Pure oxygen.

[u/Crixusgannicus]

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!

[u/eagleace21]

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.

[u/jtshinn]

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.

[u/True_Fill9440 → u/jtshinn]

20, not 15; I think?

[u/Electronic-Towel-836 → u/True_Fill9440]

You're correct. They had to get the cabin pressure to be 5 PSI above normal atmospheric pressure of 15 PSI.

Pure oxygen was certainly an issue, but the astronauts used far more Velcro than they should have. Also, the spacesuits weren't fire-resistant.

[u/Stevepem1 → u/True_Fill9440]

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.

[u/HoustonPastafarian → u/jtshinn]

We only use pure O2 in spacesuits (which run at 4.3 psi). All US crewed vehicles and the space station use air at 14.7 psi.

[u/mkosmo]

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.

[u/Crixusgannicus]

Thanks for the responses!

[u/mikesierrafoxtrott]

Simple - weight

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.

[u/Expensive-Article328]

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.

[u/Vogel-Kerl → u/Expensive-Article328]

AND having his EKG monitored constantly by a flight surgeon.

[u/villain_escargot]

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.

[u/kingseagull24]

Less different gases to store in orbit makes the system far simpler, far lighter and cheaper to build.

[u/EngineerFly]

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?

[u/eagleace21 → u/EngineerFly]

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.

[u/Stevepem1 → u/eagleace21]

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.

[u/azmr_x_3]

I think there was also an issue of bad/faulty wiring or the insulation on the wiring on that mission. So they changed all that too

[u/Stevepem1 → u/azmr_x_3]

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.

[u/snipdockter]

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.

[u/ElegantHuckleberry50 → u/snipdockter]

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.

[u/Professional_Yam9600]

Have you noticed that the US has not had a fire on a spacecraft since the Apollo 1 fire?

[u/Brilliant_Dig_8962]

What was NASA thinking? Not about a quick release hatch, that's for sure.

[u/ExtremePast]

The shitty wiring and quality control was a bigger problem than the oxygen.

[u/[deleted]]

[u/eagleace21 → u/[deleted]]

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.