Wednesday spacewalk to remove failed coolant pump
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 10, 2010
NASA managers Tuesday approved plans for a second spacewalk Wednesday to deal with an unexpected ammonia leak that blocked removal of a faulty coolant system pump aboard the International Space Station during a spacewalk Saturday. If the plan works, the old pump will be removed and a new pump installed and hooked up during a third spacewalk Sunday.
NASA managers decided late Tuesday, however, to delay the start of the EVA by one hour, letting the astronauts get a bit more sleep and giving engineers more time to fine-tune robotics procedures. Under the revised timeline, Wheelock and Caldwell Dyson will switch their spacesuits to battery power at 7:55 a.m. to officially kick off the excursion.
For identification, Wheelock, call sign EV-1, will be wearing a suit with red stripes around the legs. Caldwell Dyson, EV-2, will wear an unmarked suit. This will be the fifth spacewalk for Wheelock, the second for Caldwell Dyson, the 13th station EVA so far this year and the 149th since assembly began in 1998.
The ammonia pump in coolant loop A failed July 31, leaving the space station with just one coolant system to dissipate the heat generated by the lab's electronics. The loop A failure forced the astronauts to implement a widespread powerdown to prevent critical equipment from overheating.
Four spare pump modules are on board and NASA planners quickly developed plans for two spacewalks to remove the old pump and install a replacement. But those plans went awry Saturday when the spacewalkers ran into major problems disconnecting one of four ammonia lines.
After struggling to close a quick-release fitting on the M3 line, the astronauts reported a significant leak. The quick-disconnect was reopened and the spacewalkers headed back to the airlock for routine decontamination procedures.
Engineers suspect a problem with one of two internal valves on the outboard side of the pump module.
For the second spacewalk, Wheelock and Caldwell Dyson first will close quick-disconnect valves on both ends of the starboard one, or S1, truss segment where the pump module is mounted. With those valves closed, the ammonia line leading to the leaking connector will be isolated.
Flight controllers planned to reduce pressure in the line before the spacewalk began, clearing the way for Wheelock and Caldwell Dyson to install a vent tool to release any residual ammonia trapped in the pump and its plumbing.
Then, if all goes well, the now-evacuated connector can be disconnected and moved out of the way. After unplugging five electrical cables, Wheelock and Caldwell Dyson will loosen four bolts and remove the old pump module, temporarily mounting it on a stowage fixture at the base of the station's robot arm transporter.
The replacement pump module would be installed and reconnected during a third spacewalk Sunday. The quick-disconnect valves that were closed to isolate the leaking connector would be reopened at that point and flight controllers would be free to re-activate coolant loop A.
Flight controllers are considering an alternative scenario for Wednesday's spacewalk that includes an initial attempt to simply close the M3 quick disconnect in hopes that whatever caused the leak Saturday is no longer present. If no leak is observed, Wheelock could detach the M3 ammonia line straight away and press ahead with the pump's removal.
If the leak reappears, the astronaut would reopen the quick-disconnect and press ahead with work to isolate that section of the line. It is not yet clear whether flight controllers will, in fact, clear Wheelock to spend the time needed to reach the M3 valve as opposed to simply isolating the line right away as currently planned.
Here is an updated timeline of major events (in EDT and elapsed time; best viewed with fixed-width font):
EDT........HH...MM...EVENT 03:00 AM.............Crew wakeup 03:35 AM.............Crew hygiene break 04:45 AM.............EVA preps continue 06:15 AM.............Spacesuit purge begins 07:20 AM.............Airlock depressurization begins 07:55 AM...00...00...Suits to battery power 08:00 AM...00...05...Post depress/airlock egress 08:15 AM...00...20...Setup 08:40 AM...00...45...EV-2: Close S1 outboard quick-disconnect 08:55 AM...01...00...EV-1: Vent line routing 09:25 AM...01...30...EV-1: Inboard S0/S1 jumper QD closed 09:40 AM...01...45...EV-2: M1 ammonia line restrained 09:55 AM...02...00...EV-2: Start venting 10:05 AM...02...10...EV-1: Close M2 ammonia line 10:35 AM...02...40...EV-1: Close vent tool 10:40 AM...02...45...EV-2: Vent tool cleanup 10:50 AM...02...55...EV-1: Close and demate M3 ammonia line 11:00 AM...03...05...EV-2: Cover and restrain M3 11:10 AM...03...15...EV-1: Break torque on failed pump module bolts 11:30 AM...03...35...EV-1: Retrieve adjustable grapple bar 11:30 AM...03...35...EV-2: Release failed PM electrical cables 12:00 AM...04...05...EV-2: Release failed PM bolts 12:15 AM...04...20...EV-1: Remove failed PM with AGB 12:25 AM...04...30...EV-2: Remove failed PM 01:00 PM...05...05...EV-1: Move failed PM to stowage fixture 01:10 PM...05...15...EV-2: Break torque on new PM; cleanup 01:30 PM...05...35...EV-1: Cleanup/airlock ingress 01:55 PM...06...00...Airlock pre-pressurization