Model Rocket - Model Rocket Motors

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 generally 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, although a few F blackpowders motors have been made.

The physically largest blackpowder model rocket motors are typically E-class, for black powder is very brittle. If a large black powder motor is dropped, or is exposed to many heating/cooling cycles (for 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 more quickly than it should, producing greater than normal internal chamber pressure inside the engine. This pressure may exceed the strength of the paper case, causing the motor to burst. A bursting motor can 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 ratings 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 type of propellant is similar to that used in the solid rocket boosters of the space shuttle and is not as fragile as black powder, increasing motor reliability 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 assemble 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 significantly less than single-use motors of the same impulse. Secondly, assembly of larger 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.

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 in essence 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.