ESA GNC Conference Papers Repository
LAUNCH VEHICLE STIFFNESS BUDGET SENSITIVITY TO CONTROL STABILITY CONSTRAINTS
In the design phase of a new launch vehicle, the stiffness budget analysis represents an essential task. Indeed, during its flight, the launcher is subject to significant external forces which may lead to instability. As a consequence, the stiffness of each item of the launch vehicle has a fundamental impact on the definition of the global bending modes parameters and on the fulfilment of the launch vehicle control stability requirements. In this context, this article focuses on a sensitivity analysis to the stiffness budget robustness, obtained by varying each items stiffness and computing the launch vehicle worst-case stability margins. The VEGA-E (Evolution) launcher has been considered as a benchmark for the activity under study. Indeed, this launcher is currently in the second development phase and lends itself well to this type of analyses that could lead to improvements in its design. The analysis proposed in this paper allows to graphically assess which parameters mostly affect the fundamental bending modes eigenfrequencies and shapes and to approximate the impact of stiffness budget changes in terms of launch vehicle performance in view of mass saving optimizations. A set of stiffness budget alternates has been considered and an uncertain Linear Fractional Representation (LFR) of the launch vehicle has been defined for each different design case. It is worth to underline that, since the slosh masses are not deterministically known by the Finite Element Model (FEM), the bending modes have been computed considering three different values slosh masses (minimum, nominal and maximum). Moreover, considering that the payload mass represents another factor that adds variability to the bending modes parameters, two different payload configurations (minimum and maximum) have been considered in the FEM model. With this set-up, the structural stiffness needs have been verified with respect to the control related requirements by examining all the considered data sets from the stability point of view. Since the stiffness variations mostly affect the bending modes, for each case taken under study a tuning of the bending filter has been performed using the structured H? technique. To fasten up computations, it has been assumed that the worst-case tuning condition is to be the least controllable design point according to the aerodynamic to thrust efficiency ratio. Results have shown that this strategy makes it possible to ease the controllability constraint in the stiffness budget optimization, allowing a possible improvement in terms of performance and guaranteeing at the same time the fulfilment of the required worst-case stability margins.