Abstract: A method includes: during a first time period, pumping propellant: to a set of moderator coolant channels to cool the moderator; through a set of fuel elements to heat the propellant to a first outlet temperature; out of a thrust nozzle to produce a first thrust; and locating the reflector at a first closed position to reflect neutrons toward the set of fuel elements to maintain a fission rate of the set of fuel elements above a threshold fission rate. Additionally, during a second time period: locating the reflector at a second open position to reduce the fission rate of the set of fuel elements to below the threshold fission rate; and pumping the propellant through the fuel elements to heat the propellant to a second outlet temperature less than the first outlet temperature and out of the thrust nozzle to produce a second thrust less than the first thrust.