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.
Long
history
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.
Conquering
"stiction"
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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