During launch of a rocket, there are many transient vibrations. Some are critical for the survival of the payload (the satellite). Our client needed new tools to analyse these transient vibrations. We had to go further that the state of the art algorithms even in academia to satisfy our client’s needs.
To precisely identify the impact of transient vibrations on the satellite, it is mandatory to know the dynamic properties of the rocket launcher at the moment the vibrations occur. It is also necessary to identify the vibration source. This identification becomes critical in the case of increasing vibration levels without any clear explanation of the root cause of this increase (more rigid or less damping structures or a higher vibration level at source).
Before this work, there were no tools enabling modal analysis in transient conditions (or non-stationary conditions) without access to the structure's input. All operational modal tools are based on a stationary input hypothesis.
Cornis developed a blind identification method (or operational method) coping with transient vibrations and allowing system and input identification. The modal properties of the structure are obtained and the characteristics of the input are also obtained.
Cornis used a simplified parametric model of the structure and the input it was subject to. The behaviour of the structure under transient input was analysed. Thanks to the mathematical tools based on analytical signal theory, phase and amplitude information were extracted. This extraction enables a fit with the parametric model. This fit enables a precise identification of the structures modal properties (that are difficult to identify since they evolve during flight due to the loss of propellant mass) and to quantify and characterise the input it was subject to.
Thanks to this novel method our client is now able to quantify the resonance frequencies of the rocket structures but also the evolution of the input frequencies. Our client has been able to refine the interpretation of in-flight data and was able to take the right decisions to counter the increase in violent transient vibrations.