Model Rocket Motors and Engines

Most small model rocket motors are single-use engines, with cardboard bodies and lightweight molded ceramic nozzles, ranging in impulse class. Model rockets generally use commercially-manufactured black powder . These motors are tested and certified by the the National Association of Rocketry, Tripoli Rocketry Association or the Canadian Association of Rocketry. The motors come in standardized sizes A, B, C, D and E. The diameter of each size is A 13, B 18, C, D and E 24mm. They contain a black powder propellant.

The maximum sized motors are typically E-sized, 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. During ignition, the internal chamber pressure of the engine may exceed the strength of the paper case, causing the motor to burst. This can channel the blast pressure through the rocket's tubular body with effects ranging from a simple ruptured tube to the violent ejection (and occasionally ignition) of the recovery system.

Larger rocket motors thus customarily employ composite propellants made of ammonium perchlorate, potassium nitrate, aluminum 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 reliability. These motors range in impulse from size B to O. Composite motors produce more impulse per unit weight (specific impulse) than do black powder motors.

Reloadable motors are also available. These are commercially-produced motors requiring the user to put 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 a short length of pyrogen-coated nichrome, copper, or aluminum wire pushed into the nozzle and held in place with flameproof wadding, rubber band, a plastic plug or masking tape. On top of the propellant is a tracking delay charge which produces smoke but essentially 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.

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

 

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