Computer glitches impact station attitude control
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: June 12, 2007
A Russian navigation computer aboard the international space station hung up today, triggering a chain reaction of computer miscues in the midst of work to unfurl two huge new solar blankets.
The arrays were successfully deployed, but the computer glitches forced flight controllers to maintain the station's orientation using shuttle rocket thrusters instead of the lab's stabilizing gyroscopes. That, in turn, resulted in a less-than-desirable attitude and a loss of solar power from a left-side array, which could not properly track the sun. Work to reboot the Russian computers also apparently triggered a false fire alarm that briefly prompted concern on the ground.
Late today, flight controllers at the Johnson Space Center in Houston came up with an untried technique for switching orientation control back to the station's control moment gyroscopes without going through the Russian command-and-control computers. The gyroscopes can safely control the station's orientation to maximize power generation without the stress, strains and exhaust plume contamination that goes with using rocket thrusters.
But moments after switching back to the gyros, an alarm sounded and flight controllers asked shuttle commander Rick Sturckow to re-enable Atlantis' digital autopilot once again while engineers worked on an alternative approach.
"Our attempt to take attitude control obviously didn't work," astronaut Megan McArthur radioed from Houston. "We'll be working on a secondary plan and will get you words as soon as we have a plan."
Joel Montalbano, a mission operations manager at the Johnson Space Center, said Atlantis had plenty of rocket fuel and, in a worst-case scenario, could maintain the orientation of the combined vehicles for a full week if necessary.
But engineers were confident it wouldn't come to that and by 9 p.m., they had successfully re-established gyroscope control and the left-side array was tracking the sun as required.
Computer glitches aside, program managers were thrilled with the successful deployment of the new S3/S4 solar arrays on the right side of the space station's main power truss.
"As a program guy, I'd be remiss if I didn't tell you what these solar arrays mean to us," said Mike Suffredini, NASA's space station program manager at the Johnson Space Center in Houston. "This set of solar arrays will provide about 20 kilowatts of power to the international space station. That's useable power. It provides quite a bit more than that, but some of it is siphoned off to take care of its own systems.
"That equates to something like eight to 10 households," he said. "So it's a significant amount of power we bring to the space station now. In addition to that, it's going to allow us to take our next steps, which begin early tomorrow morning and that'll be the retraction of the 2B array."
The station's power system includes four sets of solar arrays, two on each end of a truss that eventually will stretch more than 300 feet from tip to tip. A set of arrays known as P4 was added to the left side of the truss last September and the Atlantis astronauts added the S4 arrays that were unfurled on the starboard side of the truss today.
A third array - P6 - was launched in 2000 to provide power during the early stages of construction. NASA plans to move P6 to the left end of the power truss later this year. One of its two solar wings - P6-4B - was retracted during a shuttle flight last December. The other wing - P6-2B - is scheduled for retraction this week.
But the December shuttle crew ran into major problems retracting P6-4B and ultimately had to stage an unplanned spacewalk to coax the blankets back into their canister.
This time around, the astronauts plan a slower, more deliberate approach, giving the array more time to settle out between retraction attempts in a bid to ensure the blankets fold smoothly. If they don't, astronauts Patrick Forrester and Steve Swanson will provide manual assistance at the start of a planned six-and-a-half-hour spacewalk. The primary goal of the excursion is to complete the activation of a rotary joint designed to keep the new S4 arrays face-on to the sun.
Back at the Johnson Space Center, engineers are testing different techniques for securing a pulled-up insulation blanket on Atlantis' left-side Orbital Maneuvering System rocket pod that was spotted after launch Friday. The shuttle crew's mission has been extended two days and a fourth spacewalk added to allow time for repairs.
John Shannon, chairman of NASA's Mission Management Team, said today he has not yet decided which repair technique to use or even when to use it, holding open the options of attempting the blanket repair during the third spacewalk Friday or deferring it to a fourth and final excursion Sunday.
One technique calls for a spacewalking astronaut on the end of the shuttle's 50-foot-long robot arm to push the blanket corner down and sew it to an adjacent blanket using steel thread. Another technique calls for the astronaut to use a surgical stapler to secure the blanket. Another option is to simply push the blanket down and leave it at that.
Shannon said tests will be conducted overnight in a wind tunnel in California and furnaces at the Johnson Space Center to see which repair technique holds up best in the re-entry environment.
Otherwise, Shannon said, the shuttle's heat shield appears to be in good condition. Engineers are still assessing post-launch "spikes" in data from wing leading edge impact sensors on the shuttle's left and right wings.
"Last night, there was a one-G indication on our wing leading edge impact detection system," he said. "Last night it was the sensor between the right (wing) panels 11 and 12. The day before we got a one- half-G reading on the sensor between left panel 7 and 8.
"On previous flights, we've had several indications exactly the same. We don't exactly understand what is going on. We think we're getting some thermal settling, or thermal deformation of the support structure to the wing leading edge. People don't feel like it was an impact because no other sensors around that area pick it up. There's no ringing of the structure at all, just kind of a little click."
After Atlantis undocks from the space station next week, the astronauts will conduct a now-standard "late" inspection of the shuttle's heat shield "and if there were anything in those areas we would pick it up," Shannon said.