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Spacewalkers begin second EVA to perform power repair
Posted: September 2, 2012

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Floating in the Quest airlock module, astronauts Sunita Williams and Hoshide Akihiko switched their spacesuits to battery power at 7:06 a.m. EDT (GMT-4) to officially begin a planned six-and-a-half-hour spacewalk to make a second attempt to install a replacement power switching unit.

Credit: NASA
Without the device, electricity from two of the station's eight solar arrays cannot be utilized, a shortfall that, left uncorrected, would have a major impact on station operations.

Equipped with a set of MacGyver-like homemade tools, including a toothbrush attached to the end of a metal rod, the astronauts will attempt to clean and lubricate balky bolts that jammed during a spacewalk last Thursday, preventing them from attaching the replacement main bus switching unit.

For identification Williams, call sign EV-1, is wearing a spacesuit with red stripes while Hoshide, call sign EV-2, is wearing an unmarked suit. This will be the sixth spacewalk for Williams, the second for Hoshide and the fourth for station astronauts so far this year.

After last Thursday's marathon eight-hour 17-minute spacewalk, Williams moved up to 24th on the list of most experienced spacewalkers with 37 hours and 34 minutes of EVA time. Two hours and 12 minutes into today's excursion, she will move past Peggy Whitson as the world's most experienced female spacewalker.

The 220-pound main bus switching unit, or MBSU, is one of four used to route 160-volt power from the space station's eight huge solar array wings to transformers that distribute 120-volt power to the lab's myriad systems. MBSU No. 1 suffered an internal failure of some sort several months ago and while it still delivered power, it could no longer be commanded.

As a result, Williams and Hoshide removed MBSU No. 1 during a spacewalk last Thursday, but they were unable to bolt a replacement into its enclosure on the forward face of the station's main solar array truss. One of the two bolts in question, used to pull the box down onto cable connectors and interlocking cooling fins, jammed for some reason well before the box could be tightened down.

Williams reported seeing metal shavings in the bolt housing when the faulty MBSU was removed. She attempted to blow the debris out with compressed nitrogen, but the astronauts still were unable to bolt the replacement MBSU in place.

Rather than risk stripping the threads or shearing the bolt off with the power tool used to drive it, the astronauts were told to leave the box in place, partially installed and tethered to the truss, while engineers at the Johnson Space Center in Houston debated various repair options. The unsuccessful spacewalk ran eight hours and 17 minutes, the third longest in NASA history.

Working through the Labor Day weekend, engineers radioed up instructions for homemade tools that Williams and Hoshide will use during their second spacewalk to more thoroughly clean and lubricate the bolt hole and threads.

A spare bolt will be driven into the housing to help clean its threads and a toothbrush attached to the end of a metal rod will be used to clean the threads on the MBSU bolt. The astronauts will take special care to align the box before driving the bolts to ensure cooling fins and connectors engage properly.

The timeline allows four hours for the replacement work. If the MBSU is not plugged back into the station's electrical grid by then, Williams and Hoshide will bring it back to the airlock for additional troubleshooting inside the space station. In that case, engineers would work on possible techniques for jury rigging electrical connections and cooling to get around the bolt problem.

With MBSU No. 1 out of action, the station cannot tap into the power produced by two of the lab's eight solar arrays. In an unrelated issue, a direct current switching unit, one of eight used to route array power downstream to the MBSUs, tripped off Saturday afternoon, taking a third array out of the power grid. The DCSU issue likely will be dealt with during yet another spacewalk.

The station's six-member crew is not in danger because of the electrical outage. But the loss of power, if left uncorrected, would quickly have a major impact on station operations, including robot arm work and research.