Cob: rocket stove question

[Royce Hays]

We'd like to run the flue of a rocket/J stove under an outdoor courtyard
area but am not
sure how long of a flue/pipe that I can get away with.  I know that the
length will be determined
by how hot the fire burns (the hotter, the longer the flue, I believe).
A releated question to
this is how long should I run the pipe under/through floor/benches
before allowing it to vent
up and out so as to not have a glowing hot pipe sticking out of the
ground?

What materials have folks tried that work well for the combustion
chamber and nearby
piping such that the chamber won't burn through due to the high heat in
a few years?

Also curious about how much curvature/bending I can get away with.
We're primarily
using this to heat the earthen floor and earthen benches in an
outdoor/covered courtyard area.
Ideally, I'd like to run the pipe down the length floor on one side and
back on the other and provide smaller branch sections of the pipe that
could run up into benches and reconnect
with the main pipe.  Don't know if/how that'll work but that's why I'm
asking.  Rough area
is 20' x 10' area with bench areas mostly on the perimeters.

Thanks in advance.

-- Royce (royce at hays5.com)