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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