I've been lurking and reading up on this fabulous setup for efficient burning and heating. 

What I wonder is how durable the secondary burn chamber is?  Does it need replacing often?  It seems to bear the brunt of the heat and I would think that it would take a toll on the metal, especially in a weather-exposed outdoor situation.

What have been your experiences?  Do you need to design it to be replaceable?

I'd suggest buying the book.  "rocket mass heaters:  Superefficient Woodstoves you can Build (and Snuggle Up To)" by Ianto Evans and Leslie Jackson.  We just did ($18 new on amazon!  used copies seem to be more expensive, sometimes MUCH more expensive, I don't get it?) and all of our questions were answered.

The barrel element was a concern for us as well.  That part of the stove isn't terribly difficult to rebuild, and from what I can tell they last a descent amount of time (indoors).  Six years and still going strong at the last count, for a stove built, used, and described by one of the authors.  Depends on how often it's used, of course.  I'd think that an outdoor one should be given some kind of protection from the elements over its head for longevity's sake, not to mention the comfort of the fire stoker. 

well ive not burned through one yet and i have tried my best

best estimate so far is ten years but this depends on the barrel type; a barrel designed for non flammable products is thinner and less durable than one for a flammable material. We suspect that the ones for flammable products will last longer. Experiments are in the works for home made secondary burn chambers out of 1/4" plate but as yet we have not come to a conclusion.

one consideration is that while you can easily replace the barrel best practice is to get a barrel that is as undamaged as possible (no dents or rust). mount it with care and use your stove normally ( as a researcher I push mine hard with different fuels because i want to be able to answer this sort of question. yours should not get bright red for the first third of the barrel nor white hot under normal use)

I've seen stainless barrels before, and Chrome-Moly barrels too, and idea how these might affect longevity? Stainless resists oxidization fairly well and I know that CM is tough and that Molybdenum is used to affect heat resistance in other alloys but I don't know if CM has enough to hold up any better than plain old steel. Do we have any metallurgists around?

Why can one build the secondary chamber out of inexpensive fire brick? Cover the outside of the fire brick with cob and there you go. Is there a rule that a barrels be used?

Just a thought to ponder ,,,,,

What about using a combination of fire bricks and cob to make the combustion chamber? For example, could you use cardboard forms to make the shape, enclose it with bricks and cob, and then let the cardboard burn away?

Another thought I had was that in place of the steel barrel, you could perhaps use a large ceramic pot of some kind. I have seen large ceramic pots about 3' tall and about 2' in diameter that look like they could do the job. Any thoughts?

Alex

give it a try. let us know. no problem with ceramic pots or bricks or what have you, only differences are the way the system sheds heat, how it performs over time, cost, labor.

personally i dont understand the impulse to redesign the thing before you even spend any time with one that is built as designed to work as is.

I've wondered about this too.
  Through some trade long forgotten I aquired two coal fired water heaters from a very old hotel.  The boilers consist of a double walled cast iron body a little over 3 feet tall and just under two feet in diameter. Wall water jacket thickness about 4 inches. Weight well over 200 lbs. Original fire box and chimeny a seperate lower unit. Looks a little like R2d2 from "Star Wars"
I thought about using them for their intended purpose but just don't trust the idea. I think that filling the water jacket with sand or casting investment and using it as the barrel, would create a great heat sink and work as the secondary chamber.