Unpowered Approach and Landing Guidance with Normal Acceleration Limitations

Abstract

F UTURE reusable launch vehicles (RLV) may greatly benefit from advanced guidance and control (AG&C) technologies. Hanson [1–3] has argued that AG&Cwill greatly improve safety and reliability by successfully returning an RLV that is plagued by aerosurface failures, poor vehicle performance, and larger-thanexpected flight dispersions. Advanced guidance methods have been developed for the ascent, entry, and landing phases of an RLV [3– 10]. Approach and landing (A&L) is a critical flight phase that brings the unpowered vehicle from the terminal area energy management (TAEM) phase to runway touchdown. The Space Shuttle A&L guidance scheme uses a two-phase (steep and shallow) reference flight path, which has proven to be effective for low lift-to-drag (L=D) vehicles [11]. Shuttle guidance also relies on a small set of fixed A&L reference profiles, and therefore it may not be well-suited for scenarios with large trajectory dispersions, poor vehicle performance, or actuator failures. Schierman et al. [9] and Kluever [10] have developed onboard trajectory-reshaping algorithms for the A&L phase of an RLV. Both algorithms recompute a new reference A&L path in the presence of winds, aerodynamic uncertainties, and extreme trajectory dispersions. This Note presents an A&L guidance scheme for an unpowered vehicle with diminished maneuverability in the vertical plane, namely, limited normal acceleration capabilities. An RLV with limited normal acceleration will have difficulty transitioning from the initial steep glide slope to a shallow flight path required for touchdown. The proposed guidance method computes a new reference trajectory such that load factor is minimized during the A&L phase. Trajectory planning is accomplished by performing a series of 1-D searches by iterating on a single guidance parameter and numerically propagating multiple paths to touchdown conditions. Numerical results are presented to demonstrate the effectiveness of the guidance method. System Models