Wanted: Simplest possible version of a Rocket Mass Heater for inside insulated greenhouse

Hi Rocket Mass Heater Experts:
I'd like to build the simplest possible version of a Rocket Mass Heater inside my "Greenhouse in the Snow", where unfortunately the passive Geothermal system has failed.
The current problem with this goal is information overload – I just don't have the time to watch a pile of movies and read through a truckload of documents on rocket mass heaters.
Could someone please point me to the place where I am most likely going to find the info I need for a simple rocket mass heater?
If I can build one directly into the ground (or at least partially into the ground), it would be ideal.
Note that the greenhouse is also heated by my large outdoor boiler, but that thing eats a lot of firewood when it has to heat 2 dwellings, hot water tanks and the greenhouse.
If I can do at least some heating with a rocket mass heater in the greenhouse, it would take pressure off the outdoor boiler and reduce my wood usage.
Thank You!

For fast and simple I suggest an 8" J-Tube with a stratification bench.
It can be partially buried.
Perhaps using the Walker-style half-barrel design

The Wisner's book "Rocket Mass Heater Builders Guide"  is the go-to book on J-Tube construction.

One of Ernie and Erica's 8" brick J-tube combustion cores feeding a semi-buried half-barrel bell would work nicely for a greenhouse.

One mistake I have seen people make is to run the chimney outside underground with a short uninsulated stack. You will get much more reliable operation if your chimney (can be just a stovepipe) rises from the bell straight up through/near the peak of the roof. It will not be so hot that it endangers any plastic on the roof, and if it leaks a little it will probably not hurt your greenhouse.

If your soil in the greenhouse isn't insulated from the soil outside the greenhouse, I think putting the rocket in the ground will be very inefficient.

How is the passive geothermal set up?

Good point on ground insulation... I think it has to be considered whether the ground heat into a cool greenhouse outweighs the loss into the ground from a warm greenhouse...

A bell will have most of its heat at the top, so may lose much less heat into the ground than a buried duct would. It would probably be a net benefit to have a layer of insulation under the bell cavity.

I remember seeing photos long ago of a wood framed greenhouse, with a wood burning "stove" along the north wall.  It was just a "bench" made of bricks with a stove door, perhaps reused from an old cast iron stove.  The exit pipe at the far end of the "bench" turned out the end wall.  There was no bell or chamber.  It appeared to be built right on the ground.  One photo showed several people having tea, with a "billy" being heated on top and seed trays on the bench near the exit.

The greenhouse looked to be maybe 30 feet long and perhaps 12 to 14 feet wide.

Peace

Hi Everyone:
Just to answer some questions.
The greenhouse is 60' long by 17' wide.
We only dug down about 2 feet rather than 4, because our water table is so high. Instead we put 1 foot thick styrofoam insulation 8 feet wide all around the greenhouse, covered with about 1 foot of soil, so it is well insulated.
The passive geothermal is just 10 pipes (4" non-perforated, corrugated pipes) buried in a 250' loop, 8 feet deep, with both ends emerging inside the greenhouse and a blower on top of one end. The idea is to circulate that air through for cooling in summer and heating in winter. Unfortunately the pipes have all failed – we are not sure if they got crushed or cracked by the weight of soil and water seeped in or condensation built up, but we have been unsuccessful at pumping them out, so no air is circulating through them at this point.

I think I still have an old Franklin wood stove kicking around somewhere, maybe I can make use of that somehow, instead of a barrel?
I did order the Wisner's book.

Not what you asked about, that geo thermal loop might not be crushed, "just" full of water.
Depending on how tight the geothermal loop is , you might be able to get a fish-tape through it, and use that to pull in  some 1/2 or 3/4 inch PEX.

Anyhow, Deane's post made me think of  this:
Turning a wood stove into a mass heater to warm a greenhouse

It's not a rocket, but it is a mass heater.
Since you have a woodstove, you are well on your way to making something similar.
For the bell I would probably use steel drums with cob/stone/bricks on the inside and/or.