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Imagine there is a micro-city built on the ocean floor in the pacific ocean covered with a durable dome with a 1 mile radius to keep water out. At this moment let us assume that the dome holds up beautifully thanks to the super-tough reinforced concrete developed to stand up to TSAR bombardment.

The problem is: how do submersibles get in and out of this dome city? Air pressure inside dome is 1 atm and the water pressure is approximately 1000 times stronger.

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    $\begingroup$ What is the problem you are having here exactly? What aspect about entry/egress have you gotten stuck on? $\endgroup$ – MichaelK Oct 3 '17 at 7:57
  • $\begingroup$ Also, what's the air pressure inside? How deep, exactly, is it? $\endgroup$ – Mołot Oct 3 '17 at 8:00
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    $\begingroup$ Are you sure about 1000atm external pressure? In water you get abut 1 atm/10m which means your city is about 10km under water, we don't have many trenches that deep. $\endgroup$ – ZioByte Oct 3 '17 at 13:20
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    $\begingroup$ The air pressure inside doesn't (and shouldn't) be at one atm. Humans can survive in significantly higher pressures (>70 atm if you adjust the air composition). The trick at that point is to not move between different pressures too quickly. $\endgroup$ – Samuel Oct 3 '17 at 18:04
  • $\begingroup$ I referenced this Quora question on The maximum depth a human body can go... on another WB question. It's actually kind of amusing to read. As we approach 100atm, it starts to be unclear whether there is any mixture of gas that isn't toxic or narcotic enough to prevent functioning. $\endgroup$ – Cort Ammon Oct 3 '17 at 19:04
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You can use the concept of a decompression chamber with two doors, one to the outer, one to the inner environment.

When it is open to the outer environment it is closed to the inner. Submersible can enter the environment from the outer (or leave to it).

When the outer door is closed the pressure can be adjusted (and some water removed) to match the inner pressure and the door can be opened on the inner side, allowing the crew to reach it.

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    $\begingroup$ Yes an air lock would be the best bet (the only bet?) at this depth. It would be relatively easy to enter and leave the dome if a series of large watertight doors were operated one at a time in sequence along an entrance tube way. Multiple doors would be very likely on such a large structure to provide an additional safety factor. $\endgroup$ – Slarty Oct 3 '17 at 9:25
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    $\begingroup$ Would not be easy or fast, The time and energy needed to remove all that water would be significant. Docking via a porthole interface would be the fastest, docking via a bottom entry pressure bubble would also be quicker though difficult to maintain the conditions. $\endgroup$ – anon Oct 3 '17 at 14:35
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    $\begingroup$ Instead of completely removing all water try releasing your sub into a lake inside the dome. That way as long as your subs aren’t huge you can release the high pressure water into the lake to then be pumped out at your leisure. $\endgroup$ – Joe Bloggs Oct 3 '17 at 15:24
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    $\begingroup$ @anon, there's no "all that water" involved. Water is essentially incompressible, so matching the pressure is a matter of removing only a tiny amount of water -- for a reasonable submarine and a minimally-sized decompression chamber, you're looking at only a few liters. $\endgroup$ – Mark Oct 3 '17 at 17:13
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    $\begingroup$ @anon Squashing water is not like squashing air or a sponge. It's more like squashing a rock. Litres is perfectly reasonable - if you pressurise water then leave a tiny gap, a little bit squirts out really, really, really, really, really, really, really, really, really fast. $\endgroup$ – wizzwizz4 Oct 3 '17 at 18:16
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I suggest flooding your dome.

No, wait, hear me out.

Your subs, essentially, want to be able to get in and out of the dome. Moving them in and out of water is messy and inefficient.

Your dome, too, will have to pump out water occasionally from a ‘sump’ (I’d call it a bilge, but this isn’t a ship), so you must have pumps capable of moving water out of the dome. Trouble with these pumps is theyll have to do an awful lot of work to pump out into 1000 atm, which will make them slow.

If the base of your dome is flooded with water at sea level pressure then you give the pumps a bit of a buffer that can happily absorb a sudden influx of water.

Your subs can then simply sail from the outside into a water lock with an inner and outer door (preferably ones that open sideways, or if you’re not just opening the doors to move water around, outwards). Once the sub is in you can close the outer doors and use the water pressure inside the lock to force water into the flooded base of your dome. If you allow the water out through carefully chosen vents you can minimise currents against the sub. If you want an example of this try looking up canal locks.

Once the pressures (very quickly) equalise you can open the inner doors and let the sub in. Your sump pumps can then take their time with the extra water, and thanks to how incompressible water is the level of your sub marine submarine pen won’t even rise that much (say your water lock is 20m by 10m by 10m you’ll be adding just under 100l of water to a lake a mile wide; or less than 0.1 mm rise).

Operating in reverse is similar, but this time you let water in from the outside, taking care that either your sub is strong enough to deal with the watery jackhammer you’re applying (hydraulic shock is not the same as water pressure) or that you let the water in slowly. Keep the lock open for when you want to return.

Plus you get a lagoon under the sea, and that’s pretty cool.

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There are a few options:

Basically a dry dock air/water lock. Your dome would need a gate carved into the side. The gate would open allowing subs into a chamber. The gate would close and water would be pumped out of the chamber and filled with atmosphere. The chamber could then be accessed by personnel. This method has the added advantage that external maintenance could then be easily performed on the sub.

A Docking system just like space shuttles and the ISS, you could have a standardized docking system that connects the dome to a port on the sub, thus docking subs outside your dome.

The other method that is not possible with your system:

Bottom entry sub pen if you pressurize your dome to the point water cannot flow in from the bottom (much like a bucket of air pulled into a pool) subs could come in from the bottom, surface and be docked.

All of these methods are currently used one or the other today in marine technologies.

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  • $\begingroup$ You could have a bottom-entry high-pressure sub pen chamber, with an air lock into the main habitable area. Thus the docking mechanism would not have to resist water, only air. Robots could service the sub in an atmosphere, not water, and thus not be impeded by buoyancy or the resistance of water. Repairs could be made to the sub using 'open panel' techniques if the insides of the sub were brought up to the same air pressure.. $\endgroup$ – Justin Thyme Oct 3 '17 at 14:26
  • $\begingroup$ You could but that is a hybrid of the water lock, also only the water lock provides full repair capability. The other solutions could result in flooding of the main cabin which could be unacceptable. $\endgroup$ – anon Oct 3 '17 at 14:31
  • $\begingroup$ What I had in mind was a sub pen, with the water and atmosphere at 1000 atm. The sub could be raised into the atmosphere chamber, still at 1000 atm, clear of the water. It would be all engineering. A human could not survive in such an air chamber, at 1000 atm, but robots could. And no, you could not bring the chamber atmosphere down to 1 atm, as the water would still be pressured to 1000 atm. $\endgroup$ – Justin Thyme Oct 3 '17 at 15:30
  • $\begingroup$ you could do that but if you are using robots you might as well do most the repairs in the water and only dry dock/lift the sub for specific repairs. $\endgroup$ – anon Oct 3 '17 at 17:30
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I suggest two types of docking systems:

  • Personnel docking: the submarines may dock via "door to door" system which allows personnel and cargo to pass through a connection while the submarine stays outside. Think of astronauts docking to a space station. This is the most efficient docking system as maintenance is seldom needed. That's why this docking method will be used most of the time.

  • decompression chamber large enough for a submarine: a second dome contains an air pocket at depth pressure and may remain open at the "seaside". When personnel leaves the submarine through this large chamber, two steps are necessary: the seaside entry and the habitat-side are sealed. Decompression is achieved by pumping air into a storage tank and then the habitat door is open. The water is barely compressible and is pumped out (into another tank) only when drydock maintenance operation is needed.

Points of interest:

1- i have found that using storage tanks for the air in the airlock is easier than having to pump water in and out into the open.

2- I based my assumption on water compressibility as explained in this Wikipedia article. https://en.m.wikipedia.org/wiki/Properties_of_water#Compressibility

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  • $\begingroup$ Yes. The key benefit of a distinct "personnel" port is the ability to maintain pressure at 1 atm. for the people moving between the city and the submersible. Once you expose your people to high(er) than usual pressures, you have to decompress them slowly, or they get the bends. $\endgroup$ – papidave Oct 3 '17 at 16:46
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Have you tried this thing as a kid, when you are swimming in a pool or at a lake, where you take a small bucket, turn it upside down, and competed for who could push it down the farthest? It is hard, since the bucket will hold air inside itself, which wants to float. However, it is possible to force it under and hold it down. The air does not come out of the bucket unless you flip the bucket around.

Your underwater bio-dome is basically a giant semi-spherical bucket. Its structure gives it weight, plus it requires a handful of ultra-strong cables anchoring it to the sea floor, probably all the way to bed rock. The air may need to scrubbed (as on the ISS) and replenished from time to time but it will not escape. Here is a crude doodle that I made in paint:

enter image description here

My doodle omits the "city" part, but I would expect buildings could either be on stilts from the ocean floor, or suspended from the top of the dome, or both.

Now that we see this, the answer to your question is almost trivial. The submersibles enter and exit from underneath.

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    $\begingroup$ OP requires 1atm inside dome, 1000atm outside. In your design the pressure is, of course, equal. It also wouldn't need the "super-tough reinforced concrete developed to stand up to TSAR bombardment" $\endgroup$ – ZioByte Oct 3 '17 at 13:16
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    $\begingroup$ pasting from @Slarty's fine comment on now-deleted similar proposition, because I like how it is phrased: " The ocean at that depth is pressurized to 1000 atmospheres the air inside the dome is at 1 atmosphere. As soon as the hatch was opened to the sea the water would flood in until the dome was almost full. The domes atmosphere would be compressed into a head space in the top of the dome at 1000 atmospheres pressure." $\endgroup$ – Willk Oct 3 '17 at 13:28
  • $\begingroup$ I would not want to try living inside this dome. $\endgroup$ – Joe Bloggs Oct 3 '17 at 15:53
  • $\begingroup$ @ZioByte is entirely correct. I had missed that aspect of the OP's requirements. This deserves the down-votes it has received, but I am going to leave as it helps to demonstrate some ideas, even if it fails to satisfy the parameters of the question. $\endgroup$ – cobaltduck Oct 3 '17 at 15:57
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    $\begingroup$ This idea is called a "moon pool". $\endgroup$ – Samuel Oct 3 '17 at 18:04
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You have two transitions to make here, if you want the vehicle to come into the dome. One is high pressure to low pressure, the other is air to water. Thus, you would need a chamber big enough to hold the vehicle, and then pump water out and air in. You then have the decision to make - do you pump the water back into the sea (high pressure) or into the dome (low pressure)? When the vehicle leaves, the problem is just one of engineering - designing the chamber so that it would handle the inrush of high-pressure sea water of its own accord.

So your decision is an external docking method, like the ISS, or an internal hanger/bay system, like Star Trek. The external system would be faster, and energy-efficient, but the second system allows for maintenance of the vehicle and loading external weapons.

But a consideration - is the dome truly a dome? Is it a sphere, or a hemisphere? Is it half a mile high, or a mile high? A hemisphere (actually, a distorted hemisphere, somewhat like a Hershey's kiss) would handle the external stresses better - take them to ground. It makes a difference, as the water pressure at the top of the dome would be considerably less than at the base, and would be the better place to put your docking mechanism. Less stress.

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    $\begingroup$ I see no reason to require an air-water transition. A submarine operates in water, therefore it is simpler to have a water-lock which allows the submarine in, depressurize to 1atm, and let the submarine out inside the dome still submerged. The submarine can then surface and dock normally, and maintenance areas be designed outside of the security critical water-lock. $\endgroup$ – Matthieu M. Oct 3 '17 at 15:01
  • $\begingroup$ @ Matthieu M. It depends on the demands of the plot line. $\endgroup$ – Justin Thyme Oct 3 '17 at 15:20

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