I've had a huge interest in masonry heaters for years, but while searching for some things recently I ran across these rocket mass heaters. Now, to me, the concept seems identical, with the only difference being how the heaters are designed to be fed and the size. It seems these rocket mass heaters are built around a central metal barrel with a bunch of heavy stuff around it where a masonry heater is like a RMH taken to 11. It's built out of more traditional firebricks or soapstone and looks like a more traditional wood stove or fireplace, and they're often much, MUCH bigger. Do I have that about right? Or is there some design difference I'm completely missing?
You have the basic idea right. They both use a small hot fire, heating a large mass.
One big difference is Masonry heaters are insurance company approved, (if built by an approved mason) Rocket Mass Heaters except in a few cases are not.
Masonry heaters have a mandatory second skin of brick for safety. RMH's can but normally do not.
Rmh's however are not limited to just a barrel and a bunch of heavy mass.
The standard rmh that you have seen is known as a J tube design.
Newest RMH technology has moved on to what is known as a "batch box" rmh . These generally use an all brick vertical bell . Similar in appearance to a Russian masonry stove.
Here is a link to the best batch box information site. http://batchrocket.eu/en/
Oh, one last difference. A proper masonry stove is around $10,000 . A RMH can be built for $200 If you bought new bricks and the best building supply's, MAYBE you could spend $800 - $1000
Check things out , here at Permies & over at Peter Bergs site batchrocket EU .
I think you will be impressed!