Spacewalkers to swap out depleted tank in station truss
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: February 13, 2008
Astronauts Rex Walheim and Hans Schlegel are suiting up for a planned six-and-a-half-hour spacewalk today to swap out a 550-pound nitrogen tank used to push ammonia through the station's main cooling system.
Walheim, anchored to the end of the space station's robot arm, will carry the new tank, about the size of a small refrigerator, from the shuttle Atlantis' cargo bay up to the left side of the station's main solar power truss. He and Schlegel will temporarily mount the tank on an attachment fitting, remove the old tank and "temp stow" it to one side. After installing the new tank and hooking up electrical cables and high-pressure flex hoses, Walheim will carry the old unit back to the shuttle for return to Earth.
Today's spacewalk is the 103rd devoted to space station assembly and maintenance since construction began in 1998. It is the fourth EVA for Walheim and the first for Schlegel, a 56-year-old father of seven who became ill earlier in the mission. He had to sit out a spacewalk Monday to prepare the new Columbus module for installation but he said Tuesday he was ready to go for the second excursion.
"I feel really great right now," he said. "I'm, of course, a little bit anxious because (this) will be my first EVA."
Here is an updated timeline of today's activity (in EST and mission elapsed time; includes revision E of the NASA television schedule):
EDT........DD...HH...MM...EVENT 02/13/08 04:45 AM...05...14...00...STS/ISS crew wakeup 05:20 AM...05...14...35...EVA-2: Airlock repressurized for hygiene break 06:15 AM...05...15...30...Flight director's update on NASA TV 06:30 AM...05...15...45...EVA-2: Airlock depress to 10.2 psi 06:35 AM...05...15...50...ISS daily planning conference 06:55 AM...05...16...10...Columbus outfitting continues 08:00 AM...05...17...15...EVA-2: Spacesuit purge 08:15 AM...05...17...30...EVA-2: Spacesuit oxygen pre-breathe 08:25 AM...05...17...40...Columbus SSC activation 09:05 AM...05...18...20...EVA-2: Airlock depressurization to vacuum 09:35 AM...05...18...50...EVA-2: Spacesuits to battery power (begins spacewalk) 09:40 AM...05...18...55...EVA-2: Airlock egress 09:40 AM...05...18...55...Station robot arm (SSRMS) supports 09:55 AM...05...19...10...EVA-2: Nitrogen tank removal from payload bay 11:15 AM...05...20...30...Crew meals begin 11:45 AM...05...21...00...EVA-2: Nitrogen tank installation on P1 truss 01:50 PM...05...23...05...EVA-2: Old nitrogen tank stowed in payload bay 03:30 PM...06...00...45...EVA-2: Cleanup and airlock ingress 04:05 PM...06...01...20...EVA-2: Airlock repressurization 04:15 PM...06...01...30...Spacesuit servicing 05:30 PM...06...01...45...Mission status briefing on NASA TV 05:35 PM...06...02...50...EVA-3: Tool prep 07:15 PM...06...04...30...ISS crew sleep begins 07:45 PM...06...05...00...STS crew sleep begins 08:00 PM...06...05...15...Daily video highlights reel
While the spacewalk is going on, station commander Peggy Whitson, Dan Tani, Leopold Eyharts and Yuri Malenchenko will continue work to activate the new Columbus module, repositioning experiment racks from their launch positions. The crew is ahead of the timeline for their part of the activation sequence, but engineers at the German control center new Munich have run into problems activating the lab's computer system.
"Today, the crew's going to be moving some of the racks from their launch locations to the permanent location in the module and continuing to move other equipment," said station Flight Director Ron Spencer. "On the ground commanding side, we're a little bit behind. We had a problem yesterday commanding to some of the computers inside the Columbus module and we're still trying to work through those problems and determine what the cause is and figure a solution. So right now, we're about two-and-a-half hours behind on the ground commanding side from what we expected to have finished yesterday. But the crew is way ahead."
Walheim and Schlegel spent the night inside the Quest airlock module at a reduced pressure of 10.2 pounds per square inch. The so-called "camp out" helps purge nitrogen from the bloodstream and prevent the bends after working in NASA's low-pressure spacesuits. If all goes well, the astronauts will switch their suits to battery power around 9:35 a.m. to officially begin today's spacewalk.
"It sounds so simple, just exchange a nitrogen tank," Schlegel said in a NASA interview. "(But) it's about a yard times a yard times one and a half yards, and the weight is roughly 500 pounds. It's quite a heavy thing. You cannot just put it on your back and move and get it there. ... The installation includes mechanical connections, electrical connections and then nitrogen (lines), highly pressurized, and after that, you have to close thermal covers to keep it protected from thermal influences by the environment."
The nitrogen tank assembly, or NTA, is loaded with about 80 pounds of nitrogen pressurized to 2,500 pounds per square inch. Unlike the unit it is replacing, the new NTA can be refilled in orbit.
After exiting the airlock, Walheim and Schlegel will make their way over to the shuttle's cargo bay. Leland Melvin and Stan Love, working at a robotics control station inside the Destiny laboratory module, will operate the station's robot arm.
"I will get the arm ready to ride and then jump in and basically clip my heels into the robot arm," Walheim said in a NASA interview. "Leland and Stan will have a chance to drive me around to my work site. My first work site is in the payload bay where I'll take the brand-new nitrogen tank and extract it from the payload bay ... and then we'll take it out to the P1 (port 1) truss where we're going to swap it out.
"We'll put it on a ballstack, which basically holds it in place, and then ... we'll pull out the empty nitrogen tank. I'll pull that out on the arm and then we'll temp-stow that one, also on a ballstack. So we'll have the two of them basically temporary stowed. Then I go back and grab the new one and put it into the truss and then we can start doing the electrical and nitrogen connections in the front and the back of that tank. Once I'm done with that, I take the empty nitrogen tank and we put it back in the shuttle payload bay so we can bring it back home and use it again."
Asked about the difficulty of manually carrying a 550-pound component, Walheim said "it's not too hard. I've found in space things are fairly stable when you hold on to them, especially if they're big, a large mass, you know, several hundred pounds, up to over a thousand pounds. They're fairly stable.
"If you want to get them moving, you just give them a little bit of force and they start moving, but then you've got to stop them, too," he said. "So the main thing is just to hold them loosely - a loose grip, don't over control them - and just hang on and be aware when you're starting and stopping on the arm."
If time is available today, the astronauts will place thermal covers over the keel pins that held the Columbus module in Atlantis' cargo bay during launch.