openMotor

OpenMotor is an open-source internal ballistics simulator for rocket motor experimenters. The software estimates a rocket motor's chamber pressure and thrust based on propellant properties, grain geometry, and nozzle specifications. OpenMotor can be used both on Windows, Mac, and Linux. See the instructions here on installation and a tutorial.

BurnSim

BurnSim is an internal ballistics simulator for rocket motor experimenters. It is Windows only and cost money. BurnSim is now free for students.

AFCESIC

AFCESIC stand for Force Chemical Equilibrium Specific Impulse Code

Download here and follow the instructions in the zip

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