Table 2.2. Criticality of failures in the execution of basic functions, a. basic functions of the docking device; b. category of criticality of failures; c. docking; d. preservation of docked state; e. undocking; f. 4- most frequent; -r-/ — maybe; — rarely. Analysis of the reliability of docking devices must consider the execution of all three basic functions. The results of analysis (Table 2.2) show that when special care is used, one can create equipment which insures a docked state and undocking. The majority of elements and mechanisms in the docking mechanisms participate in the execution of two or even three functions. Analysis shows (Table 2.3) that failures in virtually all basic elements of the docking mechanism, mechanisms of the docking frame, and several other elements are in categories IT or ITT. Thus, it is necessary to take measures to insure the reliability of all these elements. There are elements whose reliability is especially critical, primarily the locks of the docking frame. The reliability of the docking device is primarily insured by effective construction of the principal scheme and construction, which should match the given functions and operations; the use of simple and logical solutions; the use of successful, earlier tested and verified schemes, units, and elements, etc. However, due to multi-functionality, and the possibility of various type of failures of the same elements, full redundancy is virtually impossible. Moreover, redundancy to increase reliability in the execution of some functions may decrease the reliability of the execution of other functions. For example, redundancy to increase the reliability of docking frequently leads to a decrease in the reliability of undocking (see section 2.1.5).
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