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.
Typical on-pad global modes (order-of-magnitude)
| Mode | Frequency (Hz) | Notes |
|---|---|---|
| 1st lateral bending | ~0.2–0.3 Hz | Strongly affected by hold-down arms |
| 2nd bending | ~0.6–0.9 Hz | Tower coupling sensitive |
| Higher bending | >1.5 Hz | Less critical pre-liftoff |
| Torsion | ~1–2 Hz | Depends on tank fill state |
These are significantly lower than in-flight values because:
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The vehicle is constrained at the base
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Effective boundary conditions are neither fixed nor free
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Service arms and umbilicals add stiffness and damping
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Wind loading dominates excitation
Reference
Description and Performance of the Saturn Launch Vehicle’s Navigation, Guidance, and Control System (NASA TN D-5869)
See also
Beam Bending Natural Frequencies & Mode Shapes
– Tom Irvine