Build a rocket stove

A rocket stove burns small wood so completely and so hot that it will boil a large pan on a double handful of twigs, with almost no smoke. In a long emergency it is the difference between fuel being a daily problem and fuel being something you pick up on a walk.

It is also the most misunderstood build in preparedness. Most homemade ones do not rocket, and the reason is nearly always one of the two rules below.

Why it works

An ordinary campfire burns wood and lets most of the energy escape as unburned smoke and radiated heat in every direction. Smoke is fuel you failed to burn.

A rocket stove does three things differently:

1. It burns the wood in a horizontal tunnel, feeding only the ends into the fire, so the wood burns at one point rather than along its length.
2. It exhausts through a tall insulated vertical riser. Hot gas rising in that chimney creates a strong draught, which pulls air in through the feed at speed. That accelerating airflow is the roaring noise the design is named after.
3. The riser is insulated, so it stays extremely hot, and the smoke and unburned gases coming off the wood ignite a second time inside it. That secondary combustion is where the efficiency comes from and why there is almost no smoke.

The result is a fire running at 700 to 1,000°C in a tube, with the heat delivered to one place instead of the sky.

The two rules that matter

Rule one is the one people break without noticing. A restriction anywhere throttles the whole system, and an enlargement slows the gas and kills the draught just as effectively. If your burn tunnel is a 100 mm pipe, the riser must be 100 mm too, and the feed opening must present the same area. A stove that is "a bit bigger at the top for a better chimney" will not rocket.

If you must vary shape, keep area equal. A 100 mm round pipe has an area of about 7,850 mm²; a square tunnel of the same area is roughly 89 mm by 89 mm.

Rule two is the one people skip because it is fiddly. An uninsulated steel riser radiates its heat away, the gases cool below ignition temperature, secondary combustion never happens, and you have built a slightly efficient smoky stove. Insulate the riser with wood ash, perlite, vermiculite, dry sand as a poor substitute, or ceramic fibre if you have it, and the same stove transforms.

A useful check on whether you have both right: a working rocket stove is nearly smokeless once up to temperature, and it roars. Visible smoke from the top means one of these two rules is broken.

Proportions

For a household cooking stove, a 100 to 150 mm system is the sweet spot: enough power for a large pan, small enough to light quickly and feed with twigs.

The tin can build

Buildable in an afternoon from scrap, and it works properly if you respect the two rules.

What you need: a large tin (a 5 litre oil can or a paint tin), a smaller tin for the riser, a short length of pipe or a third tin for the feed, insulation (wood ash is free and works well), tin snips, gloves.

1. Make the riser. A steel tin or pipe of your chosen diameter, open at both ends, three to four times the diameter in length.
2. Cut the feed hole low in the side of the large outer can, sized so the feed tube presents the same cross-sectional area as the riser.
3. Fit the feed tube horizontally into the hole so it meets the base of the riser at a right angle. This joint is the burn tunnel. Keep the area constant through the bend.
4. Fit a shelf inside the feed tube, a flat strip of metal splitting it horizontally. Wood rests on top, air comes in underneath. This single detail improves performance dramatically, because it guarantees the fire gets air even when the feed is full of wood.
5. Stand the riser in the outer can, centred, and pack the gap with insulation. Dry wood ash is excellent, free, and what most builds should use. Perlite or vermiculite from a garden centre is better and light.
6. Cap the top of the outer can around the riser so the insulation stays in and the exhaust exits only through the riser.
7. Add pot supports on top: three bolts, tabs cut and bent up from the can, or a metal trivet. Leave the pot gap described above.

The brick build

Better, longer-lived, and it needs no cutting. This is the version worth having if you expect to be cooking outdoors for months.

Sixteen ordinary bricks and no mortar builds a working L-shaped rocket stove:

1. Lay a base of four bricks, flat, as a platform.
2. Build a channel on top: two bricks forming the sides of a horizontal tunnel, one across the top, leaving a square gap of roughly 90 to 110 mm. That is the burn tunnel and the feed.
3. Build a vertical chimney at the far end of the tunnel, four bricks per course laid to leave the same square gap in the middle, three or four courses high.
4. Split the feed with a flat piece of steel or a thin brick so wood sits above and air flows below.
5. Pot supports on top: two bricks laid to leave a gap, or a metal grate.

For a permanent version, insulate the chimney by building a second brick skin around it and filling the cavity with wood ash or perlite. That converts a decent stove into a genuine rocket.

Use fire bricks or dense engineering bricks if you can. Ordinary soft house bricks work but spall and crack over months of thermal cycling.

Running it

• Light from the top of the riser if you can, or with a small handful of tinder at the tunnel mouth, and let the draught establish before adding more. A rocket stove that has not warmed up will smoke; once the riser is hot it clears.
• Feed small and often. Thumb-thick sticks, a few at a time, pushed in as they burn back. This is a stove that eats twigs, hedge trimmings, offcuts and pine cones, and does badly on large logs.
• Push the wood in, do not pile it. The fire lives at the tunnel end; the feed is a magazine, not a firebox.
• Do not damp it down. There is no air control. It runs flat out or it goes out, and that is the design working correctly.
• Dry wood only. Wet wood costs energy to boil the water out and produces smoke that the riser then has to burn. See seasoning firewood in this domain.
• Expect to sit with it. A rocket stove needs feeding every few minutes. It is a cooking fire, not a heater you can leave.

Efficiency is genuinely striking: a litre of water to a rolling boil on a handful of sticks in a few minutes.

When it will not rocket

Almost every failure is one of a short list.