Parts are not just parts when they're part of the International Space Station

September 27, 2002

Each part of the International Space Station is a handcrafted masterpiece of aerospace engineering.

Some of the most important parts, however, will never fly in space. They are the "test articles" -- test parts that get pushed, pulled, shaken, blasted by loudspeakers and much more to ensure that the real hardware can stand up to the rigors of launch and years in space.

One of those test articles is the obscurely named S1-STA (Starboard 1 Structural Test Article), a twin to the S1 (starboard 1) Truss. The S1 Truss is scheduled for launch aboard Space Shuttle Atlantis on October 2 to be attached during three spacewalks to the growing international research complex.

Primarily built of aluminum, the S1 Truss segment is 45 feet long, 15 feet wide and 10 feet tall. Fully outfitted, it weighs over 27,000 pounds. It is one of nine similar truss segments that will serve as the Station's main backbone, measuring 356 feet from end to end when fully assembled. The Space Station's labs, living modules, solar arrays, heat radiators and other main components will be attached to the truss.

The S1-STA test article is a structural duplicate of the S1 Truss. NASA also built test articles for the Space Station pressurized modules, such as the U.S. Destiny lab and Unity connecting node module.

"It's not unusual for any manufacturer to build a test version of real hardware," said Lanny Upton, a test engineer at NASA's Marshall Space Flight Center in Huntsville, Ala., where both the S1 and S1-STA trusses were partly assembled and tested.

"But test articles are particularly important in developing space hardware, which is often one-of-a-kind equipment designed on a computer and is difficult or impossible to return to the factory if it fails," Upton said. "We can subject the test article to flight-like conditions so we can be 100 percent confident in the performance of the flight hardware when it reaches space."

The primary structure of the S1-STA truss test article was built by The Boeing Company in Huntington Beach, Calif., and flown to the Marshall Center, where the company added secondary structural components and outfitting items such as tubing, brackets, cables, said Alex Pest, a Boeing manager who oversaw assembly of both the test article and flight truss. It then underwent "modal" testing at Marshall - using machines that precisely strike the structure, like a piano key strikes a chord, to look for natural vibration frequencies that could be damaging. It was flown to Johnson Space Center in Houston to be blasted with sound from large speakers to simulate the Shuttle's enormous acoustic energy during launch and ascent. After that, it was returned to Huntington Beach to undergo pushing and pulling using hydraulic pistons to simulate the stresses the flight truss would have to endure during launch, landing, docking, temperature extremes and other aspects of its anticipated flight life.

The S1 flight truss primary structure also was built by Boeing in Huntington Beach and flown to Huntsville, where brackets, cable trays, fluid tubing and other secondary components and outfitting items were added. In Huntsville, it was screened for manufacturing flaws, including pressure- and leak-checking tubing and electrical checks for cabling, before being shipped to Kennedy Space Center, Fla., for final hardware installation and testing.

"It turns out both the flight article and the test article are almost equally important," Pest said. "We take the combination of all the worst case events and try to stress the test article to meet those worst case events. Obviously, we don't want to subject the actual S1 flight truss to those worst case tests."

The S1 Truss segment's particular job is to provide structural support for half of the Space Station's radiator panels, which cool its complex power system components. The panels use a liquid ammonia coolant to draw off heat from the Station's solar array wings - four photovoltaic modules that turn sunlight into electricity, which power the Station. The coolant is delivered through a series of tubes to the radiators, where the excess heat is expelled into space. But the S-1 is also much more than just a metal tower, which made the job of testing it more critical.

"The term truss is really a simple way of describing a very technically complex part of the Space Station," Upton said. "In fact, the truss is more like part of a building minus the walls. It may not be a 'room' like the Station's pressurized lab and other modules. But it has structural framework, cables for power, plumbing for fluids, utility trays, power outlets, brackets for fluid storage tanks, a communications antenna, video cameras for watching the outside of the 'building,' and even a track that spacewalking astronauts will use to travel along the truss."

The S-1 Truss also is divided in half by a Thermal Radiator Rotary Joint that allows the heat radiators to remain pointed away from the heat of the Sun, Upton noted.

Boeing employees and support contractors built more than 140 wiring harnesses and welded hundreds of feet of metal tubing to carry fluids for the radiator panels, said Pest, who oversaw the completion of the S-1 Truss. Workers designed and installed 600 pieces of multi-layer insulation to shield the S-1 truss from extreme temperatures. They installed a track for a cart that astronauts will use to move easily along the truss.

They built 34 umbilical mating adapters and 11 video cameras - including two of each for the S-1 segment -- that will be fitted along the full length of the Station framework by the time it's completed.

The adapters will serve as plug-ins for spacewalking astronauts' power tools, while the cameras will give astronauts inside the Station a full-length view of the Station's exterior. They also installed a new antenna on the S-1 truss that will markedly improve communications between the Station crew and ground controllers.
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For digital images or for more information about the International Space Station, visit the Marshall News Web site at http://www.msfc.nasa.gov/news

International Space Station S1 Truss Facts
· 45 feet (13.7 meters) long
· 15 tons (13.6 metric tons)
· One of 11 sections of the Integrated Truss Structure
· Attached to the S0 Truss, the first Station truss segment to be launched
· Supports the transmission of 92 Kilowatts of power
· Contains 15 miles of power wires, 426 feet of pipe, and 1,671 feet of fiber optic cable
· Heat radiators can reject the equivalent of the air conditioning systems from three-to-four 2,000-squre-foot homes.
· The area of three radiators equals the size of one tennis court.

NASA/Marshall Space Flight Center News Center

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