Wanting to build my first rocket stove that I designed.

I want to build my own design of rocket stove with some ideas I came up with. He is the basic plan. The Idea is to build a stove that will burn a small fire many hours very efficiently. It will use some heavy wall pipe and a 55 gal. drum as well as 8" stovepipe. It will also use a fan to help create a draft to get the fire started as well as dampeners to control the burn and turn the fan air off. Looking for any ideas on if there is something I missed or suggestions on how to improve this idea.  I plan to build a small scale working model this summer first to test it out before a make the large scale one in my home. I don't have any sources of clay near my home. So I was thinking of using concrete, but not sure how well it would handle the heat.

Hi Rick, and welcome to permies.

Interesting concept....Have you looked at some tried and true designs for a rocket mass heater? There are some things about this one that deviate from the well-known standards and could cause you a bit of trouble:

1)  That long horizontal run of the flue is not a good idea. The fewer bends and departures from vertical in your chimney the better it will draw. Ideally, you'd go out the back of the mass and straight up through the roof. You probably won't need the fan if you build everything according to fully tested and debugged plans.

2) No obstructions in the flue. If you choke down the draft near the end, you're creating conditions for smokeback and reverse drafting. We want clear pathways and as little friction as possible for the exhaust gases to keep things flowing.

3) Concrete will work away from the combustion zone, but it's really hard to tweak if you build it, run it for a while, and decide you want to change something. Cob is nice for this because you can break it up and reuse it. Consider a stratification chamber (a bell) instead of the piped mass.

Cob does not need a huge amount of clay. Just enough to make it sticky. Our soil here is mostly silt with about 10% clay and it makes brilliant cob. You can get a bag of cheap pottery clay or fireclay and mix that with your dirt to make it fit for purpose.

Phil Stevens wrote:


1)  That long horizontal run of the flue is not a good idea. The fewer bends and departures from vertical in your chimney the better it will draw. Ideally, you'd go out the back of the mass and straight up through the roof. You probably won't need the fan if you build everything according to fully tested and debugged plans.

Phil,
Do you mean the Part between the labeled L and the fan or the part labeled J?

I'm referring to the horizontal section from the 90-degree bend after the dampener and leading to the tee with the fan.

Phil Stevens wrote:I'm referring to the horizontal section from the 90-degree bend after the dampener and leading to the tee with the fan.

OK tnx, another question.  Doesn't cob turn rock hard after it is fired? I don't see how you could reuse it.

Consider a Consider a stratification chamber (a bell) instead of the piped mass.  (a bell) instead of the piped mass.

Also, I dont know what stratification chamber or piped mass means.

Hi Rick, there are hundred of detailed builds on this forum thread, some feature very basic construction methods others use the latest high tech materials.
So the best know workable formula for a J tube design is based on a 1-2-4 format ie the feed tube is 1 foot the burn tunnel is 2 feet and the riser is 4 feet.
We try to avoid any deviation from the system size, so if you build a six inch diameter feed tube then the burn tunnel, riser and chimney will also be six inch diameter.
You want to avoid as many bends as possible and ideally there would not be any elbows or bends after the barrel exit.
You will get a better performance using a bell rather than a piped mass.
It is possible to deviate from the tried and tested dimensions but not recommended especially for a first build.

Your feed magazine is not likely to just sit there cold and drop wood into the combustion zone; as heat rises, the wood in the magazine will get hot and eventually start burning, and if you then open it to add wood, it will strongly burn up out of the top opening. If the bottom air inlet stays open for this operation, it will be an even more effective chimney shooting flames into your face. Part of the rationale of a J-tube is to draw inlet air through the wood to pull flames down.

The feed tube needs to be no more than 1/3 the height of the heat riser, 1/4 is better. I think the best proportions are around 1:1.5:3 for feed, burn tunnel floor, and heat riser. This lets the riser predominate while letting the feed be tall enough for firewood to fit completely inside so it can be capped off in an emergency or to regulate airflow.

A nitpick: I see your drawing shows flame in the burn tunnel/riser base, then smoke in the riser, then more flame in the donut at the top of the riser. I know people have talked about a "reburn" at the top of the riser, but from my experience that is not how it happens. The air and fuel gases mix in the burn tunnel and more in the riser, and all of the combustion happens there. Sometimes the flames end well before reaching the top of the riser, sometimes they do reach the top depending on conditions, but they do not burn again after leaving the riser.

You list heavy steel tube/pipe in the combustion core. This will not work for the long term, as if you get to efficient combustion temperatures, the steel will spall and corrode and be destroyed often within a season or two. Also, the steel will pull heat from the fire and transmit it elsewhere, cooling the fire and making combustion less efficient. The better insulated the steel is, the faster it will be destroyed. You need to use refractory materials in the combustion core, various masonry or ceramic materials are good for different parts of this. You indicate a 6" combustion core which will work, but an 8" or 10" channels and chimney after. It is true that the burn tunnel should be the smallest part of the system if all are not constant, but you are setting up for more draft than the core can handle and you might need to throttle it with a partial cover on the inlet for best combustion. You want just enough air to burn all the fuel; more air will just dilute the mix and cool the flame.

You don't mention the space this system is intended to heat. A 6" J-tube system can heat a small house or large room in a real winter climate, while an 8" J-tube can heat an average house that is compact and not chopped up into little rooms. This is assuming the structure is reasonably well insulated. Some more information will help us give the best advice here.

Some very useful information has been given by our top rocket scientists.

To add: A small scale model won't behave the same way as a full scale model.
The smallest reliable size for a J tube is a 6" system. Any smaller and gas flow friction becomes much harder to overcome leading to smoke back.

A "small fire burning many hours" is not the way a RMH works.
Hot and fast is the way to a clean efficient burn.

As labeled in your diagram, all of "J" piping could be eliminated and instead just be made into an open chamber. This is known as a bell or stratification chamber. (A search on permies will bring up much information about them). Your vertical exit pipe then could run straight up through the roof. This pipe should have its opening very close to the bottom of the bell.

Glenn Herbert wrote:

A nitpick: I see your drawing shows flame in the burn tunnel/riser base, then smoke in the riser, then more flame in the donut at the top of the riser. I know people have talked about a "reburn" at the top of the riser, but from my experience that is not how it happens. The air and fuel gases mix in the burn tunnel and more in the riser, and all of the combustion happens there. Sometimes the flames end well before reaching the top of the riser, sometimes they do reach the top depending on conditions, but they do not burn again after leaving the riser.

I have seen this talked about in many of the sites I have done research at before I drew up my plans. I'm not sure one could prove or disprove this theory without making one out of glass, so you can see what happens. I was just assuming it was a fact due to my lack of knowledge on the subject.

You don't mention the space this system is intended to heat. A 6" J-tube system can heat a small house or large room in a real winter climate, while an 8" J-tube can heat an average house that is compact and not chopped up into little rooms. This is assuming the structure is reasonably well insulated. Some more information will help us give the best advice here.

The area I plan to heat in cold winters here in N. Idaho is about 1400 sq. ft. with a half basement and I think I'm planning to put this in the basement with heating/AC ducts to the main floor.

Fox James wrote:Hi Rick, there are hundred of detailed builds on this forum thread, some feature very basic construction methods others use the latest high tech materials.
So the best know workable formula for a J tube design is based on a 1-2-4 format ie the feed tube is 1 foot the burn tunnel is 2 feet and the riser is 4 feet.
We try to avoid any deviation from the system size, so if you build a six inch diameter feed tube then the burn tunnel, riser and chimney will also be six inch diameter.
You want to avoid as many bends as possible and ideally there would not be any elbows or bends after the barrel exit.
You will get a better performance using a bell rather than a piped mass.
It is possible to deviate from the tried and tested dimensions but not recommended especially for a first build.

I'm an inventor and I never just go off the plans of others in anything I do. I have built everything from a builders foam mini bass boat

to a complex trebuchet from my own plans having never done it before.

I would much rather experiment and fail, if that is how it turns out with my own designs. If I fail I try to figure out why, make the needed changes, and try again. That's the only way to come up with truly new ideas. I do however listen to opinions of others on subjects I'm not skilled in and make tweaks to my design if I think it's needed.

Ok Rick, that is fine, you have seen our advice and opinions so I guess the ball is in your court now …. Good luck and lets us know how it goes.

Rick A. Lawrence wrote:I have seen this talked about in many of the sites I have done research at before I drew up my plans. I'm not sure one could prove or disprove this theory without making one out of glass, so you can see what happens. I was just assuming it was a fact due to my lack of knowledge on the subject. .

My suggestion would be to build one and then report back on how it worked.

I have a peephole in the side of my bell at the level of the riser top, and I see the flames mostly ending a bit below the top of the riser, occasionally reaching above the top by a few inches. I have never observed reignition above the top of the riser.

Fox James has made glass-topped systems, and could report what he has seen.

When invention moves from relatively benign devices into the realm of flames, combustion products and potential for explosion, it's a good idea to heed the designs and advice gleaned from other's experiences. At the very least, it can minimize the chances of seeing "it wasn't supposed to do that" inscribed on a tombstone.