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In a series I remember that a group of people were trying to make a colonization ship and they had a big problem: the air. Ships (like every object) aren't perfect and they have micro fissures in the armor or in the edges of two plates of hull and by that fissures the air slowly escape.
By a normal ship it isn't a big problem, because the air losse is very little and they can refill oxygen in other station, but for a colonization ship (who has a travel time of several hundred of years) they can't "refill" air.

I first thought that it was possible but then I remember that I read one time that NASA's fuel tank for rockets are only filled some days before the launching because the pressure of the hydrogen and it's small size were able to go through matter, so the tanks usually lose around 1% of their fuel. Maybe that could also happen with oxygen.

My question is: Is it possible to make an almost perfectly sealed ship? (You can use technology above some centuries) (With almost perfectly I mean the same air could be inside the ship for several millennia, not the rest of the eternity).

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    $\begingroup$ Engineering does not work this way. You don't make a perfect bridge, you make a maintainable bridge. You don't make a perfectly sealed ship, you make a ship with good enough seals and plan to replace the lost gas; for example, you may want to carry liquefied oxygen and nitrogen. And don't plan to store helium or hydrogen for a very long time; they diffuse through most materials. $\endgroup$ – AlexP Jul 24 '17 at 8:38
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    $\begingroup$ Use a planet. You'll lose some air but not enough to matter over the course of the journey. $\endgroup$ – Whelkaholism Jul 24 '17 at 9:39
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    $\begingroup$ I hear that Maxwellian daemons can be hired for such tasks, but their fees are almost infinite. $\endgroup$ – can-ned_food Jul 24 '17 at 10:31
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    $\begingroup$ @AlexP, the question is perfectly practical. He does use the "perfect" hyperbole, but specifies that he means "several millenia". So the question is "how would one construct a ship that keeps its athmosphere for several millenia". With "technology some centuries in the future". OP, I'd suggest you remove the "perfect" wording, it is a bit misleading. $\endgroup$ – AnoE Jul 24 '17 at 10:36
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    $\begingroup$ Anything will escape through anything given enough time. I remember how a local company thought someone was stealing their platinum until they realized it was escaping through the hull of their reactor. $\endgroup$ – Raditz_35 Jul 24 '17 at 15:25
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You face a few challenges...

Cort's answer provides the math for an ideal system. But your system won't be an ideal, sealed, sphere. It will therefore likely outgas faster. In part because it isn't an ideal sphere. And in part because things fail over time.

Case in point. I used to own a high-cost medical device that was sealed. It was rated to be water proof up to about 10 meters. Not that much, really. However, after normal use for a few months, that rating was destroyed. It got wet, it got ruined. Because the seals weakened due to casual wear and tear.

Your station must be able to withstand the general abuse your ship will take. That abuse takes many forms:

  • Micro-meteor impacts at your cruising speed.
  • Friction wear from cycling open/closed any exit portals.
  • Aging parts
  • Design faults
  • Stresses while under acceleration or as thrust vectors change

Solving these will require multiple approaches.

  • Use the best materials and manufacturing techniques
  • Design seals in depth, so if one seal fails, another can take the pressure
  • Constant monitoring for micro-leaks, so they can be patched before they become mega-leaks
  • maybe designing some kinds of self-sealing materials that can flow into and fill any micro-leaks

And all of this must be crazy simple to maintain while under flight. No one can just fly down to your local neighborhood Ship-Mart for spare parts (Shop smart, shop Ship-Mart).

I suggest you make at least five independent hulls. Pressurize each to some degree with a high-weight noble gas. Noble gas, because they don't like to react with other things. And partial pressure so that there is less pressure differential between the vacuum of space and the pressure vessel full of people. It wouldn't need to be anywhere near 1 atmosphere pressure. Just enough to help ease the pressure (pun intended).

These exterior safety hulls should be designed in some sort of honeycomb grid of cells, so that a rupture to one cell doesn't empty the entire hull. This also greatly increases the structural strength of that hull.

Or you could store water ice in at least one of these hulls, to provide a radiation shield / water supply / hydrogen+oxygen supply as needed.

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In a word, no. Perfect seals simply do not exist in the real world. Fortunately, neither do perfect ships. Even if you had perfect seals, you wouldn't be able to cruise the skies for all eternity because you'd eventually break down as you impact small particulate matter. Entropy always wins.

Fortunately, for practical purposes, you can do okay. You mention hydrogen, and hydrogen is indeed quite special. It's far smaller than anything else, and is notorious for doing evil things in high vacuum setups. Normal steel is typically sufficient for most gasses, but hydrogen can diffuse right through it. That's why every vacuum chamber you see is made out of stainless steel (and thus costs more than my house!).

The most important thing you can do is minimize the seals, and minimize thermal effects. Don't have any fancy seals like those which can rotate or which can open. Focus almost entirely on joints like the copper knife-edge seals they use on high vacuum setups. These are joints that feature a knife edge which cuts into a copper gasket to create a very strong seal. These seals are trusted in high vacuum situations, so they should be good for you.

Also, make sure you pay attention to thermal effects. As long as the ship is at relative equilibrium, you won't see too many microfissures.

For some perspective, you can look at the high vacuum community. These aren't the normal vacuums you're used to. Most of us deal with low vacuum, which might bring the pressure down from a normal 760 Torr down to 100 Torr. The high vacuum community likes to operate in the nanotorr region and below. At these pressures (or lacks of pressure), everything outgasses. They actually care about this because the tiniest flow in will ruin their experiment. From documentation, one can expect stainless steel to "outgas" at $3\cdot10^{-13} \frac{Torr\cdot Liter}{sec\cdot cm^2}$. This means that you expect gas to flow through the steel at roughly this rate. You can use that number to determine how long the pressure can remain in your ship.

Let's make up some numbers. The ISS has a volume of about 1000 cubic meters (1,000,000L). If we made it a sphere (the best shape for minimizing losses), it'd be about 12m in diameter, so it would have a surface area of about 2000 square meters (2*10^7 cm^2). Multiplying/dividing these through, along with that constant for stainless steel, and you get $6\cdot10^{-12}\frac{Torr}{sec}$. That's your pressure loss per second. That's $0.000185274 \frac{Torr}{year}$, or $0.185274 \frac{Torr}{millennia}$. If you started with atmospheric pressure (760 Torr), it would take 4 million years to deplete out, using these rough estimates.

You will have better results with larger spheres, so you can easily get into the 10s of millions of years. But it's not perfect.

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    $\begingroup$ @Erik no. No seal is ever perfect. Not against any of the gases. It's only that hydrogen is worst of them, so it works as decent "worst case scenario", and numbers are relatively easy to find. $\endgroup$ – Mołot Jul 24 '17 at 9:27
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    $\begingroup$ Primary contribution to outgassing are release of adsorbed and absorbed gasses, not permeation through. This outgassing drops with time, until potentially other effects (permeation and sublimation) take over. Permeation should sharply drop with material thickness. In short: it looks to me that you are using wrong data, quoted number is mostly for desorption, not permeation. $\endgroup$ – M i ech Jul 24 '17 at 9:43
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    $\begingroup$ How is called this property? I want to the values of other materials. $\endgroup$ – Ender Look Jul 24 '17 at 15:32
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    $\begingroup$ @CortAmmon Actually, that makes your numbers useless. They don't answer the question at all. Principle is correct in that permeation does exist, though practically limited only to Hydrogen, but your estimation bears NO relation to mechanism you describe. There is no way to infer real permeative leak from your answer. Furthermore, question is concerned with atmosphere, where hydrogen escape is a non-issue. Hydrogen is mentioned specifically to ask if oxygen is subject to same principle. While it's certainly interesting piece of information, IMO you didn't actually answer question at all. $\endgroup$ – M i ech Jul 24 '17 at 18:46
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    $\begingroup$ @erik hydrogen scooping doesn't actually work. The thickest gas cloud in outer space (ie a hydrogen nebula) is eight times closer to a perfect vacuum than we've created in a lab here on Earth. $\endgroup$ – Draco18s Jul 24 '17 at 21:52
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After reading Cort's impressive answer; I'd offer an alternative, to "nearly" perfectly sealed. Construct a cover that fits over the ship; as close as possible with the constraint of being only two pieces with a single seal between them (as small as possible). Or for practicality, as few pieces as possible with seals as obvious as possible. Make the cover of glass diamond$^1$ and stainless steel. Then pressurize the gap between the cover and the ship to match (or very slightly exceed) the ship's pressure.

$^1$ added: The OP allows future tech; present tech allows us to deposit diamond film and use high pressure to create gemstones; presumably future tech will be able to make pure diamond windows and thick diamond films for the cover described.

The point here is to use some non-toxic commonly available gas (the most common are hydrogen, helium, oxygen, nitrogen, neon, in that order) to pressurize the gap between the cover and the ship wall. Neon is probably your best bet, it is non-toxic and chemically inert, meaning it forms no compounds (unlike nitrogen and oxygen which both form compounds), and has an atomic mass of 20 (vs. 1 and 4 for hydrogen and helium resp.)

As Neon outgasses from the cover, it can be scooped up for replacement purposes. This makes it a good "sacrificial" gas, i.e. we may leak neon, but we don't leak our oxygen and other special recipe of gasses inside the ship that sustain life comfortably.

To the extent that Neon ingasses to the interior of the ship; it is non-toxic and we can filter it out for re-injection into the gap (the ship walls can have ports for this; remember only the outer shell needs to have as few joins as possible).

The advantage of the cover is also maintainability; with just a few simple straight seals that are easily accessible, we can mount equipment there to monitor the seals for leakage and fix them with relative ease. Such equipment can operate in a vacuum; the communication can be by magnetic field fluctuation, acoustic or radio wave through the cover without penetrating it. The same goes for other sensory equipment the ship may require, or antennae, lasers, telescopes, dishes, armaments, etc.

Of course the entire outer cover is sacrificial, as well. In the event of damage by space debris it can be repaired; but because it is not pieces bolted together and has no "components" other than the single seal (or a few simple seals) repairs can be hard welds and permanently fused glass melted into place.

In dock near planets the neon gas can be depressurized and re-liquefied for storage (yes, neon of all elements has the narrowest range of temperatures for liquefaction; just a 5.5F degree window, but we have future science on our side!). Then the cover can be detached; perhaps stored in space while the ship heads to the planet surface. Of course the ship would still be constructed to be pressurized itself, and would suffice in an emergency (like the cover being breached by an impact that does not breach the ship's hull); but it can be designed with many components for maneuverability, landing, loading and unloading cargo or passengers and so on.

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    $\begingroup$ It won't be "perfect", which is what the question asks for. There will be diffusion between the outer and inner hulls, at approximately the same rate as between a single hull and vacuum, as the gases seek to find their correct partial pressure. Some of this diffused gas will then diffuse through to vacuum. You can probably shave off a couple of orders of magnitude from an already small number, but it still won't be "perfect" $\endgroup$ – nzaman Jul 24 '17 at 12:54
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    $\begingroup$ @nzaman No there won't, the gap between the outer and inner hull is pressurized and filled with neon gas, it is not a vacuum. We have already stipulated that "perfect" is impossible, short of magic. perhaps with tech a few centuries from now, we can make the outer hull from perfectly formed diamond. After all, we can already deposit diamond film on a surface. But sure, quantum tunneling alone prevents perfection. Given that, the only answer to this question is "NO", but to help the OP, further explanation on how to get asymptotically closer to perfection is in order. $\endgroup$ – Amadeus Jul 24 '17 at 13:24
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    $\begingroup$ Yes, but there's no oxygen or nitrogen, so as far as those gases are concerned, it's a vacuum and they'll move to fill it. Another way of looking at it is that there is no movement of $N_2$ and $O_2$ from the inter-hull gap inwards but there is one from the outward. Dynamic equilibrium will be reached when the two flows are equal, and that requires a sufficient amount of those gases in the gap. Until equilibrium is reached, $N_2$ and $O_2$ will keep leaking into the hull gap and some part will escape through the outer hull--but that will be a couple of orders of magnitude less than otherwise. $\endgroup$ – nzaman Jul 24 '17 at 17:13
  • $\begingroup$ @nzaman I don't believe that is how it works; nitrogen and oxygen aren't magical thinkers, they don't care if "their kind" is elsewhere or "recognize" their own kind. This is mass and size, pure and simple, and you are engaged in magical thinking. $\endgroup$ – Amadeus Jul 24 '17 at 18:12
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    $\begingroup$ @Amadeus: You are the one engaged in "magical thinking" unfortunately. Not only do nitrogen and oxygen not recognize "their kind", they don't even recognize backpressure. They will permeate outward no matter what. The reason that equal partial pressures cause the loss to stop is not because the movement stops, but because movement rates in both directions are equal and cancel-equilibrium! When you think about the problem correctly, it does indeed matter what substance is outside causing the backpressure, because now you have oxygen leaking out, and neon leaking in, and they don't cancel. $\endgroup$ – Ben Voigt Jul 25 '17 at 2:19
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It just goes to my mind, not sure how practical would be:

How about an additional layer of hull? The gap between both layers can be wide enough to send a robot/a man in a space suit to perform repairs/maintenance. And use of vacuum pumps to gather air back under the internal layer.

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  • $\begingroup$ This is already popular in hard SF. In The Expanse, season 2, you could see it in action. In other works it happened as well. $\endgroup$ – Mołot Jul 24 '17 at 10:55
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The issue is refilling the lost air (and presumably other materials), so what you need is a way to carry sufficient quantities of replacement elements but without adding excessive mass to the ship (which makes engineering more difficult, costs more energy to accelerate, decelerate or make any course changes and so on)

Fortunately, there is a way to achieve this. Since the ship will be in the hard radiation environment of space, you need massive shielding. If the ship is moving at any appreciable velocity, interstellar dust, gas molecules and so on will be impacting the hull and slowly eroding it. So the ship needs to be both massively shielded and have some sort of protective armour in the front to protect against erosion in the direction of travel. The "ideal" shape of the ship would resemble a golf "Tee", with the wide end up front acting as a "wake shield" and a massive cylindrical sheath over the remainder of the ship.

enter image description here

Typical golf tee

In order to keep the rest of the mass down. this shielding is made of ice, and serves double duty, both as the shield and as a reservoir of hydrogen and oxygen to supplement the life support system. In the cold of interstellar space, a thin metalized foil cover over the outer surface is probably all the protection you need for the ice. Since water, oxygen and hydrogen are not sufficient in of themselves, the ice is mixed with other frozen "ices", such as methane, CO2, nitrogen and so on, so tapping the ice reservoir provides many important elements for the life support system.

If your recycling is efficient enough, then the amount of mass being drawn from the ice shield reservoir will only be a fraction of the total amount of ice actually available. Drawing the ice from the rear (near where the engines would probably be) doesn't sacrifice the protection over the rest of the ship overly much, and if necessary, some excess heat can be leaked into the hull to allow the ice to "flow" like a glacier to cover or recover thinned out or damaged spots in the protective shield.

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Over the long term, materials degrade under environmental stress (and therefore in unpredictable ways). They require endless, regular observation and repair. "Intelligent" materials (e.g. impregnated with nanomachines) or structural integrity systems could automate the job of keeping materials in top shape, provided they have ready access to an unlimited supply of repair material, appropriate infrastructure for removing waste, and the ability to completely maintain and/or replicate themselves.

Assuming the ship structure and materials manage zero gas loss when their state is within some margin of ideal that this system is capable of maintaining, you'd be fine. Even if the structure or materials do permit some small loss in their ideal state, this kind of system (especially nanomachine-impregnated materials) could also enable a kind of active reverse-osmosis by consuming energy to repel escaping gases back into the ship.

Unfortunately excessive damage would still lead to rapid gas loss before the automated maintenance system could repair it, and given enough time such damage is virtually guaranteed. But as others have suggested, replacing the lost gases would make up for it, and a system that already has the infrastructure and materials to fabricate repairs to containment systems could use its resources to generate lost gases as well.

All of this assuming we're within a few centuries of cost-effective, micro-/nano-scale matter-energy conversion, which is probably the only way to solve all of these problems (maybe even just one of them) with a single system.

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