Vibration Response Analysis: Spacecraft Jitter

Space based observatories are sensitive to flight vibration, where onboard mechanisms such as reaction wheels create mechanical vibration that degrades the optical response.

Major lessons learned:

• Use a multiplicative Model Uncertainty Factor (MUF) to account for errors in the Finite Element Model (FEM) and other models.

• Reaction Wheel Actuator (RWA) disturbances should be represented by a speed-squared model, applied to a system FEM that includes the RWA dynamics, and incorporating gyroscopic moments.

• Flight realizable isolators have various physical characteristics that reduce isolation performance, which must be correctly modeled; additionally, the system design should be assessed and potentially modified so that the isolator can achieve its design performance.

• Use a coupled damping model for systems with a combination of lightly damped and highly damped modes, for example a system with isolators.

• Stepper motors are always present on flight systems, can be the largest mechanical disturbance, and their induced disturbance is a strong function of the operating step rates.

• High value missions such as NASA Class A (Hubble, James Webb, or Nancy Grace Roman Space Telescope), should perform a physical parameter Monte Carlo vibration response analysis to account for parameter uncertainties.

• Large monolithic optics can bend quasi-statically under flight vibration loads, at frequencies well below their first mode, which can be the dominant source of wavefront error, so bending of optics should be included when analyzing the optical response of systems with large optics.

More to come on these topics in future posts.

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