It seems there are really two versions of rocket stoves. Those like my little stovetech, which are really just primary burn chambers with a horizontal feed, and a more robust design with something like a refractory chamber where the combustion gases get superheated and result in a more complete combustion. The little ones tend to make a lot more soot. Whereas the ones with the chamber tend to burn quite clean, produce more btus per fuel consumed, but are not altogether as portable.