Ablative cooling us an incredibly simple way to cool a liquid rocket engine, when compared to regenerative cooling. But ablatively cooled engines are (presumably) hard to refurbish.

Why not inject a spinning film of water into the chamber? The centrifugal force will keep the cool water adhered to the wall, and get rid of the hot steam. All you need to do for a reflight is refill a tank. Plus, because of it's low molecular mass, I think water has a higher specific heat capacity than pretty much any ablative material.

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    $\begingroup$ But how do you make sure that the water film is free of holes for all the time between ignition and engine shut down? The spinning speed of the water should be maintained from the injection up to the end of the nozzle. How many water injectors do you need to guarantee sufficient cooling? $\endgroup$
    – Uwe
    Apr 16 at 17:06
  • $\begingroup$ @Uwe only the first set of injectors, or a least that's the idea. It's a question for a reason. I would expect the film to maintain its momentum... $\endgroup$
    – Abdullah
    Apr 16 at 17:11
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    $\begingroup$ Isn't this just film cooling with more complexity? $\endgroup$ Apr 16 at 19:05
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    $\begingroup$ There is another cooling method than ablative: using the rocket fuel for cooling of the combustion chamber and the nozzle. No extra tank for water is needed and no extra pump. Some engines even use a small part of the fuel at first for cooling and to burn within the chamber after that. Burning water after use is impossible as we all know. $\endgroup$
    – Uwe
    Apr 16 at 22:11
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    $\begingroup$ "But ablatively cooled engines are (presumably) hard to refurbish." are there any ablatively-cooled engines that are refurbished? Water film seems superior to ablative for an engine you'll want to reuse, but propellant film seems superior to water film (and has higher TRL) in that case as well. $\endgroup$
    – Erin Anne
    Apr 17 at 2:36

1 Answer 1


Something like this is done already in some liquid engines, but with propellant, not water, and not spinning (I don't think you can reliably spin anything inside a rocket engine, is too chaotic of an environment), but injected through the chamber walls.

In regenerative cooling, you circulate propellant literally through the inside of the engine walls, in both the cone and the chamber, and in some cases you can do something called film cooling, where the propellant is re-injected into the chamber or throat, or really wherever you need it, through holes in the walls.

Everyday astronaut has a very clear explanation in this article.

Everyday astronaut engine cooling rundown

Now I don't really think you can pull this off with water, it will likely boil off way before it even reaches the chamber (even ignoring regenerative cooling), you need cryogenic propellants, and you also need constant injection of new coolant because it will be quickly washed off by the combustion process.

The reason this is not always done even though it is probably cheaper from an operations and refurbishing perspective is simply because it is a more complex system, and the more pieces an engine has the more things can go wrong, the more expensive is its design, and likely requires longer design cycles. So at the end is a trade-off.

  • $\begingroup$ If water is under pressure, above the chamber pressure, it will boil at a much higher temperature than 100 °C. $\endgroup$
    – Uwe
    Apr 18 at 13:36

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