Help in my first rocket stove heater project

HI,
I have a steel bell with such dimensions (view attachment 1). Height is, obviously, too long compared to diameter.
I have been inspired by the "Liberator rocket heater" model (attachment 2). Unfortunately I don't have the dimensions of this model
Concerning the burning chamber, I am planning to insulate it from inside with a 2 cm refractory cement layer cast
So my questions are:
- What is the best height for a 40 cm bell ?
- What about the dimensions of the J tube ?
- Is castable refractory cement a good insulation material? and is 2 cm thinkness OK ?

Hi Elyes,

Welcome to Permies.

Hi Elyes, it seems to me that you are asking about the riser. The riser is insulated and is where the final combustion of the super hot gases occur. A bell is like an upside down sink, where the gases radiate their heat and then sink to an exit path. As far as the proportions, I am sure one of the RMH experts here will answer.

Elyes Kallel wrote:HI,
I have a steel bell with such dimensions (view attachment 1). Height is, obviously, too long compared to diameter.
I have been inspired by the "Liberator rocket heater" model (attachment 2). Unfortunately I don't have the dimensions of this model
Concerning the burning chamber, I am planning to insulate it from inside with a 2 cm refractory cement layer cast
So my questions are:
- What is the best height for a 40 cm bell ?
- What about the dimensions of the J tube ?
- Is castable refractory cement a good insulation material? and is 2 cm thinkness OK ?

Hi Elyes.... Welcome to the Permies RMH forum.
What size of building are you trying to heat? Dimensions will vary according to whether you choose a typical 6" or 8" J tube.
Do you have experience with casting? I know several seasoned builders here have and it seems a lot harder way to start a first build.
Have you instead considered fire brick?
What is the shape of your steel bell? Round or square?

Gerry Parent wrote:

Elyes Kallel wrote:HI,
I have a steel bell with such dimensions (view attachment 1). Height is, obviously, too long compared to diameter.
I have been inspired by the "Liberator rocket heater" model (attachment 2). Unfortunately I don't have the dimensions of this model
Concerning the burning chamber, I am planning to insulate it from inside with a 2 cm refractory cement layer cast
So my questions are:
- What is the best height for a 40 cm bell ?
- What about the dimensions of the J tube ?
- Is castable refractory cement a good insulation material? and is 2 cm thinkness OK ?

Hi Elyes.... Welcome to the Permies RMH forum.
What size of building are you trying to heat? Dimensions will vary according to whether you choose a typical 6" or 8" J tube.
Do you have experience with casting? I know several seasoned builders here have and it seems a lot harder way to start a first build.
Have you instead considered fire brick?
What is the shape of your steel bell? Round or square?

What size of building are you trying to heat?

Re: about 60 m²

Do you have experience with casting?
Re: No, but I know some masons experienced in firebrick chimneys

Have you instead considered fire brick?
Re: I prefer beginning with steel as a first project


What is the shape of your steel bell? Round or square?

Re: Round

Steel for the combustion core is a bad idea. Lots of youtubers do it, but they hardly ever show how it worked a year later. People who have reported on it almost universally say the steel is destroyed within a season or two, assuming they built and operated it at proper RMH temperatures for complete combustion.

You need to use refractory materials for all of the inner core, whether castable or firebrick or ceramic fiber. You can use steel as an outer form to contain the insulating core material, but there is no advantage to using welded heavy steel for that. Sheetmetal works fine.

A 15" diameter bell over the heat riser is cutting it close for clearances. A 6" system with a 1" ceramic fiber riser will have an outside diameter of 8" or so, leaving an average of 3 1/2" clearance all around. Any bulkier riser material, especially firebrick, will barely be able to fit inside a 15" barrel at all. A standard and cheap 55 gallon barrel works fine and will last plenty long. You don't need heavy steel for this part.

If you want to use your existing cylinder for a bell, there is practically no such thing as too much height. A 6" J-tube may well heat your 60 m2 (600 ft2) space fine, depending on your climate. How cold does it get, and for how long? How well insulated is your space?

If you use standard firewood (generally 16" in the US), you would want the feed tube of your core the same depth so the wood fits completely inside it. This is important for emergency capping as well as for regular partial covering to regulate air supply. Following what I like best for proportions and using US standards, 1:1.5:3 would give you a 16" feed tube, 24" burn tunnel floor, and 48" heat riser. This is just barely possible to fully clean ashes from using a short scoop/rake as long as you don't have huge hands. You would have a 12" burn tunnel ceiling which would give plenty of working clearance with your metal bell.

Firebrick for the feed and burn tunnel, insulated all around, with a 5 minute riser (1" ceramic fiber blanket fitted inside a sheetmetal jacket) on top of the firebrick, would work well for your situation. You don't need to be an experienced mason for this, you can lay up the firebricks dry as long as you hold them in place with a sturdy containment.

I am getting a lot of good information from the wood oven movie.  Like I never really thought about the barrel and what I should look for.  As well as how to not blow myself up if there are still volatile gasses in it.  I recommend watching the movie, but here's an excerpt to get you a quick peek: https://permies.com/s/barrels

You may be able to find really good pointers in the Freaky Cheap Heat movie. You can get it for $1 if you back the Kickstarter that is launching soon. (Plus you get a load of other resources!) Here is a clip: