Today's spacewalk to finish work on paddle wheel joint
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: November 22, 2008
Astronauts Heidemarie Stefanyshyn-Piper and Stephen Bowen are gearing up for a grueling seven-hour spacewalk today to finish cleaning and lubricating the international space station's damaged right side solar array rotary joint. Engineers in Houston, meanwhile, are continuing tests to troubleshoot problems with the station's new urine recycling system.
Today's spacewalk, the 117th devoted to station assembly and maintenance since construction and the third of four planned by Endeavour's crew, is scheduled to begin around 1:45 p.m.
While the work outside is going on, managers, engineers and flight controllers at the Johnson Space Center in Houston will be assessing test results and plans for coaxing the station's urine processor into normal operation. An operational water recycling system is crucial to NASA's plans for increasing station crew size from three to six next May.
The shuttle Endeavour's mission could be extended one day to give engineers more time to resolve the problem and collect samples of processed urine for return to Earth. As of this writing, however, it's not yet clear whether an additional day would be sufficient to resolve the problem with the urine processor's centrifuge motor.
Flight controllers activated the processor Friday after troubleshooting an alarm that halted operations the day before. The UPA ran normally for two hours, but then shut down. The motor in the centrifuge of the UPA's vacuum distillation system began slowing down while drawing more power. Telemetry indicated a hardware problem as opposed to a software glitch.
The processor was re-activated late Friday and briefly operated to collect additional data. It was re-started again this morning, around 9:15 a.m.
The astronauts do not have a spare distillation unit on board. If a replacement is required, NASA's plan to increase crew size from three to six next May would have to be put on hold until a new unit could be launched and installed. The next shuttle mission is scheduled for launch in February, but it's not known if a new distillation unit could be ready by then.
Even if a new unit could be ready by then, three months of testing and water analysis would still be required before any astronauts would be allowed to sample recycled water.
But a centrifuge replacement is a worst-case scenario and engineers remain hopeful they can get the water recycling system up and running.
"The two main units we've been working on, the water processing assemby and the urine processing assembly, are in different states," station flight director Brian Smith said early today. "The water processing assembly is doing well and it's currently running right now and processing condensate. The urine processing assembly's had a couple of hiccups since we tried to activate it a couple of days ago. It's currently not running.
"There was a problem yesterday that caused it to shut down. The problem, we believe, is in a sub unit called the distillation assembly. The centrifuge is in that assembly and we saw an unusual signature related to the speed of the motor of that centrifuge as well as the current driving it. We saw the speed go down and the current go up. That sometimes is indicative of some kind of blockage of the spinning motor.
"But we really haven't nailed down the exact root cause yet," Smith said. "We did conduct a test overnight where we brought the unit back up and we ran it again and collected some more data and that data is currently being reviewed by the engineers. We won't do anything else until that data is reviewed and we get better understanding of what's going on.
"As far as how that impacts our goals for the water recovery system, of course we wanted to take samples and bring those back home and we are still on track to do that. The original plan was for the water recovery system to process both urine and condensate, collect that together and run it through the water processing assembly to produce water that we could then sample. We're not going to get the contribution from the urine processing assembly that we had planned for by this flight day. But we've been able to come up with a plan to make up for that by generating extra condensate."
NASA's Mission Management Team will meet later today to discuss the schedule and, presumably, whether to extend Endeavour's mission.
Stefanyshyn-Piper and Bowen, meanwhile, are preparing for a seven-hour spacewalk to complete the cleaning and lubrication of the station's right-side solar alpha rotary joint, or SARJ. The station is equipped with two SARJ joints to rotate outboard solar arrays like giant paddle wheels to track the sun as the lab complex orbits the Earth.
The starboard SARJ has suffered extensive damage and degradation due to a lubrication breakdown. The joint's 10-foot-wide drive gear is held in place by 12 trundle bearing assemblies featuring pressure-loaded rollers that grip the toothed gear on three faces, or bearing races. Two of those, known as the datum-A surface and the inner canted surface, are in good shape. But the surface of the third race ring, the outer canted surface, has been ground up by excessive friction, producing extensive metallic debris.
To reduce rolling friction and minimize additional damage to the outer canted surface, the Endeavour astronauts are replacing 11 of the joint's 12 trundle bearings - one was replaced on an earlier mission - cleaning the race ring with scrapers and grease-impregnated wipes to capture debris and, in the process, applying a fresh layer of lubrication.
During today's spacewalk, Stefanyshyn-Piper and Bowen will attempt to remove and replace six trundle bearing assemblies, or TBAs, to complete the bearing swap outs.
"It's going to be challenging," Stefanyshyn-Piper said Friday. "We have a lot of work to do, still a lot of trundle bearing assemblies to change out and a lot of the race ring to clean. ... Steve and I will be very, very busy out there."
During the first spacewalk Tuesday, Stefanyshyn-Piper lost a $100,000 tool bag overboard. Two of the crew's grease guns were lost, forcing the astronauts to improvise. The original plan called for the spacewalkers to remove a TBA and then lay a bead of lubricant down on the outer canted surface with a grease gun. The grease was intended to capture debris as the astronauts chipped away ground-on fragments with a scraper tool.
Because of the lost grease guns, Stefanyshyn-Piper used a different technique in her second spacewalk Thursday, using grease-impregnated wipes to provide the lubrication needed to capture scraped-off debris. Mission managers later approved the use of a heat-shield repair grease gun if necessary, but Stefanyshyn-Piper said she preferred to press ahead with the wipes instead.
"I think at this point, having looked at both the NOAX (heat-shield repair) gun and also having two EVAs under my belt so far, I think the greased wipes seems to be the most practicable way to go," she said. "The wipes do lay down quite a bit of grease on the surface and it's sufficient grease to scrape up the debris that's there. It requires a little bit more meticulous work because you don't have quite as much grease there, but it is sufficient and you can get by. That's the way we're going to press going into EVA-3. Definitely, having two EVAs, I've learned a lot of things, I've changed my techniques ... so I'm ready to go into EVA-3 and get going."
If the spacewalkers get all the bearings changed out, flight controllers will conduct a test overnight, putting the starboard SARJ in "auto-track" mode for two full orbits. Sensors will measure vibration levels and motor drive currents to determine if the cleaning and lubrication reduced stress on the joint. The test is planned for 3:40 a.m. Sunday,.
The test is designed "to see if the cleaning and lubrication had any noticeable effect and we'll have sensors programmed to record vibrations during the time of the rotation and we'll also have a video camera trained on the joint itself," Smith said.
"In the past, we have been able to notice vibrations as the joint is moving so we'll look for that in the video and we'll also be looking at the current in the telemetry to see what the signatures there look like. Now, if we don't get everything done today on EVA-3, then we'll have to readjust whether it makes sense to perform that test overnight. You certainly don't want to rotate that solar alpha rotary joint if you haven't gotten all the cleaning done. ... So whether or not we do that test tonight depends on the results from the spacewalk today."
Here is an updated timeline of today's activity (in EST and mission elapsed time; includes revision H of the NASA television schedule):
EST........DD...HH...MM...EVENT 08:55 AM...07...13...00...Crew wakeup 09:30 AM...07...13...35...EVA-3: 14.7 psi repress/hygiene break 10:15 AM...07...14...20...EVA-3: Airlock depress to 10.2 psi 10:40 AM...07...14...45...EVA-3: Campout EVA preps 11:50 AM...07...15...55...Cargo module transfers resume 12:10 PM...07...16...15...EVA-3: Spacesuit purge 12:25 PM...07...16...30...EVA-3: Spacesuit prebreathe 01:15 PM...07...17...20...EVA-3: Crew lock depressurization 01:45 PM...07...17...50...EVA-3: Spacesuits to battery power 01:50 PM...07...17...55...EVA-3: Airlock egress 02:05 PM...07...18...10...SDRM transfer 02:05 PM...07...18...10...EVA-3: Setup 02:15 PM...07...18...20...EVA-3: SARJ cleaning and TBA R&R 04:15 PM...07...20...20...ISS: Crew meal 05:15 PM...07...21...20...ISS: Potable water dispenser work 08:15 PM...08...00...20...EVA-3: Cleanup and ingress 08:45 PM...08...00...50...EVA-3: Airlock repressurization 08:55 PM...08...01...00...Spacesuit servicing 10:10 PM...08...02...15...Evening planning conference 10:30 PM...08...02...35...Mission status briering on NASA TV 11/23/08 12:25 AM...08...04...30...ISS crew sleep begins 12:55 AM...08...05...00...STS crew sleep begins 01:00 AM...08...05...05...Flight day 9 highlights 03:40 AM...08...07...45...Starboard SARJ auto-track test 07:30 AM...08...11...35...Flight director update 08:30 AM...08...12...35...HD flight day 9 highlights 08:55 AM...08...13...00...Crew wakeup