# Inertial dampers

## Main Question or Discussion Point

I would like to know what ways are there to get rid of the inertia problem when it comes to sci-fi concepts such as Iron Man and spaceships. I'm actually more interested in how could it be possible for a spaceship to accelerate to a very high G without causing its inhabitants a mass concussion. And I guess the answer could be applied to the Iron Man question as well.

More specifically, if that is possible to begin with, is the solution limited to some field generator or could it be possible to say, create your spaceship or suit out of some different metal or whatever have you ?

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DaveC426913
Gold Member
I would like to know what ways are there to get rid of the inertia problem when it comes to sci-fi concepts such as Iron Man and spaceships. I'm actually more interested in how could it be possible for a spaceship to accelerate to a very high G without causing its inhabitants a mass concussion. And I guess the answer could be applied to the Iron Man question as well.

More specifically, if that is possible to begin with, is the solution limited to some field generator or could it be possible to say, create your spaceship or suit out of some different metal or whatever have you ?
Not quite the solution you were hoping for, but it should work:

Immerse the crew in a fluid with a density similar to water.

Take it a step further and saturate the fluid with oxygen, so the crew can have their cavities filled with it and still breathe.

Some sci-fi stories have gone so far as to fill the bridge with fluid, so crew are free to move about. Added bonus: uplift dolphins, and they can serve as crew too.

If Iron Man's suit (and cavities) were filled with oxygen-saturated fluid, he'd be pretty well protected, so long as the hard shell didn't literally cave in when struck.

This is amazing video that proves the point elegantly.

The gold starts at 3:20.

tnich
Homework Helper
Not quite the solution you were hoping for, but it should work:

Immerse the crew in a fluid with a density similar to water.

Take it a step further and saturate the fluid with oxygen, so the crew can have their cavities filled with it and still breathe.

Some sci-fi stories have gone so far as to fill the bridge with fluid, so crew are free to move about. Added bonus: uplift dolphins, and they can serve as crew too.

If Iron Man's suit (and cavities) were filled with oxygen-saturated fluid, he'd be pretty well protected, so long as the hard shell didn't literally cave in when struck.
I was thinking of that solution, too, but realized that it doesn't solve a fundamental problem. Jet pilots black out at around 9g acceleration. This does not happen because they slam into a bulkhead; they are strapped into their seats. The problem is that their blood tends to pool at the lowest point of their bodies. The blood vessels there expand to accommodate the increased volume of blood. This means that the blood drains out of their brains, or pools at the backs of their heads. Surrounding the body with water might prevent that. I think the idea is that immersing the body in fluid (including body cavities normally filled with air - like lungs) will keep external pressure on the blood vessels and keep them from expanding. That kind of works, unless you want the blood to circulate throughout the body.

Imagine the heart as a pump pushing blood uphill. Under the acceleration of gravity, it can generate enough pressure get the blood through your arteries up to your brain. Under much higher acceleration, it could not. Pressure $P=\rho ah$ where $\rho$ is density of the fluid, $a$ is acceleration, and $h$ is height. At some value of $a$, the downward pressure of the column of blood reaching up to the brain is higher than the upward pressure the heart can generate, and the heart cannot move the blood. By the same token the muscular action that creates pressure to push the blood from your legs up through your veins back to your heart could not do that under acceleration many times that of gravity.

DaveC426913
Gold Member
I was thinking of that solution, too, but realized that it doesn't solve a fundamental problem. Jet pilots black out at around 9g acceleration. This does not happen because they slam into a bulkhead; they are strapped into their seats. The problem is that their blood tends to pool at the lowest point of their bodies. The blood vessels there expand to accommodate the increased volume of blood. This means that the blood drains out of their brains, or pools at the backs of their heads. Surrounding the body with water might prevent that. I think the idea is that immersing the body in fluid (including body cavities normally filled with air - like lungs) will keep external pressure on the blood vessels and keep them from expanding. That kind of works, unless you want the blood to circulate throughout the body.

Imagine the heart as a pump pushing blood uphill. Under the acceleration of gravity, it can generate enough pressure get the blood through your arteries up to your brain. Under much higher acceleration, it could not. Pressure $P=\rho ah$ where $\rho$ is density of the fluid, $a$ is acceleration, and $h$ is height. At some value of $a$, the downward pressure of the column of blood reaching up to the brain is higher than the upward pressure the heart can generate, and the heart cannot move the blood. By the same token the muscular action that creates pressure to push the blood from your legs up through your veins back to your heart could not do that under acceleration many times that of gravity.
This is only true if the vessel containing the fluid is allowed to deform. If it is not allowed to deform, then there is nothing preventing the heart from pumping blood around because there's no pressure differential. The fluid outside the blood vessel is pushing down just as much as the fluid inside the blood vessel, cancelling any pressure diff.

Think of a simple rigid, fluid-filled sphere. It will experience no internal pressure differentials under acceleration. There will be no circulation of fluid, since every part of it will experience the same forces.
This also means a pump pushing fluid around inside that rigid sphere will have no more trouble than if it were not accelerating at all.

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tnich
Homework Helper
This is only true if the vessel containing the fluid is allowed to deform. If it is not allowed to deform, then there is nothing preventing the heart from pumping blood around because there's no pressure differential. The fluid outside the blood vessel is pushing down just as much as the fluid inside the blood vessel, cancelling any pressure diff.

Think of a simple rigid, fluid-filled sphere. It will experience no internal pressure differentials under acceleration. There will be no circulation of fluid, since every part of it will experience the same forces.
This also means a pump pushing fluid around inside that rigid sphere will have no more trouble than if it were not accelerating at all.
Suppose you put that rigid fluid-filled sphere of radius $r$ on a very large planet with gravity $n$ times that of Earth. Regardless of conditions outside the sphere, inside the sphere gravity still acts. So it seems to me there has to be a pressure differential inside the sphere of $P=2\rho ngr$. The rigidity of the sphere would allow the fluid inside to exert a different pressure at the top than at the bottom of the sphere.

DaveC426913
Gold Member
Suppose you put that rigid fluid-filled sphere of radius $r$ on a very large planet with gravity $n$ times that of Earth. Regardless of conditions outside the sphere, inside the sphere gravity still acts. So it seems to me there has to be a pressure differential inside the sphere of $P=2\rho ngr$. The rigidity of the sphere would allow the fluid inside to exert a different pressure at the top than at the bottom of the sphere.
What would you expect to happen then?

Would you expect a certain mass of fluid near the bottom to spontaneously start moving upward (since it's going to go from high pressure to low pressure)?

That would set up a perpetual cycle. If that were true, we could extract useful energy from it simply by inserting a dynamo in the flow. You posit a perpetual motion device, extracting useful work out of gravity, despite being stationary.

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tnich
Homework Helper
What would you expect to happen then?

Would you expect a certain mass of fluid near the top to spontaneously start moving downward, while other parts move upward, setting up a perpetual cycle? If that were true, we could extract useful energy from it simply by inserting a dynamo in the flow.

You posit a perpetual motion device, extracting useful work out of gravity, despite being stationary.
How did this turn into a perpetual motion device? All I am saying is that gravity acts the same way inside a rigid sphere as it does outside. Gravity does not create perpetual motion outside, so why would it do so inside the sphere?

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DaveC426913
Gold Member
How did this turn into a perpetual motion device? All I am saying is that gravity acts the same way inside a rigid sphere as it does outside. Gravity does not create perpetual motion outside, so why would it do so inside the sphere?
There's no doubt that gravity still acts - the point is that it acts on all parts equally.

Why would fluid in the bulk of the body have any trouble moving about?

Water at-depth is not harder to move through than water near the surface.

I'm pretty sure that, if I took a 90 foot straw down to 100 feet depth, and held it vertically, I would not have any more trouble blowing water through it than if it were horizontal.

tnich
Homework Helper
There's no doubt that gravity still acts - the point is that it acts on all parts equally.

Why would fluid in the bulk of the body have any trouble moving about?

Water at-depth is not harder to move through than water near the surface.

I'm pretty sure that, if I took a 90 foot straw down to 100 feet depth, and held it vertically, I would not have any more trouble blowing water through it than if it were horizontal.
I agree, but try blowing water upward through a straw when you under very high acceleration.

DaveC426913
Gold Member
I agree, but try blowing water upward through a straw when you under very high acceleration.
Gravity/acceleration = same thing. (That's why you used the gravity analogy, isn't it?)

You are forgetting that the pressure that is in my mouth is also on the outside of my mouth (remember, I'm immersed in fluid - it is not just in the straw), which means I don't have to fight it. It's already compensated for.

tnich
Homework Helper
Gravity/acceleration = same thing. (That's why you used the gravity analogy, isn't it?)

You are forgetting that the pressure that is in my mouth is also on the outside of my mouth, which means I don't have to fight it. It's already compensated for.
If you are lying on your back pushing water upward through a straw, you are working against a pressure differential due to the length of the straw. $P=\rho ah$

DaveC426913
Gold Member
If you are lying on your back pushing water upward through a straw, you are working against a pressure differential due to the length of the straw. $P=\rho ah$
You are misusing the formula. It does not apply to you and the entire straw being immersed in the water. It applies to pushing the water above the mean water level - i.e. out of the water.

Or, to put it another way, a straw whose upper end is underwater has a head height (h) of zero. (Which is why my immersed pond pump does not care about h.)

Or, another way: set h to zero and solve for P.

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tnich
Homework Helper
You are misusing the formula. It does not apply to you and the entire straw being immersed in the water. It applies to pushing the water above the mean water level - i.e. out of the water.

Or, to put it another way, a straw whose upper end is underwater has a head height (h) of zero. (Which is why my immersed pond pump does not care about h.)

Or, another way: set h to zero and solve for P.
The human body is not a uniformly dense fluid. Fatty tissue is less dense than muscle. Muscle is less dense than bone. How would they be affected by high acceleration?

DaveC426913
Gold Member
The human body is not a uniformly dense fluid. Fatty tissue is less dense than muscle. Muscle is less dense than bone. How would they be affected by high acceleration?
Proportional to the difference in density.

Fats are about 90%ish as dense as water.
Air is about 0.12% as dense as water (that's a decimal in there).
So fat is about 705 times denser than air.

That means the effect of acceleration on fatty tissue will be about 705 times less than on air.
Or, put another way. Fat will be affected by acceleration about 1.1 times as much as the water surrounding it.

i.e. A couple of orders of magnitude difference.

Bone is 1.0 to 1.2 times denser than water. So, still, a couple of orders of magnitude closer to water than air.

A jump or fall from a height can put many Gs on the body, without causing damage. So, multiply the Gs of a normal jump by about 700, and that's how much you could take in a spaceship.

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tnich
Homework Helper
Proportional to the difference in density.

Fats are about 90%ish as dense as water.
Air is about 0.12% as dense as water (that's a decimal in there).
So fat is about 705 times denser than air.

That means the effect of acceleration on fatty tissue will be about 705 times less than on air.
Or, put another way. Fat will be affected by acceleration about 1.1 times as much as the water surrounding it.

i.e. A couple of orders of magnitude difference.

Bone is 1.0 to 1.2 times denser than water. So, still, a couple of orders of magnitude closer to water than air.
So I guess that would put a limit on the acceleration and/or the time under acceleration the body could stand before the connective tissue gave way. Still a pretty good solution.

DaveC426913
Gold Member
So I guess that would put a limit on the acceleration and/or the time under acceleration the body could stand before the connective tissue gave way. Still a pretty good solution.
Sure, but it's pretty big.
A good jump off a ladder can give you many Gs. Make that ladder 720 times higher, and you should survive it, without injury if you were encased in a fluid.

Should I take that the field/different material "solutions" are not on the table? It sounds rather odd. Is this problem that fundamental that it just can't be overcome? Faster than light travel for example at least has some solutions, speculative as they are.

On a side note, could a gas (not necessarily one we know of) replace this fluid? Also, in the Iron Man movies, his head seems pretty close to the walls of the suit. Is it not true at the amount of liquid between his face and the helmet needs to be increased proportionally to the amount of G's he/she is experiencing?

Should I take that the field/different material "solutions" are not on the table? It sounds rather odd. Is this problem that fundamental that it just can't be overcome? Faster than light travel for example at least has some solutions, speculative as they are.
You might "impale" the crew with tiny needles that would prevent movement of their internals. It might be tricky to enable heartbeat, eye movement etc. Submerging is probably better...
Sadly there's no material that would insulate against acceleration. You can invoke gravity plates though, that's pretty common in sci-fi although not possible with known physics.

On a side note, could a gas (not necessarily one we know of) replace this fluid? Also, in the Iron Man movies, his head seems pretty close to the walls of the suit. Is it not true at the amount of liquid between his face and the helmet needs to be increased proportionally to the amount of G's he/she is experiencing?
The layer of fluid can be as thin as you can make it, say under 1mm. You might have trouble speaking though, but I guess Iron Man can build a sensor to detect muscle tension in the throat and reconstruct speech from that.
I'm not aware of a gas as dense as water. I believe if a human were subjected to oxygen or nitrogen atmosphere as dense as water it would cause some damage to cells, although I'm not sure what exactly.
Nitrogen cannot be used for depths higher than 30 meters or so, because people feel drunk. In a recent video, Cody's Lab felt funny after inhaling xenon, which you'd normally expect to do nothing. https://en.wikipedia.org/wiki/Nitrogen_narcosis

Edit: In fact you don't need a fluid. If you can make the suit tight enough, it can support the body as well. Some company is trying to make space suits that work on this principle (saw it in the TV a few years ago). The suit would need to fill your nose, mouth and lungs though for high G.

DaveC426913
Gold Member
Should I take that the field/different material "solutions" are not on the table? It sounds rather odd. Is this problem that fundamental that it just can't be overcome? Faster than light travel for example at least has some solutions, speculative as they are.
What causes inertia is still a bit of a mystery.

On a side note, could a gas (not necessarily one we know of) replace this fluid? Also, in the Iron Man movies, his head seems pretty close to the walls of the suit. Is it not true at the amount of liquid between his face and the helmet needs to be increased proportionally to the amount of G's he/she is experiencing?
Remember,
1] what causes tissue damage isn't the acceleration, it's the distortion - as different parts accelerate at different rates, causing them to pull apart.
2] if the fluid is the same density as his head, then his head parts don't get distorted.

Edit: In fact you don't need a fluid. If you can make the suit tight enough, it can support the body as well.
Problem there is that the body naturally changes shape just to move normally. Looking at the simplest case of a rigid suit that exactly contours your body, you wouldn't be able to move a muscle. You wouldn't even be able to breathe.

I would like to know what ways are there to get rid of the inertia problem when it comes to sci-fi concepts such as Iron Man and spaceships.
Even if you use the solution above, what about his organs?

First to declaim that I am well awared of the fact that this is a physics forum. But I just want to say: If your problem is around Iron Man, then forget about it.The studio which created the character is the same studio that made a movie about a wizard(i.e, doctor strange) which its content will be a complete nonsense on this forum. So perhaps they don't care about the science behind it. They just want to be cool.

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russ_watters
Mentor
Even if you use the solution above, what about his organs?
My suspicion is that this problem of organs re-arranging has been under-stated. The human body has one large and several smaller air cavities inside, and as a result the strain of higher gravity can be significant. Your diaphram has to be able to move the organs in your abdomen out of the way in order to fill up your lungs, while your esophegas needs to stay open. I'd be surprised if we could go over 20g's this way.

Also, a person wouldn't be very functional in this state.

And also, there doesn't really seem to me to be a need for such fast accelerations, so I'm not sure I see much value. In a half hour at 1g, you'd exceed the fastest robotic spacecraft we've ever launched and in a week you'd fly past Saturn at 2% of the speed of light!

russ_watters
Mentor
On a side note, could a gas (not necessarily one we know of) replace this fluid?
Gases are fluids and what makes liquids better for this is that they are not compressible, so you don't have to pressurize them, but if you do they don't change their density under pressure. Adding an absurdly high pressure to the issue would just be another unnecessary layer of complexity.

russ_watters
Mentor
Should I take that the field/different material "solutions" are not on the table?
The term "field" as used in sci fi is practically gibberish/technobabble.
It sounds rather odd. Is this problem that fundamental that it just can't be overcome?
It would seem so.
Faster than light travel for example at least has some solutions, speculative as they are.
Not really. The pop-sci stuff you read about FTL travel and communications tends to leave-out or downplay the fine print that you still can't violate Relativity. There's not a lot of value in travleing to Alpha Centuari almost instantly if you have to sit on the launch pad in a bubble for 5 years before you leave. You may as well have traveled conventionally.

Also, my understanding is that the idea is purely mathrmatical; nobody knows how to actually do it.

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DaveC426913
Gold Member
Even if you use the solution above, what about his organs?

First to declaim that I am well awared of the fact that this is a physics forum. But I just want to say: If your problem is around Iron Man, then forget about it.The studio which created the character is the same studio that made a movie about a wizard(i.e, doctor strange) which its content will be a complete **** on this forum. So perhaps they don't care about the science behind it. They just want to be cool.
I think you may be missing the point of the discussion. It's not about the movie or the Ironman suit.

The point is whether one can, in principle, protect the body from high acceleration. An Ironman-like suit is nothing but an example for illustrative purposes.

Read the responses above, a number of which address the issue of form-fitting acceleration protection.