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Walk-around of one of the two pyrotechnic igniters mounted in the nozzle extension's igniter bosses.

Picture 2 of 5.

Note the four electrical contacts inside the end of the igniter.

Fuel-rich turbine exhaust gas would enter the nozzle extension from the turbine exhaust manifold. This gas, relatively cool compared to the combustion gases of the main combustion chamber, would cool the double-walled nozzle extension.

The inner wall of the nozzle extension was comprised of shingles with gaps where the shingles overlap. The turbine exhaust gas exited the nozzle extension through these gaps, providing film cooling for the inner nozzle extension wall.

The pyrotechnic igniters would ignite this fuel-rich gas, although I'm unclear as to why this was necessary. Perhaps it was so that the gas was already burning and didn't suffer a hard ignition when it entered the main combustion chamber's exhaust stream.