joints of the frames; 4) substantial damage of parts which insure hermeticity of the joint. That is why, in the planning of these mechanisms and elements, one must study their working capacity especially carefully and take all measures to eliminate or reduce the number of potential failures, for example, duplication of the drive of docking mechanisms, reduction of the number of protruding elements on the joint of the ring and frame, etc. When there is a failure in the drive of the docking mechanism, it is possible to use the APDA in the passive state to fire the ring and erect additional passive guides. This structural variant was provided in the initial stage of work on the Apollo-Soyuz project. 4, For some types of docking failures (substantial disintegration or damage of the joint and other interacting elements) the problem is solved by the use of a back-up mooring, for example, on long-term stations (Skylab). 5. In addition to the cases in points 2 and 3, it is also expedient, where possible, to use functional redundancy, that is, to use mechanisms not only for their direct purpose, but also to carry out additional operations. For example, in the Soyuz APDA the drive of the docking mechanism, in addition to its main operation of extending and retracting the ring, also does the required equalizing; the Apollo docking device performed coupling without linkage (see section 1.5.7). 6. Redundancy is widely used in electrical automation circuits, and in the commutation of drives and indicators, and this is usually combined with redundancy of the sources or supply buses with individual current shields. Drives, sensors, terminal contacts, etc. are. also duplicated. The circuits are constructed with a consideration of redundancy, as a rule, in accordance with the general principle of construction of electrical supply and control of the spacecraft as a whole. 7. The latches of the docking mechanism should be duplicated, as this increases the reliability of linkage when they jam in the compressed position. However, the probability of disengagement is increased in complete jamming of the latches. Nonetheless, this redundancy is reasonable, since partial jamming of the latches is more likely. Compression of the latches may occur due to a force of impact of the spacecraft reaching several thousand or even tens of thousands of newtons. On the other hand the introduction of latches in the disengagement is
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