Water heating with a rocket stove: I found these videos of different styles which seem to work.
There was another guy from Poland who lives in Bristol, England. He had got to the point of having on on-demand, mains water fed hot water from his rocket stove but he recently took all of his videos off-line. I think the heat exchanger was a thin steel water jacket (about the diameter of a length of scaffold pipe) connected to the water mains, this he put in a separate chamber, next to but not in the heat riser (which would cool the rocket down). He was making great advances, shame he has now gone off-line...