First rocket

Well I thought I would make a test rocket to use as an outdoor cooking area, and show my wife that it really does work and is safe. It rockets fairly well but when I put longer sticks in they burn up and smoke out the burn chamber a bit. I Cast the burn chamber out of cement and perlite with a little furnace cement. The riser is just cement and perlite. The rise has developed a vertical crack the entire length so will have to see how it holds up. The burn chamber is 7"x 6'' by 10" deep. the burn tunnel is 6' and from the bottom of the chamber to the top of the riser is 40". Any comments/ideas for improvement? I forgot to add that I used 6" duct for the center of the riser mold and did cut the top off at the concrete.

Clinton; Cement core & riser are doomed to fail. They can't take the high heat. Next build , use fireclay & perlite with some refractory. Oh and I hope thats a typo about a 6' burn tunnel... add a peter channel to help with the draft. And if you don't have one ,get a copy of version 3 rocket mass heaters by ianto evans.