Thrust is how hard of a push the rocket produces. Burn time is how long the fuel in the motor takes to burn while pushing gases out of the back of the rocket. The impulse of a rocket motor is the area under the thrust-time curve. This means the impulse is a number for how much force the rocket makes and for how long. The impulse of a black powder motor is used to tell what size class it is in. Motors are divided into classes from 1/4A to E. These classes hold rockets with impulses from 0 to 40 Ns (Newtons*seconds). Other kinds of model rocket motors can be named up to an ‘H’, which is up to 320 Ns.