Abstract: Within the range of a rocket/launcher system, the influencing factors on the initial disturbances of rockets are analyzed. Differential equations of motion for the rocket during the semi-constraint period are constructed using the Newton-Euler method. Estimation of initial disturbances of the rockets with vibrations of the directional tubes are given. The results of estimation are compared with the simulation results applying the dynamics simulation software ADAMS. There exist only minor differences between the results by the proposed method and those from the software ADAMS, when the clearance between the rocket and the directional tube is small enough. The differences become larger when the clearance is larger. It is proved that the results by the proposed method is accurate enough when the clearance is kept small enough.