Engineers refine repair
options for solar array
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: October 31, 2007
Repairing a mangled space station solar array is now NASA's top priority because of concern the ripped, partially deployed blanket could pull apart under the stresses and strains of normal operations, possibly forcing a future crew to dump the panels overboard, NASA officials said today.
"Given the fact we could potentially damage this array if we leave it in this configuration, and if we damage this array enough, we could potentially not have it available for the life of the program, this then becomes our priority."
Earlier today, NASA managers called off plans for a spacewalk Thursday by Scott Parazynski and Doug Wheelock to inspect a contaminated solar array rotary joint on the right side of the station's main power truss. Instead, Wheelock and Parazynski, a former emergency room physician, will attempt a daring bit of space surgery to effectively sew up the two rips in the P6-4B solar blanket on the far right side of the power truss and possibly cut away a fouled guidewire.
While engineers have not yet finalized the repair plan, the general idea is to put Parazynski on the end of a shuttle heat shield inspection boom attached to the space station's 50-foot-long robot arm. With the space crane based at a work site at the far left end of the power truss, Parazynski could be maneuvered into position near the damage site on the P6-4B array.
Wheelock, meanwhile, likely would be anchored at the base of the damaged array to provide verbal guidance cues to arm operators back inside the Destiny laboratory module.
Engineers are still debating the details of the surgery, but one option would be for Parazynski to insert pre-made tabs that work like cufflinks through holes in the blanket slats that were used to secure the panels during launch. Fold-out latches, like the wings of a cufflink, would prevent a tab from pulling back out of a hole. The other end would be inserted through the corresponding alignment hole in an adjacent slat.
The idea is for the connected cufflinks, or something like them, to carry the 70 pounds or so of tension the blanket experiences when the array is fully extended. It is that tension that provides the necessary structural stability and, in this context, the force that could pull the ripped slats apart if nothing was done to strengthen the area.
"There is a lot of work ahead of us on this," Suffredini said. "The team has worked extremely hard. We've got a lot of great ideas. ... We feel pretty good about our chances of getting out for an EVA-4 on Friday. But we haven't actually made a determination that's what we're going to do. We've asked the team to go work this. Later on, late tonight, we're going to look where we're at. If we have the procedures in place and we're comfortable we have a plan that we can implement then we will formally ask the crew to implement that on EVA-4 on Friday."
Covering all the bases, Suffredini said "what I'd like to have if at all possible is another opportunity to go outside if we need it."
"First, we do EVA-4 on Friday and we get as much work as we can get done," he said. "If we're successful, we can call it a mission and let the crew stand down, we'll get the shuttle all configured for return (and) we'll let the shuttle go home. ... If, on the other hand, we don't quite get all our work done (on EVA-4) - and that's a distinct possibility - then we'd like to have the option to pursue an EVA-5 on Sunday."
In that case, Discovery's mission - already extended one day because of now-canceled plans to carry out a rotary joint inspection - would have to be extended an additional day.
"So where we are as a program is, the team is off trying to come up with way to approach the array, to clear the snag and then perhaps install some load-bearing straps, if you will, to take on the load so we can redeploy the array," Suffredini said. "If we can't get all our work done on Friday, we'll sit down with the shuttle team and the ops team and look at whether or not we can gain ourselves a second EVA on Sunday and have the shuttle leave one day later."
Parazynski and Wheelock will carry out the proposed Friday spacewalk, with Parazynski, one of NASA's most experienced spacewalkers, on the end of the 50-foot-long orbiter boom sensor system.
In normal operations, the shuttle's robot arm latches onto a grapple fixture at one end of the boom. The other end carries a laser scanner and camera for use in heat shield inspections, along with a fitting that can accommodate an astronaut foot restraint. The station's arm is not compatible with shuttle grapple fixtures and instead will have to pick it up using a station fixture mounted at the center of the boom. That will extend the station arm's reach by about 25 feet instead of the full 50.
The station arm currently is parked atop the space station's mobile transporter at work site 8 on the far left end of the main solar power boom. If the repair plan is approved, the arm will be moved back to a central worksite Sunday to pick up the OBSS, currently mounted in Discovery's cargo bay. The transporter then would take the station crane and the OBSS back to work site 8 to await Parazynski and Wheelock on Friday.
It is not yet clear who would carry out a second spacewalk Sunday should one be required. But Suffredini made it clear NASA needs to fix the torn blanket and a second spacewalk opportunity would provide time to complete any unfinished work or perhaps correct any additional problems.
"The snag is providing a sort of ripping function on the blanket," Suffredini said. "Right now, we're supposed to be able to distribute 70 pounds of load across a 15-foot hinge and we're missing about three feet of that hinge. Not only are we trying to distribute that load across the remainder of the hinge, but you also have a high stress area right where you see the rip beginning.
"Given that configuration, the fact that we have sort of a forcing function the way it's being ripped up, we believe we're in a condition where we could, over time, tear the blanket further. And if we do enough damage to the blanket we could potentially get in a configuration where we couldn't stabilize the array and if we can't stabilize the array, we'll have to figure out what to do about that and we don't have a lot of options. The most likely option is we'd have to jettison it. So before we get to that position ... we've made it a priority to go repair it. It's stable right now, we've got time to go work this problem."
In yet another major change for mission planners - and the Discovery astronauts - the Mission Management Team today approved a revised landing strategy that would move re-entry next week from before dawn to the afternoon. The change will require the astronauts to adjust their sleep cycles and fly a trajectory that will carry them over the heartland of America, a northwest-to-southeast flight path NASA has avoided since the 2003 Columbia disaster. The change will permit additional landing opportunities.