Space station reboost, circuit breaker glitches assessed
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: November 30, 2006
Space station engineers believe a Russian rocket firing to boost the lab's altitude, needed to permit the shuttle Discovery to dock on flight day three of its upcoming mission, was aborted Wednesday because of the station's unbalanced mass.
The aborted rocket firing Wednesday would have boosted the station's orbit enough to permit FD-3 dockings every day through the Dec. 17 end-of-year launch target. But the planned 18-minute 22-second firing was terminated after just three minutes and 16 seconds, apparently due to the unbalanced mass and because the station's orientation was not constant.
"That consistent orientation, or attitude, is required for the reboost to continue to avoid any tumbling or excess stresses on the mechanical systems of the international outpost," NASA spokesman Kelly Humphries said today from space station mission control at the Johnson Space Center.
The station is asymmetrical at present due to the addition of a new set of solar arrays on the left end of its main power truss during a shuttle flight in September. The reboost maneuver Wednesday, using engines in a Progress supply ship attached to the back end of the Zvezda command module, was the first since the new arrays were added.
"Attitude managers have been watching over the systems and looking for opportunities that they could find a good configuration of the thrusters that would allow for another reboost attempt," Humphries said. "No decision yet."
The shuttle Discovery is scheduled for launch Dec. 7 to re-wire the space station, allowing it take advantage of the new port-side solar arrays. But Discovery must dock on the third day of its mission to complete all the planned work.
While the Russians investigate reboost options, NASA engineers are troubleshooting a glitch Tuesday in which a circuit breaker popped open during tests of new solar array control software.
The new port-side arrays are designed to rotate like a big paddlewheel once operational, allowing them to stay face-on to the sun as the station circles the globe. During Discovery's mission, the solar alpha rotary joint, or SARJ, that physically turns the huge solar wings will be turned on for the first time.
But one of two circuit breakers that routes power to the SARJ motors tripped open Tuesday during a test of new software designed to adjust the alignment of the motors and the drive gear.
Engineers are conducting tests to determine whether the trip was caused by the software or by some sort of hardware problem. Based on preliminary testing, they suspect it was software related. But troubleshooting is not yet complete.
The issue is critical because without a fix, the system would have no redundancy. A second remote power controller trip during the upcoming space station re-wiring work could trigger a blackout aboard the lab complex.
"The work ongoing now is to gather additional data what might have happened in the software associated with that solar array rotary joint," Humphries said. "Later today, they are going to pull down some data from the multiplexer-demultiplexer computer that transmits the data back and forth between that remote power controller and the truss structure."
A software-related data communications glitch involving the MDM could be responsible for the trip. But troubleshooting is not yet complete.