CAPE CANAVERAL - Only one of four electronics boxes that could fail on Discovery and destroy the docked orbiter and the International Space Station will be replaced before the shuttle flies to the outpost in July, officials said Wednesday.
The other three won't be fixed until after the mission because NASA has no additional spares and the chance of catastrophe is extremely remote - somewhere between 1 in 10,000 and 1 in one million.
"We think that it is safe to fly as is, but we have the opportunity to change out one of the boxes and reduce the risk, so we're taking the opportunity to do that," said Kyle Herring, a spokesman for NASA's Johnson Space Center in Houston.
The potential problem lies within the shuttle orbiter's Reaction Control System, which consists of 44 jet thrusters in its nose and tail. The system is designed to steer 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 the shuttle commander's stick, the ship's computers or Mission Control.
NASA safety studies show that an inadvertent thruster firing could tear apart the station and a docked shuttle, triggering rapid depressurization of both spacecraft and killing all aboard. The risk was pointed out in a 2005 FLORIDA TODAY review of agency documents obtained through the Freedom of Information Act. The records showed a push by internal safety panels to make changes to reduce or eliminate the risk.
To guard against inadvertent firings, astronauts routinely power down the thruster system when a shuttle orbiter is docked at the station.
But NASA safety studies showed the thrusters can fire even when power to them is turned off. Short circuits in other shuttle systems could trigger an unintended firing if associated wiring is bundled with electrical lines leading to the Reaction Jet Drivers. Other potential causes: transistor failures or short circuits within the boxes.
Engineers recently discovered a phenomenon that could lead to short circuits within the boxes. Tin components within them are susceptible to developing very fine metallic extrusions called "tin whiskers."
These extrusions "could be conductive, and in extremes, they could short mechanisms," Herring said.
Despite the finding, NASA managers this week concluded it would be safe to fly in July with Discovery's four existing Reaction Jet Drivers.
Herring said there is no evidence components within them actually have tin whiskers. Also, the boxes have never been opened and subjected to the type of handling known to induce development of the metallic extrusions.
However, NASA does have a single spare box outfitted with components made of metals not susceptible to the phenomenon. So it will be installed in Discovery.
"The consensus was we have a pristine box, so let's buy down the risk even further by changing out the box," Herring said.
Tin components in the other three boxes will be replaced after the July flight. Time-consuming tests will be required prior to Discovery's subsequent flight.
Electronics boxes associated with two other shuttle systems also are susceptible to the phenomenon. But component replacement work is being put off until after the July flight. The boxes have backups that can be pressed into service in the event of a failure.
NASA took steps to prevent inadvertent thruster firings before Discovery launched last July. A computer software patch was designed to shut down inadvertent firings within 1.3 seconds -- or before structural loads on a docked shuttle and the station increase enough to cause serious damage.
The patch will be inadequate once station assembly resumes. Thruster firings shorter than 1.3 seconds will generate enough force to cause catastrophic damage as the station grows.
Additional steps have been taken since last July.
Chief among them: Extra inspections of wiring in bundles containing electrical lines to Reaction Jet Drivers. Special measures are being taken to protect the wiring from chafing, a move meant to prevent short circuits.
A permanent fix - redesigning the Reaction Jet Drivers - would take three years and cost $36 million. NASA's shuttle fleet is scheduled for retirement in 2010.
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