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 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.
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?
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.
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.
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!
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:
PI day is 3.14 (march 14th) and is also einstein's birthday. And this is merely a tiny ad:
five days of natural building (wofati and cob) and rocket cooktop oct 8-12, 2018https://permies.com/t/92034/permaculture-projects/days-natural-building-wofati-cob