Various sources consider heat emission and heat management a major issue when it comes to setting up "realistic" space battle scenarios.

I have a few questions in regards to this:

  • Heat is a byproduct of creating power via various means. Say my starship is powered by a central reactor, the reactor will transform some kind of fuel/material into energy to power its various systems. And it will likely emit a ton of heat as a byproduct of said process.

Does this mean the reactor heat emission is the primary source of heat, or would, for example, firing a laser gun, also emit a sizeable amount of heat which would have to be take into consideration? Would firing some kind of rail gun or mass driver emit heat, as compared to a laser?

  • If I need energy and my reactor emits heat while creating something like electricity, why can't I simply surround my reactor with something like a mantle of solar sails, so I could harness the heat and transform it into power, in effect circumventing ANY heat issues in the process? I'm thinking what you consider heat management would in this example simply lead to a smaller reactor with less heat and power output, but the heat would be converted into power, setting off the smaller initial reactor size / efficiency.

  • Do conventional, chem engines emit heat or is this a moot point since the heat is generated outside of the starship, i.e. at the thrusters, and hence of no issue to the starship?

  • 4
    $\begingroup$ What qualifies as a "sizeable amount of heat" will depend on the effect that the heat's presence causes. For example, the Mass Effect video game series postulates that stealth systems are difficult because any amount of heat will be detected against the absolute zero of deep space. (That's what they said in the game, dunno how realistic the claim is IRL. Cosmic Background Radiation might provide a small degree of interference?) If there's a specific story or "effect of space battles" that you're thinking of, it might help us decide what does or does not generate "too much" heat. $\endgroup$
    – Steve-O
    Commented Feb 26, 2017 at 13:55
  • $\begingroup$ Basically everything on a space ship would generate heat to a certain degree. The reactor core or engines might well be the biggest source, but computer systems, moving mechanical parts, lights, etc would all give off a certain amount of heat. Life support systems would generate heat on purpose to keep the interior of the ship at livable temperatures. Even the crew members themselves will radiate heat. (Well, humans will. Aliens are up for debate, I suppose.) $\endgroup$
    – Steve-O
    Commented Feb 26, 2017 at 13:58
  • $\begingroup$ Asking multiple questions within a single question is likely to get this closed as "too broad". Just warning you. $\endgroup$
    – SRM
    Commented Feb 26, 2017 at 14:48
  • $\begingroup$ Also see projectrho.com/public_html/rocket/…. $\endgroup$
    – praosylen
    Commented Feb 26, 2017 at 16:18
  • $\begingroup$ Charles Stross explains it all: antipope.org/charlie/blog-static/2016/03/… $\endgroup$ Commented Feb 26, 2017 at 22:15

5 Answers 5


There are multiple questions in this one question, so I'm tempted to close as "too broad", but I can answer one of your questions, so I'm going to:

If I need energy and my reactor emits heat while creating something like electricity, why can't I simply surround my reactor with something like a mantle of solar sails, so I could harness the heat and transform it into power, in effect circumventing ANY heat issues in the process?

Ever heard of a Matrioshka Brain? https://en.wikipedia.org/wiki/Matrioshka_brain

An even deeper description is found as part of the Orion's Arm archive. Warning: I've lost friends in Orion's Arm. You start reading one article, and you just lose yourself for days amid the awesome visions of possible futures. PACK A LUNCH before clicking on that link.

This is basically a Dyson sphere, then another sphere that lives off the heat output of the inner sphere, then another sphere around that, and so on. There's a lot of material out in the world about ways of harnessing the heat output of one process to fuel the next process that needs energy. So, at its heart, your proposal is a sound one.

The problem is that ultimately, at the end of the shells, the last shell has to actually bleed heat off to space or the whole system melts down. So while you can use your reactor for thrust and then use its heat for making tea for the crew (to use an easy example), at some point, you're going to have to seep heat to space.

Note that even the biological processes of your crew are adding heat to the system that will have to be vented. Every active process on your ship adds to the heat. Sure, this pales in comparison to the heat of your engines, but it has to be addressed or your crew will bake.


@SRM addresses the question of reclaiming heat as power. You need a heat differential to do that. Your hot thing gets cooler and the cool thing gets hotter. Space does not count as the cool thing. On a spaceship, you have a limited supply of cool things that you can heat up to generate power.

"Venting heat" as steam involves losing mass to space; @Sherwood Botsford proposes using steam for that. If you have plenty of mass that is fine. If mass is precious, less so. With current tech, mass is expensive because you have to push it up off of Earth first.

If you are not jettisoning hot mass somewhere else your option to ditch heat is radiation. Other methods useful on earth entail transferring heat to handy mass - convection (gas mass) or conduction (nongas mass). Neither is available in space. Satellites have things like increased surface area from which to radiate, and maximize reflectiveness (minimizing heating from incoming radiation).

You have a spaceship. Unless you have a https://en.wikipedia.org/wiki/Reactionless_drive you will need to throw mass of some sort behind you as propellant. Hot things have just as much mass as cold things. Maybe your engine could serve double duty and also serve as a heat exhaust? If you are a lover of steampunk space scifi you could use Sherwood's idea and push your ship along on a plume of superheated steam.


To address your indirect question...

The reason people bring up heat is cuz it accumulates and it's very visible. Heat radiates very slowly away because it needs a medium to disperse through and there isn't much in space. So you end up holding it with little being disapated, however, so long as you are moving you can just dump it out through venting gasses. If you're not moving you just create a gas cloud around you that continues to keep the heat around you.

The other issue is that ideally, if you're in a military craft, you'd like to not give away your position, but due to heat issues you light up like a christmas tree. This is why a lot of sci-fi stories use heat sink to collect all the heat and then just eject it periodically, rather than radiating it or expelling it via gas. Then you can use the Heat sinks as decoys if you really wanted to. Of course the issue with the heat sink method is that it means you keeping all the heat in a given place and causing it to become very hot, which if you're in that place or have electronics that can't stand high heat, you probably don't want to do, but it is a tactic that could be used for short periods of time.

The other answers give you the direct answers... Basically, every process generates heat. Even the coldest functioning process generates heat. Heat is movement. If you're doing something, you're moving. Everything that is doing something is heat. It really is that simple. You can mitigate heat to a certain extent with knowing that. Every little bit helps and so a ship would be designed with as few moving parts as possible. Of course that practical engineering too. Everything that moves has a high chance of failing. In this case you just have an extra-critical reason to do it, due to the heat.


You have two sources of heat:

  • The generation of heat to power. The theoretical limit comes from Carnot.

(Temp in -Temp out)/Temp In.

For hydrogen fusion, in theory you can have very high efficiencies due to the initial reaction being millions of degrees.

Postulate superconducting coils for the reactor, and a small fraction of your energy is lost to heat.

  • Waste heat from weapons and the activity within the ship

Lasers are notoriously inefficient at present. Something like 10%?

I would suggest using waste heat to boil water, pressurize to super heat temperatures, then vent it to space. It would cool very rapidly, and condense into fog. Water and heat management would be coupled. If you have enough pressureized storage to keep it for a while, then radiate at leasure, you save water.

Note that if a high pressure tank ruptures, it is sudden death for people in the area. Read up about high pressure steam safety.

Another useful tool for moving heat around are heat pipes. Take a tube, pump all the air out of it. Put a wick and suitable liquid in it. This becomes almost a superconductor of heat. The wick moves liquid to both ends of the pipe. The hot end evaporates the liquid, the cold end condenses the liquid. For water it works well from 0C to about 120 C assuming under 60 psi pressure. Propane works well for lower temp ranges. CO2 has a very wide range. The max range for a given liquid I think goes from its freezing point in vacuum to its triple point.

Some variation on a heat pipe can be used to radiate waste heat into space.

Remember that heat runs downhill -- from a higher temp to a lower temp. You can 'pump' it out but this also generates more waste heat. Space ships will have large radiators operating at around room temperature. This will produce large quantities of far infrared radiation. This radiation will be one of the ways to detect the enemy. Shielding this radiation, and beaming it in directions where there are no enemy detectors will be part of the game. Radiation goes up with the 4th power of the temperature. Double the temp, and you need a radiator only 1/16th the size.

You will need at least a conceptual understanding of thermodynamics to pull this off plausibly. (There is reason that engineers refer to ThermoGodDamnics. I suspect this is one reason why most space opera just ignores the problem.)


If I need energy and my reactor emits heat while creating something like electricity, why can't I simply surround my reactor with something like a mantle of solar sails, so I could harness the heat and transform it into power

It is all about Power Density

Military ship needs high power density (HUGE!!!). It needs it for:

  1. manuevering as thrust $F = P/I_{SP}$ ($P$ is power, $I_{SP}$ is specific impuls). You want to have very high specific impulse, because that's the only way how to achieve high velocity using limited storage of propelent (see Rocket Equation for details). So yur power requrement is $P = F I_{SP}$ where both $I_{SP}$ and $F$ you want to have as high as possible. But if the ship should be maneuverable it should be also very light, because maneuvering is about acceleration $a = F/m = P/(mI_{SP}) $. So $P/m = aI_{SP}$ ... and $P/m$ is your power density.
  2. weapons obviously you want to have as much power for weapons as possible, not only because you want to deposite as much energy to the target with your laser (and ablate its armor), but also you want to reach the far distance target fast with your railgun, which means your projectile must have high muzzle velocity. And muzzle velocity scales $v_{muzzle} \approx P^3$ (I don't want to derive it here, but believe me). Yet, you still want to keep the weapon light for the same reason as the whole ship should be light - it must be manueverable!

You can use waste heat as power source for next-process, but ...

Clearly you can e.g. use waste heat irradiated from engine of Daedalus Project to heat some watter, produce steam, than use turbine to make electricity. It is Molybedenum sphere of 1000m radius 1600K hot, that is a lot of waste heat! (According to Stefan-Boltzman law it is 2*pi*1000^2 [m^2] * 1600[K]^4 * 5.6e-8 ~ 2.3 TW of power (i.e. like 2000 decent nuclear powerplants ) or half of current humanity electricity consumption). => I guess you would need some sort of heat engine (biler, turbine etc.) of size of these 2000 nuclear power plants. Perhaps you can miniaturize it a bit (using better materials, like steel instead of concrete), but still it would be HEAVY.

Run, It is a Pyramide scheme ! :-)

all this only illustrates general picture which is a cascade (Djoser pyramide)

  • every step of this cascade (pyramide) is at lower temperature (using waste heat from cooler of previous step)
    • Therefore it needs more heavy machinery and space (e.g. area), because heat engines operating at lower temperature needs to be bigger for the same power

Why it is so?

One problem is Carnot efficiency $\eta = 1-T_C/T_H$, so in order to have high efficiency you need high difference between Heater temperature $T_H$ and cooler temperature $T_C$. If you have low efficiency you need to both produce and cool lot of heat, but you gain just little work.

More importaint problem is density of Heat flux. Heat flow differently in different environment/setting. But everywhere heat flows faster at high temperature. This makes sense, because Heat is transfered by some particles (e.g. moleculers of gas, electrons of metal, or photons from radiator). The higher temperature the more energy there is per particle, and the faster the particles are (=> higher flux or current of particles). It is loosly connected to 3rd law of thermodynamics but not very clearly, so I guess my explanation whit speed of particles is more clear.

The difference is sometimes HUGE, just consider how much area of radiators you would need to cool your steam powerplant (using waste heat from Daedalus main engine). So you have 40% Carnot efficieny, threfore you gain 0.92 TW of useful work and 1.38 TW of waste heat in form of hot watter (Assume T~100C=400K), so using Stefan-Boltzman law from above you need ~1e+9 m^2 of radiators ( that is square with side 33 km ) ! => You can do that, but your space-ship will be significantly heavier.

in effect circumventing ANY heat issues in the process?

You cannot. you always need cooling, because every usefull device on the ship need to do work and work can be only created at the expense of dissipating some heat (=cooling). This is absolutely general and essential 2nd law of thermodynamics. It has something to do with information (you don't have full information about position and velocity of every particle of photon, so you cannot employ them exactly as you want), seem Maxwell Daemon (but I don't want to go to much into theoretical physics). The best what you can do is expressed by Carnot Cycle and Carnot efficiency mentioned above. You may think this is something specific for steam engine (as I was thinking when I was young), but It is not, Carnot efficiency is absolutely general law for any device which turns heat into work.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .