I am thinking about a stove build in my caravan where space is really at a premium. So I'll be forgoing the bench, but I'm still interested in having some mass. I'm inspired by the "Minnie Mouse", which has stacked bricks around the inside of the barrel, and also Uncle Mud's CottageRocket, with the gases inside the barrel channeled through pea gravel before exhausting lower down.
To give you some context that may help in thinking of ways to DIY your own heater:
Each of those builds you mentioned are based on earlier mass heater designs which came before rocket stoves. The Brick-Inside-A-Barrel design was originally from Finnish, Swedish, and Russian contraflow heaters, which had a more-or-less open space in the barrel's internal volume, with firebricks stacked radially around the center riser.
The "pebble"-style heat-storage device is based on ancient Steinofen — which have much better gas flow and thermal capacity when larger (2-4inch sized) rounded or irregularly-shaped rocks are used, rather than actual "pebbles" or pea-gravel.
Other variations on the general style include sauna stoves... and the more-shapable designs are based on the "Gabion" of welded-wire mesh holding the vertical wall shape of the stacked rocks in place. **Something like 1"-opening stainless steel welded wire mesh (used for gardening to protect plants / excluding wild animals from access) would work and would be a lot more sturdy/durable than common chickenwire.** (
definitely do not use the typical outdoor construction gabion material, galvanized welded wire in high heat areas. Offgassing metal fumes from heated / burning galvanization is really, really toxic.)
www.gabionwalls.com.au/blog/how-to-build-a-gabion-basketCob inside the barrel would probably not be a great option for 3 reasons:
• Cob actually has very low thermal storage capacity relative to its volume and mass. Considering that we're limited by the space of the barrel's internal volume, selecting a material like firebrick or basalt rock
which have much more thermal capacity than cob does would make a better use of that limited space.
• Cob tends to crumble when exposed to high heat and direct gas flow... so using it to shape channels inside the barrel could be severely problematic when the cob eroded, fell, and filled-in the open channels intended to allow the passage of flue gasses. That would block the exhaust stream, choke the stove, and fill the caravan with smoke.
• Cob is fairly insulative and is slow to conduct heat. That's useful on mass benches, but harmful to the function
inside the barrel... the purpose of which is to act as a rapid heat-exchanger immediately downstream of the riser's top-end. The gasses fall in a barrel because they hit the top of the barrel (which is thin steel) and are forced to spread laterally. This rapidly dumps heat from the flue gas—> into the steel—> and immediately into the air in the room. Having shed heat so quickly, (and also being pushed along by more hot flue gas continuing to come up behind it out of the riser) the gasses condense and fall along the sides of the barrel —
where they continue to transfer even more heat into the barrel sides as they descend.(‡)
For that last reason (‡) listed above, placing (insulative) cob on the *inside* of the barrel's sidewalls would defeat the heat-transfer ability of the steel barrel. A liner of dense firebrick on the barrel interior would be a lot more efficient at capturing the heat as it flowed past, and then re-radiating that heat through the steel barrel later.
For more thermal storage capacity *outside* the barrel wall, you could use a gabion-style surround filled with stones similar to the modern "HUUM" sauna stove designs.
This design would need a flue exit low on the side. There won't be enough room in the barrel to contain 1) the rocket's insulated riser, 2) the thermal mass for capturing & storing heat
inside that barrel, 3) the necessary air gap channels between all the thermal mass to allow a relatively quick flow-through passage of exhaust gasses, and 4) the chimney flue extending down through the barrel lid all the way to the lowest point of (cooled) exhaust gas collection.
It's going to be really important to ensure that the flue gasses aren't constricted by the internal mass that they're flowing through & around. Gas flow velocity within a gas path divided into many multiple channels tends to slow with increasing resistance — the more that pathway is divided into smaller & smaller holes, the more the resistance and the slower the gas velocity. So with that in mind, you'll need to ensure that the open-spaces between your brick/stone thermal storage mass are equal to a total
Cross Sectional Area *greater* than that of your rocketstove system size. I would guess you'll need approximately 2x CSA (or more) to prevent draft stalling. I think you'll probably also want to go with a 4"-4.5", (or 5" maximum) J-tube system size. (So, for example, if you use a 4" J-tube: you'll want to ensure that the total open space between the bricks (at any given height "cross section" through the barrel) do not total less than 8 inches [system size] of open space.)
CSA is the 2-dimensional measurement of how wide (how much open area) there is in the (assumed circular) flue path, expressed in units
2 (square inches here.)
Area of a circle = (pi x diameter ÷ 2)
2 4" diameter system size CSA = (3.14159 x 4 ÷ 2)
2 = 12.57 square inches
8" diameter system size CSA = (3.14159 x 4 ÷ 2)
2 = 50.27 square inches
So for a 4" J-tube, I'd plan to have a minimum of 51 square inches total open space throughout the mass (at any given horizontal level throughout the barrel, from the top of the riser to the bottom flue exit.)
It would be a good idea to dry-stack the brick in the barrel and test the unit, first, to ensure it draws well, before mortaring anything into place. That way you can remove some mass to widen the overall CSA of the descending flue pathway if you need to.
If you end up using rocks instead of stacked bricks, you'll probably need even greater than 2x CSA, because the gasses will be forced through and around non-linear pathways — sometimes having to be forced a few millimeters
back upward to travel around the irregularly-shaped pieces of mass... and the back-pressure created by such a gas flow will need to be given a safe margin of extra open-space.