After the torque limiters, the torque is  input into a secondary gearbox. The  purpose of the secondary gearbox is  threefold: 1) to shift the torque axis into  the proper position for the mechanism  shafts to pick up; 2) to further increase  the torque provided to the fold  mechanisms and latches; and 3) to  split the single torque input from the  torque tube into two separate and  mutually exclusive torque outputs, one  for the fold mechanism and one for the  latches. Control of the relative motion  of the two outputs is performed by a  mechanism that is a separate  subassembly contained within the  secondary gearbox called the timing  mechanism. From the secondary  gearbox outputs, shafts deliver the        Figure 3.  V-201 Port Fin Stow ed  torques to the two fold linkages and the three latch linkages. The forward secondary gearbox delivers  torque to the forward linkage, forward latch, and mi d latch. The aft secondary gearbox delivers torque to  the aft linkage and aft latch.  

Cams on the shafts engage limit switch assembly pairs . There are five limit switch assembly pairs; one  for each of the two fin actuation linkages and one for each of the three latches. Signals from these are  used by the fin fold software in the V-201 computer  to shut the motor(s) down and provide feedback to  the crew on a laptop in the orbiter. For the stowed position, the motor is shut down when a signal from  each of the two limit switch assembly pairs indica tes both linkages are locked overcenter in the stowed  position. For the deployed position, t he motor is shut off when a signal from at least one switch of each of  the five limit switch assembly pairs indicate s both linkages and all three latches are fully engaged. 

The linkages and latches lock the fin in the deployed position as well as compress the fin fold line  environmental seals. The environment al seal is based on the shuttle landing gear door seal. An outboard  thermal barrier consists of three layers of a al umina-boria-silica continuous filament braided tubular  sleeving over a knitted Inconel X750 wire spring tube filled with silica mat or batting. The thermal barrier  is capable of withstanding the high temperature of entry. However, it is porous. An internal Teflon coated,  Dacron stiffened silicon rubber pressure seal prevents the pressure difference from the interior of the fin  to the exterior of the fin from ingesting the plasma through the thermal barrier. This pressure seal cannot  take the high temperature; theref ore, the thermal barrier and pressure seal are both required for a  complete environmental seal. The fin is covered with tiles on the outboard surface and leading edge. The  inboard surface is covered with a blanket except fo r tile hinge fairings around the protruding hinge lugs. 

A counterbalance mechanism was designed to be install ed internal to the upper fin to make 1-G ground  fin actuation more representative of  on-orbit use. This mechanism is removed for flight. The mechanism  will not be discusses in this paper. 

Fin Actuation Mechanism Linkages  Fin Actuation Mechanism Linkage Description   The fins are folded via two four-bar linkages that toggle over-center in both the stowed (folded inboard)  and deployed (locked in outboard for atmospheric flight ) positions. The upper fin is driven by a crank on  the lower fin that is attached to the upper fin via a turnbuckle coupler. The turnbuckle consists of a center  sleeve with a right-hand thread on one end and a left-hand thread on the other. Two M81935/1 rod ends  with spherical bearings are threaded into the sleeve. Hence, when the turnbuckle linkage is installed with  the rod ends pinned they cannot rotate; rotating the sleeve will extend or shor ten the turnbuckle. The  365