You don't. All of the major motor producers have had accidents where lives were lost. Do you think you can be safer than Aerotech, Estes, etc.? Focus on the aerodynamics of model rockets. You'll have more fun that way.
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At the very bottom is the nozzle, after that is the fuel. Depending on how big the rocket is and how much fuel it needs determines the size of the chamber
They are often more polluting, less efficient, and the whole rocket has to withstand the immense heat of the burning propellant. If it can't cope, something may give, and spew burning fuel and/or...
There are two kinds of solid fuels commonly in use today. The first is a mixture of black powder and solid sulfur. The second is called composite rocket fuel, since it's so much newer, it's very...