Abstract:
A temporary control fin stop system employs a housing coupled to a vehicle. At least one tang is coupled to the housing and positioned to engage a trailing edge of a fin. The tang is ablatively erodible at a predetermined temperature induced by a flight profile of the vehicle to allow unconstrained motion of the fin.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with Government support under (F33615-03-9-2422) awarded by the Department of Defense. The government has certain rights in this invention 
     
    
     BACKGROUND INFORMATION 
       [0002]    1. Field 
         [0003]    Embodiments of the disclosure relate generally to aerodynamic surface locking systems and more particularly to embodiments for a physical control lock to prevent undesirable motion of an aerodynamic surface during a boosted launch with ablative erosion of the locking elements to release the surface and lock geometry for break free operation upon powering of surface controls. 
         [0004]    2. Background 
         [0005]    Launch of boosted two stage vehicles from a carrier aircraft may be somewhat forceful in order to achieve successful and safe separation from the carrier aircraft. Control surfaces on the vehicle are typically unpowered during launch and the surfaces must be maintained in a neutral or defined position to avoid inadvertent cocking at angles which might cause uncontrollable flight or inadvertent striking of the carrier aircraft with attendant crew safety issues. Mechanical control locks typically require a complex mechanism for activation and may provide an additional failure mode. Frangible locks or pyrotechnically disengaged locks may produce debris pieces which are large enough to be a potential hazard to the vehicle. 
         [0006]    It is therefore desirable to provide a structurally simple and cost effective control locking system which maintains control of the aerodynamic surface until control system power is applied. Additionally, it is desirable to provide reliable disengagement of the locking system at a predetermined flight phase. 
       SUMMARY 
       [0007]    Exemplary embodiments provide a temporary control fin stop system employing a housing coupled to a vehicle. At least one tang is coupled to the housing and positioned to engage a trailing edge of a fin. The tang is ablatively erodible at a predetermined temperature induced by a flight profile of the vehicle to allow unconstrained motion of the fin. 
         [0008]    The embodiments provide a method for constraining an unpowered control fin by attaching a housing to a booster exterior surface and inserting stop tangs into the housing to extend for contact with a control fin. The stop tangs are then ablatively eroded to allow unconstrained motion of the control fin. 
         [0009]    The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a side view of an exemplary air vehicle on which the present embodiments may be employed; 
           [0011]      FIG. 2  is a partial side view of the air vehicle of  FIG. 1  showing details of and installation of an exemplary embodiment; 
           [0012]      FIG. 3  is a detailed side view of the exemplary embodiment; 
           [0013]      FIG. 4  is a detailed front view of the exemplary embodiment; 
           [0014]      FIG. 5  is a top view of the exemplary embodiment; 
           [0015]      FIG. 6  is a detailed isometric view of a stop tang employed in the exemplary embodiment; 
           [0016]      FIG. 7  is a side view of the stop tang; 
           [0017]      FIG. 8  is a top view of the stop tang; 
           [0018]      FIG. 9  is a front view of the stop tang; 
           [0019]      FIG. 10  is an isometric view of the exemplary embodiment; 
           [0020]      FIG. 11  is a detailed side view of the exemplary embodiment of the control fin stop assembly with a match drill tool installed for spacing of the stop tangs and fin; 
           [0021]      FIG. 12  is a detailed side view of the exemplary embodiment with a match drill tool removed after installation of the control fin stop assembly; 
           [0022]      FIG. 13  is a bottom view of the installed control fin stop assembly engaging the fin; 
           [0023]      FIG. 14  is a graphical representation of exemplary aerodynamic heating for ablative erosion of the stop tangs; 
           [0024]      FIG. 15  is a side view of the air vehicle showing range of motion of the control fin for shearing of the uneroded material of the stop tangs; and, 
           [0025]      FIG. 16  is a flow chart depicting the method for fin control using the disclosed embodiments as a control fin stop assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The embodiments described herein provide a passive control fin stop system for precluding unwanted motion of a control fin during a launch sequence of an air vehicle such as an air launched, boosted, two stage high speed vehicle. The control fin stops incorporate a dimensionally stable housing fabricated in metal or carbon matrix composite (CMC) and finger like protrusion that inhibit control fin movement. The finger like protrusions referred to herein as tangs have a material composition selected based upon the launch and initial boost phase environments of the flight. The selection of the tang material utilizes low temperature material properties seen at high altitudes to restrain unpowered control fin movement prior to launch while providing a low melt point to erode/ablate from aerodynamic heating during the boost phase of the vehicle flight. The control fin stop tang material and shape are designed to be passive and failsafe. The passive control fin stops arrests fin motion under aerodynamic load and random vibration in case of inadvertent control fin locking mechanism failure. The control fin stop tangs melt away before the fin is commanded. However, the control fin stops also yield at room temperature to be sheared by a powered/commanded control fin and do not impede the functionality of a powered actuator. 
         [0027]    Referring to the drawings,  FIG. 1  shows an exemplary air vehicle  10  on which the embodiments to be described herein may be employed. Air vehicle  10  is a multistage test vehicle having a booster  12  and a hypersonic primary vehicle  14 . The primary vehicle  14  has one or more control fins  16  for aerodynamic control. As shown in  FIG. 2 , a control fin stop assembly  18  is attached to the booster  12 , as will be described in greater detail subsequently, to engage each control fin  16 . 
         [0028]    The control fin stop assembly  18  is shown in detail in  FIGS. 3-5 . A housing  20  is employed for attachment to the booster  12 . A pair of tangs  22  extend from a forward surface  24  on the housing  20 . For the embodiment shown, the housing  20  is attached to the booster  12  using bolts  26 . Each stop tang  22  is coupled to the housing by engaging a portion of the tang in a relief or shaped cutout  28  in the housing. For the exemplary embodiment, each cutout  28  has a trapezoidal shape with a narrow side opening  30  onto the forward surface  24 . Details of exemplary stop tangs  22  are shown in  FIGS. 6-9 . Each stop tang  22  has a body  32  shaped to be received in the cutout  28  in the housing  20 . The trapezoidal shape of the body  32  prevents the tang from being withdrawn axially from the cutout  28  through the narrow side opening  30 . A narrow end  34  of the body  32  provides a neck on which a contact head  36  is attached. As seen in  FIGS. 3 and 5 , the contact head  36  extends forward of the forward surface  24  on the housing and provides a contacting surface  38  for the control fin as shown in  FIGS. 6 and 7 . A reaction surface  40  is provided on the contact head  36  which engages the forward surface  24  of the housing  20 . Force exerted by a control fin acting on the contacting surface  38  of the cantilevered head  36  urges rotation of the head on the neck  34 . Reaction surface  40  acting on the forward surface  24  prevents rotation of the head  36  substantially resolving any rotational forces imparted by the control fin into shear across the neck  34 . Head  36  has a blunt forward end  42  and the head tapers from the neck as seen in  FIGS. 6 ,  8  and  9  to reduce volume of the head; both attributes enhancing the ability for ablative erosion of the head to free the control fin as will be described in greater detail subsequently. 
         [0029]    The assembled control fin stop assembly  18  as shown in  FIG. 10  provides opposing stop tangs  22   a  and  22   b  between which the control fin constrained. Separation width of the stop tangs  22   a  and  22   b  provides clearance for normal control fin offset or minute uncontrolled motion during the constrained phase of the flight. As seen in  FIG. 11 , placement of the control fin stop assembly  18  on the booster  12  may be accomplished using a centering tool  44  in which a trailing edge  46  of the control fin  16  is received, the centering tool  44  in turn being received between the stop tangs  22   a  and  22   b . Match drilling of bores for bolts  26  may be accomplished with the centering tool in place or with bolts employing self centering devices, the bolts may be tightened with the centering tool in place for exact positioning of the control fin stop assembly  18 . As seen in  FIG. 12 , with the centering tool  44  removed after assembly, the trailing edge  46  of control fin  16  is substantially centered between the stop tangs  22   a  and  22   b . However, stop tang  22   a  prevents unconstrained upward motion of the control fin  16  by contacting a top surface  48  of the control fin and stop tang  22   b  prevents unconstrained downward motion of the control fin by contacting a bottom surface  50  of the control fin. As seen in  FIG. 13 , the control fin stop assembly  18  positions the stop tangs  22  sufficiently outboard of the surface mold lines of the booster  12  and primary vehicle  14  to assure engagement of an inboard portion of the trailing edge  46  of fin  16 . 
         [0030]    In operation, the air vehicle  10  is carried aloft by a B-52 or comparable carrier aircraft and launched at a predetermined altitude. Typical temperature of the exterior of the air vehicle  10  and control fin stop assembly is approximately −30° F. during prelaunch carry at altitude as represented in  FIG. 14 . A primary function of the control fin stop assembly is to prevent undesired control fin movement during the launch sequence and immediately after launch to assure clean separation from the carrier aircraft. The structural strength requirements for the stop tang material at prelaunch conditions must be sufficient to withstand uncommanded control fin forces if the control fin actuator is inadvertently unlocked and the actuator is in the unpowered or passive condition. Upon launch, the air vehicle  10  accelerates rapidly providing significant aerodynamic heating. Material selection for the stop tangs  22  is also made based on desired ablative erosion of the stop tang heads  36  within the acceleration time of the air vehicle prior to separation of the booster  12  from the primary air vehicle  14  to allow unconstrained motion of the control fin upon separation. 
         [0031]    Thermoplastic materials provide the desired combination of low temperature strength and desirable melting temperature for ablative erosion of the tang head. In one exemplary embodiment, low density polyethylene (LDPE) is employed as the stop tang material. Alternative materials employed in various embodiments may include High Density Polyethylene (HDPE), Polypropylene (PP), Polystyrene, Polyvinyl Chloride (PVC), Acrylonitrile Butadiene Styrene (ABS), Ionomer (Surlyn) or Acetal Polymethyl Methacrylate (PMMA). LDPE has a melting temperature of 232° F. as shown in  FIG. 14  by trace  51 . The LDPE stop tangs attain the melting temperature due to aerodynamic heating at between approximately 16 and 31 seconds after launch as shown by traces  52  and  54  with an average of approximately 21 seconds as shown by trace  56 . For an LDPE embodiment of the stop tangs having a neck area of 0.302 in 2 , a reaction surface extending 0.148 inches beyond a neck width of 0.151 inches normal unpowered control fin forces of 121 lbs could be sustained at the launch temperature conditions. However in the event of lack of ablative erosion, actuator movement of the control fin in the powered condition occurring approximately 27 seconds after launch with an initial trim of 3° as shown in  FIG. 15  with fin position  16   a  or controlled actuation from unconstrained positive to negative control fin positions at approximately 29.8 seconds after launch as shown by fin position  16   b  and  16   c  respectively results in forcible contact with sufficient shear force to shear the stop tang neck at a predetermined load for free control fin movement upon separation of the primary air vehicle  14  and booster  12 . 
         [0032]    As represented in  FIG. 16 , the embodiments disclosed provide a method for constraining an unpowered control fin on a flight vehicle by attaching a housing to a booster exterior surface, step  1600  and inserting stop tangs into the housing, step  1602 , which extend for contact with the control fin. The housing and stop tangs may be positioned relative to the control fin with a centering tool and fixed in position, step  1604 . During launch, the stop tangs are ablatively eroded, step  1606 , to allow unconstrained motion of the control fin. If the stop tangs are not completely eroded or if ablative erosion has not occurred, shearing of the stop tangs is accomplished by powered actuation of the control fin, step  1608 . 
         [0033]    Having now described various embodiments of the invention in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present invention as defined in the following claims.