Welcome, Ernie and Erica! Waiting on the edge of my seat for the RMH DVD set to mail! Thanks so much for all the work you do.
My question is if you've ever used phase change salts for your mass instead of rock or cobb? Also, have you ever used water jackets around the base of the bell and along the exhaust flue (fully vented to the atmosphere to avoid "boomsquish" of course) for heat mass?
We haven't done this ourselves, but there has been some discussion of this approach on the ProBoards forum. (Particularly for testing temperature curves). I'll let Ernie or Kirk tell you more about those experiments.
I myself really like the earthen masonry for its simplicity, performance, non-toxic nature, and low maintenance. But I realize that other approaches might appeal to other operators.
What got me thinking along these lines was a set of pictures I saw where someone put a rocket stove in their RV. I was like, "you need some mass for that RMH buddy".
Then I thought through the logistics of RVs. You don't want to haul them heavy. In fact, if possible you drain the water tanks and septic before moving. And voilà, I instantly thought of a drainable water jacket to use in lieu of heavy masonry. If it's vented indoors, (water side I mean,) it'll add humidity during cold weather, it'll act similar to the cobb as far as a heat mass, and it can be lightened quickly when it's time to move the wagon.
What excites me about the concept is now there's a truly portable rocket mass heater that may be bulky, but not prohibitive weight-wise. The potential for putting RMHs in homes on a temporay basis becomes wide open.