PURPOSE: To make it possible to obtain an on/off modulator having intermediate frequency characteristics between the PWPF modulator and the DR modulator by providing the PWPF modulator with an adder, variable gain element, and on/off modulator variable gain selection command generation section.

CONSTITUTION: The thruster control system includes a control torque command calculator section 1 for computing a control torque for attitude of a space flight body, an on/off modulator 10, a thruster drive circuit 3 operative in response to the output from the modulator 10, and a plus thruster 4 and a minus thruster 5 driven by operation of the drive circuit 3. The on/off modulator 10 is formed as an on/off modulator having variable frequency characteristics and this modulator 10 is constructed including an adder 12, a variable gain element 13 and a variable gain element 14 in addition to the comparator 6, primary delay element 7 and dead-band equipped hysteres relay 8 included in the conventional WPF modulator. The adder 12 is disposed between the primary delay element 7 and the relay 8. The on/off modulator has the same characteristics as those of the DR modulator from the standpoint of the output from the variable gain element 13.