Falling out of an airplane in a tank of water
July 18, 2016 8:31 PM   Subscribe

If a person were in a super strong tank of water and then got tossed out of the airplane, would they die on impact (assuming that the tank was strong enough to not break when it hit the ground)? How would it work? I tried to Google this, but am not sure what exactly I am looking for.
posted by Literaryhero to Science & Nature (30 answers total) 4 users marked this as a favorite
I think the explosive shock would rupture a human being in multiple places, as well as the deceleration breaking things.

You might also try asking Randall Munroe: https://what-if.xkcd.com/
posted by nickggully at 8:40 PM on July 18, 2016 [2 favorites]

The way to not die on impact is to go from "falling" to "landed" without accelerating (well, decelerating, depending on your point of view) too hard. High acceleration == bad for squishy humans. One way to survive is to land on something "squishy" that can slow you down gently. A pile of cardboard boxes, for instance.

Unfortunately, water isn't squishy at all so I don't think it will have any protective effect, and your person is just as doomed as if they'd been dropped without the tank.
posted by BungaDunga at 8:43 PM on July 18, 2016 [10 favorites]

They would likely die on impact.

The salient thing that has to happen when you hit the ground is for your speed to go from whatever you were falling with to zero. That happening is not the thing that necessarily kills you; it's that large change of speed happening over a short time. So, you either need to have your falling speed be lower (i.e. parachute) or have the change in speed spread out over a larger time (e.g. a big air bag or net). The water may provide some extended time for changing speed, but water isn't very compressible, so it won't help much.
posted by Betelgeuse at 8:47 PM on July 18, 2016

You should expect the same result as if the person was in a super strong tank filled with concrete.

tldr; strawberry jam
posted by HiroProtagonist at 8:52 PM on July 18, 2016 [11 favorites]

Falling in a body of water would roughly approximate falling onto a body of water. I.e. strawberry jam, quoth HiroProtagonist above.
posted by so fucking future at 9:26 PM on July 18, 2016 [1 favorite]

The shock wave from the landing would travel through the water and kill you instantly, in the same way it kills the fish when an explosion goes off in the water. You need the deceleration process to take a long time upon landing, and the only solution would be an enormous bouncy castle.
posted by tillsbury at 9:26 PM on July 18, 2016 [4 favorites]

It seems to me (warning: I am not an expert and I've forgotten half my high-school physics) that buoyancy would be the main force working to save the person's life. The person would have to be buoyant enough in the fluid to counteract the impulse force of the tank hitting the ground. Even with enough buoyancy to not slap the bottom of the tank, they might suffer some internal trauma.

Of course, if the person is too buoyant (thought experiment: the tank is filled with sand or something) they will stop too quickly, just like hitting the ground without the tank. I suppose the key is to adjust until the person almost, but not quite, hits the bottom of the tank hard enough to cause injury.

For an interesting science-fiction version of something similar, see Haldeman's The Forever War. When a warship has to execute extreme-high-gee maneuvers, the crew are basically vacuum-sealed in coffins, with their abdominal cavities filled with a dense fluid to keep their internal organs from liquefying.
posted by neckro23 at 9:32 PM on July 18, 2016

Oh, to clarify, I didn't mean that the person was floating ON the water, I was thinking more like suspended IN the water. Like imagine a guy with neutral buoyancy in the middle of a giant water balloon. Still, I like tillsbury's explanation of dynamiting fish. That makes sense to me.
posted by Literaryhero at 9:55 PM on July 18, 2016

So I get what you're saying. It's not just the buoyancy, it's also that the water tank would stop, leaving you traveling through the water which has some drag, and maybe you'd slow to a stop in a tall enough tank. A couple of problems I see with that - relative to the water, you would first go from 0 to [some high speed through the water] in a very short time. It would be like belly-flopping onto the surface, I think. Also, the shock waves from when the tank hit the ground would be rapidly transmitted through the water like ripples on a pond and probably kill you.
posted by ctmf at 10:16 PM on July 18, 2016 [2 favorites]

I suppose the key is to adjust until the person almost, but not quite, hits the bottom of the tank hard enough to cause injury.

This is the crux of the reason why I can see how people think this would help. The idea that the person in the water would drift down through the water and decelerate gently is entirely flawed. Because the tank of water has just hit the floor, so the falling tank has suddenly stopped, so the water has suddenly stopped, so there are shock waves in the water travelling upwards that would kill the human during and at the same time as the person would be trying to float down through the water to slow themselves. It simply wouldn't help.

The nearest thing to helping is if you can remove the shock waves within the water by making the water static, in which case you are trying to drop a human from a great height into a body of water, which we already know only works from a certain height, because over a certain falling speed of the human the energy required to displace the water to allow the human to enter is greater than the human body can resist and things get broken.
posted by Brockles at 10:18 PM on July 18, 2016 [2 favorites]

Like imagine a guy with neutral buoyancy in the middle of a giant water balloon.

To absorb the impact, the water has to dissipate energy. There are two ways that this can happen in a fluid: it can compress and release energy through the work to compress the fluid or the fluid can displace, causing viscous drag.

Water does not compress in any sense that matters here. It will transmit a shock as well as any solid.

Since the tank is also sealed, water can't displace around the person. Normally if you fall into water, it takes a lot of energy to drive the water away from you. This is essentially the friction of moving water, called viscous drag. This can be quite effective. It takes a lot of force to keep moving through water compared to air. But the box is sealed and rigid. Unlike a baloon, it can't flex (which would also allow for some drag at least). In the box , the water has nowhere to go, so viscous drag isn't help our falling diver either.

So the water will very efficiently transmit the shock of impact through to the person inside, with almost no dampening. Unlike water, we contain voids and spaces that can compress, and that shock will do a lot of damage through the body. The person will die of that shockwave, as it causes something like a whole-body bruise. Strawberry jam time.
posted by bonehead at 10:51 PM on July 18, 2016 [4 favorites]

For the same reason too loud a noise can kill you - the stuff on the inside is traveling at a different velocity then the stuff on the outside for a brief but deadly period of time.
posted by ptm at 10:54 PM on July 18, 2016

To add insult to injury, the terminal velocity of a tankful of water is probably going to be a lot higher than that of a free-falling human. Which would make it even worse than falling out of the airplane without a tankful of water.
posted by mr vino at 11:08 PM on July 18, 2016 [1 favorite]

Your brain would slam against your skull extremely hard.

Assuming constant deceleration over the space of a meter, for a person falling at the terminal velocity of a skydiver, 200kph:

Your skull+brain would initially be moving at would moving at + 200 kph, decelerating to 0 kph in the space of a meter or so, which would take ~0.018 sec.

(d = v*t)
t = d/v = 1 meters / 200 kilometers/hour = 0.018 sec

Acceleration = Δvelocity / Δtime = (0 - 200 kilometers/hour) / 0.018 seconds
= 3086.4198 m / sec^2
= 314 g.

Note that 150g is a hard hit in football.

However, I'd guess most of the deceleration is over ... half a meter or so?
t = d/v = .5 meters / 200 kilometers/hour = 0.009 sec

Acceleration = Δvelocity / Δtime = (0 - 200 kilometers/hour) / 0.009 seconds
= 629 g.
posted by sebastienbailard at 11:43 PM on July 18, 2016

This video gives you a good idea of what happens to any air cavities in our body when a large shock wave goes through incompressible water.

Now, that being said, I wonder what would happen if you were using perfluorocarbons for liquid breathing (think of the movie "The Abyss"). Since we are mostly water, would the shock wave just pass through with no (or little) ill effects? Hmm...
posted by qwip at 2:32 AM on July 19, 2016

This is actually a case where, rather than having the tank be super strong, you actually want it to break as easily as possible. The explosion of water as the tank bursts is actually dissipating a lot of energy. The more the water splashes, the less your internal organs splash.

(No idea if this effect is sufficient to actually make this survivable, though)
posted by firechicago at 4:25 AM on July 19, 2016 [6 favorites]

Even if you were suspended in the water in the tank, where would you go once the tank hit the ground? There is nowhere for the water under you to go. You might as well just be hitting the ground. The deceleration would pulverize your internal organs and kill you.
posted by carter at 7:17 AM on July 19, 2016 [1 favorite]

The explosion of water as the tank bursts is actually dissipating a lot of energy. The more the water splashes, the less your internal organs splash.

Yeah, the main difficulty of this thought-experiment is determining where, exactly, all the energy from the impact will end up.

The shockwave is a good point. My initial thought was that there wouldn't be a shockwave -- such a shockwave is simply a non-uniform distribution of pressure, and in order to have high-pressure areas you need corresponding low-pressure ones, and where would that reduced pressure come from in a closed hydraulic system, since water is (more or less) incompressible? Then I realized that the person's body would be that negative-pressure area, since some (important!) parts of it are plenty compressible. Eek.

Would this be enough to kill you? The human body can withstand some pretty big pressure fluctuations (although this might cause brain injury, it'd be survivable). It looks like the maximum survivable pressure is about 3 PSI. I'm not sure how you'd calculate this -- the total energy generated by the impact divided by the surface area of the human body, perhaps?

So really, we're back to filling up your abdominal cavity with space goop. You'd have to protect the brain somehow too. It's not looking good for the person taking a budget vacation from the orbital colonies.
posted by neckro23 at 9:30 AM on July 19, 2016

Cavitation. Your inertia shoves you down (as much as your compressible body parts allow), then the water above you cavitates (creates a temporary vacuum), then, hating a vacuum the way it does, the water slams into you like a speeding locomotive. This probably happens right about the same time the shock wave under you meets your underside.

Jelly in a meatsac.
posted by mule98J at 9:56 AM on July 19, 2016 [1 favorite]

It would be neat to try this with an egg (in whatever liquid it would float in) and a high-speed camera at the impact point.
posted by ctmf at 1:21 PM on July 19, 2016

This is actually a case where, rather than having the tank be super strong, you actually want it to break as easily as possible. The explosion of water as the tank bursts is actually dissipating a lot of energy. The more the water splashes, the less your internal organs splash.

(No idea if this effect is sufficient to actually make this survivable, though)

Intriguing: wouldn't it necessarily be possible to hypothesize a case that has the perfect, um, breakability, such that for a given volume of water and height of fall, it would dissipate just the right amount of energy to perfectly cushion your landing?
posted by progosk at 1:42 PM on July 19, 2016

(And it would likely resemble a fairly sturdy balloon, more than any actual case, wouldn't it?)
posted by progosk at 1:44 PM on July 19, 2016

This is actually a thing! Since pressure in an incompressible fluid is evenly distributed, submerging a human in fluid with equal density to the human body (~water) can apparently keep a human functioning at 10G (an acceleration of ~100m/s^2), and presumably makes even higher stresses survivable. If you can fill all the internal cavities with that fluid, the survivable acceleration increases.

So with a suitably designed tank (that would deform/break to help dissipate the impact energy), I think this should be pretty survivable.
posted by russm at 1:05 AM on July 20, 2016 [2 favorites]

I love you, russm. That is the best best answer.
posted by Literaryhero at 11:29 PM on July 21, 2016

Well, not really. It sounds like the one that most fits what you want, but it is not the whole story. Because that answer/research refers only to sustained g, not impulse forces, which is the biggest issue when dropping a tank onto the floor. Yes, pressure in water is equal when the forces are static but your scenario doesn't use static forces until the box is already on the ground, and water is a very efficient transmitter of the shock waves and transfer of forces while it is establishing that equal pressure, which is bad news for the meat sack floating in it. There is a world of difference between 'the body can cope with sustained 10g' and 'the body can cope with an instantaneous force of 10g unprotected'. 10G directly applied to the body as a pulsed force is not the same as a gradual sustained g-loading at all. The build up of g in that test is gradual (rate of turn g in cars, aircraft and even space craft is far slower in application of force than a crash impulse).

The crash impulse is the biggest issue. The first half second of the impact is where the damage is done - in my work I have seen crash impulses of 20-30G from crashing a (deformable) car while strapped into a moulded and formed seat and had people survive without any damage and sometimes with a little whiplash. But everything is designed to reduce that initial crash impulse, just as much as it is in a road car. Everything has a bit of give and stretch and the driver is wearing a helmet with padding and a head restraint (HANS device) to reduce the effects of the crash impulse.

In your example, the crash impulse would be the part of the initial impact from the milli-second the edge of the box touches the ground and during it's initial deformation (think it through in slow motion) and whenever it failed or crushef. There is ABSOLUTELY NOTHING in your 'box and water' system to reduce the crash impulse at all - every deformation of the box (or lack of) is transmitted to the water and almost instantly straight to the human because of how water works. The impulse g would be far above a survivable level unless you made a large deformable structure part of the equation. So you'd need to make the box itself take the impact, because you can't have the crash impulse transmitted to the water, because that would kill your person inside. Once you have modified your container to do that, basically, it's not the water that is doing any good, it is the box.

So the only way to make the crash impulse survivable for the person in the water is to remove the crash impulse from getting TO the water. So if you don't do that, the person dies, and if you do, it's not the water that is helping, but the box design, in which case the box could be empty - the water becomes irrelevant.
posted by Brockles at 9:38 AM on July 22, 2016 [2 favorites]

Ok, I think that actually Brockles has the best answer, and one that is also easily understandable to the average idiot (me). I'm going to mark this one resolved. Thanks everyone!
posted by Literaryhero at 5:04 AM on July 23, 2016

The example that I thought of that would help explain it, is the depth charges used to hunt submarines - they only need to drop an explosive somewhere near a sub in the water to sink them, which shows a hugely capable method of shock wave transfer. The box hitting the floor would be akin to that kind of energy impact, with only water being between the person and the shock wave.
posted by Brockles at 8:30 AM on July 23, 2016 [1 favorite]

This might work slightly better with fat, as it is more compressible than water out to about 100 MPa or 14.5 thousand PSI.

There are two additional big differences between being in a tank vs dropping onto a body of water. One is that not knowing the time of impact could prevent tensing up which makes for a slight reduction in damage (may be mroe than offset by other issues). Two is that the stresses are more evenly applied to the whole body at once rather than an uneven application of force -- I think this would translate into bones in fewer pieces but maybe more stress on internal organs / brain.
posted by BrotherCaine at 1:18 PM on August 3, 2016

Ok, I just thought of this question again for some reason, and now I think you'd get squished like a bug. At the moment of impact, from the perspective of the tank, there's a force pushing the bottom up (the ground) and a force pushing the top down (momentum). That's a compressive force on the container. The water inside would resist compression, and there would be a massive water pressure spike all around your compressible body.
posted by ctmf at 11:16 AM on August 6, 2016 [1 favorite]

Further to Brockle's very comprehensive answer, the example linked to by russm is about attempting to "keep a human functioning at 10G"...i.e. maintaining sufficient alertness thoroughout in order to pilot an aircraft/spacecraft, which mainly involves some means of keeping oxygenated blood from being centrifuged away from the brain. That's quite different from the G-forces that are survivable without major injury, even if you black out momentarily. For example, the current US CPSC standard for bicycle helmets is that they must attenuate impact force upon the skull to no more than 300 G*.

*As the medical science regarding concussive brain injuries moves forward, the validity of this standard is subject to debate.
posted by wutangclan at 12:16 PM on October 8, 2016

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