Update 1-10-17: There is one additional limit to consider. Below about 3 psi pressure of pure oxygen, the absolute humidity is so low, it causes the lungs to dry out, crack, and bleed, if exposure is long-term. For a pure oxygen suit, the exposure time is actually fairly short, and the pressures proposed in this article (2.67 to 3.81 psia) are pretty close to the limit anyway, so they should not present a problem. The two-gas habitation atmospheres, where exposure is long-term, are proposed to be much higher (6.8 to 10.6 psia at 30% oxygen by volume), thus avoiding the problem completely.
Update 2-16-16: Had the wrong plots for Figures 10 & 11; now corrected.
This article updates and supersedes earlier articles on this subject. Those are:
Update 2-16-16: Had the wrong plots for Figures 10 & 11; now corrected.
This article updates and supersedes earlier articles on this subject. Those are:
1-15-16 Astronaut Facing Drowning Points Out Need for Better Space Suit
11-17-14 Space Suit and Habitat Atmospheres
2-11-14 On-Orbit Repair and Assembly Facility
1-21-11 Fundamental Design Criteria for Alternative Space Suit Approaches
Up to now, space suits have been designed with 100% oxygen atmospheres inside at suit pressures equaling or even exceeding the partial pressure of oxygen in Earthly air near sea level. That is probably “overkill”, which makes the design of more supple space suits difficult, whether they are conventional “full pressure” suits or the alternative mechanical counterpressure (MCP) suits. Space station atmospheres have usually been two-gas: oxygen with nitrogen for dilution, similar to Earthly air.
There are four things of importance to consider when running numbers for suit and habitation atmosphere design: (1) the displacement of dry breathing gas pressure by water vapor pressure inside the moist lungs, reducing the effective partial pressure of oxygen driving diffusion of oxygen across the lung tissues into the blood, (2) how much or how little oxygen is really needed to stay functional, (3) reducing the fire danger posed by oxygen enrichment, and (4) reducing or eliminating the “pre-breathe time” necessary to blow off dissolved nitrogen (or other dilution gases) from the blood when transitioning from a multiple-gas atmosphere to a pure oxygen suit. These are independent of suit type.
Water Vapor Displacement Effect
The movement of oxygen, nitrogen, other dilution gases (if any), and carbon dioxide across lung membranes is a diffusion process, driven by the differences of partial pressures, including those of the dissolved gases in the blood. Unlike those, the water vapor in the lungs is dominated by simple evaporation of the liquid phase at body temperature, attempting to reach the equilibrium value. This approach to equilibrium (a number straight from the steam tables) is not perfect, but it’s not a bad estimate either, and it is easy to compute, when non-equilibrium is not.
Having non-equilibrium water vapor pressure reduces the actual water vapor pressure, and thus reduces dry gas displacement. This leads to slightly-higher oxygen content inside the wet lung than one would calculate for equilibrium. Thus equilibrium design is a lower bound on oxygenation: you actually do slightly better than that in real life. It’s easy and it’s slightly conservative: a good tool.
Human body temperature is 98.6 F (37.0 C), at which the equilibrium vapor pressure of water is 0.061921 atm. That is the value you subtract from the supplied dry breathing gas pressure to determine the partial pressure of that dry breathing gas inside the wet lungs. The volume percentage of oxygen in that breathing gas, applied to the in-lung partial pressure of dry breathing gas, is then the partial pressure of oxygen in the air inhaled into the wet lungs. It will be less than the partial pressure of oxygen in the dry breathing gas. But, this wet in-lung oxygen is your real design criterion.
How Much or How Little Oxygen is Needed?
This depends upon whom you ask. The USAF says pilots must use supplemental oxygen when flying in an unpressurized cockpit above 10,000 feet. The USN is worried more than USAF about loss of night vision, something well known to be sensitive to hypoxia. So USN says to use supplemental oxygen above 5000 feet. The FAA says that pilots flying above 10,000 feet for more than 30 minutes, and that pilots flying above 14,000 feet for any time at all, must use supplemental oxygen. These FAA requirements apply to all civil aircraft, commercial or private.
Most of humanity lives within several hundred feet of sea level: call them “flatlanders” for convenience. There are lots of people living in cities near 5000 feet elevation. Flatlanders visiting these elevated places have essentially zero problems acclimatizing almost immediately. There are also quite a few cities around the world at 10,000 feet. Flatlanders visiting these elevated places require nothing to a few days to acclimatize, leading to few if any problems. Max airliner cabin altitude is also 10,000 feet.
There are a very few cities in this world located near 15,000 feet above sea level. The people who live there are well-acclimatized, but visitors might require significant time to acclimatize, and will experience some mild problems until they do. But they eventually do acclimatize, on a time scale of days to weeks, usually. Designing to a 15,000 foot equivalency is certainly feasible, therefore.
There are no cities, but there are a very few rural herders, and some mountain climbers, at 20,000 feet who use no supplemental oxygen. Not very many flatlanders can acclimatize to this, and it takes months or years to do so for permanent residency. Yet almost no one dies from this exposure, especially if it is not prolonged. So that’s too high for practical design equivalency, but it is survivable.
What that suggests is that space suits and habitat atmospheres should be easily feasible and quite acceptable anywhere between sea level and 10,000-to-perhaps-15,000 feet elevation-equivalent oxygenation. Based on the water vapor displacement discussion above, the criteria by which to measure this is the wet in-lung oxygen partial pressure, not the oxygen partial pressure in the dry ambient air at whatever equivalent altitude that we are considering.
This is a judgmental factor, governed by simple human experience. Most supplemental oxygen systems range from 50% to 100% oxygen, with well-known fire dangers. Most hospital oxygen is 60%, also with well-known fire dangers. NASA’s rough rule-of-thumb is that nothing over 30% oxygen is safe enough. For this article, I will use the 30% max oxygen criterion.
Both oxygen and nitrogen dissolve in the blood. If you move to another atmosphere at lowered oxygen and nitrogen partial pressures, what’s in your blood must come out of solution, a process that requires time. The bigger the differences in partial pressures driving the solution and un-solution rates, the faster the process. Except, if you try to do this too fast, the gases form bubbles in your blood before they can diffuse into your exhalation. This is “the bends”, which can be quite damaging, even fatal.
A person in a space habitation with a multiple-gas atmosphere must generally put on a suit with a pure oxygen atmosphere. The nitrogen (and any other dilution gases) in his blood must come out of his blood, but without forming bubbles. This takes time spent breathing pure oxygen at near-habitation pressures, before reducing the pressure to the suit design point and going outside. This time is called the “nitrogen blow-off” or “pre-breathe” time.
There is a rough rule of thumb derived from deep sea diving experiences, used by NASA, that says that if the ratio of nitrogen partial pressure to suit atmosphere pressure is 1.2 or less, then the required pre-breathe time is zero (decompression may safely be immediate). Whether this applies separately to all dilution gases, or collectively to the sum of their partial pressures, is perhaps still unclear, but a conservative design approach says apply it to the collective sum. So that is what I do here.
What is Required Not to Asphyxiate at High Altitude or in Space
You must have sufficient oxygen pressure in your lungs to oxygenate the blood. When that oxygen pressure is larger than the local ambient atmospheric pressure at altitude, a vented oxygen mask cannot work: you must then breathe oxygen at pressure-above-ambient. This is called “pressure breathing”, and it makes you subject to two classes of serious, even fatal, problems.
The immediate effect is from trying to breathe at about 2 psi or more above ambient pressure. This ruptures the lung tissues, and is called “pneumothorax”. It is irreversible and fatal: you literally drown in your own blood. This risk is well known among divers; it is why you never, ever hold your breath while diving with a breathing rig. Rising as little as 4 feet while holding your breath can kill you.
One of the longer-term effects is something called “edema”, which is body parts painfully swelling up with gases and fluids driven out of the blood. These go into the spaces between cells in the other tissues, which is what causes the swelling. For small exposed body parts, this takes a while: something like 30 minutes for a hand or foot exposed to vacuum, when the rest of the body is not. It can happen quicker with whole-body exposure: something like 10 minutes.
The other longer-term effect is simple blood pooling in uncompressed extremities, like the arms, and especially the legs, driven there by the pressure differences. You faint pretty quickly from this, something like 10 minutes’ exposure at most, 5-10 seconds at minimum. It’s quite similar to high gee-exposure in flight: blood pools in the legs, leaving your brain starved, and you faint.
So you must have pressure-breathing of oxygen, but your body must also be compressed by the same pressure, and this has to be fairly-evenly distributed. This body compression is called “counterpressure”. Many people were hurt, disabled, even killed, learning these rules.
History of High-Altitude Suits and Space Suits
There are two known ways to achieve the counterpressure: (1) put yourself inside a gas-tight balloon so that the gas pressure you breathe is also the counterpressure applied to your body, or (2) use a breathing gas helmet and use very tight garments to mechanically apply the counterpressure all over your body. The first approach is called a “full pressure suit”, and the second approach has been implemented as something called a “partial pressure suit”, or more generically, a “mechanical counterpressure” suit (MCP suit). The body simply does not care how the counterpressure is applied.
The first successful means of pressure-breathing at unsurvivable altitudes was a full pressure suit literally adapted from a deep-sea diver’s hard-hat diving dress, in the mid 1930’s. This was literally a rubber balloon with cloth inside to ease friction donning the suit, and protective canvas on the outside to prevent ruptures to the rubber balloon layer. Unlike the deep sea application, this suit is inflated above ambient pressures, which stiffens the garment immensely. You’re sealed inside, so moisture builds up from breath and sweat. And you are well-insulated and so cannot get rid of body heat. Movement was extremely restricted in such suits, and they were extremely uncomfortable, even debilitating. A person wearing one could actually do very little in the way of useful activity.
Starting in the late 1950’s, the advent of high-altitude spy planes and the beginnings of human spaceflight required something better than the partial pressure suit with its short protection interval. The full pressure suit was updated with better joints that flexed easier, and by the Apollo program, water-cooled underwear was added to help keep the astronaut cooler as he worked in the suit.
The suits and life support backpacks got very large and heavy, because of these heat and moisture control requirements. Mobility, while better, was still quite restricted. Particularly difficult is doing anything but rather gross tasks with the clumsy, bulky pressure gloves. The higher the suit pressure, the worse this problem is. Such suit designs went from 200 pounds of suit and equipment on the moon with Apollo, to nearly 400 pounds in the shuttle, and on the space station today.
Suits like this are designed as little “personal spaceships” surrounding the astronaut. The one garment must protect against any and all conceivable hazards. Requirements can be contradictory.
This kind of thing simply will not work very well for astronauts visiting the surfaces of other worlds (moon, Mars, etc.). Movement and joint flexion are still too restricted. The suits are heavy enough and clumsy enough to limit the ability of an astronaut who has fallen to get back up without help. If the suit pressure is lowered, these restrictions ease slightly, but there are limits to how much change can be made. But, this is where we have been since about 1960.
Except, for some extremely interesting experiments in MCP, that were never seriously developed.
Experiments in MCP
In the mid to late 1960’s, Dr. Paul Webb, an expert in high-altitude crew survival, conducted experiments with an improved MCP suit approach. He substituted elastic compression garments for the clothing drawn tight by inflated air capstans. At that time, the elastic materials used in pantyhose were brand new, and he used them. This suit design approach was intended for Apollo on the moon, but he could not get the design fully developed in time. It’s been mostly forgotten ever since.
These materials are not gas-tight; they are quite porous. You sweat right through them to cool, just as with Earthly clothing. There is no gas pressure in your clothing, your skin is really actually exposed to vacuum, but your clothing squeezes you, thus countering your breathing gas pressure. That’s all that is really required. See Figure 3. The backpack contained liquid makeup oxygen in a Dewar, plus controls.
No water-cooled underwear is needed, which eliminates the risk of drowning when your underwear springs a leak (something seen at least twice now with full pressure suits).
Puncture a full pressure suit, and you will die if it deflates before you can get inside. Puncture an elastic leotard (or even the old partial pressure suit), and there is no leak: there is no gas in the clothing to leak out. If the hole is under about 0.1 inch in size, you may safely ignore it. Simply sew it up later, once inside. If it is bigger, it still won’t kill you. You can avoid local vacuum injury to the exposed skin by a simple tight wrap of something like duct tape. Fix it later when you go inside, by sewing it up.
These elastic leotards are easily launderable; full pressure suits are not. That is a crucial advantage in dirty, dusty places like the moon and Mars, and for repeated long-term use anywhere.
And Webb changed the design philosophy: think of this as a pressure-breathing helmet and vacuum-protective underwear, over which you don whatever protective outerwear fits the task at hand.
Figure 4 – The MCP “Elastic Leotard” Provides Very Excellent Mobility
Dr. Webb’s experiments clearly demonstrated the suppleness and mobility achievable in this MCP elastic approach. The fact that his test subject was not wearing the insulated coveralls and hiking boots needed for the moon or Mars makes no difference. Those apparel items are something bought at Walmart or its equivalent. They do not need to be incorporated into the vacuum-protective underwear. Nor would they have added any significant impediment to the test subject wearing the elastic leotard.
The back bend maneuver shown in Figure 4 is completely impossible in today’s full pressure suits. Yet this test subject in an MCP suit could do the maneuver. Plus, the much smaller and lighter backpack evident in Figure 3 would not have compromised this ability.
Likewise, the ladder-climbing exercise in Figure 5 would also be virtually impossible in today’s full pressure suits. It was barely possible in the much-lighter (but still cumbersome) Apollo moonsuit. And again, the smaller MCP backpack would not have impeded this activity.
Figure 5 – MCP Suppleness Allows Performance of Tasks Impossible in Full Pressure Suits
In Dr. Webb’s design, there are elastic compression gloves and booties, so no body parts are left uncompressed. If you compare these gloves to the full pressure suit gloves in either Figure 2 or Figure 1 above, you can see at a glance these are far thinner and far more supple.
The elastic leotard MCP suit is not a single one-piece garment. You put it on in many separate pieces and layers. In particular, you can don and doff the gloves without disturbing the suit compression on the rest of the body. There is no risk of losing breathing gas pressure, as there is none anywhere in the suit, except the helmet and breathing bag.
Given the 30 minute interval before swelling starts in a vacuum-exposed hand, this presents the possibility of doing very fine tasks completely bare-handed in space, on a short-term basis. The only other requirement to do this is workpiece temperatures that will not cause thermal injury to the bare hand, and that has nothing to do with the suit design.
This work by Dr. Webb on the elastic leotard MCP suit was partly funded by NASA, so they knew by 1968-1970 that this kind of suit would work, and that it offered some very attractive characteristics. But NASA has never seriously followed up on this development, not back then, nor ever since. And yet, this is the kind of suit we really need for long term activities on the surfaces of other worlds, such as the moon or Mars. Dr. Webb tried over the years to get private funding without success, and has now died.
NASA has in recent years provided grant monies to Dr. Dava Newman at MIT to continue work on a variation of the elastic leotard MCP suit. This is academic research at relatively small funding levels, not a major engineering development effort. Dr. Newman’s variant looks at fewer layers of more-tailored elastic properties, something not available in Dr. Webb’s day. Her version is still quite a supple-looking “skinsuit”, shown in Figure 7.
Dr. Newman has since been hired to work directly at NASA. Yet, there is still no major development effort going on for MCP spacesuits. Insiders tell me the astronaut office is afraid of MCP because they have been erroneously told that skin exposure to vacuum is fatal. There’s simply no excuse for that, given Dr. Webb’s experimental success almost 5 decades ago.
Running the Numbers on Suit Pressures
I looked at a sea level-equivalent design, and a design equivalent to 10-15,000 feet, as bounds on what we might really do for spacesuits. This applies to either type (full pressure or MCP). There has to be a little leakage margin for the suit or the breathing system. You want the leaked-down pressures to still be adequate enough to get back inside without help.
I based my calculations on wet in-lung partial pressure of oxygen as the criterion, as discussed above. I used a spreadsheet to calculate the dry air pressures, and the displaced in-lung wet pressures for air as a function of altitude on a US 1962 standard day. Then I looked at pure oxygen at atmospheric pressures as a function of altitude, and did the wet in-lung displacement on those. The image of that spreadsheet is given as Figure 8. At the bottom of the image, I have converted the design point data into a variety of units of measure for the reader’s convenience.
For the low-pressure suit design, I looked at 10 to 14,000 feet air, highlighted green in the spreadsheet. There’s a yellow highlight on the 15,000 feet conditions that I used for the leaked-down condition. I settled on a 12,000 foot design. It’s pure oxygen counterpart is also highlighted green, at about 40,500 feet. The corresponding leakdown is about 42,000 feet. The wet in-lung oxygen partial pressures for this design are 0.120 atm nominal and 0.107 atm leaked-down, for a 12% margin.
Looked at the more usual way, the “low pressure” suit has a nominal suit pressure of 0.182 atm (2.67 psia), and can safely leak down to 0.169 atm (2.48 psia). The “sea level oxygen” suit has a suit pressure of 0.259 atm (3.81 psia), and can safely leak down to 0.236 atm (3.47 psia). They are both pure oxygen suits, requiring dessicant, carbon dioxide absorbent, and makeup oxygen. The margins look different using those numbers, at 7.7% low-P and 9.7% high-P. But, comparing wet in-lung partial pressure of oxygen is the more proper measure, and the more proper basis for calculating leak-down margins. So that is the way I did it, not the usual way.
Running the Numbers on Habitation Atmospheres
The minimum habitation air pressure obtains when you set its wet in-lung oxygen partial pressure equal to that in the suit. In other words, you let the suit drive the habitation atmosphere selection. You can always set it higher, but you should not set it lower than suit oxygenation. Doing this calculation as a function of design suit pressures, across a range of them, allows you to see the trends in what is important and what is not. You also do it for a range of oxygen concentrations (volume percentages) in the habitation, as the parameter on a parametric plot with multiple curves.
There are two questions to investigate: what effects (if any) suit pressure selection might have upon habitat pressure, and also upon the pre-breathe time requirements. Again, I did this in a spreadsheet, images from which are given in Figure 9.
The illustrated sequence of calculations is simple and straightforward. Start with the suit pressure, which is pure dry oxygen. Do the water vapor displacement subtraction to determine the suit wet in-lung oxygen partial pressure. Make that value the habitation wet in-lung partial pressure of oxygen. Use the volume percentage oxygen to figure the wet in-lung partial pressure of nitrogen, then sum the gases to obtain the wet in-lung partial pressure of dry air. Then add back in the partial pressure of water vapor to determine the habitation atmosphere total pressure before any vapor displacement occurs.
I did this for the normal Earthly air oxygen percentage of 20.94%, for the max allowable percentage acceptable from a fire risk standpoint (30%), and finally an even higher oxygen percentage that meets the “no pre-breathe time” rule of thumb. That last value is just about 41%. This data can also be plotted in the same basic format to visualize the true nature of the pre-breathe trends.
I plotted habitation atmosphere pressures versus suit pressures in Figure 10, parametric on oxygen percentages. For the highest suit pressure and lowest oxygen percentage, the minimum required atmosphere pressure can be right at sea level standard pressure, the rest are reduced-pressure.
I plotted the ratio of habitation nitrogen partial pressure to suit pressure (the pre-breathe factor) versus suit pressure in Figure 11, again parametric on oxygen percentages. Those are relatively less sensitive to suit pressure, but more sensitive to habitation oxygen percentage: richer oxygen is closer to feasibility for no pre-breathe time, and the stronger effect of the two.
You can eliminate pre-breathe time (at any feasible suit pressure) by using just about 41% oxygen, unacceptable from a fire hazard standpoint. Otherwise, use the greatest oxygen percentage (30%) that you safely can, and also use the lowest credible suit pressure (a weaker effect, but still significant) to achieve the minimum required pre-breathe time.
Figured at the nominal suit pressures for the “sea level oxygen” and low pressure designs discussed above, the corresponding habitat atmospheres are described (and converted to a variety of units of measure) in the data of Figure 12. Again, this is a spreadsheet image.
Figure 12 – Results for Preferred Suit Designs and Oxygen Percentages
I do not have a way to calculate the actual pre-breathe time requirement from the pre-breathe factor, something needed when that factor exceeds about 1.2. But, the larger the factor, the longer the time, that much is certain. So there really is some benefit in terms of pre-breathe time to using the lowest credible suit pressures.
Checking the Upper Limit
There is one final thing to check during pre-breathe activities: upper limits on the pressure of the pure oxygen. With no more than 1 atm of habitation pressure, this is not a problem. The upper limit for pure oxygen exposure is known to be 1 atm from deep-sea diving. Pure oxygen exposure becomes fatal at 2 atm, although this does take several minutes to occur. The usual first symptom is convulsions.
#1. Habitations should probably use a two-gas atmosphere that is oxygen-enriched (to the fire safety limit), basically an oxygen-enriched synthetic air: 30% by volume oxygen and 70% nitrogen. That is a simple and easy thing to do.
#2. The minimum habitation atmospheric pressure depends upon the selected suit design pressure, ranging from near 0.462 atm (6.8 psia) for the low-pressure designs, to 0.720 atm (10.6 psia) for the higher-pressure designs, all when using the 30% oxygen composition.
#3. The range of credible suit pressures extends from 0.182 atm (2.67 psia) to 0.259 atm (3.81 psia).
#4. Using the lower suit pressure design decreases required pre-breathe time, but not to zero.
#5. The suit pressure designs and habitation atmospheres recommended here apply to either full-pressure suits or MCP suits.
#6. Far better mobility in a much lighter and more versatile suit design can be achieved using MCP; so these suits need to developed into a routinely-reliable form as soon as possible!
#7. Raising the habitation pressure above minimum increases pre-breathe time required somewhat.