After playing with batch boxes for a bit I thought I should build a j-tube to compare. This is a 1:2:4 ratio j-tube with a P-channel, tripwire, and kicktail. It is built from clay, sand, wood ash, and straw. It has a 4 inch square feed with a 4 inch round riser. I was 12 minutes into a burn when this happened:
I have had this fear with the J-tube design since I first started researching rocket stoves. Since I am still new to these things I thought I would ask for some opinions on what went wrong.