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Vibro-Accoustic Analysis
The sources of vibro-acoustic excitation are mainly due to noise generated by the launcher during ignition, lift-off and atmospheric flight. The vibro-acoustic environment is usually a dimensioning load case for light-weight structures like antenna reflectors and solar arrays. For equipments mounted on the panels of a spacecraft, the dynamic response of the panels to the acoustic load leads to high and, often dimensioning, random vibration loads.
All the equipments of a spacecraft must successfully go through qualification mechanical tests before they are allowed to be flown. For the vibro-acoustic environment qualification, random vibration test is generally specified as qualification requirement for equipments mounted on panels. As for larger appendages, acoustic test in reverberant chamber is the preferred choice.

These equipment specifications have to be derived in the early design process with very limited information on the actual configuration of the spacecraft. Thus, specifications need to be adequate but not too conservative.

Early definition of the specification based often on heritage of test data from other spacecrafts and on empirical data [ECSS-E-10-03A].  

Definition of the specification based often on heritage of test
Definition of the specification based often on heritage of test data
In some situations (e.g. spacecraft of unique dimension or shape like the example shown below), neither the heritage nor the empirical data are considered adequate, the vibro-acoustic analysis is then used.
Spacecraft of unique dimension or shape
Spacecraft of unique dimension or shape
Also, sometimes at a later stage during the spacecraft development, vibro-acoustic analysis is performed in order to re-evaluate the equipments’ specifications. This occurs when certain design constraints of the equipments lead to failure to meet the original specifications. Thus, a more precise evaluation is needed.
The technique used in the analysis for the low frequency range (< 500Hz) is based on the established boundary element / finite element coupling method. The structure of the spacecraft or equipment is considered as elastic body whose dynamic motion caused by the sound pressure load is fully coupled to surrounding the media. The picture below shows the comparison of test and analytical responses at a location on the sun shade of HERSCHEL S/C.
Analysis for low frequency range
Analysis for low frequency range
For what concern the high frequency range, the Statistical Energy Technique [SEA] is used. Due to the high modal density of the structure at high frequency, the dynamic response is often expressed as a mean value for a given substructure. One typical application of the statistical energy technique is on the evaluation of the pressure level inside the fairing of a launch vehicle. The figure below presents the works performed on the fairing of VEGA LV.
Last update: 12 March 2013

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