stovepipe and insulation only

[chronictom]

Is there any reason I can't just take 8" stove pipe and two elbows, put them together in the 'J' shape, pour some vermiculite (or ashes on the bottom of a box, put the pipe on the bed then fill all the way around it with more ash or vermiculite?

Keep the feed tube at 2 foot vertical, the exhaust tube 6', with the top 4 feet out of the insulation (or put another box around just it and insulate it as well....

That would be the most basic example of the rocket itself no?

[Donkey]

Works just fine. Not likely to last a long time, single wall pipe ain't the most resilient.
Ashes work great for a short time then eventually, will settle into a packed layer at the bottom. Better to use vermiculite or pearlite / clay.

Smooth piping can create laminar flow conditions. Might want to install some kind of "turbulator".

You will like the roaring speed that pipes can provide!