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"Thickness required to satisfy assumed acoustics environment "Stringers sized to accommodate bending rigidity requirement M = Monocoque, SS = Skin-stringer

"Thickness required to satisfy assumed acoustics environment "Stringers sized to accommodate bending rigidity requirement M = Monocoque, SS = Skin-stringer

Mass decreases with a shorter cylinder, but the stability requirement becomes more critical as the radius increases. More iterations with a shorter cylinder and additional stringers would be appropriate. A more massive example would make the skin-stringer design more attractive.

11.6.8 Mechanisms and Déployables

Aerospace mechanisms can be divided into high- and low-cyclic applications. The former, such as antenna gimbals or solar array drives, require frequent or constant articulation. The latter restrain a payload on launch or retrieval, or they propel the payload to the deployed or restored position. Figures 11-46 and 11-47 show examples of these mechanisms. The design is complete only when principles of mechanics and environmental considerations lead to a producible and testable spacecraft The most challenging requirements for mechanisms are those that demand precision pointing and a long operating life.

Fig. 11-46. High-Cyclic Mechanism, Rotary Actuator Assembly and Components. An example of an aerospace mechanism requiring precision pointing (motor driven).

Fig. 11-46. High-Cyclic Mechanism, Rotary Actuator Assembly and Components. An example of an aerospace mechanism requiring precision pointing (motor driven).

Requirements. Typical spacecraft requirements for aerospace mechanisms are as follows:

High-cyclic mechanisms —Antenna pointing and tracking —Solar array pointing and tracking —Attitude control reaction wheels —Boom extensions

Low-cyclic mechanisms

— Antenna launch retention

— Antenna deployment

— Solar array retention

— Solar array deployment

— Contamination cover removal

— Spacecraft/launch vehicle separation

Prime Mower (Gsaihead Motor)

Prime Mower (Gsaihead Motor)

Fig. 11-47. Low-Cyclic Mechanism, Solar Array Retention Mechanism. An example of an aerospace appendage retention mechanism.

The MEL-A-83577 [1988] specification for moving mechanical assemblies gives us important technical guidance. The functional requirements for the mechanisms derive from mission requirements and resolve into torques or forces and operating rat». An operating rate profile, as shown in Fig. 11-48, establishes the payload articulation or deployment rate. This profile determines the maximum angular acceleration, a. Once we have determined the payload moment of inertia, MOI, we can compile the mechanism's operating torque, T = a (MOI). For rough torque sizing, we can add a 20% friction torque to the operating torque. The constant-speed part (s2) of the operating rate profile, represents the mechanism operating torque because there is no acceleration during this phase. With the two operating points known, we can generate a torque-speed curve (see Fig. 11-49). This linear curve establishes the stall torque and theoretical no-load speed for the mechanism. When these mechanism-performance characteristics are arithmetically manipulated by the mechanical advantage of a gear train, the new performance characteristics represent the principal motor requirements. With the mechanism's stall torque now known, we can do first-order approximations of the mechanism parameters using Fig. 11-50.

As an example, a solar array with moment of inertia, MOI, must be deployed by rotating from a stowed position to a locked position in time, t. This time period involves accelerating the array to a maximum rate, then decelerating to the lock. Therefore, the operating torque (operating point 1) equals moment of inertia, MOI, times acceleration (ij -rt/2). In the absence of other running friction data, we can assume operating point 2 is 20% of operating point 1. Extrapolating to a stall torque (assume 200 N*m) lets us use Fig. 11-50. If the mechanism had a 200 N-m stall torque, we can see that the mechanism mass will be about 18 kg, require 90 W of power, and have a volume of about 7,800 cm3. As a guideline, mechanisms should have a 100% torque margin to provide for uncertainties of friction, payload inertia growth, and thermal effects.

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