How Things Work
Space Station Truss
- By Adam Pitluk
- Air & Space magazine, March 2007
(Page 2 of 3)
“We were able to pack the truss segments right up to the limit on the space shuttle,” says McCann. “We used every ounce of ascent capability.”
Traveling to the station, truss payloads are cocooned tightly inside the shuttle cargo bay, solar arrays folded. Once in orbit, the arrays open like 200-foot wings. When the ISS is completed, the truss is expected to generate more than 80 kilowatts of power.
The power systems also come with external cooling systems necessary to dissipate the excess heat the power systems generate. The waste heat comes from the switching units and transformers that regulate power to make sure each station system has enough juice.
The switching units, for example, can run for only an hour before reaching their 113.9-degree-Fahrenheit limit, while the transformers can endure 143 de-grees before needing to shed heat.
While the space shuttle uses freon to keep its electrical systems and avionics cool, the ISS truss uses liquid ammonia cooled to 37 degrees and pumped through pipes and loops. Ammonia was chosen because of its stability and low freezing point.
The cooling system needs to be in place before the new pair of solar arrays, delivered in December 2006, can be brought to life. “Not just the power system, but the ‘plumbing’ also needs to work for this to happen,” says Joy Bryant, program manager for Boeing’s space station program.
Furthermore, any work on the electrical systems must be done without interfering with the crew’s experiments, construction schedule, or safety. “It’s kind of like leaving power on in the house and rewiring the east wing,” Bryant says.
Last September’s shuttle delivery contained a literally pivotal piece of hardware, one that is necessary to maintain power for the final configuration. Astronauts installed a 2,500-pound joint, built by Lockheed Martin, that will enable the solar arrays on Port Sections 3 and 4 to turn 360 degrees in order to stay aimed at the sun.
“Since the station orbits the Earth and maintains the same orientation relative to the sun, the arrays have to rotate so they can track the sun,” says Lockheed spokesman Buddy Nelson.
“Imagine that the station is one of the seats on a Ferris wheel. As the wheel goes around, the seat retains the same orientation,” he explains. “The station essentially goes around the Earth once every 90 minutes, and the motors on the rotary joints turn the arrays at the same rate.” Nelson says the piece of equipment is unique among the many others on the station.
“The section joint is one of the largest ever made, and this is the first time one has had to operate in the harsh environment of space,” he says. “Almost everything on the ISS is distinctive.”