Lots of info on them, including real world experiences here on SM. Hit up the search at the top right.
Played around with them and in the right circumstances they could be an asset. Nothing "new" about the idea as this is the same thing as a Dakota fire pit, or Viking iron ore fire pit.
Yep, until early 1900's, a modified system was used to melt all cast iron and was small and simple enough that 5 or so people could make wood stoves etc. At that time the skill was in the pattern making and the ramming of sand in the molds. Loaded in charcoal or coke, coal could be used but the sulfur etc tended to mess with the iron, got a good fire going, had a tyre to blow a blast of air into the fire, gassified it and the hot gases went up thru the next layer of scrap iron to cast, it melted and formed a pool on the ashes, put in another layer of coke and scrap and drew off the first. New Mexico State U in Las Cruces did it up until about 1960 in the engineering classes then went to al. A lot of info on the net, but if you have interest in Viking wrought iron, early iron making in the USA, casting iron, blacksmithing , etc, it is an awfully deep and time consuming rabbit hole. Saw a picture of people casting cast iron banks in China a couple years ago and they had one where they were dumping in baskets of fuel and scrap and doing the whole thing in a very primitive shop.
The only think I do not like about the thermal mass stoves is that you pretty much have to let the fire run wide open to prevent creosote buildup and that is not really practical when you burn it for 5-6 months straight. You don't want to have to start a new fire in the morning and then again when you get home every day. We keep ours shut down with no air 90% of the time and just keep adding wood to it. I rarely start a new fire unless we go out of town. Obviously we get some major creosote buildup but I just knock it down into the stove and burn it because the pipe pretty much just goes straight down. Those thermal mass stoves usually have the smoke routed through a maze in the stove to capture all of the heat and they need maximum air to insure that everything is burned before it leaves the firebox so it does not buildup creosote in the maze.
If it has a sideways fire, I like it! I build them in the outside fireplace out of fire brick and sand in the winter and disassemble them again in the spring. Good practice. What were you looking to do with it? The rocket stove is a different critter from the rocket mass heater, although the breathe fire the same way. What do you have for fuel? Side feed? Vertical feed? Indoors or outside? Lots of stuff here and elsewhere. You might also like the T-LUD (top-lit updraft) stove. They burn wood pellets, chips...just about anything. (I did fire one on dried cat poop just to see if it could be done. It can.)
I worded the subject line wrong. Rocket mass heater is what I'm looking at, to heat our off grid cabin. The cabin is going to be a smallish footprint of probably 25x25, but I'm doing a full basement under and a loft above to expand usable sq footage to around 1450. I would like to use thermal mass and passive solar design to make it as energy efficient as possible. Tons of research and decisions left to be done/made. I just saw the rocket mass heaters and liked the idea but wanted to hear from those who've used them. I see what Ben is saying about not being able to choke it down and prevent having to re start a fire, but my understanding is that starting a fire in one is simple and the radiant heat from the mass (cob or whatever you use) keeps letting off warmth during that downtime. That's in a perfect world though I guess.
Ernie and the folks over at Permies have been doing different designs of thermal mass heaters for many years. Plenty of design and build videos out there. I never have any trouble getting one lit off, but it helps if you have something that's going to create a lot of heat to start the draft at the inlet. I have a bellows in case I don't have wads of dry newspaper. A few blasts of air and things start going sideways... which in this case is a good thing. If you want to get an idea of how fast a top feeder can heat stuff up I'll offer this: I built it as an experiment, and it's a wood gobbler, but it'll eat 8' 4x4 lumber and anything else you can stuff down its throat. What you see is basically an upside down thermal mass heater with firebox, chimney and heat riser all packed together without any burn tunnel or extended manifold to heat a thermal mass. It gets HOT right now, but it cools off quickly if not fed. Did I mention that it's loads of fun to burn stuff in!
