Pendulum Mode Control in the Dynamic Analysis of Lift-off of Launchers

Predicting the vibrational behavior of a launcher and its payload during lift-off is of prime importance to guarantee the integrity of the system when excited through thrust and acoustics loads. The payload and launcher structures are shell constructions that experience significant prestress due to gravity, acceleration under the action of the thrust and internal pressure in fuel tanks and boosters [

1

], [

2

]. The prestress effects are accounted for in vibration simulation through additional stiffness contributions. Some of the prestress contributions (such as the total apparent gravity forces) cause the zero eigenfrequencies of the rigid body modes to become negative and therefore render the model unstable. In practice the launcher is stabilized by a Thrust Vector Control such that the rigid body motion of the launcher has low positive associated eigenfrequencies.

In this contribution we will recall a numerical procedure to stabilize the prestressed launcher model [

3

]. Introducing an explicit rigid degree of freedom and applying a proper change of basis the rigid body motion of the system can be modified. The procedure will then be interpreted as a collocated controller to bring the rigid modes back to a zero eigenvalue. This is not exactly how real thrust vector controllers are implemented in the system. However, based on the analysis of a simplified thrust vector controller, we will show that the numerical procedure affects the vibrational dynamics of the launcher in a way very similar to the a real thrust vector and can therefore be used in numerical simulations.

We will discuss the consequence of different choices of sensor and actuator locations in the numerical procedure and give results obtained for the lift-off analysis of an Ariane 5 launcher.