Glenn Herbert

For a straight wall, I would echo your concern. However, the sinuous form of the plan will keep it from tipping over. A dry-laid stone wall would still want some batter on a wider base, but the cob will hold itself together. What I am concerned about is the bare top of the cob. In a very dry area it might be okay, but with lots of freezing and snow, I would expect the surfaces to take some erosion damage by spring. Hopefully the builders know the local conditions well enough to be confident of its durability. I would have designed it to have a shingled type of rooftop to shed the majority of water.

Note that the animated graphic referred to as a pocket rocket is actually a Cottage Rocket, developed by Chris McClellan ("Uncle Mud"). It is intended to heat small spaces cheaply and reliably. The pocket rocket is a fast-burning outdoor stove with no mass.

To get the efficiency of a rocket core but still have it temporary, you might be able to do something like the cottage rocket but with only enough brick/cob mass that you and your family can still pick it up and carry it to/from the garage or storage shed. You can get fireproof mats commonly made for in front of stoves to keep the floor safe from the feed opening.

The tricky part would be connecting the exhaust to the chimney while keeping the existing damper functional when the rocket is not connected. A plate fitting the damper frame with a good gasket, holding the damper open when it is in place, and connected to the rocket exhaust pipe, might do the job. It would take a skilled builder to do that safely. The lack of a proper insulated stainless chimney liner would not cause creosote buildup in this case because there would be no creosote to worry about.

Do you know what the current chimney looks like internally? The smaller the better for this purpose, as there will be less volume for gases to loiter and get chilled and heavy. If the liner is big, like 8 x 12 or 12 x 12, it may be a bad idea to have any rocket combustion core because you may be at risk of cold plugs from damp cooled gases. Or a big liner might let warmer gases rise while cold ones fall, setting up some circulation that allows enough water vapor to escape from the top.

A wood stove may have the same issues, cooling too much for good draft in a large chimney volume, while also depositing lots of creosote when it very likely cools below creosote condensation temperature. I don't think you can safely use the same chimney for fireplace or wood stove.

If you need a gentle enough swale to drive over easily, I think you need to use a backhoe or excavator from the side to scoop one bit at a time. Are you going to be running this frequently or continuously all year, or even all growing season? If so, I would be concerned about driving over it and leaving ruts. Is the soil so solid that once cut, it will not rut? Or the "sand" so firm that water does not make it softer?

I think the stove gasket is the best choice for the bottom edge of the barrel, though I would have gotten a thicker version which would be easier to assure a good seal with. I would set the barrel on the fresh cob and wiggle it a bit to clearly mark the location, then carefully make a groove following the mark so the gasket is positively held in place.

I think the mineral wool strips would do well as the seal for the barrel ridge. I would form the cob to meet the ridge while the barrel is sitting on the base cob; then when the barrel is sitting on the gasket there will be a small gap below the ridge that can be packed with mineral wool. A blunt knife blade or the like can push the wool snugly into the gap without distorting the barrel.  Or you could lay an even strip of mineral wool on the slope of the cob ridge so that it seals there when the barrel sets down onto the base gasket.

Your core versus bell sizing is problematic. A 4" batch box, extrapolating from the chart at batchrocket.eu, would take about 25 square feet of ISA, and a J-tube is less powerful for a given size so might equate to a 3" batch box, or maybe 15-20 sf at most. Your bell has about 30 sf not counting the barrel, and I think the barrel at around 8-10 sf will radiate most of the heat before it gets to the bench. The bench may never get very warm.

You can still add a P-channel to your J-tube feed, just a piece of sheetmetal with edges bent to assure a small clear air path down to the top of the burn tunnel. There are examples elsewhere on Permies.

Nice work!

You didn't mention how superinsulated your house is; from talking about using that big kitchen stove, it seemed like you needed an average amount of heat to keep the place warm in winter.

A J-tube feed really should be tall enough to hold your typical fuel and have a brick slid over the top for draft control and emergency shutoff. Depending on a downstream damper for flow control is a dangerous idea, especially if it is possible to severely restrict flow with it. If someone closes it while there are still live coals, you could build up carbon monoxide in the room. Draft control is best done with a closure for the top of the feed tube.

I do have concerns that a small system like yours may take a long time to warm the mass you have. It may be hours after you light it that the cobbed bench starts to get warm. The partial barrel over the riser will throw some instant radiation; only use will show how much.

You are right that you need more exit airflow, but you could easily get that by making little pillars all around the top instead of a flat top with small grooves in it. I might try six pillars with equal spacing and height as their thickness. Put them toward the outside edge of the top of the chimney so there is just enough inner/upper surface to properly support the smallest pot you want to use.

If you want the J-tube feed to stick out of the front of the wood box, an 8" core can work fine. Given the framing between the brick box and the doorframe I would go with 3" of cob over the brick - enough to be sturdy but not take too much space or too long for heat to start to come through it. My recommendation would be an 8x8 feed opening, 8" x 9" high burn tunnel with 8-10" long roof, and 8" diameter riser from perlite-clay. You can line the floor with an inch of strong cob, or pieces of firebrick split 1 1/4" thick, to get the right dimension without cutting lots of firebrick. From my experience, you want about 2" thickness on the riser which gives 12" outside diameter. The simplest way to make this is with a piece of 8" sonotube and a pair of 6" stovepipe sections bent out and snapped together to make a 12" sleeve. As it commonly comes in 2' lengths, I would advise three pieces of stovepipe with one cut in half and fitted to itself, giving a 36" height to set on top of the burn tunnel walls. 36 + 8 = 44", which is just enough for an 8" J-tube with 16" high feed tube. See my RMH build for riser construction details.

You need some insulation under the core so the floor doesn't suck heat out of it; an inch or two of perlite-clay under the core would do the job nicely, with scraps of 1 1/4" firebrick at intervals to stabilize things. To strengthen the core, especially the feed tube, I would surround it with at least 4", 6" would be better, of cob with some perlite mixed into the inner couple of inches. This would give a pretty good mix of strength and insulation. I would use a good amount of straw in the main/outer layer, around the feed and around the brick box, so you don't get large cracks.

For the bell bench, I would make walls of 2" thick block as you mentioned, and internal piers of 4" hollow block on end to support the roof, spaced as widely as you can while supporting all the roof panels. I have found 16 x 16 or 18 x 18 concrete pavers at Lowe's or Home Depot, and those supported all around the edges and at corners in the middle will make a good base for a cob seat. The bell cavity will have the coolest air at floor level, but I would still put in an inch of perlite-clay with a smooth hard top for cleanability, after placing structural block.

A 4" J-tube core which is what you are describing may or may not function well depending on exact details, but in any case is much too small to do you any good. What is the character and size of your existing chimney? If at least 6" metal or 8x8 flue tile, and the chimney draws positively when first started, I believe you could put an 8" J-tube core in, which would give enough heat to warm the space well without feeding it all day.

I have a bunch more to say but have to run now. I will post again this afternoon with details.