Each latch hook is driven by a four-bar linkage that toggles over-center to prevent the latch from  disengaging. The linkage consists of a crank attac hed to hook via a coupler link. The entire latch  mechanism is housed internal to an integrally mach ined hinge fitting. Due to limited available volume and  access for assembly, the coupler-to-hook connecti on is a unique design that does not require an  additional pin (Figure 5). The crank drives the hook to engage the lug attached to the upper fin, compress  the seal until the upper fin engages a shimmed hardstop,  and the coupler link is overcenter to lock the  latch hook. As the crank approaches the hardstop, a cam on the input shaft trips the limit switch.   Latch Mechanism Testing   For all joints, the X-38 project required a positive margin  of safety on the design load with a 1.5 factor of  safety and a 1.15 fitting factor applied (or 172.5% of the design load). The mid hinge/latch analysis could  not show positive margins with this fitting factor app lied. The lead analyst agreed to waive the fitting factor  if the joint was tested to destruction. The test asse mbly shown in Figure 6 was tested to destruction. It  failed at the splice joint below the hinge line at a load equal to 171% of the design load. This closely  matched the finite element predicted failure load and location. However, this margin must be reduced due  to thermal considerations which results in an 8% decrease in strength for the aluminum hinge fitting. This  reduces the margin of safety to 5% above the safety factor of 1.50. Reference “X38 V-201 Fin Mid  Hinge/Latch Qualification Test Plan” and “X38 V-201 Fin Mid Hinge/Latch Qualification Test Report”. 

Figure 6.  Fin Hinge/Latch Qualification Test Unit Load  applied  here Failure Point

Latch Mechanism Lessons Learned and Observations   • Avoid shim s. Requires significant time to adjust small increments.   • If you need shim s, use lam inated shim s and m anufacture extra shim s. Laminated shims allow small  increment adjustment 0.05 to 0.08 mm (0.002 to 0.003 in.). However, it is easy to remove too many  laminations when pulling the shims; therefor e, it is important to have extra shims.   • Negotiate interfaces carefully.  The failure point was a stress conc entration that was originally away  from the load path of the mechanism. However, as  the mechanism evolved,  the load path shifted  closer to the stress concentration leading to a lower load capability.   • Solid Film  lubricants are often applied too thick.  It is a good idea to include a burnishing procedure in  installation and/or rigging specifications. Often it is as simple of sliding the shaft into a mating bushing  on the bench top to remove excess lubricant. This has been seen with Everlube and Tiolube. 

Fin Actuation Mechanism Driv etrain  Power Driv e Unit Description   The PDU gearbox consists of two planet ary gear trains in series providing mechanical logic that allows  multiple inputs. Translational gearing after the planet ary gearing output is used to transfer the torque to  the required output shaft axis of rotation. The firs t planetary train has one gearmotor attached to the sun  gear and a second gearmotor attached to the ring gear allowing either gearmotor the ability to  independently drive the first planetary train of the gearbox. The carriage output of the first planetary train  369