2 vs 1 gas systems

[PhantomWolf]

I know that this is more "Spacecraft" than "Apollo" but...

With a single gas system you have the advantages of less chance of explosive decompression, no decompression for spacewalks, only a single tank system and equipment required, no equipment to monitor and adjust gas ratios, and less weight. It would seem that the only disadvantage comes in the depressurisation and pressurisation during launch and landing, though Apollo solved this with a single pressure valve.

So... what are the reasons to use the two gas system instead of the single gas one?

[brobertsumc]

Fire.

[Obviousman]

Here ya go: Space-cabin atmospheres. Part IV - Engineering tradeoffs of one-versus two-gas systems

hdl.handle.net/2060/19670020669

(Will open a PDF file)

[macapple]

The two gas vs 1 gas environment was a key problem in the ASTP program as Apollo ran a 100% O2 environment at 5PSI and the Russians ran 80:20 Nitrogen and Oxygen environment at 14.7 PSI (earths atmospheric pressure).


1. Weight - Its much easier having a single air source and in apollo this was resolved as both power and life support could be provided by a single source. With improvements in technology most space activity inside space stations and craft is done at 14.7PSI dual gas. 100% Pure Oxygen environment for a long period of time (months etc) is very bad for the old human lungs and can result in premature health problems

science.howstuffworks.com/question493.htm

2. Space Walking - Both the US and USSR used 100% Oxygen for any EVA. If the Astronaut is in a 14.7% PSI dual gas environment and has to go to 100% Oxygen they have to suit up and breath pure oxygen for upto an hour before moving out so they dont get the bends . All the Apollo guys has to do was suit up equalise the cabin pressure and off they go. Much quicker if there is a major failure or a need for an EVA cutting down precious mission time.