I was thinking about the problem of waste heat in a large military space craft. It seems beyond a certain technological point, the ability to dump waste heat becomes the largest problem in quickly accelerating a craft, or even in using many weapon platforms. Even non-biological computers do not function well beyond ~100 celsius, which means you are very limited in how much energy your engines could create at any one point, especially within smaller ships. So how feasible would something like an endothermic chemical reaction as a radiator be vs something much simpler like a diamond or copper radiator?

Obviously once you run out of the materials needed to create the reaction, it would no longer be effective, so is there any known reaction that might make it worth it to keep large quantities of material on board a ship to dump heat during high acceleration? If not might some endothermic reaction theoretically be possible? In other words, are there theoretical limitations on endothermic reactions beyond the processes that we are aware of?

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    $\begingroup$ Real-life spacecraft use heat pumps to concentrate the heat into radiators which then radiate it into space as electromagnetic waves. Why doesn't this work for the "large military spacecraft"? Just make the radiators as large as needed. $\endgroup$
    – AlexP
    Commented Sep 24, 2022 at 2:28
  • $\begingroup$ @AlexP, see the science-fiction tag, note that Real Life cannot be an overriding limitation on any question unless specifically requested, and methinks large radiators would be an easy target on a military ship. $\endgroup$
    – JBH
    Commented Sep 24, 2022 at 2:32
  • $\begingroup$ The first thing to do in appraching this question is to realize there is no such thing as 'heat'. What we call 'heat' is all in our minds, it is a construct our mind creates to explain temperature sensations. Only living things experience what we refer to as the sensation of heat. The scientific phenomena we call 'heat' is in reality the rapid movement of molecules and atoms. Harness this movement, you have mastered the problem. $\endgroup$ Commented Sep 24, 2022 at 22:23

4 Answers 4


I think you will struggle to remove much heat using endothermic chemical reactions. The best option (and it's not that good) is to arrange an onboard cold sink. For example ultra cold water or ammonia ice linked to a large volume vacuum chamber. Heat could be absorbed by raising the temperature of the ice to its melting point, more energy could be absorbed by melting the ice in latent heat of fusion, more heat could be absorbed in increasing the temperature of the liquid to its boiling point and more latent heat again to boil the liquid into a gas. Beyond that the options are limited a small amount more could be absorbed by heating the gas or at the cost of destroying the cold sink, the gas could be vented to the vacuum of space absorbing more heat.

  • $\begingroup$ Note that you could potentially direct the heated fluid to your propulsion system, and make use of it as reaction mass. Regeneratively cooled rocket engines actually do just this, they cool components of the engine with the cold propellant flow before combusting it, which both lets them stay cool and lets them get use out of what would otherwise be waste heat. This is only useful when your engines are running, but if you're producing excessive amounts of heat due to intense combat, they probably are. $\endgroup$ Commented Sep 24, 2022 at 23:32
  • $\begingroup$ In an endothermic reaction, two (or more) chenicals are mixed to form another chemical, using heat energy to do it. SInce 'heat' is 'molecular/atomic vibrations', what this means in reality is that the molecules/atomic vibrations in the product are less 'intense' than they are in the original chemicals. I assume that the excess molecular/atomic vibrations become part of the 'binding energy' of the new substance. The limitation is the total mass of the reactants. Amonium nitrate and water is used in cold packs, but at 325 J/g that is a LOT of cold packs to cool a ship. $\endgroup$ Commented Sep 25, 2022 at 17:38
  • $\begingroup$ Whether a chemical reaction occurs is determined by Gibbs free energy G which is dependant on enthalpy and entropy (deltaG = deltaH - T * deltaS). Enthalpy H (heat energy + work done in expansion) is usually the main driver of most reactions but entropy S (effectively degree of disorder) also makes a contribution especially as the temperature T increases. So heat can be absorbed by a reaction if there is a sufficiently large increase in entropy to balance it. $\endgroup$
    – Slarty
    Commented Sep 26, 2022 at 7:37
  • $\begingroup$ Although entropy and enthalpy are terms bandied about in science, there is no clear fundamental substance to them. Their scope begins and ends with their definition, and the apprpriate terms in equations behind these definitions. They are only empiracle. Like the concept 'heat'. I am reminded of the centripital/centrifugal force conundrum, are they terms defining and refering to something that does not indeed exist, but should be more appropriately addressed using a different approach? Personally, I think there is an as-yet-undiscovered boson-field behind the concept the terms try to explain. $\endgroup$ Commented Sep 26, 2022 at 12:34
  • $\begingroup$ Gibbs free energy is a well established concept that is widely used in chemistry and thermodynamics to calculate if chemical reactions are feasible and at what temperatures: chemguide.co.uk/physical/entropy/deltag.html en.wikipedia.org/wiki/Ellingham_diagram $\endgroup$
    – Slarty
    Commented Sep 26, 2022 at 15:59

Clever idea and there's nothing stoping you from using it!

So long as you don't take the time to explain the endothermic reaction, or how you "reset" that reaction, I think it's a great simple-explanation for how waste energy is disposed of on your ship. I like the idea!

Some basics to consider:

  1. Mass is bad when it comes to space. As mass increases, the energy you need to move it increases. That means you're generating more heat to carry around the stuff you're using to remove that heat.

  2. Consumables are also bad in space. Recycling good. Consumables bad. Consumables mean you need to stop to replenish them. Many worldbuilders forget that economy is as important as technology. The more consumables you have the more expensive your ship is to operate.

Having said all that, and appreciating the use of the Science-Fiction tag, consider two unobtanium substances that behave endothermically when brought together, but also re-seaparate once enough heat is applied. Yup, perpetual motion! Ignore that. What we now have is a recycling system that can consume your heat and doesn't require extra cost. Because it's recycling, it justifies the extra energy needed to move it around.

Your alternative is to use a technology like a thermoelectric generator that consumes the heat in a constructive way. Just a thought.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Sep 27, 2022 at 4:31

Phase change is endothermic.

  1. Collect matter from space. Any solid chunk will do. Chunks of metal would be nice. Water or ammonia would be fine too.

  2. You heat the matter. It changes from solid to liquid to gas to plasma. All that heat is now in the form of matter bits flying about.

  3. Let those hot bits fly out of your ship. Let them find their way out in the big universe. Let them take the heat with them.

  4. GOTO 1

From my answer to this closed question Closed off habitat surrounded by extreme heat, is it impossible in the long term without handwavium?

Iron fusion is endothermic.

This is not an endothermic reaction in the chemical sense, but it is endothermic. Fusion of elements below iron releases energy and fuels stars. Fusion of iron or elements higher in the periodic table absorbs energy and kills stars.


Higher mass stars will switch from helium to carbon burning and extend their lifetimes. Even higher mass stars will burn neon after carbon is used up. However, once iron is reached, fusion is halted since iron is so tightly bound that no energy can be extracted by fusion. Iron can fuse, but it absorbs energy in the process and the core temperature drops.

This process is why stars die: when iron fusion eats enough energy that the temperature drops and they collapse on themselves. You could have a little iron fusion in your fusion drive to moderate temperatures.

  • $\begingroup$ So if Ironman fused he would be a really cool dude? $\endgroup$ Commented Sep 25, 2022 at 17:41

These things nearly always wind up being trade-offs. You pay for what you need at the expense of other things. It is not possible to make a ship that will survive arbitrarily much enemy fire.

You only have a finite ability to pack what amounts to armour. Passive protection of any kind gets categorized as armour. Unless the now-heated chemical can be used as a weapon of some kind. There are SF devices that get charged up by enemy weapons, providing the energy the attacked ship requires to fire its own weapons. But put that aside for now.

Some parts of your ship will be more sensitive to temperature change. So you give them the most protection. That would require a detailed analysis based on data from ships that survived battle.

The analysis has to be correct. If a ship gets shot up and comes home, then the parts that got shot away are the parts it can live without. Those are the parts that do NOT need more protection. If the captain's cup holder gets shot away the ship probably still comes home. If the engines get shot up probably it does not come home. So the repair yards will repair the captain's cup holder a lot more often than they will replace shot-up engines.

So you pack the parts that do need protection, the engines, with extra protection. And you tell the captain "Suck it up!" when he complains about his cup holder.

Two basic patterns will apply: ablative and absorptive.

Absorptive means it takes the shot and recovers. Probably it will reject the heat using some sort of radiators. So you would need heat pumps of some kind. Maybe the chemical you use has the ability to change states if you compress it, for example. So later, after the battle, you can rest up and cool off. This is probably appropriate for larger ships that have room for lots of equipment to perform these tasks. Possibly the radiators are stowed until needed. Possibly they are inconvenient when they are extended, maybe interferring with maneuvers. Maybe they can be jetisoned in an emergency. That means the ship is severely hampered at cooling off.

Ablative means you lose some of the chemical each time you get hit. The chemical is ejected and takes the heat with it. So you need a new supply of the chemical to recover. This is a "trope" in science fiction. You can easily find this by googling. Usually it involves a layer of some material that boils away when it gets hit by an energy weapon. This carries away the energy. And some forms provide a temporary cloud of material that obscures the area. The cloud may or may not absorb additional weapon fire while it lasts, or it may just give obscurring cover. Ablative is probably for smaller fighter type ships that cannot afford the extra equipment.


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