posted 12-23-2011 06:54 PM               

The reason I ask is I know Apollo used a 5 psi internal atmosphere of pure oxygen during its space missions. I know Skylab's internal pressure was pretty low, likely close to 5 psi (it had to be as a docked Apollo capsule couldn't handle too much increased pressure or it would rupture like a balloon).

I've been going on the assumption that it was still pure oxygen, based on what I've read and I know the pressure was kept low as the Skylab astronauts mentioned they sometimes had trouble communicating without headsets across the lab to one another if somebody was in the ATM area and another astronaut was on the "floor" of the OWS.

But, I've also heard that it may have been an oxygen-nitrogen percentage to mitigate both a possible fire danger and due to medical concerns of breathing in just oxygen on a long duration space flight of over two weeks.

posted 12-23-2011 07:04 PM               

In my research at McDonnell a lot of their design was based on Mercury/Gemini. Did you know a Gemini hatch was used in the Skylab airlock?

posted 12-23-2011 07:25 PM               

...inspired oxygen partial pressure equal to 22.6 kPa (170 mm Hg), inspired nitrogen partial pressure equal to 10 kPa (75 mm Hg), inspired water partial pressure equal to 1.3 kPa (10 mm Hg), inspired carbon dioxide partial pressure equal to 6.7 kPa (5 mm Hg), although the nominal composition was inspired oxygen partial pressure equal to 24.1 kPa (188 mm Hg) and inspired nitrogen partial pressure equal to 10.3 kPa (77 mm Hg) at a total pressure equal to 34.4 kPa (258 mm Hg). This atmosphere was planned to provide approximate sea level equivalent alveolar oxygen partial pressure.

It seems to make a lot of sense to use this type of atmosphere on such a long-duration mission both from a physiological and safety standpoint.

posted 12-23-2011 08:08 PM               

For Apollo after the Apollo 1 fire, it was determined that a fire in space wouldn't burn much faster than one at a normal atmospheric pressure one in an oxygen nitrogen environment. Flames don't experience convection in the same fashion as on the ground. So if a fire broke out, it could be extinguished relatively easily. Apollo 1 was different though as we were talking about 15 psi pure oxygen on the ground AND normal convection took place as things burned.

The solution for Apollo launches after Apollo 1 was to pressurize the spacecraft with oxygen and nitrogen on the pad. The astronauts themselves would breathe pure oxygen in their suits). So as the spacecraft rose into orbit, the pressure would bleed down to 5 psi and by the time the astronauts removed their helmets, it would be nearly pure O2 at 5 psi for the rest of the trip.

Indeed nitrogen would have added complexity and weight to an already weight strapped spacecraft. Plus, use of nitrogen potentially means the need to pre-breathe pure oxygen before a spacewalk to prevent the bends (at least when going from a sea level O2 N2 pressure as on shuttle to a 5 psi suit pressure). I imagine for Skylab if any pre-breathing was done, it wouldn't amount to much since the pressure was the same or slightly reduced (and you likely wouldn't see nitrogen bubbles forming like in a bottle of carbonated soda with the cap taken off as a sealed bottle has a higher internal pressure than atmospheric pressure).

MOL was going to go with a helium oxygen mixture at 5 psi since helium is a less dense gas than nitrogen (and requires less complex equipment to manage since it is inert). But, helium also causes the vocal cords to constrict, affecting voices. If NASA had used it, imagine the humor that would have taken place during the first Apollo broadcasts enroute to the moon, or Neil Armstrong's famous words if he sounded like Donald Duck.