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.