Questions on building earthen rocket stoves for developing world

Hi, I'm trying to build some rocket stoves that can be used by the community living around me here in Zimbabwe. It seems that the ones made of tins or metal don't last long and I like the idea of making them completely of earthen materials. So two possibilities making unfired rocket stoves from a mix of clay, sand and sawdust for insulation, I was wondering about additives that can be used to stop cracking and making it last longer? I read on one website that potash water and sugary water can be added is this a good plan?
Also is it better to do one chimney of an insulated mix of clay and sawdust or is it better to have a chamber between two cylinders and fill with loose ash?
The other possibility is making more portable fired clay rocket stoves, but again what can be added to make the clay fire resistant and long lasting?
Is there a formula for the sizes of the openings and chimney that make it the most efficient?
Thanks

If you have access to straw, you can make an insulating cob that will have good strength too. The straw nearest the inside will char and leave air spaces, while the outer layers of straw cob will give strength to hold everything together. Depending on how big the stove is and how hot it is fired, you may actually get the inner layers to become pottery.

There are formulas for size and length, depending on the particular layout you are using. I don't have time now to go into detail, but others will be able to point you to resources.


You might want to try the sawdust cob inner layer with a straw cob outer layer. If you are making an L-shaped combustion chamber, you can make two forms for the inside that you can pull out when the cob is stiff enough to support itself (but not so stiff that it will crack). If you are making the more efficient J-tube combustion chamber, removable forms will be trickier. I have some ideas though... will post later when I have time.

Thank you for the help, I have made five different stoves of different sizes and different mixes to see which works best. They take a long time to dry in the shade to stop cracking so I have now tested 3 of them.
I made 4 L shaped stoves the two I have tested work very well, I'm waiting for the very thick walled one to dry to test and I need to make a base for it. I used sawdust in one and sieved straw from cow, donkey and elephant dung for insulation in the others.

I made one J tube stove where I used pipe to shape around, I cut a circle in the front to pull out the middle pipe and this now has a cover that serves as an ash dump. This one doesn't work as well as the L shaped stoves, I didn't know what the best sizes where for each section so I just tried it out. So maybe if I can get some information on the best ratio for each section I will try again. I have attached a photo.... Nope it's not working I will post photos and link to blog next week so that you can see what I built.

The J-tube design is best suited to larger sizes; anything smaller than 6" diameter equivalent is trickier to make work right. 4" is about the smallest that skilled or lucky builders have reported to work well.

The recommended J-tube proportions are 1:2:4 for vertical feed tube, horizontal burn tunnel and vertical heat riser. You can measure along centerlines or outside edges of the channels, whichever is easier for you. A J-tube is not likely to scale down enough to be easily portable, as the smallest I think would work well would be about 4" diameter, 8" feed, 16" burn tunnel and 32" riser. If you can make a smaller one work, excellent! And let us know with full details.

The idea of pulling the J-tube burn tunnel form out the front and having a closeable cleanout port is what I was thinking of, so good for you that you came up with it on your own.

Thanks for the dimensions will have to play around. I've posted some pics on my blog of the rocket stoves I have made so far. Really enjoying the L shaped ones they are very efficient.
http://dancelikeanelephant.blogspot.com
Will be making a few more and will see which work best

Zim, there's a thin grass, rather long, which is thiner than agricultural straw. Which we see on every documentary about eastern africa, rift valley, okavango part. theis may be would be nicer to use i think.

About as thin as the straw they make panama hats from.