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that's visible is actually an extension that was put onto the F1 engine to increase its efficiency at higher altitudes.

Because the Saturn V first stage, also known as the S1C, it gets out of the lower atmosphere very quickly,

and as it gets into the rarefied atmosphere, efficiency of the rocket engines becomes of big importance.

So the extension was put on to make the engine more efficient at these higher altitudes.

But to keep that extension cool, that is to keep it from actually melting, this hot exhaust gas is pumped into the extension.