Hoping and praying

With the super cold winter and high propane prices here in Michigan this year Im nearly broke. While researching a wood stove I both found the rocket heater and that I did not have enuf money left to install a conventional wood stove. I have begun building a small rocket heater and am hoping and praying this is not some type of hoax LOL. Im also hoping some of my design features are not destined to fail. Maybe some of you could help me with this. I started with a 13 gallon air compressor tank. My J tube is firebrick cemented together. My concerns are that my horizontal part of J tube is slightly smaller than the vertical part and the exhaust is 6 in. diameter stove pipe which is bigger than either. Can this create a problem. Also is there a min. and max. for distance between barrel top and top of J tube

Joe Sylwestrzak wrote: My concerns are that my horizontal part of J tube is slightly smaller than the vertical part and the exhaust is 6 in. diameter stove pipe which is bigger than either. Can this create a problem. Also is there a min. and max. for distance between barrel top and top of J tube

As far as I can see, the burn tunnel being slightly smaller wouldn't pose a problem.
Yes, there's a minimum size for the top gap. Take the cross section area of the riser and divide this by the circumference of the same. The resulting figure is the minimum gap. Most rocket mass heaters tend to run better with a slightly wider gap. As for a maximum, I think there isn't. I've used a top gap of 2 ft in a 4" Ianto Evans' system which ran like mad.

From what I can gather and understand about basic combustion, the minimum gap is there to provide a steady combustion. Think of the gap as the exhaust pipe on an automobile. If the exhaust pipe is not fully open, the engine runs "sloppy" as back pressure is created. If it is completely blocked, the engine snuffs itself out. The same holds true for a rocket stove and any furnace. Of course I am only using knowledge I gained while growing up burning wood for heat in a commercial wood furnace as well as automobile knowledge.

Thanks very much