A useful distinction among spin-stabilized vehicles can be made between spin-stabilized vehicles in the more narrow sense and dual-spin vehicles. In the first case, the entire spacecraft is spinning about some axis; in the second case only a portion (usually the more massive one) does so. In a still wider sense of the term, momentum-biased spacecraft, that is, spacecraft that incorporate a fast spinning momentum wheel (Sect. 6.5.2), can also be classified as spin stabilized. In all cases, the orientation in inertial space of the spin axis is maintained by the gyroscopic effect that is produced by the spinning part.
The attitude control of spin-stabilized spacecraft (Fig. 6.2a) is simpler than it is for three-axis stabilized vehicles. This advantage can be offset, however, by the dimensional restrictions that are imposed by the available launch vehicle payload space (i.e., the size of the "shroud" or "fairing"). Because the maximum possible solar cell area of spinning vehicles is proportional to their diameter and length, high-power communications and broadcasting satellites with their high demand for electric power, when spin-stabilized, can become too large for available launch vehicles.
Spinning spacecraft can be either prolate (Fig. 6.4a) or oblate (Fig. 6.4b). As discussed in Sect. 6.3.2, the precession of prolate bodies is unstable in the sense that the nutation angle tends to increase. Oblate bodies are stable in this sense. However, the attitude control of spinning vehicles requires more than merely a stable precession at a constant nutation angle: a control is needed to reduce to near zero the nutation angle that may have been caused by a disturbance.
In spite of their basic instability (which can be corrected by nutation dampers; see Sect. 6.6.3), prolate spacecraft are often preferred because of their better geometrical fit to the launch vehicle payload space.
Horizon sensors can take advantage of the spin. Mounted on the side of the vehicle, the line of sight—by virtue of the spin — can be made to sweep over the earth or planet (Fig. 6.17). If a stepping motor is used in addition to change the line of sight in the fore-aft direction, a single sensor can serve for the sweeps A to A' and B to B' indicated in Fig. 6.8.
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