How is the airlock vented?
The shuttle’s airlock is vented directly overboard to space. We do not save the gas for reuse because the hardware (pumps, valves, etc…) would weigh significantly more than just loading the extra nitrogen and oxygen each mission. For the International Space Station however, nitrogen and oxygen are a valuable consummable and we want to vent as little as possible overboard. The station airlock will have pump that will depressurize the airlock from 14.7 psia down to 1 or 2 psia. This gas will be pumped back into the main volume of the station, and the remaining gas in the airlock will be vented overboard.
