Station crew begins spacewalk for inspections
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 18, 2007
Space station commander Peggy Whitson and flight engineer Dan Tani began a planned 6.5-hour spacewalk today to inspect two critical solar array repositioning systems that are not working properly, curtailing the lab's electrical power.
For identification, Tani is wearing a suit with red barber pole stripes around the legs while Whitson's suit is unmarked. Tani's call sign is EV-1 and Whitson is EV-2.
At the Kennedy Space Center, meanwhile, engineers are preparing to pump super-cold liquid hydrogen rocket fuel into the shuttle Atlantis' external tank to troubleshoot an elusive problem with low-level engine cutoff - ECO - sensors that derailed two launch attempts Dec. 6 and 9.
Special test equipment has been spliced into the ECO sensor circuitry to help pinpoint a presumed wiring or connector problem that only shows up after the system has been exposed to liquid hydrogen at minus 423 degrees Fahrenheit.
Fuel loading is scheduled to begin at 7 a.m. Assuming a problem area can, in fact, be identified and repaired, NASA managers hope to make a third launch attempt Jan. 10 to kick off a long-awaited mission to deliver Europe's Columbus research lab to the space station.
To provide the electrical power ultimately needed to support Columbus and two Japanese research modules scheduled for launch in February and April, the station's huge solar arrays must be able to rotate and turn from side to side to keep the panels face on to the sun as the lab complex circles the globe
The station is equipped with two massive solar alpha rotary joints, one on each side of the lab's main power truss, to do just that, slowly turning outboard arrays like giant paddle wheels.
Each solar blanket also is equipped with a so-called beta gimbal joint to turn the arrays from side to side, like changing the pitch of a propeller, to maximize electrical output.
The port-side SARJ is operating normally as are the four beta gimbal assemblies on the four left-side solar wings. But the starboard SARJ is locked in place because of excessive vibration and recently discovered internal metallic contamination and bearing race ring damage.
In addition, one of the two beta gimbal joints on the right side is now locked in place because of circuit breaker trips Dec. 8 that may have been caused by damage resulting from space debris or a micrometeoroid. During today's spacewalk, the S4-1A beta gimbal assembly inspection is first on the crew's to-do list.
As for the starboard SARJ, engineers suspect one or more of the 12 trundle bearings that press against the 10-foot-wide race ring with 1,000 pounds of force could be causing the observed damage. Data from instrumentation shows vibrations were highest near bearing assembly No. 5.
Whitson and Tani plan to remove up to 22 thermal covers around the race ring for a detailed inspection of all the bearings. They also plan to remove trundle bearing No. 5 for eventual return to Earth aboard Atlantis.
"After we've inspected under as many panels as possible, we will bring in trundle bearing No. 5, unless we find one that we think was more of a problem," Whitson told reporters during a news conference last week "The ground has data that suggests that maybe that's where the problem is. But if we can visibly tell it's a different one, we'll bring in the one that we think is the troublemaker."
The astronauts will start by removing the double-wide thermal panel over one of two motors called drive lock assemblies, or DLAs. That panel, No. 20, also will expose trundle bearing No. 5 to view. The astronauts plan a so-called "pull test" to gather additional strain data from nearby sensors.
Whitson and Tani then will remove a double-wide cover No. 5 to expose the second drive lock assembly and nearby bearings. Splitting up, the spacewalkers will remove eight more double-wide covers before turning their attention to the single covers.
Along with visually inspecting the SARJ mechanism, the spacewalkers will take digital photographs, use a mirror to look on the back side of the race ring and use adhesive tape to collect samples of previously observed metal shavings.
Whatever the problem might be, NASA needs to fix it and restore the right-side SARJ to normal operation as soon as possible.
The SARJ features two identical drive gears and two redundant drive motors. In a worst-case scenario, spacewalking astronauts could install fresh bearings on the undamaged race ring and reposition the drive lock assembly motors. The other option is to clean up the contamination that's present on the damaged race ring, fix whatever is causing the problem and resume normal operation.
"Once they have more data, they can make a better assessment of which of those approaches we should do, whether we should clean up the current race ring or just shift over," Whitson said. "Obviously, shifting over (to the other race ring) involves a lot more software changes and limits us on the timing. So the guys on the ground will have to make that decision. What we're providing (Tuesday) is additional data.
"I think either one's doable," she said. "To me, in my mind, I think it would be probably, from an astronaut's perspective, easier to just shift to the other race ring rather than trying to clean it up. But we don't know yet how easy that's going to be to clean up."
Kirk Shireman, deputy manager of the space station program at the Johnson Space Center, said no such decisions will be made until engineers have a better idea of what might be wrong.
"All options are still open," he said. "The big key question for us is what caused the issue, because we're hesitant to go to the only remaining ring we have on that side without understanding why this occurred. If you moved to the other ring and had the same problem, that's a really big issue for us. So we need to understand what's the root cause, what caused it to start and then propagate. We're working really hard to get that answer.
"There are some ideas, all kinds of ideas about how we could continue to use this ring for a while and then transition to the other ring," he said. "I don't think we've taken anything off the table at this point. I will tell you that most folks are thinking that we will go to the outboard ring at some point in time, it's just a matter of exactly when it makes sense to go and do that. It's not a very easy thing to do. It's a very EVA-intensive task and we do give up some redundancy."
A more immediate problem is fixing the S4-1A array's beta gimbal assembly, which is currently locked in place in an orientation that limits the panel's ability to generate electricity.
During routine operations Dec. 8, two circuit breakers tripped, possibly the result of a space debris impact that might have damaged the mechanism that allows power and data to flow through the rotary joint used to turn the array about its long axis. The trips also could be due to damage in critical cables.
Whitson and Tani plan to inspect the beta gimbal assembly and associated cables to look for signs of damage before moving onto the SARJ inspection.
"The idea is, we'll conduct the EVA right now, the SARJ inspection and the BGA inspection, and we'll learn what we need to learn," Shireman said. "Then we'll find the most opportune time to go fix it, not only the BGA but hopefully the SARJ. It really depends on how our analysis comes out. We'll be (working) to figure out exactly how long we can go with the BGA locked and the SARJ restrictions we have in place."