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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.