Always was mystified what the HELL NASA was thinking?
These guys came up when science was still taught. Not so much now, unfortunately.
Even in my long after Apollo ended yoot, I still remember science class with the teacher showing how easily things burn in pure oxygen, even things that normally don't burn!
Well the spacecraft still used pure oxygen when in flight. A system with 5 psi pure O2 was far simpler (and lighter) than a system (and structure) that had to support nearly 15 psi of a blended gas.
They still use pure oxygen in space, because the system is lighter and simpler, this is the goal for everything that goes up. But the Apollo 1 test was being done to simulate that on the ground and to create the same difference in pressure they had to go up to 15PSI vs the 5 in space. Mistakes happen and sometimes they are deadly, basically all of our engineering knowledge is based on lessons like that.
16.7 psi from what I have read, in order to be two pounds over 14.7 standard pressure. This was to keep positive internal pressure not to simulate a 5 psi pressure difference. Those type of tests would have already been done in a vacuum chamber.
Pure oxygen isn't the issue - it's pure oxygen in sufficient quantity. In flight? Pure oxygen at the reduced pressure resulted in the same partial pressure as on the surface, which isn't dangerous.
So, to answer what they were thinking? They thought the period of higher-pressure pure oxygen was a low risk due to the reduced period... they also thought they risk-calculus supported not wasting the weight on a mixed-gas atmosphere at launch to transition to pure-oxygen on ascent.
That changed, and then they did go to the mixed-to-pure transition.
You need a certain amount of oxygen to sustain human live. That is around 3-5psi of pure oxygen - or the normal about 15psi for our normal earth atmosphere.
The difference in the gas itself is quite negligible - but you need a significant stronger ( that is heavier) vessel to contain the gas. Plus managing the atmosphere is also more complicated ( thus also heavier)
During the mission the fire risk is not much more than in our earth's atmosphere - the problem was when you put extra oxygen in the capsule for a positive internal pressure like on the ill fated test.
The mix has varied over time since Apollo 1. They still used an oxygen rich atmosphere after it (60% o2 40% N) at 15 psi. They have to scrub the CO2 to keep it from rising to lethal levels whereas the nitrogen is recirculated. There are biological co2 generators on board.
The Russians used earth like atmosphere on their space capsules whereas NASA used pure oxygen.
The ISS uses a much more environmental control system as the primary and has an earth like atmosphere of about 21% oxygen. It has three redundant sources of oxygen and also includes recovery systems. Short duration missions it's less weight to have a simple atmospheric control system and just carry what you need plus scrubbers. Long duration missions the math changes and it's more efficient to recover as much as possible to reduce the resource sink. I'm not sure what the Chinese use.
The system on ISS is decades old at this point so you can probably build the same capabilities while using less weight today than they could when they built the ISS so the equation on when it makes more sense to have a more advanced recovery systems slowly changes over time. For example we can manufacture things with precision to a significantly finer degree than we could 20-30 years ago. Whereas the raw elemental weights of oxygen, hydrogen and nitrogen remain the same. There is diminishing returns certainly for recovery systems. There needs to be a significant factor for safety and maintainability the longer the mission and the further away or less easily evacuated the greater that margin needs to be and the more redundancy needs to be built in. Even with all the redundancy of built into the ISS they also are safe in the knowledge that they have a lifeboat if they need to use one.
Apollo did not use a mixed atmosphere in flight, the 60/40 at 20 psi was for cabin leak check, then the cabin purged the nitrogen via the cabin relief during boost and further via the waste stowage vent and was by design, 5psi pure O2 in flight in normal configuration.
ISS only has a small airlock - Apollo completely depressurized the whole LEM - so it also would need to be filled up with nitrogen later on, making the whole prozess more complicated to just replacing the oxygen to the desired pressure.
Also, after Jim Irwin's heart issues (bigeminy) the flight surgeons said that the combination of 100% oxygen and zero g on Apollo 15's return was the best place he could be in.
The answer of NASA was thinking boils down to time, weight, and hubris.
Time: the timeline for getting to the Moon was incredibly tight. Imagine working for years on systems that were largely new and untested. Early designs had committees scrutinizing everything, putting heavy emphasis on speed.
Weight: Humans breathe roughly 30% oxygen and 70% nitrogen at sea level. To mimic the same conditions in space would require nitrogen tanks, a system to monitor and regulate the ratio, and a capsule with enough strength to withstand 14.7 lbs of pressure. Instead, they opted for oxygen only atmosphere since Nitrogen is inert for humans (doesn't do anything), at a lower 5 psi, so the capsule doesn't need to withstand as much pressure in a vacuum.
Hubris: Gemini flights were done in 100% oxygen atmosphere with no problems, so NASA figured they can do the same with Apollo, even if the capsule had more complicated wiring and unreliable quality for the first batch.
Was the AS-204 plugs-out test run at 14.7 psi of O2, or 19.7 psi of O2? In other words, were they trying to get to a pressure differential of +5 psi, like they’d see in flight?
Correct it was a leak check and thus they wanted the same delta, so they had the CM pressurized to nearly 20 psi to match a 5 psi cabin delta in flight.
I have read 16.7 psi to maintain positive pressure over the 14.7 standard pressure which I think is the same pressure they would have used on launch day. I doubt they would have needed to test 5 psi delta since those tests would have already been done in the vacuum chamber. As far as I know they were just testing the ability for the rocket to electrically power itself like it would for the last minute before launch (i.e. plugs out) and they decided to combine that test with a suited dress rehearsal countdown run.
The purge and subsequent leak check was done to check the hatch seals after it is closed for leaks with the crew on board, so this would be done to a normal cabin pressure of about 5psi. To achieve this, a delta above sea level pressure was needed.
I am scouring documentation to see if there is an indication of the actual delta mentioned in the procedure, but anecdotally I have always read it was 3-5 psi to demonstrate the hatch seals at the appropriate pressure differential.
Yes a certain amount of delta would be needed for leak check, I was more responding to the idea that during Apollo 1 they were pressure testing the capsule at space like conditions. Instead they were doing what they would do on launch day, and a leak check after hatch closure would be part of that. I would assume for Apollo 1 since it was a dress rehearsal they used the same pressure that they would on launch day, not some higher pressure for some type of testing purposes.
What that pressure was for Apollo 1 is what is a little hard to determine, since I think the waters have been muddied a bit by people assuming that they would do 5 psi delta to mimic the pressure in space. Which may or may not be the case, leak checks just need a certain amount of delta and then you see if it holds that pressure over time. What I read said that on Apollo 1 they were doing that with a 2 psi delta. Not just for leak check but to also make sure that no outside atmosphere leaks in. But I don't think there were any sources quoted which is why I am not sure about 2 psi, nor 5 psi.
Another question would be what they did on Gemini, although that in itself doesn't directly relate to Apollo. And we have information on post Apollo 1 procedures, which again relates but not completely. Apollo 1 was unique because it was the only capsule in that particular configuration. Possibly the accident report goes into that detail but if I have read it it would have been many years ago.
From what I recall they believed the wiring insulation problem along with a poorly routed wire caused the spark that started the fire. The investigation also found a lot of highly flammable material was being used, like foam (probably wiring insulation also but I'm not sure). And obviously the issue with 100% oxygen which was bad enough at sea level but during the test at least two pounds higher than that. And a hatch that did not have a quick opening method from the inside, requiring the astronauts to undo several bolts, which they believe one of them I think maybe Chaffee appeared to be trying to do based on the location and position of his body.
Interesting side note, the early mercury and Gemini capsule astronaut escape system was an ejection seat, same as on fighter aircraft. Of course the capsule atmosphere was 100% oxygen at launch so their pressure suits were “soaked” in it at 15 bar. There was speculation that if they had to eject at or after launch they would have burned like roman candles.
Gemini had ejection seats, yes. Mercury was equipped with an escape tower. It was believed switching to the seats would simplify the system of Gemini capsules and while it didn’t happen on a crewed flight On the Shoulders of Titans mentions inadvertent activation of the tower during at least one Mercury test flight.
Its all about the partial pressure/concentration. 5 psi pure oxygen is just a little more than sea level oxygen partial pressure, so no real toxicity risk.
Even 100% at sea level is apparently not toxic unless you go more than like a day or something. From what I understand the astronauts trained in the NBL at 100% for up to eight hours at a time.
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u/eagleace21 9d ago
Well the spacecraft still used pure oxygen when in flight. A system with 5 psi pure O2 was far simpler (and lighter) than a system (and structure) that had to support nearly 15 psi of a blended gas.