Long article about Tin Whiskers
Ramon
April 27, 2006
NASA will swap box on shuttle
Agency can't change all 4; remote risk to station, orbiter
BY TODD HALVORSON
FLORIDA TODAY
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.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke
Sent: Friday, April 28, 2006 2:13 AM
To: [log in to unmask]
Subject: [TN] NTC Thank you for all of your support for Pushback
Just a note to say happy Friday to everyone on these 3 industry links.
And of course a big Thank you - I truly appreciate the overwhelming
support emails that I have received since I put up the Pushback web site
on Tuesday night.
There have been a number of additions to the web site since then and
many companies and organizations have linked the site to theirs.
Many people requested links and a logo so, this evening, at the
prompting of Randy Morrison of PENTAGON-EMS I have generated the lead
free "chaos butterfly" artwork. I hope that you like the logo... it was
designed to give you all a smile:
http://www.rohsusa.com/
If you do visit the site don't forget to visit the new T shirt section.
Have a great Friday all.
John Burke
Santa Clara CA
-----Original Message-----
From: John Burke
Sent: Wednesday, April 26, 2006 9:07 AM
To: 'TechNet E-Mail Forum'; 'SMART Group smart-e-link'; 'Leadfree'
Cc: John Burke
Subject: NTC Thank you for all of your support
Good morning electronics industry!
For all of those people out there who cannot understand WHY we are
removing the lead in solders I have put up a web site as both a
discussion as well as an informational forum.
It makes some points and provides a feedback vehicle for those among us
who feel that it was not necessary and actually may be environmentally
detrimental.
And whether you agree with it or not please take the time to read
through it - the feedback is valued. If you like it, pass this email
along to your Colleagues.
Here is the link:
http://www.rohsusa.com
Kind regards,
John
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
