Quantification of the Effect of Simultaneous Tri-Axis over Sequential Single-Axis Vibration Testing

The launch mechanical environment presents a multi-axis random vibration excitation to the launch vehicles and spacecraft payloads. However, ground vibration testing of these systems abides by sequential, single-axis inputs, enveloping flight vibration levels along individual axes. The actual vibration direction depends on the excited mode shapes, and in general it is not precisely aligned with any given reference axis of the article. The response acceleration could be in any direction, with strong local variations expected, depending on both the properties of the structure of interest and the applied loads. In this paper, it is first shown that matching all response acceleration components due to a realistic environment by single-axis testing is, in general, impossible. Subsequently, minimum amplification factors for single axis to match realistic reference environments are quantified, such that both acceleration responses and elemental stresses were covered for the whole spacecraft in at least one of the uniaxial tests. This procedure was repeated for five different realistic spacecraft models. It was found that a consistent amplification factor and, respectively, overtesting, were necessary for the single-axis excitation sequence to properly validate the articles' survivability against the true mechanical launch loads.