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In terms of context, this airlock would be used on airships which fly at a height in which oxygen levels are too low for humans to live. Within the hull of the airship, stored oxygen would be pumped inside and kept within through insulated walls. However, many still need to go out on to the deck. This would need to be done without the oxygen inside being lost and the inside of the ship being depressurized, and thus, an airlock is needed.

However, as this takes place in a semi-medieval world, would something like this be possible to construct given the technology and knowledge available?

Note that the people creating this would have advanced knowledge of chemicals and gasses as a whole, as well as mechanical technology that consists of levers and pulleys.

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    $\begingroup$ They have no advanced knowledge of chemicals and gasses, yet they manage to build huge airships capable of rising up 8000+ meters, as well as storing and pumping oxygen..? How? $\endgroup$ – dot_Sp0T Nov 24 '18 at 6:24
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    $\begingroup$ If they can separate, pump and store oxygen, an airlock should be a cinch. Just scale up the evacuation chamber of the pump $\endgroup$ – nzaman Nov 24 '18 at 6:43
  • $\begingroup$ Are these magical mideival airships also at equal pressure to the air outside (just with a richer O2 content to keep people conscious)? It's much easier to keep o2 in when it's not actively trying to escape! $\endgroup$ – Cort Ammon Nov 24 '18 at 18:27
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Leather and grease

Let's forget about the impossibility of flying and concentrate on the question of 'can you build an airlock?

These huge leather bellows show that it is possible to make airtight structures with leather.

enter image description here

This sort of bellows has to contain flap valves that allow passage of air in only one direction.

enter image description here

Airlocks would simply consist of an arrangement of flap valves. Small valves would be used to equalise pressure slowly. Large flap valves supported by wooden panels would act as doors. Of course, every time the outer door was opened some air would be lost to the outside but that's true of high-tech space airlocks anyway.

The flaps would need to be kept supple and airtight by the application of a suitable grease. Goose grease would be ideal.

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    $\begingroup$ Bellows didn't hold pressure for any amount of time though, they would contain it briefly while working. $\endgroup$ – Tim B Feb 13 at 17:23
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Yes, it is possible to use medieval technology to make an airlock.

Goldbeating is an ancient technology. It can be used to make thin, continuous sheets of metal, including cheaper metals like copper or brass. The interior of the airlock can be "gilded" with this "imitation leaf" to minimize air loss through the walls. Ideally, the leaf would be protected from damage on both sides by sturdier materials.

Large pieces of leaf can be attached to adjacent pieces of leaf by multiple-layer crimps. A practical modern-day example is use of two side-by-side sheets of crimped-together aluminum foil to protect the bottom of a roasting pan from drippings. This makes it much easier to clean the roasting pan after the meal is cooked.

The gilding is not a structural material. Instead, it is a thin layer over structural materials that are porous, leaky, or outgas too much. Tongue-in-groove plywood or the materials suggested by Hippeus_Lancer could be used as backing layers. Studs or other structural framing could back-up the sheet goods. The hardest part of the design would by allowing for expansion and contraction of the backing layers, without tearing holes in the gilding. Fortunately, laminated wood is an ancient technology that mitigates expansion and contraction.

The gilding would be applied to the main surfaces of the doorways, but would not be used to seal around the edges of the doorways. The seals around the edges of the doorways would be air-tight gaskets.

Vacuum-tight gaskets can be made using metal. In fact, the "hardest" vacuums are retained by metal gaskets, not rubber gaskets. The fundamental technologies for making a "knife-edge" gasket are knife making, vises or clamps, and copper billets. Medieval metal workers should be capable of making such gaskets.

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    $\begingroup$ I don’t want to imagine how heavy a gilded airship must be. $\endgroup$ – Joe Bloggs Nov 24 '18 at 8:47
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    $\begingroup$ @JoeBloggs Your lack of imagination is not a grounds for objecting to this answer. With some quick math using this source (gildedplanet.com/gildingbasics.html), I found that gold leafing a ~40 sq ft area only adds 10 grams of weight, and that is with a particularly heavy metal. $\endgroup$ – Pink Sweetener Nov 24 '18 at 18:48
  • $\begingroup$ Metal leaves, especially gold leaves, can hardly withstand being inflated. Plus there is still the problem of welding one to another, and having a door to access the environment. $\endgroup$ – L.Dutch Nov 24 '18 at 19:46
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    $\begingroup$ @PinkSweetener : Good to know, though I’m curious as to why you thought my comment was an objection. The concept of entirely golden airships is cool enough that I dont care how much weight holding it adds. $\endgroup$ – Joe Bloggs Nov 24 '18 at 20:34
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    $\begingroup$ @JoeBloggs well, I don't know about gold airships... but I can show you a real-life working lead balloon. Now all we have to do is wait until someone tries it with gold leaf... bring on the solid gold airships ;) $\endgroup$ – Samwise Nov 24 '18 at 21:57
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No, you can't build something like that with medieval technologies.

First of all, a medieval ship is made of wood. Assembling wood to make it airtight can hardly work, for a series of reasons:

  • large manufacturing tolerances
  • deformation over time
  • deformation under stress (a pressurized vessel acts like a balloon)

Same holds for doors and windows. Moreover, while you could use pitch for sealing the gaps between fixed parts, you would have no way to seal the gaps with a movable part (like the door and its frame).

Then we come to the other problem: lacking any electronic you could only pump Oxygen with no control on the flow.

Pumping Oxygen in an environment made of wood with no control is a really poor idea, as the slightest ignition source can cause a huge fire. And guess what they used in medieval time for lighting applications? Flames!

Last problem: how would you separate Oxygen from the air? You have just elementary pumps and loose tolerances for metal crafting, and storing liquid gases requires way more than that.

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  • $\begingroup$ What do you think of Jasper's idea? $\endgroup$ – Pink Sweetener Nov 24 '18 at 18:50
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Modern airlock uses rubber seals.

Your people can use leather with some padding inside, or same material as the airship envelope, inflated using the same means.

Couple questions for you: once out of airlock, how will the person breathe? Oxygen tanks are way beyond medieval technology. Maybe they can trail an air hose, but that requires better materials than door seal

Also, if air is too thin to breathe, it might be too thin to support an airship. You would need to carry composed gas and realise it into the envelope when going up, and when going down, you would waste it into outside air, or try to pump it back into the tanks. And compressing air is beyond medieval technology.

Finally, how well you move around without an engine? Pedal power?

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A small pool of water between inside and outside where the hull touches down to the surface of the water. You could also build it as a water-filled, U-shaped tunnel where one end is on the inside and the other on the outside. Crew members have to dive through it to enter and exit the ship.

Of course this adds quite some mass (water is heavy) and as others have pointed out in the comments, if you can build an airship and store oxygen you can also build an airlock.

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  • $\begingroup$ How do you prevent water to boil in the (near-)vacuum side? $\endgroup$ – Eth Feb 13 at 12:42
  • $\begingroup$ @Eth: Good point, depends on the pressure and temperatures of course. OP didn’t specify. $\endgroup$ – Michael Feb 13 at 12:46
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    $\begingroup$ I think this answer is valid, since this is an air ship it can't be zero pressure outside - otherwise it can't fly! $\endgroup$ – Tim B Feb 13 at 17:25
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Since your airship is still in atmosphere a perfect seal isnt neccesary unless your pressurized ship plans to stay up that high for days at a time. Using medieval materials like tar, pitch, cork, and rubber can give you a reasonable, but not perfect seal. You’ll be leaking atmosphere, but thats not a huge problem unless you plan to stay up indefinitely.

If your people have mastered airship technology then perhaps they have pressure vessels as well? Several canisters of pressurized air could help extend the length of time you can spend up so high.

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It might be possible to create an airlock using pitch. Pitch is the black, tarry substance used by Egyptians in 3000 BC to make the bottom of their boats water-tight. Assuming some alien technology already built the hull of the spaceship, the medieval humans could make a large air lock door out of plate armor pieces, with the edges sealed using pitch. Cam levers (like bicycle quick-release latches) can apply pressure to the door. Hollow reeds can be used to carry air into the airlock. Blacksmith bellows can be used to pump that air.

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    $\begingroup$ At room temperature (or even colder in the upper atmosphere), pitch is somewhat brittle. It could be used to permanently seal a door by pouring it around the edges while hot, but it would be likely to shatter when the door was opened, and wouldn't reseal cleanly. $\endgroup$ – duskwuff Nov 24 '18 at 22:12

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