Solar wing tears during deployment
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: October 30, 2007
Engineers are scrambling to recover from a solar array hang up that ripped a two-and-a-half-foot tear in one fragile panel as the hinged blanket was pulled from its storage box today. The disheartening, hard-to-reach hang up occurred as the Discovery astronauts were "95 percent of the way to a perfect day," as one NASA official put it, moving the 17-ton P6 solar array truss segment to its permanent mounting point on the far left end of the lab's main power truss.
"In the configuration we are in today, right this moment, we're basically providing all the power we need from this array," said Mike Suffredini, space station program manager at the Johnson Space Center in Houston. "When I left (mission control), we were about 3 percent shy of the amount of power being provided by the other array. And that's indicative of the fact that we're not fully deployed.
"What that means is we haven't damaged the (power) feed wires," he said. "So that's great news. ... Structurally the array's in fine shape, so we have plenty of time to go sort out this problem."
But until the problem is resolved, the P6-4B array cannot be fully extended and locked in position with the 75 pounds of tension on the blankets that is required to provide structural stability. And until that issue is resolved, the left-side arrays - P6 and P4 - cannot be rotated continuously to track the sun, which limits their ability to generate power.
Given that the station's right-side arrays are now locked in place because of concern about unusual contamination inside a rotary joint, the blanket tear on the outermost right-side array was an especially disheartening setback. NASA hopes to launch a long-awaited European research lab aboard the shuttle Atlantis on Dec. 6 but Suffredini said he would not be comfortable pressing ahead until this latest problem is dealt with.
"We have a lot to talk about," Suffredini said. "We will talk about options, about EVA-4, about EVA-5, when we need to plan to do these EVAs, what we need to accomplish, what the priorities are, what we know today, what we'll know tomorrow, can we come up with reasonable plans to implement? So all this is forward work. We haven't had a lot of time to sort it out."
The combined station-shuttle crew currently plan two more spacewalks: One Thursday to inspect the contaminated rotary joint on the right side of the main power truss; and a final outing Saturday by station commander Peggy Whitson and Yuri Malenchenko to prepare the newly installed Harmony module for its move to the front of the space station after Discovery departs.
Harmony will serve as the attachment point for Europe's Columbus research lab in December and Japan's Kibo lab modules, scheduled for delivery in February and April. Every day added to Discovery's mission will trigger a corresponding delay for work required to prepare Harmony for Columbus and Suffredini said he wants to preserve the Whitson/Malenchenko spacewalk if at all possible.
But he said the content of either spacewalk could change depending on what engineers come up with for addressing the P6-4B problem. And he would not rule out an additional mission extension if necessary.
"We have a lot of options," he said. "We're in a good configuration to sit here and work through this problem. So from our perspective, if you have to deal with this sort of thing, this is the way we'd like to be, we like to be in a nice, stable config while we think through our options. So that's what we'll do over the next few days."
The blanket problem developed at the end of an otherwise successful day, one in which spacewalkers Scott Parazynski and Doug Wheelock, working with robot arm operators Stephanie Wilson and Dan Tani, successfully attached the 35,000-pound P6 solar array truss segment to the left end of the station's main power truss.
The station design incorporates four huge sets of solar arrays, two on each end of a truss that will span the length of a football field when fully assembled. Each array segment - P4 and P6 on the left, or port, side and S4 and S6 on the right side - features two 110-foot-long, 15-foot-wide "wings" made up of two folding blankets supported by central self-assembling masts that extend outward on opposite sides.
Two blankets making up the 2B side of the newly installed P6 solar arrays were successfully re-extended today after the truss was connected to the power truss. But the astronauts aborted extension of a second set of panels after noticing an apparent guidewire hangup and a jagged tear in one of the two remaining blankets.
The second set of blankets was more than three quarters extended at that point. In television views from the station, it appeared as if several slats making up the blanket might have gotten hung up on a guidewire, increasing tension in that area and ripping a two-and-a-half-foot tear in a seam. But it was not immediately clear what might have gone wrong.
Astronauts had major problems retracting the P6 arrays during previous shuttle visits in preparation for this week's 145-foot move from a central mounting point to its permanent position on the left end of the power truss. Grommets on the sides of the blankets hung up on frayed guide wires during the retraction process, requiring spacewalking astronauts to provide a manual assist. Eventually, the blankets were coaxed back into their storage boxes.
Whitson said the ripped seam between to slats apparently occurred in the same area where a frayed guidewire was noticed during the earlier retraction.
"And Houston, Alpha on one, just some further information," she called. "When we were deploying it, we noticed there were more dynamics around the bay 11 point where we were expecting there might be that problem with the frayed line. And at that point, we still thought we were OK. But it appears that maybe that was the extra motion we saw.
"We didn't abort because we didn't see the tear. Unfortunately, the sun angle was such that it was in a place that we just didn't see it. You know, we paused it later, but by that point in time the problem area was behind the (robot) arm in one view and the sun angle in out other view was such that it covered it up. So, it was a combination of really bad sun angles, but I think we did see it, you know, we saw the extra motion when it probably happened and it probably happened as we were pulling it out of the blanket box."
"Hey, no worries Peggy, we had good video, too, and we were keeping our eye on it, so that's just the way it goes," astronaut Kevin Ford replied from Houston. "We appreciate you doing the abort when you did, that was awesome. And we'll just keep working this."
To ease tension on the torn blanket, commands were sent to retract the mast a few feet while engineers debated their options and the astronauts took additional photographs to document the array's condition.
"There was a point late in the shift where I thought the entire thing was going to go off with(out) a hitch," said lead flight director Derek Hassmann. "Before we start talking about the surprises and the things that didn't go quite as well as expected, I'd just like to emphasize that the spacewalk itself was just unbelievably successful. We were able to accomplish all the planned objectives, including the P6 attach to P5, which we knew was going to be a tough and challenging activity. ... All of the P6 attach went by the book, extremely well."
But the P6-4B hangup cast a shadow on the day's activity.
"That was a little bit of a disappointment at the end of an extremely successful day, but we've got folks already having meetings, talking about our go-forward plan from here," Hassmann said. "So I expect that tomorrow we'll have a much better handle on our approach to that solar array."
A major problem for NASA is that P6 is so far out on the power truss that the station's robot arm cannot reach far enough to get an astronaut to the area where the damage occurred. To either fix the tear or release the snag, the astronauts likely will have to retract the P6-4B array far enough to provide access. If engineers can figure out a way for the station arm to pick up the shuttle's 50-foot-long heat shield inspection boom, known as the orbiter boom sensor system, it may be possible to get a spacewalker farther up the mast to the damage site.
"We're going to have to get this area where we can get to it," Suffredini said. "There has been a discussion about the OBSS. Unfortunately, the grapple fixture on the end of the OBSS is not a station grapple fixture, it's the shuttle arm grapple fixture. It's a little different than the space station grapple fixture and in fact, the guys are going off to find out if they can grapple it with the SSRMS (station arm). All of this is forward work. My guess is you'll hear us talk about trying to retract the arrays a little bit to try to get them within reach."
Until the problem is resolved, the P6-4B array cannot be fully extended and its blanket subjected to the normal 75 pounds of tension need to provide structural stability because of concern the force could pull the blanket apart at the tear. Without that structural stability, the arrays cannot be rotated to track the sun. And if the arrays cannot rotate, they cannot generate the power required for the upcoming addition of the European and Japanese research modules.
That's in part because of contamination found in the solar alpha rotary joint, or SARJ, that is used to turn the station's right side arrays. Metallic contamination was discovered in the massive joint during a spacewalk Sunday and as a result, mission managers extended Discovery's flight by one day, deferred a heat shield repair demonstration and devoted an entire spacewalk Thursday to a more thorough inspection.
During today's spacewalk, Parazynski inspected the left-side SARJ to provide a point of reference and found no visible contamination. Suffredini said the problem with the right-side SARJ could involve one of 12 so-called trundle bearings, which press against a bearing race with 1,000 pounds of force, or the two drive motors that engage a 10-foot-diameter bull gear to turn the outboard arrays.
There are no spare drive motors in orbit, but two spare trundle bearings are available if it turns one one of the 12 in the right-side SARJ is the source of the contamination. But it was unclear whether a replacement operation would be attempted during Discovery's mission even if the problem is clearly identified Thursday.
While Suffredini held open the option of adding solar array repair work to Thursday's spacewalk, engineers appeared to favor sticking with the SARJ inspection and using the additional time to come up with a plan to address the array tear.
"We need power out of P6 in order to go ahead and live with the SARJ problem," Suffredini said. "This is an additional challenge for us. Station is a robust vehicle and we have many, many options of how to deal with the problem. And so it's not a situation where anybody's particularly panicked. But if, on the other hand, what we want to do is get this fixed to a point where we can continue assembly the way we had planned, this is a significant challenge for us to deal with.
"But we will deal with it. If we get the array down and we cut the snag and we figure out how to reinforce it, we'll deploy the array. It's giving all the power we need, that's all we need out of that array, is power. It doesn't have to look good, it just has to get us power. It's not about style points at this point with this array. We need it out and we need about as much power as it provides today. Any one of the fixes we have considered ... will be plenty of power for us as a program."
In the meantime, he said, "I don't want to do any more damage to the array than has already been done."
"We need to first figure out what the condition is, how we're snagged, whether that snag is putting us at risk of ripping further or worse in my mind, ripping the power cables," Suffredini said. "Before the shuttle leaves during this docked mission, we will know what we're dealing with and we'll have assessed what to do on the next shuttle flight by then. ... But right this instant, I wouldn't want to plan (the Columbus mission) until I know better what's going on with this array."