Urine processor repair attempt being planned
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: November 22, 2008
Engineers troubleshooting problems with a centrifuge in the distillation assembly of hardware designed to convert urine into potable water aboard the international space station believe a relatively simple fix might resolve the trouble. Station commander Mike Fincke will attempt a repair job Sunday, removing vibration dampers from the centrifuge and locking the unit in place. In so doing, engineers believe, thermal expansion after the unit runs and warms up will no longer cause a specific sensor to interfere with the spinning centrifuge.
"The experts on the ground have been meeting throughout the day," lead Flight Director Ginger Kerrick told reporters late Saturday. "They believe we have isolated the source of the problem to the way the centrifuge is mounted in the distillation assembly. It is on isolators and they believe that that is a contributer to the signals that they're seeing that caused the UPA to shut down.
"Their proposal is to remove those isolators and hard mount the distillation assembly. So folks are working on that procedure right now, it'll be scheduled on Mike Fincke tomorrow and we think it's going to be about two hours long. And after that, we will send some commands to it, they're working on that procedure as well, and we'll see if that solves the problem."
The urine processing assembly is a critical component in hardware delivered by the shuttle Endeavour that is designed to convert urine and condensate into potable water. The recycling system is required for NASA to boost station crew size from three to six next May.
The part of the system that processes condensate has been working relatively well, but the UPA has suffered a string of start-up glitches, the most significant being the shut down of the centrifuge in the vacuum distillation assembly after running for about two hours.
Analysis of telemetry led engineers to suspect a thermally induced interference between a speed sensor and the centrifuge that causes the motor to slow down and draw more current than expected.
The centrifuge is mounted on dampers to reduce vibrations and sound as it rotates. Engineers believe the dampers are "possibly allowing the (distillation assembly) to set itself up at a frequency that ultimately causes it to shift around, allowing one of the centrifuge speed sensors to come in contact with the spinning centrifuge," Kerrick said later.
That, in turn, causes the motor to work harder and draw more current, triggering a shut down.
"There is a thermally induced physical interference that seems to be occurring at the exact same time in the processing," Kerrick said at the briefing. "It's also aggravated by the fact that this system is on the isolators. They think that hard mounting this device will resolve the issue."
Even with the on-again, off-again behavior of the UPA, the astronauts successfully collected the first samples of processed water today for return to Earth aboard Endeavour. The objective was a sample comprised of 30 percent processed urine and 70 percent processed condensate. What they got was 90 percent condensate and 10 percent processed urine. But Kerrick said that was sufficient for engineers on the ground to evaluate the system's performance.
With samples in hand, and with work to install a potable water dispenser on track, Kerrck said it does not appear the Endeavour astronauts will need an additional docked day at the station.
But she confirmed there are no spare centrifuge units on the ground and the hardware aboard the station must be coaxed into normal, or near normal, operation to permit the planned boost to six full-time crew members next May. She said engineers are studying options for supporting an expanded crew if Sunday's repair attempt fails and the processor can only be operated for short periods between cool downs.
"There is a second way we can run the urine processor," Kerrick sad. "Right now, it fails after about two hours of running. We may be able to run it for about an hour and 45 minutes and process urine that way, that's how we got the initial 10 percent versus 90 percent distribution of urine to condensate we have right now. So that is an option. It requires a different set of procedures for cooling down and starting up and folks are working on those as well. So if this hardware fix doesn't work, there is a potential for us to process more urine during the mission using that technique."
As for supporting six crew members with that technique, "those numbers have not been crunched yet," she said.
"We did ask that today, because if this is as good as it's going to get, we do need to be able to answer that question. Fortunately, we have at least until (the next shuttle visit in February) to answer that question. Folks will definitely be going off and studying that."
NASA requires 90 days of testing and analysis before the new system can be declared operational. What impact a lengthy startup delay might have on plans to boost station crew size next spring is not yet known.
While the astronauts inside the space station spent the day Saturday working with the water processing gear and moving equipment to and from the shuttle, astronauts Heidemarie Stefanyshyn-Piper and Stephen Bowen were carrying out a spacewalk to wrap up servicing of the station's right-side solar array rotary joint.
Over the course of three spacewalks this week, the astronauts have been methodically cleaning and lubricating the joint's 10-foot-wide drive gear and replacing trundle bearings to reduce friction and vibration. One of the three bearing races on the gear has suffered extensive erosion because of a lubrication breakdown. By cleaning off metallic debris, installing new bearings and lubricating the races, engineers hope to be able to resume occasional sun-tracking to improve power generation.
Going into Saturday's spacewalk six trundle bearing assemblies remained to be installed. Stefanyshyn-Piper and Bowen managed to replace five of them and left the drive gear with 330 degrees of its bearing races cleaned and lubricated. Lead spacewalk officer John Ray said the remaining 30 degrees, and installation of the final trundle bearing assembly, will be shoehorned into a fourth and final spacewalk Monday.
"I would say the crew executed as perfect and EVA as I've ever seen," Ray sad. "I mean everything went really well, they were right on top of their game right out the door and they just stayed at a very steady, even pace and they got everything we had planned on for the EVA and a little bit more.
"They were getting very close at the end to getting to the point they could finish up the starboard SARJ, but they weren't quite going to make it. We could see that coming and the day was getting pretty long. So although the suits were doing really well and the crew was doing really well, we decided to go ahead and call it a day. We've got plenty of time on EVA-4 to go out and finish up the starboard SARJ."