I am building a bunkie out of a 20 foot shipping container. We are going to live in the bunkie 6-7 months of the year while we build our timber frame house. I was thinking of putting a rocket stove in as a way to heat the bunkie in the spring and fall. The bunkie will have a covered outdoor kitchen where the burn chamber and feed tube are located. The chimney will run into the bunkie on the floor and up the far side wall and back outside. The whole floor will then have sand put in to cover the 4 inch chimney pipe and then concrete over that.
The idea being, in the cooler weather of the spring and fall, the floor inside will be warmed by the mass and the outdoor covered kitchen area will have the actual stove where to heat will take the chill off there as well as have the barel top to be used for heating water and such.
My questions to those that have made these are:
1) Does this seem doable. Will it heat the floor mass with 4 inches of sand and probably 2 or 3 inches of concrete.
2) What is the maximum length the chimney pipe should be? Running it 20-30 feet under the floor if at a gradual slope from one side to the output. Is that to far?
A 4" diameter system is very small for a rocket mass heater, which is what you are making even if the primary purpose is cooking. Heaters this small require everything to be done just right if they are to work at all, and even then it will not put out a lot of heat. I think you would be much better served by building a 6" diameter system, which is the smaller of the two standard sizes and is likely to work as long as you basically do things right. A 6" RMH can support about 35' of horizontal duct run in the mass, minus 5' for every 90 degree bend. So an in & back run would have a maximum length of 25', or about 10' in, 5' at the end, and 10' back. If you want to exit at the far side, you could have 5' in, a 90 to run the length of the floor with 10 or 15' run, a 90 and 5' of run to the far wall and up. The duct in the floor will probably make a 2' wide strip somewhat warm, but will not affect the floor beyond that. 4" ducts are too small to carry the exhaust, and larger ducts would require a thicker bed of mass.
Sand is not a good mass material, as the tiny airspaces give it insulating properties. Mixing it with just enough clay to make it solid will give a good thermal mass.
I think you might get better results by making a 10' long cob bench on the inside with the duct doubling back inside it. This will give a positive warm place to sit, lie, sleep, and radiate some heat to the rest of the space. All of the mass will get warm and be beneficial instead of only a 2' strip of a whole floor.
Lastly, if you are really planning on living in this and heating with the RMH, having the fire where you can't see or hear it without going outside will make it much trickier to tend the fire and keep it going steadily. About half of the heat from a standard RMH is radiated from the barrel, and you will get only the slow mass heat on the inside.
You might be best off building a standard RMH inside, and a rocket stove outside. This would not preclude cooking soup etc. on top of the inside barrel. It is possible to build an effective rocket stove with cooktop entirely from cob (except for the steel plate and the chimney pipe), so it does not need to be expensive to make two systems, considering that they are not intended to last decades but just a few seasons hopefully while you build your house. If you want an example, I have posted pictures of a cob rocket stove I built.