Power-generating solar wings to be unfurled today
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 20, 2009
The Discovery astronauts and their three space station counterparts plan to carefully extend a newly attached set of solar arrays today in a critical operation that will complete the station's U.S. power system after eight years of construction. The starboard 6, or S6, solar array truss segment, the station's fourth and final set of power panels, was attached during a spacewalk Thursday, plugged in, activated and prepped for array extension. Its arrays will stretch 240 feet from tip to tip when fully extended, doubling the power available for science from 15 kilowatts to 30 kilowatts.
"We're looking forward to a wonderful day in space highlighted by the deployment of the S6 solar array wings, which really is going to bring the station to full power," astronaut John Phillips said to mission control after crew wakeup. "It's going to be a full-up effort from the ground and the crew on orbit and we're really looking forward to working with you."
Along with extending the S6 solar arrays, the astronauts also plan to install a new urine distillation assembly centrifuge in the station's water recycling system, replacing a DA that failed shortly after installation late last year. Getting the water recycling system up and running is critical to long-term station operations and plans to support a full-time crew of six. The new unit will be installed late today, starting around 5:30 p.m., with testing and activation on tap this weekend.
Solar array deployment originally was planned for Sunday, but it was moved up two days after mission managers decided a focused inspection of the shuttle Discovery's heat shield, work that would have been carried out today, was not needed. Overnight, flight controllers deployed the arrays one mast bay to make sure there was no interference and to begin thermal conditioning. The actual array deployment procedure will begin around 10:50 a.m.
Because of problems with past array deploys - the folded blanket slats can stick together after being packed in their storage boxes for years - the astronauts will attempt deploy in a stepwise fashion, first heating each array in sunlight and then extending them halfway before another "bake out." Only then will each panel be extended the rest of the way.
"We will start by maneuvering the international space station and the shuttle into what's called a solar-inertial attitude," said flight director Kwatsi Alibaruho. "What that really means is that we will maneuver in a manner such that one side of the space station is always pointed at the sun at every point in its orbit. That's particularly important because of what we have to do thermally with the solar arrays."
"In the first phase of solar array deploy, we'll deploy the solar arrays to approximately 45 to 50 percent of full deploy. We'll extend the solar array mast and as the mast extends, the blankets, which are currently folded in these blanket boxes, they will start to unfold like an accordion unfolds when the latches are released.
"As the solar arrays unfold, there's a phenomenon that we call 'stiction' that we're concerned about. Essentially, whenever you have brand new plastics that have been packed for quite a while there are polymers and other chemicals in these plastics that like to stick together. The solar arrays are coated with such substances and what happens is, beyond the 45 percent to 50 percent point, the sticking effect tends to create excessive tension on the mast deployment mechanism. And if the tension exceeds the recommended limits, then you could have problems with the guidewires and the tension reels on the mechanism.
"In order to alleviate that sticking, at about the 45 percent, 50 percent point of deploy in that solar-inertial attitude, we'll be maneuvered such that we'll get constant sun on the solar arrays and as the sun lands on the arrays, what we have seen in the past is the panels simply release. As they warm up the polymers and the plastics on the solar array coating, they become less sticky and they release on their own without creating excessive tension on the mechanism.
"As they release, as we thermally condition the arrays, we'll then deploy ... to the full deploy point. At that point, we'll set the mast mechanism in full tension mode. This process is slow and somewhat gradual. We expect it will take us about three full orbits, basically about four-and-a-half hours, to get both solar arrays deployed."
Complicating the procedure, the station's gyroscopes cannot be used to maintain the proper orientation during array deploy. Instead, Russian rocket thrusters at the rear of the station will be used when the arrays facing in the opposite direction are extended and shuttle thrusters will be used when their oppositely aimed counterparts are deployed. The thruster changes are required to avoid "pluming" the panels with exhaust gases.
Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision G of the NASA television schedule):
EDT........DD...HH...MM...EVENT 03/20/09 07:43 AM...04...12...00...Crew wakeup 09:13 AM...04...13...30...ISS daily planning conference 10:13 AM...04...14...30...Maneuver to solar array deploy attitude 10:13 AM...04...14...30...Distillation assembly unpack/transfer 10:48 AM...04...15...05...Solar array deploy operations begin 10:58 AM...04...15...15...Channel 1B array to 49 percent 11:43 AM...04...16...00...Channel 1B array to 100 percent 12:28 PM...04...16...45...Channel 3B array to 49 percent 01:13 PM...04...17...30...Channel 3B array to 100 percent 01:28 PM...04...17...45...Station arm (SSRMS) maneuver 01:58 PM...04...18...15...PAO event 02:18 PM...04...18...35...Crew meals begin 03:28 PM...04...19...45...Spacesuit component swap 04:18 PM...04...20...35...Equipment lock preps 04:58 PM...04...21...15...Tool configuration 05:30 PM...04...21...47...Mission status/MMT briefing 05:33 PM...04...21...50...Distillation assembly R&R (part 1) 05:58 PM...04...22...15...Tool audit 06:43 PM...04...23...00...Distillation assembly R&R (part 2) 07:43 PM...05...00...00...EVA-3: Procedures review 08:13 PM...05...00...30...Distillation assembly pack/transfer 08:43 PM...05...01...00...Evening planning conference 10:08 PM...05...02...25...EVA-3: Mask pre-breathe 10:53 PM...05...03...10...EVA-3: Airlock depress to 10.2 psi 11:13 PM...05...03...30...ISS crew sleep begins 11:43 PM...05...04...00...STS crew sleep begins 03/21/09 12:00 AM...05...04...17...Flight day highlights 07:43 AM...05...12...00...Crew wakeup
Phillips will push the button that starts and stops solar array deployment. His shuttle crewmates and station counterparts will be monitoring a dozen camera views, ready to shout out an abort order if any unusual behavior is detected.
"There are two solar array wings and each of them has two solar blankets, two big long solar panels," Phillips said in a NASA interview. "So there's four solar panels in all and they are in what's called 'blanket boxes,' these complicated metal boxes. The ground preps the whole thing. The ground unlocks the boxes and does a lot of checks and then they turn it over to us.
"We've got the entire shuttle crew working on this. We've got 12 TV monitors looking at different views. We've got a guy on the shuttle, six guys on the station and I. It's a big team effort. When we unfold these arrays, they're coming out of the boxes and they're pleated together. The pleats are flattening as they come out of the boxes. You don't want to put too much tension on them but you have to put enough tension on them to unfold them.
"We've learned some lessons over the years in how to do these," Phillips said. "This is, should be the final such operation in the history of the station, I hope. We've got six people stationed around monitors on the station, one on the shuttle. Two guys are just watching the tensioning devices on the boxes that the arrays are coming out of. Two people are counting the number of bays or the number of panels that have unfolded. I'm the guy who pushes the button that says "deploy."
"We will deploy each solar array wing half way out and then I will abort that deploy. We do this during orbital day time. It takes about five minutes to do half the array and then we've got around 30 minutes to bake it out, where we let the sun get on it. That will keep the panels from sticking together. Then we deploy it the rest of the way.
"And then the next solar day we do the other side. This is the fourth time we've deployed solar panels but what's very interesting is that these have been in the boxes for a long time. Once side has been in the box for five years, the other side about eight years. During that time they haven't been stretched out. Now the folks at Kennedy Space Center have taken some measures to minimize problems that might occur but we're still anticipating that some of these pleated together panels might stick and might not come apart so easily."