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View of the interior wall of the nozzle extension.

The F-1 engine nozzle extension features a double-walled construction. The interior wall is comprised of 23 rows of overlapping shingles. Turbine exhaust gasses, fed from the turbine exhaust manifold and routed between the walls of the nozzle extension, enter the main engine exhaust flow through gaps between the shingles. Since the turbine exhaust gasses are substantially cooler than the main exhaust gasses, this provides film cooling for the nozzle extension.