Some drain lines, leading from the engine's turbomachinery to the exit plane.
Note the engine's aspirator. The turbine
exhaust is routed from the turbine, passing through the heat exchanger, through
the turbine exhaust duct toward the rear of the engine, to this shell around
the thrust chamber's exit plane. Here the turbine exhaust enters the engine's
main exhaust stream and is carried away. If the fuel-rich exhaust gasses were
to linger around the aft end of the missile, they could cause base heating
issues or even afterburning.