Current and future developments in reusable launch vehicle technology demand on-board, real time autonomous abort handling. If an engine failure occurs, the guidance system must quickly determine a footprint of reachable landing sites and select a robust site. A robust site is one which maximizes the vehicle's ability to reach that site in the presence of multiple uncertainties. After selecting a robust landing site, the guidance system must successfully bring the vehicle to a safe landing. A versatile approach for abort guidance is presented here. The guidance problem is posed as a nonlinear model matching problem. Reference profiles are replaced by a reference dynamic model defined by the nominal vehicle dynamics and a nominal guidance algorithm. The guidance system is built around this reference model. Vehicle capability is examined in the model based framework. A footprint definition and trajectory design algorithm using the nominal bank angle for range control evolves naturally from the reference model. A tracking algorithm uses bank angle and angle of attack perturbations to ensure that vehicle states track the reference model states. Large disturbances and initial condition errors are rejected by a trajectory redesign algorithm. The trajectory redesign algorithm operates continuously in flight to control vehicle range by adjusting the nominal bank angle. The use of a reference model leads to a simple method for analytic evaluation of landing site robustness as a function of control saturation. The algorithm is developed and verified for the X-34, a reusable launch vehicle currently being designed and built by Orbital Sciences Corporation. The system is verified in a three degree of freedom simulation environment. Multiple test cases, including a variety of abort scenarios and parametric uncertainties, verify system performance. Thesis Supervisor: Professor Wallace E. Vander Velde Title: Professor, Department of Aeronautics and Astronautics Technical Supervisor: Owen Deutsch Title: Principal Member Technical Staff, C. S. Draper Laboratory, Inc.