Add second air intake?

Has anyone tried adding a second air intake to the first chimney to create a secondary burn? Just curious how that worked out?

A J-tube style RMH doesn't need a secondary air intake there. It has enough oxygen for complete combustion, it just needs sufficient turbulent mixing for oxygen and fuel gases to meet in hot conditions. The sharp right-angle turns in the J-tube have been found to do that well, especially with some refinements near the beginning of the burn tunnel, the "p-channel" and "tripwire". Search on those terms for details.

If you want to experiment, you can't get any feel for how well this would work without first building one by the book, then adding just that one modification. Anything else will be meaningless in research terms.

To follow what Glenn says.

J tube rockets always have too much excess oxygen in their flue gases. And furthermore, if you add air at that point, it's verry bad, because you would be cooling the fire. Which would decrease the efficiency of the burn.

to create a secondary burn

what for? what the problem you trying to fix?

Joshua Crosby wrote:Has anyone tried adding a second air intake to the first chimney to create a secondary burn?

A secondary burn already takes place, within the highly insulated heat riser (in the diagram, component C).