Ideal construction of a rocket stove for cooking?

Hi all, I need some advice on constructing a rocket stove for cooking. This would be for high heat and fast cooking, so fuel efficiency is not a concern. I have a lot of scrap wood to get through.

1. I would like to build one that is self feeding, is a K type the best solution?

2. What are the ideal dimensions or ratio of dimensions?

3. Is making the air inlet narrower than the burn tube to increase the air velocity a good idea, or should it be constructed out of the same size material?

4. I have 150mm/ 6" box section steel lying around, I thought it would be a good idea to use this.

5. Are there any design considerations worth making?

6. Anything else I should know before attempting this?

Many thanks for your help.

Hi Ray;
Welcome to Permies!
I'll try to answer your questions for you.
1) No, a J-tube design is better, less prone to burning backward up the feed tube, than a K-design.
2) 6x6" feed tube 16" tall, a 6x6" burn tunnel with a 12" roof, and a 6x6" riser no less than 36" tall.
3) All internal sizing needs to remain the same dia.
4)Steel can be used but will quickly degrade, clay bricks work much better and firebricks are the brick of choice if you can get them.
5 & 6)  We have a poster here at Permies that specializes in building Rocket cookers, his name is Fox James.
Here is a link to one of his posts  https://permies.com/t/226771/messing-vermiculite
If you click on his name it will take you to his profile and then click on the number of posts under his name, there you will find a list of all his posts.
Fox experiments and builds some outstanding cooking stoves, I am sure you will learn much by reading his posts.

Unless your scrap wood is all straight and smooth, fuel can easily hang up on the edge of a slanted feed tube. So lumber yes, sticks no. A J-tube has less of an issue with this.

To get good combustion with less smoke (desirable regardless of efficiency concerns), you want the flame path long enough that big flames don't reach the top of the riser. A J-tube has a horizontal burn tunnel which increases the flame path for a given riser top height over a K-style. It also has more sharp changes in direction which increase beneficial turbulence.

For J-tube dimensions, I like a 1:1.5:3 ratio measured along the outside edge for feed tube, burn tunnel and riser. You could probably make the feed a bit taller and burn tunnel a bit shorter for cookstove use.

As a long steady burning unit, I like the L-tube style. It can accommodate long fuel completely within the burn tunnel so no danger of it hanging up or falling out, and no smokeback risk. I have used about 30" length and rise.

I would not make the air inlet, if that is somehow separate from wood feed, smaller than the combustion zone to try to get a jet of air. You probably want air restricted some at the entry so there is not a huge amount of excess air which will just cool the fire.

thomas rubino wrote:Hi Ray;
Welcome to Permies!
I'll try to answer your questions for you.
1) No, a J-tube design is better, less prone to burning backward up the feed tube, than a K-design.
2) 6x6" feed tube 16" tall, a 6x6" burn tunnel with a 12" roof, and a 6x6" riser no less than 36" tall.
3) All internal sizing needs to remain the same dia.
4)Steel can be used but will quickly degrade, clay bricks work much better and firebricks are the brick of choice if you can get them.
5 & 6)  We have a poster here at Permies that specializes in building Rocket cookers, his name is Fox James.
Here is a link to one of his posts  https://permies.com/t/226771/messing-vermiculite
If you click on his name it will take you to his profile and then click on the number of posts under his name, there you will find a list of all his posts.
Fox experiments and builds some outstanding cooking stoves, I am sure you will learn much by reading his posts.

Hi thanks!

I've also read that a J style is better, but K style seems to be more popular. Why is that?

I have 6" and 4" steel box section lying around so it will cost me nothing to build. It won't see daily use, so I have no worries about it degrading.

I've had a quick look at Fox James. It appears that he has used building materials other than steel box section. I don't know if that's relevant? I'll give it a more thorough look later.

Glenn Herbert wrote:Unless your scrap wood is all straight and smooth, fuel can easily hang up on the edge of a slanted feed tube. So lumber yes, sticks no. A J-tube has less of an issue with this.

To get good combustion with less smoke (desirable regardless of efficiency concerns), you want the flame path long enough that big flames don't reach the top of the riser. A J-tube has a horizontal burn tunnel which increases the flame path for a given riser top height over a K-style. It also has more sharp changes in direction which increase beneficial turbulence.

For J-tube dimensions, I like a 1:1.5:3 ratio measured along the outside edge for feed tube, burn tunnel and riser. You could probably make the feed a bit taller and burn tunnel a bit shorter for cookstove use.

As a long steady burning unit, I like the L-tube style. It can accommodate long fuel completely within the burn tunnel so no danger of it hanging up or falling out, and no smokeback risk. I have used about 30" length and rise.

I would not make the air inlet, if that is somehow separate from wood feed, smaller than the combustion zone to try to get a jet of air. You probably want air restricted some at the entry so there is not a huge amount of excess air which will just cool the fire.

It's all timber, not branches and sticks.

Why an L style over a K style? Is K style not favourable just because of smokeback? Would having a lid covering the feed tube solve that?

Hey Ray;
I do not know why some believe the K -style is preferable, perhaps it is similar to reinventing the wheel.
Plenty of folks have built using steel tubing and as a cooking stove, it does the job.
You will want to insulate your steel to keep the heat going up the riser, rather than radiating away off of the tubing.
The bottom line is you can make anything work if needed.

Fox does not use metal at all in his stoves and after reading some of his posts you will see why... he has progressed into making fully functional long-lasting works of art.
His work is an example of how a simple rocket cooker can be improved.

Ray,
 Tom has given or noted some excellent advice.. But just to help along, Lets ask a few more questions,

Is your intention for the stove to be a "one pot" type, or having a cooking surface for several?

As main stove?

Being new, do you know what/why steel can kinda work for awhile, but not really long term?  (a clue here is the spalling that occurs in the riser, IF YOU truly have it working well)

There are specs for these stoves fully developed and sized, have you found those yet?

Lastly, and this is just personal advice that every new builder should at least, know about.   When you have at hand, a known working dimension, that is what it truly is.  KNOWN to work and generally work well.    In your research, you will often find, those that started building and decided to make "just one change"  which in turn often makes several more changes necessary and often precludes a complete tear down to start over.

Remember, if you keep with the known internal dimensions, quite often you can alter the external dimension to your specs (cosmetic), but the heart is there and ready to beat to your cooking drum.

Best of success.   And keep firing questions, no bad ones out there.