How Things Work

If the International Space Station were a city, then the truss that connects its modules would be, among other things, its power plant, public transportation, and closed-circuit surveillance systems.

More than simply the frame for the other ISS components, the truss is a complex system on its own—a novel feat of engineering that is the backbone of the largest man-made structure ever put in orbit.

The main purpose of the truss is to hold solar arrays, their batteries, and necessary support systems, but it is also studded with sensors, antennas, ports for experiments, footholds for spacewalking astronauts, and a rail system for moving man and machine.

When completed in 2010, the ISS will be four times the size of the Russian Mir, with the living space of a five-bedroom house. The 310-foot truss and attached acre of solar panels support this internal space and its tenants.

The truss, made primarily of aluminum, is being bolted into place like a massive, zero-gravity Erector set. Spacewalking astronauts have been putting pre-fabricated pieces together by hand, with help from the largest of the station’s remotely controlled robotic arms.

Since all the hardware is installed on Earth, as opposed to being assembled in orbit, fewer time-consuming spacewalks are needed.

The station is required to support a six-person crew and supply power for the scientific experiments, says David McCann, a Boeing structures engineer who works on the program at NASA’s Johnson Space Center in Houston. (The company is in charge of the design and construction of all the U.S.-made parts of the station, including truss segments and solar arrays.) The mandate for a six-person habitat and numerous experimental labs determined the station’s size and power requirements.

Engineers designed the truss as 11 easy-to-assemble pieces (eight are shown here) to be hauled by the space shuttle, the only heavy lifter available at the time.

“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.”

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
platforms.

Engineers have placed mobile footholds around truss worksites that allow the crew to move them to different locations during spacewalks.

“One of the beauties of the [truss] is that we could lay in a lot of flexibility,” McCann says.
The world’s orbital outpost continues to grow, launch by launch. The European Union provides labs and logistics-support vehicles, Russian craft can ferry astronauts back and forth, and a piece of Canadian robotics places the newly arrived parts together.

It may take a village to raise a child, but it takes an entire planet to raise a space station.