As the Space Shuttle program draws to a close, NASA faces the challenge of developing the transportation system for future human space exploration. Given current debates on the goals of the space program, it is critical to ensure that comprehensive but transparent exploration of available options for future launch vehicle developments is available. Evaluation of launch architectures requires the assessment of options over a range of dimensions, which can be broadly grouped into technical performance, time to initial capability, cost, and satisfaction of stakeholders' various needs. The challenge is to fairly compare a broad range of architectures across these dimensions, in a way that empowers high level decision makers. This paper describes a tradespace exploration study which provides preliminary design of families, by exploring hundreds of concepts using coarse system evaluation metrics. The paper investigates the trade-offs associated with stage propellant selection, launch vehicle configuration and other relevant design parameters. Furthermore, the study considers potential LEO-class vehicles derived from the heavy lift vehicle in order to deliver early benefit from the heavy lift vehicle and provide an ongoing affordable LEO service. Architectures are evaluated through a broad set of metrics, including engineering performance, compliance to stakeholder needs and goals, compatibility with existing ground infrastructure and off-design performance. The technical assessment methodology is validated against existing launch vehicles. The paper demonstrates how a field of 192 possible launch vehicles can be transparently reduced to seven possible designs on technical considerations, almost all of which are represented by particular viewpoints in the current debate, and how further narrowing the design space requires weighting competing stakeholder priorities. The preferred concepts emerging from the study are a 2.5 stage 7/8.4m diameter all-cryogenic vehicle and a 7/8.4m diameter 2.5 stage LOX/LH2 vehicle with solid boosters as preferred baselines for further conceptual study. The paper further shows how conducting this coarse tradespace exploration early in the process can inform decision-making on the future space transportation infrastructure. The paper closes with some suggestions for future work in the area.