In the past, I did a lot of research on US WWII submarines. One thing of particular interest to me was the procedures for keeping the air breathable during submersion, which was later used in the Apollo program. US subs were not equipped with snorkels in WWII, so they were entirely battery powered when under. Diving was more of a ‘stealth mode’ than anything else. The cabin held about 12 hours of breathable air for the crew.

Now, mammals use what are called ‘bellows lungs’, where the fresh air coming in mixes with stale air going out. This means in practice that there’s multiple levels of loss of efficiency, most notably, that you only tend to get about 25% of the oxygen out of the air you breathe. Obviously, this is sufficient for us (most of the time!) but leads to the unfortunate situation where the CO2 in your air now is too high for you to get at what oxygen is left. There’s an equilibrium point.

To counter this, the sub crews would use Lithium Hydroxide (later used as part of the “mailbox” on Apollo 13) as a consumable to remove carbon dioxide from the air, so they could get at the oxygen that was left. Four cannisters, each about the size of a jerry can, would buy the crew about 2 more hours, up to a total of 18, at which point the oxygen was too wispy to get at. Then they had pressurized oxygen tanks in the sub they could use for 12 more hours, then another 6 from the Lithium Hydroxide. Total of 36 hours.

However, avian lungs actually use a circulatory design; air flows into the lung on a one-way trip, fresh air moving through the system in one direction always on its way to exhale. This, in addition to the use of counter-current flow of oxygen against blood means that birds can achieve much higher oxygen extraction, sometimes above 90%, as covered here. This is why they can fly above Mount Everest, where we need oxygen supplementation.

However, the downside is that because birds get almost all the oxygen in a single go, Lithium Hydroxide wouldn’t be an effective way for them to extend air; there’s nothing left to get at because they basically got it already, which then is a major problem for their submarines and spacecraft.

With some help, I realized that what they could do on longer-range craft that can’t rely on logistics is have spinal bays on the craft. As they move along, they can capture asteroids and comets, or pieces thereof, as able, and bring them into the bays. Once inside, they can break them down to get at the water ice contained, which they can break down into oxygen for breathing, and hydrogen for the fusion reactor. What’s left afterwards can be dumped, possibly for propulsion under Newton’s Third Law, or used as ammo for a particle beam/mass driver.

Hope this little deep dive was fun!

-Arrow