Propellant Grain
The propellant utilized in amateur experimental rocket motors may be simple in composition, being comprised of two main constituents -- fuel and an oxidizer. Such is the case with the "sugar" based propellants. Experimental composite propellants, on the other hand, may have a composition that is fairly complex, and may contain oxidizer of various mesh sizes, polymer binder, and even metals such as aluminum or magnesium. Curing agents, phase stabilizers, and solvents may be other additives included in small percentages.
For any propellant, additives may control the burnrate, either to accelerate or to slow the rate. An opacifier may be added to absorb heat that may otherwise be transmitted through a translucent grain resulting in unpredictable burning.
Regardless of the composition, however, all propellants are processed into a similar basic geometric form, referred to as a propellant grain. As a rule, propellant grains are cylindrical in shape to fit neatly into a rocket motor in order to maximize volumetric efficiency. The grain may consist of a single cylindrical segment (Figure 1), or may contain many segments. Usually, a central core that extends the full length of the grain is introduced, in order to increase the propellant surface area initially exposed to combustion.
Figure 1 -- Hollow cylindrical grain
The core may have a wide variety of cross-sections such as circular, star, cross, dog-bone, wagon-wheel, etc., however, for amateur motors, the most common shape is circular. The core shape has a profound influence on the shape of the thrust-time profile, as shown in Figure 2.
Figure 2 -- Core shapes and influence on thrust curve
Figure 8 -- A24-A3 (Nov.2007) and A24-B1 (May 2008) motor firings.
