CAPE CANAVERAL, Fla. - After years of preparation, engineers will shut the doors - literally - on a pair of massive International Space Station trusses and solar arrays when they close out the space shuttle Atlantis' payload bay today.
The next time those doors open, Atlantis and its 17.5-ton addition to the ISS should be in orbit following a planned afternoon launch set for Aug. 27. Weighing in at 34,977 pounds (15,865 kilograms), the bus-sized truss segments and arrays are the largest shuttle payload aimed at the ISS to date.
"It's like saying goodbye to your baby, you know we've been dealing with this for so long," Chuck Hardison, site manager for Boeing's ISS Florida operations, said Wednesday. "But we're ready to see it leave the nest."
Atlantis' STS-115 mission is NASA's first major ISS construction flight since late 2002, due to delays caused by the 2003 Columbia accident.
At the heart of the spaceflight is the $371.8 million Port 3/Port 4 (P3/P4) segment, which is actually a 45.3-foot (13-meter) pair of already connected pieces that will be attached to the port - or left - side of the station's main truss. The hefty addition will be mated to the end of the Port 1 (P1) truss since the station has no Port 2 (P2) segment, which was cut from its design in the 1990s, and readied for activation during three spacewalks.
The 16-foot (4.8-meter) wide hexagonal P3 truss is composed largely of aluminum braces and bulkheads, and contains a series of brackets, fittings and attachment points for spacewalk equipment, experiments or spare equipment. It, like P4, measures about 15 feet (4.5 meters) in height.
The P4 truss is the power plant capable of pumping out 23 kilowatts - enough for six average homes - of usable power when deployed. Two cylinders at the end contain the pop-up solar array masts, each of which is sandwiched between two boxes that hold 120-foot (36.5-meter) mylar blankets that unfurl like an accordion to reveal a paper-thin layer of solar cells on one side, Boeing engineers said, adding that P4 also has a radiator to keep everything cool.
Connecting the P3 and P4 truss is the Solar Alpha Rotary Joint (SARJ), a motor driven set of 10-foot (3-meter) wide wheels that allows the P4 truss and other outer segments to rotate freely from the ISS and track the Sun for optimum power generation.
When unfurled and activated, the new arrays will have a wingspan of about 240 feet (73 meters) to expose about 64,000 solar cells to the Sun and double the power output of the ISS.
It's been a long road to the launch pad for the P3/P4 truss.
Assembly of the segments began in 1997, with Boeing designing the P3. P4 was designed by what is now Pratt and Whitney Rocketdyne Power and Propulsion, with the segment's vast solar arrays built by Lockheed Martin. By 2000, both segments had arrived at NASA's Kennedy Space Center here in Florida for integration and launch preparations.
The path has been so long, in fact, that in 2005 engineers replaced the truss' 12 batteries - which were eight years old at the time - with new ones that can last through 2016, NASA has said.
The old batteries will be stripped for parts to be used on future ISS equipment, Hardison said.
When the space station's first U.S. solar array deployed during NASA's STS-97 mission in 2000, spacewalking astronauts and flight controllers were in for a surprise. After years packed away in their boxes, the solar blankets on the first array stuck together as they deployed in a low-tension mode prompting a tension line to slip off its spool and requiring a spacewalk repair.
"I think Joe was the first person to see the array as it was starting to stick," STS-115 commander Brent Jett - leader the STS-97 mission - said of spacewalker Joseph Tanner, who is also on Atlantis' next flight. "I think his comment was, 'That doesn't look good.'"
Flight controllers decided to deploy the second STS-97 solar wing in a high-tension mode and warm the array in the Sun to ease any stiction issues.
"The stiction, we found, depended on temperature so now we're allowing the solar arrays to heat up," said NASA's launch package manager Hubert Brasseaux.
That same approach will be used during the STS-115 solar panel deployment, he added.
Late Wednesday, pad workers were expected to open Atlantis' cargo bay doors in order to perform a few final tasks on the solar array batteries. As the work concludes today, engineers are hoping it will be their last glimpse of the payload.
"We hate to see it go, but we don't want to see it again," Hardison said.
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