Research On Guidance And Control Technology Of Suborbital Reentry For Reusable Launch Vehicle

Suborbital flying test is used to demonstrate key vehicle and operational technologies applicable to reusable launch vehicles (RLV) on high altitude and high speed. RLV will be air-launched from a carrier aircraft and will be accelerated by onboard engine to speed above Mach 6 and altitude 72km. An unpowered entry will follow, including a suborbital reentry which flight from the apogee to the interface of Terminal Area Energy Management (TAEM). The thesis aims to resolving guidance and control problems on suborbital reentry with strict flight constraints and complex dynamic characteristics.The altitude and velocity of suborbital reentry are between orbital reentry and TAEM. The suborbital reentry is divided by three sequential phases: Alpha Recovery, Nz Hold and Alpha Transition. The Alpha Recovery phase commands a constant angle-of-attack of large magnitude to arrest the initial sink rate and set up the normal load. The Nz Hold phase is activated when the normal load approaches the pre-defined value and attempts to maintain loads. Once the sink rate is sufficiently small, the Alpha Transition phase brings the angle-of-attack and normal load to smaller values and delivers the RLV to TAEM interface box. The thesis studies guidance technology by sequential phases, and researches on lateral control technology with large angle-of-attack.The guidance design of Alpha Recovery and Nz Hold is to handle the dynamic pressure constraint. Since the sink rate dominates the dynamic pressure rate on this regime, arresting sink rate by large angle-of-attack and large normal load is the most effective method to reduce dynamic pressure. To satisfy flight constraints, the trajectory programming based on angle-of-attack and normal load command is present. Considering the effect of initial state error and aerodynamic uncertainty to dynamic pressure, an autonomous guidance is proposed to adjust the alpha and Nz command to initial altitude and close-loop guidance method are proposed to adjust Nz command to sink rate error or track sink rate trajectory on Alpha Recovery. When the initial altitude and velocity go beyond certain range, the guidance is impossible to satisfy dynamic pressure constraint. The limit of guidance ability could be described by the upper-limit of initial state, and the algorithm for upper-limit is present.To make the guidance more adaptive, a predictor-corrector method is developed, which integrate the equations of motion, evaluate the peak of dynamic pressure, modify (iterate) the alpha and Nz command to remove the error of dynamic pressure. The algorithm has cleared the risk of convergence because of the linear correlation between the peak of dynamic pressure and Nz/alpha command used in the trajectory. The guidance of Alpha Transition phase is to satisfy the terminal altitude and velocity conditions. To avoid skip and assure flight distance, the condition to enter Alpha Transition phase should be tuned to altitude, Mach and normal load. The function of the switch criteria can be established through hundreds of simulations. To guarantee the TAEM interface constraints, an analytic trajectory solution based on dynamic pressure programming is presented for the Alpha Transition. The method to schedule the profile of dynamic pressure via altitude is present. Then an analytic algorithm is proposed to calculate the trajectory parameters based on dynamic pressure.There are control problems such as the Dutch roll instability, roll reverse on Alpha Recovery and Nz Hold due to large angle-of-attack. The yaw rate feedback to rudder is of none effectiveness in damping the Dutch roll, and aileron to rudder interconnect cannot improve LCDP. The lateral/directional control concept essentially reverses the traditional role of the lateral force effectors. The feedback of roll rate to aileron is used to damping the Dutch roll while rudder is used to damping roll mode and maintaining the roll attitude. The simulation shows the control strategy is adapt to the flight characteristics of large Mach and large alpha.At the start regime of descent the rudder power is very limited, and yaw RCS are used. The RCS control law are designed, with great computational advantage, for a linear system and then applied in a nonlinear way with a revised pulse-width modulation based on principle of equivalent accumulated impulse. The simulation result shows the validity of this method.Last, high fidelity simulation platform is developed based on the concept of"equivalent flight control"and entire nonlinear flight simulations are completed. The results demonstrate that the guidance scheme and control scheme is reasonable, the guidance law and control law is robust.