There are three common types of simple, black powder, charcoal-tailed, rocket motors: cored, nozzleless, and end-burner. A cored rocket motor is the traditional motor, typically with a clay nozzle, and a hollow core going up through the nozzle and into the fuel grain for some distance. This is how the typical skyrocket is constructed. If you take the nosing off the bottom of a commercial skyrocket and look up into the end of it, you'll see about a half inch tall ring of clay, which forms the nozzle, and then a longer core going up into the black fuel grain....