Rocket Stove Principles
Dr. Larry Winiarski
Aprovecho Research Center
- Insulate, particularly the combustion chamber, with low mass, heat resistant materials in order to keep the fire as hot as possible and not to heat the higher mass of the stove body.
- Within the stove body, above the combustion chamber, use an insulated, upright chimney of a height that is about two or three times the diameter before extracting heat to any surface (griddle, pots, etc.).
- Heat only the fuel that is burning (and not too much). Burn the tips of sticks as they enter the combustion chamber, for example. The object is NOT to produce more gasses or charcoal than can be cleanly burned at the power level desired.
- Maintain a good air velocity through the fuel. The primary Rocket stove principle and feature is using a hot, insulated, vertical chimney within the stove body that increases draft.
- Do not allow too much or too little air to enter the combustion chamber. We strive to have stoichiometric (chemically ideal) combustion: in practice there should be the minimum excess of air supporting clean burning.
- The cross sectional area (perpendicular to the flow) of the combustion chamber should be sized within the range of power level of the stove. Experience has shown that roughly twenty-five square inches will suffice for home use (four inches in diameter or five inches square). Commercial size is larger and depends on usage.
- Elevate the fuel and distribute airflow around the fuel surfaces. When burning sticks of wood, it is best to have several sticks close together, not touching, leaving air spaces between them. Particle fuels should be arranged on a grate.
- Arrange the fuel so that air largely flows through the glowing coals. Too much air passing above the coals cools the flames and condenses oil vapors.
- Throughout the stove, any place where hot gases flow, insulate from the higher mass of the stove body, only exposing pots, etc. to direct heat.
- Transfer the heat efficiently by making the gaps as narrow as possible between the insulation covering the stove body and surfaces to be heated but do this without choking the fire. Estimate the size of the gap by keeping the cross sectional area of the flow of hot flue gases constant. EXCEPTION: When using a external chimney or fan the gaps can be substantially reduced as long as adequate space has been left at the top of the internal short chimney for the gasses to turn smoothly and distribute evenly. This is tapering of the manifold. In a common domestic griddle stove with external chimney, the gap under the griddle can be reduced to about one half inch for optimum heat transfer.