So, normal humans can tolerate up to 4g of acceleration.

If I had the tech to freeze and thaw people, would being frozen aid in tolerating high-g forces, or would the brittleness of your frozen body lower your tolerance?

Maths is not required, but encouraged.

What I would like to understand is how being frozen would affect your tolerance to G-Force.

  • $\begingroup$ Need definition of "sustained" for this, because 5Gs for more than a few minutes isn't going to be tolerable. NASA's values indicate that sustained tolerance stops (for physiological reasons) at ~4Gs, because of blood pooling and similar problems). Maybe in the context of fighter pilots in G-suits? $\endgroup$ – jdunlop Aug 7 '17 at 19:55
  • $\begingroup$ Changed the question to just 4Gs. I'm thinking interplanetary/interstellar travel. But even asleep and whatnot, humans will die under accelerations that make the travel time feasible. So I was wondering if Cryogenic Freezing would allow more acceleration with less damage to the body. $\endgroup$ – Douglas Aug 7 '17 at 20:10
  • $\begingroup$ The travel time and definition of "sustained" depend on how many more gs you can tolerate when frozen. If it's 5 extra g then it's a fair bit shorter than is possible now. 20g and we're getting into "Regular jaunts to Jupiter" territory. $\endgroup$ – Douglas Aug 7 '17 at 20:12
  • $\begingroup$ It would seem like having the ability to freeze and revive your travellers would have benefits well beyond increased acceleration tolerances, then, since if physiological aging stops while frozen, you can take your sweet time getting wherever you are going. $\endgroup$ – jdunlop Aug 7 '17 at 20:17
  • $\begingroup$ The problem with that is that time on earth would pass. My setting involves a lot of trading, and thriving markets need goods quickly. $\endgroup$ – Douglas Aug 7 '17 at 20:22

Something to keep in mind about ice: small pieces of it, or thin sheets of it, are very brittle. Large, thick pieces of it, however, are quite strong. Ice at freezing temperature has the hardness of lead, and around -70 C, it is as hard as titanium. This is good news for you, as most cryogenic options can lower object temperatures as far as -80 C

I'm operating under the assumption that the frozen human in question will contain no empty pockets of gas and be up to 75% solid, frozen H2O.

Bearing that in mind, the human body is more flexible and inherently resistant to damage because of that. Much like a flexing bridge, the human body can absorb and distribute force rather than deflect it completely. So I looked into the breaking strength of solid Ice.

This study contains a helpful graphic on page 3 of the various breaking strengths of ice found in various parts of the world. This ice naturally forms with a lot of impurities. I feel this is a fair comparison, as humans that are cryogenically frozen are mostly made of water but contain up to 25% other compounds.

The table gives quartile strengths from 200 to 800 lb/in^2. A good median across all sites on the table is 500 lb/in^2, or 87500 pascals.

In order to compare this to the human examples you give above, I converted the max Gs experienced to pascals as well. In order to do this, I used an average human surface area of 1.7 m^2. After that, I took your initial max Gs multiplied by the average human mass to get our force applied (G Force = Gs *(9.81 * 70)).

Lastly, using the assumption that the forward-facing body of a human is roughly 1/3rd the total surface area, I solved the following for a given human form undergoing the G force you listed:

Max Pressure = G Force / Area

Max pressure (forward/backward) = 4*686.7 / 0.51 = 5385.9 pa

To solve for a feet-first acceleration:

Max pressure (up/down) = 4*686.7 / 0.272 = 10098.52 pa

Here we see that the max pressure of a thawed human is much less than that of ice. A fully-frozen human will benefit greatly from that added structural integrity, and be more than capable of surviving much greater forces. Using this breaking strength for Ice, up to 8 times the acceleration, or as much as 32 Gs (313.92 m/s^2).

  • $\begingroup$ The problem with water is it expands slightly upon freezing. Would the cells of the body survive this? $\endgroup$ – Aric Aug 8 '17 at 14:52
  • $\begingroup$ That is my handwave. I've already done an awful lot to ensure realism is maintained in my setting. I think advances in cryogenics are something that is small enough to gloss over without to much detail. After all, if we authors, writers, and GMs could figure it out, we wouldn't need the scientists! $\endgroup$ – Douglas Aug 9 '17 at 10:34
  • $\begingroup$ I would assume properly frozen humans would be very similar to Pykrete: solid, fibrous material reinforcing a block of ice. If we can somehow freeze people without destroying the cells, we can become really, really tough. $\endgroup$ – Zhehao Chen Sep 14 '17 at 23:14

I would say that part of the question would be whether you could freeze individuals in or filled with some sort of non-expanding liquid. If the lungs and major cavities are supported, brittleness ought not to be a factor, just the actual strength of the tissues and the surrounding material.

As an explanation - your hypothetical traveler is frozen in their acceleration couch. It's suspended from a shock absorber (magnetic field, liquid, or mechanical) which prevents sudden, very high-G events (impacts) from shattering them. However, sustained G-force acts as a compressing force, pushing the body up against whichever surface is orthoganal to the direction of acceleration and "behind" the traveler. If their lungs, as the largest empty cavity in the body, are filled with (chilly) air, and the force applied is sufficient to substantially reduce their volume, the question would be whether their brittle flesh is then sufficient to withstand the force of compression - which is the second part of your question. If, however, the lungs were flooded with a biologically-inert liquid, itself frozen along with the traveler, then the force isn't applied to the structure of the body, but just to the individual tissues, and the concern is the tissues' crush strengths, rather than their strength as structural components.

In that situation, while I couldn't give you an actual figure without knowledge of the properties of frozen flesh I do not have, I would imagine you could exceed normal acceleration tolerances substantially, since most of the normal problems associated with high-G have to do with blood moving about the body, or bad things happening to the vitreous humours of the eye or compression of the brain.

The problem would be finding a fluid with which you could flood the body's easily-accessible cavities that wouldn't have negative effects a) when doing the flooding, and b) when defrosting your traveler.

  • 1
    $\begingroup$ So I need to knock them out, fill them with biologically inert liquid, then freeze them. Cracking. $\endgroup$ – Douglas Aug 7 '17 at 20:18
  • $\begingroup$ Luckily, my setting has a lot of advanced tech, so I can handwave the mechanism for filling their bodies up a little. Say "quantum tunneling" or some such. $\endgroup$ – Douglas Aug 7 '17 at 20:20
  • $\begingroup$ I would stay away from "quantum tunneling", since it doesn't mean anything to do with large quantities of matter. If you've got the tech to kill someone and then revive them, though, you could delay the freezing step by a minute or so and fill them the old fashioned way. (That way it wouldn't be uncomfortable for the traveller, aside from the 'cessation of life' bit.) $\endgroup$ – jdunlop Aug 7 '17 at 20:25
  • $\begingroup$ Problem with that is that one of the main themes of the campaign I'm running is that "you can't beat death". $\endgroup$ – Douglas Aug 7 '17 at 20:26
  • $\begingroup$ When I said "tunneling" I meant "teleportation". That part doesn't need to make sense. It's just detail I'll not be able to deal with. $\endgroup$ – Douglas Aug 7 '17 at 20:27

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