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A short while ago I visited a real life dissected rocket and it came to my attention that its nozzle is ribbed across its surface.

Why would engineers build it this way and what are the advantages of such a particular design with more even surfaces?

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Rocket engine nozzles often are cooled by pumping (some of) the propellant through pipes inside the nozzle wall or outside of it before they are burned inside the engine. An example of this is the Saturn V's F-1 rocket engine. This is called regenerative cooling. It's likely these "ribs" you were seeing.

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    $\begingroup$ Some nozzles of that time were welded from a lot of parallel small tubes for cooling. But this method required a lot of labour and it was very difficult to make all welds airtight or gastight. There is a much better way to build those cooling channels in a combustion chamber and the nozzle. The inner wall is made from copper and the channels are milled. The channels are filled with an electrical conducting wax and the outer wall is build by galvanic deposition of nickel. The wax is removed by heating. That way, the cooling channels are build without any welds. $\endgroup$ – Uwe Feb 12 '17 at 9:40

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