Spacewalker finds debris in critical station truss joint
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: October 28, 2007
Space station engineers are scrambling to determine the source of unexpected debris in a critical solar array rotary joint and considering whether to order an additional, more thorough spacewalk inspection to figure out what sort of downstream repair work might be necessary.
The international space station's right-side solar alpha rotary joint, or SARJ - a 10-foot-wide, 2,500-pound motorized gear used to turn outboard arrays to keep them face on to the sun - needs to work normally to generate the electricity required by the growing space station's myriad systems.
The Discovery astronauts attached a new module to the station Friday that will serve as the attachment point for European and Japanese research labs scheduled for launch in December, February and April.
Mike Suffredini, manager of the space station program at the Johnson Space Center in Houston, said the starboard SARJ can be locked in place in the near term while engineers study the contamination problem and its possible solution. He said even with the starboard S4 solar arrays locked in place, they would generate enough power, in concert with two set of arrays on the left side of the station's main truss, to permit launch of the European Space Agency's Columbus module Dec. 6 as planned.
But at some point over the next few months, the problem must be resolved or the station might not be able to provide the required power when two Japanese research modules are attached in February and April.
"We have lots of time to work through this problem, it's not an immediate issue," Suffredini said. "The system is robust in terms of providing the power we need. We know how to operate around it so we can get all the power that we require. So we can pamper, if you will, the joint while we sort through the anomaly and make sure we fully understand it before we go back to nominal operation."
The space station ultimately will be equipped with four sets of huge solar arrays, two on each end of a beam the length of a football field. A SARJ on each side of the truss rotates the outboard solar arrays like giant paddle wheels as the station circles the Earth to maximize electrical generation.
The station currently is equipped with one set of arrays on each side of the power truss: S4, or starboard 4, on the right and P4 on the left. A third set of arrays, P6, initially was mounted at the center of the station to provide power during the initial stages of construction. That 35,000-pound segment, with its arrays stowed, was detached today and will be installed on the far left end of the power truss during a spacewalk Tuesday. The fourth and final set of arrays, S6, is scheduled for launch next Fall.
The port SARJ has operated normally since its installation last year and the starboard joint, installed last June, showed no immediate problems. But just under two months ago, engineers began noticing higher vibration levels and power usage. While the port SARJ motor operates at an average of 0.1 amps, its counterpart on the right side has been averaging 0.2 to 0.3 amps with peaks up to 0.9 amps.
Believing an insulation panel might be rubbing against something in the mechanism, NASA managers added an inspection to today's spacewalk. Astronaut Dan Tani took a look around the perimeter of the joint and saw nothing out of the ordinary. But when he removed one of 22 insulation blankets to look inside, he was surprised to see what appeared to be metal shavings on the main bearing race and drive gear.
"We have been watching a slight increase in currents on the starboard SARJ joint," Suffredini said. "The currents indicated to us some friction increase in the joint, so that's why we asked the ops team if they would take a look. We did find what seems to be an indication of some particulate that we would not expect in the joint.
"In addition to that ... the flat surface right below the gear is the race the bearing runs on, it looks kind of mottled and it we would expect it to look shiny. There's about 1,000 pound force on that surface, so any little bit of particulate on that surface could potentially cause additional drag."
Each SARJ features two huge drive gears, one inboard and one outboard, and two drive lock assemblies, or motors, and associated electronics. Both DLAs can be positioned to engage the drive gear by remote control and both are on the outboard side of the starboard joint. Only one DLA is used at a time.
"This is a very low current motor that's driving these joints, very high precision joints," Suffredini said. "So any little bit of particulate, of course, can have an effect on the ability of the DLA's to overcome the friction in the joint. Of course, if the friction gets high enough, then we can't drive the joint. Now we're not at that point today, but it was important for us to start looking at it because we have had some peaks as high as .8 and .9 amps."
Tani described the debris in the joint as metallic shavings. Suffredini said engineers on the ground believe it may be aluminized mylar insulation, a tinfoil-like material on the interior of the 22 thermal blankets mounted around the joint. The blankets are anchored on the outboard side of the joint and extend across the drive gears like diving boards with about a half-inch of clearance. They rotate around the SARJ as the outboard truss rotates.
Suffredini said if any of the foil under one or more thermal blankets was rubbing against the outboard bearing race or gear, it would be damaged and that could explain the debris seen today. Finding the source of the debris may require removing all 22 thermal shields. How to clean up the debris that's already there could be a more difficult problem.
"We'll spend quite a bit of time trying to figure out what we should do next relative to this investigation," Suffredini said. "In the meantime, our plan is to limit use of the SARJ in what we call the auto-track function, simply the function that allows it to follow the sun as we go around the Earth. We have completed our assessment of the remainder of this mission and believe for the most part, we can keep it parked. We'll have to reposition it every so often in order to deal with a couple of thruster firings and, I believe, for undocking. But repositioning is not a big impact.
"In the meantime, the team is off assessing the ability to park the SARJ for the (period between Discovery's mission and the December launch of Columbus) and for the docked timeframe for 1E (the Columbus mission). My belief, and don't take this to the bank because the guys haven't finished their analysis, but I believe ... we'll have the power we need to limit the movement of the SARJ to the greatest extent possible and still be able to accomplish the objectives of our future missions.
"So, we have time to go work through this anomaly," he said. "We'll talk a lot about whether or not we want to modify one of the EVAs during (Discovery's) flight in order to look some more. I think the prevailing thought is the team would like to go ahead and take off the remainder of the covers. There's 22 in all, so we've looked under one, perhaps look under the other 21 and perhaps find an area where perhaps some portion of the mechanism is rubbing on one of the panels. If not, to get a complete assessment of the entire mechanism to decide what our future plan is. So the team is off working that right now."
The Discovery astronauts currently plan spacewalks Tuesday, Thursday and Friday. Tuesday's excursion is required to attach the P6 solar array to the far left end of the main power truss, a critical operation, and it would appear unlikely NASA would opt to add additional SARJ inspections to the crew's already busy timeline.
The mission's fourth spacewalk is a relatively short excursion to test a heat shield repair technique. The day after that, station commander Peggy Whitson and cosmonaut Yuri Malenchenko plan to stage a spacewalk to get the newly installed Harmony module ready for its eventual move to the front of the lab complex after Discovery departs.
Mission managers could opt to defer the tile repair demonstration and instead have spacewalkers Scott Parazynski and Doug Wheelock conduct an additional SARJ inspection in its place. To have more time for planning, managers also could opt to replace the tile repair spacewalk with Whitson's, extend the mission a day and stage a revised fifth spacewalk to look into the SARJ. No decisions have been made one way or the other.
"We would like very much to get (the Columbus) mission accomplished before the Christmas holidays," Suffredini said. "So It would be our intention to try to do whatever we can during this docked mission. If we don't figure out a way to recover nominal operations with the SARJ, we would figure out how to use it in a limited capacity to get us through the next stage and the next flight and then look at perhaps a subsequent EVA" during a future mission."
In a worst-case scenario, spacewalking astronauts could remove the drive motors and the 12 trundle bearings that press on the outboard bearing race and reposition the components on the inboard gear of the joint, effectively replacing the entire assembly. But that would require multiple spacewalks and is strictly last resort."
"If we decide this damage is great enough we don't want to live with it long term, we could choose to totally reconfigure that joint," Suffredini said. "I suspect that would be at the very end of our list of things to talk about. If we can figure out the source of the problem and reduce the contamination, if we can operate the joint without the vibrations that we're getting - and it's really not the current, it's the vibrations that are the long-term concern for us - then we'd probably try to live as is."
Vibration is more of a long-term concern, he said, because it could lead to life-limiting metal fatique.