All you need is enough air flow so that the smoke and gases from the burn hit the wall at the end of the air run mixing to create a second fuel and then a flue tall enough to get hot enough to burn it.

Some say the flue needs to be insulated for it to qualify as a "rocket" but seeing such stoves in operation...it appears to me that these all pipe designs create their own insulation by thermodynamically creating vertical layers within the flue - allowing the smoke and gas to get hot enough to create that second burn by tunneling up the middle surrounded the somewhat cooler air that is kept both hot enough and cool enough to maintain itself and insulate the super hot flue burn by its hot contact with that second burn on one side of the layer and contact with inside surface of the walls on the other because they do burn hotter than one would expect, just going by the most common theory behind rockets. And that's the only explanation I can think of that covers all the bases.