Naked Science Forum
Non Life Sciences => Technology => Topic started by: clueless on 31/08/2015 16:51:26
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When the Enola Gay dropped Little Boy on Hiroshima, how come the EMP didn't disable electronic devices on the aircraft? Was it because the Enola Gay was high in the air? Thanks.
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Vacuum tube electronics is much less suceptible than semiconductors to EMP damage, as long as you can prevent the pulse getting into the primary circuit of a radio receiver. Since aircraft have radar transmitters and are frequently subject to lightning strike, avionics receivers are already protected from input overload.
There was an interesting discovery on the 1970s that a Sukhoi fighter-bomber flown to the west by a defecting pilot, was full of "archaic" electronics which he descibed as "power-hungry but inherently nuclear-proof".
There was a lot of interest in the 1980s in making tanks and suchlike nuclear-proof by designing circuits that would switch to safe mode when irradiated by gamma photons and neutrons. The specification was that the machine should remain operable for 20 minutes after a nuclear shell detonation, as this was the likely lifetime of the crew. It was also discovered that a number of critical circuits in strategic missiles were degraded by exposure to the warheads' emissions in storage.
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The fireball from a nuclear explosion is a basically spherical ball of conductive plasma, expanding through the Earth's magnetic field. This will induce currents in the plasma, but this considerable electric field will almost cancel, outside the fireball.
This is why the first nuclear test outside the Earth's atmosphere (https://en.wikipedia.org/wiki/Nuclear_electromagnetic_pulse#Starfish_Prime)caused a surprising amount of EMP damage - there is a different mechanism at work. Gamma rays from the explosion strike electrons in the atmosphere almost simultaneously, across a wide area. This causes a planar electric wave of extremely high frequency to propagate downwards through the atmosphere. These high frequencies can be picked up by small lengths of wire, and can enter small gaps in metal shielding to destroy electronics within.
This test caused electrical supply outages in Hawaii, 1500km away.
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I recently had the opportunity to visit the Nagasaki Atomic Bomb Museum. They convey the horrors of nuclear war by means of personal accounts, photos and artefacts caught in the explosion.
The immediate effects from this low-altitude detonation were due to radiation from the fireball and shockwaves from the blast. The delayed effects of radiation appeared over subsequent days and weeks.
Due to the density of air at this low altitude, gamma rays from the explosion would have been stopped very close to the source. This means that the EMP would be severely attenuated.
It is estimated that (for the valve-based electronics of the time), the EMP would not damage a plane that was beyond the blast radius of the bomb. See: https://en.wikipedia.org/wiki/Nuclear_electromagnetic_pulse#On_aircraft