Crew struggles to get balky array retracted enough to permit other critical work
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 13, 2006
Continuing running notes...
Going into their third daylight pass since the retraction work began, the P6-4B solar array wing remained partially retracted with 17.5 40-inch bays of its central mass still extended. Fifteen start-and-stop cycles were needed to get there. Flight controllers ultimately agreed with a suggestion by Discovery commander Mark Polansky to re-extend the mast to 19 bays and then employ a start-and-stop technique to allow any unwanted motion to damp out before it could cause a problem.
By limiting the re-extension to 19 bays, flight controllers were assured of at least 10 feet of clearance between the P6-4B wings and the newly installed P4 arrays on the end of the station's main solar array trust. The goal of Discovery's mission is to re-wire the space station to take advantage of its permanent power system. But to do that, the P4 array must be able to freely rotate to track the sun. And for that to happen, the 4B wings of the P6 array must be retracted at least 40 percent - about 19 bays.
The astronauts told flight controllers they did not believe extending the mast to 19 bays would be enough to clear the problem. But they said in their view there was little risk they would be unable to get back to the 19-bay point. Flight controllers then told them to try it anyway, Williams extended the mast two 19 bays and retracted to 18 before calling an abort.
The same problem developed.
"What we see is the beginning of the same sort of pattern," said station commander Michael Lopez-Alegria. "We stopped at 17-and-a-half bays. And the probelm that we see is simliiar, although not as pronounced. So what we think is indicated is that this problem is, in fact, asociated with this particular spot. I guess that's good news and bad news. Bad news because it's a problem. Good news because it's inside 19 bays.
"What we think we'd recommend is going back out to 19 again and then come back in just to try to gain a little more insight into the repeatability of it. And then if it repeats like we think it will, then reassess at that point. But it may be worthwhile at that point to go beyond 19 and then stop at some point and come back in. We're not clear on how far to go beyond 19 but we can cross that bridge when we come to it."
"That's a great plan, Mike, and we concur entirely with it," said Stephen Robinson from mission control.
The mast was extended to 19 bays and another retraction commanded.
"Retracting, ready, ready, now," astronaut Sunita Williams called at 4:39 p.m. Two and a half bays later, "aborting!"
"Houston, we're just a smidge past 17 bays showing," Polansky said. "It looks like it's very repeatable."
After discussing it further, the astronauts and flight controllers mutually agreed to extend the array until guide wires once again showed the proper tension even if that ultimately meant extending past 19 bays. At that point, wherever it might be, the astronauts would retract in small steps, hoping to maintain the proper tension."
And so, at 4:54 p.m., Williams began re-extending the mast, stopping again at 19 bays. She then retracted a half bay and stopped to evaluate the motion of the slats. After letting the motion damp out, whe restarted retraction. This time, slats on the other panel making up the array wing appeared to fold the wrong way, forcing another abort with 17 and a half bays extended.
The mast was re-extended to 19 bays to clear the folding problem. Another retraction was started and this time, the slats folded the wrong way on the other panel.
Trying a different approach, a decision was made to extend the mast with stop-and-start movements, i.e., to command an extension followed by an immediate abort in hopes the shorter-period, jerkier motion might help.
Lopez-Alegria reported that approach didn't work, saying the modest ripples set up in the blankets was not enough to clear the problems.
At that point, as the shuttle-station complex sailed into Earth's shadow for another night pass, flight controllers told the astronauts the goal was now to get the mast back inside of 19 bays to make sure they could eventually press ahead with plans to start rotating the P4 arrays and priming the solar array truss cooling system with ammonia for the long-planned electrical re-wiring work on tap Thursday.
"We're thinking in terms of EVA solutions we might be able to apply on EVAs 2 or 3, but our near-term goal will be in the next day pass to try to get down to less than 19 bays."
Moving back into daylight around 5:40 p.m., Lopez-Alegria asked for an update.
"The team has made the decision to redeploy beyond 19 until the fold problem has cleared," Robinson said. "Let's hear youyr side and then we'll tell you what we're thinking."
Lopez-Alegria said the array apparently had two problems, one involving random folding issues and the other caused by a tension issue in the guide wires. He suggested repositioning the arrays slightly to provide a better view to help determine if the latter is a cause or an effect.
While engineers thought about that idea, the crew was cleared to re-extend the mast as required to clear the folding problem.