Some of the important characteristics of an electrically suspended reaction sphere for space vehicle attitude control are discussed, and data presented on the capabilities and limitations of the reaction sphere concept. Some of the factors which must be considered in determining the relative value of the reaction sphere are discussed with relation to a representative system appjication. Data which have resulted from experimental and analytical investigations are presented.A TTITUDE control systems for space vehicles usually rely upon the principle of reaction to provide the control torques necessary to rotate the satellite axes to a preferred orientation. These control reaction torques are generated through the expulsion of gas to provide a thrust vector, or through the torque applied to a member free to rotate relative to the satellite vehicle.An implementation of the latter is three single-axis reaction wheels mounted in the satellite so that their momentum vectors lie on orthogonal axes. By properly exciting each reaction wheel, a torque of the desired magnitude can be generated around any arbitrary satellite axis containing the satellite center of gravity.A reaction wheel is essentially an electrical motor designed with a rotor having a large moment of inertia and an efficient torque producing capability over the entire operating speed range. The reaction wheel is a more efficient means for generating torque than reaction jets when the torque requirements are for a large number of cyclic torques (l). 3 The reason for the greater efficiency of the reaction wheels under these conditions is that energy for the reaction wheel can be obtained from solar energy. The reaction wheel also has an upper bound on the angular momentum which can be stored; hence, there is the limitation to cyclic disturbances. Reaction wheels are also efficient for controlling random disturbances.The conventional reaction wheels described have the following problem areas: bearings, three wheels required (one for each axis), and gyroscopic cross coupling between axes.The reaction wheel bearings must either be within a gas filled, sealed enclosure or they must be capable of operating in a vacuum environment. In either case, the bearings must be rugged to withstand the launch vibrations. Enclosing the reaction wheels in a pressure container will result in windage losses in addition to the bearing friction losses, both of which must be supplied by stator excitation.The reaction sphere eliminates the forementioned problem areas for reaction wheels, but retains the reaction wheel ad-