Build a Little, Fly a Lot: An Affordable, Evolutionary Approach to Flexible Path, Lunar Surface, and Beyond

The premise examined in this paper is that of incremental development of modest pieces of infrastructure for extended human exploration, which are affordable in parallel with an active flight program. As new capabilities come online, the missions undertaken will be increasingly ambitious, moving beyond low Earth orbit to geostationary orbit, EarthMoon libration points, and circumlunar flights. Indeed, this first tier of missions are shown to be accomplishable with the basic development of a minimally-sized human-carrying spacecraft and human-rating of existing evolved expendable launch vehicles (EELVs) for access to space. Since the nonrecurring costs dominate programs with low flight rates, and costs are roughly proportional to vehicle scale, it is a basic axiom of this study that new launch vehicle development programs should not be undertaken unless no feasible alternative exists. The veracity of this assumption is demonstrated through comparative system analyses of programs with and without new launch vehicle development. This paper builds from a previous publication, which did the basic definition of a modular architecture for human space exploration. Three basic building blocks were identified: a 4900 kg crew module, a 7000 kg in-space Orbital Maneuvering Stage, and a modified OMS specialized for lunar landing, the Terminal Landing Stage. The small size of each of these elements reduces up-front nonrecurring costs, and allows the use of multiple vehicles as necessary to perform any desired mission. This paper continues the investigation of the modular architecture concept, using the baseline Constellation lunar architecture for comparison purposes. Conventional wisdom dictates that the larger number of mission elements for the modular architecture will produce an unacceptably low mission reliability, particularly for a complex mission such as lunar surface exploration. A detailed reliability analysis was performed, which demonstrated that the modular architecture easily accommodates spares in real time, resulting in an overall mission reliability which is actually higher than that for the baseline Constellation mission. This paper also refines the design reference mission for the modular architecture, as well as a revised program cost estimate incorporating more detailed cost data which has recently become available. With an aggressive annual flight manifest including four space station crew rotation missions, two lunar landings with preliminary cargo deliveries, and a “flexible path” mission in cislunar space, the steady-stage flight operations budget is estimated at approximately $3B/year, leaving the majority of the current NASA budget to support advanced research and future missions.