The potential payload gains due to airbreathing propulsion are evaluated for a one-and-one-half stage reusable vertical-takeoff ballistic space transporter with a gross mass of 155 Mg. The system consists of a returnable ring of turborocket/ramjet engines added as a half-stage to the structure of a 130 Mg, single-stage, vertical-takeoff-vertical-landing, pure-rocket transporter considered in an earlier industry study. High-pressure topping-cycle rockets are used part-time in parallel with the airbreathers, with hydrogen/oxygen propellants used for both. With fixed takeoff mass and airbreathing engines, the main variables were the rocket thrusts and burning times, cycle change and staging points, and the injection angle at 100-m altitude. State-of-the-art technology was assumed. Trajectories free from constraints, except acceleration limits, allowed optimal propulsion system use. Propulsion performance maps, ascent trajectories, and weight breakdowns are given. Payload fractions up to 7.8% of gross mass were obtained for the airbreathing vehicle, which is very high for this size class compared with expendable two-stage rocket transporters.