Dan Bates

I've been reading about RMHs for a few years now but can't wrap my head around the use of a thin barrel within inches of a high temperature riser where metal is taboo. Does the barrel burn out? If so what is the life span of a typical barrel?
I've considered ideas like attaching ceramic fiber board bolted to the top of the barrel, but it seems that might diminish the heat sink properties of the barrel. Also if the fasteners failed it would block the riser, snuff the fire and cause smoke inside the house.
I've also seen a steel plate set inside the rim on the top of the barrel to add an extra layer.
Is there any value in this, or is barrel burnout not a real world concern?
I don't know if my first RMH will be this summer or 20 years from now, but I'm trying to plan ahead...

I've seen plans for a similar RMH by Ernie and Erica Wisner called "The Annex". It was 6" pipe and a little longer, but had the same number of bends. I have no hands-on experience but from all I've read the critical part is a well insulated riser and good heat transfer cooling the exhaust on its way back down to the bench. This should essentially double the convection force, pushing the exhaust through the pipe in the bench. I would be more concerned about appropriate scaling, sizing the riser to the pipe, than the number of bends or how close they are to each other.
Please note this is armchair engineering but I've read enough real life experiences that I'm confident it should work as long as the other components are well designed and properly scaled to the system.
I do have a copy of the plans for "The Annex", but I'm not sure if I have the right to distribute it. I got it through a free promotion but I have seen it for sale.
Try this link to get it for free.
http://richsoil.com/annex-plans.php

I've been thinking about ways to insulate the riser in a rocket stove so that it gets hot enough to be efficient. I was thinking that the size of the barrel might limit the thickness of the insulation. The barrel deflecting the exhaust around the outside of the riser also limits the materials you can use for insulation. I got to wondering if there was a reason for this coaxial setup instead of a parallel rig. I realize the falling exhaust in the barrel is hotter than the outside air and probably reduces the need for insulation. If the riser were not in the barrel how much would the heat loss be increased? How much more insulation would be needed to compensate? Right now this is all armchair theory, but I do hope to start tinkering with scrap stoves which may end up heating a barn or garage soon. Attached is a sketch of the idea.