In a composite propellant solid rocket motor, the igniter should be inserted up to the top of the propellant grain (the end furthest away from the nozzle).

In a black powder propellant solid rocket motor, the igniter should be inserted at the nozzle end of the rocket motor.

In a hybrid rocket motor, the igniter should be inserted up to the top of the fuel grain just below the injector head (the end furthest away from the nozzle).