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I read somewhere that prolonged G forces (even 2 Gs) are not tolerated by human physiology and that this ultimately limits our ability to sustain space travel. Are there any tactics to reduce G force stress on the body?

enter image description here G-Force numbered https://www.newscientist.com/article/mg20627562-200-maxed-out-how-many-gs-can-you-pull/

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    $\begingroup$ The first part of that may be true (that sustained G forces kill you) although this would be a better question if you could give your source. On the other hand current rockets are only able to sustain that kind of acceleration for a few minutes, so it's not really a problem. The scope of possible space travel would massively increase if we could sustain 1G for hours or days (or even years) and only once that is achieved would there be much point in looking at the problems with sustaining 2Gs. $\endgroup$ – Steve Linton Apr 3 at 13:23
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    $\begingroup$ What Steve said. Human space travel is not limited by G force vulnerability, except during launch and landing. But once you are out of the atmosphere, fuel is so precious that we use the most gentle, efficient accelerations that will work, and even those accelerations are only momentary. $\endgroup$ – Wayne Conrad Apr 3 at 13:37
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    $\begingroup$ See related How fast will 1g get you there? $\endgroup$ – James Jenkins Apr 3 at 16:50
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    $\begingroup$ Roundtrip times at 1g, including subjective time for a relativistic traveller upload.wikimedia.org/wikipedia/commons/f/f5/Roundtriptimes.png $\endgroup$ – JollyJoker Apr 4 at 10:48
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    $\begingroup$ I'm guessing you got this notion from Phil Plait (a.k.a. The Bad Astronomer). Well in this case he earned his nickname. Phil was badly roasted on his own forum. Oddly enough I can't find Phil's mangled physics on YouTube. $\endgroup$ – HopDavid Apr 4 at 13:56
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The problem isn't so much that humans cannot sustain high G forces for any extended length of time: The problem is that rockets cannot. If a rocket could sustain 1 g acceleration for a bit over a day, we could go to Mars in a bit over a day. It instead takes several months to get to Mars because the rockets used to get there only fire for a few minutes. The spacecraft then coasts all the way to Mars. Just a few hundredths of a g of sustained acceleration would cut the trip time to Mars down to a week or so.

The chemical engines currently used to propel spacecraft on interplanetary trajectories coupled with the tyranny of the rocket equation are the key reasons rocket cannot sustain high accelerations for an extended length of time. There are some promising low thrust / high efficiency (high specific impulse) technologies such as ion thrusters that might help humans get beyond the Moon. Ion thrusters are in use now, but none are quite ready for prime time when it comes to human spaceflight. There are some promising high thrust / somewhat high specific impulse nuclear technologies that might be useful; these are mired in politics.

Other than science fiction, there is no known technology that could take humans beyond the solar system.

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    $\begingroup$ I disagree with your last sentence we have the tech to get humans beyond the solar system. Getting there and back in a single human life time would be a totally different question/answer. +1 for the rest of the answer though $\endgroup$ – James Jenkins Apr 3 at 16:47
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    $\begingroup$ @davek Your max speed is lightspeed, though as we near it the energy required to accelerate further steadily climbs - So your basic premise is sound but isn't relevant until we're working in very large fractions of C - or never an issue at all, with present technology. $\endgroup$ – Saiboogu Apr 3 at 18:36
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    $\begingroup$ @davek you stop accelerating in a plane because the drag from air resistance is equal and opposite to the thrust from the engines at some speed, since there's no air in space there's basically nothing to stop you accelerating more until you get close the speed of light and relativistic effects become significant $\endgroup$ – llama Apr 3 at 19:19
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    $\begingroup$ @jpmc26 - I was referring to ion thrusters. The problem is they're currently of such low thrust that the mass of humans and the life support systems needed to power them would require ridiculously large amounts of electrical power, which would entail even more mass. Ion thrusters are great for geosynchronous satellites and smallish probes to the asteroids. They're not quite there yet for human spaceflight. $\endgroup$ – David Hammen Apr 3 at 22:51
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    $\begingroup$ @davek The source must be making some assumption about the amount of reaction mass you are able or willing to start with. An ion engine is, in fact, a rocket like any other, just one with a very high exhaust velocity. Accelerating to to 90 km/s with current ion drives would involve about 90% of the starting mass of the spaceship being reaction mass, but if you could somehow manage to start with 99% reaction mass, you could achieve 180 km/s. $\endgroup$ – Steve Linton Apr 4 at 20:07
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Ignoring the major point that human tolerance of G forces is not the limiting factor on space travel, plenty of thought has been made on how to counteract G forces, not least by 60s sci-fi writers.

You can find more information than you ever wanted at Projectrho on this topic.

The general gist: for lowish accelerations like 2 G, you don't need to do anything special to the human body, just make sure you're lying either prone or on your back, and remaining disciplined about your breathing.

For higher Gs, like 5G+, you need to carefully manage the human body, putting it in a gel-like cocoon of similar density, and substituting air for a breathable liquid. Any differences in density can result in the denser parts of the body tending to 'settle' towards the back of the ship, and so must be avoided where possible.

Of course, such measures to counteract G forces can only ever be necessary with the use of nuclear or antimatter propellant. Chemical propellants do not burn for long enough to require such measures.

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    $\begingroup$ Best answer. This actually addresses the question, flawed as its premise is. $\endgroup$ – user45266 Apr 4 at 5:27
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    $\begingroup$ In fiction, balance with gravity from mass you carry along, like the classic 'sailboat carrying its own fan' -- scifi.sx or tvtropes (warning! warning!) at 'Inertial Dampening'. (And in another McAndrew/Roker story, Sheffield also has the solution to propelling this monster -- self-energy of interstellar vacuum. Sure.) $\endgroup$ – dave_thompson_085 Apr 4 at 6:18
  • $\begingroup$ Just install reactionless thrusters. Lots of SciFi spaceships have them. :-) $\endgroup$ – Carl Witthoft Apr 4 at 17:28
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    $\begingroup$ He was exposed to those G forces briefly. The question is about longer duration G-forces. 30G is definitely not survivable over the period of a day. $\endgroup$ – Ingolifs Apr 4 at 20:47
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    $\begingroup$ Going past the 60's...Most modern SciFi seems to admit G-dampening/G-compensators/G-Generators are A Thing in spaceflight, but don't go into any details about how they do it. $\endgroup$ – T.E.D. Apr 4 at 21:51
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This is way beyond foreseeable economic possibilities, but the physics is sound:

Gravity is a surefire, scalable, elegant way to counteract G forces from acceleration.

A planet-sized spaceship with its own gravitational pull of 5 Gs could accelerate at 4 Gs, people living towards its tail would only experience the difference, one G.

(note that I'm talking about a ship roughly 5 times the mass of Earth, minus density differences)

The same is true for a ship with 100 Gs accelerating at 99 Gs.

Edit: moving the people through tunnels in the ship towards the front of it would allow for keeping the one G experience as propulsion slowly shifted to breaking.

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    $\begingroup$ Of course, then you have the problem of high-G loads when you stop accelerating. And you probably want to decelerate once you arrive at your destination, which is even worse for our hapless passengers. $\endgroup$ – chepner Apr 4 at 20:35
  • $\begingroup$ @chepner Put them on the orbit of their planet-ship, then cut off the acceleration. They'll be in microgravity. $\endgroup$ – kubanczyk Apr 4 at 21:13
  • $\begingroup$ Why not just be in orbit the entire time? Then you don't need a larger planet, or have the acceleration tied to the gravitational pull of the planet. $\endgroup$ – chepner Apr 4 at 21:22
  • $\begingroup$ When you stop accelerating you need to move further away from the <strike>planet</strike> spaceship. Gravity strenght decreases the further you are away. Two pairs of limit quarters (one on the ground, one really high up) could solve this. And to decelerate your turn the thing around. Not the plant/ship, but you move to the opposite side of the planet and use another pair of engines. $\endgroup$ – Hennes Apr 5 at 11:47
  • $\begingroup$ Since gravity is only space-time curvature, maybe antimatter could help in warping up the space and create artificial g loads :| $\endgroup$ – Prakhar Apr 5 at 17:21
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G Force is a function of acceleration. Gravity works on a mass to pull it toward another mass. Large masses have higher levels of gravitational attraction. The force of gravity on Jupiter and Saturn is stronger thatn that on earth. The moon less than on earth.

On earth gravity is a force that continues to pulls us down toward the center of the earth. The physical surface stops that acceleration. Our weight is the measure of that force acting on our mass.

Acceleration is a change in speed. When coasting (no acceleration nor deceleration forces) then there is no g-load (weightlessness in space).

Accelerating in a car, plane or spaceship causes G-Loads. Again, it is the acceleration that is causing the load. Banking an airplane in a 60 degree bank will cause g-loads on the body due to centripetal force. Looping and airplane will do the same. An inside look causes positive g-load while and outside loop causes negative g-load. Both are measured by effect on the body. When upright, positive g-loads causing blood to flow out of the head toward the feet and negative g-loads causing blood to flow from feet to head. human bodies tolerate positive g-loads better than negative. Lying down, like in many fighter jets help mitigate the impacts as more of the body is level.

So toleration of space travel is a combination of tolerating g-loads during accelerating and deceleration phases and weightlessness (absence of acceleration) periods which tend to affect muscles, bone densities, etc.

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    $\begingroup$ G force isn't a function of acceleration. it is acceleration. $\endgroup$ – Ingolifs Apr 5 at 0:56
  • $\begingroup$ the force you experience IS a function of acceleration. $\endgroup$ – Hobbes Apr 5 at 7:09

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