Take one of the four soup cans (soup can #1) and trace the shape onto No.10 can. I cut my hole slightly above the bottom of the can to make it easier to avoid dealing with the bottom of the can when cutting and shaping the hole. Seems to have worked out fine.
I then punched a number of holes into the circle so I could get the tin snips in there to cut the circle out. Punching that many holes turned out to be a waste of time as a single hole would have been fine. The tin snips cut through the can quite easily.
I used a pair of needle nose pliers to bend the edges of the hole back to get the final shape, but in retrospect I would have probably just relied on the sharpie outline and cut the hole wider to begin with. I was somewhat conservative when cutting and widening the hole took some effort. Later can fittings were made easier by just cutting the hole to the right size to begin with. I was afraid of cutting too big to begin with and therefor spent a considerable amount of time fidgeting with it.
Step 2: Mark and cut hole in soup can #1
You are going to need to have a hole cut at the same height in soup can #1 so that a later soup can (soup can #2) will go through the No.10 can and directly into soup can #1 to form an elbow of sorts.
To do this, I placed my first soup can into the No.10 can and snugged it up next to the hole I had cut in the No.10 can. I then took a Sharpie pen and traced the hole I had already cut in the No.10 can onto the soup can. The photo is hard to make out but it is a picture of the soup can inside the No.10 can with a faint black marker outline of the hole on the soup can.
Pro tip: Make sure you have fitted a can through the No.10 can hole to make sure it fits through cleanly but somewhat snug. you don't want any big gaps, but you also don't want to trace a hole onto the soup can that is already to small to fit the next soup can you will be inserting into it.
Pull the soup can out of the No.10 can and cut out the shape drawn on it. I punched a starter hole with the hammer and nail again. I cut the shape and then rounded the edges back a bit for a snug fit
Step 3: Cut bottom off of Soup Can #2 and fit as elbow
Take your second soup can and remove the bottom. You can do this with a can opener if you want, but I just used my tin snips. It was actually faster this way. The bottom of these cans is not like the top anymore so they don't open well with a can opener.
This (soup can #2) will be the horizontal burn chamber of the Rocket Stove.
You will want to "Dry Fit" the first two soup cans together to make sure they will fit once you assemble them inside the No.10 can. Once they fit well you can assemble these two cans inside the No.10 can.
Soup can #1 with the hole you cut into it should be placed into the center of the No.10 can with the hole in the soup can lining up with the hole in the No.10 can. The open end of soup can #1 should be facing up so that you can insert soup can #3 into it as the chimney in a later step.
Soup can #2 should go through the hole in the No.10 can and into the hole in soup can #1 to form an elbow of sorts.
Step 4: Cut and fit the Chimney from Soup Can #3
Take your third soup can (soup can #3) and remove the top and bottom and then slit it vertically from top to bottom. You will fit this as the chimney stack, inserting it into soup can #1 that is sitting inside the No.10 can.
I found that I needed to slice off about a quarter of an inch worth of excess can in order to crimp it down small enough to neatly fit inside of soup can #1. Seemed like a lot to take off just to squeeze down inside the circle of the other soup can, but it did.
I also would recommend cutting a small arch into this can where it intersects soup can #2 coming in from the side. Otherwise the chimney will be too tall when finished. I do not have a picture of this, but when I first put it all together I noticed that the chimney stuck up too high for placing anything on top. It needs to be about a quarter inch below the top of the rim of the No.10 can for proper use. I removed my chimney and cut the arch and it fit perfectly. You could just cut the can down a little and leave the arch out. It would still work fine as long as you have the proper height of the chimney in relation to the rim of the No. 10 can.
Fit the chimney properly and you are ready to move on.
Step 5: Cut a hole in the lid of the No 10. can for the chimney
Take the chimney (soup can #3) and trace the shape onto the lid of the No. 10 can. You will want to cut it out so that the chimney can pokes through the top of the stove.
This is fairly straight forward. To punch the starter hole however, it is a good idea to place the lid on a piece of wood to give the nail something to drive into. Doing it on a hard surface does not allow the nail to penetrate and doing it on dirt will bend the lid. I used a scrap piece of lumber.
Snip the hole to size and test fit the chimney.
Step 6: Assemble, Fill with insulation material, seal the lid.
With all of the parts for the stove cut and fitted, you will want to assemble the burn chamber (Soup Can #1) and the elbow (Soup Can #2) and chimney (Soup Can #3) inside the No.10 can.
Once assembled, you will want to fill the space left over with an insulator. I used Perlite, but any insulator will work fine. You can use sand, clay, foam insulation, vermiculite, ash, anything with good insulative properties. I use Perlite, in my garden, so I had it on-hand. It is basically the same sort of tiny foam balls you find inside a bean bag chair, but I don't think that is what is used inside a bean bag chair. It just looks like them. Anyway... you can get it at any home improvement store or garden center.
Fill the area somewhat slowly, taking time to tap and rattle the can so the insulation settles into the crevices around the soup cans. Not too much though as you don't want to jar the assembly apart. Just a gentle rapping on the outside of the No.10 can should do the trick.
Fill the No.10 can to about one half inch below the rim.
You will want to use your tin snips to cut vertical slits in the No. 10 can from the top rim down about one half inch. This will create tabs that you can then fold down to hold the lid in place. I cut 8 slits so that I had four tabs that folded down and four that stayed upright. The four that fold down hold the lid down and the four that are upright will hold the cooking vessel above the flame.
Once the can is filled with insulation, place the lid on top and fold down the tabs.
You can see from my picture where this first time, my chimney is too tall and sticks up above the height of the vertical tabs. I took it apart and cut down the chimney so that it stuck up above the lid about a quarter of an inch. That leaves a gap between the opening of the chimney and the top of the No.10 can tabs.
Your stove is assembled.
We just need to cut out a small fuel shelf and you'll be ready to fire it up.
Step 7: Make a fuel shelf from Soup Can #4
With the final soup can (soup can #4) you will want to make a small shelf that will fit into the mouth of your stove. This is what the wood will rest on and at the same time allow air to travel into the stove from the bottom.
You will want to cut a "T" shape that is roughly the same width as soup can # 1 with a little wider portion at the top to keep it from sliding all the way inside the stove.
It is easier to see in the photos than to describe it in words.
I will admit that while I originally used a fourth standard soup can, I found it to be inadequate for holding the fuel. I resorted to cutting up a larger can (Chunky sized) so that my fuel shelf was longer and protruded out of the can further. This allowed the weight of the wood that was sticking out of the can to rest comfortably on the shelf without tipping it up and dumping the fuel onto the ground. You will see in the finished version what I mean.
Cut the can to size and slip it into the mouth of the stove. You should have a gap at the top and at the bottom. Without the gap at the bottom the rocket stove will not work.
Step 8: Light that bad-boy up!
Caution: While at first the stove is cool enough to hold in your hand, the outside does heat up and should not be touched with your bare hands. You will be severely burned.
To get the stove going I used some wadded up pieces of regular printer paper shoved into the chimney and lit on fire. This worked fine but I found that the stove really performed well when I used a little bit of paper and also placed a piece of wood down the chimney at the beginning.
The whole reason this system works is due to the thermal differences between the chimney (HOT) and the fuel shelf (COOL).
Getting the whole chimney nice and hot made it work right off the bat. On a second burn I had forgotten this step and it would not hold a flame. Once I put a little fuel down the chimney it worked VERY well. Once the fuel in the chimney burned down low enough then the fuel from the side was all that was needed to maintain a very hot flame.
It burns with little smoke (once lit and burning) and uses very little fuel. I used standard wood shims from a home improvement store that I got for under $2 for about 25 of them. They burn really easily. I broke them in half and they fit well on the fuel shelf.
I cooked some canned chili on the stove (see pictures) with only two of the shims for fuel and probably could have done it with just one.
You will see in the photos that I had a first burn with the tall chimney and small fuel shelf. The photos at the end where I am cooking the chili have the lowered chimney and the larger fuel shelf. I hope those details are not lost in the pictures as they really did make a difference in how well it worked overall.
From start to finish once I had all the cans and tools, it took me roughly an hour to cut, assemble and have my first fire. The whole thing only weighs about a pound or so. It's small and compact. It gives off a lot of heat with very little smoke. Uses little fuel, easy to get lit and easy to put out. My only caution would be that the outside does get hot so you would want to be careful what you put it on and not to touch it once it is burning.
Also, though I did not wear any gloves, nor did I get a single nick or cut, I could see how that could happen easily if you are not careful. A small file to smooth the edges of the cut tin as well as some gloves would not be bad idea. I just didn't really need them myself.
I think this would make an excellent Boy Scouts project or, if you are like me, just a fun little weekend activity with a nice little functional result.
I think a future modification will be to punch a couple of holes in two of the upright tabs and use a wire handle from an old paint can to carry it around with.