Rocket Stove

Posted by on June 27, 2015

Last time we did chickens, we needed to buy a gallon of white gas (camping fuel). That set us back nearly $18 for just one gallon of fuel! This is how we get our big pot up to 140-150F for scalding the chickens (helps to remove their feathers).

It is vital we have this, but damn expensive and worth another look at alternatives… Enter the rocket stove.

The basic concept has been around as long as mankind ha been taming fire. The high tech understanding, measurements, and testing began in the 1970’s, which is when it got its modern name… Rocket stove.

There is a similar phrase you may hear about… Rocket mass heater. This is sort of a rocket stove with some upgrades that can keep you warm in winter. In this post, I am focusing on the stove that can be used for cooking. You can boil a gallon of water with only a dozen pencils worth of wood. Don’t quote me on that last bit, but you get the idea.

I have researched this online for a long time and have some understanding of ovens and fireplaces. I have read through many conflicting posts like this one and quite a few scholarly papers, which this is not… But I am going to summarize what I have read in technical terms as well as a kindergarten translation, where necessary.

A rocket stove (or mass heater) is so named because of the very positive airflow as you tap into the laws of thermodynamics.

Several goals of rocket stoves are to use very little fuel, create a smokeless fire, and oh yeah they can be built from almost any materials found wherever you are. These are in heavy use in parts of the world that have few resources made from mud and grass or from cheap hardware store parts or professionally welded and crafted from the finest of materials delivered to your doorstep.

My virgin project will be made from a mix of concrete and vermiculite packed in a 5 gallon plastic bucket, with soup cans and oatmeal containers for the burn chamber.

The vermiculite is an awesome insulator and will keep virtually all of the heat in the burn chamber. This will burn secondary gasses created by the wood that normally escape from an open fire. This is similar in principle to my high efficiency fireplace insert. That unit is designed to be highly efficient in transferring potential wood energy into kinetic heat throughout my house.

I am not a concrete guy at all. I have heard/read many recipes for making insulating concrete. I will be using a mix of 4 parts vermiculite to 1 part fast setting concrete, or 80% vermiculite to 20% concrete. I have seen mixes with more and mixes with less, but this will make a good first try.

The 5 gallon plastic bucket is more of a mold than actual stove material. Cut twice from the top to the bottom (leaving the bottom intact) will allow me to peel the mold away and reuse the bucket with some duct tape to hold it together. It is 14″ tall with an 11″ diameter. A 3″ diameter hole will be cut in one side of the bucket to allow a primary horizontal burn tube.

A 3″ diameter by 4.5″ tall soup can will go through this hole to the center of the bucket. It will be an inch or two above the floor of the bucket to allow a solid base. Both ends of the soup can will be cut out to allow free flowing air. This is part of a mold for the concrete and vermiculite to form around, leaving an air hole.

Centered in the bucket will be two oatmeal buckets. These are 5″ diameter and 9″ tall. The rule of thumb for the vertical burn chamber is 3 times taller than wide. With a 5″ diameter tube, I really want 15″ of height here. The 5 gallon bucket is 14″ and I want a couple inches on the bottom, so I will either need to create a form around the top of the bucket, or sacrifice my perfect design formulae.

The 3″ can and the 5″ oatmeal tube almost perfectly match up to a 1:1.5 ratio for the primary burn chamber to the main cha!mber dimensions. This draws air through the primary chamber quickly, like bellows on a forge. It also allows for expansion of hot air and lengthens the time exhaust spends in the chimney. The heat is held in that tube by the insulation, so very high temps are achieved and the residual gasses are burned as additional fuel.

You are getting more fuel from your wood than you would with an open flame…. Hotter temps, less fuel, no smoke.

The horizontal tube gets a shelf. Wood sticks go on the top of the shelf and are fed into the stove while air flows freely underneath getting pre warmed and flowing into the burn chamber with limited restrictions.

Only the tips of the sticks burn, like with a match. They give off gasses that burn since they are mixed with the heat that is trapped in the flue, instead of radiating away from a typical chimney and cooling the exhaust. This creates enough heat to quickly boil water or cook a meal with the amount of wood you would normally use as kindling to get an open cooking fire going.

My final plan is based on both good and bad examples I have seen on forums, YouTube, and google scholar. Even a few of the bad designs I have seen had at least a nugget of brilliance to them.

Any madness with tape, the cardboard oatmeal containers, etc will be burned out by the initial firing. Assuming this will be like a new fireplace, it will need to go through a series of progressively hotter and hotter fires to prevent cracking.

The very first fire will be a few pieces of crumpled paper, then let it cool, then a few small pieces of wood, then let it cool, then get a good little fire going for a bit, then cool… Then let her rip to see how well it boils water!

For around $10 in materials and some ‘waste’ parts heading towards the trash anyway, we can get incredible heat from the sticks that blow out of the trees in my yard, rather than paying good money for fracking gas or expensive electricity. Remember, the intense heat in the burn chamber burns the gasses leaving very little pollutants from the exhaust.

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