Due to the complex disturbances and uncertainties in return process

the attitude system of vertical takeoff vertical landing (VTVL)-reusable launch vehicle (RLV) is nonlinear and high dynamic. To solve the nonlinear attitude tracking control problem with complex disturbances in the return process of VTVL-RLV

a composite fast attitude tracking control method consisting of double-order power fixed-time convergence sliding mode surface and fixed-time convergence disturbance observer (FxTDO) was proposed. The FxTDO was introduced to obtain high precision approximation of the bounded matched disturbance and avoid the influence of the observer on the performance of the control system. In order to achieve fixed-time convergence of attitude tracking controller

the double-order power fixed-time convergent sliding mode surface was given based on double-power correction terms. By introducing the disturbance approximation to directly compensate the matched lumped disturbance in double-order power fixed-time convergence sliding mode control law

the discontinuous control term was removed to reduce the chattering phenomenon at permissible loss of convergence accuracy. Simulation results of the aerodynamic deceleration flight phase revealed the effectiveness of the FxTDO and the proposed FxTDO-based fixed-time convergence sliding mode tracking control method.