Tonight's spacewalk to test shuttle tile repair option
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 20, 2008
Astronauts Robert Behnken and Michael Foreman are gearing up for a six-and-a-half-hour spacewalk today to replace a faulty space station circuit breaker and to test a new heat shield repair technique that represents one of NASA's final post-Columbia safety upgrades.
"I consider it to be kind of the last thing we're going to do on the return-to-flight tile and (wing leading edge) repair tasks that we took on," said shuttle Program Manager John Shannon. "We have high confidence in it, but this will just be the final activity that we'll do to verify that's indeed a good repair capability."
NASA plans to launch the shuttle Atlantis in late August on a final Hubble Space Telescope servicing mission, a flight that cannot take advantage of safe haven aboard the space station if major heat shield damage occurs. While a second shuttle will be standing by if a rescue mission is required, a successful test today would give NASA added confidence about dealing with any heat shield damage that might occur on the Hubble mission or any other shuttle flight.
This will be the 108th spacewalk devoted to station assembly and maintenance since construction began in 1998, the ninth so far this year and the fourth of five planned for Endeavour's crew. For identification, Behnken, call sign EV-1, will wear a white suit with no markings. Foreman, EV-2, will wear a suit with broken red stripes around the legs.
The spacewalk is scheduled to begin at 6:28 p.m. The first item on the agenda is to replace the faulty circuit breaker, known as a remote power control module, or RPCM. As it now stands, two of the station's four control moment gyroscopes, or CMGs, are tied to a single RPCM. By replacing the faulty unit, the astronauts will restore lost redundancy.
But to do that, flight controllers first must power down numerous station systems, including one of the lab's two external ammonia cooling loops.
"Right now, CMG-2 and I think it's CMG-3 are both hooked up to a single RPCM," said lead flight director Mike Moses. "Both are being fed by one unit so if that one failed, it would take down two CMGs. We prefer them to each be on their own to provide redundancy.
"The power downs that are required, a good analogy is if you're about to go do electrical work in your house and change out an outlet, you want to turn the circuit breaker off upstream. This IS the circuit breaker, so we have to go one step further and take down the power distribution units upstream, which feed power to a whole lot more than just this bank of circuit breakers. We require two inhibits, so we have to take two things upstream down to verify that the EVA crew will be safe while they're working with this hardware."
Behnken will change out the circuit breaker in the central S0 solar array truss segment while Foreman moves to the Z1 truss atop the central Unity module to reconfigure an electrical patch panel.
With the RPCM installed, flight controllers will reactivate the powered down systems while Behnken and Foreman make their way back to the Quest airlock module to retrieve the equipment needed for the heat shield repair test. The work will be conducted on the bottom of the Destiny laboratory module.
"We'll actually take a couple of bags of equipment with us, all the things that you might have in your garage if you were going to do some spackling and some dry-wall repair," Behnken said in a NASA interview. "We'll have that stuff in the bag. We'll have scrapers and brushes and all that sort of equipment. We'll also have some engineering equipment. If you wanted to really understand how well of a repair job you're doing, we're taking some thermometers and a camera and things like that so that we can actually assess the progress of the repair material and our repair technique and see how we're doing with it."
The repair material, known as STA-54, looks like thick, pink silly putty. It is made up of two compounds that are mixed together in a pressure-driven applicator gun just before they exit the nozzle. The gun, called a tile repair ablator dispenser, or T-RAD, will be operated by Foreman, wearing an STA-54 reservoir attached his emergency jetpack.
"Mike Foreman will get in a foot restraint to give him good reach and access and restraint while he's working," said Moses. "He'll be the one actually dispensing the goo into the samples. Bob Behnken will be there to assist with cleanup and handing tools back and forth as he needs them."
Foreman will fill in a variety of cavities in a tile sample board. The astronauts will use pads and brushes to smooth the material out before it cures and hardens.
Some of the cavities were machined while others represent actual foam impact or ice damage. Engineers will dissect the repaired tiles after they are returned to determine the mechanical properties of the material, it's ability to adhere to the underlying tile and how much it swells due to bubbles that form when the materials are mixed in the gun just before application.
"There are a number of different sizes and shapes of samples and really, this correlates to the different objectives we're trying to get out of this test," said Zeb Scoville, the lead spacewalk planner. "Some of of our tests are going to be involving a study of the material itself, how it adheres to tile substrates, how it expands, if it bubbles, what sort of density it's going to have. Other objectives of this test are really to focus on how well the crew can operate and perform. It's one thing to be able to repair a very evenly machined sample. It's another thing to have a divot or pock mark that's been cut by an ice impact or foam damage. So we've modeled a couple of tile damage samples to represent things we've seen on previous missions or on ground testing.
"There's really one main reason why we're trying to perform this test," Scoville said. "On the ground, we were able to develop techniques in a vacuum chamber to see how the material would react and then we performed tests on NASA's zero gravity airplane to understand how the repair process works in a zero gravity environment, albeit for a short period of time. But being able to combine both the vacuum and the zero gravity aspects of these together is what we're trying to figure out here."
When the two compounds making up STA-54 are mixed, a chemical reaction causes bubbles to form. On Earth, those bubbles typically rise to the top. In space, they may be more evenly distributed throughout the material. This is a critical question because it could affect the material's ability to protect damaged tiles from the heat of re-entry.
"One of the big questions we have, in zero gravity are those bubbles going to rise to the surface or are they going to act more like a bread loaf as it bakes with the gas expanding in the material and being evenly distributed bubbles that then cause the surface to rise up over the top?" Scoville explained.
If the "bread loaf effect" causes the material to swell up above the surface of the surrounding tiles, it could disrupt the airflow across the belly of the shuttle during re-entry, causing more severe downstream heating.
"The surface smoothness is a big key in understanding how this will react during a re-entry scenario," Scoville said. "If you have a lot of bubbles and expanded ridges and what not, this can disrupt the airflow ... and cause a turbulent flow transition, which can cause downstream heating and damage the orbiter on re-entry. So being able to understand how this material's going to react and expand and what we can do to control that is really one of our primary objectives of this test."
Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision G of the NASA television schedule):
EDT........DD...HH...MM...EVENT 02:13 PM...09...11...45...EVA-4: 14.7 psi repress/hygiene break 03:03 PM...09...12...35...EVA-4: Airlock depress to 10.2 psi 03:23 PM...09...12...55...EVA-4: Campout EVA preps 04:53 PM...09...14...25...EVA-4: Spacesuit purge 05:08 PM...09...14...40...EVA-4: Spacesuit prebreathe 05:58 PM...09...15...30...EVA-4: Crew lock depressurization 06:28 PM...09...16...00...EVA-4: Spacesuits to battery power 06:33 PM...09...16...05...EVA-4: Airlock egress 06:38 PM...09...16...10...EVA-4: Setup 06:58 PM...09...16...30...EVA-4 (Behnken): RPCM R&R 06:58 PM...09...16...30...EVA-4 (Foreman): Patch panel 07:28 PM...09...17...00...EVA-4: T-RAD worksite setup 08:48 PM...09...18...20...EVA-4: T-RAD demonstration 10:33 PM...09...20...05...EVA-4: Tool cleanup 11:13 PM...09...20...45...EVA-4: Payload bay ops 11:58 PM...09...21...30...EVA-4: Node 2 ACBM launch locks removal 03/21/08 12:18 AM...09...21...50...EVA-4: Cleanup 12:38 AM...09...22...10...EVA-4: Airlock ingress 12:58 AM...09...22...30...EVA-4: Airlock repressurization 01:13 AM...09...22...45...Spacesuit servicing 03:00 AM...10...00...32...Mission status briefing on NASA TV 04:58 AM...10...02...30...ISS crew sleep begins 05:28 AM...10...03...00...STS crew sleep begins 06:00 AM...10...03...32...Daily video highlights reel 10:30 AM...10...08...02...Flight director update on NASA TV 01:28 PM...10...11...00...Crew wakeup