Overall view into the engine's thrust chamber and nozzle
The thrust chamber itself is the lighter-colored part, while the nozzle
extension is a darker grey (almost black). Note the silver ring at the aft
end of the thrust chamber; this is the fuel return manifold, where fuel sent
down the regenerative cooling tubes turns around and is routed back up to the
fuel manifold around the engine's injector.
Also note the gaps between the fuel return manifold and the nozzle extension.
Some of the relatively low-temperature gasses from the turbine exhaust manifold are dispersed through
these gaps, providing film cooling for the nozzle extension.