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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/

Muze
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Daaood
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    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. – Steve Linton Apr 03 '19 at 13:23
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    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. – Wayne Conrad Apr 03 '19 at 13:37
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    Prolonged G forces, even 2 G or less could only produced in a centrifuge on Earth. Rockets in space are limited to a few minutes. There is no available technology for a duration of hours or days. But a constant 1 G accleration would not limit our ability to sustain space travel much more than 2 G. Both are pure science fiction today. – Uwe Apr 03 '19 at 14:39
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    See related How fast will 1g get you there? – James Jenkins Apr 03 '19 at 16:50
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    Roundtrip times at 1g, including subjective time for a relativistic traveller https://upload.wikimedia.org/wikipedia/commons/f/f5/Roundtriptimes.png – JollyJoker Apr 04 '19 at 10:48
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    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. – HopDavid Apr 04 '19 at 13:56
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    Just pointing out that every human on the planet is under a constant 1G acceleration all the time. If we had a manner of propelling a craft at 1G, it would be just like standing on Earth, we just need to make sure things are oriented the correct way for comfort/function. – Rozwel Apr 04 '19 at 18:39
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    There’s excellent evidence that sustained 1G will eventually kill you, given that everyone who has ever died has died under the effects of Earth’s gravity! – Tim Apr 06 '19 at 09:35
  • Yes there is, as seen in Galaxy Quest! https://i.stack.imgur.com/b3kQr.gif – uhoh Apr 12 '19 at 23:46
  • Here is the thing. You only experience G-Force when your in a ship that is in constant acceleration. The key is energy. If we find a system that allows us to travel farther based on energy. We can get to mars or even to the edge of our solar system within weeks. The key is interval burst of acceleration on a hourly basis. possibly 10 sec's of 2g's a hour. For Example. If a Ship did 20's of 2 g's within 2 hours. Their rate of Speed would be 880 Miles a Hour In Space cause nothing is slowing them down. Plus you won't be experience G-Force cause your ship is speeding at a constant rate . – numerical25 May 27 '21 at 12:56
  • Which means as long as the ship is moving at a constant for most of the time, it doesn't matter how many time's you burst in to 2g's a sec. This was the whole plan behind the Orion Constellation Project. NASA was going to use Nuclear Bombs to burst the ship in to 1 or 2 g's every hour or have other day to increase acceleration per hour without killing the astronauts. This would make for Exponential Acceleration over a matter of hours – numerical25 May 27 '21 at 13:04

5 Answers5

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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.

David Hammen
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    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 – James Jenkins Apr 03 '19 at 16:47
  • Interesting, I always thought we coasted in an effort to conserve fuel and reduce lift off weight. Continuous thrust would only take us to some "max" speed, in which acceleration would not be sustained but decrease as we got closer to the max. Think of flying in a plane, at take off you feel the acceleration, but once you are cruising, the engines are still producing thrust, but you are not exactly accelerating. Even being in the vacuum of space, you aren't going to accelerate to light speed and beyond (with tech we currently know exists). – dave k Apr 03 '19 at 17:15
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    @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. – Saiboogu Apr 03 '19 at 18:36
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    @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 – llama Apr 03 '19 at 19:19
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    Getting into orbit would be a fair bit more efficient with higher accelerations -- as a rough estimate, each second you spend accelerating toward orbital velocity costs you 10 m/s in gravity drag. – Mark Apr 03 '19 at 20:02
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    @llama - Well, nothing but the fact that you would definitely run out of propellant long before you got anywhere close to relativistic speeds... – Darrel Hoffman Apr 03 '19 at 20:04
  • @Mark Note that if you go above terminal velocity, you do end up less efficient because you're burning extra fuel on fighting against excessive atmospheric drag, but this is very rarely a real problem – llama Apr 03 '19 at 20:54
  • "There are some promising low thrust / high efficiency (high specific impulse) technologies..." Could you name one or two in your answer, just to give an idea of which ones we're talking about? – jpmc26 Apr 03 '19 at 21:23
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    @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. – David Hammen Apr 03 '19 at 22:51
  • "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" - Sustain it for about a year, and you'd be traveling at the speed of light and could at easily visit the nearest star and come back within a single lifetime. – aroth Apr 03 '19 at 23:52
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    Although the answer is correct, it doesn't answer the question. Yes, the question concerns something that isn't a practical problem for the foreseeable future, but it's still a clear question. – JollyJoker Apr 04 '19 at 07:25
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    @Saiboogu And even that is only true for an observer from the "outside" (e.g. your origin or destination planet). As long as you keep accelerating, the trip is going to take a shorter time proportionally - the only difference is that you'e going to have less agreement with others about how long the trip took. – Luaan Apr 04 '19 at 10:24
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    FTR, in principle rockets could sustain high g forces just fine, the problem is just that they get exponentially expensive. – leftaroundabout Apr 04 '19 at 12:24
  • @llama The airplane example was to demonstrate the g forces. Even without air resistance, you can't just continually increase speed. See this quote from Nasa: "Modern ion thrusters are capable of propelling a spacecraft up to 90,000 meters per second (about 200,000 miles per hour (mph))." Once you get to this speed, further attempts to accelerate will not produce any more speed, that is the current max possible speed for that thruster. Source: https://www.grc.nasa.gov/www/ion/overview/overview.htm – dave k Apr 04 '19 at 19:30
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    @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. – Steve Linton Apr 04 '19 at 20:07
  • @davek That source is absolutely assuming the limited amount of mass we can carry in fuel (inert gas). Assuming our supply of argon were limitless, we could easily accelerate until we experienced relativistic effects. – Dylan Apr 04 '19 at 20:38
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    @aroth If you sustain 1 g acceleration over a year, you would not be traveling at the speed of light. You may reach 90 or 99 or 99.9 or 99.99 % of the speed of light, but you will never reach 100 %. But when you travel at 90 % or more, hitting interstellar dust may destroy your starship. – Uwe Apr 04 '19 at 21:34
  • Technically, yes. Though I assume that if we have engines that can sustain a constant 1g of thrust for a year or longer, even as velocity climbs very close to C, we can also have things like deflector shields or some other dust-abatement measures. Perhaps harnessing the energy from high-velocity dust collisions is part of how the hypothetical engine generates its power. :) – aroth Apr 05 '19 at 00:34
  • @aroth There are a couple reasons that's not feasible. First, maintaining a constant acceleration of 1G in the solar system's reference frame would require an increasing amount of acceleration in the reference frame of the spacecraft, eventually reaching a level of G-forces which humans can no longer endure. Second, any energy harnessed from high-impact dust collisions would necessarily be less than the amount of kinetic energy you'd lose from colliding with the dust in the first place. – Ajedi32 Apr 05 '19 at 16:52
  • Your comment on Mars; you can go further. It is ~4 years ship-time to Alpha Centauri at 1G. It is 600 years ship-time to the other side of the milky way at 1G (~400 to the core). This accounts for relativity. – Yakk Apr 05 '19 at 18:05
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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.

Muze
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Ingolifs
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    Best answer. This actually addresses the question, flawed as its premise is. – user45266 Apr 04 '19 at 05:27
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    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.) – dave_thompson_085 Apr 04 '19 at 06:18
  • Just install reactionless thrusters. Lots of SciFi spaceships have them. :-) – Carl Witthoft Apr 04 '19 at 17:28
  • John Paul Stapp did survive 30 G+ without a gel-like coccoon of similar density. – Uwe Apr 04 '19 at 20:42
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    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. – Ingolifs Apr 04 '19 at 20:47
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    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. – T.E.D. Apr 04 '19 at 21:51
  • I gave that a +1 for the breathable liquid. There's no such thing as inertial compensators, and there isn't going to be. And no matter how well you pack the human, an air-filled lung will be a weak point, as the lung will collapse. – Greg Apr 06 '19 at 21:52
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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.

Emilio M Bumachar
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    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. – chepner Apr 04 '19 at 20:35
  • @chepner Put them on the orbit of their planet-ship, then cut off the acceleration. They'll be in microgravity. – kubanczyk Apr 04 '19 at 21:13
  • 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. – chepner Apr 04 '19 at 21:22
  • When you stop accelerating you need to move further away from the planet 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. – Hennes Apr 05 '19 at 11:47
  • Since gravity is only space-time curvature, maybe antimatter could help in warping up the space and create artificial g loads :| – zephyr0110 Apr 05 '19 at 17:21
  • Or maybe a giant double ring that rotates opposite to each other, and the passenger can toggle between two rings to remain on cancelling g-force side. ( maybe multiple rings to allow smoother transitions while velocity change) – zephyr0110 Apr 05 '19 at 17:25
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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 than that on Earth. On the Moon, it is less than on Earth.

On Earth, gravity is a force that continues to pull 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 an 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 the head. Human bodies tolerate positive g-loads better than negative. Lying down, like in many fighter jets, helps 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.

No Nonsense
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Steve
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Look, can I ask everyone to think outside of the spacebox? If problems beyond our way of thinking are addressed, then problems encountered while zipping around our backyard become less than trivial. For example, it would seem that there are several processes and techniques to achieve higher than normal acceleration ($1 \frac{\text{mile}}{\text{s}^2}$ is doable) So what artificial environment can be maintained for a year? That's how long it would take to reach $c$ (speed of light) at said velocity. While there, it would be a really slick way to conduct certain experiments like finding out for sure if $c$ is our limit or not or if the time crystal based pump can actually affect the relationship between space and time. "You may say I'm a dreamer."

No Nonsense
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Milton Nicol
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