A rocket vehicle behaves as a free-free beam during flight. The vehicle’s body bending modes can be excited by wind gusts, aerodynamic buffeting, thrust offset, maneuvers, etc. The bending motion can interfere with the vehicle’s control system. The guidance, navigation and control algorithms must be designed with sufficient stability to withstand these perturbations and the resulting elastic body response.
Description and Performance of the Saturn Launch Vehicle’s Navigation, Guidance, and Control System (NASA TN D-5869)
– Tom Irvine