The nozzle shall not be subject to excitation at its natural frequency of vibration.

The stiffness of all parts of the nozzle should be designed so that their natural frequencies are higher than the natural frequency of the hydraulic actuator system. If the nozzle natural frequency is almost equal to the natural frequency of the actuator system, coupling of the nozzle and the actuator system will occur and will produce instability. If the nozzle natural frequency is less than the natural frequency of the actuator, coupling with the guidance system will occur. Further, the nozzle natural frequency must be greater than the natural mechanical frequencies of the motor and vehicle to ensure that no coupling that could cause destructive failure of the nozzle results.

The natural frequency of the nozzle and motor assemble should be measured prior to static firing.  The assembly should be subjected to a frequency range determined from consideration of the control system response, but if this is not known, it is recommended that a frequency response tests, the natural frequency must be calculated. When the natural frequency is known, a notch filter should be incorporated into the control system to suppress vectoring commands at or near the natural frequency.