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| Propellant Grain Background |
This page discusses Propellant Grain basics for the Rocket Motor Simulation Module to help the user select appropriate grain geometry and chemistry for a particular application.
The selection of grain geometry must be based on the required thrust vs. time, but must also consider grain and airframe strength, heat transfer, erosion, and missile size requirements. An infinite series of thrust vs. time profiles can be generated by selecting various:
If the grain composition is assumed to burn at a constant rate (mm/sec. or in./sec.), the shape of the grain (geometry) determines whether the burn surface is progressive (increasing) with time (and hence increasing thrust), neutral (neither increasing or decreasing), or digressive (decreasing).
Generally, the exterior of the motor grain is coated with a combustion inhibitor, preventing burning on the external surface of the grain. Grains with external combustion inhibitors or grains which are externally bonded to the internal surface of the motor tube transfer less heat to the interior walls of the motor tube, thereby allowing the walls to be tube walls to be thinner and lighter. External burning grains require heavier walls and/or insulation to prevent severe heating.
Combustion gases formed at the forward end of perforated motor grains must pass over the exterior surface(s) of portions of the grain located closer to the nozzle. It is desirable to keep passages through the grain sufficiently large that gas velocities are low, thereby avoiding unwanted erosion of the grain surfaces.
The chemical characteristics of grains will be discussed in general in the following pages of the tutorial.
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