Spaceflight Now: STS-92 Mission Report

Astronauts set the stage for more power aboard station
BY WILLIAM HARWOOD
SPACEFLIGHT NOW

Posted: October 17, 2000

  Duo
Spacewalkers Leroy Chiao and Bill McArthur work on the international space station today. Photo: NASA TV/Spaceflight Now
 
Two spacewalkers installed a pair of transformer-like devices aboard the international space station today that will step down and regulate the voltage produced by a huge set of solar arrays scheduled for installation in December.

The DC-to-DC current converters - called DDCUs in NASA-speak - were attached to the side of the newly installed Z1 truss by astronauts Leroy Chiao and William McArthur during the early stages of a six-hour 48-minute spacewalk.

It was the third station assembly and outfitting spacewalk in as many days for the Discovery astronauts and the second for Chiao and McArthur. A fourth and final spacewalk by crewmates Jeff Wisoff and Michael Lopez-Alegria is scheduled to begin around 10:50 a.m. Wednesday.

Along with installing the 129-pound DDCUs, Chiao and McArthur also completed electrical connections between the space station, the truss and a new shuttle docking port that was mounted on the station Monday and attached a second toolbox on the truss for use by future assembly crews.

Their final task was to remove a keel pin from the Z1 truss that was used to help secure it in Discovery's cargo bay for launch. The pin is no longer needed and had to be repositioned to avoid interferring with installation of the P6 solar arrays in December.

The solar arrays will be bolted to the top of the Z1 truss to supply power to the station's U.S. modules and to supplement electricity generated by solar arrays on the Russian Zarya and Zvezda modules.

The panels making up the P6 array will generate 167.9 volts initially and around 144.4 volts after 15 years of service. The two DDCUs installed by Chiao and McArthur today will step down that voltage to the steady 124 volts DC needed by station systems.

"It's a little bit analogous to the fact that power from the electrical company comes on these large high-voltage power lines," McArthur said. "And so you have to reduce the voltage. And that's sort of what we're doing.

  Nose
The nose of Discovery with the space station's Unity module to the right as seen by a camera on the shuttle's robot arm today. Photo: NASA TV/Spaceflight Now
 
"We're also trying to keep it regulated to a very steady level so that we don't see power fluctuations in our equipment inside the space station"

The DDCUs will not be activated until after the arrays are installed, but flight controllers carried out tests today that confirmed they are healthy and ready for use.

"We had a good checkout of the DDCUs," radioed Lisa Nowak from space station mission control. "Great job getting those hooked up and we're complete with our ground steps."

From a power perspective, the P6 arrays initially will produce 30.8 kilowatts. As the solar cells age, the output will drop to around 24 kilowatts after 15 years.

With the DDCUs in place, Chiao and McArthur re-routed electrical cables from Discovery's docking port, known as pressurized mating adapter No. 2, to the port installed by the crew on Monday. That port, known as PMA-3, will be used by Endeavour in December.

PMA-2 will not be used for a shuttle docking until after the U.S. laboratory is installed in late January. During that flight, PMA-2 will be removed from the Unity module's forward hatch and temporarily mounted on one of its two unoccupied side hatches.

After Destiny is attached to Unity's forward hatch, arm operator Marsha Ivins will reposition PMA-2 on the forward end of the laboratory module for future shuttle dockings.

Chiao and McArthur began the Discovery crew's third spacewalk at 10:30 a.m. when they switched their spacesuits to internal battery power. The spacewalk officially ended when they began repressurizing the shuttle's airlock at 5:18 p.m.

  Arm
Astronaut Leroy Chiao rides the robot arm. Photo: NASA TV/Spaceflight Now
 
This was the 92nd spacewalk by U.S. astronauts and the 53rd of the shuttle program. Total shuttle spacewalk time is now 331 hours and 14 minutes. Of that total, 62 hours and 38 minutes have been devoted to space station assembly in five construction missions.

The final major item on the crew's agenda today was to carry out the second of three reboost maneuvers in which the shuttle's jets are fired, or pulsed, to raise the station's altitude. The goal today was to gain an additional two miles miles or so.

Today's spacewalk was remarkably trouble free and the astronauts were able to stay ahead of their timeline most of the day.

During work to break out tools and set up equipment in the early moments of the excursion, McArthur noticed a cap that normally fits over the shuttle Discovery's airlock depressurization valve floating away into space.

"OK, I see the airlock cap floating away," he called. "Do you see it? The depress valve cap. ... It just bounced against Z1. I'll just kind of watch it for a moment... I don't think it's going to come back in this direction, but I'll just watch it."

Flight controllers asked the astronauts to make sure the valve cap could not make its way into the structure of the newly attached Z1 truss. McArthur said there was no problem.

"It was a nice billiard shot," he said. "It caromed off the corner of Z1 and then off the arm and now it's become the latest addition to the tiny bodies orbiting the Earth."

The small cap apparently broke free of its tether attachment. It is redundant and its loss will have no impact on Discovery's mission.

  PMA-1
Chiao works near the top of the U.S. Unity node and right below the Russian Zarya module. Photo: NASA TV/Spaceflight Now
 
Chiao and McArthur then pressed ahead with the DDCU installation work, pausing frequently to enjoy the view from 240 miles up.

"What part of the world are we going to be flying over in daylight," McArthur asked as Discovery raced toward an orbital dawn over the south Pacific Ocean.

"It looks like sunrise is right about over New Zealand," someone on Discovery's flight deck replied.

"Whoo!"

"Bill, I'm going to try to get some shots of you," Chiao said, looking down into the shuttle's cargo bay from his perch on the end of Discovery's robot arm. "I think I'll be able to get some good angles coming down."

"Yep. Shots of me just hanging around," McArthur joked.

"If I can get my camera out of this bayonet fitting..."

"Oh man, I apologize Leroy, I wish I hadn't run out of film."

"That's OK."

"Aw, it's spectacular! You just look so neat! Can you see the mother ship?"

"No."

"Oh good God, the orbiter's on its side! Everything's going to fall out!"

"Did you stow your camera Bill, when you went by the airlock?" Chiao asked.

"I did."

"I'll tell you what, If you want I'll hand you this camera because it's a bit of nuisance, after I come down there. I'll take some good shots of you."

A few moments later, McArthur once again commented on the sight of the station's new antenna boom extending out into space.

"That dish does look pretty up there, doesn't it?"

"Yeah, that's a neat sight," Chiao agreed. "I hope those pictures turned out over Gibralter."

"Absolutely," McArthur said. "Well, Leroy, how would you like to live on this thing for a few months? I think that would be pretty cool."

"Pretty cool."

"It would be neat, yes it would be," McArthur sighed. "Livin' in space..."

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