Oxygenation Issues for Habitats and Space Suits

search code 08072025;  search keywords space program,  spacesuit

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The following is an evaluation of oxygenation issues for space habitations and oxygen suits,  without using any models for in-lung oxygen partial pressure.   Only what is in the breathing gas to be inhaled is considered here!   That way,  the issue is not clouded with the effects of breathing gas displacement by in-lung water vapor,  or by in-lung carbon dioxide,  or by in-lung dead-end volume effects.  Not everyone agrees on the efficacy of those models.

Shown in Figure 1 is a list of selected air pressures versus altitude from a US 1962 Standard Day atmosphere model,  which is identical to the ICAO Standard Day model up to about 65,000 feet.  The breathing gas to be inhaled is either Earthly air at 20.9 volume percent oxygen,  or 100% oxygen in a vented face mask,  with delivery at the altitude’s pressure.  The corresponding oxygen partial pressures were computed and included,  along with a description of the circumstances,  and an indication of the duration of the exposure. 

Figure 1 – Oxygenation Limits Short and Long Term

Partial pressure of oxygen is important,  because it is related to the partial pressure of oxygen in-lung.   The difference between in-lung partial pressure and that in the blood,  is what drives the diffusion of oxygen across the lung membranes into the blood. 

The duration of the exposure is very important to determining what levels of oxygen partial pressure are suitable.  What I tried to identify as short-term criteria relate to the military and civil altitude requirements for going on oxygen in unpressurized airplane cockpits.  Use of such oxygen can be for many hours exposure,  and it makes sure the pilots are fully cognitive,  up to the upper altitude limits for such vented masks.

Those upper altitude limits for vented oxygen face masks are “fuzzy”.  Different people quote different values,  usually close to 40,000 or 45,000 feet.  Below that upper altitude “limit”,  you are “good” for hours.  Above it,  exposure time with high cognition is limited to only minutes or even seconds.  These are typically very short zoom-climb experiences,  peaking at 50-some thousand feet altitude.   One needs a pressure suit to stay up there. 

For long-term criteria,  the experiences of populations living at high elevations are very informative.  There are two things known that are relevant:  chronic hypoxia effects that manifest as “chronic mountain sickness”,  and an increase in pregnancy and birthing difficulties above the low-elevation rates,  presumably also due to chronic hypoxia.  Both start above around 2500 m elevation,  and get worse as elevation increases.  Below 2500 m,  there seem to be no detectable chronic mountain sickness symptoms,  and the rates of difficulties with pregnancy and birthing seem indistinguishable from those at sea level.    

Maximum oxygen partial pressures are limited by enhanced fire dangers above 21 volume percent near sea level,  and seem to be “OK” up to about 0.83 atm partial pressure for several hours flight time,  using the Navy criterion for going on oxygen at 5000 feet.   The ultimate “fatal exposure” limit (1+ atm) derives from experiences with oxygen in diving. 

All of these things are shown in the figure,  and highlighted in different colors.  The overall conclusions I drew from this are in a small table at the bottom of the figure.  I chose partial pressure limits for long-term exposure suitable for habitations,  and short-term exposure limits suitable for hours-long work shifts in pure oxygen space suits.  The space suit criteria further divide into full cognition,  versus mere survival (with presumed cognitive impairment if longer than a few minutes).

Turning to the space suit issue,  Figure 2 shows the suit pressures in a variety of units of measure,  corresponding to the oxygen partial pressure criteria already identified above,  and also as corrected upward to compensate for a 10% pressure leak-down during a long shift.  These would obtain,  if there were no other effects to consider,  but there are! 

Experience also shows that breathing oxygen at low pressures causes a loss of water from the tissues in the lungs and respiratory tract.  This drying-out of tissues can cause bleeding,  which is a very serious problem indeed!   

That same experience suggests that there is a minimum suit pressure below which there is a tissue-drying problem,  and above which there is no problem.  This is a bit “fuzzy”,  but the value most quoted is 3.00 psia (corresponding to 0.2041 atm).  This is the value to which the 10% leak-down factor needs to be applied for a higher design suit pressure,  in order to avoid tissue dry-out during a long work shift,  even with leakage.

The result is the final table at the bottom of the figure,  showing the “min suit design” pressure,  the 10% leak-down pressure for long work shifts,  and the short term survival criteria with full cognition,  and with impaired cognition,  after only minutes. 

Figure 2 – Oxygenation Issues for Pure Oxygen Space Suits

Returning to the two-gas mixture habitat atmosphere issue,  we have a good minimum partial pressure criterion for very long-term exposures:  near 0.15 atm,  per the discussion above.  However,  there are three other things to worry about when setting the habitat breathing gas mixture,  presumed to be oxygen and nitrogen.  These are:  (1) a pressure leak-down over time,  to be compensated when detected,  (2) the enhanced fire danger of higher percentage oxygen,  but which is offset by lower total pressures,  and (3) avoiding pre-breathe time,  if possible,  when going from the two-gas mix to the pure oxygen suit.    

I picked the partial pressure of oxygen in the standard atmosphere table at 8200 feet (2500 m) as the “exact” long-term criterion.  That partial pressure of 0.1551 atm is indicated in the first calculation with green highlighting,  near the top of Figure 3.  Just below it,  I ran the compositions of air as it was known in the 1960’s,  air as it is today with increased carbon dioxide,  and a synthetic air (2-gas mix) at the same oxygen content as today’s air.  This includes evaluations of molecular weight,  gas constant,  densities at 1 atm and 59 F (15 C) and also at 1 atm and “room temperature” 77 F (25 C). 

From those densities and the mass fraction of oxygen,  I computed the oxygen concentrations in kg/m³,  at 1 atm and 77 F (25 C) as “room temperature air at sea level”.  That value is 0.2738 kg/m³.  That would be the max oxygen concentration allowable in the habitat breathing gas,  to keep the fire danger as no more hazardous than that of “room temperature sea level air”.   It is based on an overall Arrhenius reaction rate model.

I also checked the oxygen concentration of 40% hospital oxygen at sea level and room temperature,  known to be a severe fire hazard.  It is about twice the concentration of oxygen in ordinary air at sea level and room temperature. That is near the bottom of the figure,  result highlighted in blue.  One would expect those fires to propagate twice as fast.

The habitat atmosphere calculations start with design values for gas composition and total pressure,  and include a leaked-down set of values.  These are on the right of the figure,  with much blue highlighting,  and user inputs highlighted yellow.  Based on prior work,  I chose to investigate what I call a “rule of 43” 2-gas mix atmosphere of oxygen and nitrogen.  The oxygen is 43 volume percent of the mix (leaving 57% nitrogen),  and the total pressure is 43% of 1 standard atmosphere.  The resulting oxygen partial pressure (0.1849 atm) exceeds the criterion 0.1551 atm,  which is a lower limit,  and the calculated oxygen concentration (0.2418 kg/m³) is less than the criterion value,  which is an upper limit. 

Just above that calculation is an estimate of the minimum suit pressure,  based on the nitrogen partial pressure of 0.2451 atm,  divided by the “no pre-breathe” criterion of factor 1.2.  That produces a 0.2043 atm min suit pressure,  which meets the “no tissue dry-out” criterion of 3 psia,  and also far exceeds the cognition limits.  That would be the minimum suit pressure you can use with no pre-breathe interval to blow off nitrogen:  you can just don the suit and go right out of the airlock.  Any higher suit pressure also qualifies.

Down in the lower right corner of the figure is the habitat leak-down analysis,  set by the min partial pressure of oxygen.  It says we can leak down 19% in pressure at the same oxygen percentage,  and still meet all criteria for safety.  Even the fire danger is OK:  the oxygen concentration reduces as pressure reduces,  depending as it does on density.

Figure 4 below summarizes these results in one place.  The “rule of 43” habitat atmosphere allows the use of rather low-pressure space suits without any pre-breathe requirements,  and provides plenty of leak-down margin,  while at the same time keeping the fire spread danger similar to that in sea level room temperature air.  The recommended suit pressure still meets the 3 psia tissue dry-out criterion as leaked-down 10% after a long work shift.

Figure 3 – Oxygenation Issues for Space Habitats

Figure 4 – Results for Combined Habitats and Space Suits

Final Remarks

There are other habitat 2-gas mixtures and pressures that would qualify.  Not all of these produce space suit designs with min pressures as low as the one found here. 

My selection of the “rule of 43” habitat atmosphere is based on previous work I did trying to meet all these criteria while getting as low a suit pressure as possible.  It is also an easy specification to remember.  Further,  that lowered suit pressure is important for 2 very compelling reasons: 

First,  a higher suit pressure not only is more difficult to design,  it also stiffens like a sports ball at higher pressures,  greatly reducing the mobility available to its wearer. 

Second,  higher suit pressures pretty much rule out suit designs based on mechanical counter-pressure (MCP),  since the materials and design practices for MCP are unavailable in any form that might be donned and doffed with reasonable ease at higher pressures.

References:

All these listed references are prior studies posted at http://exrocketman.blogspot.com.  For rapid access,  there is an archive tool on the left side of that page.  All you need is the posting date and the title.  Click on the year,  then the month,  then the title if need be (such as if other articles were posted that same month). 

1-2-22                  Refining Proposed Suit and Habitat Atmospheres best case and easiest-to-remember cases,  plus an independent estimate of the utter min suit pressures feasible

1-1-22                  Habitat Atmospheres and Long-Term Health adds a long term hypoxia criterion for the habitat in addition to short term criteria for the min-P suit

3-16-18                Suit and Habitat Atmospheres 2018

11-23-17              A Better Version of the MCP Spacesuit?

2-15-16                Suits and Atmospheres for Space

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

12-13-13              Mars Mission Study 2013

1-21-11                Fundamental Design Criteria for Alternative Space Suit Approaches

Where You Get Your “Facts” Matters!

This is another one I found on LinkedIn that I thought I would post here.  As readers of this blog already know,  I often say “you are entitled to your own opinions,  but you are NOT entitled to your own facts”. 

Most of the totally-unregulated right-wing social media,  many cable television news outlets,  and most infamously on broadcast and cable television Fox News,  are all well-known purveyors of lies and conspiracy theories.  About 30% of which was planted there by the Russians to divide us,  and successfully,  as near as I can tell.  My opinion of the veracity of Fox News has been in the toilet for about 30+ years now.  To me,  they are “Faux News”.

So,  if you who get your “facts” from such sources want to be smarter than most dogs,  please come out of your echo chambers and look around elsewhere!

On the new Iran war

Update 7-1-2025:  The Waco "Tribune-Herald" newspaper today published a version of the original article just below,  as a board of contributors opinion piece (I am on that board).  It was hardly edited at all.  The recent update below was not included,  not being in the original submittal as a column.

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Original article written 6-23-2025:

It would seem likely that we are starting down the path to another long Mideast war,  with no clear end in sight,  despite what government officials claim.  Netanyahu started this,  desperately wanting Trump to join in,  despite his campaign pledge not to,  and he did!  Israel did not have the means to hit the deeply buried facilities,  and we did,  which is why this has taken place. 

Iran’s regime is a set of terrorists,  masquerading as religious clerics,  using a private army (the Revolutionary Guard) to stay in power as a dictatorship,  and abusing religion to justify to their captive people the evils they have long done.  They have been sending out terrorist proxy armies to do their dirty work for about 4 decades now.   I think most people would have to agree with that entire assessment. 

There never was any realistic hope of negotiating an end to the Iranian nuclear weapon program,  because we are dealing with terrorists,  not any sort of people who value logic or the safety of their country and its people.  The previous nuclear agreement only slowed their pace,  until Trump abrogated it in his first term. 

It accelerated since then.  They now have a stockpile of reportedly-60% enriched uranium,  which was evacuated from the sites before Trump bombed them! 

So all the pieces for an Iranian atomic bomb are still in place,  despite the bombing,  including the terrorist government so desperate to have those weapons!  Contrary to the claims in the news reports,  you do not have to have 90% enrichment to build an atomic bomb!  Anything over 50% will work! 

Although,  for the same yield,  much smaller devices can be built at 90% than 50%.  This is no secret,  it’s been in public libraries since I first read it there as a young boy,  more than 65 years ago. 

Regime change in Iran was the one and only realistic hope of actually putting a stop to the Iranian atomic bomb!  And now Netanyahu and Trump have made that very much harder to accomplish,  turning the Iranian people against us by bombing their country,  and killing civilians in the process.  Plus,  the regime’s leaders have now gone into hiding,  so that we cannot easily take them out anymore. 

The right way to have done this would have been to support the Iranian people to rise up and do their own regime change from within,  without making enemies of them by bombing their country!  They would just need help overcoming the Revolutionary Guard.  Which without a war going on,  actually could have been done with targeted air strikes.

The Iranian people tried insurrection once before,  during Obama’s administration,  but failed,  because we failed to aid them.  Now,  such a scenario is no longer possible.  The only regime change now possible will have to be imposed from outside,  with boots on the ground,  and against a civil population rallying to defend their government from foreign attackers. 

You can “thank” Netanyahu and Trump for that really ugly prospect!  Although,  “blame” is a better word.

Any child who has passed grade school social studies (taught up to standard,  not just “to the test”) could have predicted some,  or even all,  of this.  But apparently neither Netanyahu nor Trump understood the likely results of their actions,  and apparently they still don’t. 

That conclusion is based on what they have done,  not anything either of them said.  Remember,  you can always tell when a politician is lying:  his lips are moving. 

I think both our countries have the wrong governments.  The Iranians are not the only ones in need of regime change.

Update 6-25-2025:  The cease fire between Iran and Israel seems to be holding,  after some initial violations.  I am surprised but pleased to see it.  It means maybe we have avoided getting into another endless war,  at least for a little while.

Meanwhile,  intelligence reports are still quite sparse,  but some seem to indicate that what I said above is true:  the Iranians managed to move out some of their enriched uranium and related equipment before the attack took place.  That is a thing that President Trump really hates to hear,  because it directly conflicts with his chest-thumping. 

Long term,  do not expect this cease fire to lead to any sort of permanent peace!  You are not dealing with normal political people as Iran’s government.  You are dealing with fanatical terrorists!  They will say anything and do anything,  that lets them continue their evil activities,  including building nuclear weapons.  Their history over the last 4 decades says so. 

The only permanent solution here was,  is,  and always will be,  regime change in Iran!  Unfortunately,  that will henceforth be hellishly difficult,  now that there has been a direct war to alienate the civil population.  Plus,  the ruling mullahs and the Revolutionary Guard private army that keeps them in power,  both survive intact.

Photo from Maxar showing immediately-visible damage: 

Other photos taken a day or two before the strike show a lot of unusual roadway truck activity.  Presumably,  that was truck traffic evacuating the materials and equipment,  anticipating a coming strike. 

Update 6-29-2025:  The continuing variance among intelligence reports within and without the government very strongly suggests that the US bombing did great damage to Iran's nuclear weapon efforts,  but did not "obliterate it",  as Trump insists.  In fact,  he gets quite angry with those who disagree with him over this,  clear evidence of his fragile but enormous ego.  

I think the truth lies somewhere in between the extremes of the various assessments,  including ALL of them,  even the first one Trump claimed to be "leaked".  We set them back by somewhere between a few to several (or many) months.  I also rather think that those Maxar photos showing heavy truck traffic at the Fordow site just before the strike,  indicate that the Iranians got at least some of their enriched uranium out,  and hid it elsewhere.  And maybe some centrifuges,  too.

Bear in mind,  please,  that further enrichment beyond their 60% is not necessary to build a workable nuclear weapon,  it's just that the weapon will be very large and heavy,  much like our first generation devices were.  If too heavy for their rockets,  they could always deliver it as a suicide truck bomb.  Think about that!

Regime change in Iran is the only long term solution for this threat,  exactly as I said above in the article!   And Trump and Netanyahu have made that very difficult indeed,  by starting this war,  cease fire or not.  The Iranian people will now tend to rally behind their government against foreign attackers,  despite its brutal history of oppressing them.  That's just human tribal nature. 

Starship Explosion

Update 7-5-2025:  search code 23062025.

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From AIAA’s “Daily Launch” email newsletter for Monday,  6-23-2025.  This ship was intended for Flight Test 10.  It blew up before they ever ignited the engines.  Quote: 

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SpaceX traces Starship test-stand explosion to failure of pressurized nitrogen tank

By Mike Wall published 3 days ago  (on Space.com)

"Initial analysis indicates the potential failure of a pressurized tank known as a COPV."

 

SpaceX thinks it knows why its newest Starship spacecraft went boom this week.

The 171-foot-tall (52-meter-tall) vehicle exploded on a test stand at SpaceX's Starbase site late Wednesday night (June 18) as the company was preparing to ignite its six Raptor engines in a "static fire" trial.

 

A day later, SpaceX narrowed in on a likely cause.

 

"Initial analysis indicates the potential failure of a pressurized tank known as a COPV, or composite overwrapped pressure vessel, containing gaseous nitrogen in Starship's nosecone area, but the full data review is ongoing," the company wrote in an update on Thursday (June 19).

 

"There is no commonality between the COPVs used on Starship and SpaceX's Falcon rockets," the company added. So, launches of the workhorse Falcon 9, which has already flown 75 times in 2025, should not be affected.

 

The Starship explosion did not cause any reported injuries; all SpaceX personnel at Starbase are safe, according to the update. People living around the site, which is near the border city of Brownsville, shouldn't be worried about contamination from the incident, SpaceX said.

"Previous independent tests conducted on materials inside Starship, including toxicity analyses, confirm they pose no chemical, biological, or toxicological risks," the company wrote. "SpaceX is coordinating with local, state, and federal agencies, as appropriate, on matters concerning environmental and safety impacts."

That said, the explosion did damage the area around the test stand, which is at Starbase's Massey site (not the orbital launch mount area, from which Starship lifts off).

"The explosion ignited several fires at the test site which remains clear of personnel and will be assessed once it has been determined to be safe to approach," SpaceX wrote in the update. "Individuals should not attempt to approach the area while safing operations continue."

Wednesday night's explosion occurred during preparations for Starship's 10th flight test, which SpaceX had hoped to launch by the end of the month. (Static fires are common prelaunch tests, performed to ensure that engines are ready to fly.) That timeline will now shift to the right, though it's not clear at the moment by how much.

The incident was the latest in a series of setbacks for Starship upper stages. SpaceX lost the vehicle — also known as Ship — on the last three Starship flight tests, which launched in January, March and May of this year.

Starship's first stage, called Super Heavy, has a better track record of late. For example, on Flight 7 and Flight 8, the huge booster successfully returned to Starbase, where it was caught by the launch tower's "chopstick" arms as planned.

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My take:  if the description “in the nosecone” for the location of the COPV is correct,  then it is located very close to the oxygen header tank (as the version 1 with 1200 metric tons propellant capacity was laid out),  which is also in the nose of the vehicle,  ahead of the “cargo bay” area.  Such a COPV explosion would easily rupture that oxygen header tank.  Compressed gases drive great explosive violence (with shrapnel) when such vessels burst.  See Figure 1.

Figure 1 – Inboard Profile of Starship Version 1

There would seem to be an oxygen header tank transfer piping line down the windward “belly” of the cargo bay section,  based on descriptions I have read.  In the explosion slow-motion video,  the cargo bay splits open through its heat shield,  right where that transfer line supposedly is,  with gush of something white (not fire) bursting through,  followed immediately by an explosion engulfing about the top half of the vehicle,  and a second or so later by a second explosion seemingly centered lower down.    

The main propellant tanks below the cargo bay would be the main methane tank forward,  with the methane header tank located inside,  at the base of that tank,  and finally the main oxygen tank,  just ahead of the engine bay.  The upgraded version 2 has a bigger propellant capacity,  but should be laid out similarly.  

I would hazard the guess that the COPV explosion and bursting oxygen header tank somehow put a large force on the transfer line,  which split open the belly at the cargo bay,  allowing liquid (and vapor) oxygen out through that split,  as well as releasing a few tons of liquid oxygen to fall down on top of the main methane tank. 

My guess is that spilled header oxygen and vented methane vapors are much of the first explosion.  Bear in mind that the impact of a few tons of liquid oxygen on the top of the main methane tank would rupture it as well,  adding some fuel to that first explosion pulse.  That first explosion pulse would massively rupture the main methane tank,  and also likely the main oxygen tank below it.  That’s the second pulse of the explosion,  which was larger and longer,  reflecting the larger mass of reactants. 

All of that scenario is just an educated guess on my part. 

As for the nitrogen tank,  said to be a “COPV”,  or “composite overwrapped pressure vessel”,  maybe that is not the right choice this early in the flight test program.  Such a design is a metal shell that is simply too thin to hold the pressure,  overwrapped by a yarn or fabric-reinforced composite material,  to bring it up to strength at a lighter weight. 

Here’s the problem:  no composite material has a large plastic (post-yield) strain capability.  If the COPV over-pressures for any reason whatsoever,  failure will be sudden,  without any warning!  Maybe a heavier all-metal nitrogen tank,  one with much more plastic strain capability,  would be a better choice until the other bugs all get worked out.  At least you could see it stretch before it explodes.  You do not want to fly even experimentally,  with too many possible failure modes!  See Figure 2.  

Figure 2 – Stress-Strain Curves for Low and High Plastic Strain Capability

Lots of things look good “on paper”,  but there are a lot of other things to worry about,  many of which cannot be put on that paper.  This is where the “older hands”,  with many years of school-of-hard-knocks experiences,  can be effectively very much wiser than youngsters fresh out of school.  SpaceX has no “old hands” on its staff:  they hire no one over about age 40 or 45.  There’s no gray heads visible anywhere in that organization.

PS:  note in Figure 1 the "typical" inert mass of the Starship Version 1 upper stage,  as 120 metric tons.  I do not have a figure for the Version 2 inert mass,  but it simply cannot be very much different from Version 1!  They are all built the same way.  The hearsay bandied about on the internet,  about 80 tons,  or even less,  is simply BS!

History Does Not Repeat, But It Does Rhyme!

This is an improved variation on something I already posted.  It was created by putting two photos together into one illustration,  and then I labeled it appropriately,  using epithets I found on-line (that others came up with). 

We all already know what evils this regime is doing.  So,  the image I created needs no further explanation!  Too many do not learn from history! 

Update 6-18-2025:  My wife found this one on her Facebook page.  It’s too funny not to add here.

Trump Cognition?

The vast difference between understanding what the job of President really is,  and what Trump actually does and says,  makes one wonder what he really understands about anything.  If he’s this bad now,  how bad off will he be 4 years from now?  Think about THAT!

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Hegseth the Incompetent

This one makes fun of the security breach mess created by the demonstrably-incompetent Pete Hegseth,  whom the Senate should NEVER have approved for Secretary of Defense!  The only one I can think of who is even more demonstrably incompetent is Robert F. Kennedy Jr. as Secretary of Health and Human Services,  although there are several others very nearly as bad as Hegseth!  

Kennedy the vaccine denier is going to get a lot of people killed by doing lot of very stupid things.  Probably even a greater death toll than Musk and his DOGE are going to cause.  And that’s a lot of dead people!  

It starts with Hegseth sending the Marines to the LA protests.  One has to wonder who actually paid the hired rioters to cause trouble.  Could it actually have been the Trump cult who hired the “ringers”,  trying to make Trump opponents look bad?  (That’s almost what Jan. 6 2021 was.)

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