It is clear from the pictures, that the nozzle extension of Merlin and Rutherford vacuum engine aren't actively cooled.

What is the material of construction for those nozzle extension to withstand higher temperatures, for the entire flight duration?Rutherford engine

Merlin engine


On the Merlin Vacuum engine, the nozzle extension uses a Niobium alloy. The nozzle is cooled via film cooling:

The exhaust from the turbopump is injected just above the nozzle extension and helps shield it from the engine exhaust.


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