Spacewalkers set for final excursion of the mission
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: November 24, 2008
Astronauts Stephen Bowen and Robert "Shane" Kimbrough are preparing for a six-and-a-half-hour spacewalk today to finish work on the space station's damaged right-side solar array rotary joint and to carry out preventive maintenance on its port-side counterpart. The spacewalkers also plan to manually retract a berthing latch on the Japanese Kibo lab module, install GPS antennas and thermal covers and mount a new external TV camera.
The spacewalk is scheduled to begin at 1:45 p.m., but the astronauts likely will start early if they complete preparations in time (an updated timeline is posted below).
While the spacewalk is going on outside, the astronauts inside the space station will be wrapping up final equipment transfers to and from the shuttle's Leonardo cargo module while engineers at the Johnson Space Center in Houston continue troubleshooting problems with a newly installed urine processing assembly, or UPA. Urine recycling is critical to NASA's plans for boosting the station's crew from three to six next May.
The urine processor assembly is designed to run about four hours at a time converting urine into potable water using a complex vacuum distillation technique. During test runs Friday and Saturday, the motor driving a centrifuge in the distillation sub-system slowed down after about two hours, forcing shut downs. Telemetry indicated the problem involves physical interference between the spinning centrifuge and a speed sensor, possibly caused by thermal effects and/or a frequency mode the device gets into after extended operation.
In a bid to reduce the unwanted oscillations, station commander MIke Fincke and Endeavour astronaut Don Pettit removed rubber vibration dampers on the distillation unit housing Sunday and bolted it directly to its mounting rails in water processing system rack No. 2. A third test run then was attempted Sunday evening. The unit ran for about an hour longer than in the first two tests, but it ultimately shut down with the same motor-slowdown signature."
Later today, starting around 12:50 p.m., Fincke will make another adjustment to the distillation unit housing in an attempt to coax the unit into normal, or near normal, operation.
"We tried running the UPA twice," station flight director Brian Smith said early today. "We had the same signature where it shut down due to high motor current and ... a slower speed on the centrifuge. After the second time, commander Mike Fincke performed a maintenance activity on the distillation assembly. And the maintenance activity involved him changing the mounting configuration (by removing rubber vibration dampers). He modified that to change the nature of the vibrations.
"We ran the urine processor assembly after that modification was made, we got to the two-hour mark and that was where we had seen the failures the last two times," Smith said. "We got to the two-hour mark (Sunday night) and we saw a similar signature as to what we had seen before, but not quite as bad. And the urine processing assembly was able to get through that two-hour mark and we saw the signatures level off to something a little more normal. But unfortunately, about 53 minutes after that, or about two hours and 53 minutes into the run, it failed again with exactly the same signatures we had seen in the previous two failures."
When the distillation assembly vibration dampers were removed Sunday, six bolts were pulled out but only four were put back in. Today, the other two bolts will be re-inserted to tighten up the mounting even more.
"We came up with another maintenance procedure along the same lines as the first one," Smith said. "We're going back to the mounting scheme for the distillation assembly and instead of four bolts mounting it, we're going to add two more into holes that are pre-existing, we're going to take two bolts and we're going to drive those in there to hard mount the distillation assembly into the rack and further modify the vibration nature as the centrifuge spins. And then we'll run the urine processing assembly again and see if that helped."
Today's spacewalk will be the 118th EVA devoted to station assembly and maintenance since construction began in 1998, the 19th this year and the fourth and final excursion for Endeavour's crew. Going into today's excursion, the shuttle crew had logged 20 hours and 34 minutes of spacewalk time, pushing the station's total to 739 hours and 23 minutes.
For Kimbrough, today's spacewalk timeline is unchanged. But Bowen's timeline was modified to allow him to complete work on the station's starboard solar alpha rotary joint that was left unfinished after the crew's third spacewalk Saturday.
Servicing the right-side SARJ joint was a major objective of Endeavour's mission. The 10-foot-wide drive gear in the massive mechanism is designed to rotate outboard solar arrays like big paddle wheels to automatically track the sun as the station orbits the Earth. The port SARJ has been operating normally, but one of the three bearing races on the right-side drive gear has suffered extensive erosion because of a lubrication failure. The damaged race causes increased vibration levels and the joint can no longer be used in auto-track mode.
During the first three spacewalks the astronauts cleaned and lubricated 330 degrees of the drive gear's race rings and replaced 10 of 12 trundle bearing assemblies. One was replaced last June. During today's EVA, Bowen will install the final trundle bearing and finish cleaning the last 30-degree segment of the bearing races.
Kimbrough, meanwhile, will apply lubrication to the port-side SARJ as preventive maintenance. Smith provided a detailed description of the replanned spacewalk:
"A few things have been added on to this timeline Steve and Shane will be going out conducting this spacewalk," Smith said. "Steve's going to finish up the starboard SARJ activities. We left (thermal) covers 17 and 18 off after EVA-3 and we had removed trundle bearing No. 3. He's going to replace the trundle bearing after cleaning the area and then lubricating the remaining part of the race ring and then put the covers back on. That will complete all the starboard SARJ work we had planned for the mission.
"When that is complete, he's going to go off and do a new task that was added. This is the exposed facility berthing mechanism structural latch contingency activity. A few days ago, the Japanese control center had checked out the exposed facility berthing mechanism in preparation for (a shuttle mission next year to attach an external experiment shelf). And one of the structural latches deployed but did not retract and they need to get it retracted. They tried some on-orbit troubleshooting with the crew to get the computers to get that latch to retract. That was unsuccessful, so Steve will go out to that work site and he'll just drive a bolt that will then cause that latch to retract and that mechanism should be in the correct configuration for the 2J/A assembly mission. While he's there, he's going to put a cover over that mechanism. He took that off during EVA-1 to allow for this checkout to occur, so now it's time to put that back on, it's a thermal cover.
"And then he's going to install GPS antennas onto the Japanese module. This was originally part of EVA-4. These antennas are needed for the HTV mission, the Japanese transfer vehicle, that will launch in 2009.
"While Steve is doing that, Shane is going to be working on the port solar alpha rotary joint, the port SARJ," Smith said. "We're going to lubricate all 360 degrees of that race ring. He'll do 180 degrees, then he'll go off to a different work site and he'll install a new TV camera that will be mounted permanently outside. We have three of them on the space station right now and this will be a fourth one. This one is being added to provide views of the rendezvous and the berthing of HTV next year.
"When he's complete with that, he'll return to the port SARJ and finish lubricating the rest of the race ring," Smith said. "And all those tasks were originally in EVA-4 so Shane's timeline has remained unchanged."
Assuming Bowen gets done with the starboard SARJ today, flight controllers plan a two-orbit test early Tuesday to check the operation of the joint. The test is scheduled to begin around 5:30 a.m.
"The test is very simple to execute," Smith said. "We're just going to command the joint to go into what we call auto-track. And in that mode, it's just going to automatically track the sun and do two full revolutions. While it's doing that, we're going to have sensors programmed to record the vibrations on the space station. We're also going to have one of those external TV cameras trained on the joint as well as the truss segment. We have in the past noticed when that joint is moving, vibrations in the truss, it's very easy to see. We're going to be looking for a reduction of that, a reduction of vibrations and we're also going to be monitoring the current of the motor used to drive that joint. we're looking for the proper current signature, which would be a reduced amount of current from what we've seen in the past."
Here is an updated timeline of today's activity (in EST and mission elapsed time; includes revision I of the NASA television schedule):
EST........DD...HH...MM...EVENT 08:55 AM...09...13...00...Crew wakeup 09:30 AM...09...13...35...EVA-4: 14.7 psi repress/hygiene break 10:15 AM...09...14...20...EVA-4: Airlock depress to 10.2 psi 10:40 AM...09...14...45...EVA-4: Campout EVA preps 10:55 AM...09...15...00...ISS daily planning conference 12:10 PM...09...16...15...EVA-4: Spacesuit purge 12:15 PM...09...16...20...Cargo module transfers resume 12:25 PM...09...16...30...EVA-4: Spacesuit prebreathe 12:50 PM...09...16...55...UPA maintenance 01:15 PM...09...17...20...EVA-4: Crew lock depressurization 01:45 PM...09...17...50...EVA-4: Spacesuits to battery power 01:50 PM...09...17...55...EVA-4: Airlock egress 02:05 PM...09...18...10...EVA-4: Setup 02:30 PM...09...18...35...EVA-4: Port SARJ thermal cover removal 03:00 PM...09...19...05...EVA-4/EV3: Port SARJ first lubrication 03:05 PM...09...19...10...EVA-4/EV2: Starboard SARJ servicing 03:35 PM...09...19...40...EVA-4/EV3: TV camera installation 04:50 PM...09...20...55...EVA-4/EV3: Port SARJ second lubrication 05:35 PM...09...21...40...EVA-4/EV2: Kibo capture launch retract 05:35 PM...09...21...40...EVA-4/EV3: Port SARJ thermal cover installl 06:05 PM...09...22...10...EVA-4/EV2: Kibo capture system cover install 06:25 PM...09...22...30...EVA-4/EV2: Install Kibo GPS antenna 07:05 PM...09...23...10...EVA-4/EV3: Get ahead tasks 07:10 PM...09...23...15...EVA-4/EV2: Kibo robot arm clean up 07:45 PM...09...23...50...EVA-4: Cleanup and ingress 07:55 PM...10...00...00...Pettit GLA scavange 08:15 PM...10...00...20...EVA-4: Airlock repressurization 08:25 PM...10...00...30...Spacesuit servicing 09:20 PM...10...01...25...Configure MPLM racks 10:25 PM...10...02...30...Evening planning conference 10:00 PM...10...02...05...Mission status briefing on NASA TV 11/25/08 12:25 AM...10...04...30...ISS crew sleep begins 12:55 AM...10...05...00...STS crew sleep begins 01:00 AM...10...05...05...Flight day 11 highlights 05:30 AM...10...09...35...Starboard SARJ auto-track test 07:30 AM...10...11...35...Flight director status report 08:30 AM...10...12...35...HD flight day 11 highlights 08:55 AM...10...13...00...Crew wakeup