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We have a lot of experiences in prior decades with oxygen suits in the 3 to 4 psia range. There is no cognitive problem in that range. But, below about 3 psia, you run into lung and nasal tissue dry-out problems, except for very short exposures, measured in minutes.
The main point about suit pressures is that the closer you are to 3-4 psia, the easier it is to do mechanical counterpressure suit design. Which should be as MCP underwear, worn underneath unpressurized garments to provide thermal and mechanical protection. With known material technologies, these start becoming infeasible for suit pressures above 4 psia.
In recent years, I have seen oxygen suit pressures creeping toward, or even exceeding 8 psia. There is no need for this, except to eliminate pre-breathe with something resembling Earthly air at near sea level pressures in the habitat. Synthetic air (oxygen-nitrogen mix) at 1 atm pressure has partial pressures of 3.077 psia oxygen, and 11.619 psia nitrogen.
Using the NASA pre-breathe criterion on a nitrogen pressure that high produces a min oxygen suit pressure of 9.682 psia. See why there has been suit pressure "creep" upwards?
You do NOT need that much habitat pressure, nor do you need oxygen partial pressure at sea level values! Nor do you have to have only 20.94 volume % oxygen in your mix! There are plenty of people doing just fine living and reproducing at higher altitudes. Altitudes to 2500 m (8200 feet) produce pregnancy and birthing trouble rates indistinguishable from those at sea level. So says history dating all the way back to the Spanish colonies in South America.
The only issue you run into when increasing the oxygen % in your mix is fire danger. But that depends upon concentration in units same as density, not just %. If you lower total atm pressures, you lower those concentrations for reducing fire danger.
Try my rule-of-43 atmosphere! 43% by volume oxygen, at 43% of an atmosphere pressure, in the 2-gas mix. 6.319 psia in the hab. The oxygen partial pressure is 2.717 psia, just about the same or a bit higher than that at 2500 m on Earth. The nitrogen partial pressure is 3.602 psia, which divided by the no pre-breathe criterion is a min oxygen suit pressure of 3.001 psia, right in line with the known min limit to avoid drying out tissues too fast. And the oxygen concentration equal to or less than that of room temperature air at sea level, for no worse fire danger than on Earth.
How is that not a good solution to habitat and oxygen suit pressures? On Mars, the moon, out in space, pretty much anywhere! And it makes MCP suits eminently feasible, too boot!
GW
Last edited by GW Johnson (2025-12-27 16:41:24)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Then there is the other issues created by Spacesuit Compatibility Requirement which has happened with the use of the ISS from the partners and with the crewed space x dragons.
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