Good information on the chain of decisions that set up Apollo 1 for the disaster: "Why Did NASA Still Use Pure Oxygen After the Apollo 1 Fire?" http://www.pops

Good information on the chain of decisions that set up Apollo 1 for the disaster: "Why Did NASA Still Use Pure Oxygen After the Apollo 1 Fire?" http://www.popsci.com/why-did-nasa-still-use-pure-oxygen-after-apollo-1-fire http://www.popsci.com/why-did-nasa-still-use-pure-oxygen-aft... (Warning: Horrible crap foisted on you via Javascript if you have that enabled.) tl;dr: 1. Designed Apollo to used a mixed gas (nitrogen/oxygen) atmosphere. 2. Needed weight savings. Mixed gas requires complicated, heavier, support. 3. NASA changed to pure oxygen over the objection of the North American Aviation engineers. Justification: a. Pure oxygen was used in the past with no disasters. b. The pressure in orbit was 5psi. At 5psi, fires are controllable. 4. Launch pad test with a closed hatch required atmospheric pressure + 5psi to get the differential pressure of space. 5. Spark + high pressure oxygen + fuel = disaster. Added: There were a lot more contributing factors such as the schedule pressure (wiring issues probably caused the spark) and the lack of explosive bolts on the hatch which prevented the astronauts from escaping quickly. Bonus tl;dr on the resolution: They pressurized the capsule with nitrogen/oxygen on the pad, so it was safe (other than being on top of a massive bomb). As the capsule ascended, they bled off the nitrogen/oxygen atmosphere to maintain the 5psi differential pressure (the astronauts were breathing oxygen via their space suits so it did not matter to them). Once they were in space, the atmosphere was maintained with pure oxygen at +5psi.