How Much Inhibitor?

On the Sugar Shot project we've been talking about, among other things, how much inhibitor is actually necessary to inhibit the propellant surface. I suspect that it's less than what most of us would guess. A few years back I made several motors that were designed around a readily available source of casting tubes, ie paper towel and toilet paper rolls. The motors worked great and have been fired with Fructose, Sorbitol, and Xylitol based propellants. These "casting tubes" with a wall thickness of just 0.020" seemed to do the job quite well. It makes me wonder just how thin one could go. In the last photo, the kraft paper casting tube can be seen inside of the red rosin paper used as insulation. I was expecting to be able to use a bit more insulation, but in practice the fit was tighter than I was expecting. I considered making slightly smaller diameter casting tubes to allow more insulation, but that goes against the original design intent. I then considered switching to a different insulation, but with the relatively thick casing wall and short burn time the casings on this motor seem to come through a firing none the worse for wear. There are other thin insulation material that I may try in this motor in the future.