This is a simplified rocket simulator to test different rocket configurations. The model solves the rocket equation using the explicit Euler method. The density of the atmosphere is calculated up to an altitude of 85 km. Based on this, the atmospheric drag of the rocket or stage is calculated using a constant cw value. By conserving the angular momentum, simple orbit maneuvers can be performed. As an option, the Earth's rotation at the respective launch site can be taken into account for the orbit calculations.

More information on orbital mechanics can be found here, e.g.: http://www.braeunig.us/space/orbmech.htm