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It's easy to imagine how would a water-breathing aquatic race outfit its spaceship - take a normal spaceship, insulate it, fill it with oxygenated water, possibly add some enhanced turbines to pump water so that users will be able to stay at the consoles without having to swim all over all the time.

But what would air-breathing merfolk do? They can't just be thrown into water or they'll drown. Having a mix of water and air would appear to be very problematic in an interstellar environment.

I initially conceived the problem when trying to come up with a mermaids' starship, but it would also concern anthropomorphic otters, dolphins etc. - in general anything that feels the most comfortable when swimming in water but requires gaseous air to live.

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    $\begingroup$ "It's easy to imagine how would a water-breathing aquatic race outfit it's spaceship" but hard to imagine a water-breathing aquatic race building a spaceship. Ditto any kind of sea-folk... $\endgroup$
    – RonJohn
    Commented Mar 19, 2018 at 19:45
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    $\begingroup$ I would imagine they would do the same thing as many other species: wear a space-suit. I assume they would have relatively comfortable in-ship suits, it minimizes the volume of air/water you need to run through lifesupport systems and you could have lifesupport ports in the ship where the inhabitants can sporadically switch out their waste and C02 for clean stuff, probably best to have switcheable gear so you have spares if the lifesupport malfunctions. A pet peeve of mine is space warfare without space-suits, its like Naval combat without lifesuits on hand and an easier time getting into 💦 $\endgroup$
    – Demigan
    Commented Mar 19, 2018 at 20:42
  • $\begingroup$ Biggest challenge im spaceship design for a water-dwelling species: youtu.be/r7sdJtgQ7_8 $\endgroup$
    – The Square-Cube Law
    Commented Mar 20, 2018 at 11:20
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    $\begingroup$ M. A. Golding already alluded to this, but... required reading: Startide Rising, which features a spaceship crewed by an aquatic, air-breathing species (dolphins, specifically). $\endgroup$
    – Matthew
    Commented Feb 20, 2020 at 15:51

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Assuming some sort of "Dolphin with hands" type of creature, and no artificial gravity, the simplest way to arrange this is to have a rotating torus half filled with water.

The rotation of the torus keeps the water "down" in the "floor", while there will be an airspace between the surface of the water and the "roof" of the torus.

An added bonus of this arrangement is that water already provides an effective shield against cosmic radiation and other threats, as well as provides a thermal buffer. In "The Millenial Project", the author considered a water shield 5m thick would keep radiation inside at a level similar to sea level Earth. For humans, this would mean a set of structures 5m apart filled with water surrounding the living space; for your aquatic creatures they would actually be living inside it, making some of the engineering easier. Since much of the life support would be living in the water as well (creating a closed ecosystem), in some respects the aquatic species might have an easier time of building a functional spacecraft or colony.

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    $\begingroup$ But -- remembering that dolphins have no legs -- how will these dolphins-with-hands develop fire, much less the technological underpinnings needed to build the spaceship? $\endgroup$
    – RonJohn
    Commented Mar 19, 2018 at 20:26
  • $\begingroup$ @Ronjohn assuming they start some tool usage and begin scientific advances, they would eventually get to material and chemical sciences required. Similar to humans using boats they would develop land-traveling vessles and probably need ways to build land-based mines as well. At some point they would invent ways for air-travel, probably for military purposes first. Then they develop sattelites for communication and such, and eventually reach a tech level where they consider the gargantuan task of water-filled space vehicles. Robotics and space-mining might be required first to stockpile water. $\endgroup$
    – Demigan
    Commented Mar 19, 2018 at 20:57
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    $\begingroup$ @Ronjohn and we have neither wings nor Fins, yet we fly and swim and use subs. No species evolved for space travel and yet we are there. And we found chemicals that burn underwater, no reason they wouldn't find those or that they would find all the burny things above water and do the research. Yes their tech would use a divergent path to ours, but that doesnt mean its impossible. $\endgroup$
    – Demigan
    Commented Mar 19, 2018 at 21:24
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    $\begingroup$ I remember a Larry Niven story "The Handicapped" with a protagonist who works for a company that makes (thought controlled?) prosthetic arms and hands for hand less intelligent beings like dolphins - the "handicapped". In David Brin's Uplift Universe it is common for intelligent beings to genetically modify sub intelligent beings to full intelligence. No doubt the process could also involve giving them technology and teaching them how to make and use it. So even if intelligent sea dwellers might be unable to create technology on their own, they could still have it. $\endgroup$
    – M. A. Golding
    Commented Mar 19, 2018 at 21:40
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    $\begingroup$ Since the OP has specifically asked how they will build spacecraft, it can be assumed they already have undergone the tech revolutions needed to build such things $\endgroup$
    – Thucydides
    Commented Mar 20, 2018 at 4:31
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I like this question, space mermaids don't come up often enough. Your astronauts can breathe the same stuff the divers in The Abyss breathed: perfluorocarbon. It's not without problems IRL; human lungs can't circulate PFC fast enough to remove carbon dioxide. But if you're not doing hard scifi that can be handwaved away easily enough: your mermefolk aren't human and their lungs don't have to have the limitations human lungs do.

You can read more about liquid breathing here.

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    $\begingroup$ This method is also used by the Liir from the Sword of the Stars universe - think space dolphins, with manipulating tentacles around their beak. Their ships are filled with an oxygenated liquid medium to allow them to breathe. Liir spacefarers call themselves "The Drowned", because they live in liquid for the rest of their lives and never breathe air again. $\endgroup$
    – Werrf
    Commented Mar 22, 2018 at 19:26
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This is probably not so hard as you think, and the natural answer would lead to the development of the 'right' form of space propulsion technology insofar as it would be scalable beyond interplanetary flight full on into interstellar. That said, it's not all as simple as it sounds in the OP. There is afatal flaw in your original assumption.

It's easy to imagine how would a water-breathing aquatic race outfit its spaceship

This actually isn't the case. Air breathers have the benefit of needing a gaseous atmosphere, and that atmosphere has one crucial advantage over liquids; it's compressible. This allows the body to take a much higher level of G force in that atmosphere because the gas doesn't actually add to the problem (actually it does (Boyle's Law) but not in quantities that significantly contribute to the problem).

As an experiment, put a hamster in a cage and shake it. Check your hamster; it'll be bruised, sore and more than a little angry, but it'll most likely be alive. On the other hand, put a lid on a fishbowl containing a goldfish, and then shake it.

There's a VERY good chance your fish will be dead.

(When I say 'Experiment' by the way, I'm not actually recommending you do this. Please don't try this at home and if curiosity persists, please consult your mathematician.)

The reason for this is that water isn't compressible and therefore transfers the full brunt of all its kinetic energy into whatever IS compressible in its path, and in this case it's the goldfish. Water is also very dense, meaning that the mass striking the goldfish with every shockwave of the shake is phenomenal by comparison to its normal environment. The goldfish won't survive.

You can actually remove a cork from a wine bottle by repeatedly knocking the base of it against a wall or a tree. I don't recommend this either because it bruises the wine, but my point is that any living thing taking massive G force stresses in a liquid is unlikely to survive.

Even your Merfolk will be in the same boat, but the answer (provided you can get out of the planetary gravity well somehow) is simple; some form of constant acceleration engine.

NASA is already working on two possibilities in this space; Ion and Plasma engines. These are designed to provide constant, relatively gentle thrust by comparison to conventional chemical rockets. These would be far more efficient for long distance travel than chemical rockets (although they're not powerful enough to get out of the gravity well) and the constant thrust ALSO provides for that simple answer for your merfolk; gravity.

Well, not really gravity, but the constant acceleration is going to gently push everything to the back of the spaceship. This will mean that all the water is going to drift to the back of the ship. If the ship's internal chamber is only 80% water, then the front of the spaceship is 'up', and your merfolk swim that way to breathe. You recycle the atmosphere at the tip of the ship, and the water gets filtrated et al at the bottom.

You will probably need space suits of some form for those short term attitude changes or at the half way point when you turn the ship around and start to decelerate, but for the vast majority of the trip, you have a natural chamber of air at the 'top' of the ship for your merfolk.

You still have the problem of getting out of the gravity well, but that could be done with a space elevator under the circumstances. So; you have a space elevator to get you out of the gravity well, then a continuous thrust engine to provide light gravity and orient your ship. For exploration, this is perfect. For combat, you're in trouble because rapid changes in velocity or impacts on the side of the ships don't have to breach your ship to be fatal. But, that's the price of being a water borne species I'm afraid.

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Who says you need the water at all? They could move through an air-filled ship same as a human astronaut: by pushing off the walls or climbing ladders.

You're assuming they absolutely need to swim to move, but humans don't get to use our natural method of locomotion in space, so we've adapted. So can they. In fact they could do the same thing we do.

Mermaids could probably navigate the ISS as it is. They have two arms, and that's enough. Otters have two arms but they'd need miniature ladders (dawww). Dolphins as they currently exist would have trouble because they can't grab things, which is a necessary part of climbing ladders or re-orienting yourself for the next push. However you said they were anthropomorphized, and if they're space-faring then they must have some 'hands' either evolved or technological.

There is the problem of discomfort or even drying out like a beached whale. The solution is to keep the air humidity high and also have a dedicated saunas or pool rooms where they can go rejuvenate. The pool room could simply have a nozzle on the wall they can suck air from.

In case the simplicity and safety isn't convincing enough: consider that this would require no re-designs of systems to make them water-proof. No lifting huge quantities of water into space. And land-dwelling air-breathers can inhabit the same spaces as their aquatic friends.

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    $\begingroup$ This is what I was thinking. Just fill it with air and deal with being out of water. They'd probably have an easier time navigating it actually, being wholly at home in a 3d environment regardless of medium. $\endgroup$
    – Ynneadwraith
    Commented Mar 23, 2018 at 1:31
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    $\begingroup$ Plus you could slip in jokes like "For the first week Jane couldn't help feeling like a fish out of water on the station, which, incidentally, was an alarmingly accurate assessment." $\endgroup$
    – Jared K
    Commented Mar 23, 2018 at 3:46
  • $\begingroup$ I sense the start of a highly amusing short story there ;) 'Through a odd quirk of fate, across the countless lightyears and innumerable centuries separating our stories, coincidences align to ensurw that our erstwhile dolphin-humanoid protagonist is called 'Jane', a common name among the ape-descendants of earth. Funny how these things align when the story require it...' $\endgroup$
    – Ynneadwraith
    Commented Mar 23, 2018 at 23:27
  • $\begingroup$ "this would require no re-designs of systems to make them water-proof"... except that with the humidity and condensation, it just might. Still, that's do-able. $\endgroup$
    – Matthew
    Commented Feb 20, 2020 at 15:55
  • $\begingroup$ If these aquatic creatures were floating in air, then the air would need to be colder to keep them cool, which would require more heat-radiating surfaces on the ship, which would likely be undesirable for a spaceship $\endgroup$
    – Ichthys King
    Commented Nov 16, 2020 at 12:17
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For short missions (Vostok to Apollo-type) it has be an air-filled ship with water-filled spacesuits (similar to @Christian's suggestion).

For medium to long-term missions (ISS-type) ships can have separate air and water-filled compartments, and astronauts can split their time between them for their convenience.

For more advanced long-term missions, rotating spacecrafts (similar to @Thucydides' suggestion) can be used.

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Spacesuits filled with water. Seal and compress the helmets, or have face masks to provide oxygen. Maybe the helmet stays water filled for a set period of time, followed by a purge to provide air (if being completely submersed for some amount of time is required for survival).

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  • $\begingroup$ I think OP is talking about a spaceship with life support in which you don't have to wear spacesuits. $\endgroup$
    – nagamani
    Commented Mar 20, 2018 at 10:45
  • $\begingroup$ Well that's not very helpful then. $\endgroup$
    – Unassuming Guy
    Commented Mar 20, 2018 at 15:28

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