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Scott_Abrams t1_j11nr91 wrote

Yes. Forces applied to fluids are distributed as omnidirectional pressures as fluids are difficult to compress. Because fluids are difficult to compress, there's no appreciable change in density under high acceleration. This concept has already been applied to high G flight suits, namely the Libelle G suit, which has allowed pilots to remain conscious and functioning during maneuvers as high as 10 G's.

The practical acceleration protection limit via liquid immersion is hypothesized to be approximately 15-20 G's. Beyond that, cavities such as your lungs will collapse, so you'd need to fill your cavities with a human compatible oxygenated immersion fluid which simultaneously has a similar density to water. There is practical upper limit as the differential density of tissues inside the human body will eventually be reached, but hypothetically, if you filled a person with this fluid in all their cavities (lungs, stomach, intestines, etc.), they could survive outrageous amounts of acceleration. Of course, this is all contingent on finding an immersion fluid which is both lung-compatible and has a density similar to water, so it'll probably never happen.


thenewcomputer t1_j123r1k wrote

it's a common element in some of the harder sci-fi novels I've read, with characters being wrapped in gel suits which then fill their lungs with oxygenated foam


NotAnotherEmpire t1_j12j3xp wrote

'Harder" being the operative word.

Truly high G - rivaling or exceeding rocket launch - isn't going to happen in space. Transfer orbit trajectories are defined and a truly unbound, straight line transit can accomplish the same total velocity with less violence.

The ship would also need an engine that could do this, and the fuel to burn, and I think at that point you're left with using Orion nukes.


TheAero1221 t1_j12nl7b wrote

The Expanse does some fun stuff like this. They use fusion powered engines to accelerate continuously all around the solar system. Most of the time they burn 1g towards their destination, and then flip and decelerate once they're halfway there. Combat and emergencies get considerably more spicy though.


someonefun420 t1_j12vr0h wrote

Yeah, good show. They did a lot of things really well and made it feel really authentic.


fredoule2k t1_j13ocgs wrote

Yeah, best sci-fi since bsg. It's great that gravity realism is part of the core of the books and series lore.


thisimpetus t1_j141mvn wrote

The writers were also heavily involved in the show running which is a big part of why, even where the show deviates from the writing, it continues to feel like The Expanse.


fredoule2k t1_j13o54l wrote

In the last books, it's no a spoiler to talk about it, they are using immersion and lung filling tech while sedated in a ship designed for sustained high G travel


TeetsMcGeets23 t1_j12oxiy wrote

Wouldn’t the issue be primarily “sudden deceleration” from a high-speed interstellar flight? Like, you’re speeding up constantly through your travel through space then arrive at your destination and have to stop; which for you is essentially accelerating force acting on you from a different direction.


sg3niner t1_j12qr8p wrote

Practically speaking, you'd boost for half the trip, flip, and brake the second half.

Instant stop would squish everything.


EcchiOli t1_j13ier6 wrote

A certain episode from the Expanse series nicely dealt with this "squish" that would come from instant stop.

For those of you who haven't watched it, imagine a human strapped to a seat, his bones remained in the seat, the flesh kept on moving for a bit longer.


grafknives t1_j131g1z wrote

That is not an issue. A single digit G de-acceleration is Still extremely fast in terms of space travel.

If we ignore relativistic effect, we could accelerate/de-accelerate from half C to standstill in 25 days, while experiencing ONLY 1G.

So when flying to alpha Centauri at half C (still ignoring relativistic effects), it would take one month of speeding to C/2 with 1G, 8 years of flying at this "top speed" and one month of slowing down with one G.

G force are NOT a problem at this scale.


ChemicalRain5513 t1_j1381fu wrote

Even better, once you take into account time dilation, the distances you can cover with such a manoeuvre in a certain amound of proper tine are equal to the classical calculation. Meaning in 1 human lifetime with 40 years 1G acceleration and then deceleration, you would cover almost 1700 light years in 80 years of proper time. Of course that means everyone you knew on earth is dead.


Chilliwhack t1_j150odf wrote

Example above is talking about long distance travel though what about the equivalent of short distance fighters?


FogeltheVogel t1_j13c9wf wrote

It's not "essentially". It just is. Deceleration is just acceleration in the other direction.

You can't come to a sudden stop any more than a sudden speed.


ozspook t1_j13bxwy wrote

Why not just fill the lungs with water and oxygenate the blood externally with ECMO


PainterOfTheHorizon t1_j15q0j0 wrote

At least at the moment a person in an ECMO machine needs constant supervision by a higly trained nurse in order to avoid all kinds of problems, like forming blood clots.


mcabeeaug20 t1_j12wzlf wrote

Have you seen the old movie, "The Abyss" ? This is exactly what they do to the guy- not to withstand G-force, but the immense pressure from a Super deep dive. Great movie for the late 80's. Still love it!


th3d3wd3r t1_j13g8sn wrote

That's a cool scene when he drops the mouse in. But yeah, there are breathable fluid, but our lungs aren't able to deal with the viscosity. That alone causes traumatic injury. Also, the movie "Event Horizon", they go in tanks of fluid.


gargravarr2112 t1_j13mbw3 wrote

IIRC in the Abyss, the breathing fluid was real and the rat was actually breathing it on film (the scene got censored for the UK release due to perceived animal cruelty). Bud, however, wore a helmet full of coloured water and held his breath.

In Event Horizon, the crew still breathe gas through masks. The medic mentions in the opening scene: "When the ion drive engages, we'll be pulling about 20 Gs. Without a tank, the force would liquefy your skeleton." Another comment notes that breathing gas is still viable to about 20G, above which fluid breathing would be required.


InvisibleBlueRobot t1_j14hcca wrote

It was real for the mouse, in real life military tests the fluid is too heavy and it damages human lungs to the point of incurable issues. So we need a little work. Something lighter and easier to move in and out of the lungs but provides the same benefit.


anormalgeek t1_j15u81z wrote

Isn't it being "heavy" directly related to how well it handles the pressure though? Like if it's light enough that our lungs aren't damaged, then it's light enough to have the same issues are breathable air.


glaucusb t1_j13pil9 wrote

It is not exactly the reason but still related to the pressure. In SCUB or surface-supplied diving, the air that the diver breath has the same pressure as the level that the diver is, called ambient pressure. That means when the diver is let's say in 20 metres, they consume 3 times more air in quantity (but volume is the same). This has three consequences as the dive is deeper. (1) It requires way more and bigger SCUBA tanks (there are also closed system devices that cycles gases that are not used called rebreathers to address this issue but they are a bit risky) or stronger compressors (in surface supplied diving). (2) oxygen is toxic over 0.8atm (there is also time as a factor, here is an oxygen toxicity table so we need to decrease the percentage of oxygen and increase other (inert) gases. (3) inert gases do dissolve more in the body tissues and blood during compression (when their pressure increases) and does not leave body as fast so create bubbles that results in clogging arterials and damaging tissues and organs (which we call decompression sickness). We have a solution to all these problems in deep diving what we call saturation diving. You let divers to dive in chambers and let them decompress in these pressurised chambers for weeks after the dive on the vessels. In scuba diving, we do stops in ascending.

In Abyss, they were supposed to dive to a depth that was not done before so, it would require a lot of pressure and inert gas. Also it would be quite difficult to arrange the mix since the percentage of oxygen in the mix should be quite low to prevent oxygen toxicity. Instead, they use a liquid that they can control the mix of gases in it. They also do not need to use huge amount of gas since the diver does not need to breath in ambient temperature.

One last fact: the scene that a mouse is put into liquid and it is breathing inside liquid is a real scene without any visual effects (link here). They used a liquid that can carry enough oxygen and submerged a mouse into it.


glaucusb t1_j12ly0v wrote

There are liquids that could keep high concentration of oxygen such as perfluorocarbon (here is the liquid breathing link from Wikipedia). But their density is 1.5-2g/ml.

Why do we need a density similar to water and how similar they should be?


Wyattr55123 t1_j12pzdg wrote

I do find it amusing that liquid breathing does absolutely work and has been tested on conscious adult humans, only to discover that shock the feeling of liquid filling your lungs is really goddamn unpleasant.


Ausoge t1_j12ogtc wrote

If you were to fill your lungs with something with greater density than a human body (which is about the same as water on average) and then pull an extremely high-G manoeuvre, the heavier liquid would be more affected than your body. It'd essentially be trying to "sink" through you in the direction of the G force and probably crush something important.


Scott_Abrams t1_j12scj2 wrote

The reason why you need a similar density to water is because of differential density. With a similar density, forces will be exerted roughly equally but when there are non-uniform densities, those effects can be felt. Why does oil float on top of water? Because of the difference in density and the application of buoyant force. Oil is less dense than water and so it floats to the top. But what happens when a person has a different density than the fluid both inside (the cavities) and out? Imagine for a moment if your fluid density is more dense (you "sink") and if your fluid density is less dense (you "float") relative to the fluid immersing you. Higher/lower density doesn't really matter as the only thing that really changes is the direction you go in but regardless, you'll feel the sum total of all the different forces on your body, all at the same time, as your body tries desperately to resist the laws of physics and fails.

Humans are largely composed of fluid. Yes, the fluid is tucked away by membranes (cells) and we think of ourselves as solid, but we are still almost 60% water. Do you know how we use centrifugal force to separate red blood cells from the rest of the plasma? Same concept, differential density, only now the fluids and precipitates that comprise your being are being separated by ascending/descending density.

That's why you need a fluid with a similar density as water, because we're made of water and we need to keep that water inside our cells.


Molkin t1_j12xns5 wrote

Does the fluid have to be oxygenated? Could we bypass the lungs and oxygenate the blood using a different mechanism?


_ShadowFyre_ t1_j134qhq wrote

I think the primary issue with this (using my limited internet knowledge) is that most cardiopulmonary bypass leads to hemolysis, which kinda defeats the purpose of CPB. Additionally, there’s the issue of how to distribute the bypassed blood. To effectively reintegrate the blood, you’d either have to have an open “wound” in the chest or permanently installed synthetic transplants, both of which have their own issues. Additionally, you’d probably need anesthesia regardless of the method, which then relegates the point of doing all this as you’d need a functioning being not under the effects of the liquid/bypass to manage the anesthesia.


Blakut t1_j13fdyr wrote

you can put oxygen in your digestive system and breathe through the butt using the intestine wall which is heavily vascularized for oxygen exchange.


jhudsonj t1_j14neku wrote

First, one of the big difficulties with using a fluid medium to "breath" is that the muscular effort to move it is exhausting...The mice that they left in there for too long (more than a few minutes) died from that alone.

Second, the big problem with using Bypass or ECMO is that it requires anticoagulation...So, we're going to inject you with heparin, so you can't make a clot on a dare, and then shoot you into space! I don't see that ending well!

I should add that those are both problems that can be potentially solved. But they will require fairly big advances to pull them off.


Blakut t1_j13fb9h wrote

yes, you can actually oxygenate the blood through your butt (the intenstines) because they are highly vascularized. You do need to irritate them a bit, or scratch them, so to get more surface area to the blood vessels.


lucifer-ase t1_j131oms wrote

Some perfluorchemicals have extremely high oxygen solubility and have been proposed for liquid breathing. You dont think this has gone much beyond some animal experiments. In any case the conversion from air to liquid breathing would be absolutely brutal unless done under sedation ( both ways)


comfortablynumb15 t1_j1311vd wrote

I remembered a scene from the 1989 movie The Abyss where they used Oxygenated Perfluorocarbon on a rat for real before faking a scene where humans used it to survive going under the ocean so deep.

YouTube “Classified Breathing Fluid The Abyss” to watch it ( although I personally find it upsetting as I am a pet rat owner)


HelloMcFly t1_j13el2g wrote

Hospitals still occasionally use this stuff.


Lee2026 t1_j13fl5c wrote

Why is that being in water while an explosion happens doesn’t end well then?


Scott_Abrams t1_j169zsw wrote

Differential density. Your cavities (such as your lungs) and the gasses within get compressed when the shock wave hits and your tissues rupture when the pressure is transmitted through the water and through your body. Differential density is also the reason why things like shaped charges kill everyone inside a tank (using a shaped charge, a high explosive generates a shock wave which penetrates the armor of the tank via the Munroe effect, the energy of which is transferred to anything inside it as the pressure of the shock wave compresses and destroys your tissues) even if the tank armor remains intact (no or incomplete penetration).


ChemicalRain5513 t1_j138b3d wrote

Although is there a practical purpose for this? Only case I can think of where you would intentionally subject humans to more than a few G is in fighter jets. But couldn't we replace those with drones that can make 50 G turns?


Flaksim t1_j13c7ah wrote

Yes, but drones can be jammed or taken over by an adversary. In some cases, and depending on the payload, you'd still want someone in the actual delivery vehicle who can control and deploy their payload without a remote connection being required.


swissm4n t1_j13cpnv wrote

I met the guy who invented the Libelle G suit a few times ! He's so interesting to talk to and so down to earth.


glitch-glitch t1_j14bcms wrote

Would the density of the fluid (excluding the lung issue) make a difference in how the g forces are distributed? Like would it be better for a subject to be suspended in fluid that is less dense?


Sargotto-Karscroff t1_j14rbxn wrote

If we had the tech in theory could we not say bypass breathing with a device that oxygenates blood then fill all air gaps with a liquid/gel that's made for it's density and to be non harmful instead of focusing on breathability.

After that maybe a flight suit to contain/ keep separate the different liquids/gels. As well as maybe a liquid only feeding tube as well as what ever you will need to capture the "waste" after lol. Things needed for extended use.

I like to invent hypothetical things and try to find ways to make them real and this is a fun one, out of my wheel house to attempt though even if I still had my workshop sadly. Oh how I miss trying to build a food generator for real.


-Metacelsus- t1_j15x8m2 wrote

> they could survive outrageous amounts of acceleration

In the lab I can spin human cells suspended in an aqueous buffer at 300g and they don't die.


Ubermidget2 t1_j137j1o wrote

> fluids are difficult to compress

Not strictly true, as air is most certainly a fluid. Otherwise, nice explanation


labroid t1_j106q0d wrote

The problem is that the pressure experienced under water actually comes from gravity (acceleration). If you accelerate at 10G the pressure versus depth under water goes up 10x (relative to Earth). So being a meter under water is the same as being 10 m under water on Earth. At 100G, it goes up to 100m equivalent on Earth, which would squish you. Also, if you were upright, head just below the surface, your feet would be at 250 meters deep equivalent and your head at 2 meters equivalent, so it would squish all your feet and legs into your middle/head. It would not only kill you, but would be really awful to look at...


Bkeeneme t1_j132yr9 wrote

In the "Three Body Problem" series, the author describes vividly, what that would look like! Good read, highly recommend it.


rootofallworlds t1_j11w364 wrote


Page 178. Reports a tolerance of 16 g with immersion in water.

Page 8. Mentions 24 g.

The limit is from the air filled lungs. Studies in mice show triple the g tolerance and for a much longer time period if the lungs are emptied of air. I believe they hooked the mice up to a heart lung machine; doing that for humans purely to achieve higher g tolerance would almost surely not be considered worth the risk.

Liquid breathing is discussed in the second paper I linked but I don't think it has been demonstrated in active humans. It's been used for medical treatment but that's in the context of stuff like inducing therapeutic hypothermia. Anyway the liquid used is much denser than water so that's a problem for the g tolerance idea.


Jagid3 t1_j0zyfxl wrote

You would need to fill every air filled cavity in their body with the fluid and then they would be fine.

If you missed any they would become compressed and could possibly cause damage depending on how big they are and where they are.

Of course they would need to be able to survive in the fluid. Any normal fluids we have these days would certainly kill them.


cheeze_whiz_shampoo OP t1_j0zziel wrote

I was imagining filling the lungs with an oxygenated fluid. My brain was just hopping around and I realized I couldnt even imagine what would happen to someone suspended in liquid and undergoing acceleration.


Origin_of_Mind t1_j117dk8 wrote

Suspending an organism in a density matched fluid could be used to increase the survivable acceleration, but the gain is limited, because different constituents of the body have different density. For example, fat is 0.9 g/cm^(3), while cortical bone is 1.9 g/cm^(3). If the average density were matched by the fluid, the internal stresses due to differences in density of individual parts will remain unchanged, and these differences are of the same order of magnitude as the difference between air and body density.

So, yes, it would work, but the payoff is not too great.


Game_Minds t1_j12pvb7 wrote

To paraphrase something someone said elsewhere in here, it would allow humans to achieve ludicrous relative accelerations compared to normal human expectations, but that would still be extremely negligible acceleration compared with trying to achieve relativistic speeds

And as a third person said, the cost of transporting and manufacturing this fluid would probably significantly outweigh any gains


Origin_of_Mind t1_j12yeoh wrote

Human ability to withstand acceleration is not a limiting factor in long range space travel.

With the astronauts experiencing just the ordinary 1g of acceleration all the time they would be able to get to anywhere in the visible universe and get back to Earth in just 100 years (from the point of view of the astronauts themselves.)

But even accelerating the rocket at 1g for more than a few tens of minutes is already beyond our present technology.


Game_Minds t1_j14d8b6 wrote

I think the idea of the extreme accelerations is that you could use something like a nuclear bomb to clear the first few stages of accelerating and then your onboard fuel supply doesn't have to work as hard, but I think there are more reasons than the G forces why that wouldn't work either. We don't have the tech for much more than 1g of sustained thrust anyway, like you said. These kinds of thought experiments are the very definition of speculative lol


zyiadem t1_j118dq9 wrote

Our diaphragms are not made to move liquids, and such experiments that have explored this required a Specalized ventilator to avoid permanent damage to the body.


Game_Minds t1_j12py78 wrote

And we all know being hooked up to a specialized ventilator for extremely long periods is perfectly healthy and fine


labroid t1_j10wl8x wrote

I don't think this helps, since at high accelerations the pressure gradient in the water will probably kill you. (See longer answer below)


MidnightAtHighSpeed t1_j115azp wrote

Why would the pressure crush you? There's not really anything compressible in the human body besides air pockets, is there? Isn't the biggest problem with high-pressure diving breathing gas?


labroid t1_j116lvp wrote

The pressure gradient is the problem. If you were 'vertical' (head in direction of acceleration) the pressure on your toes and legs at 100G would be, let's see: 62.4 lbs/ft^2 / 144 in^2/ft * 100G * 6 ft = 260 PSI while your head would be 62.4/144100.5 = 21 PSI. So basically your legs will be squeezed into your chest and head. Not good. (Sorry for the stupid Imperial units...)


labroid t1_j1173bm wrote

To add more fun, if your foot is about a foot long and about 3 inches wide and a couple tall, you have about a square foot of skin, so there would be 144 * 260 psi = 37,440 lbs (or ~19 tons) of force squeezing the contents of your foot up your leg into your head like toothpaste.


mutandis57 t1_j133ukp wrote

Not to mention the blood still needs to circulate in your body. Trying to pump blood up to your head 0.5m above your heart at 100G is equivalent to your neck being 50m long. Your heart is not as strong as a giraffe's! Not gonna happen without some form of suspended animation.


mutandis57 t1_j1336dz wrote

This is why everyone thinking that water suspension will allow humongous accelerations is deadly mistaken. Being underwater at 1km IS NOT THE SAME as accelerating at 100G in a water bath. Not the same! At 1km you have 100atm of pressure on all sides. You can probably scuba dive at 1km just fine if you descend slowly and prebreathe the right gas mixture. Sitting in a 1m tall chair in a water tank at 100G, your head is at 1atm and your feet are at 100atm. This is different! You'll be crushed like a bug. Filling your lungs and other body cavities with water or perfluorocarbons or whatever will not help!

A water tank should instead be thought of as the most comfortable acceleration couch possible, that supports every part of your body with the highest softness possible and no pinch points. You'll also do best if you lie perfectly flat horizontally, instead of sitting or standing.


InTheEndEntropyWins t1_j10620x wrote

How would filling someone with fluid help?


annomandaris t1_j10lffz wrote

air compresses, so if you have air in your lungs, and accelerate, the lungs will collapse.

If there was fluid inside and outside the lungs, there would be no compression of the lungs, its like how a bottle can fall to the bottom of the ocean and not break.

But yea you have to get them all, fill the empty stomach and bowels, nasal cavaties, lungs,


Tanagriel t1_j113t53 wrote

In the fiction movie “The Abyss” from 1989, they use a breathable fluid as to endure the pressure at deep ocean. Quite well made scene in how they need to control the body symptoms of drowning when taking the fluid into the lungs. ✌️


--VoidHawk-- t1_j11no0m wrote

A similar solution has been described in a number of science fiction works.

My favorite solution was in the Hyperion cantos. Just let the meat get splattered by the massive accelerations used for travel, which then gets reconstructed once the passenger reaches their destination, thanks to a weird tech/bioform they discovered.


phred14 t1_j11puxg wrote

In "The Forever War", by Joe Haldeman, they used fluid compression to survive extreme acceleration in their starships. Most of the time was shirt-sleeves, but when they went into battle, it was "into the tank." He goes into a fair bit of gory detail about the discomforts of it. Frequently better science fiction authors will have some amount of research behind what they write, but I have no idea what research he might have done back in the mid-1970s when this was written.


slashdave t1_j11t701 wrote

In the UFO TV show, the aliens space suits were filled with blue liquid for breathing. This idea turned into a plot device later in the series (the aliens were very human looking, with a blue skin color only due to this liquid staining their skin).


5J7XM33IXN4XCQI6B2BB t1_j10yiyn wrote

No, because the force of acceleration is still transmitted through the outer surfaces of the body into the inner ones. Some kind of hydraulic fluid could help distribute the load, but ultimately this is an issue of compression and a pressure gradient forming.

A similar analogy is "magic armor" that is indestructible. If you wear the armor and hold a detonating nuclear bomb, the force of the blast will fling you away and all that will be left is a pile of goop inside the intact armor shell.


Sable-Keech t1_j124fcs wrote

It would help to a certain extent. In water, the acceleration force would translate directly to pressure such that if you accelerate at 1g you would feel the same amount of pressure as you would underwater on Earth.

So assuming you’re under a meter of water onboard the ship, you could easily go up to an acceleration of 40g since then the 1 meter depth would be equivalent to being at 40 meter depth on Earth which is an okay diving depth for recreational divers.


5J7XM33IXN4XCQI6B2BB t1_j13rtdq wrote

This is inaccurate.

The hydrostatic pressure gradient is 40x as extreme, so it's more like being submerged in something 40x as dense, not 40x deeper. For reference, molten lead is only 10x as dense as water.

The buoyant force is equal to your weight, to kee you stationary. No matter the depth, on earth, it's always about 200lb. Under 40g, it would be 8000lb.


Sable-Keech t1_j13sk1u wrote

Huh, I thought acceleration was so similar to gravity as to be virtually indistinguishable.


FlyHandler t1_j0zycjq wrote

If there were small fluctuations then yes, the fluid would slow the humans movement compared to if they were in an air-filled vessel. If the G-forces were over a prolonged time, the the humans would at some point depending on the size of the tank reach the external wall of the tank and then the liquid wouldn't help any more.

It's like if you were in a car crash and in stead of airbags you had filled the car with water. It wouldn't help much because there is not much water around you. But if you had several meters of water between yourself and a fixed boundary then you the water would slow you down before you hit anything other than water.

In outer space there isn't any turbulence since it is a vacuum, so I can't see much use there though. Also a fluid filled container would be very heavy which is a big problem to launch up into space.


labroid t1_j10xabm wrote

I respectfully disagree. If the tank were upright on the ground, and a person got in, they would float at a certain depth, as their buoyant force (which comes from water pressure which depends on gravity) matches the weight of the person (which is also dependent on gravity). When you accelerate, the person's weight goes up, and the water pressure (and bouyant force) go up exactly the same amount, so they would stay at the same depth. The problem is the water pressure gradient at high Gs would kill them.

Edit: Spelling


Game_Minds t1_j10yw5w wrote

I think this, they would actually float 'up' in the direction of acceleration if there was a difference in density between the person and the water, because as the water experiences differential pressure the body might be able to maintain internal pressures. However, when the lowest pressure at any point in the tank is above a lethal amount it would still kill you. So at mega high accelerations you aren't safe even in a very large but not infinite tank

Edit: I think this is right. The acceleration of hitting the ground only kills you because it causes forces to act across your body unevenly very rapidly, g forces from acceleration work the same way. So if you don't implode from the pressure, you're still experiencing an acceleration difference across different parts of your body inhomogeneously because the actual thing keeping you intact in the water is your skin. You would still die of g forces suspended in a liquid


labroid t1_j114f3e wrote

The person displaces their weight in water - that's the "on Earth" definition. The more general definition is they displace their mass. Their weight is m * a and the water they displace is rho * V * a (rho is density, V is volume, and a is acceleration). Where you float is therefore

m * a = rho * V * a

So you cancel the a on both sides and get

m = rho * V

(for fun note that rho * V is the mass of the water). So the general form of buoyancy is that you float where you displace your mass in water. Note this is completely independent on acceleration (g's). So the person will stay in the same position, unless compressed by the extra pressure of the pool, at which point they will "sink" (move in the direction of the acceleration vector). Of course getting compressed is the "then you die" part of the problem...


Game_Minds t1_j120vu3 wrote

The fluids in a person's body have different densities at a given pressure

It would be like putting you in a centrifuge, some of your parts like your bones would have different buoyancy than others and would experience pressure to separate out

It would be significantly less pressure than other forces acting on you and it might be too small to overcome the forces your body exerts


FlyHandler t1_j163n8y wrote

I think we need to make a small experiment to figure this out. I don't think it matters wether it's in space ore here on Earth.

Take e.g. a small plastic container and fill it with water so it's about the same density as water. Put it into a bucket of water. Rapidly push the bucket sideways to see if the bottle inside moves with the water or if it crashe sagainst the bucket wall.

Any volunteers?

If the container doesn't crash into the wall, the water provides no damping so the human would not benefit anything from being submerged.

If the container does crash into the wall the water would slow the human down but the human would hit the tank wall at some point if the acceleration lasts long enough.


labroid t1_j16938b wrote

Sideways isn't the same thing. Newton's first law says the water will slosh out and the object will try to stay still relative to 'the universe' as the glass moves. If you move the glass up, however, the buoyant forces will increase. If the object if floating, it will continue to do so.

So do your experiment accelerating upwards. Of course it will be a mess when you stop :-)


cheeze_whiz_shampoo OP t1_j0zzy9t wrote

So people would be forced 'back' against the wall of the tank but just at a slower rate compared to an air filled environment?


mfb- t1_j101hsw wrote

You can use a liquid that matches the average human density. Water is pretty close already.

Where would you need a very large acceleration? 1 g is no problem, 2 g is likely acceptable for a long time, 3-5 g is okay for the time needed to reach Earth orbit.


FlyHandler t1_j109tbu wrote

Yup, basically. And when they are pressed against the wall then the person will be subject to the same G force as the tanks movement.


Jagid3 t1_j112cnj wrote

But the force is not relevant. Unless their body shatters at some point, they are virtually the same density as the rest of the water.

By your logic, no part of the spacecraft could survive any part of the journey, because it's touching itself and the engine.


smithysmithens2112 t1_j0zyfb9 wrote

No I don’t think so. If anything, it would just make it more costly to accelerate that much more weight. It’s the acceleration that matters, not the surrounding medium. No matter if I’m in air or jello, I’m still accelerating at the same rate. The only thing that can help is not accelerating as much.


Away_Conversation_94 t1_j121e26 wrote

I think even if you are breathing perfluorocarbon the limit is set by the different densities of the human tissue that could cause you something like an aortic dissection, but I have no idea how many G's can provoke that in this conditions.

I always wondered what would happen if someone jump from the roof inside a sealed watertank. I volunteer to hold someone's beer.


newinternetidentity t1_j12lbz7 wrote

You mean what would happen if they were to fall from a rooftop height while being submerged in a liquid layer coming from a surrounding tank-like encapsulation? It's an interesting question. Most probably, they still get injured if they don't just die when the fall comes from a sufficiently high rooftop. Just hypothesizing


coyote-1 t1_j11v34b wrote

Ummm…. heavy G acceleration is not necessary. Acceleration at levels we’ve already demonstrated an easy ability to handle gets you up to high velocities quite sufficiently. If you could accelerate constantly at 1g, you’d reach Mars in something like two days. At 5g acceleration, which is something humans can handle without too much stress, it would take roughly two months to reach light speed.


schmal t1_j12rz1d wrote

If I'm on the Rossy and Inaros shows up... Heavy G acceleration becomes very necessary. IMO.


WazWaz t1_j13gt4s wrote

Indeed, the only reason we use high G on liftoff is because any time we waste getting to orbit costs us 1G negative the whole time.

Note that Special Relativity means you won't reach light speed, though it will seem as if you blow right past it as your rapidity increases.


SweetActionJack t1_j125g24 wrote

I forget exactly where I read it, maybe in the Rama book series by Arthur C. Clark, this concept is utilized to protect the human astronauts when an alien ship begins massive acceleration. The aliens place the humans in tanks filled with a very dense liquid. The humans float laying down on the very surface of the liquid, and this allows the ship to accelerate at speeds which would normally kill a human. I’ve always wondered if this would actually work.


S0litaire t1_j13btip wrote

Buzz Aldrin's book "Encounter with Tiber" they get around the high G load by effectively plugging both ends.

An expanding style foam in the rear ( supporting the empty digestive system) and a breathable liquid to support the lungs.

They are then if I remember correctly, they sit in fluid filled pod.


GuruBuckaroo t1_j11ad4l wrote

There was an interesting exploration of this topic in Robert Forward's "Dragon's Egg", which envisioned a manned visit to the orbit of a neutron star. In this scenario, the crew was kept in nearly-centralized spheres filled with water and kept in the center of their pods by piezo-electric waves compensating for the high forces. This was only for transition from extremely high orbit to a lower parking orbit, centered between 6 fast-spinning "compensator masses" that theoretically prevented them from being torn apart by the tidal forces of the star. Quite a fascinating read. I have no idea how good any of the science is, but still entertaining. It's sequel, Starquake, continues the story.


recitegod t1_j1290rg wrote

If you take an egg, seal it in water inside a can, drop it from 100FT, nothing will break. If you put me inside a sealed tank, fill it with perfluorocarbon , accelerate against a wall at 100 mph. I still die. my brain went through the deceleration inside my skull. None of my bones would be shattered though. When you drive a 2 sec car or catapulted from an aircraft carrier, your body can still feel that "something" isn't quite right inside your skull even though you don't have any nerves there.


JesseB342 t1_j13cozk wrote

What you’re describing is called a ‘gravity couch’ and the concept has been around for a long time. While liquid isn’t compressible, the air inside your lungs is so you still need a way around that. As others have said you could use super oxygenated liquid to breathe. It’s been around since the 60’s I think and was used to some success in deep sea diving to avoid bends but using it ALWAYS resulted in the divers developing a pneumonia afterwards so not really worth it.


JonJackjon t1_j10tdgs wrote

No data here but....

What would happen is:

  1. as the tank accelerated your body would rush to the "bottom" of the tank
  2. Once you hit the bottom your body will accelerate the same as if you strapped in a typical space craft seat. Only you will be up against the tank wall. Not nearly as supportive.

labroid t1_j10xs7f wrote

Don't think so. Position in the tank is determined by the person's buoyancy and the water pressure gradient, and both of those are determined by gravity on earth, and acceleration in space. The person should stay at the same position


JonJackjon t1_j11vspz wrote

Think so. The OP specified "submerged" so the buoyancy will only determine where in the tank they are.

But no matter how you look at it, if the tank and water accelerates, the body will accelerate as well. So there will be no reduction in the stress resulting from acceleration.

If you don't believe this simply take a glass of water, put something in the water. Accelerate the glass with the water (upwards). The item in the water will feel the same acceleration as the glass.


labroid t1_j11xcll wrote

Agreed. Body won't hit the bottom of the tank, but the acceleration on the body will be identical. Being submerged avoids impact concentrated pressure on contact points (as you'd get from being in a seat) but acceleration on the body as a whole is identical.


JonJackjon t1_j12j1yd wrote

What forces would stop the body from hitting the bottom of the tank?

If you held a penny midway in a glass of water, the released the penny, gravity ( a similar force) would cause the penny to drop to the bottom.


labroid t1_j13odyh wrote

I assumed the OC meant the person was floating in the water, so buoyant forces keep it at the top. When you accelerate, the person's weight increases, but the buoyant force increases exactly the same amount, so everything would stay where it was.

If it were a penny, then you are right - it would already be on the bottom and would stay there.


Game_Minds t1_j110ouz wrote

Only if the persons density changed at the same rate as the water. Our body would be pushing outwards to maintain a lower internal pressure, making us more buoyant

Makes me think of another question, would the pressure change or the marginally increased g forces kill you first in this experiment

Edit: Another hilarious question... would the water heat up? Increasing pressure rapidly in an incompressible liquid produces heating, see: refrigeration

Would you scald to death, die of a stroke when your gas-free blood still pools in the back of your skull, or die of internal pressure gradient because your cells stop working and burst


JonJackjon t1_j11waim wrote

>Edit: Another hilarious question... would the water heat up? Increasing pressure rapidly in an incompressible liquid produces heating, see: refrigeration

No, refrigeration depends on the change of state liquid <--> gas of the refrigerant.

I agree a gas will become hotter when compressed. Simply because the "heat" will exist in a smaller volume.


Game_Minds t1_j121hdb wrote

I did way too much thinking about this today lol

It would be a super small amount of heating unless the acceleration was relativistic, so negligible

The separation effect of the acceleration (centrifuge forces) would also be pretty small at all but very high accelerations

And the amount of damage caused by the acceleration would be a lot smaller without any gas at all-- BUT gas is a byproduct of a large number of our body processes like oxygen uptake, AND that acceleration would still have a meaningful effect on your body's different systems. You would still die of a stroke at pretty small accelerations

Edit: also good call! I totally forgot about the latent heat of vaporization thing being the primary factor


WazWaz t1_j13h1w8 wrote

Why? There's very little reason for high acceleration in deep space. Unless you're fighting your way out of a gravity well or slingshotting close to a planet, 10 minutes at 6G is no more useful than 1 hour at 1G.


CaptainDadJoke t1_j13h2lj wrote

yes, but its so much worse :D so while being encased in fluids does help, as some posters have mentioned, the stopping point is that your lungs will collapse under enough pressure. We can get around this factor with oxygen saturated liquids. Essentially you have to breath in this liquid that our bodies can actually pull oxygen from and dump carbon dioxide into, and you can withstand even heavier G forces. Apparently the innitial process for getting this stuff into your lungs is terrifying and stressful, once you're in the liquid you just have to kind of push in and out to breath.


Kantrh t1_j13ixja wrote

A 1986 study said you would need to use liquid breathing at around 20g's


Kronzypantz t1_j1dgpbe wrote

There is an otherwise kind of bland sci-fi series written by Jack Campbell where the first alien species humans encounter is aquatic, so they take advantage of their water filled ships to pull off more g’s than vessels filled with humans and air could safely attempt.