This model is loosely based on the Fall and Skydive models. It includes the simplifying assumptions that the only forces acting on the rocket are thrust from burning fuel, and gravity, that acceleration due to gravity is constant, the Earth is flat, the rocket is vertical, and the rocket only has one stage.
Once we have these simplifying assumptions in place, we can more easily conceive what's happening in the model.
This model starts with three stocks: position, velocity, and mass.
Position: Initial position is 0 because the rocket is on the ground and hasn't been launched. The change in position flow shows the change in position based on the velocity stock.
Velocity: Velocity in this model is initially 0, however, it quickly changes via the Change in Velocity flow, which is calculated based on the acceleration due to gravity, plus the specific impulse divided by the rocket's mass.
The specific impulse is the impulse per unit weight of burned fuel. In this model, the specific impulse term contains logic that says that (Time() < Burnout_Time) ? Specific_Impulse : 0. The length of the specific impulse comes from the specific impulse slider. As long as the time step is lower than the burnout time, the specific impulse will equal what is indicated on the slider. Once the time step reaches burnout time, the specific impulse will equal 0.
Mass: The initial value of the mass stock is 5000. The term Rocket Mass acts as an additional stock that contains the total mass of the rocket and fuel. Burnout time is the amount of time until all of the fuel is burned. The change in mass flow is constant while the time step is lower than the amount of time until burnout, and contains the amount of unburned fuel divided by the burnout time. After burnout, change in mass becomes 0.