Discovery astronauts set for one more spacewalk today
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: September 5, 2009
Astronauts John "Danny" Olivas and Christer Fuglesang are gearing up for a third and final spacewalk today, a planned six-and-a-half-hour excursion to deploy a payload mounting mechanism, replace a circuit breaker and an orientation sensor, install GPS antennas and to lay 60 feet of cable to power a new module.
This will be the 133rd spacewalk devoted to station assembly and maintenance since construction began in 1998 and the 14th so far this year. Going into today's excursion, station assembly EVA time stood at 823 hours and 50 minutes, including 13 hours and 14 minutes for Discovery's crew.
Today's spacewalk was replanned following an EVA Thursday in which Olivas ran into problems routing a power cable on a docking port where a new module is scheduled to be attached next year. A bundle of four cables was not in the expected position on the port and that task was dropped from the Discovery crew's timeline pending additional analysis. Longer jumper cables likely will be needed to hook up the cables in question.
"On EVA-2, when we got to the get aheads and discovered the cables weren't in exactly the right config we intended them to be in, we called the crew off and thought about it for the rest of the day," said space station Flight Director Royce Renfrew. "We went ahead and pulled that off the EVA-3 timeline and added some other get-ahead tasks. It's actually a fairly full timeline."
The first item on the agenda is to deploy a payload attachment mechanism on the upper right side of the station's solar power truss. The mechanism will be needed in November when the crew of the next shuttle visiting the station delivers two external logistics carriers, or ELCs, carrying critical spare parts and components.
ELC-1, which will be mounted on the left-side port-3 truss segment, will carry a solar array battery charge/discharge unit, a plasma contactor unit, a robot arm latching end effector, a control moment gyroscope, a nitrogen tank assembly, an ammonia tank assembly and a cooling system pump module.
ELC-2, which will be mounted on the starboard-3 truss attach mechanism being deployed today, will carry a high pressure gas tank, another control moment gyroscope, a pump module, a robot arm transporter umbilical system, experiment support equipment and flight attachment mechanisms.
"We're also going to go play with the rate gyro assembly," Renfrew said. "When we go around the Earth, we not only need to know where we are and how fast we're going, but one of the things we need to know to be able to do attitude determination and to essentially point the vehicle in the right direction is know how fast we're actually moving in any axis.
"So if we're pitching or yawing or rolling, if we're changing the attitude of the stack in any of those three (axes), we need to know how fast we're actually changing there. So we have some pretty high-tech equipment on the vehicle, the rate gyro assemblies. One of those has failed. On this EVA we'll go replace that rate gyro. We're also going to replace two GPS antennas, Global Positioning System antennas."
While Olivas replaces the first of two GPS antennas, Fuglesang will remove a failed remote power control module, or RPCM, in the power truss electrical system. The work requires a partial shut down of critical systems, including a stabilizing gyroscope and one of the station's two external cooling loops.
"When we go to change that out, we have to get the safing in place before the EV crew members can safely handle that RPCM," Renfrew said. "In order to get the safing in place, we have to go upstream to a different power module and power it off there. Somewhat similar to if you're in your house and you wanted to go change out a plug in the wall, you'd probably go outside and turn the circuit breaker off to make sure that's not live. Same thing here, only when we go upstream of that RPCM it turns off a lot of equipment on the truss.
"That's pretty complicated and requires a lot of choreography between the EV crew and the ground to make all that work flawlessly."
Finally, Olivas and Fuglesang will work together to run 60-foot-long cables from a distribution box in the solar power truss down to the left side of the Unity connecting module where a new module, known as node 3, or Tranquility, is scheduled for attachment early next year.
"These cables are about an inch and a half of two inches in diameter and they're really stiff," said Zeb Scoville, the lead spacewalk officer for Discovery's mission. "We've coiled up loops of this cable into maybe a diameter of about two feet. With each coil, we've taken one of these copper wire ties ... and we twist each loop with one of these wire ties.
"As they pull off one loop, they can release that one wire tie, pull off maybe just five or so feet, secure that down to structure while they hold onto the rest of it, they can move down the truss structure a little bit, undo one twist tie, get the second loop off and start working on that. As controlled as that may sound, cables have always presented a challenge in the past just because they do have a lot of memory and sort of a mind of their own. We've done our best to try to control that."
While the spacewalk is going on, returning station astronaut Timothy Kopra and station flight engineer Robert Thirsk will spend three hours or so replacing one of 16 motor-driven bolts holding the Leonardo cargo module to the Harmony module's Earth-facing port. Engineers noticed higher-than-normal drive currents in the bolt earlier and during a test this week, it jammed.
"Unfortunately, Murphy's Law holds true because the one bolt that we're after is buried under a bunch of other structure," Renfrew said. "We'll take that out, put it back together. ... It'll be about a three-hour task. That'll all be taken care of before we need to get the MPLM (Leonardo cargo module) off the vehicle."
Finally, station flight engineers Michael Barratt and Frank De Winne will spend their afternoon replacing a presumably clogged filter in the U.S. oxygen generation assembly.
Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision I of the NASA television schedule):
EDT........DD...HH...MM...EVENT 09/05 11:59 AM...07...12...00...STS/ISS crew wakeup 12:34 PM...07...12...35...EVA-3: 14.7 psi repress/hygiene break 01:24 PM...07...13...25...EVA-3: Airlock depress to 10.2 psi 01:29 PM...07...13...30...ISS daily planning conference 01:44 PM...07...13...45...ISS: Harmony nadir CBM bolt replacement 01:44 PM...07...13...45...EVA-3: Campout EVA preps 03:14 PM...07...15...15...EVA-3: Spacesuit purge 03:29 PM...07...15...30...EVA-3: Spacesuit prebreathe 04:14 PM...07...16...15...MPLM transfers resume 04:19 PM...07...16...20...EVA-3: Crew lock depressurization 04:49 PM...07...16...50...EVA-3: Spacesuits to battery power 04:54 PM...07...16...55...EVA-3: Airlock egress 05:09 PM...07...17...10...EVA-3: Setup 05:24 PM...07...17...25...EVA-3: S3 upper payload attach deploy 06:54 PM...07...18...55...EVA-3: Rate gyro assembly 2 R&R 07:54 PM...07...19...55...EVA-3/EV-1: GPS-4 antenna install 07:54 PM...07...19...55...EVA-3/EV-2: S0 RPCM R&R 08:39 PM...07...20...40...EVA-3/EV-1: Node 3 cable routing 08:39 PM...07...20...40...EVA-3/EV-2: GPS-2 antenna install 08:59 PM...07...21...00...ISS: OGS filter replacement 08:59 PM...07...21...00...EVA-3/EV-2: Node 3 cable routing 10:24 PM...07...22...25...EVA-3/EV-1: Node 1 slide wire removal 10:44 PM...07...22...45...EVA-3: Cleanup and ingress 10:44 PM...07...22...45...ISS: "Buzz" EVA video 11:19 PM...07...23...20...EVA-3: Airlock repressurization 11:34 PM...07...23...35...Spacesuit servicing 09/06 12:24 AM...08...25...00...Evening planning conference 01:00 AM...08...01...01...Mission status briefing 02:59 AM...08...03...00...ISS crew sleep begins 03:29 AM...08...03...30...STS crew sleep begins 04:00 AM...08...04...01...Daily highlights reel 09:00 AM...08...09...01...Flight director's update 11:29 AM...08...11...30...Crew wakup