# Naked Science Forum

## General Science => General Science => Topic started by: emanuel on 29/10/2008 10:55:52

Title: How many G-forces could you withstand submerged in a water tank?
Post by: emanuel on 29/10/2008 10:55:52
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...
Title: How many G-forces could you withstand submerged in a water tank?
Post by: graham.d on 29/10/2008 13:44:25
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.
Title: How many G-forces could you withstand submerged in a water tank?
Post by: emanuel on 29/10/2008 18:35:54
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).
Title: How many G-forces could you withstand submerged in a water tank?
Post by: lyner on 29/10/2008 22:30:26
The 'G Suit' worn by fighter pilots helps them in the same sort of way and they don't have to get wet.
Title: How many G-forces could you withstand submerged in a water tank?
Post by: LeeE on 30/10/2008 15:12:06
'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.
Title: How many G-forces could you withstand submerged in a water tank?
Post by: graham.d on 30/10/2008 16:05:36
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.
Title: How many G-forces could you withstand submerged in a water tank?
Post by: lyner on 30/10/2008 16:45:24
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.
Title: How many G-forces could you withstand submerged in a water tank?
Post by: emanuel on 30/10/2008 16:58:30
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").
Title: How many G-forces could you withstand submerged in a water tank?
Post by: graham.d on 30/10/2008 17:28:56
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.
Title: How many G-forces could you withstand submerged in a water tank?
Post by: LeeE on 31/10/2008 00:18:05
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.
Title: How many G-forces could you withstand submerged in a water tank?
Post by: emanuel on 03/11/2008 17:32:21
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...  :-/
Title: How many G-forces could you withstand submerged in a water tank?
Post by: Bored chemist on 03/11/2008 19:17:00
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.
Title: How many G-forces could you withstand submerged in a water tank?
Post by: samdan87 on 05/11/2008 17:47:41
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.
Title: How many G-forces could you withstand submerged in a water tank?
Post by: samdan87 on 05/11/2008 18:04:26
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.

Title: How many G-forces could you withstand submerged in a water tank?
Post by: Bored chemist on 05/11/2008 18:18:12
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?
Title: How many G-forces could you withstand submerged in a water tank?
Post by: emanuel on 05/11/2008 18:51:25
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? :-)
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: WildRose on 02/03/2017 10:32:33
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...
I know this is an older topic that hasn't had a response in sometime but here goes.

I was considering this subject and decided to google it and came across the topic.

There seem to be a log of intelligent replies so I thought I'd offer what I hope you consider is an intelligent question.

What if instead of a tank the human is simply encased in a rigid suit similar to futuristic space suits or deep dive suits?

Fill it with water and pressurize it to further reduce blood flow limitations, red out and hopefully blackout?
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: syhprum on 02/03/2017 19:58:17
I am sure a lot of research took place in German concentration camps and American prisons but is it likely that multi million dollar fighter planes will indulge in the type of combat we saw in WWII more likely they will launch missiles from a distance as the aircraft are to expensive to risk
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: WildRose on 03/03/2017 04:01:14
I am sure a lot of research took place in German concentration camps and American prisons but is it likely that multi million dollar fighter planes will indulge in the type of combat we saw in WWII more likely they will launch missiles from a distance as the aircraft are to expensive to risk
Multi million dollar fighters have been engaging in dogfights since the Vietnam Era.  Our fifth generation fighters are specifically tested for their ability to perform both in the dog fighting role as well in emergency avoidance maneuvers to shake missiles.

They are capable of aerobatics that no aircraft of the previous generations could even hope to achieve and subject our pilots to G forces far beyond what early generation fighters did.

My question wasn't necessarily related to potential dog fights and pilot survivability but that's definitely one application where it could be lifesaving.

I was specifically thinking in terms of space travel and high G acceleration and turns which as we reach further into space are going to be a big part of the equation.

I was just hoping some of the guys with the Medicine and Physiology understanding demonstrated in this thread were still around or that some new minds with similar knowledge would check in.

We're not far from manned missions to Mars where high g acceleration and maneuvers could greatly reduce the time of flight from Earth to Mars and back.
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: alancalverd on 04/03/2017 08:49:16
The problem is that the human (or adult rat) body is not homogeneous, unlike an early fetus. Sustained acceleration redistributes fluids within and around the rigid bits, and this results in hypoxia, with the possibility of longterm disability. The inability to move during a high-g period is unimportant as you can pre-program the machine to fly the desired course.

So if you want to launch humans into space from a cannon, send up some ova, sperm and an artifical placenta, and mix them when they arrive at their destination. We know the individual bits can survive for umpteen years in a cryostat, and it only takes about five years  on arrival to generate an animal capable of independent life.  Amoebae and complex invertebrates would be a better choice as they are born with independent capability.
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: CliffordK on 12/03/2017 22:15:18
That is interesting about liquid breathing.  So, can an ordinary fish withstand higher G-Forces than a human?

It might be possible to put a person on a heart/lung bypass machine, then fill the lungs with water or some other liquid.  No doubt it would generate a strong cough reaction, so one would probably have to heavily sedate the person.

One might be able to artificially change the blood pressure too.

I'm sure there is still an acceleration limit, as nothing is completely homogeneous for density.
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: WildRose on 13/03/2017 09:15:27
That is interesting about liquid breathing.  So, can an ordinary fish withstand higher G-Forces than a human?

It might be possible to put a person on a heart/lung bypass machine, then fill the lungs with water or some other liquid.  No doubt it would generate a strong cough reaction, so one would probably have to heavily sedate the person.

One might be able to artificially change the blood pressure too.

I'm sure there is still an acceleration limit, as nothing is completely homogeneous for density.
They've actually been working on liquid breathing since the fifties.

Reaserch has produced some success with lower mammals and very premature babies but they have yet to come up with a fluid capable of the necessary exchange rates of oxygen and CO2 to support an adult human more than briefly and adult lungs then react badly to being flooded when the fluid is removed.

As you can imagine it's also a very controversial field of experimentation with very few people even being willing to participate.

Fortunately science and tech have both advanced to the point that most of what's left to be worked out can probably be done via computer simulations once an acceptable fluid can be identified.

It just seemed to me that immersion plus pressurization should increase resistance to G Forces even more than immersion alone since it would help (or should help) to prevent organs from tearing loose due to those G's.

We could conceivably intubate pilots and crews long enough to get them through launch/acceleration phases to keep the lungs working even without them being able to fully inflate due to the internally pressurized suits.  As long as you have the proper balance of gasses circulating through the lungs exchange will happen with or without inflation/deflation.

That would also be much less invasive than a heart lung machine which might not be able to function properly anyhow at high G's.
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: Colin2B on 13/03/2017 10:14:45
Reaserch has produced some success with lower mammals and very premature babies but they have yet to come up with a fluid capable of the necessary exchange rates of oxygen and CO2 to support an adult human more than briefly and adult lungs then react badly to being flooded when the fluid is removed.
Perfluorocarbon seems to be used with some success. Reports I've read indicate one problem is higher viscosity than air which means breathing is hard work and rib stress fractures have been reported.
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: CliffordK on 14/03/2017 01:41:41
Perfluorocarbon seems to be used with some success. Reports I've read indicate one problem is higher viscosity than air which means breathing is hard work and rib stress fractures have been reported.
It is quite possible that with high G-Forces, one wouldn't be able to expand and contract the rib cage anyway.  One could, of course, aid breathing with a pump, perhaps driven by natural nerve/electrical impulses to the diaphragm.

How would extreme G-Forces affect the heart?  Blood vessels?  Could the shape of the major heart valves change enough to cause problems?

Looking at deep SCUBA dives, the body seems to tolerate the pressures reasonably well, other than correcting for breathing (and various breathing and gas related issues).  Perhaps that would be another thing one could try.

So, with the tank idea, what would happen if one pressurized it to say 50 ATMs while accelerating?  Would that improve the ability for breathing and gas exchange?
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: WildRose on 16/03/2017 07:55:17
Perfluorocarbon seems to be used with some success. Reports I've read indicate one problem is higher viscosity than air which means breathing is hard work and rib stress fractures have been reported.
It is quite possible that with high G-Forces, one wouldn't be able to expand and contract the rib cage anyway.  One could, of course, aid breathing with a pump, perhaps driven by natural nerve/electrical impulses to the diaphragm.

How would extreme G-Forces affect the heart?  Blood vessels?  Could the shape of the major heart valves change enough to cause problems?

Looking at deep SCUBA dives, the body seems to tolerate the pressures reasonably well, other than correcting for breathing (and various breathing and gas related issues).  Perhaps that would be another thing one could try.

So, with the tank idea, what would happen if one pressurized it to say 50 ATMs while accelerating?  Would that improve the ability for breathing and gas exchange?
You're on the same track I am.  Presumably just like with saturation diving if they stay under pressure long enough there will be a decompression period necessary after such long exposures.

I was thinking more in terms of something like a rigid dive suit for pressurization rather than flooding say the entire cabin of a plane or space vessel.

More particularly I'm thinking of long periods at high G acceleration which would be necessary to reach the speeds necessary for exploration in deep space or beyond Mars in our own solar system.

The distances become so vast that the need to accelerate to at least near light speed to reduce travel time is going to be necessary for manned missions.
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: Colin2B on 16/03/2017 17:10:26

Presumably just like with saturation diving if they stay under pressure long enough there will be a decompression period necessary after such long exposures.
As you probably know one of the limitations for scuba diving is nitrogen which saturates body tissues which then comes out of solution on ascent to form damaging bubbles. Also at depths below 40m in sea water nitrogen becomes a narcotic causing hallucinations and risk of death due to disorientation. Perfluorocarbon avoids nitrogen narcosis as it replaces nitrogen as the carrier of O2 and CO2, this also means there should be no need for extended decompression other than to equalise pressure in body cavities.
Unfortunately at partial pressures of >0.45ata O2 becomes toxic, although it might require exposures of many hours to a few days, but for pO2 > 1.6ata brain toxicity can occur within minutes to hours and this occurs at a depth of 66m in sea water which is a pressure of 7.6ata with a breathing gas containing 21% O2. If I remember correctly the toxicity is due to the compressed O2 molecules now being so close together that they collide with each other forming oxygen radicals which do the actual damage. Fluids are relatively incompressible so the O2 molecules should not be forced closer together, but I'm not sure whether this is true when the O2 is in tissue subject to the higher pressure
The other problem might be air in various body cavities e.g. stomach and gut, this gets compressed and could cause surrounding tissue to stretch. It's certainly an issue if the pressure doesn't equalise slowly on ascent. :-[

Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: jeffreyH on 16/03/2017 19:11:14
Probably a better way to get decent acceleration is to get into an elliptic orbit. Accelerate into and out of perihelion each time round until the desired momentum is obtained. I have no idea if this is feasible BTW. Since you feel no acceleration due to gravity you limit exposure to harmful effects. If it is at all possible to carry out.
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: WildRose on 29/03/2017 13:31:10

Presumably just like with saturation diving if they stay under pressure long enough there will be a decompression period necessary after such long exposures.
As you probably know one of the limitations for scuba diving is nitrogen which saturates body tissues which then comes out of solution on ascent to form damaging bubbles. Also at depths below 40m in sea water nitrogen becomes a narcotic causing hallucinations and risk of death due to disorientation. Perfluorocarbon avoids nitrogen narcosis as it replaces nitrogen as the carrier of O2 and CO2, this also means there should be no need for extended decompression other than to equalise pressure in body cavities.
Unfortunately at partial pressures of >0.45ata O2 becomes toxic, although it might require exposures of many hours to a few days, but for pO2 > 1.6ata brain toxicity can occur within minutes to hours and this occurs at a depth of 66m in sea water which is a pressure of 7.6ata with a breathing gas containing 21% O2. If I remember correctly the toxicity is due to the compressed O2 molecules now being so close together that they collide with each other forming oxygen radicals which do the actual damage. Fluids are relatively incompressible so the O2 molecules should not be forced closer together, but I'm not sure whether this is true when the O2 is in tissue subject to the higher pressure
The other problem might be air in various body cavities e.g. stomach and gut, this gets compressed and could cause surrounding tissue to stretch. It's certainly an issue if the pressure doesn't equalise slowly on ascent. :-[
Good thoughts.  For deep dives though particularly those requiring many hours at depth the work around is to us a mixture of Oxygen, Nitrogen, and Helium.  Used in the right mix and divers can remain at work hundreds of feet below the surface particularly if they have a capsue, or modern version of the old diving bell.   This is done pretty much on a daily basis around the world on offshore rigs and military divers.

We start here with the following: If one atmosphere equals about 14.6 pounds per square inch pressure, and the pressure increases 1 atmosphere for every 10 meters of depth. How many atmospheres are forcing the nitrogen into the blood stream at 30 meters (about 100 feet) and at 75 meters (about 250 feet)?

S0 using my proposed 250' dive  14.6g (Pressure of water force against the body) For units here I'll say as the diver goes down he's adding 1atm.

Therefore upon reaching the 250m depth we have 14.6*250'=We arrive at pretty much the max pressure the human can tolerate for any length of time and still function absent one of the hard suits made for the deeper depths and provide the wearer wit protection against the pressures as they go deeper.

This is where I come to my question.   Propose that using such a suit and pressuring it from the inside as Gforces build upon acceration then providing the astronauts the ability to survive the pressure.  The fluid could be water or water combind with some soluables to help reduce the direct pressure on the wearer hiving them the ability to suvive the kinds of burns necessary to reach our neighbors as we keep reaching further out into space.

Realistically we need to build a ship that is capable of approaching near light speed or the trips would have to be either generational or at least one way with everyone knowing that even if they reach their objectives at the site they are sent to, they would most likely never live to return to earth essentially just dying of old age on the trip.

To achieve that kind of speed it's going to require a rocket to just continually accelerate slowly up to whatever "cruising speed" is set.

There is another probable benefit to the heavy suits filled with fluid is that in the event of an emergency such as a small meteoroid turning up in their path requiring quick maneuvers to avoid it, the crew are well dressed to deal with the lateral turns, twists and even dives or acceleration as the fluid will just contain you in your pressure bubble one benefit of which is that as it builds pressure, it "shrinks" you from the outside in and in doing so reduce or eliminate organ tearing loose which can quickly lead to death.

Free floating in a pressurized "flooded" cabin I think posses more risk than rewards due to the expense and difficulty in attempting to make sure everything is dry and tight.

Thanks for chiming back in. Hard to find a crowd in which you can have such a conversation at all.

Best to all. WR
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: PmbPhy on 08/04/2017 11:58:32
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...

There is a limit to the usefulness of accelerating in a water tank. The pressure can't be distributed evenly throughout the brain since its encased in a hard shell, i.e. the skull. So that brain will become squashed in the direction parallel to the acceleration. It's also harder for the heart to pump blood in in the direction parallel to the acceleration. My guess is that under extreme acceleration this may become a problem.
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: jeffreyH on 08/04/2017 13:14:58
This then raises a question. In free-fall a human undergoes acceleration but does not feel the force. Therefore both brain and blood flow would only become a problem due to extreme tidal forces. We can only overcome the problem if we could apply a force globally throughout an object rather than locally at a surface.
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: Colin2B on 08/04/2017 14:15:25
We can only overcome the problem if we could apply a force globally throughout an object rather than locally at a surface.
Yes, that's the problem, you can accelerate a rigid seat pushing the astronaut but brain, blood etc will try to stay at their previous speed.
Title: Re: How many G-forces could you withstand submerged in a water tank?
Post by: PmbPhy on 10/04/2017 08:59:50
This then raises a question. In free-fall a human undergoes acceleration but does not feel the force. Therefore both brain and blood flow would only become a problem due to extreme tidal forces. We can only overcome the problem if we could apply a force globally throughout an object rather than locally at a surface.
Indeed, for a body in free-fall which is not subject to tidal forces or a body whose extent in spacetime is local , i.e. so small that the tidal forces can be ignored then there's no problem. If there's no tidal forces present, e.g. in a uniform gravitational field, then the body experience nothing different than being at rest in an inertial frame of reference in flat spacetime.

However the only force that acts globally, i.e. causes all parts of the body at the same rate relative to an inertial frame, is the gravitational force. Exceptions to this rule are when the force field is uniform and charge per unit mass is uniform throughout the body.