In our recent Campfire Blacksmithing class, we used rocket stoves to maximize the heat energy and combustion attainable using wood as a fuel source. While our stoves were made of concrete, the same concept can be achieved using almost any non-flammable building material, such as metal…or rocks! A rocket stove can totally be built using only what you find in the woods around you! In fact, the Dakota Fire Hole (aka Scout Fire Pit) uses the same concept – and even keeps the flames hidden underground in the event you’re, say, trying to avoid detection.
Rocket stoves can be built in a variety of ways, but include at least three main parts:
- Intake – This is a tube or opening that allows oxygen to enter. It can also serve as the area to feed in fuel to burn, in our case wood. (#1 in the Diagram)
- Combustion Chamber – Where the burning happens. (#2-3 in the Diagram)
- Heat Riser – Where the magic happens. The thermal column is usually built to rise vertically. (#4 in the Diagram)
If the walls are built heavily insulated, such as with concrete or earth, a whopping amount of the heat energy from combustion rises directly to the hole in the top. It wastes very little energy as compared to an open wood fire or propane stove. The version we’ll build here simplifies the construction and places the Intake and Combustion Chamber into one area.
Why is it called a “rocket” stove?
Let’s ask Bernoulli.
The Bernoulli Principle, as it relates here, says that as we force air through a smaller opening, it’s speed will increase. So if we were to narrow the hole between the fuel and the riser (as shown in the diagram), the combusting gases will rush through with a rocket effect.
“Whoosh” of a rocket engine included.
To build a simple rocket stove, you’ll need:
- A container such as a planter or 5-gallon bucket (planter pictured)
- A few feet of PVC (4” diameter for planter, 6” for 5-gallon bucket)
- 1 bag of Quikrete or other fast setting concrete mix (Note: It takes just over 1 bag of Quikrete to fill a 5-gallon bucket. Just under 1 bag to fill a tree-planter, as pictured.)
- Utility Knife (that you can abuse)
Safety Note: Concrete can burn the skin and eyes. Wear long pants, safety glasses, and gloves when working with concrete.
How to Build Your Rocket Stove
- Cut a hole in the side of your container. It should fit the outer diameter of your PVC as closely as possible. Take your time. Make it neat. (Note: Locate it 1 to 2” above the bottom of your container.)
- Cut the PVC at a 45-degree angle, so you can turn and match the angles into an “L” shape. The short side simply needs to be long enough to reach from the middle of the bucket and stick out the hole a few inches.
- Pour a layer of the (dry) Quikrete into the bottom of the container, up to the hole you cut.
- Insert the short end of the PVC into the hole, so it reaches the middle of the container. Stand the long end on top of it. Hold in place.
- Begin adding Quikrete around the vertical PVC, until it will stand on its own (should only take a few inches), pictured at left.
- Add water and mix/poke as necessary. Add Quikrete and water until you reach the top of your container.
- Important: After ~15 minutes, remove the PVC. Be cautious and certain that the concrete has setup enough to hold its shape. But don’t wait much longer than 15 minutes! If so, you may find it impossible to remove the PVC.
- Allow the stove to sit overnight. It should be cured by morning.
- Using that abuse-able utility knife, cut the container off of your new stove. I like to carefully cut straight lines up the sides of the container. (Then, with just a bit of duct tape to cover the cuts, the container can be used, and reused, again and again.)
- Set some kindling into the chamber, ignite, and enjoy!
Here, a student in our Campfire Blacksmithing course retrieves his knife blank from the core of a rocket stove.
A few items of note:
- We’ve stacked a second tube on top of the stove to create a longer thermal column
- Flames can be seen between the two pieces of concrete. That’s the thermal column/heat riser.
- Adding a long infeed tube (especially of a narrower diameter) to where the student is reaching in would create more of the draft/rocket effect of the Bernoulli principle.
- Near-complete combustion is possible with a rocket stove: It will completely incinerate wood into ash, not leaving behind chunks of charred wood – and efficiently maximizing the energy tied-up in the wood. That’s good.