Inspections added to Saturday's spacewalk
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: November 23, 2007
The crew of the international space station is set for a planned 6.5-hour spacewalk Saturday to deploy and connect a second set of ammonia cooling system supply and return lines between the lab's main solar power truss and the newly installed Harmony connecting module. While station commander Peggy Whitson finishes work to hook up a station-to-shuttle power transfer system, flight engineer Dan Tani will remove a thermal cover and two debris shields on the station's right-side solar array rotary joint to permit a quick inspection.
Using a digital camera, Tani plans to photograph the bearing race under the removed covers to help engineers determine the source of metallic contamination discovered during a similar inspection of a different part of the race during the shuttle Discovery's recent mission. That inspection was ordered because of unusual vibration and high current levels that were indicative of higher-than-expected friction in the 10-foot-wide joint.
Along with photographing the interior of the joint, Tani also will use adhesive tape as necessary to collect any readily available filings that might be present on the race ring. Figuring out what is wrong with the massive joint - and how to fix it - represents a major challenge that must be resolved over the next few months to continue station assembly.
Saturday's spacewalk, the 99th devoted to station assembly and maintenance since construction began in 1998 and the 22nd so far this year, is scheduled to begin at 6 a.m. Saturday, although it could get started up to an hour or so early depending on the pace of preparations.
For identification, Whitson will wear a suit with red stripes around the legs and use the call sign EV-1. Tani, EV-2, will wear a suit with diagonal stripes. Here is the latest timeline (in EST and elapsed time):
EST........HH...MM...EVENT 06:00 AM...00...00...Spacesuits to battery power 06:10 AM...00...10...Airlock egress and setup 06:15 AM...00...15...Whitson: Remove port ammonia loop B shunt jumper 06:15 AM...00...15...Tani: Configure vent tools 06:45 AM...00...45...Whitson: Vent and stow port shunt jumper 06:55 AM...00...55...Tani: Remove node 2 fluid quick-disconnect caps 07:20 AM...01...20...Tani: Relocate portable foot restraint 07:35 AM...01...35...Whitson/Tani: Release loop B fluid tray 08:25 AM...02...25...Whitson/Tani: Relocate loop B fluid tray 08:55 AM...02...55...Whitson/Tani: Secure loop B fluid tray to Destiny 09:15 AM...03...15...Whitson/Tani: Deploy node 2 loop B fluid tray 09:35 AM...03...35...Whitson: Vent node 2 loop B fluid tray 09:35 AM...03...35...Tani: Mate/open tray hinge quick disconnects 10:05 AM...04...05...Tani: Mate/open node 2 quick disconnects 10:15 AM...04...15...Whitson: Mate/open S0 truss segment quick disconnects 11:00 AM...05...00...Whitson: Configure loop B tray heater cable 11:00 AM...05...00...Tani: Remove starboard SARJ cover 7 for debris inspection 11:15 AM...05...15...Whitson: Node 2 starboard port preps for Columbus install 11:25 AM...05...25...Whitson: SSPTS final connections 12:05 PM...06...05...Whitson/Tani: Cleanup and airlock ingress 12:30 PM...06...30...Airlock repressurization
Assuming Saturday's work goes well, engineers at the Johnson Space Center in Houston will activate and check out the power, data and coolant connections with Harmony on Sunday, clearing the way for launch of the shuttle Atlantis Dec. 6 on a long-awaited mission to carry the European Space Agency's Columbus research lab into space. Columbus will be attached to Harmony's right-side port and tied into the module's power and cooling systems.
"From a big-picture perspective, the thing we have in front of us now is one additional spacewalk to get the space station in a configuration that will be ready to support 1E," station flight director Derek Hassmann said Tuesday, referring to the Columbus mission.
"On Saturday, we will complete the other half of the node 2 (Harmony) cooling system and then actually on Sunday, from the ground, we'll do the complete node 2 activation. We've done some (electrical) continuity checks to give us confidence all the cabling, all the copper paths are good, but the final checkout of all the node 2 systems won't happen until Sunday.
"Once we get node 2 activated and verify all the systems, the bulk of the space station configuration work required for 1E will be done," Hassmann said. "That's the last major milestone in front of us before we're ready to support 1E. Of course, there's a lot of work by the space station crew to prepare for the mission, there's pre-pack work to be done like there is for every shuttle flight, but node 2 activation on Sunday will mark what I consider the last significant milestone."
Harmony was launched to the station aboard the shuttle Discovery Oct. 23 and temporarily attached to the central Unity module's left-side port. After the shuttle departed, Whitson and flight engineer Yuri Malenchenko staged a spacewalk Nov. 9 to prepare the shuttle docking port on the front of the Destiny module for attachment to Harmony.
The docking port, known as pressurized mating adapter No. 2, was successfully moved to Harmony, using the station's robot arm, on Nov. 12. Two days later, the Harmony/PMA-2 "stack" was moved to the front of Destiny and robotically bolted in place. During a spacewalk Tuesday, Whitson and Tani connected one of two ammonia coolant loops and, running ahead of schedule, completed all required electrical connections. They also hooked up part of the station-to-shuttle power transfer system that lets docked shuttles tap into the lab's power grid.
Based on the extra work completed Tuesday, flight planners added the SARJ inspection to Saturday's timeline.
The space station is equipped with two solar alpha rotary joints, or SARJs, one on each side of the lab's main power truss. The SARJ joints rotate outboard solar arrays like giant paddle wheels as the station circles the planet, keeping the blankets face-on to the sun to maximize electrical output. Each joint features two redundant gear/race rings and two drive motors, only one of which is engaged at any given time. Twelve so-called trundle bearing assemblies are positioned around one of the two gear races to allow smooth rotary operation.
The left-side SARJ is operating normally, but the right-side SARJ is locked in place pending analysis of the contamination issue.
In a worst-case scenario, the 12 bearing assemblies and two drive motors could be moved to the redundant gear during three to four spacewalks. But engineers do not want to consider such a drastic step until they figure out what is causing the problem with the active gear and race ring.
The problem is not serious in the near term because the station's arrays can generate enough power even with the starboard blankets locked in place. But the starboard SARJ must be back in operation by next April when NASA plans to launch Japan's Kibo research module.
As with the first inspection, Tani will use adhesive tape during Saturday's excursion to collect samples of any metallic debris that might be present under thermal cover No. 7, one of 22 that protect the massive gear-and-bearing assembly. Debris from the first inspection, which was returned to Earth for analysis, came from the race ring itself.
At the conclusion of Saturday's inspection, Tani plans to leave the thermal cover and debris shields off. At some point after the next shuttle mission in December, a camera on the end of the station's robot arm will peer into the joint while it is commanded to move through a full rotation.
In the meantime, NASA is accelerating work to launch a full set of replacement trundle bearing assemblies and a new drive motor to facilitate any repairs that might ultimately be ordered. Engineers are considering a broad variety of options, ranging from the worst-case switch to the redundant drive gear to minimal fixes that would permit normal operation even with some level of damage.