Crew set for spacewalk to install truss segment
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 12, 2006
The Discovery astronauts are gearing up for their first spacewalk today, a planned six-hour excursion to bolt a new 4,110-pound spacer segment to the left side of the international space station's main solar array truss. Astronaut Robert Curbeam, call sign EV-1, and Swedish flier Christer Fuglesang, call sign EV-2, are scheduled to exit the station's Quest airlock module around 3:42 p.m.
The astronauts were awakened around 10:50 a.m. by a recording of "Waterloo" by the Swedish group ABBA beamed up from mission control in Houston.
"Good morning, Discovery," astronaut Shannon Lucid radioed from mission control. "And a special good morning to you this morning, Christer, and we wish you well as you step outside of the airlock for the first time today."
"Morning Houston, thanks a lot," Fuglesang replied. "Nice music this morning."
Curbeam and Fuglesang, Sweden's first man in space, spent the night in the station's NASA airlock with the pressure lowered to 10.2 pounds per square inch. The so-called "campout" procedure is designed to help the spacewalkers avoid the bends after working in their 5 psi spacesuits. After a hygiene break today, they will re-enter the airlock and lower the pressure again before donning their spacesuits.
This is the first of three spacewalks planned for Discovery's mission and the 74th devoted to station assembly or maintenance since construction began in 1998. Going into today's excursion, 45 Americans, 13 Rusians, two Canadians and one astronaut each from Japan, France and Germany had logged 444 hours and 14 minutes building and maintaining the lab complex.
For identification purposes, Curbeam's spacesuit features red stripes on the legs while Fuglesang's is unmarked.
Here is a timeline of today's activities (in EST and mission elapsed time):
12/12/06 10:47 AM...02...14...00...Shuttle crew wakeup 11:17 AM...02...14...30...Station crew wakeup 11:22 AM...02...14...35...EVA-1: Crew airlock 14.7 psi repress 11:22 AM...02...14...35...EVA-1: Hygiene break 12:07 PM...02...15...20...EVA-1: Crew airlock depress to 10.2 psi 12:52 PM...02...16...05...Station arm (SSRMS) to preinstall setup 02:02 PM...02...17...15...EVA-1: Spacesuit purge 02:17 PM...02...17...30...EVA-1: Spacesuit oxygen prebreathe 03:07 PM...02...18...20...EVA-1: Crew lock depressurization 03:42 PM...02...18...55...EVA-1: Airlock egress 04:07 PM...02...19...20...EVA-1: P5 launch lock removal 05:07 PM...02...20...20...EVA-1: P5/P4 attachment 06:37 PM...02...21...50...SSRMS (at worksite 7) ungrapples P5 06:37 PM...02...21...50...EVA-1: Keel pin relocate 07:37 PM...02...22...50...EVA-1: S1 camera R & R 09:07 PM...03...00...20...EVA-1: Payload bay cleanup 09:17 PM...03...00...30...SSRMS mobile transporter moves to worksite 3 09:27 PM...03...00...40...EVA-1: Airlock ingress 09:52 PM...03...01...05...EVA-1: Airlock repressurization 10:17 PM...03...01...30...Shuttle arm (RMS) powerdown 10:17 PM...03...01...30...Logistics transfer tagup 11:30 PM...03...02...43...Post MMT/mission status briefing on NTV 12/13/06 12:02 AM...03...03...15...Cameras positioned for P6 retraction 02:17 AM...03...05...30...Shuttle/station crew sleep begins 03:00 AM...03...06...13...Daily video highlights reel on NTV 04:00 AM...03...07...13...MMT briefing replay on NTV 07:30 AM...03...10...43...Station flight director update on NTV 09:00 AM...03...12...13...NASA video file on NTV 10:17 AM...03...13...30...Discovery crew wakeup
To put today's spacewalk in context, it is helpful to review the current configuration of the international space station.
The lab complex currently consists of six pressurized modules. At the back end of the outpost is the Russian Zvezda command module featuring two solar arrays and an aft docking port that can accommodate Progress supply ships or Soyuz crew capsules. An airlock/docking module called Pirs is attached to a downward-facing port on Zvezda's front end. Zvezda's forward port is attached to the Russian Zarya module, a supply and propulsion segment equipped with its own pair of solar arrays.
Zarya's front end is bolted to a pressurized mating adapter that, in turn, is attached to NASA's Unity module, a multi-hatch node with six ports. Its starboard port is occupied by the Quest airlock module while its upper, or zenith, port accommodates the Z1 truss and the P6 solar arrays that for the past six years have provided power to the U.S. segment. Unity's downward facing port is used by cargo modules brought up by the shuttle and its port hatch is home to another pressurized mating adapter that will be relocated later in the assembly sequence.
Unity's forward port is attached to the Destiny laboratory module. On the forward end of Destiny is another pressurized mating adapter used as a docking port by visiting space shuttles. On top of the lab module is the station's multi-segment solar array truss, which is mounted at right angles to the long axis formed by the pressurized modules.
The S0 truss segment sits in the middle atop the lab, flanked by the S1 and P1 truss elements. S0 houses the major electrical components of the permanent electrical system: four main bus switching units, or MBSUs, and transformers called DC-to-DC converter units - DDCUs - that serve to step down and regulate solar array power to levels needed by station equipment.
S1 and P1 house the station's two independent cooling systems, each of which include large ammonia tanks, a nitrogen gas pressurization system and a massive pump module to pushes ammonia coolant through cold plates and heat exchangers and out into deployable radiators, three on S1 and three on P1. To maximize heat rejection, the radiators are mounted on a rotating beam that can point them toward deep space and away from the sun.
In September, the crew of mission STS-115 attached two new truss segments to the left side of the solar array beam. The first, P3 (there is no P2) features a powerful solar alpha rotary joint, or SARJ, while the second, P4, includes a new set of solar arrays that stretch 240 feet from tip to tip.
The solar array truss eventually will feature two SARJ joints, one on each side, to rotate the station's solar arrays like giant paddle wheels as the lab complex circles the Earth. That rotation, 360 degrees every 90-minute orbit, will keep the arrays generally face on to the sun. The orientation of the blankets can be fine tuned by so-called beta gimbal assemblies, or BGAs, that automatically adjust the pitch of each solar array wing like the orientation of an airplane propeller can be adjusted in flight.
Up to this point, the MBSUs, their associated electronics and the two cooling loops have never been fully activated. The goal of Discovery's mission is to activate the electrical and cooling equipment in the solar array truss and to rewire the station so all of its solar array power is routed through the MBSUs.
The Discovery astronauts docked with the station Monday and used the shuttle's robot arm to pull a short spacer segment - P5 - from the orbiter's cargo bay. The 4,110-pound tress segment then was handed off to the station's robot arm and positioned near the left end of the main solar array truss.
Today, Curbeam and Fuglesang will carry out a spacewalk to connect P5 to the outboard side of the P4 solar array segment. The spacer measures 11 feet long, 14 feet 11 inches wide and 13 feet 11 inches tall. Next year, the P6 arrays now providing interim power atop the Z1 truss will be moved to the left end of the power truss and bolted to P5.
NASA planners initially planned for Curbeam and Fuglesang to manually move P5 into position and then bolt it into place. But due to an oversight, planners did not realize the current position of P4 would leave the spacewalkers with just two inches of clearance while moving the spacer into place.
As a result, Sunita Williams and Joan Higginbotham, operating the station's robot arm from inside the Destiny laboratory module, will position P5 while Curbeam and Fuselgang, positioned near P4, provide visual guidance and verbal cues.
"They move it in for the mating and then Christer and I actually bolt it manually," Curbeam said in an interview. "It doesn't have the automatic (attachment) system. They'll bring it in really, really close, we'll take the launch locks off, which actually just protect the mating bolts, and once we take the launch locks off ... we drive the bolts to permanently connect them."
Precision work with the robot arm will be required to get P5 into place. The P4 arrays installed in September have not yet been tied into the station's electrical system, but their solar cells are generating power nonetheless. Most of that power is fed back into the arrays through a device called a sequential shunt unit and dissipated as heat.
During the final stages of P5 installation today, the truss segment will pass within two or three inches of the sequential shunt unit on the P4 solar array segment, "which all the juice from the solar array goes through," Curbeam said. "Not a good thing to hit."
"As a matter of fact, that's why we ended up doing it robotically. The first idea was to grab it and put it in by hand. But the problem was, when they did the evaluation, they had the SARJ angle wrong. The sequential shunt unit was in the wrong orientation for mating. So they thought they had plenty of clearance, they said you can do this by hand. Then I went like, wait a minute, no, I'm looking at it and I see two inches of clearance, maybe and inch and a half. And they went like, no. Then we did our homework and like yes!
"I said there's no way - and it's a blind mate if you do it EVA - there's no way I can sit here with a straight face and tell (lead station flight director) John Curry that I wasn't going to hit that thing. So I told him, I said John, I'm sorry but we've got to look at a different way of doing this because I can't tell you we're not going to hit it. It was a blind stick.
"I think everyone is comfortable with the arm install now," Curbeam said. "But in the beginning, they said hey, this is not what we had planned. I said I know it's not, but I don't feel comfortable doing this EVA because if we do it ourselves we are going to hit something. ... if we hit that SSU, that could turn into a really, really bad scene quickly."
Along with attaching P5, Curbeam and Fuslegang will re-position a radiator grapple fixture and replace a faulty camera on the S1 truss segment that will be needed next year when astronauts build out the right side of the solar array beam.
Once today's first spacewalk is out of the way, the astronauts and ground controllers will turn their attention to the heart of mission STS-116: electrical bypass surgery to wire in the new solar arrays.
Overnight, flight controllers will slightly retract the left wing of the P6 solar array to make sure the retraction mechanism is working normally. Because the P6 arrays extend at right angles to the new P4 arrays on the main truss, the P6-4B wing must be retracted to provide clearance for the SARJ to begin rotating the new arrays.
Because of the delay caused by the Columbia disaster, the P6 arrays have been extended for six years and engineers don't know for sure how smoothly the P6-4B wing will retract. But if all goes well, the astronauts will complete the retraction Wednesday, setting the stage for two critical re-wiring spacewalks Thursday and Saturday.