Station solar array wing successfully retracted at last
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 18, 2006
Perched on the end of the space station's robot arm, astronaut Robert Curbeam, assisted by Swedish flier Christer Fuglesang, finally coaxed a set of balky solar blankets to fully retract today after repeatedly clearing guide wire hangups. The successful retraction of the huge array, which finally folded up like neatly pleated window blinds, was the final objective of the shuttle Discovery's mission to re-wire the international space station.
"Robert Curbeam, you do good work," astronaut Stephen Robinson radioed from mission control when a final guide wire snag was pulled free.
The P6 solar array, made up of two wings known as 2B and 4B, was attached to the station six years ago to provide interim power during the initial stages of assembly. Construction has now reached the point where the arrays need to be moved to the station's main solar array truss and wired into the lab's permanent power system.
To do that, the two wings are being folded up one at a time, with 4B scheduled for retraction during Discovery's mission and the still fully extended 2B wing during a shuttle mission next March. In September, another crew plans to move the stowed arrays to the left end of the main truss and re-extend both wings.
The Discovery astronauts attempted to retract the 4B wing on Wednesday but they were only able to pull it in about half way. Several of the slats in the folding blankets, held in alignment by guide wires similar to the cords in pleated blinds, failed to fold evenly, stopping the process in its tracks. Repeated attempts to free a presumably jammed grommet and guide wire were unsuccessful.
On Saturday, during a spacewalk by Curbeam and Sunita "Suni" Williams to finish the Discovery crew's goal of re-wiring the space station, NASA's Mission Management Team approved a fourth spacewalk to help get the P6-4B wing retracted. Curbeam and Williams, meanwhile, finished their electrical work early and ventured up to the stalled array for an up-close inspection.
The spacewalkers then repeatedly shook the solar array storage box, setting up ripples in the hinged slats of the solar cell blankets, freeing up stuck grommets and permitting their crewmates inside to retract the central mast an additional six bays. But they eventually ran out of time and had to call it a day, leaving 11 of the mast's 31 open-framework bays still extended.
For today's spacewalk, Curbeam and Fuglesang were equipped with a variety of tools, all insulated with non-conducting tape, to free grommets and guide wires as needed. The solar array will be shunted to lower its electrical potential. In addition, the station's robot arm was available to position Curbeam as required.
Once in position, Curbeam spotted a frayed guidewire that was preventing the solar array slats from folding up smoothly. Using an insulated tool similar to a windshield ice scraper, he gently flipped the slats, one at a time, to work the frayed section of guide wire through a series of grommets. That allowed the blanket panels to line up properly, much like the slats in a pleated blind. Commands were then sent to pull the array's central mast in about 40 inches, the length of one of its open-framework bays.
"OK, and Houston, we're ready to go one bay retract," shuttle skipper Mark Polansky radioed. "OK. Here we go. Ready, ready, retract."
After one bay, the retraction was halted. Curbeam reported "I have a grommet that's caught on the outboard guidewire of the aft blanket."
Fuglesang, free floating near the base of the array, shook the blanket storage box, setting up ripples in the still-exposed slats in hopes of freeing the guide wire. It moved somewhat, but did not pop free. Fuglesang shook it again, to no avail.
"Can I get one more shake?" Curbeam asked.
"You're go for another three-cycle shake."
"OK," Fuglesand said. "One, two, three..."
But the shaking didn't work and Curbeam, on the end of the robot arm, was moved up to the panel to try some additional slat flips with his scraper tools. To his surprise, guidewire was moving freely and Curbeam could not see anything obviously amiss. But the guidewire was slack, indicating a problem somewhere.
He then pulled out an improvised wire-puller tool, snagged the loose guidewire and gently pulled to take up the slack. But the cable pulled in freely, resulting in a loop of free line.
"Here we go... yes, and all that's doing is kind of pulling the outboard looseness."
He reported the guidewire was sliding through the grommets smoothly, raising questions about whether a takeup reel at the base of the blanket box was winding properly.
"Our concern is, either the reel isn't doing its job or that snag is trying to roll down until it gets very close to the reel," Robinson said.
Flight controllers then asked Curbeam what he thought about using a set of needle nose pliers to pull the frayed wire back out manually to get a feel for the motion. Then he could, perhaps, cut the fray away. Fuglesang was asked to look at the base of the blanket box and report whether he could spot the guidewire where it entered a takeup reel.
He saw no problems and Curbeam used the needlenose pliers to pull the guidewire out of the takeup mechanism on the theory that, like pulling on a stuck safety tether, that might clear up whatever was causing the problem.
"OK, here we go, I'm going to pull it out about five inches," Curbeam called. "Now I'm going to let go. And it is, tensioned, all the way out!"
"Good work!" someone said.
"The tension was very light. I pulled it out to five inches estimated and let it go and it retracted very smartly, I mean immediately."
"LIke a safety tether real," Robinson observed.
"Exactly. It took in quite a bit of line."
The astronauts then attempted another one-bay retraction. The mast pulled in smoothly, but another grommet appeared to hang up on a guidewire. Fuglesang gave the storage box two more shakes to jerk it free and the astronauts ordered another one-bay retraction.
And again, the grommet hung up. After using a longer "cheater bar" to reach the area, Curbeam cleared the snag and another one-bar retraction was ordered.
"Ready, ready, retract."
Again, a grommet appeared to hold up but this time, it freed itself.
"The aft solar array looks good," Curbeam reported. "And the forward is good," said Fuglesang.
"You are go for a single bay retract," Robinson said.
The blankets rippled a bit and seemed to hang up briefly, but as the motion damped out the slats lined back up. Another retraction was ordered, but this time, the slats hung up again.
From that point, it was retract one bay, manually clear hang ups, retract again.
Finally, four hours and 20 minutes into the retraction, only one bay remained extended from the mast canister. The spacewalkers spent a half hour inspecting the blanket boxes to make sure the slats were lined up and ready for final retraction.
"You are go for final retract per step 17.6," Robinson radioed around 6:50 p.m.
"OK, copy that. We're going to go ahead and set up for the final retract," Polansky replied. "OK guys, here comes the final retract command. Ready, ready, retract."
"Good motion," the spacewalkers said in unison as the central mast pulled in the final few feet. As the top of the array box moved down into place over the lower section, smoothly sandwiching the array slats between them, flight controllers burst into applause.
But the celebration was premature. Curbeam reported the center guidewire on one of the blankets apparently didn't fully retract and a short loop was left exposed. After closer inspection, Curbeam used needlenose pliers and attempted to gently pull the looped wire out through an exit port to permit a takeup reel to pull it in. But the wire didn't budge.
After using his pliers to pull the wire out from between two slats - and a folded over grommet as well - Curbeam was asked to pull on the guidewire again in a bid to pull out the slack.
"OK, all loop's gone, no wire down between the glass panel," he replied. "It is in tension."
"Beamer, we don't have TV," Robinson said a moment later. "Do you feel the guidewire is now not looped and completely clear and the way it should be?"
"Most definitely," Curbeam replied.
"Robert Curbeam, you do good work," Robinson said.
After a final inspection, commands were sent to engage latches, firmly locking the solar array blanket boxes closed to complete the retraction.
"Step 31, we are go for SABB latching," Robinson said at 7:30 p.m., five-and-a-half hours into the spacewalk.
"OK, the forward blanket box is completely latched," Curbeam reported. Houston then confirmed both boxes were latched, saying "Great job by everybody up there and down here on the ground. "We do have confirmed latch."
In the end, it took 71 mast motor activations over four days to fully retract the P6-4B solar array.
The spacewalk began at 2 p.m., but by the time the astronauts got up to the array and ready to work, they were in orbital darkness. Coming back into sunlight around 3:15 p.m., Curbeam outlined his plan of action.
"And Houston, what I plan to do, just looking at this, it looks like if I push just a little on the hinge panel that should free up that grommet. Just a little on that outboard hinge panel ... and I think you'll see this whole thing loosen. Then I'll give a little back bend just to make sure the wave doesn't come out and get too close to me."
"We copy that. Sounds like a good plan. Tell us what tool you plan to use, Beamer," Robinson called from Houston. "And just to let you know, that guide wire is a three-strand wire."
"Oh great," Curbeam said. "I think you've got two left. I'm going to start off with the scraper tool then I'm going to do a knee bend just to get away from any resulting wave. ... And here we go."
He flipped a slat and then saw the frayed section catch in the next grommet.
"The guidewire is catching on every single grommet," Curbeam said. "Now it's caught on the next inboard grommet. I'm going to lift that one up. Here we go on the next grommet. ... I'm going to have to loosen each one individually because like I said, each grommet is catching up on that part. So the next grommet's coming with your concurrence."
"OK, got two grommets that time," Curbeam said. "Yeah, you can see probably on (my helmet camera) now that part right there, that I'm pointing to, that part of the guidewire is frayed and that's what it's hanging up on."
He then released one grommet after another.
"As soon as that frayed part gets in the reel, though, we should be OK," he reported. "One more grommet... and another... and another... yeah, the tensioning reel is taking in the frayed part."
A few moments later: "OK, it is past all the grommets and well clear of anything it should hang up on. I am putting away the scraper tool... the scraper tool is stowed... and I'm as low profile as I can be. If you guys want to leave me here, that's fine or if you've gotta take me clear, that's fine."
The robot arm then moved Curbeam a few feet away while his crewmates inside prepared to send the first retraction command.
"And Houston, just so you know, it was only one strand that was popped," he reported.
"OK, Beamer, it's great to have an explanation for why it was hanging up," Robinson replied from mission control.
Curbeam said he did not expect the wire to cause any problems during the array's re-extension after it is moved to the end of the station's main solar truss next September.