Dynamic Lateral Entry Guidance Logic

Abstract

Lateral motion of an entry vehicle is controlled by the sign of its bank angle, determined by the entry guidance system. The conventional technique for changing the bank sign is based on prespecified threshold values in the heading error of the vehicle with respect to the landing site. A new automated lateral guidance logic is developed based on the crossrange, which is found to be a more suitable parameter than the heading error for lateral guidance. The present guidance logic determines the bank reversals by constantly evaluating information from the reference crossrange profile, current crossrange, and estimated actual lift-to-drag ratio. Near the end of the trajectory, a noniterative numerical predictor decides whether a final bank reversal is needed to null the heading error. This algorithm enables the entry guidance system to fly a wide range of missions and vastly different bank-angle profiles and provides reliable and good performance in the presence of significant aerodynamic modeling uncertainty. All of these tasks can be accomplished without requiring manual tuning of guidance parameters or using an excessive number of bank reversals. Extensive high-fidelity simulations with different mission scenarios and significant dispersions are presented to demonstrate the performance of the proposed method. I. Introduction T HE entry guidance system for a lifting entry vehicle controls the bank angle and angle of attack during the atmospheric flight from the entry interface at about altitude 120 km until the velocity decreases to Mach 2‐3 (Ref. 1). The guidance commands are generated by tracking a reference trajectory which is designed either in preflight planning, 1 or potentially on board, as recent efforts have attempted to achieve. 2−4 In the traditional approach, the reference to be tracked represents the desired longitudinal profiles as in the case of the shuttle, where the reference is a drag-acceleration-vs-velocity profile that equivalently defines the range-vs-energy condition. 1 Tracking the longitudinal profiles determines the magnitude of the bank-angle command. The sign of the bank-angle command, on the other hand, is changed to the opposite whenever the heading error with respect to the targeted landing site exceeds a prespecified threshold. This forced change of bank sign is referred to as bank-angle reversal. The traditional approach works well when the actual flight goes as planned in pre-mission analysis and the actual bank-angle magnitude remains close to the reference value. When the bank profile flown is significantly different, the bank-reversal threshold criterion will require adjustments, typically made through simulations. The situations where a very different bank-angle profile may be necessary include entry from a different orbit, landing at an alternate landing site, variations in entry conditions leading to much different downrange and crossrange, and contingency entry missions (such as on-demand entry and aborts) that are not planned. The recent advances in trajectory-planning algorithms would potentially allow on-board generation of a reference trajectory based on the actual conditions. However, the corresponding bank-angle profiles could be drastically different from the nominal one. Similar situations can develop in the presence of significant aerodynamic modeling mismatch, where flying even the nominal longitudinal profiles may necessitate a bank-angle profile considerably different