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.

A brief comparison is made between opposition class (with three impulse return), conjunction class, Venus swing-by to Mars and Mars-Venus double stopover trajectories. Vehicle configurations based on NERVA-I engines of varying life and restart capability are considered. It is found that the opposition and Venus swingby trajectories to Mars yield competitive mission performance; the double stopover entails about 30-percent penalties in initial mass and trip time; and that the minimum-weight vehicle uses a single, restartable engine.

A brief comparison is made between opposition class (with three impulse return), conjunction class, Venus swing-by to Mars and Mars-Venus double stopover trajectories. Vehicle configurations based on NERVA-I engines of varying life and restart capability are considered. It is found that the opposition and Venus swingby trajectories to Mars yield competitive mission performance; the double stopover entails about 30-percent penalties in initial mass and trip time; and that the minimum-weight vehicle uses a single, restartable engine.

Comparing Nuclear Propulsion Technologies for Crewed Missions to Mars Current chemical propulsion systems are limited in their ability to support crewed missions to Mars due to their low propellant efficiency and long mission times. Nuclear propulsion technologies offer a potential solution to these challenges, as they are capable of producing higher thrust and propellant efficiency than chemical rockets. This paper compares three nuclear propulsion technologies for crewed missions to Mars: nuclear electric propulsion (NEP), nuclear thermal propulsion (NTP), and nuclear fusion propulsion (NFP). Each technology has its own advantages and disadvantages, and the best choice for a crewed Mars mission will depend on a number of factors, including mission requirements, cost, and risk. NEP systems are the most mature of the three technologies, and they have already been used successfully in a number of space missions. NEP systems use a nuclear reactor to generate electricity, which is then used to power an electric thruster. NEP systems are very efficient, but they produce relatively low thrust. As a result, NEP systems require longer mission times than other nuclear propulsion technologies. NTP systems are less mature than NEP systems, but they offer the potential for higher thrust and shorter mission times. NTP systems use a nuclear reactor to heat a propellant gas, which is then expelled through a nozzle to produce thrust. NTP systems are more efficient than chemical rockets, but they are also more complex and expensive to develop. NFP systems are the most advanced of the three technologies, but they are also the most speculative. NFP systems would use the fusion of atomic nuclei to generate energy, which could then be used to power a variety of propulsion systems. NFP systems have the potential to offer even higher thrust and shorter mission times than NTP systems, but they are still in the early stages of development.