Do-It-Yourself Rocket Stoves, by E.B.

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I live and prep on a tight budget – at least for the time being. I am lucky to have a fireplace in my home, not a woodstove, nor the room for one, but at least a fireplace. It’s better than no fireplace, but rather inefficient for heating or cooking. It would do in a pinch, but a rocket stove would greatly reduce the amount of wood needed to cook a meal. On my budget even $100 is a lot right now and I began looking into building my own rocket stove. The knowledge is out there, and “improved stoves” are being made in many third world countries to reduce fuel use and increase efficiency. The number of sizes, shapes and applications for improved stoves is incredible. I decided to try to make my own, and the money I saved could then be spent on something I couldn’t make for myself (like ceramic water filters, for instance).

Why DIY?
There are some very good reasons why learning to do-it-yourself (DIY) with an improved stove is a good idea. Not least of which is the cost. Cost was the initial factor for myself, and remains a benefit for me. Not only can I make myself low-cost rocket stoves, I can make them for extended family members and gift them as emergency kits disguised as camping stoves. In learning how they operate, you can also troubleshoot your stove, fine tune it to the task you require, and fix your problems yourself without resorting to someone else’s customer service. The fact that you are learning a new skill, and a potentially lifesaving one at that, is another great motivator. In a disaster scenario the ability to boil water efficiently is essential. Fuel is likewise inexpensive or free. The aftermaths of disasters almost always will have broken lumber which provides a useful and readily available fuel, or they can burn previously overlooked fuel such as branches of smaller diameter (around the thickness or a finger or thumb works well in my little stoves) that may have been considered too small to be worthwhile issuing in a proper wood-stove. I have purposely tried to make all my rocket stoves out of locally available and free re-purposed materials, or very inexpensive materials which are readily available. I have found some ‘non-free’ materials really help the process, and create a superior product (like JB Weld) but I also want to be able to make them out of the most basic materials around if needed.

The Science of Improved Stoves
There are a number of principles that go into making a good rocket stove. From what I know, the most important is that they need to be hot. Very hot. The goal is create a clean, complete burn that burns the combustive gases and the particulate (smoke). For that you need temperatures that go beyond your simple three stone fires. Most of the other principles are a part of trying to create that heat required for a good burn. High temperatures equal full combustion. For that reason, improved stoves need an insulated combustion chamber, top keep the heat in. Pre-heating the combustion air makes a hotter fire. By having a shelf for the wood to sit on, air can move underneath freely, providing all the oxygen needed, and is heated by the fire before being burned. Thus cold air is not as likely to get into the main combustion chamber and reduce the temperature, keeping it hot.

A flange or shroud can be built to surround your cooking pot, forcing the hot air to move further along the side of your pot, transferring more heat to the pot and cooking your food faster.
Another principle is that the air volume in needs to equal the air volume out. Sounds simple when your inner chamber is a consistent 4” diameter, but when adding a shroud you have to make sure there is enough area/space in between the pot and flange for the equal volume of air to escape through the top. If not, you get a backdraft and smoke pouring out of the bottom of the stove.

Construction Basics
The basic design of any improved stove starts with an “L”-shaped combustion chamber. Combustion is meant to take place at the right angle corner of the “L”. The chimney/upper part of the “L” must be long enough to allow time for complete combustion of the gases. On the bottom of the “L” you typically run a wire shelf for the fuel (wood) to rest on. This allows air to enter horizontal portion of the “L” freely, beneath the fuel, and pre-heats the air before it gets to the actual combustion area.

Around this “L” is the outer sleeve. The sleeve should surround the vertical portion of the “L” completely, with a few inches gap in between to fill with insulation. You will need to cut a hole in the side of the sleeve for the horizontal “L” section, and in the top for the vertical “L” section. There are lots of ways and styles in which to do this. The main idea is that the sleeve holds the insulation against the “L”.

Three DIY Stoves
I made my first stove out of an empty white-gas/naptha can and some old drain pipe from a downspout on the house. I cut a 90 notch in the drainpipe, angled at 45 degrees to its length, with tin snips and folded it to make a simple 90 degree bend in the drainpipe. I then traced the entrance hole to fit the drainpipe on the narrow side of the naptha can and punched it out with a chisel and tin snips. I removed the top of the can with a can opener and tin snips, and traced and cut an exit hole for the vertical chimney portion of the drainpipe. I then sealed it with insulation (more on that later). I slid the top of the tin over the chimney drainpipe, and let it set and dry. I used a cut section of another drainpipe, drilled with a bunch of holes at one end, and slid into the horizontal section of the “L” as a rack to place the fuel on that also allows air to enter underneath and pass through the drilled holes directly into the coals of the fire. A wire rack for the pot to sit on top completed the stove, and allows for the smoke and flue gas to escape. So far this stove has performed fairly well. Not perfect, but an encouraging first attempt.

My second stove was a better built model, loosely based on a plan from the internet. Two #10 cans, one with top removed and one with top and bottom removed (with a can opener) become the body of the stove. I taped them together end to end along the interior of the can with duct tape. This allowed me to use JB Weld to join the cans together, which form the outer sleeve. When dry I removed the tape. The inner sleeve I made from purchased 4” stove pipe, though I later found that standard food tins are also 4” diameter, and could also be used. Having a 90 degree elbow stovepipe section greatly decreases the work involved, and makes the stove look much more streamlined inside, but I am confident I could work up a 90 degree elbow out of 4” food tins as well. Again, I cut holes for the 4” combustion “L” tube on the side of the lower can and on the removed lid of the upper can. Fill with insulation, replace the lid, add a fuel platform and pot rack, and viola. It has performed much better than the first, I think because there is more insulation, and perhaps because the height-to-diameter ratio of the combustion chamber is better. Either way it produces less smoke.

The third stove I tried came from a Webster called Practical Action (practicalaction.org) and is a build-in-place stove in the backyard made of simple red bricks. The principles are the same, creating an “L” shaped combustion chamber, tall chimney, insulate and create a fuel platform and pot rack. I like this type of stove particularly for use as a summer shack for outdoor cooking. It is easy to make, and can be made out of just about any sort of bricks, earth, or rock. Some in-place improved stoves get pretty fancy, similar to the old homesteading wood ovens, and have chimneys that vent right out of the house. The brick version is very similar to the survivalist “adobe stoves” that are dug into the side of clay hill side.

For any stove, efficiency can be greatly improved if you use a flange or shroud to surround your pot when you cook. It works by holding the hot gases close to the sides of your pot as they rise, rather than the gases just heating the bottom of your pot. I was able to make shrouds for a few pots, but they do have to be tailored to the size of your pot to maximize efficiency. I made mine by cutting a hole slightly larger than the diameter of the chimney into a round sheet of metal (mine came from a big popcorn tin). I put that over the chimney, and the pot rack on top of it. On goes the pot, and surrounding the pot I made a shroud out of the sides of the popcorn tin. It sits right on the bottom sheet, and I bend it to fit around whatever pot I use. I tend to leave about a ½-inch to 1 inch gap between the shroud and the pot. These work on the same principles that you see with many ‘windscreens’ for single burner back-country stoves like the MSR Whisperlite.

Insulation
I have tried three types of insulation in my first stove – wood ash, sand, and my favourite a concrete-perlite mix. Wood ash was messy, and though perhaps a very good insulator, it kept leaking out of the gaps in my stoves. You also have to be sure to use completely burn wood ash, or you may get an unintentional smoldering fire inside your stove. Sand was the second choice, and it worked well, but again leaked through any sloppy joints. I found that a mix of concrete and perlite (a soil additive from a gardening store) was the best all round for ease of mixing and performance in the stoves. Perlite is an inert volcanic mineral that has the consistency of small Styrofoam beads. It’s light and fluffy and very insulative. I mixed it with concrete to keep it in place, and it sets well, doesn’t leak out the gaps, and adds some rigidity to the stove so your whole pot weight isn’t sitting on the JB Welded joints. It’s another concession to modern materials, but if they are available then why not use them and get a longer lasting product.
I have seen informative instructions and videos on making firebrick, the gold standard in woodstoves for insulation properties, out of sawdust and clay which is then fired in a kiln. The resulting bricks are porous (the sawdust burns away in the kiln) and extremely insulative and lightweight. However, they are beyond my expertise and resources.
I also found that whatever insulation I use, I seem to get closer to complete combustion as the stove runs a while and heats up.

Caveats and Warnings
One issue that I was concerned with was the use of non-intended materials for cooking over. Galvanized stove pipe, for example, may release toxic fumes when heated. Even the liners from some food tins are probably not too nice if you burn them. I personally don’t feel this is a major concern if you cook outside and your pot has a lid on it. I also burn a good hot fire in the stove for a good while to hopefully cook out any fumes before I use it for food preparation. If I am frying, I do not use a shroud with my frying pan, and feel that most flue gas and fumes likely blows off in the wind. To be be completely technical remember that simply burning wood and other biomass releases carcinogenic chemicals that are likely not good for human consumption in quantity. I feel it is a measured risk. Each person must make their own choices for themselves.

Conclusion
Though a store-bought rocket stove is still on my wish list, I am happy to have an in-the-meantime solution to cooking without power, and one that can be replicated over and over, or adapted to the materials at hand, and given to friends and family. I encourage you to look online for plans - youtube has a several videos on making brick backyard stoves, for example - and get cooking. Though I have yet to rival the clean burning store models in my home-made designs, I find them very useful in the meantime, and as gifts in emergency “camping” kits.

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This page contains a single entry by Jim Rawles published on August 15, 2012 12:06 AM.

Letter Re: American Redoubt Coverage was the previous entry in this blog.

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