Space Launch Environment and Vibration Testing
The pedigree of spacecraft dynamic load testing is through the one-axis-at-a-time; however, the launch vibration environment used to be a simultaneous six-axis, including three translational and three rotational. Literature suggests that the multi-axis environment induces different acceleration and stress states in the article, but fails to quantify the degree of over-/under-testing through a multi-axis shaker. The primary objective is to quantify the level of over-/under-testing through sequential single-axis and simultaneous tri-axis vibration testing. The multi-axis vibration test setup is complicated and costly, especially when the six-degree-of-freedom shaker setup is used. The secondary objective was to develop an algorithm that can induce six degrees of freedom excitation effect through single- or tri-axis shaker testing. Different spacecraft models were investigated through transient, random and shock response analysis to quantify the degree of over-testing and conclude that at least twice the base input was needed through sequential single-axis testing to envelop simultaneous tri-axis induced acceleration and stresses. This research further investigates that enveloping the resultant response, measured by a tri-axis accelerometer, must match the resultant response magnitude as well as the direction. Coupled load analysis estimates the spacecraft base acceleration levels and output of three translational and three rotational accelerations, and the effect of these rotational excitations is also studied. An algorithm is proposed using linear regression to apportion the rotational excitation effect on translation excitation. The rotational excitations are parametrised to study the effect on the responses and efficacy of the algorithm. A physical spacecraft structure was built, and a 1D and 3D-shaker test campaign was run. Generally, finite element models used to be correlated using single-axis tests; however, the 3D-shaker test was also used to correlate the model, and it was found that the 3D-shaker test correlates better compared to the 1D-shaker test. The virtual shaker concept was used to include a 3D-shaker table in a finite element model to investigate the effect of table dynamics in high-frequency mismatch during correlation. The apportionment algorithm obtained base shake input was used to test the article through a 3D-shaker, and both the algorithm and test response showed a very good match.
Multi-axis vibration testing, 3D-shaker testing, test correlation, space structures, frequency response assurance criterion, component mode synthesis, Coupled Loads Analysis (CLA), 3D-shaker test correlation
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