With something like a coil gun mass driver in mind where the more acceleration the cheaper the design in mind:

If you were to suspend the astronauts in a shallow layer of water and horizontally compared to the direction of acceleration - how many g could they survive?

I already know from a wiki article that a guy survived 46 g in a rocket sled unsuspended and horizontally oriented.

Maybe the effect is nothing, but wouldn't the water support their mass perfectly and also supply pressure preventing blood draining/vein damage?

  • $\begingroup$ Do you mean the crew would be oriented perpendicular to the axis of acceleration? $\endgroup$ Oct 18, 2014 at 13:42
  • $\begingroup$ Yes, they would feel like they were lying down. If the acceleration acting on the water alone is not enough for sufficient pressure we can also assume the water is pressurized to begin with. $\endgroup$ Oct 18, 2014 at 13:54
  • $\begingroup$ See also, sililar/related: my and other answers at [physics] : Magnetic levitation for space travel $\endgroup$ Oct 29, 2014 at 10:11
  • $\begingroup$ Didn't we have a convo along similar lines in The Pod Bay a while ago ...? $\endgroup$
    – Everyone
    Oct 29, 2014 at 11:11
  • $\begingroup$ reminds me of youtu.be/n8mK-A_0viA?t=8 $\endgroup$
    – uhoh
    Nov 15, 2019 at 5:10

3 Answers 3


Ok I will try to answer my own question as best as I can with known information. (We assume some extra mass is not a problem in our design because we have a very cheap to launch coil gun.)

An intuitive way if simplified of understanding the issues would be a balloon filled with water:

  1. If it is stretched enough the walls of the balloon burst - either through support deformation or hydraulic/water weight pressure from within.
  2. If you hold it with little support the water pressure deforms it.
  3. Shock - if pressure, compression or g differences occur it could create heat and or shear forces.
  4. Weight differences between materials become more pronounced at high g - heavier plastic will sink down compressing the balloon to a flat plane at sufficient g even if the balloon is floating in water around it.
  5. Ambient pressure may affect some materials adversely. The plastic may melt, chemical reactions could take place or the water could change phase to solid ice.

None of 3-5 were issues during John Paul Stapp's tests - which I have now read in detail.

I have ranked 1-5 above in order of what would kill us (or a balloon first). Nr. 5 would not occur unless you made the balloon of styrofoam and/or reached incredible g. We can disregard that.

Nr. 4. will probably become a problem only at 50-150+ g as the human chest would cave into the lungs under its own weight as the air in the lungs offer no structural support. Liquid suspension inside the lungs would help against this because it would reduce the mass difference between the lung space and ribs, probably pushing nr. 4 as an issue well above 100 g. Bone density is 1.85g/cm^3 so water suspension would push this limit - whatever it is - up by 117% (1.85/0.85 = 217%). We know Strapp had no issues with this whatsoever at near 50 g so knowing nothing about bone strength/other similar issues I estimate this limit at 200-300g with lung liquid suspension and 100-150g without.

Nr. 3 I do not see as an issue. This is because you could build your coil gun to ramp acceleration slowly, cool the suspension fluid and so on. This and in general careful construction should prevent any shocks. You might also construct the "launch bath" as a rotating structure so that it could always face the acceleration in the most optimal fashion. Further about 20 cm of water would at the bottom at 100g only reach 2 bar additional pressure (20 meters of water gives 2 bar). This is quite negligible.

This brings us to nr. 2: Strapp actually cracked his ribs at around 18 g with some bad harnesses when facing the acceleration/deceleration. This makes a lot of sense as what looks like simple belts in his images would at this g have supported 1.4 tonnes if he weighed 80 kilos normally! Turning the seat, better harnesses or in our case liquid suspension gets us well beyond 50g. If the liquid has the exact density of the human mass only internal differences in body mass/inability to compress evenly/negligible/without damage has consequences.

Finally the big killer: Nr. 1. Strapp had both whiteouts and redouts. The first being where blood left his eyes and the second where blood pressure burst every vein in his eyeballs. Going back to the balloon example it would be like holding a heavy water filled balloon with nothing to support it - pressure would make the walls of it burst.

Water suspension may help here to some extent, but I don't know the limits: The eyeballs would be pressurized, most of the body mass, but the skull may prevent parts/all of the brain from being fully pressurized, as body fluids cannot move unrestricted past it and it will not itself flex - especially at rapid acceleration onset. This low pressure at the back of the skull could mean veins bursting inside the brain - and any other body part not capable of "transmitting" pressure without tearing.

However we KNOW that water g-suits exist and work. These suits do not use hydraulics it seems, but the water itself. Our water suspension should work even better. Further they are used in sitting position - a MUCH more vulnerable position to g force.

Since Strapp, as far as I can tell, used NO pressure suit and still took these amazing forces (25g for 1.1 seconds with a peak of 46.2g) and walked away unhurt (his eyes healed almost completely in few days time without surgery) I have to assume pressure assistance - whether from water suspension or a g-suit - would help greatly. I think if you slowly go to high g from low g and let the pressure gradient created by the basin "settle" into the body's organs and fluids you could deal with this problem.

Upon leaving the muzzle of the coil gun however high pressure areas inside your body would suddenly have nothing pressing back - again creating bursting potential though less and of shorter duration. I would counter this by artificially increasing ambient pressure in the bath when leaving the muzzle to lessen the shock.

You could counter this by stopping the acceleration slowly, but that is not really an option with a realistic coil gun design.

With this in mind the pressure shock of going from 100g to 0g ie. 3 bar in the lowest parts of your body to 1 bar in the top of your body with no time to equalize is probably the limit. Strapp experienced something like this at only 25g, but for a much longer duration - ie. constant ~0.5 bar pressure difference in his eyeball veins for 1.1 seconds with NO dynamic counteracting pressure.

So based on guesswork, these ramblings, the numbers above, the fact that Strapp was completely fine and died at 89 years old and that humans have been said to survive brief spikes of 1000g. I would say that breathable liquid suspension could allow astronauts to launch at 100g from a coil gun and be 99.9% okay after resting in a hyperbaric chamber/seeing a doctor.

Interestingly enough a 50 g acceleration to low earth orbit speed (7.8 km/s) would use a 62 km long launch tube, of which 1.9 km could be horizontal on the ground and the other 60 km would have to be in the air suspended with gas balloons. The launch tube would curve as the velocity vector changed and would exit at about 40-45 km height where atmospheric influence would be at 1% or less (exit heat shield still needed). The launch would take 15.9 seconds. If the accelerator is all/mostly on the ground and only maglev suspension inside the airtube the astronauts would face some g for longer to pay for cheaper construction costs. Not sure how much longer/at what g the in-air tube ride would be.

  • 1
    $\begingroup$ If you remove the indentation on the numbered list it will be easier to read. $\endgroup$ Oct 27, 2014 at 18:51
  • 1
    $\begingroup$ The g-suits you refer to, use air instead of water. They work in aircraft because the G-forces are often aimed downwards, so blood wants to flow to the pilot's legs. The suit applies pressure to the lower body to reduce this blood flow. None of this helps if the G-force is towards the back instead of down. $\endgroup$
    – Hobbes
    Oct 29, 2014 at 17:02
  • $\begingroup$ Liquid breathing would do more harm than good. Your lungs contain ca. 5 liters. That's 5 liters of liquid pressing down on the lung walls and everything behind them, including the heart if I'm not mistaken. At 10 G, that's 50 kg of extra weight the body has to withstand. $\endgroup$
    – Hobbes
    Oct 29, 2014 at 17:24
  • 1
    $\begingroup$ Quote from wiki: "The g-race suit is a liquid(water) filled, autonomous and aircraft independent working full-body g-protection system." The pressure of the lung liquid is counteracted by the water pressure around you - without it nothing helps support your chest. $\endgroup$ Oct 30, 2014 at 11:21
  • 2
    $\begingroup$ @Hobbes "that's 50 kg of extra weight the body has to withstand." Why is this a bad thing? What is the weight limit of a body? $\endgroup$
    – NPSF3000
    Oct 31, 2014 at 1:29

One big issue with G-forces is the way it affects the bloodflow. At high accelerations, the heart is unable to pump the blood 'uphill'. In aircraft, this leads to blackouts around 9 G. Having the acceleration vector towards the back instead of the feet mitigates this, but you're still going to have a blood supply gradient over your body. Surviving this for one second is feasible, but having parts of the body without sufficient blood for one minute will give problems.

A G-suit as worn by pilots shows that an external force on the body can help with this a bit: it constricts the blood vessels of the lower body, making more blood available in the higher regions. This works because pilots experience G-forces towards their feet. The legs have lots of surface area relative to their volume, and major arteries close to the surface.

If the blood pools around the spine and in the back of the head, adding surface pressure is not going to help force the blood 'up'.

(Having thought about it some more, I have to rewrite this part of my answer, my initial assumption was incorrect).

A body suspended in water must have enough buoyancy to float. If you're not buoyant enough, you'll sink toward the 'bottom' of the container under acceleration, and eventually you'll be supported by a solid surface instead of by water. If you're buoyant enough to float, you'll stay afloat under acceleration.

Any water on top of the body will add pressure: a 10-cm layer of water at 50g will feel like 50 kg/dm^2; that's not healthy.

A body almost submerged in water is supported by the water, which helps mitigate some of the effects of the G-force.

From the Wikipedia article on liquid breathing:

Acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15 to 20 G. Extending acceleration protection beyond 20 G requires filling the lungs with fluid of density similar to water. An astronaut totally immersed in liquid, with liquid inside all body cavities, will feel little effect from extreme G forces because the forces on a liquid are distributed equally, and in all directions simultaneously. However effects will be felt because of density differences between different body tissues, so an upper acceleration limit still exists.

Liquid breathing for acceleration protection may never be practical because of the difficulty of finding a suitable breathing medium of similar density to water that is compatible with lung tissue. Perfluorocarbon fluids are twice as dense as water, hence unsuitable for this application.

Original source: Guyton, Arthur C. (1986). Textbook of Medical Physiology, 7th Ed., Aviation, Space, and Deep Sea Diving Physiology. W. B. Saunders Company. p. 533.

This ignores the fact that the lungs aren't the only air-filled cavity in the human body. All cavities would have to be filled with liquid to prevent collapse.

All of this only works if the acceleration is limited to one direction only. A curved launch tube would add a centripetal acceleration force, which the water would not help against. You'd crash into the side of the container.

PS this article has lots of information on the effects of G-loading on the human body.

1: bottom and top are relative to the direction of the acceleration force.

  • $\begingroup$ "G-force will push the astronaut to the bottom(1) of the water container" - Source? I regularly experience G forces immersed in liquid and this is not my experience. $\endgroup$
    – NPSF3000
    Oct 30, 2014 at 1:25
  • $\begingroup$ I suspect it depends on your buoyancy; if your density is low enough that you'll float, you'll keep floating under G-loads. If you're heavier than water, you'll sink. $\endgroup$
    – Hobbes
    Oct 30, 2014 at 6:37
  • $\begingroup$ The question is then what happens when the fluid is a similar density to your own body - does it support it? Remembering that there's nothing inherently dangerous about acceleration - if I'm in free fall at 100g's my body would just think it's at 0 g. $\endgroup$
    – NPSF3000
    Oct 30, 2014 at 8:49
  • $\begingroup$ The weight of your body is supported in the ideal case by half the surface area of your body. Water may get you closer to the ideal case, but by how much? $\endgroup$
    – Hobbes
    Oct 30, 2014 at 9:56
  • $\begingroup$ I don't think it's just about distributing the pushing force, it's about supporting the body. If I lie down in a perfect memory foam mattress, what's to stop my cheeks from pulling, my top leg muscles trying to pancake out? $\endgroup$
    – NPSF3000
    Oct 30, 2014 at 12:23

I've thought about this myself and have wondered as well, can you cheat g's if you are suspended in a mostly non compressible liquid that keeps you at 0 buoyancy (or close to it, water will do fine). Of course there will be hollow spaces like your lungs, but you could hypothetically withstand a substantially greater g load if it worked.

I don't recommend doing this, but imagine having a goldfish in a bowl tied securely to a rope, and you were to swing it around, would the goldfish experience much more if any than 0 g's? Correct me if I'm wrong, but the answer would be no, because the force is being paid for by the bottom of the container and rope holding it, but not to its occupant (unless of course an accident happens.)

  • $\begingroup$ Physics doesn't work like that. $\endgroup$
    – Mark
    Jan 9, 2015 at 0:56
  • $\begingroup$ @Mark what doesn't physics work like? The basic premise of Doc G is correct - see: physics.stackexchange.com/questions/141030/… $\endgroup$
    – NPSF3000
    Jan 9, 2015 at 7:22
  • $\begingroup$ If goldfish have a swim bladder like other fish, high g forces may destroy it. $\endgroup$
    – Uwe
    Aug 7, 2017 at 19:40

Not the answer you're looking for? Browse other questions tagged or ask your own question.