All good points guys!
Did some searching on drafting issues with rocket stoves and came across this exerp... verry interesting!
ROCKET STOVES – CONTROLLING DRAFT
(Variations in chimney height and air flow
openings in a modified “rocket stove”.)
by Damon Ogle
CONCLUSIONS AND RECOMMENDATIONS
Controlling draft at the exit point of a “rocket elbow”
is much more effective than trying to do so
from the point at which
air enters the elbow.
It is possible to produce high temperature exit gases (above 1200 F) and maintain their
temperature through the length of a long combustion chimney.
High temperature and “dwell time” by themselves
are not sufficient to produce dependably clean
combustion. It may be necessary to further mix the exit gases (either by turbulence or by other means) in
order to aid combustion in rocket stoves.
Higher temperatures and cleaner burns are sometimes obtained by partially blocking the flow of
gases through a rocket stove. It may be possible to use mixing devices or jets of air to achieve this
blocking effect and improve combustion at the same time.