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Re: Design for failure?



On Thu, 27 Jun 2002, Assi Friedman wrote:

> Most spacecraft builders think about it, but never really design their
> system to this.  So, if we build to perform, why not build to fail? A
> dead pack could mean non-eclipse operation only, what's wrong with that?

Interestingly we designed for shorted cells in PCsat.  The battery charge
regulator was designed to give up putting any solar array current into a
battery that had dropped more than 1.2 volts below minimum discharge.
Thus isolating the battery from the rail once they failed.

BUT since in PACKET mode, each packet burst takes a peak current (for
one second) that is greater than the average ouput of the solar cells,
there MUST be some battery capacity to provide that  1 second burst of
peak energy.

Once PCsats batteries get too soft to provide even that 1 second burst
(like it did on Field Day) all you hear is the beginning half second of a
packet and then a decaying dribble and then the TNC resets on the
negative going voltage transient...  So we almost got it right...  WHat we
should have done is then also had a LOW POWER mode on the transmitters
that would not have drawn more current than was available form solar
panels alone...

AND we made all sides of our satellite have dual redundant
wired panels (except one) so that we could survive any single solar
panel failure....  But of course, the one panel that failed on launch was
the unique specially configured non redundant one-of-a-kind one on the
bottom...

Well, we almost got it right...

Bob

http://www.ew.usna.edu/~bruninga/pcsat.html


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