Astronauts attaching an equipment-stuffed, 17.5-ton truss carrying solar panels and other gear onto the International Space Station orbiting about 220 miles above the earth, are staying on schedule with marathon spacewalks, but have struggled periodically with "captive" bolts that spring away, and one that was so stubborn it took two astronauts to break it free.

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In an interview Sept. 13, David McCann, Boeing�s manager of structural and mechanical systems at the space flight center in Houston, cautioned against "putting this out of proportion." He pointed out that the astronauts on the third spacewalk alone "torqued and turned over 300 fasteners" and were having great success in accomplishing their mission, but he admitted that the issues with fasteners would be looked at closely. "We will probably sensitize the crew a little bit more" about problems that could occur when working with them, he said.

McCann said Fairchild Fasteners (a unit of Alcoa) makes the troublesome bolts, called Fairchild Tridair bolts. They are used on 22 covers that the astronauts are removing to release restraining braces that protected sections of the folded-up structure from the violent vibrations of launch.

Each cover has four, captive, � in. bolts and a spring pressing up between the cover and the bottom of an oversized, 5/16 in. hex head. The spring pops the bolt up when the threads are free, but a circular retaining ring on the inside of the cover, with tabs protruding into a track on the bolt shaft, are designed to stop the bolts from backing entirely free. But on the first space walk, reportedly after astronaut Joseph Tanner set a cover aside to get at the restraining brace, the clip apparently came loose and the bolt, spring and a washer shot away.

Something similar apparently happened again on the second space walk on Wednesday, although the astronauts did not see it go. It was just no longer with the cover when he retrieved it from its tether to put it back in place.

And on the third spacewalk astronaut Steven MacLean found a bolt so seized in place that he broke a power-wrench extension trying to get it free. He switched to a different wrench, and with the help of his partner, astronaut Daniel Burbank, the two of them were able to muscle it free.

The restraints were locking a massive rotary joint at the base of a 240-ft-long solar panel array. The array was launched packed tight like a Chinese fan and successfully extended on Sept. 14. The rotary joint lets the array track the sun as the station orbits the earth every 90 minutes with its keel parallel to the surface. The joint also conducts the power generated by the solar cells to power the station�s system.

Engineers were concerned that loose bolts, washers or springs could interfere with its operation if they got into the works, but inspection showed no evidence of that. The frozen bolt could have posed another kind of problem, potentially preventing them from freeing the rotary joint so it could move, but they were confident it could have been worked around.

McCaan had a couple of ideas about what could have happened to cause the captive bolts to come free. The retaining rings are installed with a special tool that squeezes them in one direction, causing them to become elliptical so the interior tabs can pass over the threads. Of the first bolt that got away, he says "the fastener is close to where the astronaut has a tether fastened. The tether could have put a side load on it, and if the clip comes off the spring will eject it," he says. "We actually had some of the same fasteners come off when we were working on these panels at KFC [the Kennedy Space Flight Center]. They are sensitive to a side load."

He added that the Tridair bolts are not used in many places on the station.

He says the clips can also come loose if the bolt is pulled hard against them, but there was no evidence of that in either case. There was no tool on the first bolt when it cut loose, and on the second bolt, which simply went missing, the astronaut said the bolt was still on the cover when he set it aside.

"The bolt was there. He verified it to be there when he went to stow it, but when he brought it back up, the bolt was missing," McCann says.