I am also wondering if they are viable. I have plans to build a rocket forge this fall so I will let you know how it goes.
As far as dual purposing it for a mass heater, I believe that may be tricky. A forge or foundry has a specific access point for metal or crucible placement. The highest temp on a rocket stove is at the top of the heat riser so that is the best place to heat the metal. I imagine there would have to be a manifold to redirect the hot gasses into a mass and without a standard rmh barrel assembly, it may not draw properly, if you are trying to heat a bench. If you just want mass, building it with a mass around and behind the rocket stove and chimney will probably work fine. Think traditional masonry stove with the massive wall structure. Definitely something I would build outside for testing before building it inside.
Just my two cents.
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