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The classic example of a cylindrical space habitat, Clarke's Rama, has a 10 km-wide ring of water at the middle. Most depictions of this O'Neill-style of habitat do something similar, placing a large body of water somewhere inside.

The drawbacks to this design choice are fairly obvious:

  1. If the habitat is meant to move, the momentum imparted to the water needs to be accounted for when it comes time to apply the brakes. Rendezvous With Rama "solves" this with a sheer cliff on one side of the sea, against which the water sloshes. This cliff towers over the rest of the habitat and serves no other apparent purpose.

  2. The sea takes up space – nearly 20% in Rama's case – that could be dedicated to habitation, agriculture, and so on.

  3. The sea, especially if it occupies the circumference of the inner surface, is a barrier to all surface-based transportation.

  4. If the primary purpose is water storage or recreation, then arguably numerous smaller and less obstructive lakes could do the same job with fewer or reduced downsides. In fact, smaller lakes could be even more effective (e.g.: if the goal is recreational coastline, ten lakes 1/10th the size of the sea would have more than three times the sea's coastline; water stored within the hull does not pose a flood risk).

  5. Water is less dense than the surface and sub-surface material it replaces, which could throw off the balance of the habitat and cause it to spin end-over-end.

So, are there any undeniable benefits to having a large and obstructive body of water, either at the 'equator' or elsewhere, in such a habitat? Benefits must offset the above drawbacks. Assume a habitat for humans; an amphibious or aquatic species would likely ask "why all this dry land?".

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    $\begingroup$ I always interpreted the inside of Rama as reproducing an ecosystem rather than providing effective housing. Point 1. can be avoided by propelling the ship "sideways" or using smaller accelerations. $\endgroup$ – SilverCookies Feb 9 '18 at 10:50
  • $\begingroup$ water is an excellent shield against space radiation. So you could have the spaceship formed like a ring, a ocean/sea on the outside and housing/farming/whatever on the inside. $\endgroup$ – Julian Egner Feb 9 '18 at 11:18
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    $\begingroup$ @JulianEgner I'm not talking about a ring or about a water shield. $\endgroup$ – rek Feb 9 '18 at 15:53
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    $\begingroup$ @SilverCookies: Propelling the ship sideways (i.e. perpendicular to the axis of rotation for artificial gravity), when the ship is braking, would have the peculiar sensation of increasing art. gravity (when your feet are pointed towards the thrust vector) or (partially or fully) negating art. gravity (when your head is pointing towards the thrust vector). Subjecting the body of water to this fluctuating (and rotating) angle of gravity is likely going to create a massive wave pool for the durating of the braking burn. Maybe not worse than the initial issue, but not really much better either. $\endgroup$ – Flater Feb 9 '18 at 16:45
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    $\begingroup$ Isn't at least a portion of the area "under" the raised cliff used for tunnels and such? And in an inhabited RAMA, the water is used as a divider to prevent access between areas. $\endgroup$ – Michael Richardson Feb 9 '18 at 16:56
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Radiation shield

There is lots of radiation in space. There are few better shields than water. Due to its hydrogen bonds and large dipole (that is, the oxygen is negatively charged, the hydrogen positively), water interacts readily with the most dangerous radiation in space, cosmic ray protons. Furthermore, since water is full of free H$^{+}$ anyways, the reaction of the protons doesn't cause any weird effects, it just makes the water (very, very slightly) more acidic over time. It also is a good blocker of gamma radiation.

Simply build your large body of water around the habitat, to protect everyone inside. As an added benefit, water is relatively transparent to visible light. The absorption coefficient of water in the gamma range is something like 20 cm$^{-1}$, while for blue light it is 10$^{-4}$ cm$^{-1}$. 2 meters of water will block 99.9% of incident gamma radiation, while letting the same amount of blue light through (admittedly, only about 82% of red visible light will go through).

Conclusion

You can armor your space habitat with a water radiation shield while allowing most of the light through to brighten your inhabitants days. Once you have installed a large mass of water, you can keep it for other uses, as well; potable water, humidifier, fish tank, recreation, etc.

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    $\begingroup$ @JoeBloggs The tank goes around the outside. But if you think about it, with centrifugally simulated gravity, the tank is 'down' for every point on the station. So you could say the entire station is 'floating' in an ocean. All the structures in the colony have an ocean in the basement. I would say, you won't want this just at the equator, you want it everywhere. $\endgroup$ – kingledion Feb 9 '18 at 12:38
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    $\begingroup$ @kingledion: If it’s an outer sheath in a large tank I’m not sure that would count as a ‘large and obstructive’ body of water as seen in the Rama books. Not disagreeing with the premise, just not sure it answers the question.. $\endgroup$ – Joe Bloggs Feb 9 '18 at 12:46
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    $\begingroup$ @TomášZato There's plenty of light in space if you're close to a star. $\endgroup$ – Beofett Feb 9 '18 at 14:17
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    $\begingroup$ @rek: though I can imagine diving in a submersible down through the internal ocean to check on the water shield ecosystem where the deep sea creatures live in total darkness. That would be pretty cool. $\endgroup$ – Joe Bloggs Feb 9 '18 at 16:19
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    $\begingroup$ @Madlozoz Water isn't specifically good at gammas, but it is specifically good at cosmic rays, as I say in my post. It also happens to be the most common thing by mass in asteroids, comets and moonlets in the solar system, so its going to be super cheap as well. Finally, water is very hard to activate, there isn't a lot of naturally occurring radioactive water. Its the best thing to make a radiation shield out of. $\endgroup$ – kingledion Feb 9 '18 at 16:26
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Depends on your definition of benefit. In the case of Rama the lake wasn't just a lake, it was also a machine reclamation (and presumably construction) yard where machines from anywhere in the habitat could take advantage of 3d space, and if I remember correctly also had something to do with energy storage. Anyway..

By body of water I'm going to assume you mean un-tanked and visible. This immediately opens up some possibilities:

1: Passive temperature/humidity control.

Our ocean is a brilliant storer of heat. It's warm in winter and cold in summer as the thermal mass of the ocean slowly absorbs/releases temperatures. In your ship the designers would (presumably) work out the ideal heat and water vapour transfer/storage ratios (IE the surface area of the ocean to it's volume) and build accordingly. If it turns out the easiest way to achieve such a ratio is one big lake, then so be it. Arguably they could make a series of smaller lakes or actively controlled heating, but they might not want to have to duplicate the systems that allow control of the body of water, or may need a very large ratio of volume to surface that's easier to create by having one or two huge obstructive lakes rather than a series of small ones. It might even be that to conserve power they're pouring all their effort into making one large, stable body of water that then maintains the environmental conditions of the rest of the ship almost entirely passively in the same way that you can build buildings that passively control their own climate as long as a large enough body of air is available.

2: As an ecosystem.

You can't keep a whale in a duck pond for any length of time. Deep biodiversity of the kind you'd want to maintain an ecosystem requires space, but also requires interaction. If you want to bring along a heap of ocean species then you need something approximating an ocean to put them in including depths, large surface and shoreline.

3: People like oceans.

I'm not talking a recreational lake here and there: I mean a big, open expanse of water that people can go 'deep sea' fishing on. Perhaps a mandate from a senator or a populist cry from those destined to board your ship requires that there be an ocean. In that case you may as well put all the water in one place.

4: Counterweights.

I'll concede this requires two oceans, one at either end, but with creative plumbing and some undersea turbines (to transfer momentum from cylinder to ocean) you can use oceans such as this to counteract unwanted tumbling. Such a system would be more controllable in isolation, but if you need to have an oceans worth of water and all you're doing is occasionally moving momentum into/from it it may as well serve a second purpose

5: Acceleration cushioning.

If you suspend living quarters or sensitive machines in a body of water then when the spacecraft begins braking the water will provide a g-force cushion, aiding in survivability. I assume that (like Rama) the acceleration is down the axis of the cylinder. If the body of water is large enough, the acceleration small enough and you're in the centre of the ocean you may not even notice the deceleration (even though the surface might 'slosh' quite alarmingly). In a closed tank the 'sloshability' of such a large body of water is greatly reduced, which translates to more of the acceleration making it's way to whatever you have suspended. If you have a large tank of water being accelerated with an open 'sloshable' top then you may as well use it as an ocean and gain some of the other benefits noted.

All told I think that having such an ocean wouldn't be down to any one benefit: It would be a combination of smaller benefits and design considerations that, when taken together, would lead the engineers to the solution of 'sod it, lets put it in one place'. However: as I've noted in an answer to another question such an endeavour does add complexity. It would very much be a balancing act between the various engineering complexities, political expediencies and mission requirements.

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In the case of Rama and other long, single-cylinder habitats, it provides mechanical balance.

A long, thin, uniform cylinder "wants" to spin end-over-end, as this is the lowest-energy configuration. If you start one spinning along its long axis, random perturbations will cause it to gradually shift to an end-over-end tumble, dissipating excess energy as heat.

You can change this preferred balance by adding a ring of high-density material around the "equator" of the cylinder, eg. in the form of an ocean. This moves the mass distribution of the cylinder closer to the center of balance, and makes spinning around the long axis the lowest-energy option.

(An O'Neill colony doesn't have this issue, since it's a pair of counter-rotating cylinders. Each cylinder counterbalances the other, leaving the system with no preferred orientation or spin direction.)

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A large habitat is going to have a water cycle

Humans, animals and plants all cause considerable evaporation of water. This will condense on the coolest parts of the habitat and run down till they reach the "lowest point" (or in the case of habitat rotating to simulate gravity, the widest point.)

All this water should run down and collect in a single large lake, where it can be stored and reprocessed. The lake can be used for recreation, too, just like earth reservoir. Untreated water is drawn from the lake and reprocessed prior to pumping into the purified water supply, just like on earth. Care would have to be taken to keep this separate from the sewage system (just like on earth...)

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    $\begingroup$ Hmm.. I wonder what the condensation of water particles would be like in the area of microgravity along the central axis... $\endgroup$ – Joe Bloggs Feb 10 '18 at 9:55
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The larger the body of water, the more stable the ecosystem.

For example, when you are designing a fish tank, the smaller ones are much harder to maintain, because their chemistry can get unbalanced very quickly.

The larger the tank, the slower the changes, giving you more time to correct and balance your nitrate & ammonia cycles.

If you think of a star ship as a giant fish tank for people, you might want to build a live support system an order of magnitude bigger than you think you really need, so that it could house the plants for your nitrate & ammonia cycles, all your fish for food and still be drinkable.

It would also give you some wiggle room if your heating-cooling system has to go offline for some reason. It could store/absorb a lot of extra heat in a pinch.

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  • $\begingroup$ Is there a reason multiple smaller bodies of water can't stablize the ecosystem? A single large body is also a single point of failure. $\endgroup$ – rek Feb 9 '18 at 16:01
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Michael Richardson mentioned in the comment:

Security

If you have a large space habitat, then odds are there will be a combination of military and civilian populations on it. As such the giant body of water and corresponding cliff are intended to act as a non obvious wall to segregate the civilian zone from the military zone. That way the military zone can happily tend to the military work, government business, or secret mecha research without worry that a random kid will wonder in on them.

On the civilian side of the ocean civilians have free access to go wherever they want, but only at designated check points can individuals cross over between the two sections.

Warning TV Trope link

Rule of Cool

The ocean does not have a practical or functional reason for existing besides from the fact that having one raises the prestige of the space station by making it look cool. This can happen over a simple competition between two space powers and each is trying to out perform the other one:

The Union made a space habitat and now we the Federate need to top their space habitat when we build ours, as such we are going to put a giant ocean in it. Then the Union proceeds to make a space station with an even bigger ocean in it than the Federate.

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The drawbacks

Your drawbacks are somewhat flawed or incomplete. Let me elaborate:

  1. If the habitat is meant to move, the momentum imparted to the water needs to be accounted for when it comes time to apply the brakes. Rendezvous With Rama "solves" this with a sheer cliff on one side of the sea, against which the water sloshes. This cliff towers over the rest of the habitat and serves no other apparent purpose.

The mass of the water is a valid argument. Water is rather heavy, and takes a lot of energy to acc/decelerate. However, note my feedback in 4.

I'm not sure how this cliff is intended to be used. Sloshing against a cliff will not help with the braking problem as far as I'm aware. Water sloshing back and forth is a zero sum game, energy-wise. And during a braking action, any force that the cliff exerts on the water (to slow it down) means that the water's kinetic energy is being transferred to the cliff (action equals reaction, thus making it a zero sum game).

The cliff does function as a containment vessel for the water, but that's not related to the kinetic energy argument for space travel. Assuming that the space habitat is able to brake slowly for a long time, the inertia of the water may even be negligible enough to not need high cliffs (but you would need to brake really slowly to achieve that)

  1. The sea takes up space – nearly 20% in Rama's case – that could be dedicated to habitation, agriculture, and so on.

The benefit of having the sea should be weighed against the benefit of what else to do with the space. You wouldn't buy a house with a massive garage and a tiny kitchen, if you cook elaborate meals every night but do not have a car.

If you so choose, it's also not impossible to make the bodies of water subterranean (i.e. build your agriculture on top of it), unless you need open access (or line of sight) to the water.

  1. The sea, especially if it occupies the circumference of the inner surface, is a barrier to all surface-based transportation.

Am I to believe that we're capable of building a generation colony ship slash space habitat, but shudder at the thought of having to build a bridge?

Any reasonable surface-based transportation is going to condense traffic on roads (instead of driving wherever you want). It's not excessive to build a few bridges.

Keep in mind that many of the logistical constraint of transportation on Earth is related to the transportation network having evolved and expanded. Many cities (especially in Europe) were built with a horse and carriage in mind, not a truck with a container on it. It's impossible to move the buildings around to widen every road; and this is the main issue with the increased traffic load on roads.

However, the space habitat will be built at a time where everyone is already aware of transportation. Roads can be built before residential areas; and there will be no logistical issues unless the space habitat is expected to undergo major redevelopment while being habited at the same time.

A great example of this in real life can be found by comparing the US city layout to a European city layout. When most US cities were built, humans already knew the concept of traffic much better than when European cities were founded. The road system reflects that knowledge, as gridsgenerally handle heavy traffic better than Europe's snaky roads.

  1. If the primary purpose is water storage or recreation, then arguably numerous smaller and less obstructive lakes could do the same job with fewer or reduced downsides. In fact, smaller lakes could be even more effective (e.g.: if the goal is recreational coastline, ten lakes 1/10th the size of the sea would have more than three times the sea's coastline; water stored within the hull does not pose a flood risk).

If the primary purpose is water storage, then that means the water is needed. That means that even if there was no body of water; that the water would still have to be on board somehow (e.g. in small containers, all over the place).
If the water is on board anyway, and we're only considering whether we store it in a large body or keep it distributed, then the kinetic energy argument (see 1.) is moot.
The same amount of water will have the same mass, regardless of where on the ship it's located.

Looking at recreation; most of the reason why humans enjoy water-based activities is because it simulates a lower-gravity environment (due to our bodies' buoyancy). But in the space habitat, we're already in space. It would be possible to create a (near-)zero-G zone near the axis of rotation of the ship; thus creating a surrogate for water-based recreation (and arguably even more impressive by human standards, at least initially before it is considered normal).

Taking this into consideration, recreation doesn't seem like a good enough reason (by itself) to warrant bringing an additional body of water.

  1. Water is less dense than the surface and sub-surface material it replaces, which could throw off the balance of the habitat and cause it to spin end-over-end.

Water is less dense than steel (or any similar metal alloy), but keep in mind that you don't need that much steel. Steel has a very high structural integrity, and the mass of that construction can be dramatically reduced by using bracing and crossbeams, as opposed to simply using solid steel.
This is something that humans have mastered for milennia. The Ancient Egyptians already understood the benefit of a wheel with spokes (compared to a solid disk), and they weren't even the first ones to realize that.

Extending the idea of the wheel with spokes; humans have also discovered that a wheel under tension (i.e. there is a pulling force on the spokes as opposed to a pushing force) are considerably stronger. Modern day bicycle wheels are a good example of this, they are all under tension.
Remember that we're creating artificial gravity by spinning around. This is already functioning like a wheel under tension. Lucky us!

which could throw off the balance of the habitat

You're putting the cart before the horse. If the space habitat is going to contain bodies of water, that will be known at the time of building the space habitat. Therefore, the blueprints can account for the water.

It also doesn't make sense that you'd complain about the uneven distribution of water, while ignoring the presumably equally uneven distribution of housing (unless you want to house people based on body weight and constantly reshuffle them?), agriculture or people moving around. A couple of people moving around are insignificant, but what about a massive festival with people jumping up and down?

Secondly, the only balance that is needed (long-term) is an even distribution around the axis of rotation. Going by the O'Neill cylinder, all you need to do is ensure that every "arm" of the habitat contains roughly the same amount of mass.
In other words, if you give every "arm" a body of water of the same size (= mass), the balance is maintained, regardless of how big/small the lakes are.


Summary

  • Any drawback related to the space habitat not being able to house the bodies of water, are negated by the fact that the space habitat will likely be built with the bodies of water in mind. You don't build a ship, only to then decide what to put on it.
  • This includes any issues with navigating around a body of water. The roads can be built before the water is added to the ship (here on Earth, the water was there before the roads were built). This gives us the benefit of designing the roads and lakes in a way that does not compromise logistical efficiency.
  • The kinetic energy argument is valid. However, this relates to the total amount of water that is on board, regardless of whether it's condensed into large bodies of water or comparatively small water tanks. Mass is always expensive in space travel. If a space habitat is being built, everything we would want to take with us would have to have its benefits weighed against its mass (pun intended). E.g. we're not going to take statues of lead or gold, just because they're pretty. The drawback of the added mass far outweighs the value of bringing it with us. In other words: regardless of having large bodies of water or not, we are already inherently constrained in the amount of water that we can bring.
  • Functional uses of water inherently mean that the water is needed. Therefore, the sip must inherently be built in a way that it is able to transport the amount of water we're going to need.
  • Non-functional uses of water (recreation) are generally negated. We're already capable of having a zero-G (or low-G) environment in space, which is very similar to floating in water. Also, if the people are only willing to travel around space if they're able to go jetskiing every weekend; they're bringing the wrong mindset to an interstellar journey. It is more than reasonable to ignore any request for facilitating water-based recreation.

Some reasons to have bodies of water

These are just quick-fire suggestions.

  • If you put 1000 liters of water in a single 1000L tank, you'll need less material than when you want to build 10 100L tanks. The bigger your container, the less material you need (relative to the volume stored).
  • If the water is being cleaned by e.g. algae, it might be considerably easier to have a few massive colonies of algae, instead of a large number of small algae colonies.
  • Even if there is no discernible functional benefit, people might still prefer it simply because people generally like looking at large bodies of water.
  • Having a lake in the center of your area means that everyone is spaced out a bit more, thus creating less noise/light pollution.
  • An open body of water naturally humidifies the environment. The dryer the atmosphere gets, the more water will evaporate (thus humidifying the environment). Similarly, if it's already particularly humid, less water will evaporate. It's a self-regulating system that requires no power or active control.
  • Are there animals on your space habitat? (If not for food, for ecological reasons, or even just as a zoo?) People like fish!
  • Land animals, if allowed to roam freely, will instinctively still prefer a body of water. You can't expect animals to adjust to spaceship habits; they'll have a hard time adapting.
  • You could e.g. build agricultural fields over the body of water. Instead of having to ferry water around (irrigation), you can simply regulate the moisture of the soil via the water below it.
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    $\begingroup$ Re: the cliff; it's there just to stop the decelerating water from flooding the stern of the habitat. Your feedback on 2-5 ignores the outlined scenario. And unfortunately none of the suggestions at the end address the question – they do not offset the "cost" of the single large open body of water or present an advantage to multiple smaller bodies. $\endgroup$ – rek Feb 9 '18 at 16:11
  • $\begingroup$ @rek: That is because there are no significant differences between large or small bodies (assuming a given total amount of water). Space travel considerations only care about mass, structural engineering can be adjusted within physical reason. The only real issue (in nature) is that a larger body tends to run deeper (thus increasing the volume and mass dramatically compared to surface area), but there's nothing stopping you from designing a shallow yet large (surface area wise) body of water if you so choose. $\endgroup$ – Flater Feb 9 '18 at 16:22
  • $\begingroup$ @rek: You need to consider the huge difference between Earth and the habitat. Humans have had to deal with the Earth the way it was. We have limited capabilities in forming the land, we can't reasonably dig out entire mountains etc. But the mere fact that we can build a space habitat inherently means that we are able to create the entire habitat to our specifications. We are only limited by physics, and we're nowhere near physical impossibility here. The problems you are pointing out are notably easier to solve than the problems of building a space habitat in the first place. $\endgroup$ – Flater Feb 9 '18 at 16:28
  • $\begingroup$ If the space habitat is creating artificial gravity by spinning around and is going to contain bodies of water, that will be known at the time of building the space habitat and thus be accounted for. +1 $\endgroup$ – Mazura Feb 9 '18 at 23:10
  • $\begingroup$ Find my answer on this page for an idea on how to solve the water slamming against the wall problem. :) $\endgroup$ – Len Mar 7 '18 at 19:29
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1. ...Rama "solves" this with a sheer cliff on one side of the sea, against which the water sloshes...

Answered by yourself

2.The sea takes up space – nearly 20% in Rama's case – that could be dedicated to habitation, agriculture, and so on.

Sure, but if you need the water, and humans do, then you gotta put it somewhere.

3.The sea, especially if it occupies the circumference of the inner surface, is a barrier to all surface-based transportation.

Unless there are bridges and vehicular transportation.

4.If the primary purpose is water storage or recreation, then arguably numerous smaller and less obstructive lakes could do the same job with fewer or reduced downsides. In fact, smaller lakes could be even more effective (e.g.: if the goal is recreational coastline, ten lakes 1/10th the size of the sea would have more than three times the sea's coastline; water stored within the hull does not pose a flood risk).

You might be right about smaller lakes in terms of storage, but then you have to find ways of containing those so they wont create the same problems you say the bigger body would have. IMO your trading one problem for a lot of little problems, with not much benefit.

5.Water is less dense than the surface and sub-surface material it replaces, which could throw off the balance of the habitat and cause it to spin end-over-end.

But surely a technology advanced enough to create all this in the first place has already accounted for that in numerous ways.

My two favorite benefits for you are:

A) Drinking water. They have to treat it before drinking it of course, but as I said, the water has to go somewhere.

And B) Radiation shielding. To be sure, Rama didn't use its water for that purpose, but it could be used that way.

Good luck!

EDIT:

Piggybacking on Flaters answer about the water-

What if you did this to help prevent issues of momentum: Make the water slosh back onto itself as opposed to just slamming hard against the wall. Just an idea...

enter image description here

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If you have an aquatic species on board as a partner race, it would make sense. Otherwise water should remain in hydroponics, small pools, plumbing, and maybe small easily drained streams and fountains for aesthetics. There is simply no reason to have a lake.

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