The space station's solar array truss eventually will stretch the length of a football field, sporting two sets of dual-wing solar arrays on each end of the main truss. The solar array wings, or SAWs, are numbered based on their position on the station with even numbers assigned to panels on the left, or port, side of the main truss and odd numbers assigned to SAWs on the right, or starboard, side.

The P4 segment's two SAWs are numbered 2A and 4A while the P6 SAWs are numbered 2B and 4B. The S4 arrays are designated 1A and 3A while the S6 SAWs will be known as 1B and 3B.

The four sets of solar arrays are essentially identical. In each set, solar power flows from two SAWs into a sequential shunt unit. Power coming into the SSU can vary from 130 to 180 volts DC depending on a variety of factors, including blanket degradation, shadowing, etc.

SSU output can be adjusted as required, but it typically will be set at 160 volts and passed on to an integrated equipment assembly, or IEA. The SSU routes excess power back to the SAWs to be dissipated as heat and it also can be used to isolate a set of SAWs from the power grid if necessary.

Because each solar array wing powers a separate station circuit, the IEAs in each array include two sets of electronics. A direct current switching unit (DCSU), containing six high power switches, routes SAW electricity from the SSU into battery charge/discharge units that regulate the flow of power to and from six batteries, three for each SAW.

When the array's SAWs are in sunlight, the DCSU sends solar power to one of four main bus switching units, or MBSUs, mounted in the S0 truss that. Solar array power is routed through the SARJ, and also into the batteries to charge them up. As the station moves into Earth's shadow, the DCSU begins adding battery power to the flow going to its MBSU to maintain the proper voltage. When the arrays are completely eclipsed, the DCSU sends battery power alone to the MBSU in a continuous, automatic procedure.

The DCSU, the battery chargers and other components in each array's integrated electronics unit are cooled by ammonia circulated through cold plates and then routed to a single deployable radiator. Each of the four sets of arrays that eventually will be attached to the station include its own ammonia cooling system, which is independent of the main cooling systems in the S1 and P1 truss segments.

Electricity from the solar arrays - "primary power" - flows to the MBSUs on S0 and then to transformers that lower the voltage to 124 volts DC. This so-called "secondary power" is then directed to the station's myriad electrical systems using numerous electro-mechanical switches known as remote power controllers.

The S4 solar array wings are marvels of engineering, folding into shallow "blanket boxes" for launch, yet extending some 240 feet from tip to tip when deployed in space. The blankets are extended by a self-erecting central masts made up of 31 "bays," each one measuring about 40 inches deep. Boeing is the prime contractor for the S3/S4 truss segment while Lockheed Martin built the included SAWS. Here's a bit of background from Boeing:

Source: Boeing

Each SAW is made up of two solar blankets mounted to a common mast. Before 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 20 inches thick while in this stored position. The mast consists of interlocking battens that 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. Like a human torso, it has two arms when mounted on S4, and they 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 so the blankets would not stick together. This testing was completed in July 2003 and proved to be successful when the P4 solar array was successfully deployed on STS-115 in September.

When fully deployed, the SAW extends 115 feet and spans 38 feet across and extends to each side of the Integrated Equipment Assembly. Since the second SAW is deployed in the opposite direction, the total wing span is more than 240 feet.

Each SAW weighs more than 2,400 pounds and uses 32,800 solar array cells per wing, each measuring 8-cm square with 4,100 diodes. The individual cells were made by Boeing's 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 S4 module yielding a total power generation capability approaching 66 kW, enough power to meet the electrical needs of about 30 2,800-square-foot houses, consuming about 2kW of power each.

Following S3/S4 attachment to the power truss on flight day four, the astronauts plan to deploy the new SAWS on flight day five. Getting a head start, flight controllers will send commands while the crew sleeps to unlatch the solar array blanket boxes and to extend each SAW one mast bay. After crew wakeup, the SAWs will be extended, one at a time, in a stepwise fashion, first to 49 percent and then to 100 percent with a half-hour gap in the middle of the sequence to permit the sun to warm up the blankets. The idea is to avoid a problem seen during the first array extensions in 2000 when array slats stuck together and caused problems.

"What happens is these panels tend to stick together and then as the mast is being deployed, the panels would release and they would do that fairly dynamically and you can see how there's a wave that propagates up and down the array and those panels towards the base moved quickly into the blanket box and that did cause some problems with the tensioning mechanism," Beck said of the first P6-2B extension in 2000. "The crew had to go outside during a spacewalk and correct it.

"So the new technique is designed to avoid that dynamic motion. Basically what we do is we deploy the array about halfway out, we let it sit with sun shining on the panels so those panels can warm up and as they warm up, they tend to release. And so we'll sit there for 30 minutes and let the panels release. We'll deploy the 1A array first to the halfway point, wait 30 minutes with sun on it and then complete the deploy of that array. It will be repositioned and we'll do a small attitude maneuver to get sun directly on the 3A array. It will be deployed halfway, same sequence, wait for 30 minutes to warm the panels and then complete the deploy."

DATE/EDT.......DD...HH...MM...EVENT

Tue 09:08 AM...03...13...30...STS/ISS crew wakeup
Tue 10:38 AM...03...15...00...SSRMS maneuver
Tue 11:43 AM...03...16...05...1A mast deploy to 49 percent
Tue 12:23 PM...03...16...45...1A mast deploy to 100 percent
Tue 01:13 PM...03...17...35...3A mast deploy to 49 percent
Tue 01:23 PM...03...17...45...Spacesuit swap
Tue 01:53 PM...03...18...15...3A mast deploy to 100 percent
Tue 02:33 PM...03...18...55...Joint crew meal
Tue 03:33 PM...03...19...55...Crew off duty time
Tue 05:33 PM...03...21...55...SRMS moves to retraction viewing point
Tue 05:58 PM...03...22...20...EVA-2: Tools configured
Tue 06:03 PM...03...22...25...SSRMS WS-2 config translation
Tue 07:03 PM...03...23...25...Equipment lock preps
Tue 09:03 PM...04...01...25...EVA-2: Procedures review
Tue 10:03 PM...04...02...25...PAO event
Tue 10:48 PM...04...03...10...MT translates from WS-2 to WS-3
Tue 11:23 PM...04...03...45...EVA-2: Mask pre-breathe and tool config

06/13/07
Wed 12:18 AM...04...04...40...EVA-2: Crew lock to 10.2 psi
Wed 12:38 AM...04...05...00...ISS crew sleep begins
Wed 01:08 AM...04...05...30...STS crew sleep begins

"Every person on the crew will have a role because there are a number of things that we'll be watching for," he said in a NASA interview. "As the solar arrays unfurl, after we initiate the drive command and as these are unfurling, they basically just, as if you had folded up a piece of paper back and forth, we just basically unfurl the whole solar array in that way. But as it's unfurling there's a, occasionally the panels that will tend to stick a little bit because they've been boxed up now for quite some time; but as they unfurl they'll start to unstick and jostle themselves a little bit, which is normal.

"But what we'll be watching for are those that might stick a little bit harder than what we expect, and so we have what are called the tension reels, so we'll have two of our people on the crew that are going to be watching the tension reels the whole time during the deploy. There'll be two of the other folks that will be then counting the individual bays, because we deploy out about halfway, to about 49 percent of the deploy, and then we stop and let sun basically heat up the components so that we don't hit a high-tension condition. The problem there is if we hit a high-tension condition then these tension reels could lose their tension on the wires that actually hold everything in position. If that's the case, then we would have to go out and do an EVA so we're going to be watching that very carefully. The ground will then leave this at 50 percent for one day-cycle, and then we come back once we hit another day pass, then we will then deploy the solar arrays out the rest of the way to the 100 percent.

"Things we'll be watching for are the tension reels, as I mentioned, but we'll also be watching a tension bar that's at the base of the solar panel itself, and we'll be watching to make sure that is basically not moving until we get to the very last panels, and when we start pulling them out to final tension, then that bar at the base will actually separate about 22 inches. Those are the things that we'll be watching for. But, for the most part, it's really just making sure everything just deploys nominally and just in a nice, orderly sequence, nothing sticking, and nothing's really trying to jerk too much tension on the mechanisms and the base of the solar array blanket box."

During the P3/P4 attachment last September, the port-side solar alpha rotary joint was configured for operation before the extension of the two SAWs. That was because the P4 arrays had to be rotated before their cooling radiator could be deployed. That is not necessary for the S3/S4 segments, so the crew is reversing that sequence, deploying the new SAWs and radiator first before completing SARJ activation during their second spacewalk on flight day six.

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