This little low-tech wonder of high efficiency cooking was developed by an American gentleman, Dr Larry Winiarski for use in the third world to reduce firewood usage. I have also seen them referred to as right-angle stoves.
I had been aware of them for some time but made a critical error in understanding how they worked and only recently realised their true value and made one for us to use. They are ideal heat sources when the sun doesn’t shine and use sustainable, renewable biomass (wood).
In principle you take a right angle tube about 100mm in diameter or 125mm square and insulate the firebox heavily so that head from the fuel goes into cooking your tucker rather than heating up the stove. You can use thin wall steel for your right angle tube like tin cans or galvanised stove pipe, I used a bit thicker wall tube because it was what I had lying around, but also it will last longer. You can use a mixture of clay and sawdust to make insulating bricks to construct the right angle combustion chamber but you have to work out a way to fire them. By and large the steel combustion chamber works for me!
Construction details
The outer casing of the stove is easy to build out of a recycled 20 litre tin which, again, is what I had hanging around, I also had the 90mm steel tube and some ashes hanging around to use as insulation. I could build this thing for virtually nothing!
I cut a roughly 90mm hole about 35-40mm in the side of the tin about 35-40mm up from the bottom and another 90mm hole in the centre of the top. I did this by first using a set of steel dividers to scribe where the holes would be, then using an electric drill and a 2mm or so drill (all the marking get worn off my drills, comes from not tightening up the chuck enough) holes inside the circumference of the scribed circle. By jiggling and angling the drill back and forth you can sever the small webs between the holes. Then using a round file and ball pein hammer you can clean up the hole so it actually looks round! Due to the curve of the tin the hole in the side will need to be a bit wider than the scribed circle but a bit of extra filing and a bit of tapping with your ball pein will sort it out. Try your tube for fit often to make sure you don’t go too far.
That part was relatively straight forward except that I chose a Sydney heat wave day when it was 45ºC in the garage and I was sweating like a pig! I then had to cut the pipe on an angle so that when I turned the pieces around they met at a neat right angle. I figured I would do this the same as I would for a bit of square timber by marking off the section to cut with two parallel lines around the tube 90mm apart and a then marked an angle around the tube between the two lines so that I had a line at 45º around the tube. I then cut this line using an angle grinder (my favourite tool!) fitted with a metal cutting disk. This should have meant that when I turned the two cut ends of the tube around, they would meet at a neat 45º. Did I ever tell you that I don’t understand the 3 dimensional geometry of tubes? The cut surfaces had a really weird profile and lots of open space, but with my trusty angle grinder, this time fitted with a grinding disk (did I mention it is my favourite tool?) I tapered the angle off to something closer to 40º and the fit was reasonable.
My next trick was to weld the tube in the right-angle position. Ummm I’m not as good a welder as I thought I was either, but with the arc welder on 70 amps and with a couple of 1.6mm rods I managed it with a minimum of blow-throughs and curse words. It don’t look pretty but it will hold together and be a sealed joint. If I had had an oxy-acetylene set I would probably have done a better job, but I didn’t so there you go.
All of this just goes to illustrate why people buy an already formed right-angle section of stove pipe, so they don’t have to go stuffing around like I did to sort out that part of the design. All in all to cut and weld the tube probably only took me about 4 hours…………….easy! If I was doing it again I would probably make a tin-can or paper mock up first so that I could work in some easier material and get the real thing right first time. I’m a bugger for not doing that though!
I hunted through my stock of scrap metal and came up with a length of 6mm steel strap. It was somewhat thicker than I was looking for but would do the job and the width and length looked good. It had a couple of angles welded to it but judicious application of my favourite tool and they were gone. I then tack welded this piece into the bottom of the combustion chamber so that the fuel would sit on this plate but as the fuel burned, air would be drawn in underneath to ensure that there was efficient combustion. The plate should sit in the horizontal fuel chamber but not protrude into the vertical combustion chamber.
I then installed the right-angled tube into the supporting steel 20 litre container and poured in the ashes that I was going to use as insulation. I had been saving ashes from our two wood burning heaters for use in the garden but in keeping with the ethic of using stuff that I had on hand, in they went. You could use perlite or vermiculite, anything that would stand up to the heat and contain trapped air to act as an insulator. Sand and earth etc. are good thermal mass, ie they absorb and retain heat but are not good insulators so should not be used in this case.
The next thing is to set up some chocks for the cooking pot, wok or whatever to support it the right distance above the hot gases coming out of the stove chimney. To make sure that as much of the hot gases as possible are used, the pot should be arranged so that the area between the end of the chimney and the bottom of the cooking pot is the same as the cross sectional area of the chimney itself.
I use ours quite a lot with our wok for stir fries, omelettes and the like and find that the commercial wok ring that I use to support the wok over the output of the combustion chamber works really well. I have a cast iron square fry pan sort of thing I use with it for barbeques and that works well with the wok ring too.
Fuel
One of the reasons that I have taken such a shine to the rocket stove is that around where I live the parks and roadsides and park like areas have lots of tall gum trees. There trees seem to drop a constant stream of twigs and small branches which accumulate until they are hit by the council mower. These sticks are an ideal size for use in the stove, plentiful, sustainable and free but any small sticks such as cut up pallet wood or other waste timber could be used.
Infromation about how I cook with my rocket stove can be found here and details on constructing a smaller and larger rocket stove are also available.
