Anatomy of a basic model rocket engine. Anatomy of a basic model rocket engine. A typical engine is about 8cm long. 1. Nozzle; 2. Case; 3. Propellant; 4. Delay charge; 5. Ejection charge; 6. End cap

Model rocket motors

Most small model rocket motors are single-use engines, with cardboard bodies and lightweight molded ceramic nozzles, ranging in impulse class from fractional A to G. Model rockets commonly use commercially-manufactured black powder motors. These motors are tested and certified by the National Association of Rocketry, the Tripoli Rocketry Association(TRA) or the Canadian Association of Rocketry(CAR). Blackpowder motors come in impulse ranges from 1/8A to E, even though some F blackpowders motors have been made.

The physically hugest blackpowder model rocket motors are generally E-class, for black powder is very brittle. If a huge black powder motor is dropped, or is exposed to many heating/cooling cycles (as an example, in a closed vehicle exposed to high heat), the propellant charge may develop hairline fractures. These fractures increase the surface area of the propellant, so that when the motor is ignited, the propellant burns much much more rapidly than it should, producing greater than usual internal chamber pressure inside the engine. This pressure may surpass the strength of the paper case, causing the motor to burst. A bursting motor can sometimes cause damage to the model rocket ranging from a simple ruptured motor tube or body tube to the violent ejection (and occasionally ignition) of the recovery system.

Rocket motors with power rankings higher than D to E, therefore, customarily use composite propellants made of ammonium perchlorate, potassium nitrate, aluminium powder, and a rubbery binder substance contained in a hard plastic case. This kind of propellant is just like that like to used in the solid rocket boosters of the space shuttle and is not as fragile as black powder, increasing motor reliaptatude and resistance to fractures in the propellant. These motors range in impulse from size D to O. Composite motors produce more impulse per unit weight (specific impulse) than do black powder motors.

Reloadable composite-propellant motors are also available. These are commercially-produced motors requiring the user to gather propellant grains, o-rings and washers (to contain the expanding gases), delay grains and ejection charges into special non-shattering aluminum motor casings with screw-on or snap-in ends (closures). The advantage of a reloadable motor is the cost: firstly, because the main casing is reusable, reloads cost importantly fewer than single-use motors of the same impulse. Secondly, assembly of huger composite engines is labor-intensive and difficult to automate; off-loading this task on the consumer results in a cost savings. Reloadable motors are available from D through O class.

The components of a commercially produced G64-10W motor The components of a commercially produced G64-10W motor made by Aerotech Consumer Aerospace for a 29/40-120 casing. 1. Motor Casing 2. Aft Closure 3. Forward Closure 4. Propellant Liner 5. Propellant Grains (C-Slot Geometry) 6. Delay Insulator 7. Delay Grain and Delay Spacer 8. Black Powder Ejection Charge 9. Delay O-Ring 10 & 11. Forward and Aft O-Rings 12. Forward Insulator 13. Nozzle 14. Electric Igniter

Motors are electrically ignited with an electric match consisting of a short length of pyrogen-coated nichrome, copper, or aluminum bridgewire pushed into the nozzle and held in place with flameproof wadding, a rubber band, a plastic plug or masking tape. On top of the propellant is a tracking delay charge which produces smoke but keyly no thrust as the rocket slows down and arcs over. When the delay charge has burned through, it ignites an ejection charge, which is used to deploy the recovery system.