using cob to make a rocket stove

So I am trying to build a rocket stove using cob. I am using my hands to form the sides and I can't build more than three inches high at a time using hour and a half intervals or it will cave in. Has anyone tried using a tall vase with a hole cut into the base for feeding the firewood? How about using about form for building the tower then scooping out the burn chamber and heat riser?

You will get far better results by using a form to pack cob around. If you can cut wood, you can make the shape of the internal cavity from thinnish scrap wood, plywood, or whatever. Keep it hollow, and when the cob sets up, you can start a fire inside the form and burn it out, hardening the cob in the process. This is a 6" J-tube I made for a rocket cookstove:

thomas newcomb wrote:I can't build more than three inches high at a time using hour and a half intervals or it will cave in.

Your mixture is probably too wet and/or doesn't have enough sand.

I am thinking that it will work with cob in the burn tunnel, but it won't last long. An interesting experiment might be to get some refractory powder mix and use that for the cob mix around the hottest areas.

Thanks very much. I'm in Nicaragua where dimensional lumber doesn't exist. When I return to the US I'll build a dorm for next year's trip. My mix is 3.5:1 but I don't know how pure my clay is. Is the J all that necessary? Won't an L work as well? The important part is the burn chamber and heat riser?

Attempt #1

Okay, you are making a purely cooking stove, and in a less technological setting. I presume this is a teaching demo, for the locals to improve their cooking equipment? The L-tube works wonderfully if well proportioned, but operates a bit differently than a J-tube. It is not self-feeding as the fuel burns down; you need to keep adjusting the sticks. It is appropriate for those accustomed to cook directly on a ground fire, as they typically squat and can easily feed the low fire entrance. Anyone used to standing to cook will have a much easier time with a J-tube set so the pot is 2-3' off the ground, as the fire is easily visible and reachable while standing.

My experience building cob kilns is that I can build up a foot of height without stopping. Your mix must be too wet - you can make it drier as long as a new lump firmly melds with an existing one. More sand might help too, depending on your current consistency. How much straw/dry grass are you using in the mix? That is important for wet strength, and as it burns out it also leaves tiny airspaces which have some insulation value. I would use a low-grass mix on the inner surfaces for dry strength, but as much grass as you can work with around it. I have found that fine dried grass in short pieces (1-2") gives wet strength yet lets the surfaces be smooth, unlike straw. When I mix the dry grass with the clay immediately before use making a very sticky mix, the grass absorbs enough water in a short time to stiffen up the build as I am working. If the walls are thick enough for the exterior to not get very hot in use, the grass may last for some time and act like rebar, making the stove less fragile.

In the absence of milled lumber, you can use anything that can be formed into the correct shape and wrapped in a smooth covering like big leaves. A bundle of crooked twigs on the inside might even leave enough airspace to let a fire burn it out, though I haven't tried that.

Thanks again for your input. My mix has no grass. I read else where not to use it. I tried using pine nettles but it made more difficult. You are correct I am trying to make this work to become a model. Nicaraguan stoves have no chimneys and the smoky homes cause lung infections in the youth. Last year I brought aluminum rocket stoves made in South Africa and they were a hit. Hence the idea to make them using local materials

I'm sure someone had a good reason for not using grass, but I have a dozen years of experience building cob kilns with grass temper and know that it makes construction easier and does not harm the finished product, in my part of the world. It is conceivable that materials somewhere else might alter this, but I think it's worth trying.


Pine needles that I am familiar with definitely do not help - they are too stiff, do not absorb water that well, and break or break down too easily. Leaves also do not help, as they create flat planes where the clay is not sticking to itself, and when the clay dries there is a weak spot.

I'm sure you already know this, but I think it's worth mentioning that the organic materials in the mix must be dry. Maybe the person who said not to use grass meant that you shouldn't use wet grass. Dry grass is very close to straw in its action within the cob mix.

Another possible reason someone would say not to use grass in cob might be that they meant grass as opposed to straw; grass breaks down much faster than straw and would not be suitable for cob building walls which can theoretically last centuries. I recently saw pictures of someone in Denmark who salvaged some cob/adobe bricks from remodeling his century-old house and recycled them into his RMH, and the straw was still in the bricks and still doing its job.

Thanks again. I tried adding dried bunch grass that I separated out,but I couldn't make it work. Do you have a picture showing how you use the grass? I have reached 2 feet high and I'm going to stop there. It already out of reach of most Nicaraguan women. It will be hard for them to cook.

Were you planning on it staying on that platform for use? Then it is definitely too high If they expect to stand to tend the cookpot, then a J-tube will be easier to use, and in either case, its base should be set on or near ground level. The J-tube cookstove I built had about a 3' tall riser, with cooktop 3' above floor level of the user, and the riser turned out to be taller than optimal. For cooking, you want the flame to reach to the top of the riser, and a slightly shorter riser would most likely have worked better. A 2' riser above ground level would probably be about optimal for heat, and easy to use assuming there will be a pot on top.

How thick is the bunchgrass, and how wide are the blades? How long did you cut it? Longer than a few inches makes it harder to work with.
I mix the clay and water first, getting it thoroughly workable and squishy, then sprinkle a handful of grass on it and fold, knead, stretch, etc. Repeat until it is a good consistency - practice! Make sure the grass is not in a clump when you add it, but each blade touches clay.

thomas newcomb wrote:Thanks again for your input. My mix has no grass. I read else where not to use it. I tried using pine nettles but it made more difficult. You are correct I am trying to make this work to become a model. Nicaraguan stoves have no chimneys and the smoky homes cause lung infections in the youth. Last year I brought aluminum rocket stoves made in South Africa and they were a hit. Hence the idea to make them using local materials

How did the aluminum ones work after a year? Are they still being enjoyed? Is the aluminum still in good condiion after one year of rocket-fires?

That cob looks dark, and you should perhaps use less sand if you don't know the clay content. The dry odganic fibers will help hold it together hopefully

Thanks again! I wish I had spoken to you before coming down. So my rocket stove book is concerned more with thermal heating but says to lay the straw end to end and it should be the length of my forearm. When I burned my 1st attempt stove it had only a foot of rise and it was very hard to start the fire and keep it going.

You are right. It is too high. This is a temp site. I hoped to build a base when I got one to work. I will let this one dry for 2 days before putting a fire inside it.

I leave for the US on Wednesday. Looks like my demonstrations will have to wait.

John I'm sorry to say the stoves aren't used regularly old habits are hard to break and the families l gave them to could use gas stoves if they wanted. The stoves are ok with small quantities like tortillas but not large clay pots of beans or rice. My efforts with cob stovess is to incorporate them into the mud stoves they are currently using

Long straw is excellent for strength in a finished cob structure (though irrelevant in an item that will be burned), but it is considerably harder to work with than short pieces. The straw should end up going in all directions to tie all parts of the structure together - you can't really control its direction anyway when globbing and manipulating lumps of cob into a shape. The cob I have worked with could safely be fired as soon as I could handle it. Pottery needs to be bone dry before firing, but for whatever reason, cob doesn't, in my experience. A superfine clay that dries hard and tight might not be porous enough to do this with. If yours has a fair amount of sand in it, naturally or added, I wouldn't worry at all.

Have you seen the adobe/mud cooking stove thread at Donkey's forum?
http://donkey32.proboards.com/thread/1301/all-adobe-mud-cookstoves-ovens
The rest of that forum has good info too.

Now that I have a working model, I will build one with grass mixed in. Two issues. It is hard keeping the fire going. Should the fire be in the burn chamber or the burn channel? How do you clean out the ashes? The heat riser is too long to reach into and the channel too narrow. I am building today's model a foot off the ground. Nicaraguan women are smaller than five feet tall.

thomas newcomb wrote:How do you clean out the ashes? The heat riser is too long to reach into and the channel too narrow.

My burn tunnel is rectangular and I can't reach the back of it because I'm using a J setup. So I got some scrap sheet metal and bent it into an L shape. It has a slight bend in the longer part so it can slide from the feed tube into the burn tunnel. I can just reach the back of the burn tunnel using this and can then scrape the ashes towards the feed tube, then use the shorter part as a ladle of sorts to bring the ashes up.

It does tend to cause ash to fly about, making the cabin get covered with soot over time. I've started using a vacuum and that helps a lot. But if yours are for outdoor cooking then that shouldn't be a problem.

I finally got the fire burning but it is a lot of work. Integral is a 18 inch 3/4 pipe I use like a bellows to force air on the fire. Flames reach two feet high the rim of the riser. But the stove requires constant attention. I hope this improves over time.

Thomas

Have a look at this video. I have watched hundreds of rocket stove videos and this seems to be more what you might be looking for. ignore the first few seconds but I am sure you will find the concept helpful

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that didnt work very well lol just copy and paste this in your browser [youtube]https://www.youtube.com/embed/ifpOiCV53FU[/youtube]

That would certainly be better than an open fire if the chimney were vented outside, but it would not approach the efficiency of a rocket stove.

Is your new rocket stove all dry yet? It will not come up to proper temperature or draft until it is. What is the inside diameter? Is it fairly constant? How smooth is it? You currently have an L-tube, right? That wants a sheet of metal under the stick feed so air can get to the back for best combustion, according to what I have seen. The one L-tube I have built was for an oven, and is 6" diameter with a slightly taller horizontal 20-24" long and a vertical about 30" long, with an exhaust chimney on top of the oven. It burns like crazy without the air channel beneath and consumes the wood completely.

I had a good meeting with some of the locals. They had some good suggestions. One was to use black ABS 4 inch pipe as inner form for both the riser and channel. All kinds of discussion over the clay to sand ratios. They lean toward more clay but they like the feel of my surface. My inside diameter is about 4 inches. Barely enough room to squeeze my hand into the firewood tunnel not enough to put my hand down the riser. IT is an L but I don't understand what you mean by metal under the stick feed. We kept the fire burning for about two hours. Don't know if it is completely dry yet. We like the heat at the riser top long after the fire went out.

thomas newcomb wrote: but I don't understand what you mean by metal under the stick feed.

Basically you divide your feed tube into 2 chambers with a piece of sheet metal or flat steel and place your fuel (wood) in the top half of the tube and allow air to flow through the bottom.

This one is steel but it will show you the idea

Glenn what is RMH? Am Re reading entire information. Rocket stove in central America may not be the best choice of words. Think rebels. My most recent version stove looks more like a boot. I'm thinking it may have more appeal as a boot stove. Estufe bota !

RMH = rocket mass heater - the space heating version of a rocket stove. It runs the exhaust through a heavy masonry or cob mass to store all the heat and give it off slowly over the next day or so.

When Ianto Evans was developing a version of rocket stove in (I think) Guatemala for a government agency, they had the same objection, and decided to call it "estufa Rocky", as the Rocky movies were popular then.