Russians believe cause of recent Soyuz problems found
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: September 25, 2008
Russian troubleshooters believe electrical arcing between the space environment and the international space station most likely caused recent problems with explosive bolts used to separate Soyuz re-entry vehicles just before atmospheric entry.
"Our Russian colleagues have taken a number of steps," said space station Program Manager Michael Suffredini. "For the ascent vehicle, they've actually changed the pyros to a configuration that's less sensitive to this. ... In addition to that, the vehicle that's due to return home (in October) has, as you know, through a pair of EVAs, a pyro was removed therefore freeing up the mechanism."
In the two most recent Soyuz re-entries, one of five pyrotechnic separation bolts failed to disengage properly, causing the central descent module carrying the crew to remain attached to another module longer than usual. In both cases, the modules eventually broke free, but the malfunctions caused steeper-than-usual "ballistic" re-entries, shaking up the crews and resulting in off-course touchdowns.
The Soyuz currently docked to the station, TMA-12, was launched last April, after the first unusual entry but before the second. During a spacewalk this summer, Expedition 17 commander Sergei Volkov and flight engineer Oleg Kononenko released the mechanism and removed the pyro in question so it can be returned to Earth for analysis.
The Russians plan to launch Soyuz TMA-13 on Oct. 12 to ferry two fresh station crew members - Expedition 18 commander Michael Fincke and flight engineer Yuri Lonchakov - to the lab complex, along with Richard Garriott, a U.S. space tourist. Volkov, Kononenko and Garriott then will return to Earth in the TMA-12 vehicle on Oct. 23.
Suffredini said the TMA-13 spacecraft has been modified to minimize the threat of arcing.
"Our Russian colleagues have spent an enormous amount of time, a very structured effort, to create a fault tree and then work off the fault tree to come to a most probable cause," Suffredini said. "They have concluded the most probable cause to be related to the plasma environment and the delta potential between the plasma environment, the environment around space station, and the space station itself.
"The way the space station is wired together, there actually is a voltage, a delta voltage potential, between the environment around the ISS and the ISS itself and what happens over time is occasionally that potential tries to equalize. And when it tries to equalize, you get what essentially is a spark to the ISS. This is an environment we've been living in since we starting flying ISS, it's why we have what we refer to as the plasma contactor units that we operate during EVAs to prevent that from occurring during EVAs."
The plasma contactor units use xenon gas to equalize electrical potential around the station. The xenon supply is limited and the PCUs are only operated during spacewalks to prevent any chance of arcing while astronauts are working outside. Testing showed no obvious threat to the station's systems when the PCUs were turned off.
"What our Russian colleagues have determined is because of the very specific configuration of the pyros ... on the Soyuz, where it's failed to separate each time, they've determined that there's been some arcing, if you will, or equalizing of voltage in this area," Suffredini said. "And this over time has caused the pyros to be ineffective."
The culprit apparently involves insulation on the Soyuz near the pyro in question that includes an ungrounded aluminized layer.
The huge U.S. solar arrays that provide most of the station's power affect the electrically charge plasma around the lab. The recent Soyuz problems occurred after a major change in the configuration of the arrays. Another major change is expected next February, when a fourth and final set of arrays is added to the starboard side of the station's main power truss.
NASA engineers are not convinced the evidence supporting the arcing theory is conclusive, but "it certainly implies that it's a plausible scenario," Suffredini said.