Station to get new ring for damaged solar rotary joint
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: September 25, 2008
Engineers believe they have finally pinned down the cause of metallic contamination and degradation in a critical solar array rotary joint mechanism aboard the international space station, Program Manager Michael Suffredini said today. The problem apparently was caused by the premature loss or breakdown of a gold plating intended to lubricate the joint.
During the next station assembly flight in November, shuttle astronauts will replace bearings in the joint, attempt to clean up the contamination and apply a lubricant to the surface of the bearing race.
The idea, Suffredini said, is to stabilize the joint, reducing stress and vibration so it can be periodically moved to maximize solar energy generation. While the starboard solar alpha rotary joint, or SARJ, features a redundant, uncontaminated race ring, Suffredini said the long-range solution is what amounts to major surgery: astronauts will partially disassemble the joint and insert a new race ring to take over from the one that has been damaged.
"It is our intention today ... to essentially not go to outboard ops and use the redundant ring that exists on orbit today," Suffredini said. "We intend to bring up another race and we will attach it to the race that's damaged and then roll on that race and save the outboard race for later in the life of the international space station."
The space station's main power truss features two SARJ mechanisms, one on the left side and one on the right, that were designed to turn outboard solar arrays like giant paddlewheels to track the sun as the station circles the globe. The left-side of the truss is complete, with two sets of arrays in place, and it has not experienced any problems.
The starboard truss has one set of arrays in place with another set scheduled for attachment early next year. Engineers noticed high currents and vibration in the mechanism in the summer of 2007. When spacewalking astronauts took a look inside last Fall, they discovered extensive contamination in the form of metallic shavings and degradation to one face of the bearing race.
The 10-foot-wide toothed drive gear at the heart of the joint is held in place by 12 trundle bearing assemblies, or TBAs, which distribute the load as a powerful motor drives the main gear and outboard arrays. One TBA was taken off the starboard SARJ in a subsequent spacewalk and returned to Earth for analysis. It was replaced with a pristine unit.
During the next shuttle assembly flight, scheduled for launch Nov. 16, "we intend to go on the starboard side and remove the remainder," Suffredini said. "There are 12 trundle bearings, we've replaced one. We intend to remove and replace 11 other trundle bearings in order to bring those all home to help us with root cause.
"We will take that opportunity to clean up that race and then we'll also lubricate that race. What we're doing here is we're trying to modify the system just enough so that when we do have to rotate it, we minimize both the vibrations associated with the damage to the race and the contamination that's currently on the race, that helps us with structural life. In addition to that we want to reduce the amount of current required to drive this joint to make sure we never reach the maximum current the motor can drive to. So that's what these two steps do."
As a precaution, the astronauts also plan to lubricate the port-side SARJ.
"We have concluded the most likely cause of this anomaly is due to high friction, which was caused by the loss of lubrication in the joint when it flew," Suffredini said. "The way we lubricate that joint is we put a gold plating on the (TBA) rollers. This is a very soft material and over time, it kind of wears off the roller and finds its way onto the race and fills in the very small, microscopic holes and provides, basically, a lubricant that will wear over time. But it was intended to wear very slowly over time.
"We have found through a bit of research in the paperwork that was put together before we flew, and some of the information we gained from the trundle bearing we returned home, that we believe the gold prematurely came off these rollers, either because of a condition pre-flight or because of the process used to install the gold just wasn't adequate for the conditions that it saw. We have proven through testing that once you take the lubrication off this joint, it will damage the race very, very quickly."
Up to this point, the only long-range option appeared to be switching to the outboard race ring. But Suffredini said installing a new race will permit the station to continue operating with its current software and still preserve a backup in case of additional problems down the road.
"The downside to that is we have to basically separate the truss at a joint that wasn't made to separate on orbit," he said. "This is not the joint we put together on orbit, this is a joint that was assembled prior to flight and flew as an integrated truss. We referred to it at the time as S3/S4.
"We have a technique for doing that," he said. "We have to build the hardware, basically build some jack screws and we'll attach them to where we had some launch locks. And we'll basically separate this joint about 10 inches and we'll slip this new race ring in, install it and then pull it back together. To do all that, it won't happen tomorrow. It'll probably take us to late in 2010 before we have all this hardware ready to go and can get this race ring on orbit. But that is the current plan."
A detailed electrical analysis shows the station will have enough power for normal, or near-normal, operations until the new race can be installed.