Evaluation of Longitudinal Desired Dynamics for Dynamic-Inversion Controlled Generic Reentry Vehicles

Dynamic inversion is a control synthesis technique in which the inherent dynamics of a dynamical system are canceledoutandreplacedbydesireddynamics,selectedbythedesigner.Theoutputofsuchaninner-loopcontroller isthecontrol input, whichproducesthedesiredclosed-loop response.Thedesireddynamicsessentially form aloopshaping compensator that affects the closed-loop response of the entire system. This paper attempts to quantify the effect of different forms of desired dynamics on the closed-loop performance and robustness of a dynamicinversion e ight controllerfor reentry vehicles. Proportional, proportional-integral, e ying-quality, and ride-quality forms of desired dynamics are evaluated using time-domain specie cations, robustness requirements on singular values, quadratic cost, and a passenger ride comfort index. Longitudinal controllers are synthesized for a generic X-38 type crew return vehicle, using a set of linear models at subsonic, transonic, and hypersonic e ight conditions. For the candidate forms of desired dynamics and inversion controller structures evaluated here, results indicate that the form used impacts closed-loop performance and robustness and more so for some inversion controller structures more than others. The ride-quality dynamics used with a two-loop angle-of-attack inversion controller provide the best overall system performance, in terms of both time-domain and frequency-domain specie cations, and the evaluation criteria.

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