Spaceflight Now




Deployment of station's new solar arrays underway
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: September 14, 2006

Running behind schedule because of software problems, flight controllers early today began a slow, careful process to unfurl a new set of solar arrays aboard the international space station. The plan called for first deploying the huge panels just a few feet to let them warm up and decompress after years in storage. Later today, the Atlantis astronauts will send commands to fully extend the new arrays.


The first section of solar wing was unfurled this morning. Credit: NASA TV/Spaceflight Now
 
The initial deployment had been planned for Wednesday night, but software problems held up checkout of a large rotary drive mechanism designed to rotate the new arrays 360 degrees to stay face on to the sun as the station circles the globe.

One of two drive lock assembly - DLA - motors in the solar alpha rotary joint worked properly in an initial test, rotating the still-stowed arrays 180 degrees. But the drive gear in a redundant DLA appeared to be misaligned and testing was put on hold. Late last night, engineers traced the problem to a software commanding issue and after implementing a workaround, DLA-2 eventually moved the arrays through a 360-degree rotation as planned.

Finally, at 5 a.m., the first set of arrays - 4A - began slowly unfurling on ground command as its self-assembling mast pulled the top few feet of folded solar cell blankets from its box. The other array, 2A, followed suit at 5:46 a.m.

The Atlantis astronauts hope to complete solar array deployment later today in stages, sending commands to unfurl the panels first to 49 percent and then to a full 100 percent in what promises to be the most visually dramatic moment of the ongoing space station assembly mission.

Here is an updated timeline of today's activities (NOTE: solar array deploy times are uncertain and not included; in EDT and mission elapsed time):


12:15 AM	04	13	00	STS/ISS crew wakeup
03:35 AM	04	16	20	EVA tool config
05:40 AM	04	18	25	ERCA troubleshooting
06:10 AM	04	18	55	Station robot arm reconfig
06:45 AM	04	19	30	Crew meals begin
11:25 AM	05	00	10	Canadian PAO event with Jett, MacLean
11:45 AM	05	00	30	Transfer tagup
11:50 AM	05	00	35	CNN, NPR interview Jett, Vinogradov, Williams
11:50 AM	05	00	35	Infrared camera setup
12:10 PM	05	00	55	EVA-3: Procedures review
02:20 PM	05	03	05	EVA-3: Campout mask pre-breathe (Tanner, Piper)
03:05 PM	05	03	50	EVA-3: Crew lock 10.2 depress
03:45 PM	05	04	30	ISS crew sleep begins
04:15 PM	05	05	00	STS crew sleep begins (Tanner, Piper in airlock)
Each solar array wing is 15 feet wide and 115 feet long. Extending in opposite directions, the wings will stretch more than 240 feet from tip to tip when fully deployed. The so-called P4 array, or port 4 segment of the station's main power truss, is bolted to the SARJ mechanism on the outboard side of the P3 truss segment that also serves to route power and data to and from the solar panels.

When completed, the space station's power truss will stretch more than a football field from end to end. Two huge solar arrays on each end of the truss will track the sun as the station circles the globe, rotating and changing pitch as required to maximize electrical generation.

Finishing the main truss and wiring in the new arrays is a major element of near-term station assembly missions.

One set of arrays, P6, is already attached to the station, providing power for the U.S. segment of the outpost. It is attached to the Z1 truss that extends upward from the Unity module at right angles to the main solar array truss. It will be moved next August to its final position next to the P4 arrays that were attached to the station Tuesday by Atlantis' crew.

Two other sets of identical arrays will be mounted on the right side of the truss during shuttle assembly flights in February 2007 and June 2008.

Designed by Boeing, the newly attached P4 truss features two motor-driven self-assesmbling masts designed to pull out a pair of solar blankets. For launch, the arrays are folded like venetian blinds in blanket boxes measuring 15 feet long but just 20 inches thick.

The P3 truss was delivered to the Kennedy Space Center in November 1999. P4 followed suit in July 2000. The solar array blankets have not been unfolded since they were originally stowed for launch.

Prior to Columbia's launch in January 2003, the arrays were certified to operate and deploy normally when stowed for up to 45 months. Because of the extended downtime after the Columbia accident, one of the blanket boxes on P4 was removed and shipped to California for deployment tests. A replacement blanket was installed aboard P4.

As it turned out, the deployment tests went well and the blanket, which had been stowed 39 months at that point, worked normally. As a result, the certification age limit was boosted to 82 months. As of launch aboard Atlantis, the P4 blankets had been stowed for 67 and 73 months respectively.

One other consequence of the Columbia recovery and subsequent launch delays was a decision to replace all 12 batteries in P4. The 372-pound batteries were swapped out in March and August of 2005.

The solar array wings were designed by Lockheed Martin. They weigh more than 2,400 pounds and feature some 33,000 solar cells per blanket. They are designed to produce more power than the station actually needs to compensate for normal degradation as the outpost ages.

The blankets must face the sun directly for maximum electrical generation and two mechanisms are in place to do just that. The wings can be rotated about their long axis by beta gimbal joints, much like the pitch of an airplane propeller can be adjusted. The SARJ, on the other hand, rotates the arrays through 360 degrees like a waterwheel around the axis of the main truss.

The P4 integrated equipment assembly, or IEA, is a cube measuring 16 feet on a side and weighing nearly 17,000 pounds. It includes direct current converters, 12 batteries, battery chargers, control computers and an ammonia cooling system to keep the electronic gear at the proper temperature. A single set of folding radiator panels will extend 44 feet when fully deployed.

Most of the work to ready the new P4 arrays for extension "is done by the flight control team," Jett said. "The EVA crew does the physical work, in terms of, positioning the arrays and releasing all the bolts and the launch restraints. 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."

The P4 arrays are identical to P6, which was attached to the space station during shuttle mission STS-97 in December 2000. When the first blanket was deployed, engineers were surprised to see several of the blanket panels had stuck together. When they jerked free, a tensioning cable jumped its guides and required repairs on a subsequent spacewalk. For the second array's deployment, the crew let the sun warm up the array and deployed it in a so-called high-tension mode. That technique worked, and the array unfurled without incident.

For the P4 deploy, the arrays will first be extended the length of a single bay of each mast - a few feet worth - to relieve compression and to begin warming up the panels.

"The one-bay deploy is basically to start the relaxation of the blankets and to get ahead," station flight director John McCullough said Wednesday. "The time we're in an inertial attitude, the attitude where we continuously point the arrays at the sun, is limited because the rest of the space station doesn't really like that atitude.

"For this deploy, we want to have the sun in a certain cone to warm the back of the blankets and there's a limit to that. We can only be there for three revs around the Earth."

Working in stages, the astronauts will extend the 4A mast to a distance of 49 percent early Thursday, wait a half hour or so for additional solar heating, and then the rest of the way to full extension. They will repeat that process for the 2A mast. It will take about 90 minutes for each panel to fully deploy.

"We're not too terribly concerned about stiction on the deploy," said Tanner, a veteran of the STS-97 mission. "We know it might happen in certain panels. The team went hard to work after STS-97 to figure out the mechanism of stiction and what we can do to reduce it. They came up with a good operational plan to nominally deploy.

"Now if for some reason one or two panels sticks after all that, then we can go out EVA, it would be on EVA 3, and actually manually peel the panels apart for the first 40 inches or so and doing a nominal deploy after that. All of our tests say they will peel open very easily. So we're not too terribly concerned about that."

Space station Program Manager Mike Suffredini agreed that experience gained during the STS-97 mission in 2000 should result in a smooth deploy.

"The good news is we've done it twice before," Suffredini said. "The first time we deployed one of these arrays, we learned about a stiction issue that existed. After quite a bit of work during that mission, we deployed the second array in a little bit different technique, which allowed us to be successful. We learned a lot about techniques to get these arrays out without having the tension wire come loose, which is what happened when we tried to deploy the first one. It turns out after a lot of work ... we figured out a fix to go back in there to put it back in its original condition. And of course, the arrays have been fine ever since.

Using an engineering model, "we did quite a bit of testing on this stiction issue and how these arrays can stick together based on the silicon bead that's on the arrays and then we compress them for long periods of time before they go fly. And that was basically the cause of the problem. Over a long period of time this silicon would tend to attach itself to the back part of the array. And so we've done a number of things, largely operational changes but also on how we dealt with the arrays."

While the arrays were compressed for shipment to Florida, that pressure was relaxed until shortly before final preparations.

"Probably the largest changes were operational," Suffredini said. "We will partially deploy the array about one bay's worth and we'll let it warm up and sort of expand a little bit. Then the actual deploy process a little bit later will have us deploy the array halfway and then you'll see us sit for about 30 minutes as we warm it up and then we'll go the rest of the way.

"In addition to that, we're using what we call a high-tension mode. Instead of leaving the lower part of the array free to move up and down as the array gets deployed, we learned we need to hold it down against the bottom of the blanket box. So the new technique holds it down and then this particular deploy lets the heat warm things up, to allow the silicon to free itself, is the process we'll use to deploy. I have a lot of confidence in the deployment of the arrays."

The new solar arrays will not be rotated on the SARJ because of interference with the port wing of the P6 array and they will not provide any power to the space station until reconfigurations during the next shuttle mission in December.

For readers interested in a bit more detail, here is an overview provided by Boeing, the prime contractor.

Source: Boeing

Electrical power is the most critical resource for the ISS because it allows astronauts to live comfortably, safely operate the station, and perform complex scientific experiments. Since the only readily available source of energy for spacecraft is sunlight, technologies were developed to efficiently convert solar energy to electrical power. One way to do this is by using large numbers of solar cells assembled into arrays to produce high power levels. The cells are made from purified crystal ingots of silicon that directly convert light to electricity through a process called photovoltaics. Solar cells do the job, but a spacecraft orbiting the Earth is not always in direct sunlight so energy has to be stored. Storing power in rechargeable batteries provides a continuous source of electricity while the spacecraft is in the Earth's shadow.

NASA and Lockheed Martin developed a method of mounting the solar arrays on a "blanket" that can be folded like an accordion for delivery to space. Once in orbit, astronauts deploy the blankets to their full size. Gimbals are used to rotate the arrays so that they face the Sun to provide maximum power to the Space Station. The solar arrays track the sun in two axes: beta and alpha.

P4 is the second of four PVMs that will eventually be brought up to the International Space Station, converting sunlight to electricity. The first one, named P6, was brought on orbit by the STS-97 crew in November 2000. The primary functions of the P4 photovoltaic module are to collect, store, convert and distribute electrical power to loads within the segment and to other ISS segments. The P4 PVM consists of two beta gimbal/PV array assemblies, two beta gimbal transition structures, one integrated equipment assembly and associated cabling and tubing. The P4 PVM components were assembled by Boeing in Tulsa, Okla. and Lockheed Martin in Sunnyvale, Calif. prior to final assembly and testing by Boeing at Kennedy Space Center, Fla.

There are two solar array wings (SAW) designed, built and tested by Lockheed Martin in Sunnyvale, Calif. on the P4 module, each deployed in the opposite direction from each other. Each SAW is made up of two solar blankets mounted to a common mast. Prior to deployment, each panel is folded accordion style into a solar array blanket box (SABB) measuring 20 inches high and 15 feet in length. Each blanket is only about two inches thick while in this stored position. The mast consists of interlocking battens which are stowed for launch inside a mast canister assembly (MCA) designed, built and tested by ATK-Able. When deployed by the astronauts, the SAW deploys like an erector set as it unfolds. It has two arms like a human torso when mounted on P4 which are rotated outwards by astronauts during a spacewalk so they can be fully deployed. Because these blankets were stored for such a long time, NASA, Boeing and Lockheed Martin conducted extensive testing to ensure they would unfold properly once on orbit to ensure there would be no problems with the blankets sticking together. This testing was completed in July 2003.

When fully deployed, the SAW extends 115 feet and spans 38 feet across and extends out to each side of the integrated equipment assembly. Since the second SAW is deployed in the opposite direction, the total wing span is over 240 feet.

Each solar array wing weighs over 2,400 pounds and use nearly 33,000 (32,800 per wing) solar array cells, each measuring 8-cm square with 4,100 diodes. The individual cells were made by Spectrolab and ASEC. There are 400 solar array cells to a string and there are 82 strings per wing. Each SAW is capable of generating nearly 32.8 kilowatts (kW) of direct current power. There are two SAWs on the P4 module yielding a total power generation capability approaching 66 kW, enough power to meet the needs of 30 average homes without air conditioning (based on an average 2kW of power).

Spaceflight Now Plus
Additional coverage for subscribers:
VIDEO: PORT 3/PORT 4 TRUSS KEEL PIN REMOVED AND STOWED PLAY
VIDEO: HELMETCAM OF BURBANK REMOVING SARJ RESTRAINT PLAY
VIDEO: SPACEWALKERS PAUSE FOR PICTURE TIME PLAY
VIDEO: STEVE MACLEAN REPORTS LOST BOLT PLAY
VIDEO: ROTARY JOINT LOCK REMOVED BY SPACEWALKER PLAY
VIDEO: STEP-BY-STEP PREVIEW OF SPACEWALK NO. 2 PLAY
VIDEO: POST-EVA 1 STATUS BRIEFING DIAL-UP | BROADBAND
VIDEO: TANNER LOSES BOLT DURING ROTARY JOINT WORK PLAY
VIDEO: PIPER UNFOLDS SOLAR BLANKET BOXES SHORT | FULL
VIDEO: SECOND WING'S STRUCTURE DEPLOYED BY PIPER PLAY
VIDEO: FIRST SOLAR WING'S STRUCTURE DEPLOYED BY TANNER PLAY
VIDEO: STEP-BY-STEP PREVIEW OF SPACEWALK NO. 1 PLAY
VIDEO: TRUSS HANDED FROM SHUTTLE ARM TO STATION ARM PLAY
VIDEO: ARM MANEUVERS TRUSS OVER SHUTTLE WING PLAY
VIDEO: TRUSS SLOWLY LIFTED OUT OF PAYLOAD BAY PLAY
VIDEO: ATLANTIS' ARM GRAPPLES THE TRUSS PLAY
VIDEO: MONDAY'S MISSION STATUS BRIEFING DIAL-UP | BROADBAND
VIDEO: ATLANTIS WELCOMED ABOARD THE STATION PLAY
VIDEO: DOCKING REPLAY FROM CAMERA ON SHUTTLE ARM PLAY
VIDEO: SHUTTLE ATLANTIS DOCKS TO THE STATION PLAY
VIDEO: ATLANTIS' BREATH-TAKING FLIP MANEUVER PLAY
VIDEO: CREW'S CAMCORDER FOOTAGE OF EXTERNAL TANK PLAY
VIDEO: NARRATED ANIMATION PREVIEWING TRUSS UNBERTHING PLAY
VIDEO: NARRATED ANIMATION PREVIEWING THE DOCKING PLAY
VIDEO: NARRATED ANIMATION OF PAYLOAD BAY CONFIGURATION PLAY
MORE: STS-115 VIDEO COVERAGE
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VIDEO: BRIEFING ON TANK'S PERFORMANCE DIAL-UP | BROADBAND
VIDEO: TANK'S ONBOARD CAMERA LIFTOFF TO SEPARATION PLAY
VIDEO: FLIGHT DIRECTOR EXPLAINS INSPECTIONS PLAY
VIDEO: SUNDAY'S MISSION STATUS BRIEFING DIAL-UP | BROADBAND

VIDEO: LAUNCH OF ATLANTIS! PLAY
VIDEO: SHEDDING FOAM MAY HAVE HIT ATLANTIS PLAY
VIDEO: ONBOARD VIEW OF EXTERNAL TANK SEPARATION PLAY
VIDEO: INSIDE MISSION CONTROL DURING LAUNCH PLAY
VIDEO: STATION CREW TOLD VISITORS EN ROUTE PLAY
VIDEO: HOUSTON RADIOS DEBRIS REPORT TO CREW PLAY
VIDEO: POST-LAUNCH NEWS CONFERENCE DIAL-UP | BROADBAND
VIDEO: QUICK-LOOK BRIEFING ON DEBRIS DIAL-UP | BROADBAND

LAUNCH REPLAYS:
VIDEO: BEACH MOUND TRACKER PLAY
VIDEO: CAMERA IN FRONT OF PAD PLAY
VIDEO: BANANA CREEK VIEWING SITE PLAY
VIDEO: VEHICLE ASSEMBLY BUILDING ROOF PLAY
VIDEO: PAD 39B SIDE PERIMETER PLAY
VIDEO: PLAYALINDA BEACH TRACKER PLAY
VIDEO: PLAYALINDA BEACH ZOOM PLAY
VIDEO: UCS 23 TRACKER PLAY
VIDEO: UCS 11 TRACKER PLAY

VIDEO: MISSION SPECIALIST 4 STEVE MACLEAN BOARDS ATLANTIS PLAY
VIDEO: MISSION SPECIALIST 3 HEIDE PIPER BOARDS PLAY
VIDEO: MISSION SPECIALIST 2 DAN BURBANK BOARDS PLAY
VIDEO: MISSION SPECIALIST 1 JOE TANNER BOARDS PLAY
VIDEO: PILOT CHRIS FERGUSON BOARDS PLAY
VIDEO: COMMANDER BRENT JETT BOARDS PLAY

VIDEO: ASTRONAUTS EMERGE FROM CREW QUARTERS PLAY
VIDEO: CREW SUITS UP FOR LAUNCH TO SPACE PLAY
VIDEO: FINAL INSPECTION TEAM CHECKS ATLANTIS PLAY
VIDEO: ASTRONAUTS READY FOR SECOND LAUNCH TRY PLAY
MORE: STS-115 VIDEO COVERAGE
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