BurnSim ($39) is a solid rocket motor ballistic simulation program (Windows only) now in its third version.

In a nutshell, input your motor, nozzle and propellant characteristics and BurnSim calculates the Kn (kilonewton) through the burn and predicts estimated chamber pressure and motor performance. Once your motor design is in BurnSim, you can tweak the parameters such as nozzle diameter or grain core diameter and instantly see how the Kn, chamber pressure, and motor thrust is affected. BurnSim will also optimize your nozzle design.

BurnSim 3.0