Before we even begin to explain what is inside a model rocket engine, it must be stressed that you should never try to disassemble one, because they contain a propellant that is highly flammable and, once that propellant has been removed from the engine casing, it can be very easily ignited.
It is also not advisable for anyone to try to make their own engines, unless they are 100% sure that they know what they are doing. Some expert enthusiasts do make their own rockets and engines, but it is much safer for the amateur to stick with mass-produced model rocket kits and engines.
In the early days, before Vern Estes invented Mable, the very first machine for mass producing model rocket engines, early enthusiasts made their own motors, which led quite a lot of accidents, some of which were fatal. Fortunately, today, engines for model rockets are widely available and they are very affordable.
The most common type of propulsion unit is the single-use, solid propellant type of motor. Here are the components of a standard model rocket engine.
The casing is a simple tube made out of strong cardboard. Cardboard is used because it is cheap, strong and it is lightweight.
At the rear end of an engine, you will find the nozzle, which is usually made of clay or a ceramic material. The nozzle compresses the hot exhaust gasses as they are powered through it by the propellant, and that is what creates the thrust that will lift the rocket off the ground. Clay or ceramics are used for the nozzle, so that it can withstand the high temperature of the exhaust gasses.
In most standard rocket motors, the propellant that is used is black powder. When the propellant is ignited by electric igniter, or starter, it burns from the bottom up, expelling the thrust-creating exhaust. Black powder is made from sulfur, charcoal, and potassium nitrate. In the larger types of engines, a composite propellant is used, which is usually made of ammonium perchlorate, aluminum powder and a rubber-like binding material.
Once the propellant has been used up, the delay charge, which is located immediately on top of the propellant, will be ignited. At this point in the flight a model rocket will just be coasting and the delay charge will be emitting smoke and flame, but it will not be creating any thrust. The length of time that the delay charge burns will be about 3 to 8 seconds.
Once the delay charge has burned through to the top, it will ignite the ejection charge. The ejection charge is what will blow the nose cone off of the top of the casing and then the recovery system will be deployed, which is usually a parachute. When the ejection charge ignites, hot gasses are expelled, which is why recovery wadding is used to protect the recovery system from damage.
A modern model rocket engine is perfectly safe to handle and to use and, if stored properly it will last for a long time too. It's thanks to Vern Estes and his machine called Mable that we can now all enjoy flying model rockets safely.