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Chapter 1 Introduction 1.1 Problem Description Hypersonic glide vehicle (HGV) is generally a near-space vehicle that can fly at a velocity of more than Mach number 5. It is characterized by high velocity�� long flight distance and high heating rate. Therefore�� through traditional methods�� it is difficult to obtain the optimal trajectory satisfying various constraints for HGV. Reference [1] drew the conclusion that the maximum range flight scheme can be approximated by the maximum lift-to-drag ratio gliding flight. Hence�� simulations of the maximum lift-to-drag ratio flight are carried out and the trajectory and heating rate profiles are shown in Figs. 1.1 and 1.2 respectively�� where the initial altitude is 94 km�� initial Mach number is 20.4 and initial flight-path angle is 0. As shown in Figs. 1.1 and 1.2�� the maximum lift-to-drag ratio flight trajectory is a skipping gliding trajectory with a large range. Due to the severe oscillations of trajectory�� the heating rate at the stagnation point cannot be controlled and the maximum heating rate may far exceed the endurance of the vehicle�� thus leading to destructive damage to the vehicle structure. It can be seen from Fig. 1.2 that the maximum heating rate reaches 1400 W/cm2�� which is far beyond the allowable peak heating rate of 650 W/cm2. The influences of the initial altitude and the initial Mach number on the maximum heating rate for the maximum lift-to-drag ratio flight are presented in Fig. 1.3. Fig. 1.3 shows the possibility that the maximum heating rate can be controlled below 650 W/cm2 at a certain initial height and Mach number. However�� due to the constraints of each phase and the requirement of the maximum range�� the initial altitude and the initial Mach number of the entry flight may not satisfy the heating rate constraint. This means that the constrained maximum range problem cannot be addressed by adopting the maximum lift-to-drag ratio gliding flight scheme directly. As a result�� it is necessary to further study the optimal control problem under various constraints and propose a fast on-line guidance algorithm. Fig. 1.1 Trajectory of maximum lift-to-drag ratio flight Fig. 1.2 Heating rate of maximum lift-to-drag ratio flight 1.2Research Significance Owing to advance in long-range�� high velocity�� near-space flight�� low detectability and strong maneuverability�� HGV has been developed vigorously by many countries. With characteristics of high speed�� wide range of speed variation and long range in entry phase�� HGV shows strong maneuverability and wide maneuvering range. Because of its advantages in fast arrival and maneuverability�� hypersonic glider is considered to be a re-entry vehicle with wide application prospect�� which can achieve long-distance�� fast and precise strike and force delivery. This book introduces the entry dynamic characteristics and various entry guidance methods for unpowered hypersonic gliding vehicle. The trajectory of hypersonic gliding vehicle can be generally divided into the ascending phase�� transition phase�� gliding phase and descending phase. Fig. 1.3 Profiles of maximum heating rate�� initial altitude and initial Mach number corresponding to the maximum lift-to-drag ratio flight The ascending phase is similar to that of ballistic flight vehicle and carrier rocket�� where rocket is used as the booster and has a comparatively mature technology. The transition phase has characteristics of high altitude and weak control ability�� thus large angle of attack (AOA) and small bank angle control method are adopted here to guide the vehicle to the starting point of the gliding phase. And the flight mission of hypersonic gliding vehicle is mainly realized by the flight control in gliding phase and descending phase. During entry flight of hypersonic vehicle�� the flight distance is long�� the airspace is large�� and the flight Mach number varies widely. Due to the continuous increase of heat during the long-range flight��the structure of the vehicle will be under great pressure. Therefore�� the peak and accumulation value of heating rate should be fully considered in trajectory design and entry guidance�� thus it is important to find a reasonable guidance method which can reduce the peak heating rate and total heat flux during the research. At the same time�� the control of dynamic pressure is also very imperative. When the vehicle mainly flies in near space with small air density�� if the dynamic pressure is too small�� the rudder will not have the capacity to provide enough moment to stabilize the attitude of the vehicle. Moreover�� in order to accomplish some specific missions�� the gliding vehicle needs to drop down rapidly at the end of the descending phase��which may lead to large dynamic pressure and load factor. Large dynamic pressure will produce too much hinge moment for rudder structure to sustain�� and large load factor will cause damage to some weak parts of the vehicle s