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.”
A 200-foot network of rail lines integrated into the truss’ design serves as transportation for astronauts and equipment. A mobile transporter is attached to the rails, much the same way as a roller coaster connects to its tracks, and is powered by an attached cable that unspools or reels up as the mobile transporter moves along the tracks.
The tracks lead to eight designated worksites. One of the station’s robotic arms, mounted on top of the transporter, can pluck equipment or space-walking astronauts during station repairs or installation.
Also built into the truss are a slew of sensors, antennas, and ports. Most of these external boxes are part of the electrical, communications, cooling, or navigation systems. But there are also cameras and internal structural sensors to monitor the health of the station, as well as ports where temporary cameras can be mounted to oversee spacewalks. Some spare parts are also stored on truss
Engineers have placed mobile footholds around truss worksites that allow the crew to move them to different locations during spacewalks.