesuit popped open and had to be tethered in place, the astronaut left the airlock prepared Wednesday.

His crewmates covered the two SAFER latches with Kapton tape to prevent the latches from shaking loose. Even so, Fossum had to help Sellers a few times to prevent the tape from coming free and to secure the SAFER door, NASA officials said.

Wednesday's spacewalk marked the third extravehicular activity (EVA) of Fossum's spaceflight career and the sixth for Sellers. It was the 21^st spacewalk staged from the station's Quest airlock.

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