Roll Wheel

Ibl TWO-WHEEL SYSTEM <c) THREE WHEEL SYSTEM

Fig. 15-19. Alternative Momentum Wheel Stabilization Systems

Ibl TWO-WHEEL SYSTEM <c) THREE WHEEL SYSTEM

Fig. 15-19. Alternative Momentum Wheel Stabilization Systems

Momentum wheel stabilization systems are used to maintain the attitude by momentum exchange between the spacecraft and the wheel. As a torque acts on the spacecraft along one axis, the momentum wheel reacts, absorbing the torque and maintaining the attitude. As a result, momentum wheels are particularly attractive for attitude control in the presence of cyclic torques or random torques, such as in manned space stations. The wheel spin rate increases or decreases to maintain a constant attitude. Over a full period of a cyclic torque, the wheel speed remains constant. Secular torques acting on the spacecraft cause the momentum wheel speed to either increase or decrease monotonically until the wheel speed • moves outside operational constraints. A momentum exchange device (i.e., a gas jet, magnetic coil, or gravity-gradient torque) must then be used to restore the momentum wheel speed to its nominal operating value. The upper operating limit of a momentum wheel is called the saturation limit.

Momentum wheel control systems used for maneuvering operate in the same fashion as the stabilization systems. For example, consider a maneuver for an inertially pointed spacecraft Initially, the spacecraft is motionless and the wheel is spinning with angular momentum H. At some time, t, the control system is commanded to maneuver the spacecraft At this time, a transfer of momentum, AH, from the wheel to the spacecraft occurs and the spacecraft attitude begins changing. The angular momentum of the wheel becomes H —AH and the angular momentum of the spacecraft becomes AH. When the spacecraft reaches its desired attitude, the momentum transfer is reversed, the spin rate of the momentum wheel returns to its original value, and the spacecraft body momentum returns to zero. The spacecraft is now pointing at its new attitude with zero angular momentum. The Applications Technology Spacecraft-6, ATS-6, shown in Fig. 15-20, has a three-wheel momentum control system. Momentum wheels are mounted along the pitch, yaw, and roll axes and serve as prime torquers for stabilization and maneuvering.

Magnetic coil control systems can be used for maneuvers for virtually all orbits at less than synchronous altitudes (35,000 km). Magnetic control systems are relatively lightweight and require no moving parts, complex hardware, or expendables. This makes magnetic torquing attractive for space applications; however, it requires significant amounts of power, it provides slow maneuvering because of the power constraints, and its operation depends on the magnetic field configuration. Three types of magnetic torquer systems currently being used are permanent magnets, "air-"core torquing coils (i.e., electromagnets), and iron-core torquing coils. Permanent magnets are the heaviest type and are used for limited stabilization. "Air-" and iron-core magnets are used for both stabilization and maneuvering. For a spin-stabilized spacecraft, magnetic coils may be mounted either around

U> ATM SPACECRAFT

Cb) COMPONENT VIEW OF THE ATTITUDE COMTROl 8VSTEM LOCATED AT THE BOTTOM OP THE SPACECRAFT AS BEEN LOOUKO PROM THE EARTH

Fig. 15-20. The Three-Axis Momentum Wheel Stabilized ATS-6 Spacecraft

U> ATM SPACECRAFT

Cb) COMPONENT VIEW OF THE ATTITUDE COMTROl 8VSTEM LOCATED AT THE BOTTOM OP THE SPACECRAFT AS BEEN LOOUKO PROM THE EARTH

Fig. 15-20. The Three-Axis Momentum Wheel Stabilized ATS-6 Spacecraft

NUTATION DAMPER Aft SPIN PLANE MAGNETIC

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