Finally, special attitude control motors ("momentum wheels") and thrusters are planned that serve both to turn the space station in any direction and to influence its state of motion as necessary. On the one hand, this option must exist to be able to maintain the space station in the desired orientation relative to the Earth as well as in the required position relative to the direction of the rays of the sun. For this purpose, not only must all those impulses of motion (originating from outside of the system!) that are inevitably imparted to the space station again and again in the traffic with space ships be continually compensated for, but the effect of the Earth's movement around the sun must also be continually taken into account.
On the other hand, this is also necessary in order to enable the space station to satisfy its special tasks, which will be discussed later, because any changes of its position in space must be possible for performing many of these tasks and finally because the necessity can occasionally arise for repositioning the station in relation to the Earth's surface.
The attitude control motors are standard direct current electrical motors with a maximum rate of revolution as high as possible and a relatively large rotor mass. Special brakes make it possible rapidly to lower or shut off their operation at will. They are installed in such a manner that their extended theoretical axes of rotation go through the center of mass of the station.
Figure 81. Operating characteristics of a thruster motor (see the text).
Key: 1. Station; 2. Stator of the motor; 3. Rotor (armature) of the motor; 4. Axis of rotation going through the center of mass of the station.
Now, if an attitude control motor of this type is started (Figure 81), then its stator (the normally stationary part of the electrical motor), and consequently the entire station firmly connected to the motor rotate simultaneously with its rotor (armature) around the axis of the motor however, in the opposite direction and, corresponding to the larger mass, much more slowly than the rotor. More specifically, the station rotates until the motor is again turned off; its rate of rotation varies depending on the rate of revolution imparted to the rotor of the motor. (In the present case, there is a "free system," in which only internal forces are active.) Since these motors are oriented in such a fashion that their axes are perpendicular to one another like those of a right angle, three dimensional coordinate system (Figure 82), the station can be rotated in any arbitrary direction due to their cooperative combined effects.
Figure 82. Orientation of the attitude control motors (momentum wheels). The 3 axes are perpendicular to one another and go through the center of mass of the station.
Key: 1. Center of mass of the station.
The thrusters are similar both in construction and in operating characteristics to the propulsion systems of the space ships described previously. They are, however, much less powerful than those described, corresponding to the lesser demands imposed on them (the accelerations caused by them need not be large). They are positioned in such a manner that they can impart an acceleration to the station in any direction.