Simple biomass burning "rocket stove"

Last summer our church went down to Ensenada to work with a local church in Colonia San Carlos to work with the children there by doing VBS for them, and some of the other nearby places like Santa Cuquila, and Mendoza.  This last place is a boarding habitat for farm laborers working in the greenhouses that produce high volume flowers, quite possibly for export into the United States.

What caught my eye is that they are still using "primitive" cooking methods, namely burning wood over a pan or pot, propped up on several rocks.  I love that!  Something very soothing about cooking on open fire.

While surfing on YouTube, I came across what people call "rocket stove," a simple fire chamber designed to pull in air and deliver high heat, not unlike the Dakota Fire Pit (Boy Scouts of America know what I am referring to).  Except it's not an actual rocket stove, because the real thing actually includes a gassifier that produces additional biogas from the initial biomass that is in combustion.

Yet one of the benefits of this simple stove, is that it yields higher temperatures with shorter cooking time and less fuel.  In this project I thought I'd try making the cement rocket stove, molded out of a simple bucket, using fiberglass reinforced crack resistant cement concrete and vermiculite, a gardening soil additive that according to the YouTube DYI content author Gene Lonergan turns your concrete into a kind of a "firebrick chimney."

The cost of the materials in the end is fairly economical, which makes for an affordable solution for a simple stove when you need to cook in primitive conditions with limited amounts of fuel.  In Mendoza, the folks from Oaxaca had nice stacks of fire wood ready for winter.  Provided you have all the tools and a good work space, it should take one person about two hours to complete the project.

List of Materials:

1. A bucket to be used for molding, $3 at Home Depot.  
2. 4" PVC sewer pipe, $11
3. A bag of cement $6 for 80lbs (yields 4)
4. A bag of vermiculite $21 for 2 cubic ft (may yield about 3)
5. a little bit of motor oil to lubricate the sewer pipes
6. some protective gear like mask, eye protection and gloves.

Lonergan says that 80% Vermiculite and 20% concrete is the ratio that you want, which is something I really welcome in the design because concrete alone is simply just too heavy!
If one is to reuse the mold to make more in the future, the design for the materials alone is under ten dollars.

Steps:

1. Cut the PVC pipe into a clean, 45 degree angle.  You need a 12 inch circular saw to do this in one step.  I didn't so I tried to get it cut at Home Depot, but they refuse to cut PVC with their saws, due to what they consider fire hazard (store policy).  I went into a store in Tempe called Woodworkers Source, and a nice gentleman in there heard my plans and he cut it for me, except he didn't have a blade large enough to make the full cut.  I cut the rest with a hacksaw I bought a while back at a 99 Cent Store.  
2. Now you need to cut out a round hole in the bucket where the fuel chamber will seat.  A perfectly round cut is desirable, but it's tricky.  This is where your handiwork skills will come into the test.  I used a knife, being that the Home Depot buckets are of fairly pliable plastic.
3. You mix the vermiculite and the cement in the ratio of 80/20, and I was working in small batches, so as far as water goes, I put in 3 1/2 cups of water per 226 cubic inches (4 x 40oz can) per 1 can of cement.  (I think I mixed about ten batches, lost count).  The mixture should be moist but not soupy.  Too much water will yield a fragile end product.  This initial attempt is quite a lot of guesswork, so this is the part where I am hoping to God that the estimate yields a strong structure.
4. At any rate, you have to tamp the contents, packing them down well.  Gravity will do the other half of the work for you, but at each time, make sure that the pipe ends are seated correctly.  
5. After you fill it to the brim, let it sit there for about one hour.  In my case some excess water was draining out of the bottom, between the pipe and the bucket.  (a watertight seal is not necessary)  It is at this time, you can remove the pipes for later reuse.  I removed the top pipe, and waited a little longer for the bottom.
6. After the stove dries, you are to cure it while sealed in a trash bag for one week, and then let it cure for an additional three weeks before its first use.  (I don't quite understand the reasoning behind this long curing time, but I think that it will be ready just in time for our mission outing to Ensenada in mid December)