Apollo 1

[ka9q]

I think Bob's details are completely correct, so there's not much to add on how Apollo actually worked.

I can add some thoughts on their design choices. Apollo's single-gas design has been much criticized, but I still think it was the right choice once the post-Apollo 1 pad safeguards were added.

Most of the Apollo missions involved a lot of EVAs, and all but two carried a LM. It would have been completely impractical to operate the LM with an air-like mixture. It would have to be much heavier to withstand the increased pressure, the life support system would be more complex and heavier, and the astronauts would have to decompress before every EVA as they do on Shuttle and ISS. And as everybody knows, weight constraints on the LM were extreme. Since the LM was docked with the CSM on the trip out, this forced the CM to use a single gas atmosphere as well. (Apollo did eventually mate with a spacecraft using air during ASTP. It carried a complex "docking module" airlock and gas supply system to make the two compatible.)

The only part of the post-Apollo 1 design I might change is the choice of gas mixture on the pad. As Bob explained, they used 60% O₂, 40% N₂ to maintain a sufficient ppO2 once they reached orbit at a cabin pressure of 5 psia. A 60% O₂ atmosphere is a lot safer than 100% O₂, but it's still obviously worse than air. So one idea I had is to launch with ordinary air (at +2 psig to keep the cabin pressurized) and instead of bleeding down to 5 psia and holding during ascent, bleed all the way down to a vacuum. The astronauts' pressure suits, which always contain pure O₂, are simultaneously bled down to their nominal 3.75 psia and held there. Then the cabin is repressurized with pure O₂ and you're done. No need to slowly purge N₂ from the cabin over a few days, which undoubtedly wasted a lot of O₂.

I mentioned this idea to Owen Garriott a few years ago and he couldn't see anything wrong with it. Then I saw two possible problems. As flown, suit pressure decreased from about 17 psia (+2 psig at sea level) to about 5 psia rather quickly during ascent. All the N₂ had already been purged from the astronauts' bodies before launch, so this evidently did not risk decompression illness. My plan would bleed them down from 17 psia to only 3.75 psia, and this greater change would have to be studied to ensure no decompression problems. A second problem would be that the suits would become stiff once the cabin bled below 3.75 psia during ascent, and this might interfere with their ability to operate the controls. This could be avoided by holding the cabin at 5 psia during ascent, delaying the vacuum purge until they were in orbit.

Owen didn't think it was possible to get the bends on pure O₂, but I'm not so sure. It is even more soluble in blood than N₂, although the fact you can metabolize it means you can get rid of it without having to breathe it back out.

[ka9q]

O2 at one bar (atmospheric pressure) is toxic, therefore in a pure O2 environment the pressure is reduced to 1/5th atmospheric normal. There would be advantages in decompressing and recompressing a craft. Using an air mix does introduce the problems of narcosis, but reduces the problems of fire in a pure O2 system.

O₂ at 1 bar is not acutely toxic, as shown by the fact that every Mercury, Gemini and Apollo astronaut breathed it starting hours before launch when they suited up. It may have some subtle effects when breathed for a long period of time, e.g., several weeks.

The pressure is dropped mainly to reduce the stresses on the spacecraft hull (remember how the LM was built). At 5 psia, the ppO₂ is still well above that of sea level air on earth.

Pure O₂ does become acutely toxic above pressures of 2-3 bar. Convulsions occur, which are usually fatal in a diver. This severely limits the use of pure O₂ in diving, although it's useful as a final mix to help flush out N₂ at shallow depth during decompression.

"Technical" mixtures for deep sea diving must limit the partial pressures of both N₂ (to avoid narcosis) and O₂ (to avoid toxicity). This is almost always done by adding helium. I was surprised to find that on especially deep dives hydrogen is also used. It turns out that it's safe to mix with oxygen provided the O₂ percentage is kept below about 4%; that keeps it outside the flammability range. These small O₂ percentages are used only at great depths.

[raven]

Yeah, it made sense, which was why it was done. Plus, Mercury and Gemini also used pure oxygen, for example, so NASA had a rather good precedent for pure oxygen being safe and reliable. In hindsight, we know different, thanks to all the other problems with the Block 1 CSM, but that's hindsight for you.

[Bob B.]

As Bob explained, they used 60% O₂, 40% N₂ to maintain a sufficient ppO2 once they reached orbit at a cabin pressure of 5 psia. A 60% O₂ atmosphere is a lot safer than 100% O₂, but it's still obviously worse than air.

NASA performed a series of flammability tests and found that the fire hazard in a 16.2-psia 60% O₂/40% N₂ atmosphere was essentially equivalent to a 6.2-psia 100% O₂ atmosphere.  Therefore the fire risk was about the same on the pad as in orbit.  Also the Block II CM had a redesigned hatch that could be quickly opened to facilitate rapid egress in the case of a fire on the pad.  During the boost phase the crew was suited, so it was possible to vent the cabin to extinguish a fire if one were to occur.
 

[Luke Pemberton]

Most of the Apollo missions involved a lot of EVAs, and all but two carried a LM. It would have been completely impractical to operate the LM with an air-like mixture. It would have to be much heavier to withstand the increased pressure, the life support system would be more complex and heavier, and the astronauts would have to decompress before every EVA as they do on Shuttle and ISS. And as everybody knows, weight constraints on the LM were extreme. Since the LM was docked with the CSM on the trip out, this forced the CM to use a single gas atmosphere as well. (Apollo did eventually mate with a spacecraft using air during ASTP. It carried a complex "docking module" airlock and gas supply system to make the two compatible.)

Thanks for the reply. Picking through the breadcrumbs on the internet regarding the Apollo fire, this tallies with my reading and adds a little more to my understanding. It appears that the rational for a single gas system was linked to weight and system integration. Perfect sense to be found and no conspiracy.

[ka9q]

NASA performed a series of flammability tests and found that the fire hazard in a 16.2-psia 60% O₂/40% N₂ atmosphere was essentially equivalent to a 6.2-psia 100% O₂ atmosphere.  Therefore the fire risk was about the same on the pad as in orbit.

Still a little higher, since the orbital cabin pressure was 5 psi.

Flammability is a complex subject. It doesn't depend merely on ppO₂, as breathability does. The diluent gases actively carry heat away.

Also the Block II CM had a redesigned hatch that could be quickly opened to facilitate rapid egress in the case of a fire on the pad.  During the boost phase the crew was suited, so it was possible to vent the cabin to extinguish a fire if one were to occur.

The Apollo 1 review board found that Grissom had tried and failed to vent the cabin, but it would not have made a difference because the cabin pressure rose so quickly that it would still have ruptured. Was the Block II vent redesigned? Or did the combination of lower O₂ concentration and reduced fuel loading make it more likely that venting would extinguish a fire?

[onebigmonkey]

Pure oxygen at 1 bar may not be toxic, but it sure is fun! As students we had to breathe it for a few minutes to compare exercise physiology under normal air and pure O₂ conditions. There was a lot of giggling.

I've found many HBs who don't get the fact that while it was pure O₂, the pressure was so much lower (particularly important for their 'the suit would have been too stiff' line). If I remember my student biology correctly, what matters is the gradient between partial pressures on either side of the alveolar membrane that allows exchange to occur between lungs and bloodstream. The engineering and medical benefits of a low pressure pure oxygen environment seem like a very happy marriage!

[ka9q]

Pure O₂ at 1 bar should have been indistinguishable from ordinary air unless you were exercising so vigorously that you would otherwise have been limited by lung capacity. Once your saO₂ (arterial oxygen saturation) reaches 100%, extra oxygen is useless.

I recently underwent a stem cell (aka "bone marrow") transplant. One of the side effects is temporary but severe anemia; my red blood cell count fell to nearly half normal. I had expected it to feel like altitude sickness, which I'd experienced, but it was completely different. I was not short of breath but my muscles ached with even minor exertion. Insufficient oxygen reached my muscles to complete aerobic respiration, so lactic acid built up causing the pain. It faded during rest.

My resting heart rate was elevated, just as you'd expect; with fewer red blood cells in the system, my heart had to pump them that much faster around the loop.

Yet my measured saO₂ was always normal, in the mid-high 90s, because my lungs were still working normally; what RBCs passed through them were still leaving fully oxygenated. This meant that breathing pure O₂ would not have helped, and so it was never offered. Even though I understood this intellectually, I still felt like trying the experiment just to prove it to myself. Funny how your mind works when you're under stress.

[Luke Pemberton]

One of the main objections of the CTs is that pure oxygen can cause irritation of the mucous membranes rather than toxicity in the blood stream. I trawled for some information on using compressed O₂.  This link:

http://www.iecoguam.com/main/gases_oxygen_compressed.pdf

It would appear that this waves away their arguments quite nicely. Not that I understand their line, as others here have explained, a similar approach was followed for Mercury and Gemini. As the pre-cursors to Apollo I would imagine that scientists and engineers had a rational for a single gas system and learned a lot from Gemini and Mercury discussed above. Sadly they learned much more from Apollo 1.

Thank you for the discussion and replies. It has been appreciated.

[Luke Pemberton]

I recently underwent a stem cell (aka "bone marrow") transplant.

Hope your procedure went well. I donated stem cells for the Anthony Nolan Trust. Before making the donation I was injected with HGH for 5 days to stimulate the production of stem cells. My sternum felt like it was about to rupture with the pain. It was explained to me that the pain was due to the stem cell production causing my sternum to swell.

[ka9q]

I was my own donor, so I also had the injections prior to harvest. I didn't have that much pain, though occasionally my hip bone felt like it was in a vise. The nurses said that for some reason donors to others tend to experience more pain than those undergoing autologous transplants. I surmise that may be because my bone marrow was already somewhat suppressed by chemotherapy.

[JayUtah]

Flammability is a complex subject. It doesn't depend merely on ppO₂, as breathability does. The diluent gases actively carry heat away.

Yes, this was the key finding.  Even small percentages of diluents have a pronounced effect on fire propagation -- as you say, for heat-transfer reasons more than chemistry reasons.

I recently underwent a stem cell (aka "bone marrow") transplant. One of the side effects is temporary but severe anemia; my red blood cell count fell to nearly half normal.

My father underwent two, and yes I also remember carefully monitoring his hematocrit.  I also remember the size of the needle, and I'm pleased you didn't feel much pain.

[Bob B.]

Was the Block II vent redesigned? Or did the combination of lower O₂ concentration and reduced fuel loading make it more likely that venting would extinguish a fire?

I don't know if there were design changes or not.  Of course, in the Apollo I case the fire was much more aggressive then one would be in a 60% O₂, 40% N₂ atmosphere.  Also note that as the rocket ascended, the external pressure decreased and the pressure differential increased.  This would have allowed the CM to vent more rapidly and all the way down to the lower ambient pressure.

Here is the report on the CM flammability tests if your are interested:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700015953.pdf

Strangely this report is dated 1970, though the tests were obviously done earlier than that.  The following announcement appeared in the March 1968 Aviation Week & Space Technology:

"Washington - Decision to use a two-gas atmosphere (60% oxygen, 40% nitrogen) during manned Apollo on-the-pad preparations and in pre-orbital flight reflects a basic inability to make the spacecraft flameproof after 14 months of redesign that cost more than $100 million and added about 2,000 lb. to the system."

So during the Apollo 1 plugs out test, the astronauts were hooked up to the life support system and weren't breathing oxygen at 2 psi over atmosphere?

The above referenced report confirms that the astronauts were indeed breathing high-pressure 100% oxygen in their suits.  On page 2 it reads,

"After careful and extensive evaluation, it was determined that a 60-percent-oxygen/40-percent nitrogen mixture would satisfy both physiological and flammability requirements for the cabin atmosphere during the launch phase.  To avoid redesigning the suit-loop system, the suit loop would contain 16.5 psia 100 percent oxygen while on the launch pad, and the pressure would be reduced to 6.2 psia in space after the cabin pressure had dropped and the cabin atmosphere had phased into a 100-percent-oxygen condition."

The report keeps referring to the use of a 6.2-psia atmosphere in space but, from all other sources I've seen, the cabin pressure was actually 5-psia.

[JayUtah]

I don't know if there were design changes or not.

I'm not sure either, but in general you want passive flow control on those kinds of devices for safety reasons.

The report keeps referring to the use of a 6.2-psia atmosphere in space but, from all other sources I've seen, the cabin pressure was actually 5-psia.

It's 5 psia +/- a pound or so.  The hysteresis was broad.

[Luke Pemberton]

"After careful and extensive evaluation, it was determined that a 60-percent-oxygen/40-percent nitrogen mixture would satisfy both physiological and flammability requirements for the cabin atmosphere during the launch phase.  To avoid redesigning the suit-loop system, the suit loop would contain 16.5 psia 100 percent oxygen while on the launch pad, and the pressure would be reduced to 6.2 psia in space after the cabin pressure had dropped and the cabin atmosphere had phased into a 100-percent-oxygen condition."

Thanks for this Bob. It really has given me a lot to go on with. (Thanks to everyone else too.)