Nuclear Thermal Propulsion for Earth Orbit and Interplanetary Missions: Challenges and Issues.

From the late 1950s to date, nuclear thermal propulsion has the purpose of making possible missions impossible to accomplish with chemical propulsion. The use of hydrogen as a propellant and a fuel resistant to very high temperatures guarantees exceptional propulsive performance. Since the NERVA project, a propulsion system with such capabilities has proven to be the most suitable to carry out missions to Mars. Even today, the literature on nuclear thermal propulsion systems appears to be full of Mars mission studies. Little attention is given to other missions potentially made accessible by this type of propulsion. The purpose of this work is to investigate different mission scenarios that could be made accessible by nuclear propulsion and to identify the most efficient configurations of the propulsion system for each scenario. Some mission scenarios are presented, covering both interplanetary and earth orbit missions. The specific requirements that each mission imposes on the propulsion system are determined. Particular attention is given to the analysis of the safety requirements, which constitute the most stringent constraints to be respected for the use of nuclear reactors in space. A literature review identifies the most promising fuels and propellants, with a focus on the recently developed high temperature fuels. A preliminary design determines the mass, size and materials of the components of the propulsion systems satisfying the imposed requirements for the diverse missions. All the proposed configurations have associated different combinations of fuels and propellants among the one chosen from the previous literature reviews. For each mission, a trade-off is made based on the performance offered by the various propulsion systems proposed. Particular attention is given to the parameter of the system specific impulse, which weighs the specific impulse on the mass of the propulsion system. This parameter is very important in systems using nuclear propulsion, where the mass of the engine occupies a large part of the total mass of the system. From this preliminary trade-off, the nuclear thermal propulsion system configurations worth to be studied in detail and optimized emerge for each identified mission. Finally, the possible benefits in terms of system specific impulse derived by the use of the fission reactor also for power generation purposes are discussed.