Burbank, MacLean begin spacewalk No. 2
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: September 13, 2006
Astronauts Dan Burbank and Canadian Steve MacLean, floating in the space station's Quest airlock module, switched their spacesuits to internal battery power at 5:05 a.m. today to kick off a planned six-and-a-half-hour spacewalk. The goal of the outing is to complete work on a massive rotary joint that will slowly turn a new set of solar arrays to keep them face on to the sun.
This is the 71st spacewalk devoted to space station assembly and maintenance since construction began in 1998 and the second of three planned by the Atlantis astronauts. Joe Tanner and Heidemarie Stefanyshyn-Piper completed the initial setup of a new solar array truss Tuesday and will carry out a final spacewalk Friday, the day after the new arrays are unfurled.
Latest spacewalk stats available here.
Like Tanner and Piper, Burbank and MacLean spent the night inside the Quest airlock at a reduced 10.2 psi air pressure to purge their bodies of nitrogen and prevent the bends while working in 5-psi spacesuits. The so-called campout procedure shortens preparation time by about an hour.
The objectives of today's spacewalk are to remove 14 of 16 launch locks and a half dozen launch restraints to free the solar alpha rotary joint ring mechanism for rotation. Tanner and Piper removed two launch locks Tuesday as a get-ahead task and ran into problems when one spring-loaded bolt broke free. Despite some initial concern on Tanner's part that the bolt might have found its way inside the rotary joint mechanism, engineers believe it floated free and poses no threat to the SARJ. Even so, Burbank and MacLean will be on guard to prevent any similar problems today.
The SARJ features a large gear and two redundant drive motors called drive lock assemblies, or DLAs, that ultimately will rotate two sets of outboard solar arrays through 360 degrees like giant paddle wheels as the station circles the globe to maximize solar power production.
"The SARJ, the DLAs, those mechanisms, although they've been tested on the ground, we don't have any history with them on orbit," said Atlantis commander Brent Jett. "So those represent a special challenge in terms of both engineering and operations."
Said Tanner: "The big unknown is how is SARJ going to behave when we try to activate it. It should be interesting. But hopefully not too interesting."
Along with removing the remaining launch locks, Burbank and MacLean also will complete the installation of inboard stiffener struts to provide the necessary structural rigidity.
"Steve and I are going to spend the bulk of EVA 2 removing the whole series of these launch locks and launch restraints," Burbank said in a NASA interview. "We're going to spend most of our time right in the middle section of the P3/P4 truss driving a lot of bolts with a lot of power tools. After all that's done, we've removed all this hardware that's kept the P3 and P4 trusses properly oriented for launch, we're going to deploy the SARJ braces, the solar alpha rotary joint braces, a series reinforcement structures that will help to stabilize that, that alpha rotary joint on both sides. So we'll spend a lot of time doing that.
"Then, time permitting, we'll be able to do some get-aheads, some of the work we're planning on doing on EVA 3. But EVA 2's kind of different. We're going out with a different kind of a CO2 scrubber in the (spacesuits) that allow us to go a little bit longer than planned if necessary."
Here is an updated timeline of today's activity (in EDT and mission elapsed time):
EDT DD HH MM EVENT _____________________________________________ 05:05 AM 03 17 50 EVA-2: Spacesuits to battery power 05:15 AM 03 18 00 EVA-2: Airlock egress; setup 05:40 AM 03 18 25 SSRMS camera viewing 05:55 AM 03 18 40 EVA-2: EV4: SARJ prep (remove covers/restraints) 05:55 AM 03 18 40 EVA-2: EV3: SARJ prep (remove covers/restraints) 08:30 AM 03 21 15 ISS: Transfer operations 10:10 AM 03 22 55 EVA-2: Stow P3 keel pin and drag link 10:55 AM 03 23 40 EVA-2: Payload bay cleanup, airlock ingress 11:15 AM 04 00 00 MCC: SA/SARJ activation & checkout begins 11:15 AM 04 00 00 Transfer tagup 11:35 AM 04 00 20 EVA-2: Airlock repressurization 01:15 PM 04 02 00 12A EMU swap EVA-2 02:00 PM 04 02 45 Mission status briefing on NASA TV 03:45 PM 04 04 30 ISS crew sleep begins 04:15 PM 04 05 00 STS crew sleep begins 05:00 PM 04 05 45 Daily video highlights reel on NASA TV 06:00 PM 04 06 45 Post-MMT briefing on NASA TV 07:00 PM 04 07 45 MCC: 4A solar array mast deploy (1 section) 11:05 PM 04 11 50 MCC: 2A solar array mast deploy (1 section)Burbank's call sign is EV-3 and MacLean is EV-4. Burbank's suit features horizontal red dashes while MacLean's sports diagonal dashes.
Once in place at the P3/P4 work site, the astronauts will start removing the 14 launch locks, a multi-step procedure that requires them to first release an inboard clamp; remove and temporarily stow a thermal cover; remove four bolts from the launch lock; and reinstall the thermal cover.
Once that work is done and the bolts are safely stowed in a transfer bag, the spacewalkers will remove six outboard launch restraints and install the remaining brace beams needed to stiffen the inboard side of the SARJ interface.
"Dan and I spend a lot of time around that solar array rotary joint, making sure that it's ready so that that joint can move later on during the mission," MacLean said. "So we're removing all the launch locks associated with that, and that takes us a while because it was very important to minimize the vibration of that area during launch and we have several launch locks to take off. In addition, we will stiffen up the truss. ... Here we are, construction engineers, where we'll basically remove a brace and then bring it over and then reattach it, and that stiffens up the torsional mode of the truss. We do something similar on the, on the P4 side.
"We have some AJIS (Alpha Joint Interface Structure) struts that if Joe and Heide haven't got them done the day before we will do the AJIS struts on the P4 side, which again stiffen up the torsional mode of the P4 side."
If all goes well, mission controllers will activate the SARJ later today and rotate it 180 degrees to properly position the new solar arrays for deployment. The arrays will be extended one bay tonight by flight controllers in Houston as a confidence check and then fully extended by the astronauts Thursday.
"The EVA crew does the physical work, in terms of positioning the arrays and releasing all the bolts and the launch restraints," Jett said in a NASA interview. "So the physical work is done by the EVA team but then the preparation to actually make the deployment happen, all the activation sequence, and the activation of the rotary joint, is all performed by the ground team.
"Itıs a tremendous amount of work that needs to be done, simultaneously, not only during the EVAs but also during those two nights when weıre asleep. The ground teamıs working very hard to get the arrays, ready. When we wake up (Thursday), if everything goes well, the ground is going to be ready to go. We position a few cameras, so we can monitor the deployment and then we go to the computer and command the arrays to deploy. Now, hopefully, everything will go very smoothly ... and we'll get the arrays out in a couple of orbits."
But the SARJ will not begin rotating the new arrays until after a December shuttle assembly mission to complete critical cooling and electrical system changes and to retract a solar array that is currently in the way. That set of arrays eventually will be mounted just outboard of the new P4 solar panels to complete the left side of the main truss.
The official crew patch for the STS-115 mission of space shuttle Atlantis to resume orbital construction of the International Space Station.
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