CAPE CANAVERAL - Only oneof four electronics boxes that could fail on Discovery and destroy the dockedorbiter and the International Space Station will be replaced before the shuttleflies to the outpost in July, officials said Wednesday.
The other three won't be fixed untilafter the mission because NASA has no additional spares and the chance ofcatastrophe is extremely remote - somewhere between 1 in 10,000 and 1 in onemillion.
"We think that it issafe to fly as is, but we have the opportunity to change out one of the boxesand reduce the risk, so we're taking the opportunity to do that," saidKyle Herring, a spokesman for NASA's Johnson Space Center in Houston.
The potentialproblem lies within the shuttle orbiter's Reaction Control System, whichconsists of 44 jet thrusters in its nose and tail. The system is designed tosteer the orbiter in space and during the dive back through Earth's atmosphere.
Four electronics boxes,called Reaction Jet Drivers, route firing commands to the thrusters from theshuttle commander's stick, the ship's computers or Mission Control.
NASA safety studies showthat an inadvertent thruster firing could tear apart the station and a dockedshuttle, triggering rapid depressurization of both spacecraft and killing allaboard. The risk was pointed out in a 2005FLORIDA TODAY review of agency documents obtained through theFreedom of Information Act. The records showed a push by internal safety panelsto make changes to reduce or eliminate the risk.
To guard againstinadvertent firings, astronauts routinely power down the thruster system when ashuttle orbiter is docked at the station.
But NASA safety studiesshowed the thrusters can fire even when power to them is turned off. Shortcircuits in other shuttle systems could trigger an unintended firing ifassociated wiring is bundled with electrical lines leading to the Reaction JetDrivers. Other potential causes: transistor failures or short circuits withinthe boxes.
Engineers recentlydiscovered a phenomenon that could lead to short circuits within the boxes. Tincomponents within them are susceptible to developing very fine metallicextrusions called "tin whiskers."
These extrusions"could be conductive, and in extremes, they could short mechanisms,"Herring said.
Despite the finding, NASAmanagers this week concluded it would be safe to fly in July with Discovery'sfour existing Reaction Jet Drivers.
Herring said there is noevidence components within them actually have tin whiskers. Also, the boxeshave never been opened and subjected to the type of handling known to inducedevelopment of the metallic extrusions.
However, NASA does have asingle spare box outfitted with components made of metals not susceptible tothe phenomenon. So it will be installed in Discovery.
"The consensus was wehave a pristine box, so let's buy down the risk even further by changing outthe box," Herring said.
Tin components in the otherthree boxes will be replaced after the July flight. Time-consuming tests willbe required prior to Discovery's subsequent flight.
Electronics boxesassociated with two other shuttle systems also are susceptible to thephenomenon. But component replacement work is being put off until after theJuly flight. The boxes have backups that can be pressed into service in theevent of a failure.
NASA took steps to preventinadvertent thruster firings before Discovery launched last July. Acomputer software patch was designed to shut down inadvertent firings within1.3 seconds -- or before structural loads on a docked shuttle and the stationincrease enough to cause serious damage.
The patch will beinadequate once station assembly resumes. Thruster firings shorter than 1.3seconds will generate enough force to cause catastrophic damage as the stationgrows.
Additional steps have beentaken since last July.
Chief among them: Extrainspections of wiring in bundles containing electrical lines to Reaction JetDrivers. Special measures are being taken to protect the wiring from chafing, amove meant to prevent short circuits.
A permanent fix -redesigning the Reaction Jet Drivers - would take three years and cost $36million. NASA's shuttle fleet is scheduled for retirementin 2010.
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