Maximum survivable long term g-forces

Question

I assume this hasn't got a precise answer, but I was wondering if anyone had an indication of the maximum survivable long term g-forces, if the persons positioning was optimal?

For example, could you accelerate a craft 3g for say, 16 hours a day with the traveller in a prone position, and 1g for 8 hours a day so they can get up and exercise?

Or could you do even 6g in prone position for most of the day?

Like I said, I accept the answers would be estimates, but any discussion based on current research would be good.

Answer 1

It is hypothesized that the only acceleration that can be tolerated for looong time without side effects is the normal acceleration. Human subjects have been exposed to continuous high-G environment at most for seven days at 1.5 G. Although no immediate ill effects were found, extrapolation of the data to longer periods may be dangerously risky.

Relevant PubMed abstracts:

https://www.ncbi.nlm.nih.gov/pubmed/23926661 https://www.ncbi.nlm.nih.gov/pubmed/11542364 https://www.ncbi.nlm.nih.gov/pubmed/591467

I have not found any studies of chronic high-G exposure (beyond 1 hour limit). However, our esteemed fellow user cites Claude Piantadosi from Duke U.'s Center for Hyperbaric Medicine and Environmental Physiology.

Human G tolerance, like other physiological strains, is limited by different physiological factors at different levels of G stress.

There may be astronauts who can tolerate higher accelerations (and indeed, there have been tests subjecting humans to a week-long 1.5G regimen), but you don't want to stress cardiovascular system without need. If you can provide standard 1-g environment, you should do that.

Source: NASA-STD-3001 VOL 2.

Rationale: The limits in these figures represent safe levels of sustained translational acceleration under nominal and off-nominal conditions. Exposure to acceleration above these limits could significantly affect human performance for maneuvering and interacting with a spacecraft. The limits for return to Earth are lower than launch limits because crewmembers could have degraded capabilities because of deconditioning from exposure to reduced gravity. For the extreme conditions of a launch abort or emergency entry, limits are higher because it may be necessary to expose the crew to accelerations more severe than those experienced nominally. Humans are never to be exposed to translational acceleration rates greater than these elevated limits, as this significantly increases the risk of incapacitation, thereby threatening crew survival. In using figures 2 through 6, the acceleration vectors are relative to the “axis” of the upper body, particularly with a focus on a line running from the eye to the heart. However, the acceleration limit charts do not account for all body types or temporary off-axis accelerations or body positions. This is why the limits are set conservatively. Therefore, brief excursions past the limits in one axis should be reviewed and may be found to be acceptable.

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                     Figure 2 — +G_x Sustained Translational Acceleration Limits

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                     Figure 3 — -G_x Sustained Translational Acceleration Limits

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                     Figure 4 — +G_z Sustained Translational Acceleration Limits

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                     Figure 5 — -G_z Sustained Translational Acceleration Limits

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                     Figure 6 — ±G_y Sustained Translational Acceleration Limits

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6.5.2 Rotation Limits

6.5.2.1 Rotational Velocity [V2 6065]

The system shall limit crew exposure to rotational velocities in yaw, pitch, and roll by staying below the limits specified in figure 7, Rotational Velocity Limits.

Rationale: The limits in this figure represent safe levels of sustained rotational acceleration for crewmembers under nominal and off-nominal conditions. Exposure to rotational acceleration above these limits could significantly affect human performance for maneuvering and interacting with a spacecraft. The limits for return to Earth are lower than launch limits because crewmembers could have degraded capabilities because of deconditioning from exposure to reduced gravity. For the extreme conditions of a launch abort or emergency entry, limits are higher because it may be necessary to expose the crew to accelerations more severe than those experienced nominally. Humans are never to be exposed to rotational acceleration rates greater than these elevated limits as this significantly increases the risk of incapacitation, thereby threatening crew survival.

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Answer 2

For -Gz @ 1g (hanging upside-down), this article suggests anecdotally at least a few hours ( > 10⁴ sec ) : How Long Can a Person Safely Hang Upside Down?

Answer 3

CONSTANT 1 G EXPOSURE: If I have to say, the amount of variation in human body is drastic and requires a control set to fully understand the effect imparted by humans and 1 G EXPOSURE is dangerous for an untrained body any thing could happen; If someone specifically trained for withstanding G's then 1.45 G's is the max they might be able to withstand . Cardiovascular system is sensitive and can be rendered useless if put through a treacherous situation; stress tolerance is the biggest variable, I've seen cases that people experience tremendous G-force and survived to tell the tale, Coordination and communication is greatly affected if put through such situations.training can only do so much, yet we all are human's and sadly not super human's.