IV - B - 4. Attitude and Orbit Control The control system must compensate for all forces acting on the SPS, both orbit perturbations and attitude disturbances. Orbit perturbations are analyzed in section IV-A-3. The reaction control system discussion in this section includes both orbit maintenance and attitude control requirements. a. Attitude Disturbances L. E. Livingston Spacecraft Design Div. Synchronous Orbit The most important torque acting on the SPS is that produced by gravity/centrifugal gradients. For a flat solar-oriented array, there are two principal components (see figure IV-B-4-1). The first component acts about an axis normal to the orbit plane and tends to align the plane of the array with local vertical. It is cyclic with a period equal to half the orbital period. Numerically, for a circular, synchronous orbit Solar radiation pressure of about 600 N will produce a constant torque if the center of mass and center of pressure are not coincident. Microwave recoil of 22 N per antenna will produce a torque, varying in direction with antenna orientation, if the antennas are not balanced with respect to the mass center of the SPS. If the eccentricity of the orbit is not zero, the antenna angular velocity will not be constant. For the expected eccentricity of 0.04 and an 8 x 10® kg antenna, peak torque will be about 240 N-m. Variation is roughly sinusoidal with a period of one day. If a current loop is formed by large separation of positive and negative conductors, interaction with the earth's magnetic field will produce a torque on the SPS. With a total current of 450,000 A, the magnitude of the torque can be as high as 50,000 N-m per square kilometer of current loop area.
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