354 Description of the first generation latching system   The general architecture of the latching system (Figur e 2) consists of twelve latches and an equal number  of latch tabs per IBDM. All are mounted on the inside of the IBDM tunnel wall and are oriented in a radial  direction. The androgynous concept of the IBDM enables each latch to mate with a corresponding latch  tab on the side of a mating vehicle. The latches can be driven via two independent ring gears that take  care of the synchronization between all latches. Each ring gear is individually actuated by a redundant electro-motor.    Figure 2  The Latch (left) and Latch Tab (ri ght) of the first generation latching system   The latch mechanism kinematics are a combination of tw o 4-bar linkages. The latch has a roller interface  for landing onto the hook of a latch tab. The redunda ncy in the latch release is accomplished by a primary  and a secondary drive, plus a pyrotechnic solution. Additionally, a pyro-release capability can also be  implemented into the latch tab. The latch tabs  each hold a load-limiting device that becomes  geometrically positioned in line with the load path of the latch in order to have a minimal hysteresis in the load-limiter operation.   High design loads, the complexity of the design, an d the small available volume within the IBDM tunnel  have pushed the design of the latch mechanism to the limits. Even with the use of super alloys (MP35N,  Custom-455) and the selection of a cust omized bushing, only minimal positive structural margins of safety  could be accomplished.     Shortcomings of the first generation latching system

During the functional testing of the mechanism several anomalies were observed. These are the major findings:    In the nominal situation (i.e., as required), t he kinematics of the latch are designed to follow a  vertical trajectory in the final phase of latching (i.e., the roller of the latch approaching the hook of  the latch tab, see Figure 3), limiting any transverse loads on the latch mechanism. However, during functional testing, the trajectory showed a motion of the roller towards the edge of the latch  tab hook. This happened due to flexibility of the secondary drive elements of the latch that created a bigger than expected deflection of the latch linkages involved.  Test conclusion: the design concept of the la tching system appeared not to be safe against  inadvertent release of the mechanism (i.e., the la tch roller snapping of the latch tab). Therefore,  this latching system is no reliable device for a structural connection.   In order to minimize the primary drive torque to close the latch, the diameter of the free rotating  pins had to be reduced. Therefore, custom-s ized commercial parts were needed. These were  dry-lubricated bushings with a larger L/D ratio than nominal (off-the-shelf) units. It was discovered that the assumptions made for the strength verifi cation of standard parts are not valid for the  dimensions of the customized bushings. This was not anticipated by the parts supplier. The load  bearing capability of the custom-sized bushings ap peared to be lower than the load capability of  NASA/CP-2010-216272