Below is an example of of managements limited understanding of  
electronic reliability.

David A. Douthit
Manager
LoCan LLC

NASA will swap box on shuttle

Agency can't change all 4; remote risk to station, orbiter

BY TODD HALVORSON
FLORIDA TODAY



Enlarge this image


Thruster worries. 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. NASA image


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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 1 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.

Contact Halvorson at 639-0576 or [log in to unmask]
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