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Author Topic: How many G-forces could you withstand submerged in a water tank?  (Read 13951 times)

emanuel

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Humans cannot, in general withstand high acceleration very well. The main problem is that the blood tends accumulate in the direction opposite to that of the acceleration. This is for instance the limiting factor when designing fighter jets, in that there is no use making one that can turn so sharply that the pilot would black out due to the acceleration.

However, it a person is submerged in a tank of water (or some other liquid with a density close to that of human tissue) accelerating the whole tank would not put a lot of strain of the person inside, since the force would be evenly distributed as a pressure gradient in the liquid. The blood would no longer tend accumulate in the direction opposite to that of the acceleration.

So, how many “G-forces” (how many times the Earth’s gravitational acceleration) could a person in a water tank withstand?

Ten times Earth gravity? A hundred times Earth gravity? Could you envisage shooting someone into space using a cannon, as Jules Verne proposed for going to the moon?


The limiting factor is likely to be forces on the air we have in our lungs, nose, ears etc. - that like air bubbles will want to rise up through the pressure gradient. If the gradient is too strong they could rip through human tissue. But the question is how high the acceleration would have to be for this (or some other damage) to happen...

graham.d

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The problem with the blood, I believe, is to do with the ability of the heart to pump the blood to vital organs (like the brain for example) against the force of gravity. In the other direction there can be a problem with bursting vessels. I think that the tank of water helps in preventing pooling of the blood by equalising the external pressures to those being experienced inside your arteries and veins. This reduces the problem to one where you could consider the blood flow to be in rigid tubes, although I suspect it is much more complicated that this simple model would suggest. If there is as much blood above the heart as there is below then the heart does no more work than it would do normally in just overcoming the friction of the flow round the system.

I think the blood pressure problem is only one factor though. Each organ in your body has a certain mass but is held in place by other pieces of connective tissue. Acceleration increases the force trying to tear apart this connective tissue by a factor proportional to the number of g's of acceleration. This is usually not survivable beyond a certain amount and is often a cause of death in road accidents when there is a sudden stopping of the vehicle.

So I'm not sure of the answer but my guess is that the suspension in water may make some significant difference but probably not a very big one. Short duration forces, that stress the ability of organs not to get ripped off, can probably go to 80g so the water suspension system would possibly allow something between 15g and 80g. My guess would be at the lower end if it were to be sustained. We discussed the big gun idea in another recent thread and that has a number of other problems.
« Last Edit: 29/10/2008 13:46:52 by graham.d »

emanuel

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I should have said I meant sustained g-forces, as in at least one minute. NASA has found (according to http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980223621_1998381731.pdf ) that untrained subjects can withstand at least 10g sustained "forward" acceleration (upwards when lying on your back).
« Last Edit: 30/10/2008 12:05:07 by emanuel »

lyner

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The 'G Suit' worn by fighter pilots helps them in the same sort of way and they don't have to get wet.

LeeE

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'G' suite worn by military pilots help stop the pilot from blacking out at high Gs (by applying pressure to the thigh and calf muscles to restrict the amount of blood that can be forced there by High-G manuevours) but their functionality is still severely impaired.  In addition to Red-Out tunnel vision, limiting their field of view, it's not really possible to move your arms and legs very much and this lead to the HOTAS (Hands On Throttle And Stick) side-joystick controllers, where the arms rest on a support and only the fingers have to move (The initial side-stick controllers had no movement at all and just relied upon force sensors but later versions had about 1 mm of movement).  However, I've never heard if similar non-moving foot controllers (pedals) were used, although this would be logical.

In any event though, even if the pilot can doesn't black out, they will be constrained in what they can see and what they can do.

graham.d

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emanuel, my comparison with forces of short duration was to determine the strength of connective tissue holding vital organs in position and suitably connected. Assuming survival of 80g of relatively short duration, I was guessing this would be an upper limit for what could be withstood over a long duration. The lower limit of 15g is what has been withstood by well trained and fit persons for longish periods in a centrifuge. So if the water bath helps to maintain blood flow then sustained periods of between 15g and 80g should be possible, but, as I said, I think probably nearer the lower end.

lyner

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I have a feeling that the scenario around which this question is based is probably not one which would be practical for other reasons. I don't think that much is to be gained by launching much faster than is done at present. The period when the launcher is going very slowly off the launch pad is very inefficient, I know, but that could be solved by launching from a flying platform.
Most equipment used in space can be very lightweight and flimsy. If you were to launch it under excessive G force it would need to be more rugged and heavier.

emanuel

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my comparison with forces of short duration was to determine the strength of connective tissue holding vital organs in position and suitably connected.

As I see it, deforming forces will only be present on parts of body that have a different density from that of water (or other liquid that fills the tank). The force is in fact proportional to the density difference.

Bones are heavier than water, but they can sustain quite large forces compared to their weight and will probably not be the first thing that breaks.

The lungs are likely to be most vulnerable since all of the air in a lung will be at the same pressure, while there will be a strong pressure gradient in the surrounding tissue induced by the acceleration. The higher pressure on the "down" (opposite acceleration) side of the lung might eventually burst the fine capillaries in the air sacs (alveoli).

I don't know how high acceleration you could withstand before this would happen, but my hunch is still that it would be many times higher than 10 times earth gravity (10 "g-forces").

graham.d

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Yes, you are right I believe. This is quite an interesting idea. You would have to carefully match the density of the suspension fluid to the body to avoid being slammed into the sides of the containing vessel, which would definately ruin things. I suspect one problem is that we are compressible so our density would increase as the accleration occurred. I expect most of the compressibility is in the lungs though not exclusively so. As you say, the lungs may be the most vulnerable part.

LeeE

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Apart from gravity, the other accelerative forces we experience need to act upon the surface of the body and then progresses through the body.  Even though I believe that full immersion does help in practice, the risk of internal damage is still a real limiting factor.  Actually, I think water has been supplanted by gels as the optimum solution.

emanuel

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I guess you could make the experiment of putting some animal "fragile but of homogeneous density", e.g. a jelly-fish (submerged in a sea water) in a very fast centrifuge. It would presumably survive far over 100 "g-forces", since it has no low or high density parts that would be subjected to deforming forces.

I'd probably not volunteer to repeat the experiment on myself however, to see at how many g:s the lungs walls start breaking down...  :-/

Bored chemist

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I read somewher that someone (who must have had an odd outlook on life) put pregnant rats in artillery shells and fired them, then retrieved the bits. The mother rats died but the pups, "born" by ceasarian survived. The short term accelerations were calculated as thousands or tens of thousands times g.
This may well be a contender for the least useful experiment ever.

samdan87

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In G-suits, pressure bladders (balloons) are inflated in the pants sleeves of these suits to squeeze blood back into the head region. When the pilot is pulling "positive G", ie pulling "up", Centripetal Acceleration causes a force acting towards the floor of the aircraft. You can see a similar thing happening when you whirl a bucket of water overhead fast enough. The (wrongly termed) "centrifugal force" is keeping the water in the bucket. Now imagine the bucket is the pilot and the water is his blood. The blood tends to pool in his legs, causing "GLoC", "gravity induced loss of concsiousness" or "blackouts". If he is pushing DOWN on his stick (ie diving) he will experience the opposite effect, where blood is pressed into his brain causing (indirectly) a "red-out".


In fact, dragonflies use a similar principle to maintain fluid flow when in flight (they take abrupt turns which results in significant G loads). They have fluid bladders which counteract the force by squeezing back. A g suit called the "libelle" suit was invented doing something similar. Not complete immersion in water, but having water bladders running around the suit. The pressure in the lower parts of the bladder caused by the aircraft's turning squeezed bloodflow back into the upper body. The advantage was that unlike conventional pneumatic g suits, the system was totally mechanical (just the bladder) and didn't need to be hooked up to the aircraft. This also negated any response delay (the pressure acted where needed instantly because it was reacting to the same g forces). Also, while pneumatic gsuits often leave pilots with bruises on their legs, this suit didn't. And pilots wearing the suit were able to achieve higher sustained g turns (up to 12, sustained. F16s safely do a maximum of 9) than conventional pneumatic g suits.

samdan87

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Some research has been done into this area. Complete immersion into a water chamber evenly distributes the accelerating force throughout the body. The main problem would be the air cavities in the body, namely its respiratory system. As mentioned in an earlier post, the pressure gradients can cause the surrounding structures to collapse.

However there is a solution- liquid breathing. An oxygen-rich carrier liquid (usually a perfluocarbon) was used as the chemical exchange medium in place of air. It's not as outrageous as it sounds. Partial Liquid Ventillation (PLV) has been tried on humans for medical reasons, and full LV has been tried on mice.

If liquid breathing is perfected, very high accelerations will be possible. Fully immersed in liquid and with no air cavities to collapse,organ damage from deformation is minimal up to very high accelerations due to the complete immersion in a similarly dense liquid. Where the body tries to deform, the water pushes back. Little damage results- there is little pressure differential.


Bored chemist

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Liquid breathing will help a lot, but there are still air spaces- the ears for example.
The unborn rats had no airspaces so they survived.
Unless you can do the same for the pilots you won't be able to achieve the best possible effect.
Perhaps the next question is how large a set of G forces does anyone ever need to survive?

emanuel

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So:

If a liquid-immersed liquid-breathing person could withstand let's say 640g while being accelerated in a 10 kilometer long cannon (e.g. built inside near-vertical tunnel in a high mountain) he would reach escape velocity 11.2km/s and go into orbit (at least if one disregards atmospheric drag)

Any volunteers? :-)
« Last Edit: 06/11/2008 22:05:59 by emanuel »

 

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