Hi Marion, welcome to permies.
In my opinion, the story about yellow/orange flames is only partly true. That color is caused by glowing particles in the flame, in effect, those being burned. When the stove is still producing soot and depose black goo in the flue or on the pans there's unburned material left in the exhaust gases. In a rocket heater all the processes like burning volatiles (blue flames) and burning carbon are happening all at the same time. The secret is the sideways flame which is causing a lot of turbulence, fiercely mixing the combustibles with incoming air. Even more important: the flame itself is always downstream from the fuel so everything need to pass through the flame zone.
All the above is about a rocket heater, the Evans' style J-tube. A
rocket stove for cooking is a distant relative, works good most of the time but the pans and pots on the flame will get sooty. Not as much as an open fire, far from that, but nonetheless. The pot is cooling down the flame rigourously and it will get black. The j-tube rocket heater doesn't do such a thing, all is focused on completely burning of the fuel. Extracting of the heat is happening after the combustion is complete, not at the same time.
Different purpose, different construction and different results.
