WE know the original full-size bell is made from a very expensive niobium alloy, for heat resistance. But I notice that the new bell hardly glows at all - just very faintly at the extreme outer edge. The film cooling provided by the turbopump exhaust protects it completely.
So, I am wondering - are they still using that expensive niobium for the stubby nozzle? From what I can see of the bell, they could use simple mild steel and be perfectly OK.
My understanding is that it’s just the large nozzle extension that’s made of niobium as it’s radiatively cooled - the upper part of the bell and the combustion chamber is cooled by pumping propellant between the walls, so it can just be made of steel.
This is what the Merlin Vacuum engine looks like without the nozzle extension (though this is 1C): Image
It’s possible that the ‘stubby’ engine is just this.