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Rocket engines often have 2 propelants, one of which is used to cool the engine and after that the hot propellant is injected in the combustion chamber.

Would make sense to have a third propellant more suitable for the cooling process?

As an example, at high temperatures ammonia decomposes endothermically into H2 and N2 which can then be injected into the combustion chamber, but likely injected at a much lower temperature due to the endothermic process, ammonia have a much lower energy density than methane or hypergolic fuels but the decomposed products have similar energy densities.

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  • $\begingroup$ I've added the ammonia tag. Clicking it will lead to other questions about the use of ammonia in spaceflight (and natural occurrences on other solar-system bodies). $\endgroup$
    – uhoh
    Aug 9 at 1:47
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    $\begingroup$ You might find this interesting (ice forming on the CECE engine during a test): youtube.com/watch?v=smswUgtMTfA $\endgroup$ Aug 9 at 1:47
  • $\begingroup$ The fundamental problem with your question is that rocket engineers have already thought about that and -- because they aren't doing it -- have decided that while using a 3rd propellant like ammonia might provide some benefits, it also has negatives which outweigh the benefits. $\endgroup$
    – RonJohn
    Aug 9 at 15:24
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Early liquid fuel rockets kinda did this, by adding water to the fuel (V2: 75% ethanol, 25% water).

Another related example is the Viking engine, which uses water to cool combustion gases entering the pump. The water does however not end up in the combustion chamber as the engine uses a gas generator cycle.

But once you have reliable regenerative coolant cycles, it makes little sense adding a third propellant. A "better coolant" would ...require less coolant? But when the coolant is the fuel itself, there are no mass savings in using less coolant, as long as there's enough of it.
And there's typically enough of it. Only a fraction of the fuel is redirected to cool the nozzle, and expander cycle engines are even limited by waste heat!

Another side to it is that when looking for coolants that are at the same time reasonable rocket fuels, it turns out that most rocket fuels are already good coolants.

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    $\begingroup$ Does the reduction in the temperature of the fuel entering the combustion chamber help in any way? $\endgroup$
    – EduardoS
    Aug 9 at 0:31
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    $\begingroup$ Doubtful. One of the key advantages of a full-flow staged combustion engine is that the propellants enter the combustion chamber as hot gases rather than cold liquids. $\endgroup$ Aug 9 at 0:45
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    $\begingroup$ Any possible advantage is far outweighed by the plumbing, pump, and additional tanks needed for the additional coolant, added engine complexity, and the mass penalty of carrying that coolant instead of fuel. There's nothing especially onerous about using fuel to cool the engine that a special coolant would avoid...in short, this is a solution looking for a problem. $\endgroup$ Aug 9 at 1:45
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    $\begingroup$ There would definitely be fuel savings as regenerative cooling doesn't waste fuel, but does waste lots of pressure, which is generated using propellant. Less coolant would either mean a higher chamber pressure of less fuel diverted into the gas generator or similar system, both of which are extremely positive. $\endgroup$ Aug 9 at 2:16
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    $\begingroup$ @EduardoS Only if it is necessary to make your engine manufacturable with the materials and tools you have. In general, you want the engine to run as hot and high-pressure as possible, but you also need it not to melt or explode. $\endgroup$
    – TooTea
    Aug 9 at 10:06

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