Abstract:

Considering the insufficient capability of online planning and limited guidance precision of traditional three-dimensional offline, or two-dimensional onboard algorithms for terminal area energy management (TAEM) phase of reusable launch vehicles (RLVs), a three-dimensional onboard trajectory planning algorithm based on kernel extraction protocol (KEP) and differential flatness theory is proposed. Firstly, the KEP three-degree-offreedom dynamic equation is derived, where the height is deemed to be an independent variable and the dynamic pressure rather than velocity is chosen as a state variable. Secondly, three flat outputs (crossrange, downrange, and, dynamic pressure) are generated based on the flatness properties of KEP equation, and an optimal control problem (OCP) without differential equation constraints is established. Thirdly, flat outputs and their derivatives are parameterized in terms of Bspline curves, and the OCP is discretized to render a nonlinear programming (NP) problem. Finally, the resulting NP problem is solved by sparse nonlinear optimizer (SNOPT). Numerical simulations are presented to illustrate the effectiveness of the proposed approach.