The L-tube rocket stove has a horizontal fuel feed that turns 90 degrees into a vertical riser, and it is simpler, more forgiving of construction errors and easier to feed. The J-tube uses an upright or angled magazine so gravity feeds the fuel, which is slightly more efficient but harder to build and load. For survival use, build the L-tube.
Part of Rocket Stove Design
Both layouts push sticks into an insulated tunnel that bends up into a riser. They differ in which way the fuel enters.
The Fuel Path Is The Difference
- L-tube. Sticks are pushed in horizontally along a shelf and burn at their tips. The tunnel turns 90 degrees into the riser.
- J-tube. The magazine stands upright or at an angle, so burning sticks drop into the burn zone without being pushed. The overview says this is slightly more efficient because gravity feeds the fuel, but harder to build and load. The overview’s wording of the J-tube’s geometry is unclear, so the description above follows the separate Rocket Stove page’s J-tube.
The overview’s advice is to stick with the L-tube for survival: it is simpler, more forgiving of construction errors and easier to feed.
Method: Deciding
Step 1: Can you build a leak-free joint and a sloping magazine? If not, build the L-tube.
Step 2: Will someone be present to feed it? Both stoves need attendance; the J-tube just needs less pushing.
Step 3: If you build a J-tube, keep the magazine’s cross-section at least as large as the feed opening of an L-tube you would have built, and do not pack it full.
Common Mistakes
- Packing a J-tube magazine to the top. It chokes the air and smoulders, then smokes back out of the magazine.
- Walking away because it feeds itself. A smouldering or flaring magazine needs watching.
- Assuming the J-tube always saves fuel. The advantage is slight, and a poorly built one loses it.
Safety
Smoke out of the magazine means leave
A magazine that smokes or flames back is leaking exhaust into the room. That exhaust carries carbon monoxide, which has no smell, and whose early symptoms (headache, drowsiness) feel like tiredness. Move to fresh air and ventilate; do not lie down. Use the stove outdoors or with a sealed exhaust. See Carbon Monoxide Risks.
What’s Next
- Learn next: Ideal Fuel Sizes — fuel for either layout
- Combine with: Starting & Feeding — running them