Don't ever grind KNO3 with anything flammable (like sugar) in a grinder. I did that once and that was just about the last time I've seen my shack. Or my chest hair. Bad idea.
I'm using a KNO3/sucrose based propellant made by dissolving 74:36 KNO3:sucrose in hot water and then removing it, either by boiling it out or drying.
The advantage over melting it is that this way you actually dissolve both chemicals and mix them together, as opposed to melting sugar and KNO3 pebbles (no matter how well you grind it, it's still huge crystals).
The resulting composition can be melted into a sticky slush that can be easily shaped and processed into a grain. When it hardens it's relatively brittle, so the motor must be pre-heated before ignition (or use a plastifier, but that will reduce the Isp). In a bates grain config the measured Isp is around 140 s.
If you need a slower burning grain, add approx. 1 ‰ of Fe2O3 (red iron ox.) to paint the grain dark red. That way the IR heat front propagation will be slower and the core will burn slower. Uncolored grains are white and translucent like wax.
