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Comment by avian | Hacker News Reader

> Initially, NASA underestimated the electromechanical interference (EMI) projected by the light and the Apollo computers crashed when the beams were aimed at the rockets on the launch pads.

Not wanting to ruin a good story, but I find this unlikely. If it were true I would think the military would be more interested in using them as weapons for electronic warfare than search lights. More likely the lights were interfering with radio links like telemetry or voice. Or maybe they were just not conforming to some EMI guidelines NASA was using at the time and were modified as a safety precaution.

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There's a little more detail about the Apollo situation on a post [1] by Neal Strickberger on Candlepower Forum:

"Chicken wire! Turns out this was added for the Apollo program, when they used the TVS-3s to light the pad. A 20kW short-arc fed with 400Hz 3phase fullwave rectified unfiltered power produces lots of RFI at 1200Hz harmonics, and when nice and focused with a reflector it was enough to shut down the capsule. So they made a Faraday cage."

[1] https://www.candlepowerforums.com/vb/showthread.php?348441-A...

Optical people are rolling their eyes at the size of reflector required for 1200 Hz.

BEST hope possible to make the story true would be clear glass semiconductors can detect incoming light so MAYBE germanium glass tube diodes hanging out in the air could get confused, although why a moon rocket would have bare unprotected glass semiconductors on the outside is a mystery.

_harmonics_ means more than just the first odd harmonic.

With 20 kilowatts flowing it's very believable that the comb reaches quite a ways up the spectrum. And the radiator doesn't have to be tremendously efficient or focused with that kind of input power to produce significant effects in unintentional receivers. Also remember that wiring harnesses in spacecraft are probably very long, so will be good receivers at many frequencies reaching quite low.

In that era, wrapped in a tube of grounded aluminum, making an excellent faraday cage.

It's not hard to crash computers with stuff like this. I remember in college someone built a tesla coil in one of the lab rooms that would crash people's phones.

Kind of a fun story that's related. A friend of mine ran a machine shop in the early 2000s that had the world's largest plasma cutter. Apparently one afternoon the police and FAA showed up to investigate their property. The plasma cutter was generating enough interference to cause computer problems at IAH airport and on landing planes (their shop was in the flight path of the aiport.) Allegedly it had taken the FAA months to figure out where it was coming from due to the fact the plasma cutter was typically only running for a few minutes at a time.

It makes sense. Every arc is an impulse and that's wideband noise. (The FFT of a spark is basically a horizontal line.) The earliest radios were spark gap transmitters.

For an impulse, the FT is a straight line for anyone interested .

But sparks and arcs aren't impulses. They're much more interesting than that spectrally, including with significant peaks and harmonic components. These properties are things that protection equipment can use to detect and localize faults.

Sure, the PC said fast Fourier transform rather than Fourier transform so I pointed out the "underlying" identity

Agreed, of course. A spark is an approximation of an impulse. I should have said that.

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