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rk wrote:
Kevin Willoughby wrote: Does any of that explain why the S-V guidance computer worked just fine when the Apollo computer had been knocked sideways by lighting on Apollo 12? I don't know, have no data, but I always thought about just that issue. The explanation I heard was that the Apollo computer being in a smaller volume and nearer the strike was exposed to higher EMF effects than the S-V computer. D. -- Touch-twice life. Eat. Drink. Laugh. |
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Derek Lyons wrote:
rk wrote: Kevin Willoughby wrote: Does any of that explain why the S-V guidance computer worked just fine when the Apollo computer had been knocked sideways by lighting on Apollo 12? I don't know, have no data, but I always thought about just that issue. The explanation I heard was that the Apollo computer being in a smaller volume and nearer the strike was exposed to higher EMF effects than the S-V computer. Were the Saturn V guidance computers in the IU with the stable reference platform? Apollo 12 was struck twice by lightning ISTR. Were the actual locations of the lightning strikes on the vehicle documented? -- bp Proud Member of the Human O-Ring Society Since 2003 |
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"Derek Lyons" wrote in message ... The explanation I heard was that the Apollo computer being in a smaller volume and nearer the strike was exposed to higher EMF effects than the S-V computer. At that point, what would have happened if the escape rocket had been ignited? Would the system have completed the sequence and cut loose the CM? Would the rocket have been able to pull off the CM? If not, could the flight have continued without it? |
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"Bruce Palmer" wrote in message
et... Derek Lyons wrote: rk wrote: Kevin Willoughby wrote: Does any of that explain why the S-V guidance computer worked just fine when the Apollo computer had been knocked sideways by lighting on Apollo 12? I don't know, have no data, but I always thought about just that issue. The explanation I heard was that the Apollo computer being in a smaller volume and nearer the strike was exposed to higher EMF effects than the S-V computer. Were the Saturn V guidance computers in the IU with the stable reference platform? Apollo 12 was struck twice by lightning ISTR. Were the actual locations of the lightning strikes on the vehicle documented? -- bp Proud Member of the Human O-Ring Society Since 2003 digs up trusty copy of Murray and Cox ... The way it's explained there, Apollo 12 created its own lightning ... (pp. 376-7) " ... What they had done, they realized later, was to launch a 363-foot lightning rod, with the equivalent of a copper wire in the form of a trail of ionized gases running all the way to the ground. Even though there was no lightning in the vicinity before launch, Apollo 12 could create its own. And that is exactly what it did, discharging the cloud into which it had entered .... "Actually, Don Arabian's anomalies team later determined that Yankee Clipper was hit twice by lightning, once 36.5 seconds after launch at an altitude of 6,000 feet, when it discharged the cloud it was flying through, and again 16 seconds later, when it triggered a cloud-to-cloud bolt." 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. Sorry for the overlong post ... now resuming normal lurking mode ... James (who still has much to learn) |
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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 Proud Member of the Human O-Ring Society Since 2003 |
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