Allowable Aerodynamics Uncertainties Synthesis Aimed at Dynamics Properties Assessment for an Unmanned Space Vehicle

A key aspect in the success of a space project is the capability to detect as soon as possible the problems that can arise during the project development. This approach allows to optimize mission reliability, project costs and temporal delays. As an example, given the extended range of flight regimes experienced by new generation Reusable Launch Vehicles ( RLV ) demonstrators (as NASA’s X-40A, X-43 ) throughout the various mission phases, assessing the impact of the aerodynamic uncertainties on the overall system performance is of great importance. System design should be performed so that uncertainties, with particular concern to the aerodynamics ones, do not significantly affect some basic vehicle properties, such as trajectory trimmability and dynamic stability. Therefore, identifying such admissible ranges of uncertainties might be a powerful system analysis methodology which could effectively help aerodynamics database refinement and system configuration development. A methodology aimed at quantifying the admissible ranges of uncertainties in which relevant vehicle properties can be guaranteed is presented. Specifically, the properties to which we refer are Trimmability and D-Stability of the longitudinal dynamics of an RLV-shaped, un-piloted, un-powered aircraft. The latter is a property wider than simple stability, in order to take into account that vehicle instability is acceptable if the Stability Augmentation System can still enforce the desired dynamics. The approach basically reduces the problem of determining the dynamic characteristics of the complete nonlinear system to an analysis of the robust stability of linear systems subject to uncertain parameters, by means of system’s linearization around a predetermined set of flight conditions. An analytic modeling of the system dynamics is carried out applying the well known Short Period Approximation, and successively deriving the necessary conditions that enforce stability and D-stability conditions as explicit functions of parameters of interest. An application case on a winged, autonomous RLV demonstrator vehicle is analyzed, modeling uncertainties on the vehicle longitudinal aerodynamic stability derivatives. Results have shown the method’s ability to identify the maximum admissible uncertainties, and to address the areas of major concern. Specifically, maximum acceptable uncertainties in terms of CLα and CMα have been identified, and can help to identify the needed level of accuracy in the aerodynamics model. Furthermore, when open-loop stability results are compared to closed-loop ones, indications can be derived on necessary flight control system features, in terms of both robustness and capability to cope with unstable plants.