Earth-to-orbit launch system alternatives

A robust earth-to-orbit (ETO) transportation system, designed to facilitate the continuing human exploration of space, is discussed. The concept stresses larger lift capability and minimized on-orbit operations, of which the assembly and refueling operations are given priority. The modularity of the system is based on the required compatibility with the Space Shuttle vehicle, and it offers resiliency and reduced development costs. The support of lunar and Mars missions is interrelated with the mission logistics, the design of a transfer vehicle, the extent of reusability, and the required on-orbit operations. The transfer systems, the magnitude and frequency of propellant deliveries, and some human transportation requirements are considered with respect to vehicle concepts and flight rate strategies.