Compared with chemical in-space rocket engines, the NERVA nuclear rocket engine presents several unique operational characteristics. This is particularly true in the post-shutdown phase called “pulse cooldown.” At this time liquid hydrogen coolant is tank-pressure fed, the tank being pressurized by gaseous hydrogen. At low reactor cooldown power levels the tank ullage gas can be used as the coolant. Thus, there exists the operational option of using either liquid or gaseous hydrogen for coolant; this can be used to eliminate tank venting and also to minimize fluid residual weight in the tank. For a typical four-burn lunar mission it was found that the proper combination of liquid and gaseous coolant fluids gained 27 000 Ibm or 18% in payload delivered to lunar orbit, compared to the case when only liquid hydrogen was used for reactor cooldown. In addition, an increased flexibility of flight operations is indicated because tank conditions can be adjusted enroute by choice of the cooldown fluid modes.