The P6 truss structure is packed up for launch at pad 39A where workers prepare it for installation into Endeavour's payload bay. Photo: NASA

The P6 solar array segment is made up of three elements: The two photovoltaic solar wings themselves; an integrated electronics assembly housing batteries and other critical gear; and a truss section called the "long space" that includes thermal control equipment.

The P6 truss also houses a deployable radiator. Ammonia coolant lines from the U.S. laboratory module ultimately will be connected through the Z1 truss to the radiator to dissipate heat generated by the lab's electronic gear.

At launch, P6 weighs 17.5 tons and measures 16-by-16-by 49 feet. It is equipped with two solar wings, each one made up of two solar cell panels that will convert sunlight to electricity. The panels are rolled up like window shades at launch and housed in rectangular "blanket boxes" measuring 15 feet by 20 inches.

When fully extended, each 15-foot-wide blanket will stretch 110 feet. From the tip of one two-blanket wing to the tip of the other is roughly 240 feet. The blankets include some 32,400 solar cells covering a quarter of an acre.

The blanket boxes are locked to the sides of the P6 truss for launch. They must be unlatched and rotated up 90 degrees, one on either side of the truss like arms extending from a shoulder for deployment.

Each solar wing is made up of two 15-foot-wide solar array blankets separated by eight feet. In between is an ingenious, lightweight open-truss boom that will be driven open by motors to unfurl the solar cell blankets.

Each boom, when collapsed for launch, fits in a canister measuring just four feet wide and 7.5 feet long. It will take about 15 minutes to drive each boom out, one at a time, to fully deploy the solar wings.

If all goes well, that is.

One of the solar array panels is extended to full length in this factory test. Photo: Lockheed Martin

The panels and booms have been rolled up in their launch boxes for more than nine months. But engineers say they are confident both sets of panels will deploy properly.

The two solar array wings will generate about 19 kilowatts of power above and beyond what is required to charge the batteries in the electronics assembly and to operate the array's other internal systems.

By comparison, the two solar arrays on the Zvezda module generate 10 kilowatts while Zarya's two arrays provide an additional seven. Virtually all of the Russian segment power is required to keep the two modules operational with little left over for other purposes.

Power generated by the P6 solar arrays will be sent to a direct current switching unit in the P6 integrated electronics assembly. The switching unit will direct power to the station and to 12 batteries during orbital daylight.

Direct current converter units will step the voltage down from the 160 volts generated by the arrays to the 120 volts needed by station systems.

A closeup view of solar arrays. Photo: Lockheed Martin

Two independent power channels are available for redundancy, but only one is required for normal station operations.

During orbital darkness, power will be provided by 12 nickel-hydrogen batteries in the P6 electronics assembly. The 375-pound batteries will be replaced every five years. The solar arrays themselves are rated for 15 years of orbital life.

"We launch the P6 batteries totally discharged to address safety concerns," said station flight director Jeff Hanley. "Once we have the systems up and running and we have the wings deployed, then we can start the process of charging them.

"That process takes three to four orbits to complete. Hopefully by (the next day), all the batteries will be charged on P6 and we can turn our attention to actually distributing some of this power to the rest of the station."

But first, the Endeavour astronauts have to get the arrays installed and properly deployed. And it will not be easy.

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