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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. The team is looking into getting licenses. 

ProPep3

ProPep3. See the instructions here on installation and a tutorial

AFCESIC

See the instructions here on installation and a tutorial