Ejectable heat sinks for spaceships?
Radiating heat into a vacuum is a no-go, so a spaceship that generates any kind of heat is going to be in trouble. I had an idea for a potential solution - dumping your heat into a disposable heat sink component, and then spacing the component. Sort of like the heat-sink clips in Mass Effect 2, but for spacecraft.
Of course, you gotta store these heat sinks somewhere, and they would eventually run out. While this seems like a good way to introduce tension into a travel story, I want to make sure that this is a plausible method.
A typical ship that would want to use this approach is a cargo ship - it has plenty of space, but wants to maximize space for cargo over space for heat sinks. The crew is biological, so it needs some quarters. What would its heat sinks be made out of? How much heatsink stock would you need per day of journey?
Edit: Apparently, radiating heat into space is already a pretty decent method. What conditions would render disposable heat sinks necessary? How much heat would the ship need to generate, and what would require that much energy?
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Let's break this down a bit.
What conditions would render disposable heat sinks necessary?
As has been noted already: combat and stealth. Radiators can be damaged, and will give your ship a nice big heat signature that'll stand out against the vacuum of space (assuming your ship's thrusters don't already do that). Dump that heat in an internal heat sink instead and you'll be much harder to spot.
So to use your original idea, a cargo ship could use disposable heat sinks to avoid detection by space pirates or, if it's illegal cargo, the authorities. As for combat, small/medium ships could use them to avoid detection by enemies, and larger ships... I'll get onto that later.
How much heat would the ship need to generate, and what would require that much energy?
Again, as has been stated, sustained laser fire will very quickly build up a lot of excess heat that would ideally have to be dumped in heat sinks. During normal flight, the heat sinks might not need to absorb too much energy, or even any at all if your ship also has radiators - but that's a good thing, as you don't want to be ejecting and replacing heat sinks every five minutes.
What would its heat sinks be made out of?
Whatever it is, it needs to be able to store as much heat as possible, so a substance with the highest melting point or thermal capacity you can get. The obvious answer is some kind of unobtainium, but failing that you could go for tungsten or lithium, as has been suggested, or just plain old H2O.
[Just having radiators is] boring.
Honestly, I feel like just ejecting the spent heat sinks is kinda boring too. Let's weaponize them. Instead of one big heat sink, have loads of smaller ones all over the ship - they'd be easier to fit into the design, easier to dispose of and replace, and easier to use for what I have in mind here.
Taking all the above into account, the heat sinks would probably get the most use during combat scenarios. Put a futuristic railgun on your warship and you can then either eject the spent heat sinks (if it has nothing to fire them at, or to act as decoys like @D Spetz suggested), or load 'em up and fire them at enemy ships. A big chunk of metal heated to several thousand degrees Celsius and travelling at God-knows-what-velocity is gonna do a fair amount of damage.
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Use the vacuum to help cool the ship. Specifically, have several enclosed/mostly enclosed spaces that are transparent to IR and that is pumped to a vacuum. Use water to absorb the heat, and then spray the water into this vacuum chamber.
Because water boils at a very low temperature (-67 C) in vacuum, it will flash into vapor, and then crystalize into ice as the heat radiates off into space, before sticking to the chamber walls. When the chamber is no longer at sufficient vacuum then the water is diverted to the next chamber. Then the ice is collected and the chamber is pumped back into a vacuum state for the next round.
You could do this pretty easily by venting it to space once the majority of the atmosphere is pulled out. There would be some losses, and the ship may need to eat a comet once in a while.
For the dramatic tension you could have an accident vent a large amount of water to space, and so a source of water needs to be found quickly before things start to overheat.
This can be prolonged a bit by turning off non essential systems.
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