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Originally Posted by Larry Jacks
IIRC, lying flat on your back allows you to stand quite high g forces, once, but after that, less and less each time. Is that true, and if so, why?
I know of nothing that indicates G tolerance decreases after each exposure. Fighter pilots may fly for 20 years or more (with the occassional desk job for "career broadening") and they're exposed to high Gs everytime they do dogfight training (without the benefit of lying flat). There are aging factors to consider but not exposure factors.
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You're absolutely correct, Larry Jacks. In fact, frequent and repeated exposure only strengthens the muscles used in the M-1 maneuver (rapid and progressive tensing of muscles from the foot up through the abdomen to counteract the tendancy of one's blood under high G to move and pool downward.
It's usually after those "career broadening" tours of duty that their resistance to G forces has been diminished due to lack of practice.
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Likewise, the Mercury and Gemini astronauts were launched by converted ICBMs and pulled a lot of Gs. I've read that Gemini astronauts pulled over 7 Gs during launch and even more during reentry. Several of the early astronauts flew more than once.
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7 Gs lying down is nothing. Sitting in a chair it's nearly impossible to maintain consciousness for more than a minute, even if you're in terrific shape.
Immersed in water and breathing an oxygen-rich chloroflourocarbon compatible with human tissue (aka,
The Abyss) and with a density is the same as water, the human body can easily withstand 100 Gs for long periods (hours) of time.
The thing of it is, we don't have engines which can produce that much power to move both the cargo, spacecraft, astronauts, and fuel at 100 Gs. Furthermore, such a high-G is well beyond any optimal acceleration, as you'd have to beef the structure up so high the weight would cost 10x as much fuel.
Still, it might be applicable for Orion-type bomb-bursted accelerations, or long-term heavy acceleration that may yet be possible with a
nuclear salt-water rocket. And
here is the National Propulsion Laboratories article on this (admitedly at a university).