The Walden Effect: Farming, simple living, permaculture, and invention.

Rocket stove materials

Roland's drawing of a rocket stove which preheats combustion airA few of you were as intrigued by the rocket stove concept as I was, and Roland's comments sent me searching the web for more information.  Basically, I wanted to know if I could design a slightly modified rocket stove made out of found/bought materials to simplify construction.  I was also interested in any updates to the design that might maximize efficiency.

Preheating the combustion air

The drawing shown here is Roland's suggestion for preheating the combustion air to increase efficiency, in much the way that efficient space-heating wood stoves work.  A search of the web turns up contradictory pages --- folks who have tried similar methods are split on whether it increases efficiency or not.  Many sites suggest that the conventional design already preheats the combustion air by passing the air intake underneath the burning fire, so I think I'll stick with that.


Insulating the burning chamber is another important factor in rocket stove efficiency.  The official Aprovecho design calls for making your own fire bricks, which are rated at about R10 when fully assembled.  Roland's suggestion --- perlite --- has an R-value of 2.7 per inch, so four inches of loose-filled perlite placed between an inner and an outer wall could be a much easier option than making our own fire brick.  (For future reference, other folks mention using materials such as vermiculite (R2.08 per inch) and pumice (R2 per inch).)
Modified rocket stove

Body materials

I've seen various DIY rocket stove options using found or bought materials, and the ones that caught my eye used nested stove pipe.  The image shown here is my revised version of the official design made out of one big stove pipe, two pieces of smaller stovepipe, and an elbow to connect the smaller stovepipe pieces together.  As Roland mentioned, the bigger stovepipe might be replaced by a metal bucket --- otherwise, I'd have to add some kind of cap to keep the perlite from coming out the bottom.  I'm envisioning the pot sitting on pieces of rebar stuck through the exterior walls rather than welding anything together.

There's a bit of math involved in deciding how high the interior chamber should be and how much air space should be left between the pot and the skirt -- more on that later!

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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.

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Looks like a simple and elegant design. Much easier than making firebricks, in any case. :-) One detail; It might be worthwile make the contact area between the outer mantle and the horizontal pipe as small as possible. Otherwise you'd lose heat to the mantle through conduction.

As for the skirt, why not make it out of a piece of sheet metal rolled into an overlapping cilinder, with holes (like in a belt) for the rebar in different places so you can adjust it to different sizes? That way you can experiment with the air gap and use it for different size pots.

Comment by Roland_Smith Mon Feb 8 17:16:39 2010
Very good point about both the heat conduction and the flashing skirt. I was wondering how I would be able to play with the air gap to get the right draw, which seems to be one of the things that takes a bit of tweaking to get right.
Comment by anna Mon Feb 8 19:04:04 2010

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