a spacecraft has , in addition to redundant thrusters t3 , t4 prime thrusters t1 or t2 arranged to be operated one at a time which are supplied with propellant 1 via valves v1 and v2 . the thrusters are of the kind in which energy is transferred to the propellant by employing electrodes which create a current flow path through the propellant gas . a single power supply psu1 provides power to several electrically - powered thrusters . it is connected such that when actuated all the thrusters to which it is connected become charged . however , only the thruster which receives propellant is able to carry current as the propellant forms part of the current carrying circuit , and this thruster therefore draws power . this technique allows the use of a single power supply for several thrusters while eliminating or minimizing the necessity to switch power from one thruster to another .