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I was thinking of having large ships made from asteroids in my setting, and I was wondering: How effective of a heat sink would the (very thick) shell of an asteroid be? Would it be significant(probably from sheer volume), or is it a very bad idea? To clarify, the ship is built into the asteroid cause it would be hard to armor or build otherwise. I was just wondering if the asteroid it's built into could be a heat sink. It wouldn't be as good as a normal heat sink(I think), but would it be worth taking advantage of the rock that is already there?

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    $\begingroup$ Ships as in "row row row your boat" or ships as in "Where no man has gone before"? Which and how much heat you want to sink? $\endgroup$
    – L.Dutch
    Apr 24 at 15:50
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    $\begingroup$ @L.Dutch the space-combat bit suggests that rowable-rowable-rowable boats are out of scope here. $\endgroup$ Apr 24 at 15:57
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    $\begingroup$ What you suggest (as far as I can tell) is using an asteroid as a heat sink, which depends a lot on the asteroid and if you have heat-efficient propulsion (so that you can haul it around with you without overheating it). If it’s a rocky asteroid, maybe not, but if it’s an icy asteroid, it might be at least better, since water has a huge heat capacity and could absorb a lot of energy without appreciably warming up. It seems like that’d be artistic license though - you can just handwave asteroids of a thermally-convenient low-density material and go from there. $\endgroup$ Apr 24 at 16:05
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    $\begingroup$ Next WB question: How to make a human powered spaceship? $\endgroup$
    – Nosajimiki
    Apr 24 at 16:59
  • $\begingroup$ heat sink for what? engines no, thermal stability from sunlight yes. $\endgroup$
    – John
    Apr 26 at 20:42

4 Answers 4

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Really Bad

An unrefined rocky asteroid has to be about 50-100 times as thick and 15-30 times as heavy as a solid, cheap, armor-grade steel to provide similar ballistic protection. Compare it to a ship made with a more intelligently designed composite armor like what you see tanks use and you're look, closer to 5-10 times as thick hundreds of times as heavy. Not to mention it functions at least 10 times worse as a heatsink material.

An asteroid's small surface area for its size, low thermal conductivity, and high mass means that it will radiate heat 10s if not 100s of times slower than other ships of comparable weight. But the real limitation for why you should not make an asteroid ship is not so much heat as it is thrust. All the extra mass you spend on armor means you need a much bigger engine to push it than one made out of alloys, ceramics, etc. So much so, that you'll end up using more expensive metals and such increasing the size of your propulsion system than you would have had to spend making normal armor. This bigger engine also makes more heat which the rock will be poorly suited to expel; so, by choosing a rock based armor, you've created more problems than it solves.

How to design a big ship with Heat Management in mind

If you want to make a large ship that cools itself quickly and realistically, you should consider designs that have large wing or sail like protrusions used to radiate heat or something that otherwise just has a lot of surface area. Even though GeneRodenbury was famously opposed to giving his starships wings (the producers made him do it), many of the designs from his shows like the D'deridex-class or B'rel-class ships from StarTrek TNG or the Glorious Heritage class ships from Andromeda are actually really good examples of how large ships could be designed to be fairly good at radiating heat without having to go the butt ugly square fin route you see many hard sci-fi serieses starting to go these days.

How to design Space Ship Armor with Heat Management in mind

Going places in space uses up a LOT of power and therefore fuel. Even if you're a super advanced species that can do this affordably, it will still take way less energy by comparison to process metals, ceramics, and polymers than to move a ship with rock armor. So, no matter how cheap you make space flight, it will always be cheaper to engineer a good composite armor than it is to make an unnecessarily heavy ship. Also, hollowing out an asteroid requires an epic amount of labor. If you can do that, then constructing an asteroid sized hull is trivial in comparison. Also, we need to consider how to turn this armor into an ideal thermal radiator without leaving your ship vulnerable to thermal attacks.

The first thing to consider in making a giant armored ship is that the armor should be multi-layered and composed of spaced plates rather than any solid material. If a laser hits an outer material and turns it into plasma, then that plasma will block the laser from doing more harm. If you are single layered then the plasma can burn through your hull as it continues to absorb more of the laser's energy, but if you have a secondary inner hull, then the plasma will expand to fill the gap and not be able to concentrate the energy anywhere on the inner hull. Spaced armor is also effective against ballistics. If a missile, small shell, or small asteroid hits the outer hull, it will break up into a spray of plasma that will likewise have to spread out to hit the inner hull preventing it from concentrating its energy into one spot. On ships where small is important, effective spaced armor can be difficult to work in, but this is a big ship that you already want to put a really thick layer of armor on, so armor thickness is a non-issue.

The Outer Hull: The outer hull should itself be a multi layer composite. Its outermost layer needs to be a good thermal conductor with lots of surface area to maximize how fast it can radiate heat, and it should be made out of something hard with a high melting point to maximize protection against penetrators and lasers, and ofcourse, lighter is always better. Dimpled composite graphene would be ideal here, but a more simple carbon fibre or titanium-aluminum alloy would do. Your middle layer will be your plumbing that is used to transfer heat to the outer hull, It will be a series of parallel pipes that you pump a refrigerant through to move heat from the inside of you ship to the outside. Ideally it will have some self-sealing mechanism so that if it is damaged there are regularly spaced shutoff valves or a vacuum activated expanding polymer to make sure you don't leak all of your coolant off into space. Then your 3rd layer should be something that is thermal resistant. This will make sure that most of the heat you are pumping out to the hull is not just radiating back inwards, and it will also make your ship much more laser resistant. For this I'd suggest a layer of microporous silica based insulation or possibly a hydrogel. Either option is going to be very lightweight and conducts heat over 100 times slower than your average natural stone, so, just a few cm of this stuff will do more to keep lasers and heat from getting into your ship than several meters of asteroid. Lastly, you will want a thin steel backfacing. Graphene and Microporous Silica can shatter on impact; so, the backfacing will minimize how much of your own hull can become shrapnel that follows a penetrator into your inner hull.

Chobham (optional): This layer is most important if your setting has missiles and ballistic weapons. If all your fighting is done with lasers, skip this step and focus on reinforcing your other 2 hulls. When a kinetic penetrator hits armor at an angle, it does 2 things: it creates a longer cross-section for the same thickness of armor for the weapon to go through, and the reaction force on the penetrator becomes lateral instead of straight back into it which can cause the penetrator to lose energy as it deflects and drags across the armor instead of biting into and penetrating it. Because of this, sloped armor can be many times as effective as an armor plate of the same weight. So by putting angled plates of armor in between your hulls, it means that if your ship gets hit by a penetrator large enough to go through the outer hull without breaking up, that it will have a harder time getting through than against any flat armor plate. Material wise, I would suggest a titanium alloy with a ceramic coating. Ceramics are ideal for breaking up a penetrator and spreading out the impact force while the titanium alloy will give you the best kinetic energy absorption to mass ratio. Ceramics also resist heat very well which will be helpful at stopping any high energy plasma shockwaves coming from the ablation of your outer hull.

The Inner Hull: Generally, this hull is more about holding your atmosphere in than it is about keeping heavy firepower out. Its main concerns will be spalling, shrapnel, shockwaves, and already mostly dissipated plasma that slips past your outer layers of defense... but most importantly, it needs to do so without cracking. For this layer you want a thin metal outer layer. If does not need to be very thick but it should somewhat heat resistant; so, another thin plate of steel would be fine. Then you want many layers of woven kevlar (or similar polymer fibre). This will help absorb the shock wave and catch any small fragments that get through. Then you want a thin layer of tungsten alloy. Tungsten is a dense metal; so, you don't want to over do it, but it has a very high ability to absorb a powerful impact by deforming without cracking making it a good "last-stand" material to prevent your ship from springing an atmosphere leek. So even if all else fails, your inner hull is designed to dent way in without breaking.

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BUT THE OP SAID HE WANTED CHEAP ARMOR??? THIS IS COMPLEX AND EXPENSIVE, RIGHT?

Asteroid rock is worth more as an ore than you can imagine. Rare Earth elements are way more common in space than they are on Earth; so, even relatively metal poor S-Type asteroids tend to have way more precious metals in them than the richest ore deposits you can expect to find planetside. A single cubic meter of S-Type asteroid will have on average \$9,000-12,000 worth of rare Earth elements and an M-Type will have \$240,000-480,000. When you consider you'll need 10s if not hundreds of meters thick of stone to offer any meaningful level of protection, it becomes quickly apparent that it is way more economical sell the rock and buy the armor than it is to use it as armor. Building a ship out of unrefined asteroids is like finding a briefcase full of cash and using it to make a suit of paper armor.

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  • $\begingroup$ To clarify, they aren't taking rock and bolting it to the ships as armor. They are deciding that the ship is too big to armor themselves and make it out of a hollowed-out asteroid. I was just wondering if they could also use the very thick shell of the asteroid as a heat-sink as well. $\endgroup$
    – Bubbles
    Apr 24 at 17:09
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    $\begingroup$ @Bubbles Rock's lack of tensile strength means that it would have to be really thick and heavy just to be structurally sound under any sort of acceleration or vibration. Trying to use it as a meaningful sort of armor is even more unreasonable because it will have to be dozens of times as heavy as a comparably armored metal hull All that weight means that this idea would require a bigger, more expensive propulsion system than any sort of whole ship you can imagine of comparable ability. That means that any money you save on armor is going to be more than lost trying to make it move. $\endgroup$
    – Nosajimiki
    Apr 24 at 19:33
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    $\begingroup$ Don't get me wrong, such things are fine for a Science-Fiction setting where rule of cool wins. Warhammer 40K has exactly such a ship called the Angelicasta, and it is a pretty cool idea, but in a Science-Based setting where things are supposed to be at least scientifically plausible, such a ship makes no sense. By the time you throw out enough Science to make it believable, whether or not it it's a thermally sound idea already seems like a moot point. $\endgroup$
    – Nosajimiki
    Apr 24 at 19:33
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    $\begingroup$ @Bubbles They would actually be better off bolting slabs of rock on as armor. They could select the types of rock that suit their needs (or even melt and cast it to the shape they want) reject pieces that are structurally unsound, reinforce with cables and jackets, carve out cavities to reduce mass, etc. Even better would be to foam it or spin it into fibers, which could actually make it into reasonably effective ceramic composite armor. $\endgroup$ Apr 25 at 0:26
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    $\begingroup$ Also, as a heat sink for the internal heat, rock would be almost worthless, but it (or ceramics made from minimally processed rock) could be pretty effective at absorbing laser fire. The poor thermal conductivity would slow transmission of heat to the interior, while spalling and melting would remove heated material...thinning the armor, but also reducing the heat you'll eventually have to deal with. You're still likely to want to process it into a form that is more effective for its mass though, unless you're armoring a stationary platform. $\endgroup$ Apr 25 at 0:31
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Would it work as a heatsink? Well, yes. Anything can be used as a heatsink, more or less... it just needs to be colder that the thing you're trying to cool down.

Would it be a useful heatsink? It depends.

There are two magic numbers that you need to care about: thermal conductivity, and specific heat capacity.

The former is mostly important for radiators, but even for heatsinks you need to get the heat into the things somehow. Things with low thermal conductivity make for good insulators. Wikipedia has a handy list. Rock tends to have a thermal conductivity of 1-10 W/m/K. Metals tend to have conductivities of 100 W/m/K or more. This means that unless your asteroid is mostly iron and nickel, you're going to need an awful lot of plumbing to conduct heat from the hot bits into the rock, because whilst heat will flow through the rock naturally, the hot side will tend to get rather hotter than you might like. you still need to think about radiators though, because once you've heated up your whole asteroid you need to cool it back down somehow or you'll end up cooking yourself. A massive asteroid might take longer to heat up than a wispy little spaceship, but thanks to that poor thermal conductivity it'll be losing heat slowly and you're probably going to be generating it quite fast.

Heat capacity is probably the most important magic number for a heat sink... the amount of energy it can absorb as it heats up. Things with a low heat capacity will heat up very quickly. Things with a high heat capacity can absorb much more energy before reaching the same temperature as their low-capacity counterparts. Water is an excellent heatsink... even ignoring stuff like its heat of fusion or heat of vaporisation, liquid water has a specific heat capacity of 4 J/g/kg (eg. it takes 4 J of energy to raise the temperature of 1 kg of water by 1 kelvin). Metal and rock tends to have values a tenth of that, and so they can store much less energy before getting hazardously hot.

If your asteroids had a useful amount of ice in them, they could make for excellent heatsinks, so long as you were able to prevent water and steam escaping from the surface. Water is an ideal heat sinking material for any spacecraft that needs such a thing, like a warship, or something that uses lasers or particle beams to drive other smaller spacecraft. You'd still need to engineer mechanisms to evenly distribute heat through the volume of the asteroid, and that needs plumbing, but you can use a coolant loop filled with more water to help there. You'll need to mine and smelt a lot of metal, or synthesize a lot of polymers to make the ice and water tanks and plumbing for the coolant loops, and you'll still need radiators in order to remove excess heat from your heatsinks over time and really at that point it is indistinguishable from any other kind of spacecraft except yours is maybe potatoid rather than sleek and elegant.

If your asteroid was mostly made of metal, you could perhaps reform it into radiator panels and heat conductors. Iron and nickel aren't great as radiators, conductors or heatsinks, but they'll do in a pinch. You'll probably want to harvest a load of ice from somewhere though, because it'll just make everything better and easier. Luckily, space is full of ice.

If your asteroid is mostly made of rock, it won't be particularly great as a heat sink or a radiator. You'll probably need to find materials from elsewhere to provide those two features, unless you run a very low energy sort of spacecraft (and for even a quite modestly sized asteroid, that means a pitifully low thrust).

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    $\begingroup$ You also have to care about total surface area, because that impacts how much heat the heat sink can actually radiate per unit time. In space, any heat you get rid of must be radiated eventually unless you want to constantly be dumping mass to get rid of it (which is impractical for multiple reasons), and that puts a practical limit on sustainable continuous heat generation for whatever is using the asteroid as a heat sink. $\endgroup$ Apr 25 at 3:48
  • $\begingroup$ @AustinHemmelgarn - that was my least favourite part of Mass Effect 2. "Oh, no no no no, you don't have ammo. You have a clip of disposable heat sinks that are ejected from the gun when firing." $\endgroup$
    – jdunlop
    Apr 26 at 23:20
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The other answers are both absolutely correct about the physics of using an asteroid as a large heat sink and base for your ships. I would like to draw out the positive conditions where having such an arrangement might make sense:

  1. The rock must be significantly more massive than anything you are trying to do on it. You could conceivably have thermal dumping systems pretty much the inverse of a geothermal heating system, provided your heat generation is sufficiently low relative to the surface area of the asteroid.
  2. If you only generate a lot (or any) heat for a short interval of time, if the total heat capacity (Specific heat capacity times mass) of the asteroid is of a similar order to the energy produced, you could dump it all in the asteroid and wait the (long!) time it takes to radiate out while you are producing (almost) no extra heat.

A strong corollary of the first point (alluded to in the other answers) is that your normal engines are not going to be able to output much acceleration at all. So if you are happy with a ship that you can start moving and then basically just drift along only ever making micro adjustments to course, then it could work. Your living quarters and so on would have to be tiny (and the asteroid riddled with cooling pipes), and the asteroid itself probably wouldn't stand up to serious assault (such as, for example, someone throwing a slightly smaller asteroid at it) but would at least be protection from space debris.

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  • $\begingroup$ They can dump the heat, superchill the ship and hop to another asteroid before the first one's heat becomes visible from outside. $\endgroup$
    – Juraj
    Apr 25 at 9:03
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If the ship is expected to have any level of stealth, you absolutely must use the asteroid mass for cooling. Using radiators as usual will cause infrared emissions easily detectable by telescopes such as JWST.

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