Station chief gives detailed update on joint problem
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 25, 2008
Analysis of metallic contamination from a critical solar array rotary joint on the international space station indicates a "high-friction event" of some sort, possibly a misaligned bearing roller or some other like defect, has chewed up and damaged one of the surfaces of a 10-foot-wide gear and bearing race, the station's program manager said Monday.
A definitive answer to what caused extensive damage to the race ring is not yet complete and as a result, NASA managers have not yet decided what sort of repairs might be needed to restore the joint to normal operation. But Program Manager Mike Suffredini said an analysis shows the station's solar arrays can generate enough power for near-normal station operations through the rest of this year and early next with the right side solar alpha rotary joint, or SARJ, locked in place or only occasionally moved to improve power generation.
"We may have to manage how we do research, but we should be able to do the research that we planned," he said.
But this is a critical issue for the long-term health of the space station and Suffredini provided the most detailed update yet on where the ongoing troubleshooting stands.
The space station is equipped with two SARJ joints, one on each side of its main power truss. Each SARJ features two 10-foot-wide drive gears, one of which is a backup. The main gear/race ring is gripped by 12 so-called trundle bearing assemblies spaced evenly around the circumference. The trundle bearings each feature three rollers that grip different faces of the drive gear/race ring. The gear is driven by a motor called a drive lock assembly, or DLA. Each joint features two DLAs, but only one is used at a time.
The SARJ system is critical to the station's long-term viability. To provide the power necessary to run all the station's planned science operations, along with life support systems, computers, communications gear, cooling systems, etc., the solar arrays must track the sun as the station circles the planet.
The station's left-side SARJ is operating normally. But last fall, engineers noticed higher-than-expected vibration levels in the right-side SARJ and drive motor power spikes indicative of mechanical resistance of some sort.
During a subsequent spacewalk inspection, extensive internal metallic contamination was discovered in the form of apparent shavings found clumped and scattered across one face of the race ring. The surface of the ring itself appeared mottled and damaged. To minimize additional damage, flight controllers decided to suspend "auto-track" drive operations, although the starboard SARJ is occasionally repositioned to improve power generation.
During previous spacewalks, astronauts removed 17 of 22 thermal covers around the circumference of the joint to look for signs of micrometeoroid impact damage or any other issues that might explain the problem. In addition, one of the 12 trundle bearing assemblies positioned around the race ring - TBA No. 5 - was removed and returned to Earth for analysis, along with samples of the metallic debris.
During the Endeavour crew's fifth and final spacewalk Saturday, astronaut Mike Foreman removed the final five thermal covers and looked inside the joint. There were no signs of any impact damage. He also examined a small spot on the undamaged "datum A" race ring that was seen in earlier photographs. Engineers were not sure whether the blemish was a pit or a bump-like defect caused by a buildup of contaminants. Foreman said it felt rough to him, indicating it might be a depression, but with thick gloves on, the astronaut could not be definitive.
NASA managers initially held out hope spacewalking astronauts could somehow clean up the contamination and restore the joint to normal, or near-normal, operation. That no longer appears possible and NASA is expected to order a switch to the redundant drive gear at some point. But that's a last-resot sort of option. Engineers want to make sure they understand the cause of the problem first, to make sure the same thing won't happen on the backup gear.
"SARJ is going to take us a while to figure out," Suffredini said. "But we did bring home quite a bit of information on the last flight and we did learn a few things that we're still working on. The data does suggest, from the way that the material has come off of the race, that it was caused by a high-friction event. We have been postulating for a while that perhaps contamination had gotten on the race and when we rolled over it, we created a weak spot in this brittle surface that we talked about, perhaps it was damaged that way, kind of like the pothole in the road theory, and chewed it up. The problem with that theory was we chewed it up awfully quick if it started off as one small spot.
"So based on the way the material has come off and where the fracture lines are - and it is incredible to watch these guys do this work to try to do detailed analysis of these very small particles we brought home - but they've been able to conclude that ... some of these larger flakes that we brought home were the result of high-friction events. And what that tells you is that perhaps we had either a roller that was cocked or one that wasn't quite rolling as freely as we thought. There are a number of scenarios that lead you down that path. So that is a piece of information we gleaned from that.
"One of the interesting things from the last EVA, we sent the crew to go look at what we had assumed was contamination, maybe I should say we wished was contamination. The crew seems to have indicated that the spot that they saw was rough. Now that may be contamination that got pressed on to the race as we rolled over it. That is a more likely candidate, but we couldn't tell. Was it rough because it's high? Was it rough because it's low? So we'll have to think about that.
"Damage to datum A would be an interesting piece of information," Suffredini said. "To date, we've seen the damage only on the upper inclined surface. And so if you postulated that we had a roller or something that was canted wrong for whatever reason, not aligned quite right, then that would explain why only that surface was damaged. If over a much longer period of time we were starting to see the same thing on datum A, of course, that makes our theory a little more difficult. But there are still 111 branches on our fault tree and 300-plus actions left to be closed. And so our job is to give the team as much time as possible to try to sort this out."
A major question mark throughout the troubleshooting is how long station assembly and operation can go without having full auto-track solar power available. Suffredini said the latest analysis shows NASA can go ahead and attach Japan's huge Kibo research module in May as planned and even bolt on the S6 solar array segment to complete the right side of the station's power truss. That flight, known as assembly mission 15A, currently is scheduled for launch in December, although external tank delivery issues could force a delay.
In any case, the station should have enough power without any SARJ repairs until the next flight in the sequence, launch of a third Japanese component next spring.
"We have, for all intents and purposes, cleared ourselves through the 15A stage, which means we can install the S6 truss on the 15A flight and make our way all the way to the end of that stage," Suffredini said. "As you may know, the flight after that is the exposed facility for JAXA, which requires not an insignificant amount of power. So that's probably the next bump we have to get through on the road.
"In order to get through that hump and keep runway in front of us before we actually have to do something like swap races, one option is we're looking at whether we can do the analysis that lets us temporarily rotate that joint during low-power periods," he said. "Today, we try to hold it locked. And so, if we can get about 60 days worth of rotations out of that joint, then perhaps we can buy ourselves a little more time to finish our failure analysis and decide what changes we might need to make. If it's just a matter of swapping over (to the outboard, redundant race ring), I can do that almost any time. But we might decide there are some hardware changes we want to implement, which would take us a little more time.
"Today I don't know the answer to what causes this problem, we have a number of legs left to work down the fault tree where every day we get a new piece of data. We pulled all the rest of the covers off (during Forman's spacewalk Saturday) and we didn't find an MMOD (micrometeoroid debris) strike that started this whole thing. So that was important information for us. And so if we can give the team enough time, I'm convinced we can figure out how to prevent this once we go to outboard ops. So our job is to work the failure analysis as hard as we can and give the team as much time to do that before we have to swap to outboard ops. That's kind of where we're at."
Asked if the station could operate normally in the near term without an operational right-side SARJ, Suffredini said: "We may have to manage how we do research, but we should be able to do the research that we planned.
"We won't be able to line everybody up and run them at the same time, but typically crew time doesn't allow us to do that anyway," he said. "So I would not see any major implications to research during that particular stage. Somewhere in there, we'll also finish our analysis on whether we can rotate the SARJ for a certain period of time and so we'll have that in our hip pocket as well."
NASA has one spare set of trundle bearings. Suffredini was asked if NASA might consider simply installing the new bearings on the damaged race ring, preserving the option of switching to the outboard ring later if necessary.
"The race is damaged to a significant level such that my personal opinion is it would be difficult to get the structural life that we would need out of the outboard truss if we kept running along that damaged race," he said. "That is something we've looked at and thought a lot about. Of course, when we go to outboard ops we'll replace all of those bearings. So if we can figure out that that's the cause, then we have a dozen of those bearings sitting on the ground and we can certainly check them for whatever we believe the root cause is."
One option, perhaps, would be to "go ahead and do the reconfiguration to outboard ops but not rotate, get the rest of those trundle bearings home and see if we can find a smoking gun."
"But that's just one of many things that we've talked about doing," Suffredini said. "But I don't think I have an option, if I have to continually rotate the SARJ, I really don't think structural life is going to allow me to rotate on that bad race. Other than structural life, though, I think the system can drive through and our speculation is it will get easier over time as we knock the high points off that rough surface. I think the system can drive through it even with the existing bearings we have, but it'll just chew up the structural life of those quickly."
Asked if NASA had ruled out moving the bearings to the outboard race this year, Suffredini said "I learned a long time ago not to make those kind of predictions. You guys trained me that way."