## 1 Introduction  
As a fast-response, reliable, and low-cost vehicle to access space, the vertical-takeoff horizontal-landing (VTHL) reusable launch vehicle (RLV) has become a research hotspot in recent years [1, 2] The shape diagram of the VTHL RLV studied in this paper is shown in Figure 1. However, due to its complex temperature variation and wide reentry flight envelope, RLV is prone to parameter uncertainties and complex aerodynamic characteristics in the reentry phase, which makes controller design a challenging task. Hence, an advanced reentry attitude control scheme with sufficient robustness and fast convergence rate is significant for RLV to achieve a successful mission. Figure 1: VTHL reusable launch vehicle. A variety of control methods have been developed to improve the flight quality of reentry vehicles. [PERSON] et al. [3] proposed a fractional-order PID control method for hypersonic vehicle attitude control. [PERSON] et al. [4] proposed an active disturbance rejection controller (ADRC) for an air-breathing supersonic vehicle with a new smooth function. [PERSON] et al. [5] proposed an adaptive fuzzy H-infinity controller for RLV under parameter uncertainties and external disturbances. [PERSON] et al. [6] designed a backstepping technique and combined a nonlinear observer for hypersonic vehicle, which realized stable tracking of attitude. [PERSON] et al. [7] investigated a controller for hypersonic flight vehicle with robust adaptive neural networks. Compared with the above methods, sliding mode control (SMC), which is robust to disturbances and insensitive to parameter uncertainties, has attracted the most interest [8; 9]. The peculiarity of SMC is to adjust the attitude tracking errors of RLV toward a predefined sliding surface and then avoids them leaving this surface. As it is known, the traditional SMC merely ensures the asymptotic stability of the control system. Then, a finite-time SMC with solid robustness against internal and external disturbances was proposed by [PERSON] et al. [10] and [PERSON] et al. [11] to obtain finite-time convergence of states. Since the initial condition of the system determines the settling time of finite-time SMC. It may destroy the desired fast convergence performance when the initial values are unknown in practice. Thus, the fixed-time SMC (FxSMC), which has achieved great development in the past several years, can ensure that the system convergence time is independent of the initial conditions [12; 13]. [PERSON] et al. [14] presented a fixed-time nonsingular terminal SMC for RLV. [PERSON] et al. [15] presented an FxSMC to ensure that the vehicle attitudes track the desired command in a fixed time. [PERSON] et al. [16] designed a robust controller for RLV based on a FxSMC. However, the convergence time of the FxSMC is actually obtained after complex calculation rather than known before controller design, which increases the offline workload. Thus, the predefined-time stability has been widely studied in recent years [17]. The upper bound of the settling time is displayed explicitly as an adjustable control parameter [18]. [PERSON] et al. [19] designed a formulation for robust predefined-time stability of second-order system and proved the reliability of the scheme through simulation. [PERSON] et al. [20] applied a predefined-time SMC to a launch vehicle attitude control. A robust predefined-time SMC was proposed with external disturbances [21]. However, the application of these predefined-time SMC methods is limited by a singular problem, and the studies on nonsingular predefined-time SMC have rarely been reported. There is insufficient attention paid to the system transient tracking performance, although the above controllers can guarantee good steady-state control performance.