# Does Liquid Immersion protect against G forces?

I'm under the impression that if an object is immersed in water it can be protected from the effects of Gravity - or in other terms appear to be in a 0G enviroment.

Typical examples of this include the whales incredible size and diving. It's also used in the Libelle G Suit to prevent blackout (with a water filled suit).

In particular my understanding is that if a human was completely immersed a liquid of similar density, even to the extent that they were liquid breathing that they would be able to withstand considerable G forces. For example, 40G. I mentioned this as a comment to another question and was told my understanding is wrong because:

suspending humans in liquids does not magically remove acceleration force, it might merely reduce it for the length of the suspension tube and the time it would take to reach its end ... you'd be "glued" to the tail end surface

As such my question can be broken down into these parts:

• Can Liquid Suspension protect against G forces?
• What are the limitations (time, gforce)?
• Assuming we have the right liquid, are there any barriers to use?
• Even if we fill the air spaces within the human, different tissues have different densities. And so if you are thinking huge accelerations, then shear forces due to the density differences could reach fatal levels. And even considering a hypothetical creature of all the same density as the fluid, acceleration would cause a pressure gradient in the direction of acceleration. The creature would deform to the same extent as the fluid according to its elasticity. But hard to say whether such deformation would lead to the creature's demise. It's hypothetical in the first place! – docscience Oct 13 '14 at 2:46
• I'm well aware of said consideration. My understanding though is that we can still achieve considerable protection... and this is a viable idea in theory. Of course, I'm asking here for a reason. Don't make me go test it :P – NPSF3000 Oct 13 '14 at 3:38
• The tolerance will probably scale with the density difference. In air this is just the density of the body (air is negligible). For a body in saline (to match mean density), this would come down to differences in tissue (note by @docscience). If muscle is $1060 kg/m^3$ and fat is $900 kg/m^3$ (wiki: adipose tissue), I'd say you'd be looking at about a factor of six improvement, from 5g to 30g (per wiki on g-forces. Could be even higher if you're limited by blood flow and blood is closer to the mean body density than the fat/muscle difference. – user3823992 Oct 13 '14 at 4:22
• @Tildal wave... then suggest edit - which is why I linked you here. – NPSF3000 Oct 13 '14 at 22:17

Yes, it does protect against G forces because it spreads the pressure on the support surfaces of the body evenly. For example, and interesting article here

ESA (European Space Agency) have been studying this.

The article is so short, I'm going to copy it here. Link to source at the bottom

TLDR: 24G with air breathing, but suspended in liquid. 100+G with liquid breathing as well.

Is there an example design in nature for the perfect acceleration shield? In effect, there is, and it is the egg. We studied the coupling of water immersion with liquid breathing as a possible approach for the “perfect G suit”.

By completely immerging a man in a physiological water solution within a non expandable, rigid container, the increased fluid pressure developed within the cardiovascular system during acceleration is approximately balanced or even cancelled out by the gradient of pressure developed in the liquid tank outside the body. At the same time, water immersion increases tolerance to acceleration as the acceleration forces are equally distributed over the surface of the submerged body. This abruptly reduces the magnitude of localised forces and a homogenous hydrostatic response of the whole body is induced, with evident benefits for blood and lymphatic circulation. The limiting factor is the presence of air in the lungs. Once under acceleration, the immersed subject experiences an augment on external pressure, which will casue squeezing effects on his chest, until all the air present in his lungs is removed. This fact limits the applicability of the technique to a sustainable acceleration of 24 G.

In order to overcome the limit and reach the real potentials hided in water immersion, it is possible to fill the user’s lungs with a fluid. In this way there won’t be squeezing effects. The problem, then, is: how is it possible to breath with liquid filled lungs? The answer came from the field of clinical lung therapy. Here, the use of perfluorocarbon for liquid ventilation was longer studied, demonstrating the feasability and safeness of the concept.

It is hard estimating an ultimate acceleration limit possible with this set-up, but it presumably can be higher than hundreds of G. The ACT is working to assess the application of liquid ventilation for water immersed astronauts, in order to identify the space requirements and to address future studies, designed to overcome current limits of the technique.

(emphasis mine)

https://www.esa.int/gsp/ACT/projects/liquid_ventilation.html

[The article cites some more detailed papers]