I camp. I forget my propane. I like my coffee. So I need a wood burning system that can boil water. I saw the concept of a rocket stove deployed using cinder blocks, but the idea of packing 70lbs of cement bricks seemed a bit over the top, So here is a rocket stove implemented using 2lbs of stuff: metal duct work, some scrap metal pieces and such. The whole thing fits in a milk crate and you can get a super hot fire going in about 10 minutes
Step 1: The BOM.
So I had a lot of material laying around and didn't have to buy it, so my build replaces some of the items with scrap I had laying around but I'll list what I would of bought to use for the stuff I didn't have to buy. What you need is
1) 6" diameter duct work 90° elbow
2) 6" right angle converter to 10"x6" rectangle duct boot
3) 60" duct cleat
4) a bunch of #8 nuts and bolts,
5) 2 8" runs of angle iron/ slotted angle (I had some left over aluminum angle iron)
6) plate steel thicker (13 gauge? I had some sheet steel)
7) expanded steel, 12"x36" total,
Again, improvise. I made a burn chamber from a piece of metal that was a computer fan cover instead of folding some expanded steel into an open box, use what you have from the leftover bin.
Step 2: The Basic Frame
the assemble of the basic frame is 10 minutes. attach the elbow to the boot such that the joints of the elbow give you a good range of motion. drill out from top and bottom two holes so that you can attach the two parts together at the overlapping metal. Then cut up the connector piece to make lips for the rectangular box and slide them over edges to give the rectangular opening some rigidity. Use nuts and bolts to attach the legs to the outside corners of the box. I left the bolts kinda long, like 1" bolts, so that I could use the the bolts as resting and anchor points for the internal components, but that is up to you.
When you've done this you have the frame not only is the rocket stove, but it conveniently has only 3 points of contact with the ground, that really shouldn't get too hot so you should be able to put it on the table. Also the rotating nature of the elbow allows you to level the cooking surface to whatever you need.
Step 3: Internal Details
As the duct work steel is not really made for contact with fire, I wasn't sure how well it would take the burning wood, so I made a burn tray and basket out of much thicker metal I had laying around. The bottom plate is just a layer to catch falling ash, and the top is a basket with lots of air flow through the bottom. I had a metal basket (the black one) that I used, had I not had that I would of folded up a basket from the expanded steel. this basket I want mostly level and as the boot is slanted I installed two longer #8 bolts on one side to put a tilt on the basket to level it. I also added a third bolt to attach it to the burn plate.
For the burn plate I first made one out of cardboard to get the fit right, and then copied the cardboard template onto the steel to cut it out .
Step 4: More Inner Workings.
the feed for wood and air is from the 6" elbow. I cut about a 5" width strip of expanded steel for the wood feed. as a result the bottom of the 6" diameter elbow will always be a clear path for the air flow. I have the 5" strip resting on the basket. All the parts can easily come out none of them are screwed in. Note how the screws of the legs stick out and pin down the burn chamber assembly.
Step 5: Grill Top
I thought this is a clever solution to securing the grill top. I made the grill top from a cut section of the expanded steel making sure I had triangles on the edges. I folded down 4 corners of the triangles just so that they would pinch the top of the duct boot. In this way the grill top is reasonably secured to the rocket stove.
Step 6: Make a Fire
some newspaper, wood pellets, twigs, and sticks. the newspaper and twigs caught almost instantly with a match. The wood pellets took some time. at that point you feed the bigger pieces through the elbow into the burn area. Note also I took a burning leaf to the edge of the elbow and you can see the airflow in the smoke.
Step 7: Some More Pics. and a Note About Galvanized Steel
Now there is a lot of warnings about using galvanized steel in stoves. Galvanizing is a process where the metal is infused with zinc, and when heated very hot, emits fumes that are dangerous. For my project I specifically used non-galvanized duct work, but my burn plates are galvanized metal. The thing is, the exact danger of the zinc fumes is debatable, and so is the temperature that creates the fumes. The big thing though is almost all the warnings about zinc fumes are based on indoor or poorly ventilated areas.
This stove is specifically for outdoor use, aka the very best ventilation. Also it is clear that after hot and several burns, the zinc is burned off. My advice is test fire your stove several times and get it nice and hot before using it for cooking or sitting around, and if it needs to be said, don't use indoors. Here is a quote from a discussion I read about this issue:
"I read a recent post which mentioned galvanized pipe and toxicity and realized this is a persistent myth in stove circles. I have a background as a chemist, my work includes machining and fabricating, and I think I know enough about galvanized to ease some fears and help dispel the myth.First, galvanized does not give off 'cyanide fumes'. It DOES give off zinc oxide fumes, which can cause metal fume fever. Metal fume fever is an immune-response condition that goes away in a day or two as your body absorbs the zinc. It's rotten, much like the flu, but it is rarely fatal, usually only when the exposure is extreme and the person has a pre-existing lung condition. It happens to welders all the time, almost considered to be a standard occupational hazard. Welders who work in a enclosed environment and don't wear protection develop resistance rapidly.Second, galvanized steel rarely gets hot enough to create said fumes except under conditions of welding, grinding or casting. During a stove test, I managed to oxidize 4' of my galvanized flue (it was under a fume hood, just in case). White oxides formed on the pipe but none was released into the air as fume. The fume is extremely visible when welding on galvanized material, as thick white smoke.Zinc boils at 907C, ZnO at 2360C, and rapid oxidation occurs at much lower temperatures near the melting point of zinc (420C). Once the zinc has been converted to oxide, it is safe. Your stove will never reach 2360C without forced air or oxy mix. I personally use a fully oxidized white zinc-coated elbow as part of a stove I operate indoors, and it is completely inert. I have never experienced metal fume fever myself as I always am sure to have good ventilation and a respirator if I weld galvanized material.I think galvanized is actually a better material for heat risers if you wish to use thin steel. The layer of zinc oxide will help protect the steel from oxygen and heat damage, I always use galvanized for heat risers in my small stoves and it seems to hold up very well. I always fire them gently at first to oxidize the zinc rather than melt it into a pool at the bottom of the riser. Any oxide fumes will find their way out the flue and not into anyone's lungs.Finally, galvanized steel is commonly used, within code, as an exhaust duct for gas appliances (go look at your hot water tank or furnace) and even to BUILD WOOD STOVES! Ex. www.rileystove.com/products/stoves/riley/flap-jack.html is constructed with 18ga galvanized steel! No Riley customers have died yet!"
Step 8: Cooking Test
complete success! made a 2 foot full fire from just some twigs, leaves, and pine needles and one match. Really burned the metal looking for the tell-tale white smoke of zinc, nothing. Time too cook up dinner. The full enviro-brick compressed sawdust wouldn't quite fit; I could of forced it but I felt that would smother the fire, so I cut it into 4 parts and decided to try cooking on just 2 parts, aka, 1/2 a brick.