Rocket Stove

One of the things I like to focus on is how to live more in harmony with nature, and how to live without relying on the fossil-fuel based infrastructure of our society. One word for this mode of being is Permaculture, but there are many ways to describe it.

One of my preoccupations this summer is in learning to deal with fire more efficiently. How to start a fire without fossil fuels, and how to cook food without relying on fossil fuels. I came across the concept of the rocket stove which is a way to concentrate heat, enough for cooking, while burning wood very very efficiently. With a well-designed rocket stove, it’s possible to cook a meal with only a few twigs, and with very little smoke and pollution. Despite the fact that many people are using these indoors, I wanted to build one outside our front door near the house.

The basic concept of a rocket stove is that you have an L-shaped chamber that is insulated. A fire burns at the bottom angle of the L. You feed wood into the bottom part of the L, and the upright part of the L acts as a chimney. When built correctly, these burn very efficiently, drawing air up from the L and through to the top. All the heat is concentrated so that it comes out the top.

There are many different plans for rocket stoves available, but to begin with I wanted to keep things as simple as possible so that I can experiment with the design. For my first attempt I was inspired by this design that uses only bricks that are stacked together, thus allowing for easy modifications or repairs:

I used basically this same design, except I added 4 more bricks to the top to make the chimney “taller,” and I put the entire thing up on cinder blocks so that it was more accessible. Also, the arrangement of the top cinderblocks gives me a place to store processed wood for burning. The longest part by far was leveling the base so that the entire stove is level (I was picky about it so that the airflows wouldn’t be hampered by funny angles), and it came out great, as you can see in the photo here.

The base used 8 cinder blocks I had laying around, which brought it up to a comfortable height, and also gave me 2 spots to store processed wood ready to burn. Then the stove itself used 21 bricks, 20 in the main structure (2 of which are half-bricks), and the 21st is in the front on its side, which gives the sticks a platform to rest on while the burn, while still allowing air to come in from underneath.

This photo shows the sticks in the burn position. As the fire in the main chamber burns, you shove the sticks in further and further. 4 sticks the size shown here (roughly the same thickness as 2 fingers, each about 18″ long) along with some birch bark and twigs as tinder and kindling, burned for about 30 minutes. I still have to learn to be more efficient with the tinder/kindling to main fuel wood ratio so that I get the cleanest possible burns. Once the rocket stove is operating efficiently, it should burn very cleanly with almost no smoke or fumes — clean enough that this design is commonly used indoors.

After the fire had been burning for about 20 minutes, I left it unattended for a while to do some other chores. When I came back, this is what was left in the photo to the right. All in all quite a clean burn, and I look forward to getting more practice with this technique and seeing how I adapt it over time. I think I’ll make some lunch on this tomorrow, using an old cast-iron skillet I inherited from my grandmother.

Update, Apr 16

I’ve now successfully made my first meal on the rocket stove. It came out great! However, there are still a few tweaks I need to make. This first update photo shows my grandmother’s old cast iron skillet, resting on 4 small rocks leaving about a 1cm clearing above the mouth of the stove. This allows the heat transfer to happen very efficiently. You can see the sticks coming out of the feed hole below, and the fire happening within.

I think there are a couple of problems with the stove preventing it from giving the desired “rocket” effect, where the flames shoot up into the “chimney” part of the stove. First, I think the opening I have in the feed hole is too large. I need to find a brick that is half the thickness of the other bricks and put it in the bottom to reduce the size of the feed hole. The size of the feed hole is tricky, it needs to be large enough to allow adequate airflow to feed the fire, but small enough that it focuses the heat up the chimney.

Second, I think the chimney is a bit too tall (note the original design calls for a 4-brick height and I have it at 6-brick height). I had read that the taller chimneys draw better, increase the chimney effect, and cause the fires to burn hotter. This was not the case for this fire, 2 different times I had to feed some fuel down into the chimney to get the fire to burn hot enough to cook the food in the skillet.

Despite these problems, I’ll calling this first meal a success! It was a basic pasta sauce with oil, onions, celery, ground pork, celery, rehydrated dried tomatoes from last fall, mushrooms, tomatoes canned last fall, salt, pepper, basil, and oregano. Delicious!

Looking toward the future I am looking at other rocket stove designs, but this first experiment has been great fun.