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The region of expedient using of SNPS with in‐core thermal thermionic reactor (ITR) is limited by electric power level of about 100 kWe under SNPS lifetime from 3 to 5 years. At the same time the reactor power may be forced from two to three times during the period of about half a year. The mathematical model of SNPS mass dependence on a degree of forcing is given. The results of calculation of payload masses and transfer times for transfer from low orbit to geostationary orbit for two thermal reactors having emission area 1.6 m2 and 2.5 m2 are given for different types of electrojets.

The basic operating regimes of space nuclear power system (SNPS) ‘‘Topaz’’ are considered. These regimes include: prelaunch preparation and launch into working orbit, SNPS start‐up to obtain desired electric power, nominal regime, SNPS shutdown. The main requirements for SNPS at different regimes are given, and the control algorithms providing these requirements are described. The control algorithms were chosen on the basis of theoretical studies and ground power tests of the SNPS prototypes. ‘‘Topaz’’ successful ground and flight tests allow to conclude that for SNPS of this type control algorithm providing required thermal state of cesium vapor supply system and excluding any possibility of discharge processes in current conducting elements is the most expedient at the start‐up regime. At the nominal regime required electric power should be provided by maintenance of reactor current and fast‐acting voltage regulator utilization. The limitation of the outlet coolant temperature should be foreseen also.