I have been lurking here in the forums for a long time and seen your praises for rocket mass heaters..and you hope for more thermal mass.
In our country we have been using same design, basically, for at least couple hundred years. Only difference i see in design that rocket mass heater gives most of the heat instantly.
We call these by the name: pönttöuuni
Idea behind this is the same, except that pönttöuuni gives heat for one day with couple loads..
Here is the google link for some images:
http://www.ponttouunit.com/ponttouunit/kuvagalleria-uunit/
