Fig. 11-41. Comparison of Sandwich and Monocoque Construction. Thin face sheets have little bending stiffness, as Indicated by the small value of I, (A). The bending stiffness is increased by separating the faces with a low density core, (B). A monocoque wall thickness of 3.33 cm is required to obtain bending stiffness equal to the sandwich panel at three times the mass, (C). The masses per unit thickness shown are per cm using aluminum with a density of 2,800 kg/m3 and 80 kg/m3 for the face sheets and honeycomb core, respectively.

The deflection, S, and natural frequencies, f^, of simple beams are shown in Fig. 11-42 for axial and lateral applied loads [Roark and Young, 1975]. When considering only its first natural, or fundamental frequency, a structure can be idealized as a single-degree-of-freedom spring-mass system.

The spring is the structure. We can assume an equivalent beam to represent a spacecraft with a natural frequency, i11"57)

where m is mass and k = stiffness = load/deflection, also called a spring constant We find the spring constant k, using

where 5 is the deflection and g is acceleration due to gravity.

Description of a Typical Spacecraft Structure

Figure 11-43 shows the Magellan spacecraft configuration and locations of major subsystems.

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