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Will there be operating limitations regarding ascent or descent speed to avoid decompression sickness?

What altitude will require a pressure suit?

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    $\begingroup$ Are you asking from a regulation standpoint or from a medical standpoint? If this includes regulation, then you need to say for which country. $\endgroup$
    – mins
    Commented Dec 4, 2021 at 18:13

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Unacclimatized human needs around 14–15 kPa partial pressure of oxygen to breathe. Since total pressure is 15 kPa around 45,000 ft altitude, pressure suit is needed above that.

Given that decompression sickness isn't a concern in diving to 10 m, which corresponds to full one atmosphere pressure difference, I don't think it's a concern for high altitude flying with supplemental oxygen.

I don't think anybody flies unpressurized above 20,000 ft these days—even fighters are pressurized to cabin altitude around 20,000 ft, largely to provide some air-flow for equipment cooling—but in WWII aircraft did sometimes fly at 30,000 ft with no pressurization and no pressure suits, just supplemental oxygen (and heated suits, because it's brutally cold up there). So that can certainly be done without significant problems.

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1. Decompression Sickness

Altitude Decomression Sickness does occur. This article from the FAA has some interesting facts about it, when it occurs, and how to prevent it:

Altitude DCS became a commonly observed problem associated with high-altitude balloon and aircraft flights in the 1930s.

Though modern aircraft are safer and more reliable, occupants are.still subject to the stresses of high altitude flight—and the unique problems that go with these lofty heights. A century and one-half after the first DCS case was described, our understanding of DCS has improved, and a body of knowledge has accumulated; however, this problem is far from being solved. Altitude DCS still represents a risk to the occupants of modern aircraft.

Factors inducing it, and of mitigating it, are (more details in the article):

  • Altitude. Below 18,000 ft there are no serious effects, 87% of cases occur above 25,000 ft. The higher the altitude, the higher the chance of being exposed to it.
  • Repetitive exposures.
  • Rate of ascent.
  • Time at altitude.
  • Body health.
  • Scuba diving before flying.

One interesting thing about breathing pure oxygen is that it prevents and/or mitigates the effects of Altitude DCS:

One of the most significant breakthroughs in altitude DCS research was the discovery that breathing 100% oxygen before exposure to a low barometric pressure (oxygen prebreathing), decreases the risk of developing altitude DCS.

2. Pressure suits

The pressure suits that fighter pilots wear are for preventing passing out due to high g-load. During high-g manoeuvring the blood tends to want to sink away from the brain, and the extra pressure around the legs prevents this from happening to an extent. The same suits are used in fighter simulators, to give the pilot some cue of high extended g-load, which is not really possible to reproduce with a motion system.

from the wiki Armstrong link

So - can a human continue to increase altitude while breathing oxygen until in outer space? Problem is, above the Armstrong limit the fluids of our body start to boil, and mainly boiling fluid lining the alveoli in the lungs would pose a problem, with death occurring in 60 - 90 seconds. So a full body pressurised suit is required above 19 km (82,000 ft).

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  • $\begingroup$ Armstrong limit is where you need the pressure suit to be full body, but you need something well below that, because somewhere around 45,000–49,000 ft the pressure drops so much that even 100% oxygen is not enough to breathe. But you can start with just pressurized mask, then you need a suit (lungs are not designed for much overpressure, so better not risk it with mask overpressure more than a few kPa), but can have e.g. hands out… above Armstrong limit you definitely need fully enclosed. $\endgroup$
    – Jan Hudec
    Commented Dec 8, 2021 at 18:17

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