Fins

An engine hook is used to keep the motor from being forced out the aft end of the rocket when the motor's ejection charge is fired. Engine hooks are made of a springy metal so that they may be bent a little to insert and remove the motor.

Screw-on motor retainers serve the same purpose. Some rockets have motor mounts without a forward thrust ring. In this case use a motor with a thrust ring around its back end or add a thrust ring using masking tape.