A rocket stove is an efficient wood burning stove that produces more flames, less smoke, and uses less wood. The rocket stove typically has a combustion area for placing wood--horizontal tunnel and a chimney--vertical tunnel. Air comes in through the combustion area burning wood, but heat is efficiently distributed, so one does not have instant heat loss. Mudbrick made of sawdust is the most efficient at retaining heat.
The most efficient use mudbricks, mixed with clay and sawdust.
There are numerous configuration and material used:
The most basic rocket stove: