1.2.4.2 Structural Fundamentals The very large dimensions of a SPS will require an open structure for low weight and has to be 3-dimensional if high stiffness is required, (compare to I. 2.6) The most significant dynamic loading frequencies to the lar ge array are the twice daily (2.3 x 10~b Hz) and daily gravity gradient loading cycles. To keep the dynamic motions of the overall station to a reasonable level, a minimum natural frequency criterion of 10 times of the loading frequencies (2.3 x 10 Hz) should be selected. It is estimated that the control system will provide loads at frequencies which will not exceed 0.2 Hz, but there will exist potential interactions between the control system and individual structural elements. 1.2.4.3 Loads Since the mission loads were found to be very low, the transportation - and mounting - loads gain significance for the structural design. The mission loads can be divided into the following categories: A.1 Earth_grayity_gradient_forces These cyclic forces Induce a moment of about 1 million Nm to a 10 GW satellite. This moment requires a maximum control force of 100 N. Earth gravity gradient forces produce also a cyclic tension of 50 N (insignificant). In LEO gravity gradient forces are about 230 times greater compared to GEO. A.2 Solar Radiation Pressure This pressure induces a force of 600 N to a 10 GW satellite which effects mainly an orbit perturbation to a slight eccentricity. An additional shear force of 90 N will appear because of a difference in the area/mass ratios of the solar array and the antenna. A. 3 Solar_and_Lunar_Gravity_and_Earth2.s_Ellipticity These forces cause no significant structural loads but substantial orbit perturbations. A. 4 Atmospheric Drag Atmospheric forces appear only at LEO-construction sites. At an orbit of 500 km an atmospheric force of 400 N has been calculated for a 10 GW satellite. A. 5 Eccentrical Orbit Eccentricity of 0.04 of geosynchronous orbit will produce a torque of about 240 Nm. B.) Induced Loads B.1 Control System Thrusters A thrust level of 100 - 300 N is expected.
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