Editing Apollo 1 (section)

==== Pure oxygen atmosphere ====

The plugs-out test had been run to simulate the launch procedure, with the cabin pressurized with pure oxygen at the nominal launch level of {{convert|16.7|psi|kPa|abbr=on}}, {{convert|2|psi|kPa|abbr=on}} above standard sea level atmospheric pressure. This is more than five times the {{convert|3|psi|kPa|abbr=on}} partial pressure of oxygen in the atmosphere, and provides an environment in which materials not normally considered flammable will be highly flammable and burst into flame.<ref>{{Cite web |last=Benson |first=Charles D. |last2=Faherty |first2=William Barnaby |title=Moonport, CH18-2 |url=http://www.hq.nasa.gov/office/pao/History/SP-4204/ch18-2.html |url-status=dead |archive-url=https://web.archive.org/web/20160325215950/http://www.hq.nasa.gov/office/pao/History/SP-4204/ch18-2.html |archive-date=March 25, 2016 |access-date= |website=www.hq.nasa.gov}}</ref><ref name="SpaceSafetyMag">{{Cite magazine |last=Emmanuelli |first=Matteo |title=The Apollo 1 Fire |url=http://www.spacesafetymagazine.com/space-disasters/apollo-1-fire/ |magazine=Space Safety Magazine |year=2014 |access-date=April 14, 2016}}</ref>

The high-pressure oxygen atmosphere was similar to that which had been used successfully in the [[Project Mercury|Mercury]] and Gemini programs. The pressure before launch was deliberately greater than ambient in order to drive out the nitrogen-containing air and replace it with pure oxygen, and also to seal the [[plug door]] hatch cover. During the launch, the pressure would have been gradually reduced to the in-flight level of {{convert|5|psi|kPa|abbr=on}}, providing sufficient oxygen for the astronauts to breathe while reducing the fire risk. The Apollo{{nbsp}}1 crew had successfully tested this procedure with their spacecraft in the Operations and Checkout Building altitude (vacuum) chamber on October 18 and 19, 1966, and the backup crew of Schirra, Eisele and Cunningham had repeated it on December 30.<ref>[[#moonport|Benson 1978]]: [http://www.hq.nasa.gov/office/pao/History/SP-4204/ch18-3.html Chapter 18-3 – The Spacecraft Comes to KSC] {{Webarchive|url=https://web.archive.org/web/20140803002233/http://www.hq.nasa.gov/office/pao/History/SP-4204/ch18-3.html |date=August 3, 2014 }}</ref> The investigation board noted that, during these tests, the command module had been fully pressurized with pure oxygen four times, for a total of six hours and fifteen minutes, two and a half hours longer than it had been during the plugs-out test.<ref name="aibreport" />{{Reference page|page=4-2}}{{efn|The report mistakenly says "about {{frac|2|1|2}} times longer", which is clearly incorrect because the cabin had been pressurized for about {{frac|3|3|4}} hours during the plugs-out test.}}