Currently, trade-offs are being made among the various propulsion systems being considered for the Space Exploration Initiative (SEI) missions. It is necessary to investigate the reliability aspects as well as the efficiency, mass savings, and experience characteristics of the various configurations. Reliability is a very important factor for the SEI missions because of the long duration and because problems will be fixed onboard. The propulsion options that were reviewed consist of nuclear thermal propulsion (NTP), nuclear electric propulsion (NEP) and various configurations of each system. There were four configurations developed for comparison with the NTP as baselined in the Synthesis (1991): (1) NEP, (2) hybrid NEP/NTP, (3) hybrid with power beaming, and (4) NTP upper stage on the heavy lift launch vehicle (HLLV). The comparisons were based more or less on a qualitative review of complexity, stress levels and operations for each of the four configurations. Each configuration included a pressurized NEP and an NTP ascent stage propulsion system for the Mars mission
