Preliminary Results for In-Situ Alternative Propellants for Nuclear Thermal Propulsion

Water, ammonia, and other volatiles that can be used for propellant have been found on the Moon, and the technology that will be used to extract them has been laboratory tested. One of the considered propulsion systems for a crewed mission to Mars is nuclear thermal propulsion (NTP). However, current reference missions consider hydrogen as the main propellant, which is technologically difficult to store. Electrolysis units are required to process the lunar water to separate it into oxygen and hydrogen, which is only 1/8 of the mass of water mined. Due to these challenges, a preliminary analysis of alternative propellant nuclear thermal propulsion (A-NTP) expander cycle engines was made. A-NTP engine models that produced 25 000 lbf of thrust, which is comparable to the baseline hydrogen NTP engines, were constructed in Simulink for preliminary analysis, which yielded an I_sp of 320.4 s for water and 381.6 s for ammonia. Although this I_sp is lower than the most efficient chemical engines, since water and ammonia are used directly and are stored as such, a propellant tank volume decrease of up to 76.1% for water and 69.5% for ammonia is possible. This will decrease the number of launches, given that the tanks are not fully tanked at time of launch and lunar resources are used to fill the tanks completely.