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I've been looking into viable accelerations for minimally modified humans in an acceleration-gel-type chamber. Minimally modified means no permanent or significant changes to the human body. Examples that might be needed for high accelerations are filling a sinus cavity and eardrums with fluid.

For the purposes of this, the technology of the time has created a breathable, incompressible fluid with a density of X, where X can be any constant density.

Acceleration tolerance is strongly correlated to time at acceleration. For the purpose of this, 10 minutes of acceleration is desired. Bonus points if your answer includes estimates of how G-tolerance would change with time at that acceleration.

With those parameters in mind, I'm curious how fast a ship could linearly accelerate.

Initially, I considered pressure on the human body as the limiting factor, but after some research on divers, it seems like pressure probably won't be a limiting factor. The limiting factor of human divers is gas toxicity at high pressure, which might not exist in a fluid breathing medium.

I am concerned about differences in densities within the human body eventually leading to the failure of something, but I don't even know where to begin looking into that possibility.

There could be another, more obvious fail-point at high accelerations that I missed.

Related questions:

Just a brain accelerating (with significant changes)

Speculation of mice-acceleration in fluid and max human acceleration

  • This question gets quite close, but mice are significantly smaller than humans (duh), and everyone's favorite square-cubed law would be at play here. Also, I can't find the quoted parts of the study mentioned in the first answer.
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  • $\begingroup$ Immersing someone in a fluid or gel would help them survive higher pressures (e.g., diving underwater), but not really acceleration. Yes, such a fluid or gel would keep the lungs from collapsing - but you can't encapsulate the brain, heart, or any other vital organ. You can't keep blood from being forced to one side of arteries and veins. I doubt using a fluid or gel would improve human tolerance to acceleration more than a few percent - but I'm certainly not an expert. I could be wrong. But pressure ... (*Continued*) $\endgroup$
    – JBH
    Jun 8, 2023 at 2:43
  • $\begingroup$ ... affects the outside of the body first, so the fluid can ameliorate its effects. Acceleration affects everything at once (internal and external) just like gravity - and that can't be ameliorated. $\endgroup$
    – JBH
    Jun 8, 2023 at 2:43
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    $\begingroup$ Acceleration that affects everything (internal and external) does not stress the body; just like freefall in gravity, you will feel weightless. You could accelerate at a 1000Gs indefinitely in a gravity field and you wouldn't feel a thing (so long as the gradient wasn't too steep to cause undue tidal forces.) It's only when forces are externally applied and thus those forces have to be transferred through your body does acceleration get troublesome. $\endgroup$
    – Gene
    Jun 8, 2023 at 5:22
  • $\begingroup$ @Gene Your comment doesn't make sense in relation to the OP's question. Gravity is an externally applied force (ignoring the current debate as to whether or not gravity is a force). Can you show us an example of pilots experiencing a centrifuge that didn't feel a thing? Does your observation have any bearing on the issue of using fluids or a gel to moderate the effects of a very real acceleration that every astronaut feels and whether or not the use of such a fluid or gel would help cushion internal organs? $\endgroup$
    – JBH
    Jun 8, 2023 at 9:23
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    $\begingroup$ @JBH, I think the confusion is about how buoyancy and gravity would affect you while submerged in a fluid. In a thought experiment, a ball of blood that's submerged in a fluid of equivalent density would experience no net force, no matter how fast you accelerated. Unfortunately, matching the density of every tissue/fluid within the human body isn't possible, as they have different densities. I'm asking at what acceleration does the differences in density begin to matter within a human body. $\endgroup$ Jun 8, 2023 at 15:59

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I just found another question on the physics exchange asking the exact same thing here. Apparently the European Space Agency did some research on that and found that even simply emerging in water with air breathing can go up to 24g. With liquid breathing they just wrote that the limits are hard to estimate and probably hundreds of g are possible. They also wrote that in clinical lung therapy method for liquid breathing already exist using a liquid called perfluorocarbon.

Edit: An example what this could mean for a fictional story. Suppose you accelerate at a rate of 100g, or 1000m/s^2. This means starting from zero you will reach Earth escape velocity after around 11 seconds (you probably shouldn't do that on Earth inside the athmosphere), solar escape velocity after around 10 minutes and 25% of the speed of light after 24 hours of constant acceleration. Plenty enough for interstellar travel in a hard science setting.

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  • $\begingroup$ This may be the right answer, but this doesn't really help narrow down any semi-realistic maximum. This answer mostly says "there is no answer." $\endgroup$ Jun 8, 2023 at 21:34
  • $\begingroup$ @chaseleffers This aim of this stack exchange is to help create fiction, in this case scientifically plausible fiction. For this setting this answer tells you that yes, you can use much higher acceleration than humans would normally be able to withstand and gives you a rough ballpark of what current science deems plausible. I added a numerical example. $\endgroup$
    – quarague
    Jun 9, 2023 at 6:59
  • $\begingroup$ That's a good point. I accepted this answer. I feel like knowing this makes the acceleration limit of many sci-fi books much lower than they could be. I'm thinking this makes railgun-type launches with human passengers much more realistic. $\endgroup$ Jun 10, 2023 at 2:04

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