Sugar rockets, MM?
I have to say that my experiences differ from that, when making a sugar rocket with bates grains, I have found a core as small as 2 mm is enough to get the 'fire going' but it does not increase the pressure at all, the surface is just too small to build that kind of pressure.
Now... if you are combining the core with the nozzle diameter then I understand but the two may not correlate. In fact, pressing a sugar rocket with the proper nozzle then using a rod to make the core smaller than the nozzle diameter is a good way to 'sneak up' on getting a very light rocket off the pad without blowing the fins off. No need to match them up size for size.
Edited by dagabu, 07 December 2018 - 12:41 PM.