The cooling system on a specific rocket engine is usually a type of "pipe" that goes around the nozzle to not only keep the nozzle cool but also pre-heat the fuel. But the nozzle can reach thousands of degrees in heat. So why doesn't this ignite the fuel?
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11$\begingroup$ "… pre-heat the nozzle" You probably mean "pre-heat the fuel"? $\endgroup$– DarkDustCommented Sep 18 at 8:59
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2$\begingroup$ To a layman like me, the words "cooling" and "heating" seem to be used quite confusingly here. 😅 $\endgroup$– Vilx-Commented Sep 18 at 19:11
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10$\begingroup$ @Vilx-, It's pretty simple. When you use "cool" and "heat" as verbs, you are talking about removing thermal energy from something (cooling it) or adding thermal energy to something (heating it). If you're talking about a continuous process, then there does not have to be any change in temperature. E.g., a computer's CPU could be continuously heated by the electricity that it uses, and continuously cooled by flowing air, and all the while maintaining a constant temperature. The pipe that wraps a rocket nozzle continuously cools the nozzle while continuously heating the fuel. $\endgroup$– Ohm's LawmanCommented Sep 18 at 20:06
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1$\begingroup$ @Ohm'sLawman Ahh, ok, got it! $\endgroup$– Vilx-Commented Sep 18 at 23:41
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1 Answer
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It's inside a pipe with no air, oxygen, or another oxidizer.
The fuel in the cooling channel is contained, it's not exposed to air or oxygen. As such, it can't ignite. There's no way for it to burn.
What can be a problem is that when cooked in the absence of oxygen, some fuels can make soot, gummy varnish, or other solid material that could plug the cooling channel. Proper cooling system and fuel chemistry engineering is needed to prevent this -- one of the features of RP-1 rocket grade kerosene is that it is resistant to polymerizing when heated in a cooling channel.
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17$\begingroup$ "cooked" I prefer to think of it as "roasted" to bring out its robust flavor $\endgroup$– uhohCommented Sep 18 at 8:50
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16$\begingroup$ @uhoh - I prefer fuels with overtones of hickory and chocolate when roasted. Sadly, my small-batch boutique roasting rockets don't get very far off the ground. $\endgroup$ Commented Sep 18 at 17:27
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16$\begingroup$ @JonCuster Forget rocket scientists, the real money is in a rocket fuel sommelier career. $\endgroup$ Commented Sep 18 at 17:28
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5$\begingroup$ I'd add than an advantage of non RP1 fuels like Methane or Hydrogen is that coking is much, much harder. $\endgroup$– user53400Commented Sep 18 at 18:45
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18$\begingroup$ I'd add to the answer that there's a good reason the fuel is generally what's used for cooling. It might seem more intuitive to use the oxidizer because we're surrounded by air and it seems like fuels are more dangerous, but actually it's rather the oxygen that's the highly reactive part of a combustion process - and it will combust also stuff that you didn't want to oxidize. Making metal parts that withstand hot oxygen is very challenging, whereas it's easy to make them withstand hot fuel. $\endgroup$ Commented Sep 19 at 8:13