Rocket Heaters are best described in the book by Ianto Evans & Leslie Jackson".
These rocket heaters most often use a barrel to radiate at least a large portion of the heat from the combustion into the surrounding space immediately. Often the remaining heat is stored into thermal mass for gradual release later, but some designs do radiate all of the heat immediately, similar to a cast iron stove.
They are more efficent and burn cleaner without catalytic converters than most cast iron stoves, and when used with some form of 2ndary thermal storage will keep a space warmer longer.
The picture on the cover of the book to the right illustrates a typical rocket mass heater. The majority of the heat is radiated from the barrel and the remaining heat is dispersed into the cob bench via flues.
You can read more about how the rocket mass heater combustion system works by clicking here.
Because rocket mass heaters radiate a large portion of the heat immediately via the barrel or similar cover, when running, they deliver larger amounts of heat than the equivalent rocket masonry heater. However, likewise they must be run for much longer to charge any thermal mass associated with them, since most of the heat is delivered into the space as the fuel is consumed. If no cob or other material is adhered to the barrel, approximately 2/3rd's of the heat is released immediately.