Rocket Mass Heater Design - Please Help!

Hi Permies long time listener, first time caller.

Currently I'm building myself a fine woodworking workshop on an off-grid mountain-top community outside of Melbourne, Australia, and I plan to heat it with a masonry bell style RMH (hooray!), and I have some questions. Any help and input greatly appreciated.

First, some context/constraints:

- Workshop is ~50m2, walls and roof insulated with batts, concrete floor insulated with XPS sheets, which according to calculations by folks here and elsewhere, I believe means I need a 175mm or 200mm core riser.
- Floor real estate for the heater is small, i'm hoping to limit it to approx 900mm x 750mm, but the ceiling is ~2700, so i could go up
- Cash is pretty limited, I'd love to keep build cost down as much as possible (a lot of the build is already with second hand, recycled and/or reclaimed materials)
- I'd also love to keep it simple as possible, so a square riser, cutting as few bricks as possible, rectangular prism shaped bell without a bench, etc
- I want the heater to be a batch box style core housed inside a simple brick box. I'd love to load it and fire it in the morning, and not have to constantly be feeding it. I also love the look and vibe of bricks.

So, some questions:

- Core material: insulated fire brick vs ceramic fibre board (vs something else)? What's the cheapest and easiest? And is longevity/durability the exchange for that?

- Location of the flue: I've seen brick RMH's with flues inside the bell, and I've seen them outside. It seems to me that if it was inside, as soon as the burn starts it would begin heating the flue and starting draw, thus minimising (or eliminating) smoke coming into the room. However, it seems to me that this setup would also create quite a strong draw during the main burns, and would thus be drawing out hot gasses before they had given their heat to the bricks, thus making the system much less efficient. Conversely, the flue on the outside means that only gasses cool enough to reach the bottom would ever make it to the outlet, which would mean much more efficiency in terms of heat given to the thermal battery, but how does the draw start in that set up? I've been thinking about a Tee piece at the outlet, and putting in some wood shavings at the start of a burn to heat up the chimney directly? And how does the draw continue? As I understand, any flue situation relies on the temperature differential between the pipe and the surroundings, and in the scenario where the pipe is outside of the heater, and the outlet of the heater is at the bottom because that's where the coolest gasses end up after giving their heat to the brick and thus making the thermal battery a thermal battery, how does the draw maintain?

- Secondary air inlet: Does this piece of square tubing have to just be a replacement part, since it will sit directly underneath the primary burn chamber and be much hotter than steel is designed to get? I've heard of fancy expensive metals that can be used to make tubes out of, but I think in Australia it's hard to get. Love to know what folks use for this.

- Sizing the Bell: If i've read the tables correctly, if i go with a 200mm riser, I need a Bell with a 9.4m2 internal total surface area. I've read about people adding columns inside the bell to add surface area, which sounds like a good idea to me to keep overall size down. I've also been hoping to do it as a single skin for the same reason, but is that a bad idea?

- Bell lid material: What do I make the lid of the bell out of?? I've got the business end of a rocket stove pointed directly at this thing, and I can't find info about what it should be made of... I don't know much about concrete, but it seems like it would just crack? And what does it sit on for support across the span? Another reason I like the flue on the outside is a simpler construction of the lid, so there's that too...

- Bricks and mortar: What bricks for the actual Bell? Plain old building bricks? Solid/vs hollow? And what about the mortar, both for the core and for the bell?

Okay, that's all I can think of in this moment. I'm sure there's more, but it already seems like kind of a lot. If anyone has any time to answer any or all of my questions, or ask clarifying questions of their own, I'd so so so appreciate any help. And if there's a different idea it seems like I've missed, I'm open to change where I can.

Many thanks in advance,
Aeron

Hi Aeron;
I'll try to give you some answers.
First, in my opinion, do not build a full-size first-generation Batchbox with CFB.
Wood abrasion will happen no matter how careful you are.  
Use heavy firebricks; although sometimes they might crack, they rarely fail.
I recommend an external chimney using a bypass from near the top of the bell.
Internal chimneys require insulation to avoid stalling, not runaway heat.
Adding a bypass is foolproof for starting during shoulder seasons; adding a tee and building a stick fire to start the draft will quickly turn into a royal pain in the ...
The draw maintains once your bricks absorb heat, that heat has to rise. With the super-hot air coming out of the riser, the air stratifies and moves toward the bottom, where it exits the bell.
OK, the secondary air tube sit in a gap of the firebrick floor, as it has outside air rushing through it , and a layer of ash on top it is insulated from extreme heat.  The portion of that air tube that sits in front of the port has spauling issues.  A standard piece of carbon steel Schedule 40 pipe will wear out and need replacement. if you copy my quick change secondary design, changing that stub is a matter of a few minutes.  With the early design, the stub needed to be welded in place, and then the entire tube was removed, and the stub was cut off to replace it.
With light use, a plain steel stub should last you at least two seasons before needing to be replaced.
The special metal pipe you hear about is RA 330, or if you talk to me, RA253MA. Here in the US, I sell them for $60.  

Bells) Single skin bells get hotter to the touch than a double skin, but will cool faster.
Roofs are either built with firebricks sitting on T-bar angle iron, and then covered with a second layer of plain clay bricks.
Or you can use an insulation layer (Morgan super wool) and then use cement board, which also has T-bar support.
The bell is constructed with plain brick below the top of the riser; beyond the riser, the bell is constructed with firebrick.
Solid clay bricks or bricks with holes can be used.  The holes need to be filled with clay mud, or clay mortar, but not sand.
All mortar is made with one part fireclay to three parts graded sharp sand (NO ROCKS)
Using dry product (bagged clay, and bagged sand) is the easiest way to properly mix your mortar.

Here is my website https://dragontechrmh.com/. Go take a look at quick-change secondary tubes; you or a local welder can easily build your own.
Check out my studio dragon build; it could easily have been made as a single skin bell.
Take a look at my Shorty core build, it has smaller requirements in bell size and in the number of firebricks needed.

Have no fear of cutting bricks, it is quite easy.