Think outside the box re metal RMH-style cores

Good question. Ones I can recall had a metal liner/form only for the riser. All of those instances were J-tubes. I don't have specific posts at hand, aside from this one about a batch box:
https://permies.com/t/52544/rocket-stoves/metal-burn-tunnel-heat-riser#434391
"3. Chet style portal is starting to deform from heat but is functioning extremely well and certainly acts as intended
4. The riser is 1200mm 10mm wall hydraulic pipe, it has suffered spalling for 800mm of it's length, which hot spots from 100mm to 400mm particularly affected."

From my own experience, I have looked down into J-tube feeds and seen that the burn tunnel as far in as I can see is hotter than the junction of feed tube and burn tunnel, in cob-built cores which are not as well insulated as full refractory units would be.

Thanks, Max, I was looking for posts on this forum and forgot about that youtube video.

A couple more classic videos showing where metal combustion cores fail. The first has minor distortion in the feed/burn tunnel area (it's not proportioned by any standard formula), while the lower riser has holes burned in it.


Kntryhart's system was running on waste oil instead of wood, but it clearly shows where the hottest areas were, around the junction of burn tunnel and heat riser.

Glenn Herbert wrote:Good question. Ones I can recall had a metal liner/form only for the riser. All of those instances were J-tubes. I don't have specific posts at hand, aside from this one about a batch box:
https://permies.com/t/52544/rocket-stoves/metal-burn-tunnel-heat-riser#434391
"3. Chet style portal is starting to deform from heat but is functioning extremely well and certainly acts as intended
4. The riser is 1200mm 10mm wall hydraulic pipe, it has suffered spalling for 800mm of it's length, which hot spots from 100mm to 400mm particularly affected."

From my own experience, I have looked down into J-tube feeds and seen that the burn tunnel as far in as I can see is hotter than the junction of feed tube and burn tunnel, in cob-built cores which are not as well insulated as full refractory units would be.

Hi Glenn, Thanks for the info, as I suspected, it looks like the metal inserts you spoke of were just in the riser and thus do not give any indication of whether the burn tube was hotter or cooler.

As I've said before, batch boxes are "a horse of a different color"!  Has anyone measured temps in their burn chamber and the heat riser?   But I am talking about a J Tube.

Then you said you could "see" that the burn tunnel was hotter than the junction of the feed tube and burn tunnel.  What did you see? More flame?   That is what I see as well in mine but you don't see intense radiation  heat from the combustion area of any fire, but the temps of that radiation are tremendous never the less!  Such must be measured or observed by what they will melt or cause to glow.  That is how I know that the hottest temps in my core are at that junction of feed tube and burn tube.    IN an insulated system, those high temps would soon spread throughout the entire core by way of conduction and convection, but the initial hottest heat source location, I will maintain until I see evidence to the contrary is where the combustion is initiated as combustible gases meet the oxygen.

Thus this is the area on which I am concentrating my efforts to insure proper cooling away of that radiation energy which makes the steel glow in this area alone.

The other videos, (I've seen lots of them!) Show systems with insulated cores which can only spell disaster for anything made of steel as the material is forced to overheat on every firing!  A waste oil burner will burn much hotter than a wood fired one ( I know from experience!) So one cannot expect such to stand up for long to such punishment.  I am seeking to find out if a J-tube rocket can function well with a steel air-cooled core with no insulation around it. All my tests so far have indicated the only area I need to be concerned about in such a system is that at the junction of the feed and burn tubes.  More info to be gathered as I fill the integral mass (around the barrel) with a rock and sand mass. Will keep you posted.

If all you're looking for is confirmation that an uninsulated steel core can function well and be durable, you have already done that. What we have been trying to tell you is that this setup will never achieve the full combustion efficiency of an insulated core because the gases are cooled too quickly to complete their mixing and combustion. Of course your core will have its hottest spot near where the fire begins. I can tell the relative temperatures in an insulated core by the glow of the walls, or "color temperature". As a potter, I am quite familiar with the look of different temperature surfaces. The autopsy videos show not just that the temperatures are maintained into the riser, but that they increase as the gases reach the lower part of the riser, since they are still combusting.

By the way, one metal J-tube core autopsied was stainless steel of relatively similar thickness in feed, burn tunnel and riser, and its greatest damage was in the lower riser. If you won't take that as evidence of relative temperature profile in an insulated core, I don't know what else to say.

Glenn Herbert wrote:By the way, one metal J-tube core autopsied was stainless steel of relatively similar thickness in feed, burn tunnel and riser, and its greatest damage was in the lower riser. If you won't take that as evidence of relative temperature profile in an insulated core, I don't know what else to say.

Hi Glenn, If you mentioned these details before, please pardon me! I hadn't noticed a report of a system in which the entire core was steel. BTW, how thick was the steel? Where would I find this autopsy report?

That was in the "old rocket stove the autopsy" video I posted above. I think he said the feed/burn tunnel was about 1.5mm.