SARJ inspection added to Sunday spacewalk
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: October 26, 2007
NASA managers said today's installation of the new Harmony module on the international space station, and work by two spacewalking astronauts to prepare the stowed P6 solar array truss segment for its long-awaited detachment during a second spacewalk Sunday, went "extremely well." There were no problems of any significance and all of the crew's major objectives were accomplished.
While the spacewalk was going on, NASA managers decided to add a task to Sunday's excursion, a visual inspection of the massive rotary joint on the right side of the station's main power truss that turns the lab's starboard solar arrays to keep them face-on to the sun.
The station is equipped with two such solar alpha rotary joints, or SARJ units, to turn the left and right solar arrays like giant paddle wheels. The port SARJ is operating normally, but engineers recently noticed increased electrical demands and vibration in the right-side SARJ that indicate unwanted friction somewhere in the system. An alert flight controller also noticed the vibration while watching zoomed-in television views from a camera on the truss. At first, controllers thought the camera mounting might be loose. But they quickly realized the vibration was correlated with the current spikes.
"We're seeing some increased currents, which are indicative of some increased friction on that joint," said Kirk Shireman, deputy space station program manager at the Johnson Space Center in Houston. "We've collected most of the data we can collect with the sensors that we have in place and what we need to do is get some visual inspection of that joint. So ... we made a decision to send one of the crew members out to that interface during EVA-2 and actually go 360 degrees around that joint and look at specific bolts and thermal blankets and make sure they're all in (proper) configuration."
The SARJ features 22 multi-layer insulation thermal blankets that are "cantilevered over the joint," Shireman said. "So they rotate around and there's another surface that's right below it, so these blankets could be dragging on that surface and that would cause an increased drag. There's also some bolts that come out and actually were holding these blankets from the rotating side. When you undo those bolts, which we did (when the SARJ was installed), they swing out of the way. But if one of those bolts didn't swing out of the way, it could be there, it could be dragging.
"So there are a number of things we believe visually we could see just from doing this inspection. Internally, where the actual bearings and the gears and the drive lock assemblies (motors) are, there could be foreign object debris inside there, there could be a misconfiguration of not only the drive lock assemblies, there could be a misconfiguration of even some of the structure that's underneath there."
If the visual inspection Sunday doesn't turn up anything obvious, Shireman said, "it's likely we'll do additional troubleshooting in the future."
Shireman said the SARJ is equipped with redundant drive motors and redundant electronic controllers, which are equipped with circuitry that would prevent the motors from drawing too much power. The current spikes have been seen regardless of which motor was driving the joint and even when the joint was rotated in the opposite direction.
Derek Hassmann, lead space station flight director, said the SARJ issue poses no immediate threat to the station. Shuttles can dock and undock as needed and if worse comes to worse, the starboard arrays can be locked in a favorable orientation for power generation while troubleshooting continues.
"One of the concerns we have about the way the SARJ is behaving is the potential it might stall in a position that's not optimal for power," Hassmann said. "So one of the things we have been talking about ... is to find a current value at which we're going to stop rotating the SARJ and put it in a position that's good for power consumption. We've basically got constraints to find that will allow us to park the SARJ in a good power producing attitude before it would stall. As long as we can get it into an attitude that's reasonably good for power generation, combined with what the other SARJ can produce, we wouldn't have any significant power impacts that we couldn't deal with."
During a six-hour and 14-minute spacewalk today, astronauts Scott Parazynski and Doug Wheelock retrieved an S-band antenna assembly for return to Earth; prepared the Harmony module for installation; and unplugged ammonia coolant lines leading to the P6 truss segment in preparation for moving the 35,000-pound segment to the far left end of the station's main power truss. They also installed thermal blankets to protect sensitive electronic components from temperature extremes after the truss is detached.
While that work was going on, the astronauts inside the shuttle-station complex robotically locked Harmony in place on the left side hatch of the central Unity module. If all goes well, the crew will enter the new module Saturday and begin removing more than 700 bolts that added rigidity during launch. After Discovery departs, the station crew will move Harmony to its permanent mounting point on the front end of the Destiny laboratory module.
"I would ... classify this as a hugely successful day," said Dina Contella, the mission's lead spacewalk officer. "The EVA team is really happy to have conducted such a successful EVA. Scott and Wheels did a fantastic job. This was Wheels' first EVA and he did it like a real pro. So we're hugely thrilled with how the day went."
During disconnection of four ammonia umbilicals, Parazynski reported seeing a few flakes of ammonia ice spew out into space.
"There is a history with those (quick-disconnect fittings) and we have quite a bit of crew training that goes into what we would do in case there's a little bit of ammonia that comes out," Contella said. "Of course, our big worry is that we have a lot of ammonia come out, and that was not the case today. Really, he jiggled the connector and a few flakes came off and that's about all we saw. But just to be on the safe side, we went ahead at the time and had Wheels come over and look at Scott and make sure he didn't have any visible ammonia on him and ran through a calculator to make sure we were outside long enough to make sure the ammonia would bake off the suit had there been any on there. And then when we came back in the airlock at the end of the EVA, we did do some testing to verify we were not going to bring any ammonia into the cabin. It was all pretty conservative, but necessary."
During Sunday's spacewalk by Parazynski and station astronaut Dan Tani, the P6 truss segment will be disconnected from its central mounting point atop Unity. It is scheduled for installation and redeployment on the far left end of the main power truss during a third spacewalk Tuesday.