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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.

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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

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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.

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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.

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someonefun420 t1_j12vr0h wrote

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

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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.

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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.

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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

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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.

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sg3niner t1_j12qr8p wrote

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

Instant stop would squish everything.

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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.

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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.

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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.

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Chilliwhack t1_j150odf wrote

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

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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.

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ozspook t1_j13bxwy wrote

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

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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.

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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!

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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.

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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.

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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.

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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.

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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.

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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?

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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.

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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.

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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.

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Molkin t1_j12xns5 wrote

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

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_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.

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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.

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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.

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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.

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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)

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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)

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HelloMcFly t1_j13el2g wrote

Hospitals still occasionally use this stuff.

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Lee2026 t1_j13fl5c wrote

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

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Scott_Abrams t1_j169zsw wrote

https://science.howstuffworks.com/explosion-land-water1.htm

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).

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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?

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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.

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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.

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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?

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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.

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-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.

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Ubermidget2 t1_j137j1o wrote

> fluids are difficult to compress

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

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