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Old July 17th 04, 02:41 AM
Bruce Palmer
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James G. Joyce wrote:
So it sounds like both bolts hit near the top of the stack (possibly the
LES) first. Apparently, the only physical damage that reuslted from that
60,000-plus-ampere jolt flowing through the skin of the stack was to some
external instrumentation for measuring temperatures and R.C.S. reserves. It
was the induced electromagnetic fields that knocked the fuel cells off line
and tumbled the CM's guidance platform.

And, in a footnote ...

"Part of the reason the spacecraft was so affected by the lightning while
the Saturn was not involved the spacecraft's greater exposure-it was
positioned like the tip of a lightning rod-and part of it was luck, as
Arabian emphatically pointed out ... In the case of the I.U., induced
currents reached the guidance system's circuits but the computer software
kept the platform from tumbling."

I'm guessing that, in addition to being better protected, the differences in
construction and the built-in redundancy of the guidance computers in the
I.U. (thanks, rk, for enlightening me on that with your earlier post) were
why that platform didn't tumble ... thank goodness.


Those older gyros/resolvers/sliprings/torquers were very susceptible to
EMI and the platform wasn't redundant - there was only 1. Sounds like
the circuitry was sufficiently robust to protect the guts of the thing.
I wonder if the ST-124 housing had its own electrical shielding.
http://history.nasa.gov/SP-4206/ch8.htm#244

Sorry for the overlong post ... now resuming normal lurking mode ...


Not overly long at all, James, thanks for that.

--
bp
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