The "g-force" that would be felt in a turn is unrelated to gravity. That might seem counter-intuitive, but it is true. In a zero-g enviroment, anything having mass and speed would also be affected by "g-forces" if it attempted a turn. The harder the turn, the harder the forces.
In outer space gravity is used, like wings, to facilitate a turn. Gravity is what permits orbiting (which is a constant state of turning) without wings.
It is theorized that "mass" is a product of something called the "higgs field" which is believed to be everywhere. The higgs boson was recently in the news in connection with the LHC. It is the subject of intense research. Theoretically, to avoid "g-forces" one would need to interrupt or counter-act the effects of the "higgs field." In effect, become "mass-less." Again, theoretically speaking, if one understood and could control the "higgs field," one could also perform zero-g turns. Alternatively, something (or someone) made of material/energy that is "sub-higgs" would be "mass-less" as we understand mass. And would thus be unaffected by g-forces (and gravity for that matter), and capable of such abrupt turning.
Looked at from another angle, a moving body (w/ mass) would be affected by one of the laws of thermodynamics - a body in motion tends to stay in motion. The laws of thermodynamics explain effects. The "higgs field" theory is an attempt to understand what causes those effects.
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On another idea expressed (of a pilot being encased in water to avoid g-forces):
In aviation there is a similar idea that can often be found in books about flight theory. That is, 'What if birds on a plane started flying. Would the plane suddenly become lighter?' The answer is no.
For a pilot encased in water, the incompressability of water would be bad for the pilot unless he was protected somehow. He wouldn't be incompressable.
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On the idea of a diver breathing a fluid to allow for deeper diving:
I think there are already breathing devices used to do this. The idea is that the (oxygen carrying) fluid in the lungs creates a greater pressure to counter-act the great outside (of the body) pressure so as to prevent lung collapse. I believe there are functional limits though.
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On the matter of friction thru a fluid (such as air):
The Russians developed (and fielded) a torpedo a few years ago that could travel 200+ mph underwater. It does it by emitting air (or a gas of some kind) from the nose that encases the torpedo in a medium that produces less friction than water as it travels. I don't know if the West has copied that yet.
Russia is currently fielding some defense related items that reflect "outside of the box" thinking. The US, no-doubt, has toyed with the same things - often, long before the Russians, but the US (in recent years at least) has been spending to much in wars. Wars shift resources away from research and wear out already existing equipment. In the last 20 years or so, there has been a gradual shift in leading (military) technology away from the West in some key defense related areas. (I might add, China is also in on this game - often as the 'quick learner' among nations. They aren't too proud to copy (and improve upon) an existing idea.