356 The Second Generation Latching System 

Introduction   After cancellation of the X-38 pr ogram by NASA, ESA decided to independently purs ue the IBDM  development – comprising the further  optimization of the latching syst em – for implementation of this  docking mechanism into their Space Exploration plans. The second generation latch did therefore take  into account the above-mentioned lessons learned, wh ile establishing even more severe requirements for  the Exploration purposes. For exam ple, the latching system not only had to be dual fault tolerant for  undocking, but also had to become dual fault tolerant for docking.   The size of the second generation latch was enlarged with respect to the first generation latch to assess the feasibility of an increased IBDM tunnel diameter (to also allow for ISPR-racks to pass through the  tunnel instead of astronauts only). The second gener ation latch did focus on the optimization of the  primary drive and did therefore not consider the design of a secondary release mechanism in order to fulfil the dual fault tolerance requirement. Howeve r, the new overall design concept introduces a  redundant release function by the possibility to operate the latching system on the other vehicle side.       Design improvements for the second generation latching system

In order to comply with the more stringent requirement s, the following design modi fications with respect to  the first generation latching system have been incorporated:   Modification of the overall design concept, in which:   o the combination of latch and latch tab became replaced by the generic hook-on-hook principle.  Therefore each latch will mate with an identical counterpart on the other vehicle side, resolving  the necessity for the IBDM to carry the mass of unutilized elements that are imposed for androgyny (i.e., the latch tabs on a passive IBDM).   o the common drive train has been replaced by an independent motor plus gearbox for each  latch.    Latches have moved from the inside of the tunnel wall to the outside and are oriented in a  tangential direction.     A compliance element has been integrated into t he design of the latch itself. Therefore, two  mated IBDMs feature double of the latch compliance.    Inherent positive locking of the latch is incorporated.     A (primary) drive kinematics has been accomplished  that is no longer domi nated by the diameter  of the pins in the joints. Therefore, the bushings can now be correctly sized (i.e., increased) to the  latch load. For reasons of completeness, it has to be mentioned that as a negative consequence,  friction losses due to a bigger pin diameter are now a little higher compared to the first generation latch design, resulting in a lower overall mechanical efficiency.      Conservative factors of safety (1.25 for yield and 1.5 for ultimate) have been taken on the design  load stress.    The second generation latch contains a primary drive with linkage kinematics similar to the previous latch  design (i.e., a combination of two 4-bar linkages). In a nalogy to the first generation latch, this results in a  vertical trajectory for the operated hook when it approaches its mating counterpar t at the final phase of  latching.   Lessons learned from the second  generation latching system

Hardware has been built and has been subjected to stri ngent functional and structural tests (Figure 4).  The following lessons learned are recorded:    The hook-on-hook concept does guarantee a stable structural connection.     The number of moving parts in the latch drive chain has to be (further) reduced as much as  possible. Any moving part increases the risk of jamming the mechanism and more joints in the latch leads to lower transmission efficiency. A longer drive train through many linkages results in more difficult control in the synchroniz ation of the release of all latches.   NASA/CP-2010-216272