What are the effects of Buoyancy & G-force combined in this scenario?

[some bloke]

TL;DR Summary

How much differently does a body which is floating in a fluid medium experience G-force, if at all?

Hi all,

I am trying to get it straight in my head how the interactions would work with a person (or accelerometer, for simplicity) suspended in a fluid, which is itself in a capsule which is then accelerated. Let's say with a rocket, on a linear track, to avoid any circular motion dynamics.

First scenario - neutral buoyancy. the body in the chamber has the same density as the water in the chamber, so floats freely. The capsule is then accelerated at a sufficient rate for the net acceleration (combination of gravity and acceleration) to equal 2g.

f=ma, so with "a" being doubled, the force on the water would double, as would the force on the body.

= -

	=	buoyant force
	=	fluid density
	=	acceleration due to gravity
	=	fluid volume

so the force due to buoyancy will also double, as this also contains gravity. With fluid being incompressible, the density remains the same, as does the volume.

At this point, I have established that the body remains neutrally buoyant during the acceleration, which for a person would feel akin to weightlessness.

What would a person experience? Would they feel the acceleration, or would they feel only an increase in pressure as the water transfers the force it is experiencing onto them?

My thought on the subject of g-force is that it is not an acceleration which causes injury per se, but the structure of a person and the way in which they experience it - in a car, it's not decelerating rapidly which causes damage when you crash, but the fact that different parts of you do so at different times - your nose accelerates into your face when it collides with the dashboard, which then attempts to accelerate your skull, which tries to accelerate your brain. This difference is what causes your brain to slam into your skull when you stop very quickly. Blood pressure & oxygen starvation aside, is this about accurate?

I think part of my inspiration for this train of thought comes from the helium balloon in a car experiment, where it moves forwards as you accelerate instead of backward. Logically the reverse is true, and if a helium balloon is in a car accident, it won't experience the same g-force as everything else.

Any help you can give on understanding this will be welcomed. It's all purely from a curiosity point of view, I'm not trying to achieve anything except knowledge!

 

[A.T.]

Summary:: How much differently does a body which is floating in a fluid medium experience G-force, if at all?

From an earlier thread:

Here some data for mice:

http://www.esa.int/gsp/ACT/doc/MAD/pub/ACT-RPR-MAD-2007-SuperAstronaut.pdf

...when their lungs are emptied from air, the maximum acceleration reaches 3800 Gx for more than 15 minutes without any physical impairment.

Bottom line: If you fill the lungs with liquid too and the acceleration changes slowly then it can go pretty high. But it won't necessarily protect you against impacts, where the acceleration is not only high but also changes quickly, which can generate shock-waves in the fluid.

 

[some bloke]

Thanks for the link!

Wow, that's a lot of G's. I'm glad that I had it about right in my head!

For a practical application, I wonder if this could be applied to astronauts to allow a launch system which fires their ship out like a gun? Effectively a linear accelerator running up a mountain to accelerate to beyond escape velocity. Get the ship up high before firing the rockets and save a whole load of fuel, weight, and waste (dropping boosters).

If (hypothetically) we build a railgun up the side of mount Chimbaroza (highest point from center of earth) from its base, I worked out the astronauts would need to be accelerated at 10544G's up the ramp to achieve escape velocity... Something tells me this would still result in red paste Though that's 3x what the mice survived and for significantly less time... ...0.44 of a second, to be precise!

Interestingly if you built it vertically (using the mountain to support it, I guess. I'd use a different one, as it's a volcano!) then it comes in at just 2557G of acceleration, so according to the study, completely survivable! still only 0.44 of a second as well, which may be the killer!

 

[DrStupid]

then it comes in at just 2557G of acceleration, so according to the study, completely survivable!

If the astronaut is a mouse.

 

[A.T.]

If (hypothetically) we build a railgun up the side of mount Chimbaroza (highest point from center of earth) from its base, I worked out the astronauts would need to be accelerated at 10544G's up the ramp to achieve escape velocity...

Going though the atmosphere at that speed, would quickly dissipate the energy, even if the spaceship could survive it.

Though that's 3x what the mice survived and for significantly less time... ...0.44 of a second, to be precise!

"Less time" becomes a problem, if it means that acceleration changes quickly. Then you can get pressure waves in the liquid.

 

[anorlunda]

You just need to recruit tardigrade astronauts.

 

[Chestermiller]

Who says that the body is going to accelerate with the same acceleration as the fluid? The fluid is going to have to exert a viscous drag force on the body to get it to accelerate. So this is more than a fluid statics problem.

 

[jbriggs444]

Who says that the body is going to accelerate with the same acceleration as the fluid? The fluid is going to have to exert a viscous drag force on the body to get it to accelerate. So this is more than a fluid statics problem.

My understanding of the approach is that we have a kind of "g suit" which amounts to an immersed astronaut with all available air-filled cavities re-filled with a breathable fluid whose specific gravity is [ideally] a bit higher than 1.

The acceleration of the astronaut is then caused by the pressure gradient in the fluid as it is accelerated vertically by its container. That is to say, the astronaut is accelerated vertically by buoyancy.

 

[Chestermiller]

My understanding of the approach is that we have a kind of "g suit" which amounts to an immersed astronaut with all available air-filled cavities re-filled with a breathable fluid whose specific gravity is [ideally] a bit higher than 1.

The acceleration of the astronaut is then caused by the pressure gradient in the fluid as it is accelerated vertically by its container. That is to say, the astronaut is accelerated vertically by buoyancy.

After further consideration, I see what is being said, and withdraw my objection.

 

[DrStupid]

The acceleration of the astronaut is then caused by the pressure gradient in the fluid as it is accelerated vertically by its container. That is to say, the astronaut is accelerated vertically by buoyancy.

Yes, that's the basic idea. However, that doesn't work within the body because the density isn't homogeneous. The bones have an everage density of about 1.9 g/cm³ and everything else is around 1 g/cm³. With an acceleration of "just" 100 g and a total mass of 10 kg for the skeleton that results in a force of 4.6 kN in addition to buoyancy (corresponding to the weight of 470 kg). That sounds quite unpleasant.