Solar array 'wiggle' tests, possible retraction planned
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 15, 2006
NASA managers, thrilled with the success of a space station re-wiring spacewalk Thursday, plan to remotely shake and jiggle a partially retracted solar array Friday in a bid to free a hung up guide wire and coax the blankets into full retraction. One proposed test includes having an astronaut work out with a resistive exercise device to impart forces known to shake, or excite, the fragile solar arrays.
Engineers are still assessing the feasibility of a fourth, unplanned spacewalk to manually free the guide wire. But they have ruled out adding any such work to an already planned spacewalk Saturday to finish re-wiring the station. NASA managers would prefer to avoid an additional EVA if at all possible because it would force the Discovery astronauts to forego a final heat shield inspection after undocking next week.
A third option is to simply leave the array as is and attempt retraction later, either by a station crew or during the next shuttle visit in March.
"We're looking at all these options, we haven't made any decisions," Mike Suffredini, space station program manager at the Johnson Space Center, told reporters late Thursday. "Right now, the ISS is in a very good configuration relative to this array. ... We've got plenty of time in this configuration, there's no cause for alarm."
The astronauts Wednesday attempted to remotely fold up one wing of the solar arrays that provided interim power to the space station, the first step in on-going work to switch the lab complex to its permanent power system. The retraction was required to provide the clearance needed for newly installed arrays on the end of the station's main power truss to begin rotating to track the sun.
But the P6-4B array wing refused to cooperate, with its pleated blind-like slats failing to smoothly fold up. After repeated extension-retraction cycles over more than six-and-a-half hours, the astronauts finally managed to retract the blankets about halfway. That was enough to permit the newly installed P4 arrays to rotate as required. At that point, the astronauts were told to shift gears and prepare for Thursday's spacewalk.
Engineers now believe a guide wire is hanging up in a grommet due to friction and that if the array can be shaken a bit, the wire might slide free, permitting the blankets to fold smoothly.
Crew wakeup Friday is scheduled for 9:47 a.m. At some point during the next hour or so, commands will be sent to rock the arrays by rotating a central mast one way and then the other in a motion siimilar to changing the pitch of a propeller.
"If that doesn't work, there's actually another technique that will make you laugh a little bit, we have what we call the IRED (Interim Resistive Exercise Device), it's a resistive exercise device the crew uses," Suffredini said. "We found through experience a few increments ago that if they exercise at the right rate, they can excite the solar arrays. In fact, Leroy Chiao was exercising and we noticed through the camera that the array was really getting a work out."
No word yet on who might be asked to perform the ultimate power squats this time around, but if it happens it will follow a water dump scheduled to end around 1:45 p.m. Friday. First, the team will attempt to shake the arrays by rotating the mast.
"There's a water dump that we've got scheduled tomorrow (and) we didn't want to perturb that, so we wanted to see if this wiggle test will work before 10 a.m. local time (CST) tomorrow," said lead space station flight director John Curry. "So we're going to do that test sometime before 10 a.m. then we'll have a Mission Management Team meeting to discuss the results of that and then go from there."
"If either one of these techniques clears it, then what we will do probably later in the day is try this retraction technique," Curry said. "We'll probably retract it a bay or two at a time and stop and let the whole system settle out ... and see if we can't ease it back into the blanket box that way."
Otherwise, the astronauts plan to hold traditional joint crew news conference at 3:47 p.m. and to make preparations for a third and final spacewalk Saturday to complete the space station's switch over to its permanent power system.
Astronauts Robert Curbeam and Swedish flier Christer Fuglesang re-wired two of the station's four major electrical circuits during a five-hour spacewalk Thursday. Flight controllers also successfully activated a powerful ammonia coolant pump to keep the newly activated electronic gear from overheating.
"Today went as close to without a hitch as you can possibly have a spacewalk go," Curry told reporters. "From a programmatic perspective, and from an ops perspective for this particular spacewalk, this one scared me as one of those that I was worried about, all the potential contingencies that could occur, what if the software doesn't come up right, what if the pump doesn't come up right, what if the power doesn't do what it's supposed to do?
"And all that stuff worked exactly and precisely the way folks designed it and tested it and we executed the EVA and the crew, both of them, they were just machines, couldn't have gone better. ... We've simmed this particular day hundreds of times. And the fact that it went the way it did today gives me a lot o confidence that when we go to do the exact same thing again (on Saturday), that the same thing will happen. I'm hopeful, because obviously we proved a lot of things today."
Curbeam and Fuglesang finished their work an hour ahead of schedule Thursday. But even if Saturday's spacewalk by Curbeam and Sunita Williams goes just as smoothly, no solar array troubleshooting can be added to the excursion because the station's robot arm will not be in position to get an astronaut to the array.
"It just flat doesn't fit," Suffredini said.