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Like this question I'm wondering about design considerations for an intelligent aquatic species that wants to travel into space. Would they have to try and create special body suits to keep moisture content or try and fill a ship with water? Or would they have to fill up a ship once it's in space (say a space station) with multiple trips?

Size of the Ship: I am more interested in medium to larger size ships, but ones without 'Gravity fields'. Known gravity generation only. ie spinning a ship. Maybe start with something like the ISS. And going larger from there. What are things to that would go into the design?

Here's a picture from Harry Potter's Merpeople for an idea of physical characteristics ie, tail, no legs. enter image description here

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    $\begingroup$ I don't know the answer, but I know whatever it is, I feel really bad for any intelligent aquatic life that dreams of swimming among the stars. No matter how they do it, things are going to be so much harder for them. $\endgroup$ – DaaaahWhoosh Oct 15 '15 at 18:27
  • $\begingroup$ @DaaaahWhoosh I agree, that is most of what sparked this question $\endgroup$ – bowlturner Oct 15 '15 at 18:28
  • $\begingroup$ Isn't as simple as replacing the air with water? Opening the ship docks in open air is as dangerous as opening them in space. (almost) $\endgroup$ – Vincent Oct 15 '15 at 18:42
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    $\begingroup$ @Vincent Not really. If you have your door facing up in air, all of your water won't run out. On top of that, it takes a LOT of energy to get a spaceship full of water into orbit... and what about inertia? $\endgroup$ – bowlturner Oct 15 '15 at 18:52
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    $\begingroup$ This might help: worldbuilding.stackexchange.com/questions/3828/… $\endgroup$ – Tim B Oct 15 '15 at 19:40
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Basically, forget "down". An aquatic species would need that even less than humans do. Most sci-fi ships have floors and are arranged into decks similar to naval vessels. An aquatic species would have little need of a floor, as they can float anywhere they need to be within the ship without anything to stand on or grab onto.

Now, Earth fish have a swim bladder, helping to provide neutral buoyancy, and in most fish the arrangement is most stable when there's a gravitational force on the fish, with the result that fish in zero-G tend to look uncoordinated and disoriented. We've done similar experiments with many other animals, including birds and cats on "vomit comets" for short periods of weightlessness, and the results are similar; zero-G is a very novel experience, and most animals don't know how to function without a downward force acting on them.

Humans are similar in this regard, but we have advantages of sentience; we can understand what we're about to do so it isn't a surprise like it is for a cat or a bird, and we can be taught to maneuver in zero-G without having to learn it from scratch. A sentient aquatic species, theoretically, would have a similar ability to "groupthink" and thus reduce the difficulties any individuals might have.

As far as the ship design, the biggest hurdle is going to be getting water out of the planet's gravity well. The Tsiolkovsky rocket equation models a fundamental truth of rockets; they have to lift their own fuel, at least what they haven't burned yet. As a result, achieving escape velocity from an Earth-like gravity well requires the rocket to be much, much more fuel than payload. "Payload" here is basically anything that isn't fuel, including the cargo vehicle but also the non-fuel part of the rocket system like its fuel tanks, pumps, exterior sheathing, nozzles, etc., which increases as the amount of fuel does, thus requiring either even more fuel or less "useful payload", what you actually want to put in space. Our engineers, therefore, endeavor to make everything they put into space as light as it can be, especially those parts of the vehicle that are disposable and used only to house the components of the rocket itself. We also figured out multi-stage rocketry; when the fuel in a stage is gone, you can lose the containment for that stage and save that weight on the next leg. Practically every manned spacecraft was launched on a multi-stage rocket, it's just been the only technically feasible way to do it.

As one example, the Space Shuttle, one of the most effective launch vehicle designs in human history, used an orbiter that weighed 100 tonnes (100,000 kg). Inside that orbiter, another 30 tonnes, max, could hitch a ride to LEO. The total maximum launchpad mass of the STS, including the external tank and boosters, was 2,000 tonnes. That means that using the Shuttle to get 30 tonnes of stuff into orbit that you plan on leaving there requires a system that is 98.5% "unusable payload". Even considering the orbiter vehicle itself as "useful payload", being the quarters for the crew and a good place to conduct zero-G experiments before the ISS was finished, the total launch vehicle is 93.5% fuel and fuel containment.

One last thing is shock dissipation. Water is classically an incompressible medium (theoretically its density can be changed with pressure, but the ratio of force to change in volume is many orders of magnitude more than for a gas). This is typically bad news for aquatic beings when a strong force changes the water pressure significantly. "Shooting fish in a barrel" turns out to be really easy, because the shock of the bullet entering the water is enough to stun or kill the fish swimming in it, similar to the effect of a flashbang in air; you don't have to hit them with the bullet (and in fact the bullet will slow to nonlethal velocity in the span of a few feet).

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I think it would take extraordinary circumstances for an aquatic species to be capable of getting to space in the first place. Unless they are amphibious, they will have enormous mobility challenges outside of a water environment. It’s also unlikely that they would try to develop technology to mitigate that disadvantage unless their planet actually had land to explore. Even further, if we suppose that they develop sophisticated water suits capable of non-aquatic movement, that indicates that they would be inside of a spaceship that’s not totally filled with water. This would be a very difficult construction task for a species of this nature.

Their technology is also very likely to be based on fluidics. Attempting to build a spacecraft with fluidics would be hugely challenging even before considering the weight that needs to be carried into space. Discovery and advanced development of electricity would be a tall order for an aquatic species. It’s tempting to instead imagine them launching a capsule full of water, but the necessity of EVAs (even if only in emergencies) will require the development of sophisticated suits anyway.

With these ideas in hand, it seems that their best chance of being in space (however slim it may be) is with complex suits that have been through an extensive period of use, testing, and refinement on their own planet. These should provide them with the necessary mobility and life support to move outside of water and prevents needing to haul a full wet habitat with them. For longer stays, it will become necessary (if for no other reason than their mental health) to fill at least some portion of the spacecraft or station with water. Getting to water in space will be difficult until they have long-term and highly maneuverable spacecraft available, which means they’ll need to haul the water up there over multiple trips.

Once their ships are more mobile and self-sustaining, they can begin to fill larger and larger portions with water found in space. It’s likely that the water-filled sections would start with their sleeping quarters and eventually full habitat. Airlocks of some kind will be needed when moving between habitat and the rest of the ship, but it may be advantageous to keep the rest of the interior a vacuum, thereby reducing the complexity of airlocks to space.

They do have an advantage, however, in that they won’t need artificial gravity in space.

As a final thought that’s slightly off topic, I also wonder if an aquatic species would even have interest in the stars. Human beings have evolved while looking upward because we could see a vast unknown that seemed just out of reach. An aquatic species might instead evolve while looking downward — down into the uncharted depths where pressure prevents them from wandering.

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  • $\begingroup$ Why would aquatic beings have a mobility challenge? If the spacecraft is full of water it would be easier than it is for astronauts. You sort of assume transferring mass to orbit is hard, and it is, but we do it with relative ease. $\endgroup$ – FraserOfSmeg Oct 16 '15 at 0:51
  • $\begingroup$ @FraserOfSmeg A mobility challenge exists if filling crew spaces with water (for launch) is impractical. We already have severe limitations on effective payload mass and we're not doubling or tripling the weight of our crew spaces by filling them with water. Our relative ease of mass transfer certainly benefits from us not being biologically confined to the ocean. $\endgroup$ – Avernium Oct 16 '15 at 5:07
  • $\begingroup$ I'm sure the aliens that live on a planet without any atmosphere say the same about transferring so much air to orbit. $\endgroup$ – FraserOfSmeg Oct 16 '15 at 11:12
  • $\begingroup$ @FraserOfSmeg: Air is very very much lighter than water. The whole atmosphere column above a square meter, from the earth up till space, weights about the same as a mere ten cubic meters of water, not even enough to fill the space shuttle. $\endgroup$ – celtschk Jul 26 '16 at 20:37
  • $\begingroup$ A very true comment. But it's all a question of perspective. Another way of looking at it is, taking air to orbit is an infinite percentage increase in mass over not taking air (vacuum). Taking water of air is 7840% increase in mass. Since 7840 is less than infinity is a far more acceptable increase. The point I'm making here is taking what you need is acceptable, and it's certainly possible. This is world building remember, possible is fine. $\endgroup$ – FraserOfSmeg Aug 1 '16 at 13:51
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All the same concerns you would have with an aquarium, oxygen,temperature, and sanitation being the main ones. I would think this might mean that the entire ship might have current, and the creatures might think about the ship directions in terms of upstream and downstream. They also might be unlikely to consider landing on planets at all.

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The most confortable thing would be filling with water, however unlike filling a ship with air, filling with a liquid would add huge amount of weight to the ship. The extra weight would make it very expensive to make a spacecraft to reach orbit or other planets. I think unless the aliens are very small animals , some type of body suits with portable water tanks and some type of water treatment to make the suit as autonomous as possible would be a possible compromise. Maybe a mix some small liquid filled rooms for some activities, and suits for moving around in a larger ship. Most likely large leisure spaces in the station would be too expensive to fill with water.

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For a sufficiently (ie. much beyond humans) advanced aquatic species, you would not need to build a ship at all.

Saturn's icy moon Mimas has a density of 1.15 g/cm^3, whereas water is 1 g/cm^3...so Mimas is almost all water. It has a mass of 3.75x10^19 kg. Supposing an aquatic species launched a robotic mission to grab Mimas out of orbit and move it nearer to the sun. As it melted it would remain a sphere of mostly water with some other heavier elements mixed it. It would, in effect, be a perfect spaceship, simulating a 3-d ocean but with much less gravity (.006g in the case of Mimas, at the surface). Any technology developed for use the aquatic species home oceans should work just great in their new space-sphere.

The aquatic species could then insert themselves into this new space habitat and use it as a space station, or build an interstellar drive into it and go explore the stars. A big advantage of a liquid water sphere moving at thousands of kilometers/s would be excellent for absorbing the impact of random space particles and radiation, protecting the inhabitants much better than a human style spaceship with no energy expenditure on shields. A big disadvantage is that you can't accelerate faster than the acceleration due to gravity of the sphere or you will defrom the vessel to the point of splitting. So its slow moving.

Obviously, Mimas is huge, but there are smaller comets around and one of them might be a better option.

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  • $\begingroup$ Good idea, but aren't you sort of jumping right past the level of reasonably comprehensible technology and out to "miraculously advanced"? How do they develop the technology to move large space bodies without going through the "let's explore space" phase first? This seems like something a civilization might do after they had already been exploring space for some time. $\endgroup$ – JBiggs Jul 26 '16 at 19:39
  • $\begingroup$ On Earth there was a cold war and space race that rapidly pushed humans and spacecraft with minimal computer power into space, mostly for reasons of national pride. Lets say a planet developed more peacefully and only at the modern day's level of technology got interested in launching spacecraft. $\endgroup$ – kingledion Jul 27 '16 at 1:11
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    $\begingroup$ They would probably be launching strictly robotic spacecraft at this point, which would develop along some line of technological progression. Eventually they could launch a robotic spacecraft with an advanced electric propulsion that could capture a comet at a near-sun point in its trajectory into a stable orbit near the home planet (like the L1 orbit or something). $\endgroup$ – kingledion Jul 27 '16 at 1:12
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    $\begingroup$ Last note: the technological trajectory to our equivalent level of technology for an aquatic species would be so radically different from our own its hard to imagine. An aquatic species would never come up with an internal combustion engine, that uses free oxygen in the air to work. What would they use instead? Radioactive isotopes/nuclear fission driving a brayton cycle engine? They would never use steel because it corrodes in seawater. What would they use instead? Monel? Titanium? Keratin from turtle shells? Extruded basalt? They could never use electronics in seawater. $\endgroup$ – kingledion Jul 27 '16 at 1:17
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Radiation shielding. There are potential benefits to an aquatic species in spaceship design as well. For one thing, water happens to be very good at insulating against radiation, which could remove one factor that we have to face when designing any space craft designed for long term use. At a minimum, you would want to have a "layer" of "water rooms" around the outside circumference of the vessel just to be a useful radiation block for the lighter weight central parts.

Aquatic species like this would have had to create space suits for themselves long before they ever got into space, just to operate on dry land. If they existed in environments that had a higher pressure than the surface, these suits might also have to artificially "squeeze" them to keep all their organs in the right place in low-pressure environments like the surface. If they had gills, they could rely on a small amount of water in a recirculating purifier/oxygenation system (like a reverse SCUBA tank) that could be carried, so the entire ship would not have to be full of water necessarily. Alternately, if they didn't have the pressure restriction, they might fill their ship with a mixture of water vapor and some kind of lightweight, stable gas at the minimum % of water they needed to survive.

Second advantage of water on space ships: Reaction Mass. Water happens to be one of the most efficient sources of reaction mass available to move a ship in a space environment. Not only that, but there is a lot of water available in space in the form of ice (mostly in asteroid belts). A ship could be designed to take on ice from wherever it could, melt it, integrate it into it's internal water system, and use it as reaction mass by pumping some of it through a nuclear-heated rocket type system or whatever else makes sense for the technology level. Instead of both a lot of fuel and a life support system, the life support and fuel systems would be basically integrated, potentially saving complexity and space. Obviously, if you use up too much water, you could run out, but everyone knows space travel is dangerous, aquatic or not.

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