A sentient species of water-dwelling aliens achieve space-flight.

Their living quarters contain water [no free gases].

They have evolved over millions of years to regulate their buoyancy at will.


Will they be able to withstand much higher acceleration than an air-dweller.

Reason for question

If they maintain neutral buoyancy they won't sink to the bottom and they won't float to the top. Will they merely experience an increase in water-pressure? They are used to dealing with extreme changes in pressure when they dive deep and return to near the surface of their ocean. Therefore surely high acceleration won't inconvenience them.


If preferred you are at liberty to discuss the difference between turbulent takeoff through an atmosphere or smoother acceleration in a vacuum. I originally intended that the acceleration would only be along the axis of travel.

  • $\begingroup$ The ability to withstand acceleration is a result of genetics and environment. If we grew up on mars we would not be able to tolerate the same gforces as earth humans. $\endgroup$
    – anon
    Nov 20, 2018 at 22:35
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    $\begingroup$ Water has no effect on the experience of accleration. $\endgroup$
    – anon
    Nov 20, 2018 at 22:51
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    $\begingroup$ @anon - Why? G-suits act by equalising pressure around the body. Wouldn't water do the same but even more so? $\endgroup$ Nov 20, 2018 at 22:54
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    $\begingroup$ G-suits work by compressing the legs and abdomen so that the blood can't leave the head. (Their purpose is to prevent the blood leaving the pilot's head resulting in loss of consciousness.) The pilot experiences the exact same acceleration as a person without a g-suit, with the corresponding difficulty of moving their arms etc. but the g-suit gives them a chance of not fainting or dying from lack of brain oxygenation. $\endgroup$
    – AlexP
    Nov 20, 2018 at 23:03
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    $\begingroup$ Relevant related: worldbuilding.stackexchange.com/a/74060/2964 $\endgroup$
    – Steve
    Nov 20, 2018 at 23:11

7 Answers 7


The answer is no; they'll not even be able to withstand normal human acceleration limits. Not because of the pressure (induced by constant acceleration), but because of the momentum change in their environmental medium (caused by the sudden changes in acceleration, especially at the beginning of the launch).

A simple experiment that explains this (don't try this at home edit - for ethics reasons, not because I'm worried about the results); take a small fishbowl, and put a lizard in it, seal the top and shake it. Afterwards, the lizard will be sore and more than a little pissed off at you, but it will be alive. Take a fishbowl full of water and a goldfish, seal the top and shake it. Your fish will be dead, almost instantly.

Further Edit - Another analogy that you might consider here is the shockwaves caused by Blast Fishing, which also causes large changes in momentum over a short time.

Why? Because water is a non-compressible medium (and it's very dense). In any collision, the most deformable object is the one that also absorbs as much of the kinetic energy of the impact as it can. This is why modern cars are so 'flimsy' by comparison to older cars, and consequently so much safer. In an accident, the car breaks so that it absorbs as much of the kinetic energy as it can before transferring the balance to you. Older, more rigid cars don't do that and as a result many people found out what being the most deformable object in a collision actually meant right before they died.

Air is very compressible (read as deformable in this answer) meaning that it can absorb a lot of energy by comparison to its mass. Unfortunately, that doesn't mean much because its density is very low, so the two tend to balance each other out. Still, our lizard only has to worry about being the deformable object every time he hits a wall on the fishbowl. Strap him in, and he's going to fair a lot better (hence seatbelts).

Your fish on the other hand is in a medium that is not only non-compressible, but very dense. This means that it's going to take a LOT more energy to launch all that water because of the mass, and more importantly, the sudden acceleration change is going to throw all that non-compressible mass your way as soon as you start your acceleration, likely crushing you. This also means that any form of maneuvering in space or engine issues that cause significant vibrations or shaking are of concern because of rapid and large momentum changes other than the constant acceleration along your line of axis.

The problem with the reasoning in your question is that buoyancy is not the same thing as pressure. Buoyancy is relative density, whereas pressure is the force of a mass being applied against you (this is a simplification but functionally correct). Sure, the pressure deep in the ocean depths can be extreme, but it can also be introduced to your creatures slowly - they don't instantly go from 10m depth to 1000m depth, and it would kill them to do so. But, in a rocket, that's exactly what you're asking them to endure.

They'd be far better in some form of gel, that allows them to breathe but is lighter weight and can absorb most of the impact for them. Water is not the best medium to find oneself in when face with sudden acceleration.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – James
    Nov 21, 2018 at 20:10
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    $\begingroup$ your proposes experiment is a bit cheating. you advise us not to try it and i suspect you haven't either. so why should we believe your conclusion? (aka citation needed) $\endgroup$
    – ths
    Nov 21, 2018 at 20:21
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    $\begingroup$ Put differntly: As flying a rocket is like flying on a permanent explosion, flying a rocket in a fish tank is like permanent dynamite fishing? $\endgroup$ Nov 21, 2018 at 21:57
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    $\begingroup$ I'm skeptical of your claim that the lizard will escape largely unscathed after being slammed against the sides of the fishbowl as it's shaken. If this photo is legit, then this gold fish survived falling 1m to the floor in his fishbowl. The sudden deceleration at the end didn't appear to be fatal. $\endgroup$
    – Johnny
    Nov 22, 2018 at 0:40
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    $\begingroup$ I have to disagree with this answer. For one, I have ths's reservations about the result. In addition, it's completely different than acceleration in spaceflight, because the bowl is getting shaken back and forth. A bowl constantly accelerated in one direction will have no pressure waves propagating through it, as it's identical to a body of water sitting on a planet with a different gravitational acceleration. Finally, the comparison to dynamite fishing is completely meaningless, since the accelerations involved there are many orders of magnitude greater than those encountered in spaceflight. $\endgroup$ Nov 23, 2018 at 15:28

Assuming that the spaceship is totally filled with water (i.e. no air gaps) then there will be two effects experienced by the aliens.

Firstly, the effect of the acceleration on the water mass in the spaceship. For any significant acceleration there will a gradient in the water pressure along the axis of acceleration. Assume a 200 m long spaceship, at 10 g acceleration. At the 'front' of the ship, the water-pressure will be negligible, at the back end it would be equivalent to that found on Earth at a depth of about 2 km (around 200 Atmospheres). If the spaceship then decelerated at the same rate there would be an almost instantaneous reversal; of the pressure gradient (now negligible at the rear of the craft and 200 Atmospheres at the front). These pressures would scale up in a longer vessel or with higher accelerations - 500 m long and 20 g would give you an almost instantaneous 1000 atmospheres. Equivalent to almost instantaneously diving to the bottom of the Marianas trench! So if the creature relied on swim-bladders for buoyancy moderation they would be in big trouble (deep-sea fish don't to well when they are dredged up to the surface quickly). Internal baffling in the spaceship could solve this problem but then it would be very difficult to open/close doors and move around during acceleration.

Secondly, there would be the effect of acceleration within the body of the creature itself. Land-based creatures often experience significant local acceleration and jarring as a result of jumping, falling etc, so have evolved so internal organs of varying density are constrained in position against these forces. A sea creature, depending on its normal mode of transport within water may not be subjected to these acceleration/jarring effects in its normal life (consider a jelly-fish for example). So if they are used to being cushioned in water their internals may be more sensitive to acceleration than ours.

So the answer would depend on the specifics of the creature. Muscular, deep-diving porpoise - maybe. Floppy, floating jelly-fish - probably not.

  • $\begingroup$ The problem about being at the front or back of the ship can surely be solved by staying in the middle of the ship during acceleration and deceleration. $\endgroup$ Nov 21, 2018 at 8:47
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    $\begingroup$ Are you not forgetting that we are mostly non-compressible water? Sudden changes in pressure are dangerous to us because they do not give the air spaces in our bodies (Lungs, sinuses, inner ear..) time to equalise their pressure to that of the ambient pressure. Dissolved gases are a problem, only after you have had time to absorb them at a pressure relatively higher than one to which you are subsequently exposed (Decompression sickness). Assuming these aliens have evolved without such air spaces, and pressure changes are gradual, surely the best alien would be like a jellyfish? $\endgroup$
    – Flippsie
    Nov 21, 2018 at 14:33
  • $\begingroup$ Internal baffling would prevent an alien being exposed to additional pressure fom overly large bodies of water acting on them, but they will still to be exposed to g-force. The physiological effect of that is to draw blood within our circulatory system either away from or toward our brains, depending on ones orientation to the accleration. This would be the same regardless of the medium in which you are swimming (Or strapped in). An alien without a circualtory system such as ours, would surely be immune? $\endgroup$
    – Flippsie
    Nov 21, 2018 at 14:38
  • $\begingroup$ of what kind of ships are you dreaming? 500m, 20g seems wildly phantastical. i'd look more at the dimensions of a soyuz capsule. a water height of maybe 2m. $\endgroup$
    – ths
    Nov 21, 2018 at 16:20
  • $\begingroup$ @chasly In the middle of a ship of the length I suggested the maximum pressure would be half the maximum pressure at the ends. So high acceleration would still result in significant pressures. . $\endgroup$
    – Penguino
    Nov 21, 2018 at 19:45

Will they merely experience an increase in water-pressure? They are used to dealing with extreme changes in pressure when they dive deep and return to near the surface of their ocean. Therefore surely high acceleration won't inconvenience them.

The biggest problems will arise wherever there are density differences within their body. Think of putting a steel cube inside a mold of jello. As you subject them to higher pressure, nothing bad happens (there are no air pockets to be compressed).

But as you accelerate it, you are changing the forces at the boundary where the density changes. The denser steel cube wants to be at the "bottom" of the pan. As you increase the local gravitational field, the greater the stresses required to maintain out-of-order densities.

Do the creatures have bones? Sensitive, but light organs? Complex organs with multiple tissues of different densities? The greater the acceleration, the greater the forces that appear within them.

In human studies, the greatest damage at the limits reached wasn't with lungs or other aspects of empty spaces, but with the retina. That takes place entirely within an enclosed liquid container, but is still subject to damage at high accelerations.


Yes, they will be able to handle higher accelerations. This has been answered by Steve and AlexP in the comments (possibly others as well)

First, it's useful to turn the question around, why do air breathing creatures suffer from acceleration more than aquatic creatures? Imagine standing under 10g of acceleration in air. Also let's just say your blood is water to simplify things. The pressure increase at your feet: $$ \begin{align} \Delta P&= \rho g \Delta h \\ & \approx(1000)(10\times 10)(2) \\ & =200kPa\\ & =2bar \end{align}$$

Whereas the external air pressure increase at your feet: $$ \begin{align} \Delta P&= \rho g \Delta h \\ & \approx(1)(10\times 10)(2) \\ & =200Pa\\ & =2mbar \end{align}$$ There is nearly 2 bar of pressure difference between the blood in your feet and the air outside them. Lots of blood pools in your feet and legs, your heart won't be able to pump it up to your head, you lose consciousness.

If instead of air, you are surrounded by water, the pressure difference between the blood in your feet and the surrounding medium is zero, no blood pools. You stay conscious.

But there is still nearly 2 bar of pressure between your head and your feet, you might worry that your heart has to work hard to pump against that pressure gradient. It doesn't really, provided everything is incompressible. Let's simplify your circulatory system to be a simple loop. It now looks like this: $${\Huge 0}$$ With your heart as an infinitely thin pump on one side. As your heart pumps water up one side, it is replaced by water moving down the other side. The replacing water arrives at the pump at very very nearly the same pressure as the water moving up - as it's all a sealed closed loop with an incompressible fluid - so it doesn't have to overcome a high pressure difference, as it is effectively being feed water at a high pressure to begin with. This is basically how pressure suits in fighter aircraft work.

Lastly some answers suggest that the pressure changes in a deep column of fluid will kill them, this is true if they are bad engineers. If they build their space ship as a 100m continuous water column, they will have a bad time at high accelerations. If instead they break that 100m into 100 1m high sealed rooms, with no water column to the floors above, then they experience much lower pressure rises. At some point 10cm of water column above them will kill, but it'll take a lot of acceleration.

Provided they use a fluid with similar density to their blood, correctly design their spaceship for high accelerations they will be able to tolerate higher accelerations than land animals. It'll also help if they don't regulate buoyancy with an air bladder like arrangement.


Another factor that hasn't been mentioned yet--water creatures are generally going to be a lot weaker than similar land creatures. A water creature doesn't have to support it's own mass, let alone it's own mass taking a fall.


Yes, so long as they can withstand high pressures they will be able to withstand much higher accelerations than a human could (in air.)

Acceleration is equivalent to gravity, and your intuition may work better thinking of it that way (I know mine does.) Increasing gravity on a canister of water is going to increase water pressure linearly. So for example, if you can withstand 10x the pressure at 1g, you can withstand 10gs of acceleration.

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    $\begingroup$ Sorry, but this isn't correct. If you can withstand 100m of water depth (10x Earth Air Pressure) then you can withstand 10g of force IN AIR. Force , = Mass x Acceleration, and the mass of the water pressing down on you at 10gs at 100m equivalent depth of water at 1g is going to kill a normal human. We can dive to around 150m in scuba gear with the right air mix, and we can withstand around 15Gs in an atmosphere, more so in a less dense atmosphere like what was put into the Apollo spacecraft. $\endgroup$
    – Tim B II
    Nov 20, 2018 at 23:53
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    $\begingroup$ For this question we can't really assume 'a normal human'. If anything the aliens would have a physiology consistent with that of an earthly sea-creature [choose your own]. $\endgroup$ Nov 21, 2018 at 8:52
  • $\begingroup$ Would they have a swim bladder (i.e. a cavity filled with gas inside the body) like most fish? $\endgroup$
    – Deolater
    Nov 21, 2018 at 15:45
  • $\begingroup$ @Tim B II The things that hurt you in deep water scuba diving are very different than what hurts human pilots if you accelerate too fast. In the former, water crushing you is not actually a problem, since your body is pretty much water and thus incompressible. The real problem has to do with gases becoming toxic above certain pressures and details of how they diffuse into your blood. The calculation you gave is pretty much a non-sequitur that coincidentally gave a reasonable answer. $\endgroup$ Nov 23, 2018 at 16:07

if you want an advanced aquatic like species, just make the homeworld have an insanely dense atmosphere. the way buoyancy works are (tldr) you need to be less dense than what you are floating in (that's ignoring displacement and stuff) the density of water is 997kg/m³ while humans are about 985 kg/m³. now, I know what you are thinking. "if the gas will be as dense as water anyway, why don't I just limit the pressure, and just go aquatic anyway?"

the reason is a running joke in the forums of a game called 'thrive'.

ITS IMPOSSIBLE FOR A OCEANIC SPECIES TO DEVELOP TECHNOLOGY. combustion is impossible in water. they cant craft metal tools. just.... go to thrive and they will explain it better.


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