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In regenerative cooling the fuel is passed around the nozzle before being pumped into the combustion chamber right? I don't really see how this affects cooling itself, since the heat captured around the nozzle by the fuel/oxidizer seems to be pumped back into the combustion chamber which will have to be captured back by more fuel. So where is my reasoning wrong?

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You are missing how heat is distributed in exhaust.

Most of propellant ejected through the nozzle never makes contact with the nozzle surface or walls of the combustion chamber, and as result never has any chance to transfer its heat into them.

The exhaust gas primarily cools through adiabatic expansion - high pressure and high temperature both transformed into rapidly increasing volume and velocity of the ejected propellant - high-speed ejection of reaction mass; propulsion. Only a minor part of it is transferred into the walls - but that is still enough to risk damage to them through overheating and compromising durability of the materials.

On the other hand, some of the combustion energy is consumed on heating the fuels from their injection temperature to post-combustion temperature (which then transforms into propulsion). The hotter the fuel before injection the less is lost to that heating. So it's beneficial to heat the fuels - especially to boil off cryofuels - before combustion.

Since we both have some excess heat we need to get rid of, as it's dangerous to the structure, and we need the cryofuels turned from liquid to gas for combustion - we can combine the two processes, simultaneously increase the pre-combustion temperature and cool the structure. Most of the heat produced in combustion will be ejected "harmlessly" - and what excess does transfer to the structure, gets recuperated and reused by pre-heating the fuel, that energy which would be otherwise lost, pumped back into the combustion chamber as hot fuel.

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  • $\begingroup$ I always assumed that rocket fuels only turned to gas once in the combustion chamber. So when you are talking about boiling off cryogenic fuels, you don't mean before they get to the combustion chamber do you? Change of state absorbs a lot of heat, so it would be great if that could happen around the nozzle, but I don't think it could, because pumping gas is whole other animal than pumping liquid. $\endgroup$ – Johnny Robinson Mar 15 '17 at 2:59
  • $\begingroup$ @JohnnyRobinson: All depends on particular design. Usually, they turn into gas in the preburner(s). Engines burning gaseous propellants tend to be more efficient due to better mixing and more complete combustion. Regenerative cooling is quite efficient so you'll rarely see all the fuel directed through it - and using oxidizer for it is yet another pile of problems; hot oxidizer becomes far more reactive - with whatever it comes in contact with! $\endgroup$ – SF. Mar 15 '17 at 6:34
  • $\begingroup$ Now that you mention that, a full flow engine would be providing the engine with gasses wouldn't it? $\endgroup$ – Johnny Robinson Mar 16 '17 at 0:15
  • $\begingroup$ @JohnnyRobinson: Seems so - I don't think preburners exist that eject even partially liquid mix. OTOH, full flow engines are quite exotic. SSME used two preburners but both created a fuel-rich mix (propelling different pumps though), while most of oxygen was injected still as liquid. $\endgroup$ – SF. Mar 16 '17 at 1:44

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