Monday, November 17, 2014

Space Suit and Habitat Atmospheres

See also updates 11-20-14 and 11-26-14 and 1-1-15 at end.

Spacesuits are usually designed for pure oxygen “atmospheres“ supplied as breathing gas.  This is especially important to full pressure suit designs,  as lower pressures can be used with pure oxygen,  which eases the joint restriction design problem.  However,  long-term breathing of pure oxygen is detrimental to human health.  Plus,  it is a very serious fire hazard:  flammability and explosiveness are greatly amplified in pure oxygen at full atmospheric pressure. 

All of that taken together means that the habitation for astronauts (be it a ship,  a space station,  or a base on another planet) needs a suitable diluent gas.  Such atmospheres are thus inherently two (or more) gas systems.  We evolved in an atmosphere of 20.946 volume percent oxygen,  about 78% nitrogen,  and around 1% argon,  plus some other trace gases. 

Although sea level pressure is “the standard”,  there are many human communities living at altitudes where pressures are substantially lower.  Many large populations live near 10,000 feet elevation,  some near 15,000 feet,  and a very few approaching 20,000 feet.  Air pressure,  and oxygen partial pressure,  are both substantially lower at altitude. 


The pilots of aircraft are required to use supplemental oxygen when flying above certain altitudes.  The exact standards vary,  but the US Navy uses 5000 feet as the altitude above which oxygen must be used.  Most other authorities use 10,000 feet as the “trigger” for mandating oxygen.  Below those altitudes,  pilots seem to be fully functional.  Therefore,  atmospheric oxygen concentrations resembling those in the 5000-10,000 feet altitude range make pretty good criteria for the minimum life support requirements of any space suit design.  See also Figure 1.  These data were also used in “Fundamental Design Criteria for Alternative Spacesuit Approaches”,  posted this site 1-21-11.  


 Figure 1 -- Life Support Experiences

Suit Criteria

The Earth’s atmosphere has very variable humidity.  Dry air composition is 20.946 volume percent oxygen.  Water vapor acts to “displace” dry air,  so there is a slightly-lower oxygen partial pressure to breathe on very humid days versus very dry days,  at any altitude.  At human body temperature,  the steam tables show the partial pressure of saturated water vapor to be just about 47.06 mm mercury. 

Respiration depends upon the partial pressure of oxygen in the air we breathe,  not the volume percent.  This is because so many biochemical processes are driven by osmotic pressure differences. 

These two effects (water vapor displacement and partial pressure-driven osmosis) taken together say that the actual oxygen requirement,  when designing space suit and habitat atmospheres,  is the partial pressure of oxygen in the near-equilibrium saturated environment of the lungs.  This same water vapor displacement effect must be taken into account in selecting the pressure of the pure dry oxygen in the suit.  (There are other displacement effects,  but all are far less significant than this water vapor effect.)

The other factor to consider is leakage from the suit.  Suit pressures can be increased to compensate for losses that might (or might not) occur.  The “usual” factor seems to be 1.10 for leakage effects.  The suit oxygen pressure selection approach is shown in Figure 2,  Figure 3 presents the actual data.  This applies to any type of suit,  not just the full pressure suits that have been standard. 

Suit designs that supply pure oxygen at compression pressures between 147 mm mercury (10,000 feet) and 167 mm mercury (5000 feet) would thus seem to be quite adequate.  196 mm mercury corresponds to full sea level oxygen.  Those figures include the water vapor displacement effectbut not the increase for leak compensation.  Using a factor of 1.10 as leak compensation,  the design suit pressures become 216 mm mercury for sea level-based,  187 mm mercury for 5000 ft-based,  and 161 mm mercury for 10,000 foot-based designs.  

Figure 2 – Selecting Pressure in a Pure Oxygen Suit


 Figure 3 – Pressures in a Pure Oxygen Suit of Any Type.

Getting from Suit to Habitat Atmosphere

Since we evolved to breathe them,  oxygen,  nitrogen,  and argon make pretty good choices for the gases in the atmosphere of any astronaut habitat.  This is important because interplanetary missions will require living in such habitats for years at a time.  Oxygen-nitrogen is a basic two-gas system.  We might add substantial argon as an additional diluent for reducing the risk of fire. 

The problem going from a two-gas system in the habitat to pure oxygen in the suit is a requirement to “blow off” the nitrogen (and any third diluent gas) dissolved in the blood,  without risking “the bends”.  This is “pre-breathe” time,  and it is quite significant with the atmospheres used in the ISS and associated spacesuits.  There is a tested criterion for this:  if habitat partial pressure of nitrogen ≤ 1.20 suit pure oxygen pressure,  then no pre-breathe is needed. 

So,  once a design suit pressure has been selected,  this factor of 1.2 should be used to determine the maximum partial pressure of nitrogen in the habitat atmosphere.  Suit and habitat share the same in-lung wet partial pressure of oxygen.  The in-lung wet and habitat dry atmospheres share the same total pressure.  If this a multiple-gas atmosphere in the habitat,  then we know its total pressure as the sum of the wet O2 and N2 (and Argon,  if any) partial pressures,  and can figure the volume percentage of oxygen in the dry mix as its partial pressure divided by the total.  That process logic is a part of Figure 4,  with the numbers as a function of suit pressure in Figure 5. 

As a two-gas system,  all the suit pressure designs under these criteria share the same habitat atmosphere oxygen percentage:  45%.  That sounds like a high fire risk,  unless low total pressure mitigates it.  These total pressures are much lower than standard atmospheric:  ranging from only 6.86 psia for the 15,000 foot-based design up to only 9.19 psia for the sea-level-based design. 

These total pressure and oxygen percent data and the oxygen partial pressure can be used to evaluate fire risks.  I have no ready criteria for this myself.  It can be tested experimentally in test chambers right here on Earth,  however.  Adding argon lowers the percentage oxygen (but not its partial pressure),  and raises habitat total pressure.  


 Figure 4 – Scaling Up from Suit to Habitat to Avoid Pre-Breathe Requirements

Figure 5 – Basic Two-Gas Habitat Atmospheres as a Function of Design Suit Pressure

Three-Gas System for Reducing Fire Risks?

I do not know of a pre-breathe criterion factor for argon.  One might assume the same 1.2 factor over suit oxygen pressure applies,  so that one can add substantial argon to the mix,  at a partial pressure equal to that of the nitrogen. 

On the other hand,  monatomic argon has a molecular weight near 39,  versus diatomic nitrogen’s molecular weight of 28.  If osmotic rates depend directly upon molecular weight,  then argon should be slower than nitrogen to dissolve into,  and out of,  the blood.  Then perhaps the pre-breathe criterion for argon should be nearer 0.86 than nitrogen’s 1.20.  We just don’t know.  But,  that assumption lowers the argon content,  which is in the safer direction.  Results are in Figure 6.

Figure 6 – Three-Gas Atmospheres as a Function of Design Suit Pressure

Results:

#1. Go with the Navy oxygen mask criterion of 5000 feet as slightly more conservative than the 10,000 foot criterion.   Your suit (whatever its design) is then at-minimum pure O2 at 3.28 psia.  Factored up for leaks,  this is 3.61 psia.  Even full pressure suits can be somewhat supple at pressures this low. 

#2. For a two-gas habitat atmosphere,  use a pre-breathe factor of 1.20 for nitrogen as well-supported.  Use the in-lung wet suit oxygen partial pressure as the in-lung wet habitat partial pressure of oxygen.  Results are given in figure 5 for all the suit designs.  For the recommended 5000 foot-based suit design and the two-gas system,  your habitat atmosphere is 45.45% O2 and 54.55% N2,  at 6.13 psia,  with zero pre-breathe time required to don the spacesuitThe 45% O2 is rather alarming as a fire hazard.  It is at best unclear whether the low pressure of 6.13 psia could mitigate this risk.  At worst,  this is simply too severe a fire hazard. 

#3. For a 3-gas habitat atmosphere,  we use all the same design factors,  plus we set the argon pre-breathe factor at about 0.86.  Results are given for all the suit designs in Figure 6.  For the 5000 foot-based design,  we are looking at a habitat atmosphere that is 32.68 volume % O2,  39.22% N2,  and 28.10% Ar,  at a total pressure of 8.16 psia,  again with zero pre-breathe time required to don the spacesuit.  Adding the third gas greatly reduced the objectionable high volume percent oxygen.  But it is unclear whether this is enough reduction. 

#4. It is entirely possible that more argon could be used,  once a pre-breathe limit factor has been established for it.  That would act to lower oxygen percentage and raise habitat total pressure,  but would not affect the suit design. 

#5. It might also be possible to add a fourth gas (preferably an inert one,  and one not linked to the biology of life as CO2 is).    This would also lower oxygen percentage and increase habitat total pressure,  without changing the suit design.  One candidate might be helium,  which has been used in deep sea diving.  It takes longer to decompress than nitrogen in that application,  which means its pre-breathe factor is lower than nitrogen’s,  in spite of the low molecular weight. 

#6. Note that in the tables given in Figures 5 and 6,  that oxygen percentages are constant,  regardless of the suit design pressure.  Raising suit pressure will not cut oxygen percentage in the habitat,  without violating the pre-breathe factors that make EVA’s into practical operations. 

Speculation:

Assume as a wild guess that the pre-breathe factor for helium is 0.5Assume also that the pre-breathe factor for argon is closer to that of nitrogen,  for about 1.0.  Under those conditions,  a hand calculation for the 5000 foot-based suit design gives a habitat atmosphere of 27.03% O2,  32.43% N2,  27.03% Ar,  and 13.51%% He,  at 9.68 psia.  Earth-normal oxygen percentage is nearly 21%,  so surely this 4-gas atmosphere would be far less risky in terms of fire. 

Recommendations:

Determine reliable pre-breathe factors for argon and helium.

Identify other candidate gases that could be added to the mix,  and determine the pre-breathe factors for them. 

Final Note:

The atmosphere we evolved with (20.946% O2 78% N2,  and 1% argon) at 14.7 sea level psia could be a habitat atmosphere as safe as here at home,  in terms of fire risk and any long-term medical risks.  The nitrogen pre-breathe factor of 1.2 says the spacesuit pure oxygen pressure would have to be about 9.55 psi in order to avoid pre-breathe time donning the suit. 

That’s impossibly high,  even for a full pressure suit.  Our astronauts have severe mobility restrictions at about half that pressure currently.  The ISS uses near Earth-normal air,  and hours of pre-breathe time,  to use a suit at about 4.85 psi.    


Something has to change,  or these EVA-practicality issues will stymie us.  

Update 11-20-14:

The preceding calculations and graphs were made under the assumption that we wanted a minimal suit pressure,  and that we calculated habit atmospheres to match the wet in-lung oxygen partial pressure of the suit  Only 3 and 4-gas atmospheres were considered (oxygen,  nitrogen,  and argon,  plus maybe helium).

For this update,  I looked at setting habitat atmospheric oxygen to something very near Earth-normal oxygen 20.946 volume % for fire protection,  and added two more noble gases to the mix,  resulting in a 6-gas mixture.  All the inert gases added to inert nitrogen are chemically-inert noble gases.  In order,  the diluents are nitrogen,  argon,  helium,  neon,  and krypton.  There are more noble gases available (xenon and radon),  but these are becoming radioactive items,  and are to be avoided. 

I used the same pre-breathe factor of 1.2 on the nitrogen that has been proven.  The other factors are speculative.  I used a factor of 1.0 for the argon,  and 0.5 for helium and the other noble gases.  This is an arbitrary selection,  but should be in the ballpark (so something like this mix will work.)  The last noble gas (krypton) I allowed to fall a tiny bit short of its max pre-breathe factor to balance the mix at 100%. 

I did not use exactly the Earth-normal oxygen percentage,  because I could not quite achieve mathematical closure with a 6-gas mix.  But since the next noble gases are radioactive,  I did not use them.  Instead I picked an oxygen percentage very close to a nominal 21 volume percent,  which should present exactly the same fire hazard as Earth-normal air.  My number was exactly 21.28% oxygen. 

I forced the wet in-lung oxygen partial pressure of the mix to match that from sea level Earth-normal air at 149.33 mm mercury.  Removing the water vapor but retaining the mixture total pressure gives this dry habitat atmosphere:

                Gas        Pp,  mm Hg         vol%
                O2          159.34                   21.28  compare to 20.946% for sea-level Earth-normal air
                N2          191.21                   25.54
                Ar           159.34                   21.28
                He          79.67                    10.64
                Ne          79.67                     10.64
                Kr            74.55                     10.62
                Dry tot  748.80                   100.

The habitat atmosphere works out to 0.9853 atm = 14.48 psia,  very close to Earth-normal air.  The difference is 5 diluent gases at significant concentrations,  instead of just one.  None of the 5 will have large concentrations (or osmotic pressures) in the blood.  Thus,  there should be little or no risk of “the bends” going straight from this atmosphere to the corresponding suit,  assuming my pre-breathe factors are anywhere close to right.  But,  it will take experiment to find out what the real factors are. 

In the wet-lung environment,  water vapor pressure displaces part of this dry atmosphere,  lowering the partial pressures.  I assume equilibrium water vapor pressure at body temperature.  Those adjusted data look like this:

                Gas                                        Pp, mmHg
                O2                                          149.33  same as sea-level Earth-normal air in the wet lungs
                N2                                          179.20
                Ar                                           149.33
                He                                            74.67
                Ne                                          74.67
                Kr                                            74.55
                Water vapor                       47.06
                Total                                      748.80  = 0.9853 atm = 14.48 psia

We want the pure-oxygen suit to match wet in-lung oxygen partial pressures,  so that tissue oxygenation and resistance to hypoxia issues is the same,  in the suit as in the habitat.  Because the suit is a 1-gas system,  you just add the water vapor partial pressure to the wet in-lung oxygen partial pressure to determine total suit pressure. 

                149.33 + 47.06 = 196.39 mm Hg = 0.2584 atm = 3.798 psia  

Even if you add a factor 1.10 increase to counter leakage,  the suit pressure is still only 216.03 mm Hg = 0.2842 atm = 4.177 psia.  That is far lower than current practice on the ISS. 

Conclusions:

#1. It is quite feasible to have habitat atmosphere at very nearly Earth-normal pressure and Earth-normal oxygen percentages,  and yet avoid high nitrogen concentrations,  by going to a multi-gas mixture.

#2. It is very likely feasible to do this in a way that avoids the long pre-breathe times associated with the high nitrogen concentration in Earth-normal air.

#3. If you do it as a multi-gas system,  there is no need for a high-pressure space suit,  just to lower (but not eliminate) the pre-breathe times using Earth-normal air. 

#4. The lowered suit pressure requirement makes full pressure suits more supple,  and makes mechanical counterpressure suits immediately feasible with compression levels already easily achieved.

Final Note: 

My pre-breathe factors for all but nitrogen are probably only ballpark,  but certainly not exactly correct.  The research to quantify these factors correctly can be done on the ground.  Thus there is no excuse not to research this,  the payoff is simply too attractive. 

Update 11-26-14:

Once again,  my pre-breathe limit factors for all but nitrogen are guesses requiring experiment before application!  I have heard that the US Navy knows the pre-breathe factors for at least some of the noble gases that I proposed using here,  and has experience with actual use of multiple diluent gases,  exactly as proposed here.  The pre-breathe factor of 1.2 for nitrogen is in fact a US Navy value.

While not conclusive as presented here,  this does make my analysis seem at least ballpark correct.

Use the Navy's factors,  and this becomes possible as a 4-, 5-,  or at most 6-gas system.  You will be at no more at risk of fire than here on Earth with 21% by volume oxygen,  and you can use a lower-pressure,  more-supple pressure suit in the vicinity of at most 4 psia (for a sea-level-equivalent oxygenation design plus leak margin).

And,  best of all,  you need no long pre-breathe times to avoid the bends.  That is completely unlike current practice on the ISS.

Or even better than a supple full pressure suit,  you can use a really supple mechanical counterpressure suit at the same low pressure.  Such a low-pressure suit is technologically feasible right now,  and has been since 1968,  in one form or another.  3 to 4 psia compression has been well demonstrated as feasible in that approach,  with two disparate technologies.  

There is simply no excuse not to try all of this out.  The payoff,  in terms of basic astronaut mobility,  and in terms of effective human exploration capability,  is just too great.  Not to mention enabling feasible and practical designs for habitats and bases on other celestial bodies of all kinds.  And,  enabling far more effective assembly and repair operations by humans in space!

Any space agency,  public or private,  that is really interested in manned travel beyond cis-lunar space should be looking very seriously at the issues and possibilities raised here.

As for the rest of us,  we can safely conclude that agencies not looking very seriously at these same issues and possibilities,  are simply not yet serious about manned travel beyond cis-lunar space.

Update 1-1-15:

I have run across documents about what the Navy pre-breathe factor work really involved.  They looked at decompression needs coming from higher-than-1-atm pressures,  which is for deep sea diving.  What they found for that regime was that the prebreathe factor of 1.2 applies to the sum of all the diluent gas partial pressures:  sum Pp diluents / low total pressure = 1.2.  A lot of experiments were conducted to confirm this.

The low-pressure regime has not been explored experimentally yet.  When decompressing from near 1 atm to low total pressures in a pure oxygen suit,  the question is this:  does the prebreathe factor still apply to the sum of all diluent gas pressures taken together,  or can you use a separate factor for each diluent gas,  or does it correlate as something in-between these extremes?  That would be a good topic for someone research experimentally.  

However,  the Navy result is suggestive that using multiple diluent gases does you no real good,  because the factor of 1.2 applies to the sum of diluent partial pressures,  not those pressures individually.  That runs counter to what I explored above.

If that is indeed the case,  then the decompression problem revolves around this choice:  either (1) you choose a habitat atmosphere (the usual choice is Earth-normal sea level air) and design suits to match it that require no pre-breathe time,  or (2) you figure out what suits you really want,  and use the pre-breathe factor criterion to design your habitat atmosphere to be compatible.  That bounds the problem,  and the answer lies between those extremes,  very sensitive to the assumptions you actually make.

There is context to consider surrounding this choice and the assumptions.  As pointed out above,  full sea level air pressure is not really necessary.  NASA is working on full pressure suit designs with more dexterous joints and gloves,  even at higher suit pressures (in the 8-9 psia range).  NASA cannot accept oxygen concentrations beyond about 30%,  as being too much of a fire hazard.  With MCP suits,  existing technologies would only support about 0.25 to 0.30 atm compression.  MCP is inherently more dexterous,  and far more fault-tolerant of punctures and rips,  offers the possibility of being machine-washable,  but is far more difficult and time-consuming to don.  Those are just the facts of life with which we must deal.

habitat-first approach with full Earth-normal air:

Assume Earth-normal air as "synthetic air":  0.209 atm oxygen and 0.791 atm nitrogen.  Using a pre-breathe factor of 1.2 on the nitrogen partial pressure gives a required suit pressure of 0.659 atm pure oxygen.  That high a suit pressure (9.69 psia) presents extreme challenges designing-in glove dexterity in full pressure suits.  That high a compression level also rules out using MCP without major technological breakthroughs in materials.  Assuming 10% loss from leaks says your min allowable suit pressure is 0.599 atm.  With suit oxygen pressures in the 0.6 atm range,  there are medical worries about exposure to too much oxygen,  if exposure times are long.  On the ISS,  we use essentially Earth-normal air.  Accordingly,  we have long decompression pre-breathe times,  because current suit pressures are not that high.

suit-first approach using only what is needed for oxygen in the suit:

Assume your suit pressure (full pressure suit or MCP) is 0.209 atm pure oxygen,  same as Earth-normal air.  10% for leaks gives you 0.190 atm,  corresponding to the oxygen content in Earth-normal air at about 3000 feet above sea level.  That's just fine.  Using the factor of 1.2 on the design suit pressure of 0.209 atm  sets the habitat atmosphere nitrogen at 0.251 atm.  Using max allowable 30% oxygen sets the oxygen pressure at 0.108 atm,  and the habitat "air" pressure at 0.359 atm.  That level of oxygenation is still just too low,  being equivalent to Earth air at about 17,000 feet elevation.

compromise with reduced-pressure but otherwise earth-normal air in habitat:

In flying,  pilots must use oxygen above 10,000 feet elevation,  below that altitude,  they do not have to use it,  which presumes adequate oxygenation,  something proven by long experience.  The air pressure at 10,000 feet is 0.6878 atm. Assuming synthetic air at 20.9% oxygen,  that's 0.1438 atm oxygen and 0.5440 atm nitrogen.  That would be adequate as habitat air.  Using the pre-breathe factor on that nitrogen,  we get a pure-oxygen suit pressure of 0.4533 atm (6.66 psia).  That's still a considerable design challenge for dexterous full pressure suit (although NASA is working on glove designs for 8-9 psia).  It still rules-out using MCP without decompression time.  We are still very limited,  and would need decompression pre-breathe times to use MCP technology as it exists.  But this possibly could eliminate pre-breathe requirements with existing full pressure suits,  especially if refitted with more dexterous gloves.

same reduced-pressure habitat but with max oxygen allowable for fire safety:

Require the same oxygen as Earth-normal air at 10,000 feet:  0.1438 atm oxygen.  At 30% oxygen,  the habitat pressure is 0.4793 atm,  and the nitrogen is the other 70% at 0.3355 atm.  Using the pre-breathe factor on that nitrogen gets you a suit pressure of 0.2796 atm (4.11 psia).  That falls to 0.2542 atm with 10% leakage,  still quite adequate.  With suit pressures near 4 psia,  the new more-dexterous glove designs for full pressure suits will be far more effective,  and the compression levels are achievable with MCP.  Both kinds could be used with no decompression pre-breathe time requirement.

conclusions and recommendations:

What this does is bound the problem and show feasibility by making more appropriate initial assumptions.  The water-vapor displacement effect inside the lungs has not been taken into account,  and must be,  in any final design.  But those results should not be very far from these.  I'd guess the habitat and suit pressures to be just slightly higher than these bounds.

The "feasibility" shown here is threefold:  (1) we can dispense with the decompression pre-breathe times by using a significantly lowered habitat pressure,  and with the oxygen content still within known fire safety criteria,  (2) we can immediately add existing materials technology-based MCP suit designs to the mix of things available to the astronauts,  and (3) the more dexterous glove designs being developed for the full pressure suits will be even more effective at reduced suit pressures.  Those are all very favorable outcomes,  and they are also incredible enabling factors for astronauts to be far more effective when working in vacuum.

The only thing currently going on is the more-dexterous glove design for full pressure suits being done by NASA.  No experimentation is going on with habitat atmospheres on the ISS,  it's close to Earth-normal air almost by default.  MCP suits are still not seriously funded.

I see some things here that should be going on,  if we ever expect to explore other bodies like Mars more effectively.


Related Articles on this Site:

2-15-16    Suits and Atmospheres for Space (the latest!!)

1-15-16   Astronaut Facing Drowning Points Out Need for Better 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


2 comments:

  1. Private spaceflight startup here - stumbled upon this article after doing some stress test calculations for the launch vehicle.

    Kind of curious if you'd suggest using the 45.45% oxygen and 54.55% nitrogen at 6.13 psia for the atmosphere in a manned spacecraft. Think something designed with Gemini-like tolerances (i.e., 5 psia) would be able to handle that?

    Additionally, would transferring from Earth to the spacecraft require any kind of pre-breathing (assuming that system)?

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    Replies
    1. The rule of thumb is the partial pressure of nitrogen in the higher-pressure environment should not exceed factor 1.2 times the total pressure of the lower-pressure environment. If it does exceed that ratio, then decompression time for nitrogen blowoff is required to avoid the bends.

      This same ratio applies to the sum of all diluent gases for deep sea diving where pressures are well above 1 atm. The experiments apparently have never been run to see whether this is also true at pressures under 1 atm.

      So, there may or may not be an advantage to the multi-gas system in my article. Nobody knows yet.

      I see nothing wrong with using old space capsule atmospheres. They worked then, they will still work now. The only real questions are for very long-term exposures (months to years).

      GW

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