hardstop is a setscrew that is set to stop the crank  motion while the coupler link is still tensioned. A jam  nut with lockwire retains the setscrew position. This coupler link preloaded in tension provides a clamping  force to lock the fin in the overcenter position. Aerodynamic forces on the upper fin only results in  attempting to drive the linkage further overcenter . The tension preload can be increased by shortening  the coupler link or raising the hardstop on the lower fin. Reference SKK51356551 for rigging procedure.  If the test flight is aborted, V-201 will be returned in t he shuttle with the fins locked in the stowed position.  If the limit switches fail, the crank assembly modul e contains a mechanical indicator flag to provide  positive indication that the fins are locked stowed (Figure 4). It was decided not to include a mechanical  flag for the deployed position because the assumption wa s made that at least one of each of the limit  switch pairs for each latch would be working or the te st flight would be aborted. A mechanical indicator for  the deployed position would have required penet rating the TPS and a complicated linkage.  Fin Actuation Mechanism Li nkage Prototypes and Testing   The concept for this linkage is similar to the S huttle’s Remote Manipulator System Manipulator  Positioning Mechanism (MPM) pedestal linkage. Despite th is proven flight histor y, the concept was not  accepted initially. In the stowed position for launch,  the two over-center linkages are the only locking  feature that restrains the fins from  striking the shuttle door radiators. In order to further explain this  concept, a working wooden mockup was made to demons trate the kinematics of  the crank/linkage/hinge  four-bar mechanism. A full scale prototype of the fin actuation mechanism linkage was then designed and  built. The prototype uses two rectangul ar box structures to represent the interface for a portion of the  upper and lower fin structure. The box structures are hinged together and actuated by a single four-bar  linkage. The linkage included an instrumented coupler link. The prototype demons trated that the desired  tension and compression preloads could be achieved wi th the linkage. It allowed for correlation between  input torque and preload. This information was required in order to design the drivetrain. The prototype  also had mockups of the TPS to provide clear visua lization of the outboard tiles and thermal barriers, the  inboard blankets, as well as location of the pressure  seal. Later in the project, the loads changed such  that additional preload was required for the stowed position. A new sleeve with an increased outer  diameter was designed, instrumented,  and assembled in the prototype to  verify the desired preload/input  torque combination could be achieved. Every linkage used in the prototype, qualific ation, and flight units  was instrumented and calibrated and proof tested in  tension and compression. The instrumentation  consisted of strain gages wired into a full bridge designed to amplify the signal due to the strain.  

Fin Actuation Mechanism Linkage Lessons Learned   Lessons learned from the early prototype influenced the final design. The pr ototype did not include a  crank assembly module. However, due to the difficu lty of assembling the hardstops, bushings, crank, and  coupler unit on the prototype, all t hese items were combined into a si ngle sub-assembly for the flight  design. Further, it was noted that t he hardstops for the crank in the stowed position were not adjusted on  the prototype from the original nominal position. Therefore, these we re replaced with a fixed dimension  striker plate and pivoting head on the crank for the fli ght design. The prototype used fixed gussets with  setscrews to set the stow angle. This resulted in la rge holes in the upper structure. The gussets were  placed in bending which placed the lower fin rib web in bending. The setscrew had limited adjustment  capability. These drawbacks led to the drag link concept using the same turnbuckle linkages. This  concept minimized the hole size in  the structure. The turnbuckle al so had a much greater adjustment  capability. The drag links could be instrumented just as the coupler link. This aided in the rigging of the  mechanism to ensure each preload reaction was split between the drag links equally.  

Although this drag link design was more robust than the gusset design, it did lead to other challenges for  the flight design. Since the drag links retracted into the fin as it was deployed and extended as it was  stowed, the links had to be guided. The drag link fitti ng occasionally jammed on the sheet metal track.  This problem was still being resolved when the project was cancelled. 

It was noted in the prototype that if  the locking device tab was oriented incorrectly it would plastically  deform during fin actuation due to interference with surrounding structure. This caution was incorporated  into the rigging procedure for the flight mechanism. 

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