Critical spacewalk to clear way for station assembly
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 10, 2006
The most critical task planned for Discovery's mission is arguably the one that must be accomplished to permit continued assembly of the international space station: repair of a stalled robot arm transporter on the station's unfinished solar array truss that "killed itself" late last year.
Astronaut Piers Sellers and Mike Fossum plan to venture out of the station's Quest airlock module today at 8:13 a.m. to begin a planned seven-hour spacewalk to replace a cable cutter that inadvertently fired last December, along with a complex cable reel mechanism to restore redundant power to the transporter.
"Basically, it killed itself just before Christmas," Sellers said. "It's a complicated task. If we don't get this thing fixed, we can't move this truck that moves up and down the front face of the station and we can't continue with assembly. So we absolutely have to get it fixed before the next mission.
"And it's a complicated business because it's a large, difficult-to-handle object, it sits inside a bay and it's got multiple, multiple electrical connections. So there are a number of significant obstacles to getting this done. We've thought about it a lot. I think we'll manage to do it OK, the engineering team has put an enormous amount of effort into this and I think it's going to work out. But it's one of the more daunting tasks (of the mission)."
Critical, in fact. As Sellers said, the solar arrays scheduled for launch aboard the shuttle Atlantis Aug. 28 cannot be attached to the truss unless the robot arm transporter is fully operational.
Here is an updated timeline of today's activities (in EDT and mission elapsed time):
TIME HH MM SS EVENT 07/10/06 02:08 AM 05 11 30 STS crew wakeup 02:38 AM 05 12 00 ISS crew wakeup 03:38 AM 05 13 00 EVA-2: Preparations with ISS oxygen 06:13 AM 05 15 35 EVA-2: Spacesuit purge 06:13 AM 05 15 35 MPLM cargo module transfers 06:28 AM 05 15 50 EVA-2: Spacesuit prebreathe 07:28 AM 05 16 50 EVA-2: Quest airlock depressurization 08:13 AM 05 17 35 EVA-2: Airlock egress 08:28 AM 05 17 50 EVA-2: EV1 (Sellers): FGB retrieval 08:28 AM 05 17 50 EVA-2: EV2 (Fossum): APFR setup 09:08 AM 05 18 30 EVA-2: EV2: Pump module retrieval 09:08 AM 05 18 30 Station arm (SSRMS) grapples pump module 09:28 AM 05 18 50 SSRMS moves pump module to ESP-2 stowage platform 09:28 AM 05 18 50 EVA-2: EV1: Nadir IUA (cable cutter) replacement 09:28 AM 05 18 50 EVA-2: EV2: APFR reconfig and TUS setup 10:18 AM 05 19 40 EVA-2: Pump module installation 10:53 AM 05 20 15 SSRMS removes TUS (cable reel assembly) 10:58 AM 05 20 20 EVA-2: Nadir TUS removal 11:23 AM 05 20 45 SSRMS maneuvers to cargo carrier 11:53 AM 05 21 15 SSRMS TUS swap 12:18 PM 05 21 40 SSRMS moves to S0 truss 12:53 PM 05 22 15 SSRMS supports TUS installation 01:23 PM 05 22 45 SSRMS to park position 01:43 PM 05 23 05 EVA-2: TUS cable routing 02:28 PM 05 23 50 EVA-2: Airlock ingress 02:48 PM 06 00 10 EVA-2: Airlock repressurization 03:30 PM 06 00 52 Mission status briefing 04:08 PM 06 01 30 TUS checkout by mission control 06:08 PM 06 03 30 Crew sleep begins 07:00 PM 06 04 22 Daily video highlights reel on NASA TV
The transporter is a robotic cart designed to creep along rails on the front face of the station's unfinished solar array truss, carrying the lab's Canadian-built robot arm from one work station to another. At each work station, the transporter and arm can be locked down to provide stability. The long truss eventually will sport four huge sun-tracking solar array panels that will generate the electrical energy needed to support a six-member crew and a full suite of scientific experiments.
But the outer segments of the truss cannot be built without the station's robot arm and the arm cannot be moved from point to point unless the transporter is operational and has redundant power, video and data cables.
As the transporter moves along the truss it plays out or rolls up ribbon-like power and data cables. To provide redundancy, two trailing umbilical system - TUS - reel assemblies play out and rewind separate cables. The transporter was launched with powerful cable cutters in devices on the transporter itself, called interface umbilical assemblies - IUAs - in case of a jam in either TUS reel that might otherwise strand the work platform.
On Dec. 16, TUS cable No. 1 on the Earth-facing, or nadir, side of the transporter was severed when the cable cutter in the nadir interface umbilical assembly suddenly fired for no apparent reason, slamming down with 960 pounds of force. Engineers still do not know why the spring-driven cutter fired.
But the incident left the mobile transporter with just one set of power and data cables and NASA flight rules forbid its movement along the truss unless full redundancy is available. The concern is that a second failure could leave the transporter stranded between work sites and unable to be safely latched down. That, in turn, would pose a risk during shuttle dockings or other events when unwanted movement could prove dangerous.
"We have rules in place that say we do not base the (station's robot) arm on the mobile transporter during that time and we do not maneuver the mobile transporter," said Kirk Shireman, deputy manager of the space station project. "It's very important for us to restore full functionality before we continue assembly. That's because to continue assembly, we have to move the mobile transporter out on the truss, base the arm out there to actually install the next piece of truss. That's a very important piece of this mission for us, to return to full functionality."
Earlier this year, worried that the cable cutter on the upward facing, or zenith, side of the transporter might fire, Expedition 12 commander William McArthur and flight engineer Valery Tokarev attempted to disable the zenith cable cutter during an already planned spacewalk. But they were unable to drive in a "safing" bolt and rather than leave the cable in place, they removed it from the cutting mechanism and tied it off. That had the effect of stranding the mobile transporter at work site 4.
Sellers and Fossum fixed the zenith cable system during their first spacewalk Saturday by inserting a blade blocker designed to prevent any damage to the cable even if the cutter fires later. With the blade blocker in place, they re-inserted the zenith cable in the interface umbilical assembly and thus restored the transporter to single-cable operation.
That's important because the transporter must be moved from work site 4 to work site 5 this morning to get the access needed to repair the nadir cable system during today's spacewalk.
The goals today are to replace the nadir interface umbilical assembly and its deployed cable cutter with a new unit carried up aboard Discovery; and to replace the nadir trailing umbilical system reel assembly and its severed cable.
But first, Sellers and Fossum will move a large liquid ammonia pump module to a spare parts depot on the station. The pump module, part of a complex system that circulates ammonia coolant through external trusses and the U.S. laboratory module, will be installed during an assembly flight now targeted for launch in December.
With the pump module safely stowed, Sellers and Fossum will install the new interface umbilical assembly and then remove the 334-pound trailing umbilical system reel assembly. Fossum, riding on the end of the station's robot arm, will hand-carry it down to the shuttle cargo bay. Sellers will hand him the new reel assembly and both astronauts will move back up to the truss for its installation. After routing the cable back though the IUA, Sellers and Fossum will be done and the mobile transporter will be restored to normal, fully redundant operation.
"I'm excited by the TUS task," Fossum said in an interview. "It failed in December and within weeks, when we came back after ... Christmas, they had one in the (training) pool all wrapped up with a big bow on it. And we started working on this task.
"It's very important to get that redundancy in the mobile base system, the mobile transporter that moves up and down the truss. If anything was to go wrong with that remaining one we would not be able to move it to a different location. Finishing assembly, adding on pieces of the truss, the solar arrays out on the ends, requires this mobile transporter to be mobile. And so it's been a big effort."