Inter Plane Tar Y Space Asovc Synchronous Altitude

IflCROMSTEORITES

LARGELY INDEPENDENT; HIGH CONCENTRATION IN SONS REGIONS OP THE SOLAR SYSTEM

NORMALLY NEGLIGIBLE; MAY BE IMPORTANT IN SOME 9«ALL REGIONS (INTERIOR OP SATURN'S RINGS)

'ALTITUDES LISTED ARE ONLY REPRESENTATIVE; THE SPECIFIC ALTITUDES AT WHICH VARIOUS TORQUES DOMINATE ARE HtOHLV SPACECRAFT DEPENDENT.

three regions where different forces dominate. Close to the Earth, aerodynamic torque will always be the largest. Because this falls off exponentially with distance from the Earth, magnetic and gravity-gradient torques will eventually become more important. Because both of these have the same functional dependence on distance, the relative strength between the two will depend on the structure of the individual spacecraft; either may be dominant. Finally, solar radiation torque, due to both radiation pressure and differential heating, will dominate throughout the interplanetary medium.

Internal torques may also affect the attitude of the spacecraft. This ^y include seemingly small items such as fuel redistribution or tape recorders turning on and off. The internal torques can become very important, and even dominate the attitude motion, when the spacecraft structure itself is flexible. The role of flexible spacecraft dynamics is much more important in spacecraft than would be the case on Earth where flexible structures tend to be torn apart by the strong environmental torques. For example, the small environmental torques permit spacecraft to have wire booms over 100 m long, which causes the flexibility of the booms to dominate the attitude dynamics.

Because torques exist throughout the spacecraft environment, some procedure is necessary for attitude stabilization and control. Spacecraft may be stabilized by either (1) the spacecraft's angular momentum (spin stabilized); (2) its response to environmental torques, such as gravity-gradient stabilization; or (3) active control, using hardware such as gas jets, reaction wheels, or electromagnets. Table 1-3 lists the methods of passive stabilization which require no power consumption or external control. Table 1-4 lists the commonly used methods of active control which may be used for either maneuver control or active stabilization. In general, active methods of control are more accurate, faster, more flexible, and can be adjusted to meet the needs of the mission. However, active control typically requires a power source and complex logic and may require ground control and the use of spacecraft consumables (materials, such as jet fuel, which are brought from the ground and which cannot be replaced once they have been used). For those systems which use consumables, a major constraint on attitude control strategies is to use them as efficiently as possible.

1.4 Time Measurements

Fundamental to-both attitude and orbit calculations is the measurement of time and time intervals. Unfortunately, a variety of time systems are in use and sorting them out can cause considerable confusion. A technical discussion of time systems needed for precise computational work is given in Appendix J. In this section, we summarize aspects that are essential to the interpretation of attitude data.

Two basic types of time measurements are used in attitude work: (1) time intervals between two events, such as the spacecraft spin period or the length of time a sensor sees the Earth; and (2) absolute times, or calendar times, of specific events, such as the time associated with some particular spacecraft sensing. Of course, calendar time is simply a time interval for which the beginning event is an agreed standard.

Table 1-3.

Passive Stabilization Methods (i.e., those requiring no consumption of spacecraft power or generation of commands)

0 0

Post a comment