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A design study was performed to define and compare the parameters of horizontal-and vertical-takeoff reusable launch-vehicle systems to identify promising configurations for further developmental emphasis. The investigation considered both two-stage rockets and single-stage airbreathing ramjet/scramjet-powered vehicles, thus representing next-generation and third-generation configurations, respectively. The payload requirement for each vehicle was 20,000 lb delivered to a 100-n mile circular Earth orbit launched easterly from Kennedy Space Center. All vehicles were first analyzed using liquid hydrogen for the entire trajectory and then reanalyzed with liquid hydrocarbon fuel for the first stage, if a rocket, or for the low-speed trajectory segment to ramjet start for the airbreathers. The vertical-takeoff airbreathing vehicles were found to have the lowest empty weights and gross takeoff weights of all of the vehicle configurations, with three-dimensional inward-turning inlets outperforming two-dimensional inlets, and the use of hydrocarbon fuel outperforming hydrogen fuel for the launch propulsive segment to ramjet start. The best horizontal takeoff vehicle is the all-hydrogen inward-turning airbreather. For the two-stage rockets, the lightest empty weight was achieved with the use of hydrocarbon fuel in the booster and hydrogen fuel in the orbiter.

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Using a wide spectrum of completed air-breathing and rocket-powered launch vehicle baseline configurations, an assessment was undertaken to ascertain each vehicle system's scaling response to empty weight growth. To establish the growth behavior, each vehicle's baseline solution was modified for percentage increases or decreases in the baseline empty weight from 10% to 10%, after which each vehicle system was then re-solved. The identification of the trends in these solutions enabled the determination of the growth factor for each vehicle configuration. The growth factor characterizes the system's sensitivity to changes in structural weight arising from technological uncertainty. Systems with high growth factors represent a greater amount of design risk because they may rapidly scale out of control if expected technology levels fail to materialize. This understanding may also be applied to measure the extent of improvements possible from application of more advanced technology. Several other figures of merit were also used to evaluate the growth solutions, including empty weight, wetted area, and gross weight. The assessment concluded that single-stage air breathers have a higher response to weight uncertainty than two-stage configurations with the horizontal takeoff mode being more sensitive than vertical takeoff. Two-stage air-breathing configurations, whereas exhibiting lower growth factors, differed greatly from each other in total empty weight across the growth cases with the vertical takeoff mode roughly half the weight of comparable horizontal takeoff configurations. Two-stage reusable rocket configurations also show low scaling weight growth factors and empty weights and are relatively insensitive to small percentage growth changes. Nomenclature f FX = fixed weight fraction f P = propellant fraction f S = baseline scaling structural fraction f SN = scaling structural fraction GF FW = fixed weight growth factor GF SW = scaling weight growth factor W E = empty weight W FX = fixed weight W G = gross weight W P = propellant weight W PAY = payload weight W S = scaling weight W SN = new scaling weight

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