Atmospheric pressure and composition

The most critical feature of a Lunar Homestead (and any Homestead) pressure hull is that it safely and effectively contains the atmosphere of the habitat. Pretty obvious, right? So let’s determine exactly what atmospheric pressure and composition our Homesteaders need.

Atmospheric pressure and composition are intimately connected. Oxygen is the most critical component in any breathing gas (for humans at least). As long as the partial pressure of oxygen is enough to keep us alive and functional, we can play with inert gases and the total pressure. Every gas in a gas mix has a partial pressure, which is simply the pressure of that individual gas if it was the only gas present. The sum of all the partial pressures is the total pressure. All of our choices have significant advantages and disadvantages so finding the right mix is very tricky.

There are 4 key “players” in deciding on an “ideal” atmospheric pressure and composition.

• Human physiology – Not only does our breathing gas have to keep our Homesteaders alive, it also has to keep the comfortable and safe as well. There’s no point in living on Luna if you’re sick and miserable all the time. We might as well stay on Earth if that’s the case.
• Logistical concerns – We’ll have plenty of oxygen on Luna once mining and refining operations are underway. Nitrogen (or any physiologically inert gas), carbon, and water (all necessary ingredients for a breathable gas mix) are another story. Spare parts for life support equipment also have to be considered. Decisions made now will have a direct impact on the initial and ongoing expenses of each Homestead.
• Engineering requirements – Building a sustainable, long-term human (and other life forms) life support system is a huge engineering challenge. Selecting a breathing gas is just one part of the puzzle. More atmosphere pressure means a thicker hull but maybe less robust cooling fans. There’s a lot of moving parts to consider here (pardon the pun).
• The “mission” – NASA (and other agencies) chooses their breathing gas based on what they want their astronauts to do and how long they are expected to do it. Each Apollo mission lasted less than two weeks. Our “mission” is much more challenging. We need to design technology and protocols for people to live comfortably on Luna indefinitely. That’s never been done before.

We have a narrow range of pressures and oxygen mixes to work with. The two extremes are:

• One standard atmosphere – This is the pressure and composition of Earth’s atmosphere at sea level (minus the water vapor) and the maximum we should need. Most air breathing life on this planet is well adapted to this atmosphere and this would be the best choice for maximizing our Homesteader’s comfort.
• According to NASA, the minimum allowed oxygen partial pressure is 3.6 psia (24.82 kPa) with 100% oxygen (3). I’m going to consider this the bare minimum requirement for Homesteaders. This atmosphere presents significant health risks and would not be ideal.
• The sweet spot between the two – Somewhere between these two extremes is a pressure and composition that is ideal for Lunar Homesteaders (and for other Homesteaders as well). Finding that spot is the point of this research.

One Standard Lunar Homestead Atmosphere (1 SLHA)

I’m going to create a new unit of atmospheric pressure and composition. How humble of me! Anyway, this is the pressure and composition that I’ve concluded is optimal for Lunar Homesteaders based on available resources, health risks, fire risks, engineering difficulties, and comfort. I’ll probably be completely wrong and have to change it down the road. But I have to start nailing down all these parameters or I’ll never get anything done. I definitely plan on experimentally researching the crap out of the SLHA eventually.

So here it is:

78% nitrogen/21% oxygen at 70.11 kPa (equivalent to 3000 meters elevation)

I had originally thought that there was one significant variable that dominated every other consideration. The availability of nitrogen (or any biologically inert gas) on Luna. There just isn’t very much. Certainly not enough to fill hundreds (thousands, millions!) of Homesteads with a one standard atmosphere environment. Our Homesteaders would have to constantly ship large quantities of nitrogen from Earth at massive expense. Thinking further ahead, we don’t know how much nitrogen we will find in asteroids and we’ve only found a small amount of nitrogen in Mar’s atmosphere. Nitrogen seemed to be a significant bottleneck to the human settlement of the solar system if we relied too heavily on it. A 100% oxygen atmosphere seemed the logical solution (and NASA has used it on many missions).

It’s clear to me now, after all of my research, that we just can’t use a 100% oxygen atmosphere. The risks, while manageable for short-duration missions, are just too great for indefinite use. Lunar Homesteads need to be reasonably safe and comfortable places for people to live, raise a family, and enjoy life. So logistics fell to third place. The current order of priorities (most to least) is:

• Health effects – We want our Homesteaders to live long and meaningful lives. There’s no point in doing this if everyone gets cancer and dies at 40. Or if having a partially collapsed lung is a common occurrence. Their atmosphere must be as healthy as possible.
• Fire risk – Homesteaders are not astronauts on a limited duration mission. They’re going to want to cook dinner (possible ignition source) and sleep in a comfy bed with lots of pillows (possible fuel sources). We’re going to have to plan on the occasional open flame or overheated wire.
• Logistics – Shipping nitrogen from Earth is a massive pain in the ass. It is probably unavoidable anyway, even if we didn’t need it for our breathing gas. Homesteaders will need nitrogen for agriculture and some industrial processes. What they get from the regolith probably won’t be enough for these activities. They can import ammonia (NH₃) and get some hydrogen (for making water) with their nitrogen. Or they can simply buy nitrogen gas. Maybe the system will be similar to those propane cylinders found outside some stores. A Homesteader will buy a new tank of nitrogen. When it arrives they swap it out with the expended tank. Then then ship expended tank back to Earth. Lots of cool things are possible once we have Lunar Homesteads and a robust transportation system to support them.
• Engineering concerns – We know all of this can be done. We just have to be clever about how we do it. We have a lot more flexibility since mass isn’t such a critical parameter.

Advantages of one SLHA:

• It’s a well understood gas mix. Millions of humans are currently breathing it right now. It should be just as safe as breathing air at sea level.
• It should be just as comfortable as a seal level gas mix. No vocal changes, similar thermal qualities, and similar humidity.
• It has the least risk of fire. The oxygen concentration is the same as sea level (21%) while the partial pressure of oxygen is a little less.
• The risks of sudden decompression due to a high-energy impact are less. The decreased total pressure reduces the risk of sudden barotrauma to the crew and reduces the rate of atmospheric loss. And the decreased partial pressure of oxygen reduces the risk of fire caused by such an impact.
• “Normal” leakage will be less than a one standard atmosphere mix because the pressure is less.
• Engineering and outfitting the habitat will be easier. We can use off-the-shelf items because the atmosphere isn’t “exotic”. Stuff should behave in expected ways.
• The pressure hull COULD be slightly thinner because of the reduced total pressure. It won’t be because we’re going to design the whole system to handle a pressure range from one SLHA to one standard atmosphere.
• The chances for atelectasis are greatly reduced. Nobody wants their lungs to collapse.
• Less oxygen pre-breathing needed before using current space suits. This is much less of an issue for Lunar Homesteaders as we’re going to design things so they won’t have to go onto the surface much. And when they do they won’t be using suits.
• Certain plants that require atmospheric nitrogen can be grown in the habitat.

Disadvantages of one SLHA:

• We’re going to have to import nitrogen. There just isn’t enough nitrogen on Luna to meet our needs (unless we find a new local source). A 100% oxygen atmosphere is just too dangerous for long-term use.
• Homesteaders will have to spend days or weeks gradually acclimatizing to their new atmosphere if they are coming from Earth. Going directly from sea level to one SLHA would be rough and potentially fatal.
• Sudden decompression could instantly give the Homesteaders decompression injuries due to the nitrogen saturated in their blood coming out of solution. The threat is less than for a one standard atmosphere gas mix.
• The crew may have less time to resolve unexpected leaks because the oxygen partial pressure is already lower than a one standard atmosphere. It will take less time for the oxygen partial pressure to fall below the level needed for breathing.
• Air cooling equipment and Homesteaders might be a little more difficult due to the lower total pressure. Fans might have to be bigger and more robust. Noise might be an issue.
• Air circulation equipment may also have to be sized up to account for the lower pressure. Increased electrical draw and noise might be issues.
• A two gas system is more complicated than a single gas system.
• Some cooking techniques will not be possible. So we’ll just have to develop new ones!

How well does the one SLHA fulfill NASA’s guidelines for atmospheric compositions and pressures (3)? These are the basic constraints. More can (and should) be applied to increase the health, comfort, and efficiency of the Homesteaders.

• There must be sufficient total pressure to prevent vaporization of body fluids (ebullism), which occurs at approximately 6.3 kPa. This point is called the Armstrong limit (named after Harry George Armstrong). Exposed liquids (but not blood inside the body) start to boil at (or below) this pressure because the boiling point of water is at the same temperature as the body temperature. No amount of oxygen will keep a person alive for more than a few minutes at this pressure.  Experiments on animals showed zero fatalities at less than 90 seconds of exposure but mostly fatalities beyond 120 seconds of exposure (14, pg 476).
  • No problem! Our bare minimum total pressure is 15.9 kPa. The SLHA is 70.11 kPa.
• There must be free oxygen at sufficient partial pressure for adequate respiration. NASA requires that O₂ partial pressure does not go below 15.9 kPa. Additionally, the O2 concentration must be within 19.5% and 23.1% of total pressure at 101.325 kPa (dry air) for normal operations. This is called normoxic. Emergencies expand the range to 16.5-23.8% for 90 days and 15.9-23.8% for 28 days.
  • The SLHA of 14.72 kPa oxygen is below the NASA recommended minimum oxygen partial pressure. This would be a problem IF we were running short-duration missions that don’t allow people time to acclimate. Fortunately, Lunar Homesteaders CAN take the time to fully acclimate and stay acclimated for a very long duration (like permanently).
  • No problem! The SLHA runs at 21% oxygen.
• Oxygen partial pressure must not be so great as to induce oxygen toxicity.
  • No problem! The partial pressure of oxygen at one standard atmosphere is 21.28 kPa (dry air). An oxygen partial pressure of 60 kPa can lead to pulmonary toxicity and irreversible pulmonary fibrosis (10). An oxygen partial pressure of >160 kPa can cause convulsions within minutes and with little or no warning (10). The SLHA is 14.72 kPa.
• NASA requires that for missions that exceed two weeks some physiologically inert gas must be provided to prevent atelectasis (see section below). A NASA conference concluded that a 100% oxygen 5 psi atmosphere would be acceptable for missions lasting less than 30 days (21, pg 3).
  • No problem! The SLHA has 54.76 kPa of nitrogen.
• All other atmospheric constituents must be physiologically inert or of low enough concentration to preclude toxic effects.
  • No problem! The only other components would be carbon dioxide (in trace amounts) and water vapor. The plan is to design habitats and equipment that introduce the minimum amount of contamination.
• The breathing atmosphere composition should have minimal flame/explosive hazard.
  • No problem! The SLHA has slightly less flame and explosive hazard than the atmosphere at sea level. The oxygen concentration is the same (21%) but the oxygen partial pressure is less.

Additional constraints that I’ve added:

• Long-term exposure to the atmosphere should not cause irritations or damage to the Homesteaders.
  • No problem! I have no idea how many people have lived breathing this gas mix over the course of human history but I bet it has been a lot. Many millions of people are currently breathing it right now. Millions of children have been successfully conceived, born, and raised breathing this gas mix. It’s just as safe as sea level once you’re acclimated.
• The atmosphere should provide some protection against sudden decompression.
  • I’m less concerned with pressure differences between the habitat and the EVA equipment. I envision that Homesteaders won’t go out on the surface very often. When they do they’ll use hard suits or pods that operate at the same pressure as the habitat.
  • A high-energy penetrating impact should cause less barotrauma as the sudden increase in habitat pressure would be less than with a higher pressure atmosphere.
  • Air will escape more slowly from a hole or crack in the hull as the pressure is less. The addition of nitrogen helps to slow atmospheric loss as opposed to a 100% oxygen atmosphere. This will give our Homesteaders more time to fix the damage or evacuate.
• The atmosphere should minimize the need for imports from any place other than Luna.
  • The SLHA kinda fails on this point. While the SLHA saves 279.92 g per cubic meter (a 31% savings) over a sea level mix, it still uses a significant amount of nitrogen. It looks like we’re importing nitrogen from Earth unless we run with a 100% oxygen mix. And that’s just too dangerous for Lunar Homesteads.

Below are all the data and resources I collected while researching this topic. Please let me know if I missed something or got something wrong. I can use all the help I can get!

One standard atmosphere

By definition one standard atmosphere (atm), the pressure at sea level, is:

• 101.325 kilopascals (kPa) – International System of Units (Systeme international d’unites or SI)(aka Metric) – The force of one newton spread across the area of one square meter.
• 14.70 pounds per square inch (psi) – American Engineering System – Obviously the force of one pound applied over the area of one square inch.

There are other ways to measure atmospheric pressure but these are the two I’m using. Why? Because they are commonly used and they are exact. Other methods have other variables that need to be defined (such as temperature). Plus they are archaic. Why are we still measuring air pressure by how many millimeters of mercury is raises?

One standard atmosphere contains the following components (7, pg 314)

• Oxygen (O₂) – 20.9% with a partial pressure of 3.07 psi (21.17 kPa)
  • I get 21.18 kPa.
  • Other sources state 21.37 kPa (3) , 25.3 kPa (8), 22.7 kPa (13), and 3.08 psi (21.30 kPa) (7).
  • At one standard atmosphere, oxygen concentration must not fall below 11% (3)
• Nitrogen (N₂) – 78.1% with a partial pressure of 11.48 psi (79.15 kPa)
  • I get 79.13 kPa
  • Other sources state 78.60 kPa (3).
• Water vapor (H₂O) – 1.0% with a partial pressure of 0.15 psi (1.03 kPa)
  • Other sources state 1.38 kPa (at 74 ºF and 50% humidity)(3).
• Carbon dioxide (CO₂) – 0.04% with a partial pressure of 0.005 psi (0.034 kPa) – This is such a small amount that I’m not bothering with it here.

There are many good reasons to use a one standard atmosphere breathing gas:

• It’s the most comfortable mix for the crew. No vocal change, better heat retention, more humid. The denser, nitrogen rich air also helps the crew shed excess heat (this would be very important to me!). Humans (in general) are fully adapted to a one standard atmosphere environment. This would be the ideal atmosphere if all other considerations were ignored.
• There aren’t any known or unknown health risks for long-term exposure.
• The risk of fire is lower than higher oxygen mixes. And the nitrogen is an effective fire suppressant. (See Fire risk considerations)
• The loss of pressure due to sudden decompression is slower than a 100% oxygen atmosphere (16, pg 29).
• It simplifies the engineering for all the equipment housed inside the pressure hull. More standard and “off-the-shelf” items can be used. Less metal items (i.e. more flammable) items can be used. Simpler is usually safer.
• Electronic components can be air-cooled more effectively. Less dense atmospheres conduct and hold less heat and therefore need bigger cooling fans (more electricity and more noise).
• The habitat air can be more efficiently circulated than at lower pressures. The denser air is easier to move. Preventing pockets of CO and CO₂ from building up is a major concern for spacecraft operating in microgravity. Lunar Homesteads probably won’t have to worry about this as much.
• Foods can be cooked normally. Try cooking at high altitudes if you don’t think this is an issue.
• The chances for atelectasis (see section below) are greatly reduced.
• Nitrogen will be used for other functions as well as an atmosphere buffer.
  • Some useful plants, microbes, and bacteria need atmospheric nitrogen.
  • Possible fire suppressant.
  • Atmosphere purging.
  • Ullage gas storage to provide fluid and gas pressure.
• We won’t have to run many long-term experiments define all the health, engineering, and logistical impacts a non-standard atmosphere will require.
• Experiments don’t have to factor in non-standard pressure or composition (fewer variables). This is especially important if you are running simultaneous control experiments on the ground.
• Simplifies crew transfers to and from Earth. This isn’t an issue for Lunar Homesteaders, who aren’t returning to Earth any time soon and can spend as much time as necessary acclimating to the habitat environment. This IS a huge consideration for current spacecraft and the International Space Station. Having astronauts sitting around for weeks in a hyperbaric chamber in the ISS while acclimating to a lower pressure is profoundly inefficient. Plus, the chamber would be expensive to launch and maintain. Acclimating the crew on the ground before launch would involve significant engineering challenges for the launch vehicle.

With all these benefits, why wouldn’t we automatically use a one standard atmosphere? Unfortunately, it comes with a few significant disadvantages.

• A one standard atmosphere contains a lot of nitrogen. Lunar Homesteaders could have a hard time finding enough local nitrogen. (see Nitrogen considerations below)
• The higher pressure means normal leaks (through seals and fittings) lose somewhat more breathing gas than lower pressure atmospheres. Atmospheric composition doesn’t seem to have much of an effect though (16, pg 29). This means we’ll need more nitrogen to replace the stuff leaking out! Yikes! (See Leakage section below)
• The pressure habitat needs to be stronger to contain the higher pressure. The airlocks and seals as well. A 14.7 psi atmosphere requires 1.5 times the cabin-wall weight than a 5-7 psi atmosphere (14).
• A two-gas atmosphere is more complicated that a single gas atmosphere. Have to store multiple gases, more plumbing is involved, and more equipment and effort to keep the mix just right.
• Increased risk of a decompression injury (DCI) or barotrauma (damage caused by the rapid expansion of gas trapped in the body) if EVA equipment operates at a lower pressure. The higher operating pressure makes sudden decompression due to a hull rupture a larger risk as well. (see EVA considerations below)
• Additionally, a high-energy puncture of the pressure hull will cause more injury at one standard atmosphere than a lower pressure atmosphere. The energy released from the impact will compress the denser air, causing more physical damage to the crew.

Spacecraft that operate (have operated/will operate) with one standard atmosphere are:

• The International Space Station
• Soyuz
• Shenzhou
• Orion
• The Space Shuttle
  • Oxygen partial pressure on the Shuttle was 3.2 psi (22.06 kPa). Oxygen masks were required if the oxygen pressure fell below 2.34 psi (16.13 kPa) (7, pg 322).
• Mir
• Salyut
• Voskhod
• Tiangong-2

100% oxygen at 3.6 psi (24.82 kPa)

This is NASA’s minimum allowed atmosphere. It is set to this level because it is the minimum to maintain alveolar oxygen levels (13.73 kPa is the minimum alveolar oxygen partial pressure to provide oxygen equivalent to sea level) (3). No mission has ever flown with this breathing gas mix at this pressure.

Advantages:

• No nitrogen (or other buffer gas) needed. This is a huge logistical win.
• Less oxygen needed. This doesn’t really matter as Luna has plenty of oxygen.
• Some engineering requirements will be easier.
• NASA space suits operate at 29.6 kPa with 100% oxygen (4).
• Slightly less risk of fire than a 34.47 kPa 100% oxygen atmosphere.

Disadvantage:

• Many, many problems with this breathing gas. There’s just no way I could use this mix.
• All the disadvantages of the 100% oxygen at 5 psi (34.47 kPa) discussed below.
• NASA limits crew exposure to this pressure to 12 hours (7, pg 319).
• NASA limits crew exposure to this oxygen partial pressure to 14 days or less (7, pg 324).
• The least amount of time for the crew to fix an unexpected leak or puncture because the pressure will quickly fall below the level needed to breathe.

This breathing gas mix is completely unacceptable for our needs.

100% oxygen at 5 psi (34.47 kPa)

NASA has flown many missions (all of the Mercury, Gemini, and Apollo) with a 100% oxygen atmosphere at 5 psi (34.47 kPa). The primary reasons for this are: 1) simplified engineering, 2) weight savings, and 3) some protection against sudden decompression. Crew comfort was maybe a distant fourth and the missions were short anyway. Part of the problem was that the technology was racing far ahead of the biomedical science. Honestly, it was figured that the astronauts were tough guys and could just “suck it up” if they experienced any minor difficulties. The mission always came first.

Additionally, the Space Shuttle and International Space Station Extravehicular Mobility Unit (aka space suit) are operated at 29.6 kPa at 100% oxygen (4). Since the Shuttle and the ISS operate at one standard atmosphere, astronauts preparing for an EVA had to pre-breathe pure oxygen for at least 4 hours before exiting the spacecraft (4). This time could be reduced by doing light exercises during the pre-breathe and/or by manipulating the spacecraft’s atmosphere 24 hours before the EVA (4). The Russian Orlan space suit operates at 39.2 kPa and requires 1 30 minute pre-breathe (16, pg 1). See the EVA considerations section for more information.

There are some significant advantages to a pure oxygen environment:

• No need for nitrogen (or any other buffer gas). This is huge. For NASA it meant less mass they had to launch (it didn’t add up to much anyway). For Lunar Homesteaders it would mean they aren’t burdened with importing nitrogen from Earth.
• The spacecraft pressure hull could be built lighter. From 5-7 psi the mass of the pressure hull is not dependent on the internal pressure with the weight penalty increasing almost linearly with internal pressure (14). For us, less mass means less iron we need to obtain to build the thing.
• A single gas system is less complex than a two gas system. Simplicity often equals reliability. Plus, a simpler system masses less. An important point for spacecraft but not for Lunar Homesteads.
• No lengthy pre-EVA activities. (See EVA considerations below)
• Increased safety from hull punctures. A lower atmospheric pressure means the air escapes at a slower rate. I read that it would take 2 minutes for a 5 inch diameter hole to evacuate all the air from the Apollo spacecraft. That’s a huge hole. There’s a possible increase of fire risk due to hull punctures. (See Fire risk considerations below)
• Decreased leak rates. Everything leaks but less pressure means less powerful leaks.
• Sudden decompression won’t instantly give the crew decompression injuries. It’s hard to patch a hull puncture when you are doubled over in excruciating pain. (See EVA considerations below)
• It might be easier to remove carbon dioxide and other toxins from a single gas system.

Unfortunately, the disadvantages far outweigh the advantages:

• NASA states that crew exposure to 5 psi should be limited to 14 days (7, pg 319).
• NASA also states that crew exposure to oxygen partial pressures of 33.10 kPa to 60.67 kPa should be limited to 48 hours or less (7, pg 324). Apparently NASA exceeded this limit with their longer duration missions (such as Gemini and Apollo).
• One of the worst things that can happen to a habitat will be fire. Increasing the partial pressure and concentration of oxygen in the habitat significantly increases the risk of fire. The greatly increased risk of fire is one of two “deal breakers” for this atmospheric pressure and composition. NASA felt the risk was acceptable for short duration missions only. I agree. Our Homesteaders deserve an atmosphere that won’t burn them alive. See the Fire risk section for more information.
• Long-term exposure to high pressure pure oxygen causes a number of health issues. This is the other “deal breaker”. Our Homesteaders deserve an atmosphere that doesn’t damage their health. Even if it means figuring out how to deal with the nitrogen supply problem.
  • An oxygen partial pressure of 60 kPa can lead to pulmonary toxicity and irreversible pulmonary fibrosis (10). An oxygen partial pressure of >160 kPa can cause convulsions within minutes and with little or no warning (10). This oxygen pressure is well beyond what our Homesteads should be using.
  • Additionally, prolonged exposure to 100% oxygen at 101.325 kPa has led to blindness, heart problems, and unconsciousness (7, pg 319).
  • High partial pressures of oxygen can increase blood pressure and reduce heart rate (10).
  • Oxygen partial pressures in the 53.329 – 101.325 kPa range cause a bunch of respiratory and nervous systems problems (nausea, substernal distress, atelectasis, paresthesia, and bronchitis) (14)
  • Oxygen partial pressures in the 26.6645 – 53.329 kPa range can cause respiratory, renal, and hematological issues. Pulmonary and aural atelectasis have been reported. Changes in blood enzymes and in mitochondria of the liver and kidneys have also been reported. (14)
  • Apparently the retinas of children are very sensitive to higher oxygen concentrations. One paper, from 1963, states that retrolental fibroplasia has been attributed to high oxygen tensions and that oxygen in incubators (for premature infants) be kept below 40% (18, pg 44). That’s way more than what we would use but it’s still troubling. I don’t know what the current science is but I’m sure our Homesteaders don’t want their kids to have eye issues.
  • Very dehydrating. We will need to significantly increase the humidity to make it more comfortable. This will have an impact on all the equipment in the habitat.
  • Throat and lung irritation – This may not sound like a big deal but what if it was for the rest of your life?
  • There was a brief mention in one paper about the need for nitrogen for embryological development (18, pg 68). Apparently chicken embryos showed development issues when exposed to 100% oxygen at 20.0 kPa, 20% O₂/80% N₂ at 101.325 kPa,  and 100% O₂ at 101.325 kPa (18, pg 68). A 100% oxygen atmosphere will be off the table if this is true for human embryos as well.
• Much more stringent material control. We’ll need to take a very hard look at EVERYTHING in the habitat because of the increased fire risk. Things that aren’t combustible at sea level are now potential fire threats.  Materials will also behave differently in a 100% oxygen environment (off gassing, material lifespan, etc.). This is will require a massive expenditure of time and effort and is a HUGE pain in the ass. (See Fire risk considerations)
• We don’t have the research on how modern pharmaceuticals will react in the human body while exposed to a 100% oxygen atmosphere. It would be naïve to assume that there wouldn’t be any significant change.
• The loss of pressure due to sudden decompression is faster than a nitrogen/oxygen atmosphere (16, pg 29).
• Less pressure means an increase in fan power and/or larger ducts (16, pg 29). Heat exchangers also function less efficiently (15, pg 30). More massive equipment, more power needed, more repairs and maintenance.
• Pressures below 69.0 kPa results in speech being harder to understand (16, pg 31).
• Increased oxygen concentrations seem to increase the radiation hazard (18, pg 63). Oxygen always makes radiation worse because of the increased generation of oxidizing radicals (16, pg 25). I don’t pretend to understand it all. One experiment with mice showed that breathing nitrogen gave the mice 2.3-2.5 times more protection from radiation (18, pg 67). Apparently, hypothermia and hypoxia also help but we won’t use these in our habitats. One paper states that the radiation hazard caused by a 100% oxygen atmosphere at 5 psi is not significant (20, pg 115).
• Increased oxygen partial pressure and concentration may result in materials off-gassing or oxidizing faster (16, pg 31). These activities can put toxins into the habitat.
• Cooking becomes more difficult the lower the pressure gets. So does sanitation if it involves boiling water. Homesteaders would need to use pressure cookers (which present their own type of hazard).
• May increase the rate of oxidization (rusting) of our iron pressure hulls. I’ll have to research this.

Spacecraft that operate (have operated/will operate) with this atmosphere are (for a very limited duration):

• Mercury
• Gemini
• Apollo
• NASA space suits

Again, this breathing gas mix is completely unacceptable for Lunar Homesteads. The dangers and drawbacks dwarf any logistical benefits. I guess we’ll absolutely HAVE to include nitrogen into the mix (other possible inert gases have a bunch of other drawbacks as well).

74% oxygen/26% nitrogen at 5 psi (34.40 kPa)(27)

This is the atmosphere NASA chose for its “long-duration” Skylab missions. I put long-duration in quotes because the longest Skylab mission was 84 days. That was a long mission for 1973 but not for what we’re working on. I had a lot of trouble finding any data on the atmosphere of Skylab and its effects on the crew. Please let me know if I’m missing something.

Advantages:

• Less nitrogen needed (26% vs 78%) than a one standard atmosphere. This is a lot but it’s not enough to ignore the increased fire risk.
• No pre-breathe for EVA was necessary (16). This doesn’t mean as much for us because we’re going to do EVA differently.
• A lot of the health, and some of the fire, risks are eliminated or diminished as opposed to a pure oxygen atmosphere.

Disadvantages:

• A significant fire hazard still exists though. This gas mix has over 3 times the concentration (74% vs 21%) and greater partial pressure (25.46 kPa vs 21.18 kPa) of oxygen than the atmosphere at sea level. Check out the Fire risk considerations section below for why that’s troubling.
• Still has most of the pressure-related issues that 100% oxygen 34.40 kPa atmospheres have.
• Sound doesn’t travel well. Skylab astronauts stated that a loud speaking voice could only be heard about 5 inches away. Constant shouting often left them hoarse (8).
• Cough mechanism becomes less effective (8).

I just can’t get over the increased fire risk due to the significantly increased oxygen concentration. The increased oxygen partial pressure doesn’t help either. This atmosphere might be fine for highly trained and motivated people living in a spartan environment consisting of rigorously tested materials. Throw in an energetic and curious 8-year old and see what kind of chaos happens. Our Homesteaders are going to need a more forgiving atmosphere.

78% nitrogen/21% oxygen at 70.11kPa (equivalent to 3000 meters elevation)

Based on historic human habitation, 5000m above sea level is considered the highest elevation humans can permanently adapt to (5). This elevation is equivalent to an oxygen partial pressure of 11.34 kPa and a total atmospheric pressure of 54.02 kPa. This is not enough oxygen for comfortable living, even with adaptation, so we’ll have to look at a much lower altitude.

Here’s a short list of human settlements at high altitudes:

• Aucanquilcha mine (Chile) – 5950 m = 47.51 kPa total/9.98 kPa oxygen partial pressure (6)
• Mount Everest base camp (North) – 5150 m = 52.95 kPa total/11.12 kPa oxygen partial pressure (6)
• La Rinconada (Peru)(Highest known permanent settlement) – 5100 m = 53.30 kPa total/11.19 kPa oxygen partial pressure (6)
• El Alto (Highest city in the world – 1+ million people) – 4150 m = 60.44 kPa total/12.69 kPa oxygen partial pressure (6)
• La Paz (1+ million people) – 3100 m = 69.22 kPa total/14.53 kPa oxygen partial pressure (6)
• Quito (Ecuador)(3 million people)(Personal note – I stayed several days in Quito and was just fine) – 2850 m = 71.46 kPa total/15.01 kPa oxygen partial pressure (6)
• Denver (USA)(700,000+ people) – 1640 m = 83.11 kPa total/17.45 kPa oxygen partial pressure (6)

Occupational Safety and Health Administration (OSHA) specifies a minimum of 19.5% oxygen at 19.73 kPa (equivalent to 609.6 m elevation) for enclosed spaces (7, pg 323). Several space biomedical sources recommend 19.33 – 23.73 kPa of oxygen (altitude equivalent of sea level to 914.4 m), which is what over 80% of the Earth’s population breathes (7, pg 323). Finally, U.S. and Russian sourcebooks recommend an oxygen partial pressure of greater than 17.07 kPa (equivalent to approximately 2000 m) (7, pg 323). Obviously, only Denver makes the cut from the list using these parameters.

The minimum oxygen partial pressure (without acclimatization) of 16.8 kPa (equivalent to approximately 2,743 m altitude) is set by Federal Aviation Administration (FAA) and Department of Defense (DoD) requirements to reduce the chance and effects of acute hypoxia, especially high altitude sickness (7, pg 324). The maximum altitude that commercial and DoD pilots can fly without supplemental oxygen is 3,048 m (14.79878 kPa O2) (7, pg 325). Russian standards are 15.99868 kPa  to 18.66513 kPa oxygen for a maximum of 3 days (7, pg 324). Additionally, the minimum total pressure for crew not acclimatized to high altitude is 8 psi (55.16 kPa) with a minimum oxygen concentration of 11% (3).

Those numbers seem pretty restrictive since there are a lot of people living at higher altitudes. One paper states that it is possible for humans to acclimate to a minimum of 50.53 kPa (the equivalent of 5500 m altitude) (3). That seems a bit high. Another stated that the safest cabin atmosphere is 21%O₂/78% N₂ at 8,000 feet (2438.4 m) elevation (19, pg 107). This mix of pressure and composition represents the least risk of fire with the least risk of health problems according to the paper (19, pg 107).

I like 70.11kPa (3000 m altitude equivalent).

Advantages:

• It’s between Quito and La Paz, elevation speaking. Millions of people permanently live at or around this pressure.
• The oxygen partial pressure would be 14.72 kPa.
• The risk of fire would be somewhat reduced (compared to sea level) due to the reduced total pressure.
• Less nitrogen would be necessary. The partial pressure of nitrogen at this total pressure is 54.76 kPa versus 79.13 kPa at sea level. Still a lot more than the 74% oxygen/26% nitrogen at 5 psi (34.40 kPa) mix though. The partial pressure of nitrogen for the Skylab mix is 8.94 kPa.
• The habitat does not have to be as strong. This pressure is 69.2% that of sea level.

Disadvantages:

• Some physiological adaptations would be necessary. Homesteaders coming directly from Earth would have to spend several weeks physically adjusting as the pressure is gradually reduced.
• Cooking techniques would have to be adapted to the lower pressure.
• Air cooling might be more of a problem.

Fire risk considerations

Figuring out the why and how of fire is a science in itself. I don’t profess to understand most of it. Fortunately, I don’t have to for this project. I just need to quantify the fire risks for the various atmospheres under consideration.

In the most basic terms, pressure and oxygen concentration affects:

• Auto-ignition tendency – Increasing pressure decreases temperature needed (19, pg 16).
• Limits of flammability – Increasing pressure widens the range (19, pg 16).
• Reaction (burning) rates (19, pg 16).
• Flame speeds (19, pg 16).

Fire risk facts for oxygen:

• Oxygen is, not surprisingly, an oxidizing agent. It’s part of the “fire triangle” (the three components needed to start and maintain a fire):
  • Heat source
  • Fuel
  • Oxidizing agent
• For almost all combustible gases and vapors, the limits of flammability are determined by the percentage of diluent gas and the partial pressure of oxygen in the breathing mix (19, pg 102). Generally, either increasing the partial pressure of oxygen or decreasing the amount of buffer gas also increases the fire risk and the range of explosion limits (19, pg 102).
  • An inert gas can suppress burning by absorbing some of the heat, reducing the flammability of a fuel, reducing the combustion rate, and restricting the oxygen mobility (the inert gas reduces the oxygen molecular free path) (15, pg 10). All of this combines to increase the contact time and surface area required to ignite an object (15, pg 10).
• Oxygen concentration (one standard atmosphere is 20.9% oxygen).
  • Oxygen concentration (volume percent) is the primary driver for fire risk, followed by total pressure (16, pg 11).
  • According to one NASA study, flammability is more dependent on oxygen concentration than on equivalent partial pressure (12).
  • Increasing the percentage of O₂ in the atmosphere dramatically decreases the minimum pressure required for ignition (8). Experimental evidence shows that increasing the concentration of oxygen had a more pronounced effect on ignition and combustion of materials than increasing the pressure (22, pg 12). Controlling the burn rate is important but preventing ignition in the first place is paramount.
  • There is no apparent threshold for fire hazard when increasing the percentage of oxygen (19, pg 108). Increasing the oxygen content makes all the combustion parameters shift towards increasing the fire risk (19, pg 108).
  • A slight increase in the percentage of oxygen significantly affects the burning rates of many fabrics and significantly decreases the effectiveness of fire retardant materials applied to them (19, pg 102).
  • Increasing the concentration of oxygen (in a 15 psia atmosphere with nitrogen) from 21% to 31% doubled the burning rate of paper and increased the burning rate of cotton terry cloth from 0.20 cm/sec to 1.67 cm/sec (22, pg 12).
  • Increasing the oxygen concentration from 21% to 31% also increased the linear burn rates by a factor of 2 or 3 (22, pg 15).
  • Enriching the concentration of oxygen in the atmosphere makes it easier to induce ignition in materials that would not ignite in a 21% oxygen atmosphere (22, pg 15).
  • Increased oxygen content makes fire suppression more difficult (16, pg 31).  Introducing the fire suppressant into the atmosphere will reduce the oxygen concentration to the point that combustion ceases (16, pg 31). This is bad news if you are relying on that oxygen to live. Water is the only effective fire suppressant once oxygen concentrations get above 30% (16, pg 31).
  • Conventionally flame-proofed materials burn vigorously in a 30-40% oxygen environment and smothering is ineffective in suppressing the flame (17).
  • A solid or liquid in a pure oxygen atmosphere will burn hotter and faster without nitrogen to interfere with the reaction (15, pg 17). Gases, however, may or may not burn hotter (15, pg 17).
  • In a pure oxygen environment, the lower flammability limits are virtually the same as in a 20% air mix (15, pg 22).
  • Prolonged exposure to 100% oxygen didn’t make the materials more ignitable, but the surface texture and geometry of the materials had a significant effect on the rate of flame spread (23, pg 3).
• Oxygen partial pressure (one standard atmosphere is 21.18 kPa).
  • Oxygen pressures exceeding 23% of the total pressure enables normally noncombustible materials to easily burn (7, pg 349).
  • Increased pressure increased the combustibility and burn rate of materials that were not combustible in one standard atmosphere however (22, pg 12).
  • Oxygen pressure must be below 30% of the total pressure (7, pg 349).
• Gravity plays a part in fire risk as well
  • Convection doesn’t exist in microgravity, leaving diffusion as the major factor for mixing (15, pg 11). Without air circulation, combustion gases envelope the fuel, impede the flow of oxygen, and eventually stop combustion (15, pg 18). The lack of natural convection in a microgravity environment somewhat mitigates the risks (14, pg 456).
  • Partial gravity conditions (such as Luna, Mars, and spinning orbital Homesteads) might represent a more severe fire risk than Earth or microgravity due to how combustion gases react in lower gravity (16, pg 12). It is unknown how the 1/6 Lunar gravity will effect combustion characteristics. The only way we’ll ever know is to live there and find out. In the meantime, I’ll need to be a little extra conservative when it comes to evaluating the risk of fire. I’m going to have to assume that fire behaves the same on Luna as on Earth because we don’t have any other data.
• Increasing ventilation flow will decrease the risk of pockets of CO₂ or other toxic gases. Unfortunately, it increases the flame propagation once a fire breaks out (16, pg 31).
• Flames are extinguished at 16% oxygen at 101.325 kPa, which corresponds to partial pressure of oxygen with 20.9% at 77.86 kPa (23, pg 2).

Alright, so it looks like the percent of oxygen in an atmosphere is the primary driver of fire risk. The pressure of the oxygen is a secondary driver.

The first thing to determine is how safe is a 100% oxygen atmosphere. If a 100% oxygen atmosphere is too hazardous then we’ll HAVE to use a diluent (or buffer) gas, even if it’s a bigger logistical problem.

Let’s break it down:

• 100% oxygen at 1 atmosphere (101.325 kPa) (maximum oxygen and maximum pressure)
  • Absolutely no need for this. The health and fire risks are huge with no benefits.
  • One paper stated that very few materials (except most metals) will not ignite at 41% concentration of oxygen at one atmosphere (22, pg. 16).
  • The spark energies required to ignite common materials (such as clothing) are decreased more than 1000-fold in 100% oxygen at one atmosphere pressure (17).
  • The rate of burning is increased by 5-fold in a 100% oxygen at one atmosphere pressure (17).
  • The Apollo 1 fire that resulted in the deaths of three astronauts (Virgil “Gus” Grissom, Edward White II, and Roger Chaffee) was a direct result of pressurizing the capsule with 100% oxygen to 16.7 psi (2). This environment made everything in the capsule highly flammable. It was a terrible mistake and one our Homesteaders must not repeat.
• 100% oxygen at 5 psi (34.47 kPa)
  • A much more reasonable atmosphere but still a significant health and fire risk. Far too risky to be considered for Lunar Homesteads. Which sucks because now we have to include nitrogen into the breathing mix (see Nitrogen considerations why other gases were rejected).
  • Going from 1 standard atmosphere to 100% oxygen at 5 psi (34.47 kPa) decreases the minimum ignition energy by a factor of 10 (19, pg 101). This means things catch on fire easier.
  • Paper strips in a 5 psi 100% oxygen atmosphere had a lower ignition point and a 6-fold increase in burning rate (23, pg 2).
  • Experiments have shown that filter paper in a 100% oxygen atmosphere at 0.2 atm (20.27 kPa) will burn twice as fast as filter paper in one standard atmosphere (8). The extra 14.2 kPa will just increase the flammability.
  • Rate of flame propagation in fabrics is 3.5 times greater in a 100% oxygen atmosphere at 5 psi than in an airliner at 8000 ft (2438 m)(14, pg 456).
  • Ignitability, temperature of combustion, and risk of detonation are also increased when compared to conditions in an airplane at 8000 ft (2438 m)(14, pg 456).
  • 380 mm hg (50.67 kPa) 50% oxygen/50% nitrogen doubles the burn time of a fire from a 190 mm hg (25.33 kPa) ( 100% oxygen atmosphere (3).
  • Overloaded wiring will smoke and catch fire simultaneously in a 3.5 psi 100% oxygen environment (24, pg 15). Again, the extra pressure at 5 psi will increase the fire risk.
  • The sudden spike in pressure caused by an object penetrating the hull (before depressurization occurs) could momentarily increase the risk of fire (19, pg 56). The molten and vaporized material could introduce an ignition threat (19, pg 56). Additionally, the oxidation of the vaporized material could create a flash hazard in the cabin (19, pg 56).
• 74% oxygen/26% nitrogen at 5 psi (34.40 kPa)
  • I couldn’t find any literature detailing the fire risks for this atmosphere.
  • Since the oxygen content is almost 3 times higher than a one standard atmosphere mix, I extrapolate that the fire risk would also be substantially higher. Maybe not 3 times as high but still closer to a 100% oxygen atmosphere than sea level.
  • The higher oxygen partial pressure also probably represents an increased fire risk as well.
• 78% nitrogen/21% oxygen at 70.11kPa (equivalent to 3000 meters elevation)
  • Less of a fire risk due to lower pressure.
  • The substantial concentration of nitrogen will act as a fire retardant.

If our Homesteader’s health and safety is our primary concern (it is) then we have to use an atmosphere that has the same, or less, fire risk of one standard atmosphere. That leaves us with two choices. One standard atmosphere or a 78% nitrogen/21% oxygen at a reduced pressure. Atmospheres with significantly elevated oxygen content and/or partial pressure are just too dangerous for Homesteaders.

Humidity and temperature

Humidity, the amount of water vapor in the atmosphere, is a function of temperature. The atmosphere can hold increasing amounts of water as the temperature increases. Maintaining a comfortable balance in an enclosed habitat takes a bit of clever engineering. Fortunately that’s not what I’m concerned with here.

Figuring out the ideal and maximum pressure of water vapor in our breathing gas is important because we want our Homesteaders to be comfortable and safe. Low humidity causes the mucous membranes, eyes, and skin to dry out, which can cause respiratory infections (7, pg 339). Excessive humidity can result in wet surfaces, which can breed microbial and fungal growth (7, pg 339). It’s also uncomfortable, icky, and not good for equipment.

All I need to know for this project are the ideal and maximum partial pressures of water vapor for our breathing gas mix. One standard atmosphere has a humidity of 1.38 kPa (at 74 ºF and 50% humidity)(3). One paper states that the water vapor partial pressure should stay between 5-16 mm Hg (0.67-2.13 kPa)(14, pg 482). Another states that 1.0 +/- 0.33 kPa of water vapor (at 22 ºC and relative humidity of 40%) (13). The NASA Human Integration Design Handbook states that 0.19 psi (1.31 kPa) is the ideal partial pressure for water vapor (7, pg 339).

Also, NASA requirements state that the average relative humidity must be maintained either between 30%-50% (nominal) and 25%-75% (tolerable) for each 24-hour period (assuming the temperature is within the nominal range) (7, pg. 339). Values outside this range are only allowed for 1-24 hours (7, pg 339).

These pressures aren’t much compared to the total pressure in the habitat. I think for the One Standard Lunar Homestead Atmosphere I’ll use NASA’s requirements of 1.31 kPa at 40% relative humidity and 21 ºC. I’ll need to make sure the Homesteaders have the equipment to fine tune the temperature and humidity to their tastes.

Carbon dioxide

Determining how much carbon dioxide we need in our Homesteader’s breathing gas is relatively easy. NASA doesn’t have a minimum limit for carbon dioxide (7, pg 325). Too much carbon dioxide can bring on nausea, headaches, increased heart rate, rapid breathing, confusion, convulsions, and loss of consciousness (7, pg 325).

Some data:

• At sea level, air contains 0.04% carbon dioxide with a partial pressure of 0.005 psi (0.034 kPa).
• NASA’s 1000 day limit for carbon dioxide is 0.07 psia (0.48 kPa)(7, pg 327).
• The Space Shuttle limited CO₂ partial pressure levels to 0.15 psia (1.03 kPa) (7, pg 326).
• The Constellation Program CO₂ partial pressure was limited to 0.0-.653 kPa)(7, pg 329).
• One paper suggests keeping CO₂ levels below 4 mm Hg (0.53 kPa) with a maximum of 7.6 mm Hg (1.01 kPa) (14, pg 483).
• OSHA (Occupational Safety and Health Administration) standards specify that carbon dioxide partial pressures stay below 0.4 kPa (13).
• Carbon dioxide partial pressures should be maintained below .53 kPa to minimize physiological strain over prolonged exposure (26, pg. 144). Partial pressures between .53-3.07 kPa will cause mild physiological strain (26, pg 144).
• Concentrations of CO₂ at less than 0.5% showed no detectible physiological effects for long durations. Concentrations of CO₂ between 0.5% and 3% cause mild physiologic strain. And concentrations of CO₂ greater than 3% causes significant stress (14, pg 483).
• Carbon dioxide 0.5% concentrations for the atmospheres under consideration:
  • One standard atmosphere at 101.325 kPa = 0.51 kPa
  • 100% oxygen at 5 psi (34.47 kPa) = 0.17 kPa
  • 74% oxygen/26% nitrogen at 34.47 kPa = 0.17 kPa
  • 78% nitrogen/21% oxygen at 70.11kPa = 0.35 kPa
    • 0.25% concentration = 0.18 kPa

It seems to me that the ideal SLHA would have a nominal carbon dioxide partial pressure of 0.18 kPa. This equates to 0.25% concentration for a 78% nitrogen/21% oxygen mix at 70.11kPa.

Density of oxygen and nitrogen

I wanted to know just how much nitrogen each gas mix would require since we’d have to import most of it. It’s been a long time since I took chemistry in school so let me know if I screwed something up.

• Molar mass of oxygen is 15.9994 g/mol
• Molar mass of nitrogen is 14.0067 g/mol

Since both nitrogen and oxygen are diatomic in air we need to multiple the molar mass by two to get:

• Molar mass of O₂ = 31.9988 g/mol
• Molar mass of N₂ = 28.0134 g/mol

The average molar mass of a gas mixture can be determined by the following formula:

M_{gas mixture} = (mole fraction of gas 1)(molar mass of gas 1) + (mole fraction of gas N)(molar mass of gas N)

So the molar mass of dry air with a 21% oxygen and 78% nitrogen composition is:

28.5701 g/mol = (0.21)(31.9988 g/mol) + (0.78)(28.0134 g/mol)

The ideal gas law (general gas equation) is a decent approximation of how many gases behave under many conditions.

PV=nRT

• P = Pressure (Pascals)
• V = Volume (cubic meters)
• n = Number of moles of gas (moles)
• R = Ideal gas constant = 8.314 (m³)(Pa)/(K)(mol)
• T = Absolute temperature (Kelvin) – 70 deg F = 294.261 K

Let’s run the numbers for the gas mixes under consideration:

• One standard atmosphere at 101.325 kPa
  • Oxygen = 20.9% with a partial pressure of 21.18 kPa
    • (21180 Pa)(1 m³) = (n moles)(8.314 m³ Pa/K mol)(294.261 K)
    • (21180 Pa)(1 m³) / (2446.485954 m³ Pa/mol) = n moles
    • 657 moles of oxygen
    • (31.9988 g/mol)(8.657 mol) = 277.01 g O₂
  • Nitrogen = 78.1% with a partial pressure of 79.13 kPa
    • (79130 Pa)(1 m³) = (n moles)(8.314 m³ Pa/K mol)(294.261 K)
    • (79130 Pa)(1 m³) / (2446.485954 m³ Pa/mol) = n moles
    • 344 moles of nitrogen
    • (28.0134 g/mol)(32.344 mol) = 906.07 g of N₂
• 100% oxygen at 34.47 kPa
  • Oxygen = 100% with a partial pressure of 34.47 kPa
    • (34470 Pa)(1 m³) = (n moles)(8.314 m³ Pa/K mol)(294.261 K)
    • (34470 Pa)(1 m³) / (2446.485954 m³ Pa/mol) = n moles
    • 090 moles of oxygen
    • (31.9988 g/mol)(14.090 mol) = 450.86 g O₂
  • Nitrogen = No nitrogen
• 74% oxygen/26% nitrogen at 34.40 kPa (Skylab mix)
  • Oxygen = 74% oxygen with a partial pressure of 25.46 kPa
    • (25456 Pa)(1 m³) = (n moles)(8.314 m³ Pa/K mol)(294.261 K)
    • (25456 Pa)(1 m³) / (2446.485954 m³ Pa/mol) = n moles
    • 405 moles of oxygen
    • (31.9988 g/mol)(10.405 mol) = 332.95 g O₂
  • Nitrogen = 26% nitrogen with a partial pressure of 8.94 kPa
    • (8944 Pa)(1 m³) = (n moles)(8.314 m³ Pa/K mol)(294.261 K)
    • (8944 Pa)(1 m³) / (2446.485954 m³ Pa/mol) = n moles
    • 656 moles of oxygen
    • (28.0134 g/mol)(3.656 mol) = 102.42 g of N₂
• 78% nitrogen/21% oxygen at 70.11kPa (High altitude mix)
  • Oxygen = 21% oxygen with a partial pressure of 14.72 kPa
    • (14723.1 Pa)(1 m³) = (n moles)(8.314 m³ Pa/K mol)(294.261 K)
    • (14723.1 Pa)(1 m³) / (2446.485954 m³ Pa/mol) = n moles
    • 018 moles of oxygen
    • (31.9988 g/mol)(6.018 mol) = 332.95 g O₂
  • Nitrogen = 78% nitrogen with a partial pressure of 54.69 kPa
    • (54685.8 Pa)(1 m³) = (n moles)(8.314 m³ Pa/K mol)(294.261 K)
    • (54685.8 Pa)(1 m³) / (2446.485954 m³ Pa/mol) = n moles
    • 352 moles of nitrogen
    • (28.0134 g/mol)(22.352 mol) = 626.15 g N₂

OK, so some comparisons:

• One standard atmosphere requires the most nitrogen gas = 906.07 g per cubic meter
• The High altitude mix saves = 279.92 g per cubic meter (31% savings)
• The Skylab mix saves = 803.65 g per cubic meter (87% savings)
• Obviously, the 100% oxygen mix saves = 906.07 g per cubic meter (100% savings)

The next question is how much nitrogen have we found on the moon? The answer is Not Much. The Lunar Sourcebook states that nitrogen concentrations in mature Lunar soils are about 50µg/g with some soils exceeding 100µg/g (29, pg 444). That means for every gram of Lunar soil we might find 0.00005-0.0001 grams of nitrogen. Unfortunately, many of the Lunar samples were contaminated by Earth’s atmosphere before they could be tested. So we really don’t know how much nitrogen is available.

Let’s put that into context. Assume the worst case scenario of 0.00005 grams of nitrogen for every gram of regolith. To extract enough nitrogen to fill one cubic meter we would have to process:

• One standard atmosphere = 906.07 g of N₂
  • 18,121,400 grams of regolith
  • 18,121.4 kilograms of regolith
  • 18.1214 metric tons of regolith

• 74% oxygen/26% nitrogen at 34.40 kPa (Skylab mix) = 42 g of N₂
  • 2,048,400 grams of regolith
  • 2,048.4 kilograms of regolith
  • 0484 metric tons of regolith
• 78% nitrogen/21% oxygen at 70.11kPa (High altitude mix) = 15 g N₂

• 12,523,000 grams of regolith
• 12,523.0 kilograms of regolith
• 12.5230 metric tons of regolith

That is a LOT of regolith to process just to fill one cubic meter. One NASA paper puts the minimum acceptable net habitable volume for a Mars mission at 25 m³ per person (30). Of course this is for a micro-gravity that can utilize all the available space but it’s a convenient number for this paper.

For research purposes, at this stage in the game, I’ve decided that a standard Lunar Homestead habitat pressure hull will enclose 620 m³ of habitable space. Check out the Pressure Hull Shape, Thickness, and Size page for more details.

• One standard atmosphere
  • 11,235.268 metric tons of regolith to fill 620 m³
• 74% oxygen/26% nitrogen at 34.40 kPa (Skylab mix)

• 1,270.008 metric tons of regolith to fill 620 m³
• 78% nitrogen/21% oxygen at 70.11kPa (High altitude mix)

• 7,764.26 metric tons of regolith to 620 m³

Ouch. That’s a lot of regolith we’ll need to process.

Here’s my take-away from all this:

• Adding ANY usable amount of nitrogen (or any other diluent gas) means that we’ll have to import it from Earth (or other sources eventually). Even the Skylab mix would require us to process a lot of regolith. That’s if these Apollo numbers are correct. The amount of regolith we would need to process skyrockets if there is less nitrogen than we think there is.
• This doesn’t even touch on how much nitrogen we’ll need for agricultural and industrial activities. Or to make up for losses due to leaks, EVA activities, and accidents.
• According to the Lunar Sourcebook, concentrations of carbon and hydrogen could be just as low as nitrogen (29, pg 444). If we’ve got to import those two critical elements, we should import nitrogen as well.

Since the pure oxygen mix is out (too many hazards) and the Skylab mix is out (the fire risk is still too great), that leaves the one standard atmosphere and the High altitude mix. The one standard atmosphere mix might be a little more comfortable but it’s going to cost us in shipping nitrogen. The High altitude mix is just as safe and uses 31% less nitrogen. This seems like a winner to me.

Nitrogen considerations

Earth’s atmosphere contains 78.08% nitrogen gas (N₂) and 20.95% oxygen gas (O₂)(1). Yes, these numbers are slightly different that the ones I used in the “one standard atmosphere section”. They seem to just be rounding differences. Besides, I’m not going to get bent over a tenth of a percent.

Nitrogen is a physiologically inert gas (meaning we don’t metabolize it) that acts as “filler” for our atmosphere. Nitrogen adds bulk and increases our atmospheric pressure while moderating the risk of fire due to the oxygen in the atmosphere.

All habitats will need some sort of atmospheric buffer, such as nitrogen, as breathing pure oxygen for extended periods is BAD. This presents one huge problem, one big problem, and a little problem.

• While nitrogen isn’t metabolized by humans, it does become saturated in our blood and tissues. This isn’t a problem until the pressure around the body decreases enough for the nitrogen to come out of our blood and form gas bubbles. As you might imagine, gas bubbles in the blood stream is not good. Check out the EVA considerations section below for more information. This is a big problem but we have solutions to mitigate it.
• Too much nitrogen affects the human brain. This is called nitrogen narcosis. Divers breathing a standard air mix sometimes experience at significant depths. Nitrogen narcosis can impair judgement and make divers do suicidality stupid things, like trying to pass their regulator to a fish. Fortunately, this is a little problem as the nitrogen partial pressure needs to be in excess of 395.070 kPa for narcosis to kick in (7, pg 330). Only a serious problem with life support would cause nitrogen levels to get this high.
• Luna is significantly lacking in nitrogen. This is a HUGE problem. Let me explain.

The problem is that Luna is deficient in nitrogen. We’ll be able to extract small amounts from the Lunar regolith (deposited by the solar wind and from Earth). That may not be enough to support both the atmospheric and agricultural needs of our Homesteaders. Martian Homesteaders have it a little better as the Martian atmosphere contains 1.9% nitrogen and nitric oxide has been found on the surface. But they will also struggle to be 100% self-sufficient with nitrogen. And we just don’t know about asteroids and comets yet. Nitrogen could be the single biggest bottleneck for the settlement of the solar system.

One solution for Lunar Homesteads is to import nitrogen from Earth. This might not be a horrible option once a robust transportation system is in place. Shipments of ammonia (NH₃) from Earth can be broken down into nitrogen and hydrogen. The hydrogen can be combined with Lunar oxygen to form water (always useful and in demand) and nitrogen. Earth could ship nitrogen throughout the solar system if necessary. Or at least until a large supply is located elsewhere and extracted.

The other option is to reduce the need for nitrogen. We’ll never be able to eliminate nitrogen because we need it for agriculture. Nitrogen is one of the basic elements found in all living matter on Earth. Our tissues are built with nitrogen. So Homesteads are going to need a supply of nitrogen anyway. The trick is to minimize how much we need for our breathing gas.

We can reduce the need for atmospheric nitrogen by:

• Reducing the overall habitat pressure
  • Humans can tolerate a significant reduction in atmospheric pressure with few ill effects. But tolerate is a far distance from comfortable. Our Homesteaders are just steely-eyed explorers. They are also families with children. They’re going to need an environment that is better than tolerable.
• Increasing the partial pressure of oxygen (abundant on Luna) and decreasing the nitrogen content while keeping the total pressure the same.
  • One of the worst things that can happen to a habitat will be fire. Increasing the partial pressure of oxygen in the habitat significantly increases the risk of fire. The Apollo 1 fire that resulted in the deaths of three astronauts (Virgil “Gus” Grissom, Edward White II, and Roger Chaffee) was a direct result of pressurizing the capsule with 100% oxygen at 16.7 psi (2). This environment made everything in the capsule highly flammable. It was a terrible mistake and one our Homesteaders must not repeat. The partial pressure of oxygen at sea level is 21.228 kPa (101.325 kPa x0.2095=21.228 kPa). That’s the maximum Homesteads should allow.
  • High oxygen partial pressure can cause a whole raft of physiological problems. Check out the Anoxia, hypoxia, hyperoxia, and oxygen toxicity section below for more information.
• Decrease the partial pressure of nitrogen, decrease the total pressure, and manipulate the partial pressure of oxygen so it stays around 21%.
  • This is probably the best way to go. It would minimize the amount of nitrogen needed while providing enough to mitigate the risks of fire and health problems. The tricky bit is to figure out the sweet spot.
• Replacing nitrogen with another inert gas
  • The problem with this option is that there just aren’t ANY abundant inert gases on Luna that we know of. Maybe we can eventually tap into some kind of outgassing (theorized but not observed).
  • One paper states that there is little evidence to favor one inert gas over the others (20, pg 116).
  • Due to its greater specific heat, nitrogen is (marginally) the best buffer to prevent fires (20, pg 116).
  •  Plus, other gases have their own problems.
    • Hydrogen – Does NOT act as a fire suppressant!
    • Helium –
      • A thermal conductivity 6x that of nitrogen means our Homesteaders will get cold very easily. We’ll have to increase the temperature of the habitat by at least 2°-3°C (7, pg 330) or run it at 25.6 °C (7 psi) or 29.4 °C (5 psi)(24, pg 1). The amount of clothing has a significant impact on how cold someone would feel (more clothing traps more nitrogen next to the person, which allows for more efficient heat transfer)(24, pg 1).
      • Replacing nitrogen with helium increased the rate of flame spread (23, pg 3). However, ignition required more energy input (24, pg 9) and the time between an item producing smoke and catching fire is increased (24, pg 15).
      • Helium-oxygen mixes show greater leakage than nitrogen-oxygen mixes (24, pg 4). However, the helium-oxygen mixes weighed much less (24, pg 4). This would have an impact on logistics.
      • In some experiments, helium diffused into vacuum tubes and damaged some equipment (24, pg. 4). Don’t laugh. Early Lunar-made electronics may well contain vacuum tubes because they can be made locally. Try making transistors or integrated circuits with basic tools.
      • It’s a pain to store long-term. Plus, it makes vocal communication very difficult because of its low density. This may not be a significant issue in a low pressure atmosphere.
    • Neon – Actually seems to be marginally better than nitrogen for most parameters (20, pg 118).  But we just don’t have enough data on how long-term exposure would affect humans.
    • Argon – Increases the chance of a decompression injury (14, pg 472).
    • Krypton – Increases the chance of a decompression injury (14, pg 472).
    • Xenon – High concentrations act as a decent anesthetic. Increases the chance of a decompression injury (14, pg 472).
  • Making some kind of strange multi-gas mix using whatever gases we find seems like asking trouble. It’s hard enough dealing with just two gases. I don’t want to think about a four or five gas mix.
  • Since we’ll probably have to import some type of buffer gas, we may as well stick with nitrogen. We’re evolved for it and we know the long-term effects.

Maybe we’ll get lucky and find lots of nitrogen on Luna, Mars, and beyond. But for now I’m going to plan on it being a scarce, strategic resource that only Earth can provide in substantial quantities. We can always change things down the road.

Engineering considerations

Listed below are some of the engineering issues we’ll have to consider. I pulled these directly from resource 21, page 17 and added some of my own. I’m less concerned about the engineering problems than I am about the health and risk issues. Mass and weight are much less of an issue since we’re building habitats on the Lunar surface instead of spacecraft. And we’ll design things so they are easily maintained and repaired so reliability becomes less of an issue.

• Weight (mass)
  • Structure of cabin wall
  • Tankage for gas
    • Leak replacement
    • Airlock cycling
    • One-gas vs two-gas
    • High pressure, low pressure, cryogenic, chemical form (like water)
  • Air conditioning systems
    • Ventilation fan
    • Atmosphere processing fan
    • Equipment cooling fans
    • Cooling system (pumps, reservoirs, tubes, valves, radiator, and heat exchanger)
  • Reliability factors
• Transient phenomena
  • Decompression time after puncture
  • Transient overloads due to life support failure
• Power requirements
• Economic and operational factors
  • Development time
  • Use of existing hardware and equipment
  • Maintenance and convertibility
  • Crew acceptance
  • Contaminant buildup
  • Qualification testing
  • Environment for inflight experiments
  • Complexity of design and operation
  • Costs

EVA considerations

On occasion our Homesteaders will need to leave the habitat (called Extra-Vehicular Activity). We’re going to do our best to minimize this because going outside significantly increases the risk (radiation, impactors, vacuum, dust, etc.) to our Homesteaders. An additional risk we need to worry about in the pressure difference between the habitat and the EVA pressure vessel (i.e. vehicle or suit).

Just like for scuba divers or miners, a sudden decrease in ambient pressure can give our Homesteaders a decompression injury (aka “the bends”). Decompression injuries (DCIs) are caused by dissolved nitrogen in the blood coming out of solution and creating small gas bubbles. These bubbles travel through the body in the circulatory system and can become stuck, blocking blood flow. This causes all kinds of problems, especially if it happens in the lungs, heart, or brain.

The concern here is if there is a large difference between the habitat pressure (where our Homesteader’s blood has achieved nitrogen saturation) and the EVA pressure vessel. Our Homesteader will have to spend hours breathing pure oxygen to reduce the amount of nitrogen in their blood if the difference is too great. This is an annoyance at best. It’s life threatening if something happens and our Homesteader has to go EVA NOW (evacuate, patch a hole, etc.). Not taking the time to pre-breathe puts our person at increased risk for a DCI.

There are several possible solutions for this situation:

• Design the EVA vessel to handle the habitat pressure
  • Historic and current space suits can’t operate at higher pressures. They encapsulate the person in a breathable atmosphere. The problem is that they are made of fabric and are flexible. These suits become stiffer as their internal pressure increases. Even at reduced pressure they are difficult and tiring to use. Too high of a pressure and they will fully inflate and be impossible to use. Additionally, the Lunar dust will quickly degrade the fabric (Apollo suits showed significant damage after just a few days). The final nail in the coffin for inflated space suits is that they are made from high-tech materials. That’s OK for Earth-based industries but not for Lunar manufacturing. Homesteaders will have no way to repair or replace these suits without resupply from Earth. We’ll need a better way.
  • Counter-pressure suits are currently under development and show promise. These suits only pressurize the helmet instead of the entire body. The rest of the body is covered by layers of compression fabric. The compression simulates atmospheric pressure. Counter-pressure suits are comfortable and flexible. The challenge is keeping constant pressure on the joints (armpits, inside elbows, back of knees) as moving the joint creates voids between the fabric and the skin. There are some interesting solutions to this problem being researched. The bigger problem is the same as standard suits; high-tech fabrics that Homesteads will not be able to manufacture initially (or ever).
  • Hard shell pressure suits (also called atmospheric diving suits) were initially created for deep sea diving. The suit had to be thick, heavy, and constructed to exacting standards to withstand the crushing depths. A Lunar hard suit would be much thinner, lighter, and less engineered as it only needs to contain one standard atmosphere (at most). The major problem is with the joints, again. Getting a full range of motion requires a complex set of rotating metal joints. The Lunar dust won’t do these joints any good either. While a Homestead could theoretically manufacture the rest of the suit (it’s metal not fabric), I’m not sure about the joints. One major advantage of a hard suit is that it can operate with the same atmospheric pressure as the habitat. NASA is researching hard suits (AX-5) but I’ll probably take a whack at it later. Still, metal hard suits will probably be the suit of choice for Homesteaders.
  • A “pod” might be an even better choice than a metal suit. A pod is a cross between a suit and a vehicle. Its primary power will be electrical instead of human. The Homesteader will sit or stand in the pod and use mechanical manipulators instead of gloved hands. A pod is basically a very small pressure hull with a power supply (limited), life support (limited), propulsion (short-ranged), and manipulators. Since I’m already working on pressure hulls, a pod isn’t much of a leap. And there’s no reason not to make it operate at the same pressure as the Homestead (no DCI). A pod may be the best option overall for Homesteaders.
• Reduce the habitat pressure to the EVA vessel’s capability
  • This is the least desirable option. Every Homesteader is going to live in the habitat but not everyone will be allowed to conduct EVA. And EVAs shouldn’t happen all that often. The habitat needs to be as comfortable as possible for everyone. EVA can be a bit uncomfortable.
  • Doing this may allow Homesteaders to use flexible suits without significant risks of getting a DCI.
• Design Homesteading technologies and techniques to absolutely minimize the need for Homesteaders to conduct EVA, especially emergency EVA.
  • This is one of the highest priorities. Clever design, multiple redundant systems, and extensive use of basic robotics are the key to making Lunar Homesteading work. The Lunar surface is dangerous. Let the robots take the risk. Humans should be tasked with oversight (or direct control), troubleshooting, maintenance, and repair. With everything done INSIDE the habitat. Dangerous grunt work is for robots.

Anoxia, hypoxia, hyperoxia, and oxygen toxicity

Humans need oxygen. Too little, too much, and too much at too much pressure are all big problems.

• Anoxia – No oxygen in your breathing gas. No bueno. Get more oxygen immediately or die.
  • 0-0.89 psi (6.14 kPa) – Near-immediate unconsciousness, convulsions, paralysis. Death in 90 to 180 seconds. (7, pg 321)
• Hypoxia –Not enough oxygen in your breathing gas. Below a certain point you pass out and die. Above that point hypoxia causes sleepiness, headaches, and the inability to perform simple tasks (like fixing the problem)(3). Messing up the breathing gas mix or pressure can cause hypoxia. So can pressure leaks or a buildup of carbon dioxide. Falling oxygen pressure will dull the brain, making it difficult to solve the problem (or even recognize there IS a problem).
  • 2.65 psi (18.27 kPa) – Accepted limit of alertness. Loss of night vision. Earliest symptom is dilation of the pupils. (7, pg 321)
  • 2.20 psi (15.17 kPa) – Performance seriously impaired. Hallucinations, excitation, apathy. (7, pg 321)
  • 1.93 psi (13.31 kPa) – Physical coordination impaired, emotionally upset, paralysis, loss of memory. (7, pg 321)
  • 1.62 psi (11.17 kPa) – Eventual irreversible unconsciousness. (7, pg 321)
• Hyperoxia – Too much oxygen in your breathing gas. Increases the risk of fires, lung inflammation, respiratory and heart problems, blindness, and unconsciousness (3).
  • High partial pressures of oxygen can increase blood pressure and reduce heart rate (10).
  • An oxygen partial pressure of 60 kPa can lead to pulmonary toxicity and irreversible pulmonary fibrosis (10). An oxygen partial pressure of >160 kPa can cause convulsions within minutes and with little or no warning (10).
  • Oxygen partial pressures in the 53.329 – 101.325 kPa range cause a bunch of respiratory and nervous systems problems (nausea, substernal distress, atelectasis, paresthesia, and bronchitis) (14)
  • Additionally, prolonged exposure to 100% oxygen at 101.325 kPa has led to blindness, heart problems, and unconsciousness (7, pg 319).
  • Oxygen partial pressures in the 26.6645 – 53.329 kPa range can cause respiratory, renal, and hematological issues. Pulmonary and aural atelectasis have been reported. Changes in blood enzymes and in mitochondria of the liver and kidneys have also been reported. (14)
  • Long periods of exposure to an atmosphere with an oxygen partial pressure of 33.46 kPa have demonstrated changes in red blood cell fragility and cell wall permeability (7, pg 322).
• Oxygen toxicity – Breathing oxygen with a partial pressure greater than 41.3299 kPa can cause oxygen toxicity (3). This situation will only happen if something goes seriously wrong with the life support. That’s almost twice the partial pressure of oxygen at sea level.

Absorption/resorption atelectasis

Atelectasis is a complete or partial collapse of a lung or lobe of a lung which develops when alveoli within the lung deflate. There are a lot of causes for atelectasis but the one we’re concerned with here is caused by a 100% oxygen atmosphere. A person in a 3.8 psi (26.2 kPa) 100% oxygen atmosphere is 370 times more likely to experience atelectasis than breathing 1 standard atmosphere (20, pg 115).

The partial of pressure of oxygen in the alveoli at sea level is 2.01 psi (13.90 kPa) (7, pg 319) or 14.16 kPa (11). With acclimation it’s possible to live (with reduced function) at 3658 m with an alveolar oxygen pressure of 1.05 psi (7.24 kPa) (7, pg 319). At an alveolar oxygen pressure of 1.65 psi (11.38 kPa) vision starts to degrade (7, pg 320) and below this point, down to around 1.33 psi (9.17 kPa), mental performance is degraded (7, pg 321). Consciousness starts being affected around 0.67 psi (46.20 kPa) (7, pg 321).

The formula to calculate the partial pressure of oxygen in alveoli for any atmosphere is (7, pg 321):

PAO₂= FiO₂(Pb-47) – PCO₂* (FiO₂+ 1 – FiO₂/0.85)

• PAO₂= alveolar partial pressure of oxygen
• FiO₂= oxygen fraction in the breathing atmosphere
• Pb = barometric pressure of the breathing mixture
• 0.85 = assumed respiratory exchange ratio
• PCO₂= partial pressure of CO₂

Each breath of atmosphere at sea level is 78% nitrogen and 21% oxygen. The alveoli in our lungs absorb some of the oxygen (which dissolves into our blood), reducing the pressure in the alveoli. But since most of the gas in the alveoli is nitrogen, there is enough pressure to prevent the alveoli from collapsing.

Oxygen is extremely soluble in blood and quickly diffuses through the alveoli. Breathing 100% oxygen for an extended period of time means there is no nitrogen (or other inert gas) to keep the alveoli inflated as the oxygen is absorbed. Temporary obstructions (like those caused by infection) can increase the incidence of atelectasis (20, pg. 115). The oxygen can be absorbed so quickly that the sudden pressure drop at the end of each breath can cause atelectasis.

Another problem with high oxygen environments is that the alveoli use a lipid surface agent to reduce the surface tension of the liquid layer in them below that of plasma (the alveoli act as bubbles) (18, pg 52). The increase in atelectasis could be caused by the inactivation of this alveolar surfactant (18, pg 53).  This is from an old paper so I have no idea if further research was done. Apparently there was also research in aerosols (such as Alevaire) to alleviate this problem during high-g loads.

According to one paper, under normal conditions our lungs are just on the edge of collapse anyway (18, pg 54). The pressure of the nitrogen and the action of the alveoli surface agent are usually enough to prevent collapse (18, pg 54). Increasing the percentage of oxygen, decreasing the pressure, and adding acceleration loads increases the risk of atelectasis by several orders of magnitude (18, pg 54).

Minor cases of atelectasis may resolve on their own. More serious cases require a positive pressure ventilator or a large syringe (to withdraw the gas). A chest tube might be required in severe cases. Experiencing atelectasis can make a person more prone to pulmonary infections (20, pf 115). We don’t want to deal with any of this if we can avoid it.

Recommended ways to prevent atelectasis are (18, pg 54):

• Deep breathing.
• Using positive pressure (such as an air mask) to force the alveoli open.
• Breathing inert gas mix before acceleration maneuvers.
• Maintaining as much inert gas in the cabin atmosphere as possible.

While not atelectasis, occasionally a closed pocket of gas can occur in a crew member’s body (in the middle ear if not periodically equalized or in the lungs during high stress) (3). This pocket can collapse, causing the oxygen and carbon dioxide present to be rapidly absorbed (3). A diluent gas is absorbed more slowly (or not at all), helping to prevent a pocket collapse from happening (3).

There is a lot of literature regarding atelectasis during anesthesia but I haven’t found much space-related research. I’ll keep looking but please let me know if you find any.

Leakage

Most spacecraft designs use a steady state leakage rate of 1-5 lbs (0.45 – 2.27 kg) per day (mostly through the hatch and hangar seals)(24, pg 3). A standard design value for seal leakage in a N₂O₂ or O₂ atmosphere at 7 psi is 7×10^-3 lbs/day/inch while HeO₂ would be 4.21x7x10^-3 lbs/day/inch (24, pg 4). I’m not going to do the SI conversion here. Other sources of steady state leakages for a 7 psi N₂O₂ atmosphere include (24, pg 5):

• Hatches and hanger door seals – 2.85×10^-5 to 7×10^-3 lb/day/inch
• Electrical leads – 10^-7 lb/day/inch
• O-ring seal – 10^-9 lb/day/inch
• Valves – 10^-3 lb/day/valve
• Seams and joints (welded) – 10^-4 lb/day
• Diffusion through vehicle skin – 10^-14 lb/day

NASA spacecraft leakage requirements (25, pg 6):

• “Typical” leakage for pressurized module – 0.11 to 0.28 lbm/day
• Space Shuttle – 0.275 lbm/day
• International Space Station: US Laboratory – 0.114 lbm/day
• ISS: Node 1 – 0.117 lbm/day
• ISS: MPLM – 0.15 lbm/day
• Spacelab – 2.98 lbm/day

NASA pressure vessels are designed to Leak Before Burst (LBB), which means that the pressure vessel will begin to leak (in order to relieve internal pressure) instead of failing catastrophically if the Maximum Design Pressure (MDP) is exceeded (25, pg 1). The Maximum Design Pressure is determined by applying the worst two credible failures that would cause an increase in pressure (called a Two Fault Scenario)(25, pg 1). NASA mandates that the design burst factor is a minimum of 1.5, with testing conducted at 2.0 (25, pg 1).

NASA has used both positive (to prevent the hull from rupturing) and negative (to prevent the hull from crushing) pressure relief valves on its spacecraft (25, pg 2). Burst disks are used when there is a low probability of needing pressure relief and when it’s not critical that the pressure vessel is accidentally vented (25, pg 2). Obviously we wouldn’t use burst disks in our pressure hulls.

Pressure relief valves could be used though. NASA uses negative pressure relief valves (NPRV) to pressurize the crew area if the outside pressure is greater than the internal pressure (typically for ground operations and reentry) (25, pg 2). The cracking and reseat pressures for NPRVs are typically 0.1-0.3 psid (25, pg 2). I don’t see a need for NPRVs in our habitats as I can’t see a reason for the external pressure to ever exceed the internal pressure (the habitats live in hard vacuum).

Positive pressure relief valves (PPRV) release excessive internal pressure before the hull can burst (25, pg 2). A life support failure that releases compressed gas into the cabin or a fire are two ways this situation might happen (25, pg 2). Outside thermal fluctuations are another possible cause (25, pg 5) as well as leaks from any pressure vessels stored internally (25, pg 6). The cracking and reseating pressures for PPRV are usually 14.8-15.5 psid (25, pg 2). We’ll definitely need some kind of pressure relief system but I’m pretty sure we can come up with something better than just venting into space. Remember, mass won’t be an issue for habitats built on site.

Why is this important now? Because once I figure out the operating atmosphere I need to build in an overpressure safety buffer.

Data

Total pressure limits for crew exposure (7, pg 319)

• Pressure ≤3 psi (20.68 kPa) = 0 time
• 3.0 (20.68 kPa) < Pressure ≤ 4.3 (29.65 kPa)  = 12 hours
• 4.3 (29.65 kPa)  < Pressure ≤ 7.5 (51.71 kPa) = 14 days
• 7.5 (51.71 kPa) < Pressure ≤ 15.0 (103.42 kPa) = Indefinite
• 15.0 (103.42 kPa)  < Pressure ≤ 17.0 (11.72 kPa) = 12 hours
• Pressure > 17.0 (11.72 kPa) = Contingency only

Oxygen partial pressure limits for crew exposure (in Pa) (7, pg 324)

• ppO2 > 82,737 = ≤ 6 hours
• 70,327 < ppO2 ≤ 82,737 = ≤ 18 hours
• 60,674 < ppO2 ≤ 70,327 = ≤ 24 hours
• 33,095 < ppO2 ≤ 60,674 = ≤ 48 hours
• 23,442 < ppO2 ≤ 33,095 = ≤ 14 days
• 18,616 < ppO2 ≤ 23,442 = Nominal physiological range. Indefinite with no measurable impairments.
• 17,237 < ppO2 ≤ 18,616 = Indefinite with measurable performance decrements until acclimatized (after 3 days).
• 15,168 < ppO2 ≤ 17,237 = 1 hour, unless complete acclimatization, otherwise risk acute mountain sickness.
• ppO2 ≤ 15,168 = Not allowed. Supplemental O2 is required to perform tasks without significant impairment.

Resources

1. NASA Earth Fact Sheet (nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html)
2. Space Safety Magazine – The Apollo 1 fire (www.spacesafetymagazine.com/space-disasters/apollo-1-fire/)
3. Man-Systems Integration Standards – Natural and Induced Environments (msis.jsc.nasa.gov/sections/section05.htm#_5.1_ATMOSPHERE)
4. The Space Shuttle Extravehicular Mobility Unit (EMU)(www.nasa.gov/pdf/188963main_Extravehicular_Mobility_Unit.pdf)
5. Facts that Prove that Adaptation to Life at Extreme Altitude (8848m) is Possible (zuniv.net/pub/Everest2.pdf)
6. MIDE Altitude to Air Pressure Calculator (www.mide.com/pages/air-pressure-at-altitude-calculator)
7. Human Integration Design Handbook (ston.jsc.nasa.gov/collections/TRS/_techrep/SP-2010-3407.pdf)
8. Effect of Environmental Parameters on Habitat Structural Weight and Cost (www.nss.org/settlement/nasa/spaceres/II-1.html)
9. Pulmonary atelectasis in subjects breathing oxygen at sea level or at simulated altitude (www.physiology.org/doi/pdf/10.1152/jappl.1966.21.3.828)
10. Diving and oxygen (www.ncbi.nlm.nih.gov/pmc/articles/PMC1114047/)
11. vCalc (www.vcalc.com/wiki/vCalc/Alveolar+gas+equation)
12. Oxygen Partial Pressure and Oxygen Concentration Flammability: Can They Be Correlated? (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160001047.pdf)
13. Space Settlements: A Design Study – Chapter 3 – Human Needs In Space (space.alglobus.net/75SummerStudy/Chapt3.html)
14. Selection of Space-Cabin Atmospheres (http://adsabs.harvard.edu/full/1967SSRv….6..452R) (same as 21 I think. Fixing it would be a huge pain in the ass so I’ll do it later).
15. Flammable and Toxic Materials in the Oxygen Atmosphere of Manned Spacecraft (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680015284.pdf)
16. Bounding the Spacecraft Atmosphere Design Space for Future Exploration Missions (spaceflightsystems.grc.nasa.gov/repository/NRA/cr-2005-213689.pdf)
17. A Survey of Fire-Prevention Problems in Closed Oxygen-Containing Environments (www.dtic.mil/dtic/tr/fulltext/u2/675817.pdf)
18. Selection of Space Cabin Atmospheres – Part 1: Oxygen Toxicity (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19630010874.pdf)
19. Selection of Space Cabin Atmospheres – Part 2: Fire and Blast Hazards (ia800302.us.archive.org/11/items/SP48SpaceCabinAtmospheresPartIIFireAndBlastHazards/SP-48%20Space-Cabin%20Atmospheres%20Part%20II%20-%20Fire%20and%20Blast%20Hazards.pdf) – Lots of technical information on the flammability of materials and specific impact hazards. Great for when I need to worry about fire extinguishment and impact threats.
20. Selection of Space Cabin Atmospheres – Part 3: Physiological Factors of Inert Gases (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670013549.pdf) – Large section on decompression injury and prevention with various buffer gases (old but still useful).
21. Selection of Space Cabin Atmospheres – Part 4: Engineering Tradeoffs of One- Versus Two-gas Systems (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670020669.pdf)
22. Flammability in Unusual Atmospheres – Part 1 – Preliminary Studies of Materials in Hyperbaric Atmospheres Containing Oxygen, Nitrogen, and/or Helium (pdfs.semanticscholar.org/376a/aa1f861c001d10255f8aeed2e59ad79d2ab8.pdf)
23. A Survey of Fire-Prevention Problems in Closed Oxygen Containing Environments (www.dtic.mil/dtic/tr/fulltext/u2/675817.pdf)
24. Engineering Criteria for Spacecraft Cabin Atmosphere Selection (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670027991.pdf)
25. Guidelines for Developing Spacecraft Structural Requirements; A Thermal and Environmental Perspective (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040085957.pdf)
26. Conceptual Design of a Lunar Colony (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730002509.pdf)
27. Skylab: A Guidebook: Chapter 3 (history.nasa.gov/EP-107/ch3.htm)
28. Engineering Toolbox (www.engineeringtoolbox.com/molecular-mass-air-d_679.html)
29. Lunar Sourcebook (www.lpi.usra.edu/publications/books/lunar_sourcebook)
30. Minimum Acceptable Net Habitable Volume for Long-Duration Exploration Missions (ston.jsc.nasa.gov/collections/trs/_techrep/TM-2015-218564.pdf)