posted 02-19-2022 10:27 PM               

The design has been reported to have the cabin atmosphere changed to helium-oxygen in place of the pure oxygen environment of Gemini. At launch, the crew was to breathe pure oxygen in their suits while the cabin was filled with pure helium. During ascent, oxygen from the suit system would slowly bring the cabin atmosphere up to the helium-oxygen content of the station itself.

Given that these USAF MOL program designs were pre-Apollo 1 fire, it appears that the Air Force had already identified that the associated risks of pure oxygen environments in spacecraft were similar to the safe practices required for aircraft. As the program derived from Dynasoar, at some point, a helium/oxygen environment was identified as the preferred system and significant design work had been applied to provide a safe working environment for crews during long-duration spaceflight.