The simplest form of peripheral (but not androgynous) device with one discrete element is a docking device with an external docking mechanism with a rod and a receiving cone. This configuration has a number of drawbacks which in practice make this design difficult to use. The main drawbacks are: 1) asymmetry of the docking device, which complicates the placement of the spacecraft on the rocket booster under the cowl; 2) the greater effect of initial roll errors on the displacement of the rod head in the socket of the receiving cone; 3) the need to increase the length and course of the rod to avoid contact of the frames during rotations after linkage; 4) extracentrai retraction of the docking mechanism, which causes warping when the docking frames are combined; 5) there is a great potential danger of damaging the docking device and the spacecraft when there is an error in the first contact; 6) the presence of significant perturbations during impact after linkage; these perturbations lead to rotation of the spacecraft, and require an increase in the energy capacity of the angular shock absorbers; 7) a larger effect of roll rotations on the displacement of the guide rods of the docking frames when they are equalized. A continuous buffer element is usually in the form of a ring with guide protrusions to interact with an analogous APDA ring or guides of another configuration. The principal schemes with continuous buffer elements have a number of advantages over schemes with several discrete elements. The main drawbacks of docking devices with discrete elements are: 1) compensation of initial errors requires longer guide rods (by a factor of 2) than a docking device with a continuous element (ring); 2) the shock absorption process and linking with discrete elements, the shifting and deformation of which are independent, as a rule, is less favorable; 3) linkage of one or some of the discrete elements is potentially dangerous due to the possibility of spacecraft collision. The latches of the ring with guides may be made so that linkage occurs only when there is full alignment of the rings; 4) discrete elements should absorb both longitudinal and lateral forces, that is, they should contain additional transverse shock absorbers and be designed for excesses. The ring is usually installed on six rods which only work on expansion and compression; 5) a potentially more dangerous collision of discrete elements during errors which exceed acceptable levels; 6) it is more complex to insure synchronization of discrete elements during retraction and extension.
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