Rocket Stove

Hi - I just started studying in preparation to build a rocket stove. I'm confused about what materials can be used for the inner pipe of the heat riser. My hope is to be able to use either a 6" cast iron pipe (old plumbing pipe) or sheet metal piping from Menard's that is typically used for duct work in a home. Should I have any concerns about using either of these two materials? I'm planning on using a sheet metal pipe for the outer layer to contain the insulation. Thanks in advance.

IMHO,
use insulative high temp refractory materials in the combustion tube and riser, only use metal as a sacrificial form

see Rob's Rocket Heater Autopsy:

an optimised combustion "engine" will easily produce 2000F +

When building a j barrel stove is it important to make the vertical pipe larger size than the horizontal pipe? Is it possible to use the heat in a clay bed like the brick ones? Any help would be great

Can a clay flew be used in the rocket stove? Does it have to be insulated?