All of the docking that occurred during the Space Shuttle and Space Station era has used hardware  procured from Russia. The systems were developed by Russian (then Soviet) contractor RSC-Energia and  had been used previously in the Russian MIR space station complex. The Space Shuttle docking system  was known as the Androgynous Peripheral Attachment System (APAS), which is shown in Figure 3. APAS  was a variant of the Apollo Soyuz design with the petal direction reversed. The interface requirements of  the new International Docking System Standard trace much of their heritage to the APAS in terms of hard  capture and soft capture hardware geometry. The soft capture design of the APAS capture system was  largely mechanical. The three bi-pods were all mechanically connected via a triple differential. In simple  terms, pushing down on one petal pushed the other two up. This was a significant advantage in terms of  capture success but the mechanical transmissions were extremely complicated. The system needed a large  contingent of electromechanical hardware including; motors, brakes, slip clutches, fixers, eddy current  damper and 9 control boxes.  Figure 3. NASA image of the Space Shuttle with Russian APAS docking mechanism  In the early 1970s, NASA commissioned the Space Division of North American Rockwell to conduct trade  studies on docking technologies in support of the Apollo Soyuz Test Mission [2]. The contract produced  detailed analysis and sub-scale mechanisms to evaluate most of the major technology areas of docking.  Many of the technical parameters identified in this study remain in today’s docking system. The study  selected a cable retracted hydraulic actuator for the US soft capture system design. It also built and modeled  the Russian mechanical soft capture system and postulated the viability of two other architectures.   The other architectures were direct servo electric (shown in Figure 4) and cold gas pneumatic-hydraulic  soft capture systems. The cold gas system was undesirable due to a low cycle associated with the gas  consumables. Reduced weight and lower friction were identified as key improvements of a direct servo  system. The development time, weight, power consumption and reliability of the electronics needed to  control an electric platform of the early 1970s prevented this architecture from being selected. Today’s  electronics and the NDS controls approach have overcome these problems, allowing the NDS to implement  a direct electrically controlled soft capture system.  There are several unique characteristics of this system. The design employs a control approach originally  envisioned in 2003 by Boeing in support of the Orbital Space Plane Program [3]. The NDS soft capture  system externally appears very similar to APAS, but the control and effector architecture is completely  different. Unlike the APAS ball screws, the NDS linear actuators are mechanically independent of one  another, each being driven by a dedicated motor. Because of this lack of mechanical coupling, the  coordination required to accomplish capture and alignment must be provided by the controller. In most  ground-based Stewart Platform applications, the coordination involves calculating the position and  NASA/CP—2018-219887 474