Single use composite model rocket engines are made from a high temperature plastic and the fuel is a pellet of a rubber like material similar to that used in the Space Shuttle booster engines. The fuel in a composite engine is about three times as powerful as black powder so engines of equivalent power can be made in a smaller size.
A typical composite motor.
The internals of the composite engines are much the same as the black powder motors except that the nozzle and body of the engine is molded from a high-temperature plastic. The engine body contains the fuel, a smoke/timer charge, and the ejection charge.