Quasi-static loads in spacecraft are low-frequency mechanical forces resulting from the rocket’s steady-state acceleration (like gravity) combined with low-frequency vibrations that occur below the payload’s fundamental frequency. Because they act slowly enough to avoid resonant amplification, they are treated as steady or “static” forces in structural design. [1, 2, 3, 4]
The primary causes of these loads include:
- Steady Acceleration: The massive thrust of rocket engines creates continuous G-forces. The peak acceleration typically occurs just before stage burnout (when the rocket is lightest), heavily loading the spacecraft in the axial direction (along the rocket’s length). [1, 2]
- Engine Operations (POGO and Thrust Oscillation): Unsteady combustion in liquid or solid rocket boosters can create low-frequency pressure pulsations. Longitudinal oscillations, known as POGO, directly subject the spacecraft to rhythmic, quasi-static stretching and compressing.
- Wind and Atmospheric Buffeting: As the rocket pushes through the atmosphere, wind shear and aerodynamic forces create low-frequency bending. These forces act laterally against the spacecraft. [1, 2]
- Maneuvers: Rocket trajectory corrections (yaw, pitch, roll) and orbital maneuvering burns impart steady lateral and axial inertial loads.
- Transient Events: Events like stage ignition, stage separation, and fairing jettisoning can trigger low-frequency structural bouncing, which contributes to the overall quasi-static load envelope. [1]