What this step adds; in part B, the fire and its hot gasParts already in placeA 5 L pot on an L-shaped rocket stove: a 12 cm square tunnel and riser lined with fired clay and packed round with wood ash, and a clay skirt that leaves a 6 mm gap round the pot. The heat model is simple and its numbers are assumptions: the fire burns 0.9 kg of sticks an hour, air-dry wood gives about 16 MJ/kg, and 30% of the fire's heat reaches the water with the skirt on, so the pot takes 1.2 kW. Heating 5 L from 20 to 100 °C needs 5 × 4.186 × 80 = 1,674 kJ: about 23 minutes and 0.35 kg of wood. The CDC's 1-minute rolling boil (3 minutes above 2,000 m) adds 15 g, 0.36 kg in all. Aprovecho's tests found rocket stoves burn about 40% less wood than a three-stone fire, and about 50% less with a pot skirt, so the same boil on three stones takes about 0.61–0.73 kg. While the water heats the clock runs about 440 times fast, and 20 times through the boil. This is a guide to how the parts go together, not engineering drawings. Burn it only outdoors or under a flue: wood smoke carries carbon monoxide. Lift boiling water with dry cloths, and remember that boiling kills germs but does not remove chemicals.