This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
A reloadable rocket motor is a metal cylinder with screw-on end pieces.
Solid propellant and time delay are purchased separately from the motor
casing, in 'reload kits'. These kits contain all of the expendable,
non-reusable materials for a single flight. The cost of the reload is
significantly less than the cost of an expendable motor (when talking
about F sizes and up). Quite a number of reloadable motors and reload
are now certified by NAR or Tripoli. Refer to the approved motor lists of
each organization to see exactly which motors are currently certified.
YOU MUST BE A CERTIFIED MEMBER OF A QUALIFIED ORGANIZATION TO PURCHASE OR
USE RELOADABLE HIGH POWER MOTORS. See section 3.1.9, below, for
information on becoming certified to use high power reloadable motors.
WARNING: IT IS HIGHLY RECOMMENDED BY r.m.r CONSENSUS THAT YOU DO NOT
ASSEMBLE AND/OR PREP A RELOADABLE-TYPE MOTOR UNTIL JUST PRIOR
TO ITS USE (I.E., ON THE FLYING FIELD). *** UNDER NO
CIRCUMSTANCES SHOULD ASSEMBLED RELOADABLE MOTORS BE STORED WITH
IGNITERS INSTALLED ***