Compute the structural capability and compare with the applied loads to determine the margin of safety. Iterate the design as required to obtain the necessary margin of safety.

Sees. 11.6.7, 11.6.8

To illustrate the process and some of the more useful analysis methods, we have shown sizing calculations below for the simple example cylinder in Fig. 11-44. This trade study compares monocoque (skin only) and skin-stringer designs of the lightest cylinder that meets representative requirements described in Table 11-57.

Option 1—Monocoque

Sizing for Rigidity to Meet the Natural Frequency Requirement

This cylinder has uniform thickness and, by definition, no ring or longitudinal stiffeners. Using Eq. (11-57), we will find the minimum shell thickness that meets the natural frequency requirements. With =25 (axial) and 10 (lateral), E = 71 x 109 N/m2, mB = 2,000 kg (a weight of 19,614 N or 4,410 lb), and L = 10 m, we can solve to find the required cylinder A and /.

Axial Rigidity: for axial rigidity, Eq. (11-57) takes the form of case D in Fig. 11-42.

from which the required A is 28.17 cm2 and the required thickness, t, is = 0.045 cm. Lateral Rigidity: here Eq. (11-57) takes the form of case C in Fig. 11-42.

10 m

Example Cylinder Mass = 2000 kg (Evenly Distributed)

Thickness To Be Determined

Fig. 11-44. Structural Idealization for the Example Problem. In this problem, we will Idealize the spacecraft In launch configuration as a cantilevered cylinder, with all the mass of the spacecraft uniformly distributed. This Is often a good starting assumption for Initial sizing.

TABLE 11-57. Example Problem Requirements.


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