Planning and building the Rocket Stove BBQ Grill prototype

The rocket stove in it’s purest form, is simply a high temperature combustion chamber and chimney through which a lot of air and a little fuel burn in an efficient manner.  The stove itself is this combustion chamber.  What you decide to do with the heat after it comes out of the chimney is extremely flexible and it depends upon how you direct and purpose the heat produced in the combustion chamber.

In the image below, you can see the rocket stove portion of my project.  I have made mine using two 6″ diameter stove pipe elbows to create the combustion chamber, and then an additional section of stove pipe for the chimney.

Rocket Stove basic element
Two elbows and a section of 6″ stove pipe make up the heart of the stove rocket stove.

Now, let me step back a bit and explain why I am doing this because frankly I spent quite a bit of time learning about rocket stoves.  The easiest way is probably to view this video.

So through the spring and summer of 2015, I learned about and tested the concepts of the rocket stove.  I wrote about the 4 brick rocket stove, and I wrote about the importance of the chimney in the rocket stove.  After those experiments I moved forward with the chimney, experimenting to come up with the combustion chamber and chimney shown in the first image above.  As you saw from the video, my motivation is to build a new  BBQ grill.  But if you know me, you know the straight and simple path is never the one I take.

My first thought is that if I just slap a grill on top of the chimney, it is going to be so hot that I just scorch anything that I put on that grill.  Take it from me, it gets really hot on top of that chimney.  So, my next thought was that I wanted to honor the meat I was cooking, so I would divert the heat so that it wasn’t just scorched meat, but something you would enjoy eating.

The cool thing about this rocket stove stuff, is that after the heat comes out of the chimney, it has to go somewhere.  It can go up, or it can go down, sideways or just about anywhere you direct it, but it has to go somewhere.  The whole idea of the rocket stove is that there needs to be a lot of hot air moving through the system.  That means that you can get creative, and that is what I did. In fact, every time I go out to use the grill, I end up coming up with a new configuration for my grill based upon what I am trying to do.  But that is another post.

So in the next video I give you an idea of what I have done as I fire up the indirect heat layout of my rocket stove grill for the first time.

Now we have fire going in the stove.  It will take a few minutes to warm up the combustion chamber and start really drawing the air in to get it to the point where it gets the roaring sound from which it gets the name rocket stove.  While we are waiting, lets take a look at some of the design elements that I have built into this stove that gives it the ability to cook with indirect heat, but also a look forward into the many options that you can bring to this kind of project.

One of the things you will notice is that I talk about needing to insulate the chimney and combustion chamber.  This is to help get the combustion chamber really hot without losing heat to the surrounding infrastructure.  In the literature I have read, they often talk about using Perlite.   I bought the only Perlite I could find, which was infused with Miracle Grow.  I was a little leery about that, but this was a prototype so I bought one bag and gave it a try.

Hot Dog cooked.
To test it, I threw some hot dogs on. It took a while, with hot dogs it might be better to use direct heat. But, it was successful.

I poured the Perlite down along the chimney and it settled to the bottom and down around the combustion chamber.  Then I fired it up and let it get hot.  I don’t know if it was my imagination, or if there was a strange smoky smell coming from where the Perlite met the chamber.  I didn’t want to take a chance of getting that into what I was cooking, so I made some home made insulation.  I took the ashes from my fire pit, and I sifted them to remove any debris.  Then I packed the granulated ashes in around the bottom and top of the combustion chamber and chimney to seal off the Perlite and finish the insulation process.  In the picture below you can see me adding the granulated ashes around the air and fuel intake portion of the stove.  This effectively ended the problem with the smell/smoke coming from around the chimney.

insulating the combustion chamber
In this image you can see the perlite surrounding the combustion chamber, and old ashes are being added to finish the process and seal off the perlite
Insulated chimney
Sifted / granulated ashes from fire pit used to fill in space around chimney as insulation.

With the stove now put together and properly insulated, it is time to put it to a real test.  In the next post, I will go into how I configured the system to serve as both a grill and cooked using a rotisserie simultaneously.  The imagination and how you will move the bricks to put heat on meat and vegetables is the only limit to how you can use the heat from this stove.

Learning about the Rocket Stove

Burn chamber view

I have been experimenting with Rocket Stoves in an effort to build a rocket stove BBQ grill.  Well actually, I want to use it for a bunch of different types of outdoor cooking.  This post will be the first in a series of posts that walk you through my experimenting with the rocket stove.

A rocket stove is an efficient cooking stove using small diameter wood fuel which is burned in a simple high-temperature combustion chamber containing a vertical chimney, which ensures almost complete combustion prior to the flames reaching the cooking surface. Wikipedia

My ultimate goal is to build a permanent installation with a rocket stove as the heat source, but with a number of different options for cooking.  These options could include a BBQ grill, indirect heat grill, rotisserie, burner, and maybe even a pizza or bread oven.  To do this, I am going to have to start small and build my personal knowledge.  I have done a lot of research along the way, and looked at a lot of other people’s designs.  If you are interested, the Design Principles at rocketstove.org is a good place to begin.   I begin with a 4 brick stove that I will discuss in this article, and end up with a relatively flexible system that allows me to adjust bricks to customize the way I use the heat.  Along the way, I will have a handful of articles where I will share what I have done and learned.  I hope you enjoy.

Four Brick Basic Rocket Stove

Let’s start this out by saying that I did not have a budget for experimenting with rocket stoves.  That doesn’t mean I had an unlimited budget, it means I had no budget.  Most of what I do is done with bricks and cinder blocks (concrete blocks) I had around the house.

The most basic design can be built using 4 concrete blocks.  It is a basic design that requires either an “H” block or, as I did, knock one end off of one of the blocks.

4 brick rocket stove design
This shows the basic 4 brick stove design.

The first brick is simply there as a base.  I placed it so the air chambers went from side to side.  On top of this, I placed another brick that I customized by knocking the end off so that I could create a right angle for the burn chamber.  In front of the first two bricks, another brick is placed standing on end so that the openings go from the front, forming the burn chamber with the section of the second brick with the end knocked off.  The fourth brick is used to create the chimney above the burn chamber.  In the picture I have it overlapping the second and third bricks.  I did this so that it stabilized all the bricks together.

Basic chamber view
The top opening of the brick standing on end creates the opening for the wood and entrance for the burn chamber.

In the image above, you can see the burn chamber.  The  wood is placed through the top hole in the brick placed on end and into the second brick through the space where the end was knocked off that brick.  From there, the heat and smoke go straight up the chimney and out the top of the stove.  In some of the things I have read and seen, they use a shelf that the wood sits on, and allows the air to go underneath the wood into the burn chamber.

Top front view
This image shows the stove with the additional air chamber installed.

In some of the things I have read and seen, they use a shelf that the wood sits on, and allows the air to go underneath the wood into the burn chamber.  This image and the next show this kind of design.  From what I have seen, there is some debate about whether this is the best way to do it.  I haven’t taken a scientific enough approach to this to be able to answer that question.  The important thing is that the air needs to have plenty of room to come into the burn chamber, past the ends of the wood that are burning in the burn chamber, then out through the chimney.

The key is to have enough air going through the burn chamber and chimney to burn very hot, and in the process burn the smoke so that almost no smoke ever comes out of the top of the chimney.  Again, there are a lot of opinions about the best way to do this, but the things that appear to be agreed on are that the burn chamber needs to get hot, and the fire needs to go up into a chimney and burn through the chimney.  In the best designs, most or all of the smoke will be burned, and that is where the efficiency comes from.  Smoke coming out the chimney represents wasted energy since it did not burn.

Burn chamber view
This shows how the shelf allows the air to come in under the wood.

From what I have learned, this L shaped burn chamber with the air coming in under the shelf gives the person running the stove the greatest control of the burn, and subsequently also is the most efficient.  The draw back is that the wood has to be constantly shifted to keep fresh wood in the burn zone.  I think overall, that is one of the greatest issues with the rocket stove.  Not a problem, just something that needs to be considered when you are designing your stove.

Fire coming from chimney
This one shows the fire coming out of the chimney.

One of the things that I found with the four brick stove is that it is hard to get the burn chamber hot enough to get to a point of efficiency.  In the next article, I will start working with the use of a tube inside the chimney to minimize the amount of block being heated up, and hopefully increasing the efficiency of the unit.

So, what I learned here, is that the most basic form of rocket stove will work.  I did not try using it for anything, but this model would be relatively limited, perhaps the best use would be for a burner.  It wouldn’t be big enough for grilling. In a later article I will build a prototype which will be  built around a J shaped intake, combustion chamber, and chimney.  In the next posting, we take a look at the role of the chimney.