Aerodynamics Uncertainties Compliance with Desired Lateral-Directional Dynamics for an Unmanned Space Vehicle

A key aspect for 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 optimizing 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 to save costs for aerodynamics database and system configuration development. A methodology aimed at quantifying the admissible ranges of uncertainties in which some basic vehicle properties can be guaranteed is presented. Specifically, the properties we refer to are stability and D-Stability of the lateral-directional dynamics of an RLV-shaped, un-piloted, un-powered aircraft. The latter is a property wider than simple stability in that it allows vehicle instability to be accepted if the Stability Augmentation System can still enforce the desired dynamics. The proposed approach basically reduces the problem of determining the dynamic characteristics of the complete nonlinear system to the 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. Then, the admissible uncertainties region is identified by means of a numerical code, based on a polynomial approach deriving from recent theoretical results for polynomials with uncertain coefficients. An application case on lateral-directional aerodynamic stability derivatives for the CIRA USV-FTB1 autonomous RLV demonstrator vehicle is analyzed. Results show the method’s ability to identify the maximum admissible uncertainties, and to address the areas of major concern. Since the admissible uncertainties region is a five dimensional set, a geometric projection-based visualization tool has been developed to provide meaningful insight into the admissible uncertainty region shape and extension.

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