Abstract:
A floating stud assembly system of particular use in attaching insulating material to aircraft structures is described employing a stud extended through a hole in a plate. The plate comprises a pocket or cavity and an end of the stud is configured to reside in the pocket or cavity such that the stud is retained by the plate while still being allowed movement in a plane.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0001]    This invention was made with Government support under Contract No. AVTIP DO85, F33615-00-D-3052 awarded by the U.S. Air Force. The government has certain rights in this invention. 
     
    
     FIELD 
       [0002]    This disclosure relates to a floating stud assembly and method for attaching elements to one another. The disclosure has particular utility for attaching thermal protection, stand-off panels or other secondary structures to sandwich primary structures such as may be used in the design and manufacture of aircraft and spacecraft, and will be described in connection with such utility, although other utilities are contemplated. 
       BACKGROUND 
       [0003]    The Thermal Protection System (TPS) of spacecraft is an important development of the National Aeronautics and Space Administration (NASA). Broadly, the TPS of a spacecraft includes the various materials, manufacturing methods, design details, testing techniques and procedures of installation developed to protect the space shuttle vehicle from the severe temperature and heat transfer environments encountered in the various phases of each shuttle mission. A TPS typically includes three subsystems, a Reusable Surface Insulation Subsystem (RSISS), a Leading Edge Structural Subsystem (LESS) and a Penetration Subsystem (PSS). The RSISS constitutes most of the TPS and typically consists of flexible blankets and rigid ceramic-like tiles, plus the assembly concepts used to put the system together. The nature of the tiles make them extremely resistant to heat, but also brittle and susceptible to damage from structure-induced loads. Further, the tiles must be attached in a way capable of accommodating some level of error in alignment. That is, if tiles are slightly misaligned, they must not be attached in a way that pushes adjacent tiles into one another to the point that they are structurally unsound. Accordingly, tile-to-structure attaching systems have been developed that isolate the delicate tiles from damaging structure-induced loads. 
         [0004]    Current tile-to-structure attaching systems include adhesive and mechanical methods of tile attachment. While existing adhesive methods provide for a strong and flexible bond to the structure, they do not allow for convenient removal and reattachment of the tiles. Tiles in areas that require frequent access to the structure of the aircraft or spacecraft are therefore made with a hole in the center of the tile allowing for placement of a metal fastener from outside the aircraft or spacecraft to affix the removable tile to the structure. A densified ceramic plug of refractory material similar to that of the tile is then used to plug the gap between the top of the metal fastener and the outer surface of the tiles. In this fastening arrangement, an internally threaded receptacle is placed inside an oversized hole of a densified ceramic plug made of refractory material similar to that of the tile. A close-cut disk is then inserted on top of the plug and the assembly is bonded to a mating hole in the tile using an adhesive. The resulting receptacle floats in the tile and can accommodate lateral misalignment of about 0.03 in. (0.076 cm.) and azimuthal misalignment of a few degrees. 
         [0005]    While mechanical attachment systems allow for rapid attachment and removal of insulating tiles, they still require an assembly to be adhesively bonded to a mating hole in the tile. These inserts are further complicated in that they require attachment to special hard points within the sandwich structure of the vehicle. 
       SUMMARY 
       [0006]    The present disclosure provides among other things a system and method for attaching a secondary structure to a sandwich structure. One aspect of the disclosure provides a floating stud assembly system comprising a plate for attachment to the structure. The plate encompasses a pocket or cavity with a hole within the pocket or cavity. A stud having a shaft having a diameter less than the diameter of the hole and a head having a diameter greater than the hole is designed to be inserted through the hole such that the plate retains the stud, with the head located in the pocket or cavity of the plate. The plate can thus be attached to the structure and the stud retained by the plate while remaining free to move in two directions within the constraints of the hole and the pocket or cavity. 
         [0007]    The disclosure in another aspect also provides an assembly comprising first and second structures assembled together by a floating stud assembly system. The floating stud assembly system includes a plate attached to the first structure, and comprises a surface encompassing a pocket or cavity and having a hole within the pocket or cavity. A stud which comprises a shaft having a diameter less than the diameter of the hole extends through the hole. The stud has a head larger than the hole is configured to reside in the pocket or cavity and be retained by the plate. The end of the stud is larger than the hole such that the stud is retained by the plate and is allowed to move in two directions within the constraints of the hole and the pocket or cavity. 
         [0008]    The disclosure also provides a method for mechanically attaching thermal protection panels to a structure on an aircraft or spacecraft, which comprises attaching the thermal panels to the structure using a floating stud that provides tolerance for movement in two directions. In a preferred embodiment the floating stud is held within an apertured plate that has a surface encompassing a pocket or cavity; and the stud extends through the aperture, wherein the stud has a head which is larger than the aperture such that the stud is retained by the plate and is allowed to move in two directions. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. In the figures, like reference numbers refer to like elements or acts throughout the figures. 
           [0010]      FIG. 1  is a perspective view of a floating stud assembly coupled to a composite sandwich substrate, in accordance with one embodiment of the present disclosure; 
           [0011]      FIG. 2  is a perspective view showing details of the stud of the floating stud assembly; 
           [0012]      FIGS. 3(   a )- 3 ( c ) are side elevation views, in cross-section, illustrating installation of the floating stud assembly retaining plate in a composite sandwich structure in accordance with an embodiment of the disclosure; and 
           [0013]      FIG. 4  is a partial perspective view illustrating a secondary structure coupled to a composite sandwich substrate with the floating stud assembly in accordance with the present disclosure. 
       
    
    
       [0014]    Elements and acts in the figures are illustrated for simplicity and have not necessarily been rendered according to any particular sequence or embodiment. 
       DETAILED DESCRIPTION 
       [0015]    In the following description, and for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the various aspects of the disclosure. It will be understood, however, that the present disclosure may be practiced without these specific details. In other instances, known structures and devices are shown or discussed more generally in order to avoid obscuring the disclosure. In many cases, a description of the operation is sufficient to enable one to implement the various forms of the disclosure, particularly when the operation is to be implemented in software. It should be noted that there are many different and alternative configurations, devices and technologies to which the disclosed disclosure may be applied. The full scope of the disclosure is not limited to the examples that are described below. 
         [0016]    Referring to  FIGS. 1-2  and  4 , in one application of the disclosure a plate  100  with a hole  105  is utilized with a stud  120  to attach a secondary structure  300  to a primary structure  150 , typically a composite sandwich structure. Plate  100  encompasses at least one pocket or cavity  110  and is configured to be attached to structure  150 . Plate  100  may comprise a titanium or titanium alloy to allow for high temperature use or may comprise any metal, metallic alloy, ceramic, carbon composite or other material that may be formed into a plate. 
         [0017]    Plate  100  is fastened to structure  150  through any of several methods of attaching a plate to a structure, including a mechanical connector, chemical bonding, welding, adhesion or any other suitable attachment method. In one embodiment, a mechanical connector is used, and in a preferred embodiment a plurality of blind fasteners  130  are used to attach the plate  100  to the structure. In one embodiment blind fasteners  130  comprise Composi-Lok® break-off fasteners available from Monogram Industries, Inc., of Santa Monica, Calif. Referring to  FIGS. 3(   a )- 3 ( c ) the latter are installed in a hole or aperture  132  in the plate  100  and a match-drilled hole  134  in the outer skin  136  of the sandwich structure by positioning the fasteners in holes  132 / 134 , and turning a drive nut  138  on the threaded shaft  140  using a drive tool  141 . This snugs the shaft  140  in a nut  142  which draws the distal end  143  of the shaft up against the inside surface  146  of outer skin  136 , draws the outer skin  136  and plate  100  together, and distorts the deformable blind sleeve  144 . The shaft  140  is then snapped or cut off even with the top surface  148  of the plate  100 . 
         [0018]    Referring in particular to  FIGS. 1 and 2 , the pocket or cavity  110  in plate  100  contains a hole  105  and should have a depth sufficient to accommodate the head  210  of the stud  120 . In a particular embodiment, the pocket or cavity  110  has approximately the same depth as the head  210  (shown in phantom in  FIG. 1 ) of stud  120  such that the stud  120  is in contact or very nearly in contact with the structure  150  and the plate  100  when the plate  100  is securely fastened to the structure  150 . The pocket or cavity  110  also may have a specific shape so as to restrict or prevent the movement of the stud  120 . For example, the head  210  of the stud  120  may have a triangular shape and the pocket or cavity  110  may have a slightly larger triangular shape such that the head  210  is able to move translationally in a plane, i.e. in two directions, but is prevented from rotating. A rectangle, hexagon, or any other geometrical or any irregular shape may also be used in combination with the shape of the head  210  of the stud  120  to limit the rotational movement of the stud  120 . 
         [0019]    The stud  120  has a shaft  200  that is smaller than the hole  105  in the plate  100  and an end or head  210  that is larger than the hole  105  in the plate  100 . That is, the shaft  200  of the stud  120  may pass through the hole  105  in the plate  100  while the head  210  may not. The stud  120  may be composed of steel or another metal, metal alloy, carbon composite or any other material that may be shaped into a stud. Stud  120  is long enough to extend through the plate  100  and be used to attach the secondary structure  300 . The head  210  of the shaft may be shaped in conjunction with the pocket or cavity  110  of the plate  100  as described above so that the pocket or cavity  110  of the plate  100  constricts the rotational movement of the stud  120 . 
         [0020]    In one embodiment of the disclosure, the shaft  200  of the stud  120  includes a threaded portion  220  such that a secondary sub-assembly or structure  300  such as found on a spacecraft (see  FIG. 4 ) may be attached to the stud  120  with a nut, for example, a locking nut  302 . The constricted rotational movement of the stud  120  allows for tightening of the locking nut  302  on the threaded portion  220  of the stud  120 . The secondary structure  300  also may be secured to the stud  120  by a cotter pin or the like passed through a hole (not shown) through the stud  120 , or any other method of securing a structure to the stud  120 . The stud  120  may also include a flanged portion  230  upon which the secondary structure may be supported. 
         [0021]    In the context of an aircraft or spacecraft, the primary structure  150  is comprised of the body of spacecraft, typically constructed of a composite sandwich material. The stud  120  is passed through the hole  105  and the plate  110  is then attached to the composite sandwich material. This attachment is accomplished by passing blind fasteners  130  through the apertures  132  in the plate and into match drilled holes  134  in the composite sandwich material. Drive nut  138  is then turned using a drive tool  141  to secure the shaft of the blind fastener  140  in a nut  142  distorting the deformable blind sleeve  144 . The shaft  140  is then snapped or cut off even with the top surface  148  of the plate  100 . The insulation tile secondary structure  300  is then placed onto the stud and rested on the flanged portion  230 . The secondary structure  300  is secured to the stud using a locking nut  302 . 
         [0022]    This process is repeated for each insulation tile that is to be attached to the body of the spacecraft. As tiles crowd next to each other, the head  210  floating in the pocket or cavity  110  of the plate  100  allows each tile to move within the constraints of the pocket or cavity  110 . This movement accommodates any misalignment of the tiles during installation. After tightening, the screwhead may be given some protection from oxidation by covering it with a plug made of the same material as the insulating tile and designed to lie flush with the outer surface of the tiles. 
         [0023]    The stud attachment allows for simple and repeatable removal of each tile as needed. The locking nut  302  is merely loosened and removed and the tile pulled from its position on the stud. The locking nut  302  may be reusable or disposable. The removed tile is reattached by placing the secondary structure  300  back onto the stud and rested on the flanged portion  230 . The secondary structure  300  is resecured to the stud using the same or a new locking nut  302 . 
         [0024]    The present disclosure thus provides a simple and reliable system for attaching thermal protection panels to aircraft and spacecraft or the like. The system also advantageously may be employed for attaching panels together, and for attaching mounting brackets and internal systems such as plumbing or the like to a composite panel in an aircraft or spacecraft.