Black powder rocket motors are only made in small sizes. If they are bigger there is a bigger chance of exploding. They also do not perform as well if they are bigger. They are made in different sizes called classes. The classes are 1/4 A through E. If the motor needs to be bigger than this, a different type of fuel is used. This is called a composite fuel. Some examples of composite fuels are ammonium perchlorate or ammonium nitrate.