Spacewalkers shake solar wing; Another EVA Monday
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 16, 2006
Astronaut Robert Curbeam, a weight lifter in his spare time, and fellow spacewalker Sunita "Suni" Williams, a former Navy diver and helicopter pilot, took turns shaking a huge solar array storage box today in a bid to loosen up sticky grommets and fully retract unruly blankets. Despite considerable initial success, the astronauts ran into a particularly stubborn grommet on Williams' side and ran out of time, setting the stage for a fourth spacewalk Monday to complete the array's retraction.
Engineers are confident Curbeam and Swedish astronaut Christer Fuglesang will complete the job Monday, wrapping up the only loose end in an otherwise surprisingly successful mission to re-wire the international space station. Unlike today's excursion, the spacewalkers will be able to take advantage of the station's robot arm to get precisely positioned for the work and will have access to tools that likely will make it easier to reach troublesome areas.
The P6 array features two huge wings stretching 240 feet from tip to tip. Each wing is made up of two flexible solar blankets separated by a telescoping central mast. The blankets are made up of slats designed to fold up like pleated window blinds, using guidewires under tension to keep the slats lined up.
During initial retraction of the left-side 4B wing Wednesday, it appeared that one or more grommets were hanging up on the guide wires, preventing the slats from smoothly folding together. The idea today was for the spacewalks to shake the blanket boxes, causing the blankets to ripple in a wave-like motion and free up the guide wires.
But first, Curbeam and Williams had to complete work to route solar power to the station's central electrical grid. They finished about ahn hour ahead of schedule, floated up to the huge array and took turns shaking the array blanket boxes to loosen guide wires hanging up in several grommets.
"Here we go," Curbeam called, beginning his first shake. "There's five (pushes)."
"Yeah, there's three grommets that are kind of grouped together, they're just staying together, they're not moving at all," Curbeam reported. "All the rest of them seem pretty well spaced."
"You're talking the close guide wire?" asked astronaut Stephen Robinson in mission control.
"Yes, I am. That fold seems to be a bit of problem all the way across."
"Standby, we're discussing, Beamer," Robinson called. "We'd like you to try doing the very same thing you just did, only about half that frequency. It's a big structure and this will build up potentially a little closer to its natural frequency."
"OK, at about half the frequency I gave you last time, starboard and port, five cycles... there's five cycles."
"That's great, Beam," Robinson said. "You're the best at this anyone's ever been."
"Easy for you to say."
"Let us know if you see any difference in any of the grommets," Robinson said after the laughter died down.
"No, those three seem to be grouped up," Curbeam said. "I can't tell if they're binding on the two outward guide wires, but on the inside they are stuck together."
"Grommets 10, 11 and 12 outside of the blanket box, they're kind of just stuck together at weird angles where it looks like they're going to be together forever. I'm sure that wouldn't be the case when it's extended, but that's what it looks like it."
After additional shakes, Robinson said "that's an impressive amount of motion and very, very effective. Any new observations by the shuttle crew? We did not see anything change but we cannot see the grommets."
"I think we have consensus that the grommet numbers 1 through 11, if the stuck one is 12, are most definitely moving with respect to the guidewire," said station commander Mike Lopez-Alegria. "And our sense is that grommet 12 is also moving slightly away from the inner blanket box so that now it is farther away than it was before. Does that make sense to you guys?"
"Yes, it does," Robinson said.
"And I can tell you grommet 10 inboard is free now," Curbeam said.
"I have no doubt you gys are thinking about this, but it sure seems tempting to try a retract while that motion is going on," Lopez-Alegria said. "It just seems like there's some stiction on that grommet that we could do nothing but help."
Flight controllers agreed, but decided to wait until the next daylight pass to make any additional attempts. Coming back into sunlight, Curbeam shook the box two more times before Lopez-Alegria sent commands to retract the mast one bay.
"OK issueing a retract command," he called. "Ready, ready, now."
"It's coming in," Curbeam said.
"Everything looks good on my side," reported Williams.
"Ditto," said Curbeam. "Grommet 12, definitely free."
As the mast was retracted a bay at a time, repeated shaking was required to keep the guide wires freely moving in the grommets.
"We're thinking about one more bay retract," Robinson called at one point. "What's your input?"
"I think you should do it," Curbeam said. "If I see bad things happening, I'll call you all."
"Hey guys, I have a little bit of a dissenting vote," Lopez-Alegria said. "I think it would be useful to do a shake now because I'm pretty sure we're headed right down the same path and a shake now would put us in a better position to continue after that one bay."
"And Houston concurs," Robinson said. "Let's do a shake."
Curbeam then shook the array and another retraction cycle was ordered. Then another.
"Stop retract! Stop retract," Curbeam called.
"Abort!" Williams said.
"It's definitely out of plane on this side," Curbeam said. "Something is still hanging up. And guidewires, the outboard guidewires are definitely loose. Inboard seems tight for about a couple of grommets and then it goes loose. It's definitely hooking up on one grommet and I think I can see it."
"A shake may be in order," someone said.
Curbeam shook the box again, and said "OK, it's all lined up for you again. You might want to wait until that wave settles down. But it's definitely not stuck any more."
Additional retraction cycles were commanded and the blankets generally appeared to be folding smoothly. But in the end, a particularly stubborn grommet on Williams side resisted both spacewalker's efforts to shake it free.
"Beamer, you want to come over here and try it?" Williams called after repeated attempts to shake it loose. But Lopez-Alegria said he doubted Curbeam could make a difference and that Williams had induced as much motion in her blankets as Curbeam had in his.
And so, with time running out, Robinson told the spacewalkers to call it a day.
"We really commend you for a tremendous effort, an Olympian effort," he said. "We're ready for you to come back to the airlock."
In the end, Curbeam and Williams shook the array storage boxes 19 and 13 times respectively. Eight retraction cycles were ordered, leaving 11 of the 31 bays making up the mast still extended.
With the completion of today's spacewalk, the 76th devoted to station assembly, astronauts and cosmonauts have logged 463 hours and 21 minutes building the international outpost. The total through three spacewalks for Discovery's mission is 19 hours and seven minutes.