Newbie questions

Hello, I am new to rocket stove heaters and have a few questions. I plan on making my bell out of a 44" tall (w/pedestal) by 14" diameter old propane tank. I wanted to make the rest out of fire bricks until I found out the cost of them. I do have a 10' section of 6" stainless steel flexible chimney liner that I was wondering if I could use that for the riser with a piece of 8' duct around it and fill the space with fiberglass insulation? This first attempt at a rocket heater will not be a mass heater. With that size bell what sizes would I be looking at for the riser, burn chamber and feed tube? Funding for this project is limited so I want to use things that I currently have or that I can obtain locally at a reasonable price. Any help is greatly appreciated. Thank you

You could build a test setup with the materials you describe, but it would certainly burn out very quickly. I just saw a post by one of the most advanced researchers mentioning that he had made a 1/8" thick steel system that corroded out within 5 hours of full-scale burning. Go ahead and try it just to see how a rocket stove works, but don't invest a lot of time or material into this stage.

You can make a RMH core with old soft red brick if you can find that cheap (demolition site or something); it won't last as long as a firebrick one, but its life may be measured in years rather than hours.

Fiberglass insulation will melt at the kind of temperatures you find in the heat riser, but "roxul" rockwool insulation is more durable as long as it isn't actually exposed to internal riser temperatures/atmosphere.

The flexible ducting is said to make serious drag and is not good for airflow, and the riser is about the most critical flow section of the system. Straight steel stovepipe would work better, though it too would burn out before long. Don't expose galvanized ducting to high heat without first burning it off outside in the open air; the zinc coating may give off fumes at first. Once it has turned to a dull gray finish from the shiny start, it is stable.

The core proportions are recommended to be 1:2:4 feed tube to burn tunnel to heat riser, and the riser should go up to about 2" (minimum) from the inner top of the barrel (propane tank). You can make those length measurements along the centerlines or along the outside edges of the channels, whichever you want - different experts use different methods. Keep the cross section the same throughout the system. Despite what you may have read in some places, you do need a chimney to get decent draft, so once you put the tank over the heat riser you need to connect a chimney pipe the same size as the riser and going up several feet. A 5' long piece of 6" duct works well.

Glenn, thank you for your reply and information. I do not think I will waste my time using the stainless liner. The cheapest I can find 1.25" thick firebrick is, 6 bricks for $18 at Menards. Are there less expensive sources for these? The more I view videos and read information about Rocket heaters , the more I am confused. It seems that store bought rocket heaters are made out of steel and a lot of homemade ones also, but then I hear/see the stories how the steel even stainless breaks down.
When you say the proportions are 1-2-4, do you mean, the lengths or widths or both, for each component? As an example the feed tube is 6" long, the burn tube 12" long and the riser 24" long all of the same diameter?
Would you recommend for my tank size, to have a 6" diameter riser, exhaust and burn chamber?

Would it be less expensive to mold all components out of refractory cement and perlite?

Thankyou

Lots of people post about their innovative new all-steel rocket mass heater, but few of them come back with videos of how it failed after a few weeks or months. There are plenty of commercial metal rocket stoves for camping/cooking, but these are not built to get the super-hot temperatures that corrode the metal.

Your 14" diameter tank would be best suited to a 6" diameter system. All parts of the airflow path should be the same cross section, except the space between riser insulation and inside of the barrel/tank can be larger, and the "manifold" where the space inside the barrel transitions to the heat transfer duct should be significantly larger than the rest of the system (this is a common bottleneck in amateur-designed systems).

You can make the heat riser of a 6" diameter duct and a 10" diameter outer duct jacket, filled with 2" of insulation like a perlite/clay mixture. You want something that will become rigid so when the riser liner burns out the insulation can take over as the liner.

This will leave 2" airspace all around the riser for downward airflow, which is adequate.
The riser should be just a bit shorter than the barrel/tank so there is a gap of 1 1/2" minimum for a 6" system, or 2" minimum for an 8" system. You can proportion the other elements to that; the burn tunnel can be a bit shorter, if that is convenient, without causing problems.

For fire brick, go to a local brick seller. They can get you pointed in the right direction. I'm in Missouri and pay about $1.75 for a half brick 9 x 4.5 x 1.25 inches, and about $2.5 for a full fire brick, measuring about 9 x 4.5 x 2.5 inches. Rated at 2,500 F as I recall; maybe 2400 F, maybe 2600 F; something like that (higher than 2,200 F, but not as high as 3,000 F).

Internet sources are widely variable in quality of information. Be careful. The most recommended book is the one by Evans, Rocket Mass Heaters (3rd edition), which you can find at Amazon for about $20 USD. Get that and start there. That will give you a good working knowledge, and you can grow from there, and pick up more recent innovations here and at Donkey's forum too.

Terminology takes a while to get used to, and many items go by different names in different parts of the country and world, sometimes using the same words to describe different things, or different specifications. So it takes a while to get used to all that. As I say, start with the Evans book. I would also suggest watching everything on YouTube you can find that features Ernie and Erica Wisner, they are a quality source of information too, and have built hundreds of these, so they have a lot of practical experience to share.

I'm jealous - the best price I can find around here is $3 for split or full firebrick

The reason I was going to use the 100 gallon propane tank for my bell for my first attempt at a Rocket heater is, I think since it will not be a MASS heater, the propane tank is thicker walled than a 55 gallon barrel and therefore has a little more mass and may retain the heat better. If my thinking is incorrect, I do have access to a painted 55 gallon drum. Any thoughts?

Otto Werner wrote:The reason I was going to use the 100 gallon propane tank for my bell for my first attempt at a Rocket heater is, I think since it will not be a MASS heater, the propane tank is thicker walled than a 55 gallon barrel and therefore has a little more mass and may retain the heat better. If my thinking is incorrect, I do have access to a painted 55 gallon drum. Any thoughts?

Otto, i've done that with a water heater. It barely lasts much more than a barrel, may be 1/2 an hour. But they're perfectly acceptable as a "barrel", anything metal is!

I understand there are instructions along with a podcast telling how to make an 8" batchbox. Could someone please direct me to it? I can not find it. I know I am a newbie, but I am trying to soak in as much knowledge as possible. I did purchase the Rocket mass heater book and am reading it as we speak.
Thank you

Here is the thread discussing it, including complete brick layer diagrams and 3-d views:
https://permies.com/t/40007/rocket-stoves/Results-batch-box-thingy-Innovators
See Mike Cantrell's post of 10/2/14 for a lot of detailed explanations.

Here are the podcasts from the innovators' event. The second one is about the 8" batch box:
http://www.richsoil.com/permaculture/76965-304-rocket-mass-heater-innovators-event-part-1/
http://www.richsoil.com/permaculture/76988-305-rocket-mass-heater-innovators-event-part-2/
http://www.richsoil.com/permaculture/76992-306-rocket-mass-heater-innovators-event-part-3/

Thank you Glenn. Can you please tell me, how in the future, I can find post and podcast like you sent me?
Thank you

The link to the search page is at the top of this page above Cassie's video. I looked unsuccessfully for the podcast link in the forum (some are linked but not all), and finally went to richsoil.com where the podcasts are all hosted.