United States upper stages for the next decade

Abstract

United States (U.S.) upper stage development is approaching another crossroads. In the 1970 s the decision was made to fly all future payloads on the space shuttle. Work on high performance shuttle upper stages was halted after the Challenger accident, because of heightened safety concerns about liquid stages in the payload bay. The centaur G′ was ultimately developed as the centaur upper stage for use with the Titan IV launch system. In the last two decades upper stage development decisions and requirements were driven by increased payload performance demands. As we look forward to the next decade it is apparent that the situation has changed. Significant effort is underway to downsize payloads. The best mission model projections indicate that the payload performance demands have reached a maximum with current or near-term missions, and that maximum weight requirements will generally decrease in the near future. Thus the current fleet of launch systems and upper stages are expected to be able to meet all projected performance requirements through the next one to two decades. The impetus for development of a new upper stage will have to come from somewhere other than increased performance requirements. A driving need for a new upper stage is emerging from the concept and technology development work currently underway in the U.S. on the evolved expendable vehicle (EELV) and reusable launch vehicles (RLVs). The key EELV upper stage requirements are cost, operability and reliability. Clearly the EELV presents a performance challenge for a single upper stage to operate with a booster family over the complete mission model payload weight and destination range. The design requirements and trade-off considerations are quite different for a stage to operate with an RLV. In this case, manned safety and payload bay packaging become significant additional considerations which must be addressed, and reusability, with or without space basing, is an option. This paper covers the need and prospects of U.S. upper stages for the next decade by tracing the history of development and planned improvements, and identifying new requirements and challenges expected to be imposed by emerging EELV and RLV requirements. Trade-off factors, design considerations and sensitivities, as well as cost and reliability benchmarks, are highlighted through the use of representative design solutions.