RASAero II assumes the motor propellant Center of Gravity (CG) is located at one-half the motor length from the bottom of the rocket, or the bottom of the individual stage. It gets this motor length from the rasp.eng motor data, so it is important that the motor length in the rasp.eng file is correct.
The initial CG for the rocket, or for a two stage rocket the initial CG for the configuration with the booster, and the configuration with the sustainer only, is then entered.
When developing RASAero II, rather than having just another rocket design program, we put our effort into more accurate aerodynamic predictions, and more accurate altitude predictions. A summary altitude prediction accuracy plot is attached, see the RASAero web site here:
https://www.rasaero.com/comparisons-alt.htm
and other examples at
www.rasaero.com for more altitude prediction accuracy examples and comparisons with wind tunnel data and in-flight measured Drag Coefficient (CD) data.
Many rocketeers do the design lay-out in Open Rocket or RockSim to get the weight and CG, and then convert the rocket into RASAero II for the final aerodynamic predictions and altitude predictions.
Note that many of the airfoils used on high power rockets are not available in Open Rocket or RockSim, so when the rocket is converted into RASAero II the specific airfoils have to be added. Relative to the aerodynamic prediction accuracy, and altitude prediction accuracy, that’s something important to note.
One of the errors in Open Rocket and RockSim altitude predictions is that the predicted weight and CG are never updated with the actual values. Compare your predicted Open Rocket and RockSim weights to the actual weights when flown. In the end for the best accuracy you need to use the manual override values for weight and CG.
Again, many rocketeers do the design layout in Open Rocket or RockSim, and then convert the rocket to RASAero II for the final altitude prediction and final stability prediction. This is almost standard procedure on Tripoli Class 3 rockets. RASAero II reads RockSim files, Open Rocket files can be converted to a RockSim file, which then can be imported into RASAero II.
Charles E. (Chuck) Rogers
Rogers Aeroscience