Tn the development of the APDA, the American specialists rejected pneumatic springs and bellows (Figure 6.12), since in most of the course of the shock absorber it was necessary to provide a small stable force. The force of the damper F^ is proportional to the square of the velocity of displacement of the rod where is the attenuation coefficient (for this construction ); is the area of the rod ; e is the longitudinal cross section area. The area of the cross section of the by-pass aperture is decreased along the course of the rod (see Figure 6.12); as a result, the force of the damper is substantially increased at the end of the course. A characteristic of the springs has two stages; the springs which create force in the second stage are also used to compensate for a change in the volume of liquid and leaks. The minimal force of the return of the spring is 110 N. For this force the provision of hermeticity is complicated, since it requires the creation of a seal with a small (no greater than 40- 50 N) friction. A device to measure leaks is built into the shock absorber which is monitored before the launch of the spacecraft. The Apollo APDA shock absorbers were rated for work from . The coefficient of attenuation changes by about a factor of two. The mass of one shock absorber is 5.5 kg, and the energy capacity is 800 J, that is, . Four basic types of potential failures should be examined in the analysis of the reliability of the hydraulic and pneumatic shock absorbers. These failures are associated with leaks of liquid or gas and with jamming. The most critical failure is full jamming, in which docking becomes impossible.
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