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Detail of the inside of the nozzle extension, interface between the engine and the extension, and the fuel return manifold. Note the lugs welded to the inside wall of the fuel return manifold; these were used for attaching a piece of equipment when testing for leaks.

Note the channels where the turbine exhaust is vented around the circumference of the extension. The gas generator burns a fuel-rich mixture, resulting in lower combustion temperatures. This relatively cool exhaust flows down the wall of nozzle extension, protecting it from the exhaust gas from the combustion chamber. According to the F-1 Engine Familiarization Training Manual (R-3896-1) [direct link to 16.8M PDF file], the combustion chamber exhaust is 5,970°F, while the turbine exhaust is a relatively balmy 1,138°F.

Picture 2 of 3.