Patent Publication Number: US-7710348-B2

Title: Furlable shape-memory reflector

Description:
BACKGROUND 
   This disclosure relates in general to shape-memory reflectors and, but not by way of limitation, to shape-memory reflectors utilizing shape-memory polymers among other things. 
   Space antennas are designed to provide reliable RF energy reflection to a feed located at the focus of the antenna&#39;s energy collecting surface. A deployable space antenna, therefore, should likewise supply the same RF energy reflection while providing an antenna that is launched in a packaged configuration and deployed as a reflector that exceeds the size of the packaged configuration. A deployable antenna should be light weight, have a small stowage to deployment volumetric ratio, has a solid reflector surface, and be as simple as possible to deploy. 
   BRIEF SUMMARY 
   A shape-memory reflector is provided according to one embodiment of the disclosure. The shape-memory reflector may include an elastic reflector material, a shape-memory stiffener, and a plurality of radial stiffeners. The shape-memory stiffener may be coupled with the elastic reflector material in a band that encloses at least a portion of the elastic reflector surface, for example, the exterior of a paraboloid reflector. Each of the plurality of radial stiffeners are coupled with the bottom surface of the elastic reflector material and extend radially from a central portion of the elastic reflector surface toward the outer edge of the elastic reflector surface. The shape-memory reflector may be packaged in a packaged configuration that includes a plurality of reversing bends within the elastic reflector material and/or the shape-memory stiffener, and the shape-memory reflector is configured to deploy into a deployed configuration (i.e. a paraboloid) by heating the shape-memory stiffener. 
   The shape-memory stiffener(s) may include a shape-memory polymer having a glass transition temperature (T g ) that is less than a survival temperature of the shape-memory polymer. The shape-memory stiffener may also include a top and bottom face sheet. One of the top and bottom face sheets may be a portion of the elastic reflector material. The elastic reflector material may comprise a thin laminate material and/or a graphite composite material. The radial stiffeners may comprise a solid material and/or a laminate material. Heaters may also be coupled with the shape-memory stiffener. 
   A method for packaging a shape-memory reflector is also provided according to another embodiment of the disclosure. The shape-memory reflector may be initially fabricated in a deployed configuration and includes a paraboloid-shaped elastic reflector coupled with a band of shape-memory polymer at a circumference of the elastic reflector and a plurality of radial stiffeners. The method may include heating the shape-memory polymer to a temperature above T g  of the shape-memory polymer and applying mechanical loads to the shape-memory reflector such that the mechanical loads deform the shape-memory reflector into a packaged configuration. The shape-memory stiffener may then be cooled to a temperature below T g  of the shape-memory polymer following which the mechanical loads may be removed. 
   A method for deploying a shape-memory reflector is also disclosed according to another embodiment. The shape-memory reflector includes a paraboloid-shaped elastic reflector coupled with a shape-memory polymer at a circumference of the elastic reflector and a plurality of radial stiffeners, and the shape-memory reflector is packaged in a packaged configuration. The shape-memory polymer is heated to a temperature above T g  of the shape-memory polymer. Once heated, the shape-memory reflector is allowed to return to a deployed configuration, such as, a paraboloid shape. The shape-memory polymer may then be cooled to a temperature below T g  of the shape-memory polymer. 
   A method for manufacturing a paraboloid shape-memory reflector is also provided according to another embodiment. A thin elastic reflector material is provided to form a paraboloid. The elastic reflector material includes a top surface at the concave side on the elastic reflector material and a bottom surface on the convex side of the elastic reflector material. Radial stiffeners are then coupled to the bottom surface of the elastic reflector material and positioned radially about the center of the elastic reflector material. A shape-memory polymer is then coupled with the elastic reflector material at a circumference of the elastic reflector material. The circumference may enclose at least a portion of the elastic reflector material centered about the center of the elastic reflector material. 
   Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating various embodiments, are intended for purposes of illustration only and do not limit the scope of the disclosure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a three-dimensional shape-memory reflector in a deployed configuration according to one embodiment. 
       FIG. 2A  shows a back view of a shape-memory reflector in a deployed configuration according to one embodiment. 
       FIG. 2B  shows a cross-section of a radial stiffener according to one embodiment. 
       FIG. 3A  shows a cross section of a shape-memory stiffener according to one embodiment. 
       FIG. 3B  shows a graph of the shear modulus G, the complex shear modulus G*, and the ratio of the shear modulus to the complex shear modulus G*/G of an exemplary shape-memory material according to one embodiment. 
       FIGS. 4A and 4B  show a top view and a side view of a packaged shape-memory reflector according to one embodiment. 
       FIG. 5  shows mechanical linkages attached with radial stiffeners according to one embodiment. 
       FIGS. 6A and 6B  show a three-dimensional view of a deployed and packaged shape-memory reflector according to one embodiment. 
       FIGS. 7A and 7B  show a front and top view of a packaged reflector shell within a backing structure according to one embodiment. 
       FIG. 8  shows a front view of a deployed reflector shell and backing structure according to one embodiment. 
       FIG. 9  shows a top view of a packaged shape-memory polymer reflector that includes both large and small pleats according to one embodiment. 
       FIG. 10  shows a flowchart of a method for packaging a shape-memory reflector according to one embodiment. 
       FIG. 11  shows a flowchart of a method for deploying a shape-memory reflector according to one embodiment. 
       FIGS. 12A and 12B  show a packaged and deployed shape-memory reflector coupled with a satellite according to another embodiment. 
       FIGS. 13A and 13B  show a top and side view of a packaged shape-memory reflector within a clearance to launch envelope according to another embodiment. 
       FIG. 14  shows a cut away view of a circumferential shape-memory polymer coupled with an elastic reflector material according to one embodiment. 
       FIG. 15  shows a three-dimensional view of a shape-memory reflector with two shape-memory stiffener according to another embodiment. 
       FIG. 16  shows a back view of a shape-memory reflector with two shape-memory stiffeners according to another embodiment. 
   

   In the appended figures, similar components and/or features may have the same reference label. Where the reference label is used in the specification, the description is applicable to any one of the similar components having the same reference label. 
   DETAILED DESCRIPTION 
   The ensuing description provides preferred exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment. It should be understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. 
   Embodiments of the present disclosure are directed towards a shape-memory reflector. The shape-memory reflector may be adapted for space communication applications. The shape-memory reflector may be prepared and launched in a packaged configuration that requires little or no mechanical devices to secure the reflector during launch. Once in space, the shape-memory reflector may be deployed with little or no moving parts. The shape-memory reflector may be parabolic shaped in a deployed configuration and stowed in a packaged configuration that is somewhat cylindrical-shaped. The shape-memory reflector may include a surface of substantially continuous, elastic reflector material. For example, the elastic reflector material may comprise a laminate of composite polymer layers. 
   The shape-memory reflector may include a shape-memory stiffener that is used to actuate the reflector from the packaged configuration to the deployed configuration when heated above T g . The shape-memory stiffener may include a sandwich of flexible face sheets around a core of shape-memory material, for example, a shape-memory polymer and/or foam. One of the flexible face sheets may include the reflector material. The shape-memory stiffener may be attached circumferentially on the reflector material. In one embodiment, the shape-memory stiffener may be attached circumferentially with the exterior circumference of the reflector material. In another embodiment, the shape-memory stiffener may be attached circumferentially with various other circumferences of the reflector material with a radius less than or equal to the radius of the paraboloid. 
   In various embodiments, the shape-memory reflector may also include a plurality of radial stiffeners that are, for example, radially attached with the back surface of the reflector material. The radial stiffeners may extend from a central portion of the reflector material and extend outwardly toward the exterior edge of the reflector material. In one embodiment, when the shape-memory reflector is stowed in its packaged configuration, the radial stiffeners may define bend locations within the reflector material in the package configuration. 
     FIG. 1  shows a three-dimensional shape-memory reflector  110  in a deployed configuration according to one embodiment. The shape-memory reflector  110 , in this embodiment, is deployed in a paraboloid shape for directing energy to and/or from the focus at the centerline of the reflector. Various other deployed configurations and/or shapes may also be used. The shape-memory reflector  110  includes a substantially continuous reflector material  120 . The reflector material  120  may include a graphite-composite laminate with between one and six plies. Various other materials such as thin metallic membranes, epoxy films, or other laminates may be used. The laminates may include various thicknesses. The reflector material  120  may be formed on a parabolic mandrel during manufacture. The reflector material  120  may be an elastic material that is stiff in its plane and relatively flexible in bending. The reflector material may be thin enough to bend to a radius of a few inches without permanent deformation. 
     FIG. 2A  shows a back view of a shape-memory reflector  110  in a deployed configuration according to one embodiment. This view shows the convex side of the shape-memory reflector  110 . As can be seen, the shape-memory reflector may include a plurality of radial stiffeners  120  arrayed radially around a central portion of the shape-memory reflector  110 . The shape-memory reflector may also include a band of shape-memory material  140  shown at the edge of the shape-memory reflector  110 .  FIG. 2B  shows a cross section of a radial stiffener according to one embodiment. 
   The radial stiffeners  130  may be radially equidistant from each other or in any other configuration and may be attached to the convex side of the reflector material  120  in the deployed state. The radial stiffeners  130  may comprise a thicker layer of solid material, such as the same material as the reflector material  120 . The radial stiffeners  130  may also comprise plies of graphite composite laminate co-cured with the reflector material  120  during fabrication, or the radial stiffeners  130  may also comprise a strip of composite or other material secondarily bonded to the reflector material  120 . The cross section of the radial stiffener may be rectangular, as shown in  FIG. 2B , or any other shape, for example, a trapezoid formed by stacking narrower plies of composite on a wider base. 
   In one embodiment, the radial stiffeners  130  may be continuous, flexible, but non-collapsible sections. The radial stiffeners  130  may provide sufficient stiffness and dimensional stability in the deployed state so as to maintain the paraboloid shape of the reflective surface. The radial stiffeners  130  may also include sufficient flexibility in bending to enable them to be straightened during packaging. The radial stiffeners may also have sufficient strength longitudinally to react to radial tensile loads in the reflective surface that are applied during packaging. And, the radial stiffeners  130  may have sufficient local strength to provide mounting locations for launch support structures and packaging loads. 
   The shape-memory reflector  110  may also include a shape-memory stiffener  140 . The shape-memory stiffener  140 , for example, may include any shape-memory material described in commonly assigned U.S. patent application Ser. No. 12/033,584, filed 19 Feb. 2008, entitled “Highly Deformable Shape-memory Polymer Core Composite Deformable Sandwich Panel,” which is herein incorporated by reference for all purposes. In one embodiment, the shape-memory stiffener  140  comprises a sandwich including a first face sheet, a shape-memory core and a second face sheet. The first and second face sheets may include laminates or layers of composite material. In one embodiment, the reflector material  120  may comprise the first face sheet. The second face sheet may include the same material as the reflector material and may be coupled therewith. The shape-memory core may comprise shape-memory polymer foam. The shape-memory stiffener  140  may be located on the outer edge of the paraboloid surface as shown or may be located at the reflector material surface at any radius inward to the inside edge of a center hole in the parabolic surface. Multiple circumferential shape-memory stiffeners may be used at different radii. 
     FIG. 3A  shows a cross section of a portion of an exemplary shape-memory stiffener  300  according to one embodiment. This shape-memory stiffener  300  may be fabricated into a continuous band that is attached to the convex surface of the reflector shown in  FIG. 1  according to one embodiment. In another embodiment, the shape-memory stiffener  300  may also be fabricated with a plurality of discrete shape-memory cores  330  or a band that includes discrete pieces of shape-memory core  330  coupled together to form a band. The shape-memory stiffener  300  may include a first face sheet  310 , a second face sheet  320  and a shape-memory core  330 . The first and/or second face sheets  310 ,  320  may comprise the same material or similar material as the reflector material  120 . The shape-memory core  330  may be in substantially continuous contact with both the first face sheet  310  and the second face sheet  320 . That is, the core is not segmented, but instead is in mostly continuous contact with the surface of both face sheets. For example, the shape-memory core  330  may be in continuous contact with about 75%, 80%, 85%, 90%, 95% or 100% of either and/or both the first face sheet  310  and the second face sheet  320 . In some cases, however, the core  330  may comprise a plurality of discrete shape-memory cores coupled together. 
   The first and/or second face sheets  310 ,  320  may comprise a thin metallic material according to one embodiment. In other embodiments, the face sheets may include fiber-reinforced materials. The face sheets may also comprise a composite or metallic material. The face sheets may also be thermally conductive. The shape-memory core  330  may comprise a shape-memory polymer and/or epoxy, for example, a thermoset epoxy. The shape-memory core  330  may also include either a closed or open cell foam core. The shape-memory core  330  may be a polymer foam with a T g  lower than the survival temperature of the material. For example, the shape-memory core may comprise TEMBO® shape-memory polymers, TEMBO® foams or TEMBO® elastic memory composites. 
     FIG. 3B  shows a graph of the shear modulus G, the complex shear modulus G*, and the ratio of the shear modulus to the complex shear modulus G*/G of an exemplary shape-memory material according to one embodiment. The peak in the G*/G curve is defined as the glass transition temperature (T g ) of the shape-memory material. Above T g , glasses and organic polymers become soft and capable of plastic deformation without fracture. Below T g , the joining bonds within the material are either intact, or when cooling increase as the material cools. Thus, below T g , materials often become stiff, brittle and/or strong. 
   The shape-memory stiffener  140  may be a continuous shape-memory sandwich as just described. The shape-memory stiffener  140  may also include a plurality of shape-memory elements coupled together on the surface of the reflector element. The shape-memory stiffener may be a collapsible, yet strong and stiff shape-memory polymer based stiffener. The shape-memory stiffener  140  may have sufficient stiffness and dimensional stability in the deployed state (at temperatures below T g ) so as to maintain the paraboloid shape of the reflective surface. Moreover, the shape-memory stiffener  140  may have sufficient strain and strain energy storage capability at temperatures above T g  to allow packaging the reflector without to damage to the reflective surface. The shape-memory stiffener  140  may also include sufficient stiffness and dimensional stability in the packaged state, at temperatures below T g , so as to maintain the packaged shape of the reflector without extensive launch locks. Also, the shape-memory stiffener  140  may include sufficient dampening during actuation at temperatures above T g  to effectively control un-furling of the reflective surface. 
     FIGS. 4A and 4B  show a top view  410  and a side view  420  of a packaged shape-memory reflector according to one embodiment. The packaged shape-memory reflector is packaged such that the shape-memory stiffener  140  is curved into reversing bends with the peak and valley of each bend occurs near a radial stiffener  130 . The packaging is initiated by bending every other radial stiffener  130  inward, creating a roughly conical shape to the parabolic surface and stabilizing the location of a plurality of pleats in the material. The reflector may be packaged by drawing the outside of the reflector inward, forcing the pleats to become deeper and the shape-memory stiffener  140  to wrap into reversing bends. At the inside edge of the reflector, the radius would also be decreased, resulting in pleats extending from inside edge to outside edge. The shape-memory reflector may be packaged when the shape-memory stiffener(s)  140  is heated to a temperature greater than T g  of the shape-memory core. Once in the packaged configuration, the shape-memory stiffener(s) is cooled to a temperature below T g  allowing the shape-memory stiffener to carry mechanical loads and maintain the shape of the packaged reflector. 
     FIG. 5  shows mechanical linkages  505  attached with radial stiffeners  130  according to one embodiment. As shown in the figure, the mechanical linkage  505  is in the packaged configuration at  510 . Note that the radial stiffener  130  is nearly straight and in the vertical position when packaged. At  525 , the mechanical linkage  505  is in the deployed configuration and the radial stiffener  130  is in a parabolic shape as shown. Positions  515  and  520  show the mechanical linkage  505  and radial stiffener  130  in between the packaged  510  and deployed configurations  525 . Between position  515  and  520 , pressure is applied to the mechanical linkage  505 , which applies the appropriate pressure on the stiffener to reverse the bend in the stiffener. As shown in  FIGS. 6A and 6B , the mechanical linkages  505  may be provided to connect the interior edge of a deployed reflector  120  to a central hub. These mechanical linkages  505  may provide, for example, a rigid attachment between the reflector surface  120  and a spacecraft to which the reflector is mounted. The mechanical linkages  505 , for example, may also aid in controlling the packaging and deployment kinematics of the interior edge of the reflector surface  120  so as to minimize packaging strains and/or avoid non-uniform motion and material failure within the interior region during packaging and deployment. 
   During packaging and/or deployment, these mechanical linkages  505  may provide a fixed point of rotation radially inward from the interior edge of the reflector surface  120  about which the interior region of the reflector rotates during packaging and deployment. The location of this fixed point of rotation is defined far enough inside the edge of the reflector to reduce packaging strains within the reflector  110 . A mechanical linkage  505  may be located at each pleat in the packaged reflector or at some integer subset of pleats, for example, every other pleat, etc. By locating the fixed point of rotation behind the parabolic surface  120 , the inner hole of the reflector  110  can be filled with a solid, stationary parabolic reflective surface. 
     FIGS. 7A and 7B  show a front and top view of a packaged reflector shell within a backing structure  710  according to one embodiment.  FIG. 8  shows a deployed reflector  110  with a backing structure  710 . The backing structure  710  may provide deployed stiffness and/or dimensional accuracy. Moreover, the reflector may be attached to, and supported by, the backing structure  710 . The backing structure may include a number of radial arms that pivot inward for packaging and deployable truss elements to lock the arms into the deployed position. Each deployable arm may be attached to a central hub and/or a single radial position on the reflector. In addition, the inside bend of every pleat in the reflector surface is attached by a mechanical linkage to a pivot point on the same central hub. In the example shown, there are 6 arms attached to the outside edge and 18 pleats with 18 linkages at the inside edge of the reflector. Alternate embodiments could use a different number of pleats and/or a different number of arms. The arms could be attached to the radial elements inside of the outer edge, the reflector could be attached to only the linkages at the inside edge, the linkages could be at fewer than every pleat, or the arms could be at every pleat. Also, the arms could lock-out and provide stiffness and dimensional stability with various mechanical means, for example, rotation stops, linear or rotary springs, detent pins, and flexures. 
     FIG. 9  shows a top view of a packaged shape-memory polymer reflector that includes both large pleats  940  and small pleats  930  according to one embodiment. Packaging the reflector with larger and smaller pleats may allow for a more efficient package in a smaller circular space. For example, this allows the same reflector to be packaged into about a 25% smaller diameter packaging envelope. 
   According to another embodiment, the reflector could be packaged with drastically fewer pleats. For example, the reflector could be packaged with just 2 pleats forming a taco-shaped package. The circumferential stiffener would still serve to retain the stowed condition diagonals between the radial elements could be added since local curvature would be very low. 
     FIG. 10  shows a flowchart of a method for packaging a shape-memory reflector according to one embodiment. At block  1010 , the reflector is fabricated with an initial deployed shape. The reflector may also be fabricated with shape-memory stiffener or stiffeners and a plurality of radial stiffeners. This deployed configuration may provide a minimum strain energy shape for the reflector. At block  1020 , the shape-memory stiffener or stiffener(s) is heated to a temperature above T g  of the shape-memory polymer within the shape-memory stiffener. At block  1030 , mechanical loads are applied to deform reflector into a packaged shape, such as, for example, the packaged shape shown in  FIG. 4B . At block  1040  the shape-memory stiffener(s) is cooled to a temperature below T g  of the shape-memory polymer while the packaged shape is maintained with the applied loads. Following which, at block  1050 , the mechanical loads are removed and the shape-memory stiffener maintains its packaged shape due to strain energy storage in the cooled shape-memory polymer core. The reflector will remain in its packaged condition with minimal or no external loads until deployment. The pleats are stabilized for launch loading by bending stiffness of packaged circumferential stiffener. In some applications, launch restraint mechanisms may be applied at block  1060 . 
     FIG. 11  shows a flowchart of a method for deploying a shape-memory reflector according to one embodiment. At block  1110 , launch restraints, if any, are released. The shape-memory stiffener(s) is heated to a temperature above T g  of the shape-memory polymer within the shape-memory stiffeners. During this heating, the shape-memory stiffener(s) straightens out of reversing bends, allowing the reflector to return to its initial paraboloid shape with minimal or no external mechanical loads at block  1130 . At block  1140 , the shape-memory stiffener is cooled to a temperature below T g  of the shape-memory polymer. The initial stiffness and strength of the shape-memory polymer is restored upon cooling. 
     FIGS. 12A and 12B  show a packaged and deployed shape-memory reflector coupled with a non-deployable, centrally located reflector  1215 , which is, in turn, mounted on of a satellite according to another embodiment. The packaged reflector  1205  as shown in this example, is mounted on a satellite payload module  1210 . 
   As shown in  FIGS. 13A and 13B , the packaged shape-memory reflector  1205  may be stowed in a small volume  1300 , for example, for space applications. A reflector  1205  may be stowed during launch and deployed in space. A top view and side view of a packaged shape-memory reflector  1205  within it&#39;s clearance to launch envelope  1300  is shown in  FIGS. 13A and 13B  according to another embodiment. In this embodiment, the deployable shape-memory reflector  1205  could be designed such that, in its packaged state, it does not substantially shadow the non-deployable central reflector, which would enable this reflector to be used prior to deployment of the packaged shape-memory reflector. 
     FIG. 14  shows a cut away view of a shape-memory stiffener  140  coupled with an elastic reflector material  120  according to one embodiment. The shape-memory stiffener  140  is enclosed within a protective covering  1410 , such as, for example, multi-layer insulation (MLI). The MLI may be coupled with the elastic reflector material  120  using any of various adhesives  1420 . Note that, in this embodiment, the shape-memory stiffener  140  is coupled with the elastic reflector material  120 . Indeed, the elastic reflector material  120  comprises one of the face sheets of the shape-memory stiffener  140 . 
   In some embodiments, more than one shape-memory stiffener may be used as shown in  FIGS. 15 and 16 .  FIG. 15  shows a three-dimensional and bottom view of a shape-memory reflector  110  with two shape-memory stiffeners  140  according to another embodiment.  FIG. 16  shows a back view of a shape-memory reflector  110  with two circumferential shape-memory stiffeners  140  according to another embodiment. Moreover, multiple shape-memory stiffeners may be used. 
   Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, circuits, structures, and/or components may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, components, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. 
   Also, it is noted that the embodiments may be described as a process, which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be rearranged. A process is terminated when its operations are completed, but could have additional steps not included in the figure. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function. 
   While the principles of the disclosure have been described above in connection with specific apparatuses and methods, this description is made only by way of example and not as limitation on the scope of the disclosure.