Spacewalkers find damage in solar array rotary joint
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: November 24, 2007
Astronaut Dan Tani, looking inside the space station's right-side solar alpha rotary joint today, reported metallic contamination on the main gear bearing race similar to debris he saw in a different area of the joint during an inspection late last month. He and space station commander Peggy Whitson also agreed the race ring appears to be damaged due to metal-on-metal interference of some sort.
As with the earlier inspection under thermal cover No. 12, the debris seen today under cover No. 7 appeared to be magnetized and the bearing race ring appeared to be pitted and abraded in areas. Rubbing it with the finger of an over-glove, Tani reported he could not tell where his finger had touched.
"So it does look like the race ring may be pitted, or at lest somehow damaged," Whitson observed.
"I agree it's damaged," Tani added. "I don't think it's just on top."
The space station is equipped with two solar alpha rotary joints, or SARJs, one on each side of the lab's main power truss. The SARJ joints rotate outboard solar arrays like giant paddle wheels as the station circles the planet, keeping the blankets face-on to the sun to maximize electrical output.
Each joint features two redundant gear/race rings and two drive motors, only one of which is engaged at any given time. Twelve so-called trundle bearing assemblies are positioned around one of the two gear races to allow smooth rotary operation.
The left-side SARJ is rotating normally, but in recent months, flight controllers noticed unusual vibration and slightly higher current levels in the right-side SARJ. Tani, who was ferried to the station aboard the shuttle Discovery last month, looked inside the joint during an already planned spacewalk Oct. 28.
He spotted metallic contamination and collected samples using adhesive tape. Those samples later were determined to be made up of race ring material. At that point, mission managers decided to lock the starboard SARJ in place to prevent additional damage. Engineers assessing a variety of possible repair options asked for today's inspection to gather additional insight into what might be causing the presumed interference.
In a worst-case scenario, the 12 bearing assemblies and two drive motors could be moved to the redundant gear during three to four spacewalks. But engineers do not want to consider such a drastic step until they figure out what is causing the problem with the active gear and race ring.
The problem is not serious in the near term because the station's arrays can generate enough power even with the starboard blankets locked in place. But the starboard SARJ must be back in operation by next April when NASA plans to launch Japan's Kibo research module.
"I see the same damage I saw on the other panel, 12, I went to before," Tani reported. "In fact, I would say there are more shavings here. Well, I don't know if that's a fact or not, but there certainly are shavings. Again, it looks exactly like, remember the other one? Where there's obvious magnetic attraction? It is attracting most of the shavings. I'm looking on the gear, the gears are not any more dirty than the rest of the inside structure.
"The inside race, the stationary race, I can see two very clean surfaces. There is dusting on the stationary perpendicular race that is perpendicular to the base of the triangle. And the outboard race, the base of the triangle of the moving race is very clean except for the dusting of the debris. You can kind of see where the motion of the bearing along that race has sort of cleaned off a central area in the middle, you know, where it bears against the ring. But I don't see any damage. All the damage appears to be on the same surface, the angled race that faces outboard or outside of the moving race."
Tani said the shavings appeared to cluster near the trundle bearing closest to view, presumably the result of magnetic attraction. He and Whitson agreed the drive gear itself appeared to be in good condition.
"I'm looking at, I guess, trundle bearing 10," Tani said. "In general, it looks like the trundle bearing itself looks like it's in good shape. I don't know enough about the mechanism to know if I can see any rotating parts on it. ... There are shavings all over the bearing that's closest to me that I can see. Um, the majority of the shavings ... there's a big buildup of shavings in a circular magnet form just adjacent to the damaged race.
"I would say the debris is concentrated in what I would say is obviously magnetic and is very lightly dusted over most of the other surfaces I can see. If you ran your finger across it, i imagine you would see the path your finger took. Like a table that needs dusting. There are shavings on the gearing, but it doesn't like like the gearing itself is damaged."
Whitson and Tani took numerous digital photographs of the race ring before leaving the area. The thermal cover and debris shields were not reinstalled. At some point after the next shuttle mission in December, a camera on the end of the station's robot arm will peer into the joint while it is commanded to move through a full rotation.
In the meantime, NASA plans to launch a full set of replacement trundle bearing assemblies and a new drive motor to facilitate any repairs that might ultimately be ordered. Engineers are considering a broad variety of options, ranging from the worst-case switch to the redundant drive gear to minimal fixes that would permit normal operation even with some level of damage.
Today's inspected was added to the spacewalk timeline late this week. The primary goal of the excursion was to connect a second set of ammonia coolant supply and return lines to the new Harmony module to complete its activation and clear the way for launch of a European research module next month. That work was completed without incident. The astronaut are wrapping up a few final get-ahead tasks before returning to the Quest airlock module.