Study the effect of tri-axis vibration testing over single-axis vibration testing on a satellite

The satellite launch vibration environment is very harsh, and all satellites and launcher subsystems have to survive it without degradation in performance in order to perform their intended function for the success of the mission. The real launch vibration environment induces multi-axis random vibration excitation to the launch vehicles and satellites; however, ground vibration testing of these systems are generally sequential single-axis (SDOF) input vibration testing enveloping flight vibration levels. The main objective of this paper is to compare the effects of multi-axis (tri-axis) vibration with those obtained from sequential single-axis vibration testing (which is the industrial standard) and quantify this mismatch as it can be source of significant undertesting or over-testing with respect to the real operational environment. Although the issue is well known, not much has been published to quantify these differences in a realistic scenario. A satellite model was analyzed for the single-axis and tri-axis (3DOF) random vibration and it was found that, to match the 3DOF response in SDOF testing, approximately 2.5 times higher base input will be required in the longitudinal axis, which causes over-designing the article. The base input is modified for SDOF excitation to match the acceleration response of 3DOF excitation, and this modified base input could not induce the same stress state as in 3DOF excitation, which means under testing the article in SDOF excitation. Similarly, the test article undergoes a different stress state while testing in SDOF over 3DOF, and to match the peak element stresses approximately 2 times in lateral and 14 times in longitudinal base excitation needed in SDOF testing compared to 3DOF testing, which resulted in providing higher margin to sustain the SDOF testing and ultimately higher mass and cost.