A tin-can rocket stove uses three large cans, takes about 30 minutes to build and boils 2 litres of water in 8 to 12 minutes on a handful of sticks. The inner chimney can must be narrower than the outer can for the two to nest, and the gas flow area should not shrink along its path, so check the feed opening, chimney bore and pot gap before you cut.
Part of Rocket Stove Design
A rocket stove made from three large tin cans is the quickest way to see the principle work. Its cans must nest, and the gas path should not narrow along its length.
The Overview’s Build
The overview puts the whole build at about 30 minutes. It uses three large cans (#10 or institutional size, about 15 cm across):
- Outer can. Cut a rectangular opening near the bottom, 8 cm wide by 10 cm tall, for the fuel shelf.
- Inner chimney. Use a second can with both ends removed and insert it vertically into the top of the outer can.
- Fuel shelf. Cut a third can into an L-shaped feed tube connecting the side opening to the base of the chimney.
- Insulation. Pack wood ash, dry sand or crumpled foil between the inner chimney and the outer can.
- Pot support. Three bolts or stones on top hold the pot 1 to 2 cm above the chimney opening.
Three identical cans cannot nest, so the inner chimney has to be a narrower can than the outer. The overview does not give its size.
Check The Flow Areas
Gas should not have to squeeze through a smaller opening than the one before it, or the flow chokes. This is a general rocket-stove design rule of thumb, associated with the Aprovecho Research Center’s stove design principles. It is not from the overview and has not been checked against the original source here.
- Feed opening: 8 × 10 = 80 cm².
- A 15 cm chimney bore: π × 7.5² = 177 cm². That is more than twice the feed opening.
- Pot gap, 15 cm circumference π × 15 = 47.1 cm: 1 cm gap gives 47 cm²; 2 cm gives 94 cm².
At the overview’s 15 cm the pot gap is smaller than the bore. As an illustration (not the overview’s size), a 10 cm inner can has a bore of π × 5² = 78.5 cm², close to the 80 cm² feed opening, and a gap of 2 cm around it gives π × 10 × 2 = 62.8 cm², still below it. Raising the pot a little higher improves the exit; test and adjust by watching for smoke at the feed opening.
Worked Example: What The Boil Time Means
2 L of water heated from 20 to 100 °C takes 2 × 4.186 × 80 ≈ 670 kJ. In 8 minutes (480 s) that is 1.40 kW; in 12 minutes (720 s) it is 0.93 kW. These are the minimum rates of heat reaching the water, ignoring the pot’s own heating and any evaporation.
Common Mistakes
- Leaving a lacquer or zinc coating in the can. It burns off with fumes. Burn it off outdoors first and stay upwind, or use uncoated cans.
- Packing the shelf so tight that air cannot pass beneath the fuel.
- Sitting the pot directly on the rim. It blocks the exit and chills the flame.
Safety
Sharp edges, hot cans and carbon monoxide
Cut cans have razor edges; wear gloves and fold or file them. The stove is hot all over. A prototype that is cold, smoky or choked burns incompletely and makes carbon monoxide, which has no smell. Use it outdoors. See Carbon Monoxide Risks.
What’s Next
- Learn next: Brick & Clay Builds — a durable version
- Combine with: Fuel Savings Measurement — test what it saves