So the total force on the tail fins in the presence of the body is determined by multiplying the normal force on the fins by the interference factor. In other words,

                                                (CNα)fb = Kfb x (CNα)f

The fin CP is located at the same place on any two fins of the same size and shape. Since the fins on a rocket are generally all the same size and shape, the actual position of the CP does not depend on the number of fins.

Based on this,

__

X   = Xf + ΔXf

            __

where:   X = distance from nose tip reference

              Xf = distance from nose tip to top of

                      fin root

              ΔXf = distance below top of fin root

                        where normal force on fins

                        acts (see diagram)

This equation can be calculated out to be =

The total force on the entire rocket is the sum of the forces on the separate sections; so:

                                 CNα = (CNα)n + (CNα)cs + (CNα)cb + (CNα)fb

The CP for the entire rocket is found by taking the moment arm for each section (that is, the normal force for that section multiplied by the moment arm, that being the distance of the CP for that section from the nosecone tip reference) and solving for the total CP location. For our simple example where we ignore the conical shoulder and conical boat tail:

                                                                 __              __                __

    CNα = (CNα)n + (CNα)fb        and           X = (CNα)n.Xn + (CNα)fb.Xf

                                                                                    CNα

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