Airlocks

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I've been thinking about long term survival in outer space and now I'm thinking about how our exploration / exploitation of the moon might proceed.

There was an episode of Star Trek (at least I think it was Star Trek) wherein we are told the story of a colony that was established on a planet and due to some misfortune they ran short of supplies. A relief mission was sent, but failure to communicate meant the colony believed it would not arrive soon enough to save them. The leader of the colony then caused half of the colony to die which meant there were enough supplies for the remaining people to survive. Shortly thereafter the relief mission arrived, which meant the leader was pilloried as a mass murderer. If the relief mission had been delayed as expected, he would have been lauded as a hero for saving half of the colony. 

I think it was an episode of Star Trek, it was a long time ago and my memory is a little fuzzy about the details, but it brought home to me the idea of how dependent space travelers are going to be on their space ship. Earthbound explorers always had air to breath, and if you were on land you could generally find water to drink. Food can be problematic, but is often obtainable. Space travelers need to ensure they have an adequate supply of all three because there isn't going to be any air, water or food wherever they are going.

If we are going to establish a base on the moon, we are going to want to construct a factory for producing rocket fuel. I don't know how that will go, maybe some whiz kid will come up with a fancy probe they can jam in the ground and with a proper application of electricity will produce oxygen or methane directly from moon rocks. I suspect it will probably be more like earth bound mining operations: skip loaders driving around on the surface, picking up loads of rocks and carting them back to the big rocket-fuel-making-machine. Wheeled machines will no doubt need to be brought 'inside' occasionally for repairs and maintenance. To do this, we are going to need an airlock.

Now the maintenance garage could act as airlock, but it means that every time anyone comes in or goes out, all the air will have to pumped out of the garage, which would take some time. Even if we could generate enough air from moon rocks, it wouldn't be prudent to just vent the whole shop's atmosphere. You might need that air.

So I am proposing a tunnel of airlocks. Each segment would be maybe one hundred feet long with a door at each end. The doors would be like conventional garage doors so that would take up the least amount of space inside the tunnel. The one hundred foot length would allow the skip loader to drive along slowly while the doors closed in back and opened in front. Each segment would be kept at a pressure that was one PSI below the previous segment. When you open the door between two segments with a difference of one PSI, there is not going to be a whole lot of air moving from one chamber to the other. Even if they were to completely equalize during the time the door is open, it is not going to amount to a great quantity of air. 

When you get to the last chamber the air pressure will be somewhere between one and two PSI, which is not very much. If you can afford to vent that small quantity of air to the vacuum, you can dispense with the vacuum pumps you would need to evacuate that last chamber, and since the chamber is fairly large and pressure is already so low, it would probably take a very long time, time when the skip loader could be working.

So the tunnel would 1400 feet long, You might be digging it through rock, or you might make it out of plastic and erect it on the surface, or you might use a combination of both. I'm thinking any kind of substantial moon base would need to be inside one of those lava tube caverns that might be lurking below the surface. Being underground would protect you from radiation. You are going to need a tunnel to get to the surface. If the cavern could be sealed, and you could make enough air, you might possibly be able to pressurize the whole cavern, a cavern big enough to build a city. In any case, you could use an similar airlock tunnel just to carry personnel and supplies between the cavern and the surface.

P.S. Evidently, the meaning of the term 'skip loader' has changed. I ask YouTube for 'skip loaders' and I get pictures of tractors equipped with a back hoe and front end loader. To me, a skip loader has four large wheels and a large shovel on one end, much like the machine in the video. Normal skip loaders are taller, the one in video has been squashed so it will fit in a tunnel which is exactly what we would want on the moon.

Update December 2021. I was feeling under the weather yesterday, so I amused myself by thinking about airlocks and I remembered this post and thinking that the amount of pressure on the doors between adjacent sections of the tunnel would not be very much, and then I did some rough calculations and I realized that it's actually going to be quite a large amount of pressure. For instance, a ten foot square door would have 100 square feet of surface area which translates to 14,400 square inches, so with a pressure differential of one PSI you are going to have 14,400 pounds of pressure which is seven tons. The doors might need to be made of unobtanium, but they are going to need to be very stout.