Many rocket engines, especially the reusable sort, require active cooling of the throttle and combustion chamber. A portion of the fuel is split into channels which run through the combustion chamber, throat, and the nozzle. Generally it is a close loop system, so the fuel makes back to be injected into the combustion chamber.
To get max performance modern engines run hot, aka ox rich, and the regen cooling is generally not enough. So in addition to that, critical surfaces such as nozzle also get protected by injecting a thin layer of fuel. This biases combustion to be fuel heavy in localized areas which is less hot. Of course all of this happens in an extremely dynamic environment where gasses are moving at 2km/s+.
> To get max performance modern engines run hot, aka ox rich
Oxygen-rich means you have extra oxygen which doesn’t combust. That necessarily reduces performance. Most rocket engines run fuel rich because hot oxygen is a chemical terrorist.
Was actually going to post a similar comment re: NASA and the SSME engines for the Space Shuttle. This graphic shows the coolant system circulation that pumps cold fuel through the outer casing to warm it up to proper temperatures before use. [1]
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To get max performance modern engines run hot, aka ox rich, and the regen cooling is generally not enough. So in addition to that, critical surfaces such as nozzle also get protected by injecting a thin layer of fuel. This biases combustion to be fuel heavy in localized areas which is less hot. Of course all of this happens in an extremely dynamic environment where gasses are moving at 2km/s+.