Abstract:
A collapsible conductive material includes a generally mesh-configured, collapsible surface, that defines the intended reflective geometry of an antenna. A distribution of tensionable cords and ties form radial truss elements with a plurality of inflatable radially extending ribs and posts of a support structure. The antenna is fully deployed once the support structure is inflated to at least a minimum pressure necessary to place the ties and cords in tension so that the reflective surface acquires a prescribed (e.g., parabolic) geometry, which is stably maintained by the radial truss elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation-in-part of U.S. patent application, Ser. No. 08/885,451, filed Jun. 30, 1997, by B. Allen, entitled: “Tensioned Cord Attachment of Antenna Reflector to Inflated Support Structure” (hereinafter referred to as the &#39;451 application), now U.S. Pat. No. 5,920,294, issued Jul. 6, 1999, assigned to the assignee of the present application and the disclosure of which is incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to energy directing structures and assemblies, such as antenna reflector architectures, and is particularly directed to a new and improved support configuration for an energy directing surface, such as an RF reflective mesh, having an arrangement of ties and cords that are attached to and placed in tension by an inflated radial, truss-configured support structure, that facilitates compact stowage and stabilized deployment, and is therefore especially suited for spaceborne applications. 
     BACKGROUND OF THE INVENTION 
     As described in the above-referenced &#39;451 application, among the various conventional antenna assemblies that have been proposed for airborne and spaceborne applications are those which employ an inflatable medium, that may be unfurled from its stowed configuration to realize a ‘stressed skin’ type of reflective surface. In such configurations, non-limiting examples of which are described in U.S. Pat. Nos. 4,364,053 and 4,755,819, the inflatable structure serves as the reflective surface of the antenna; namely, once fully inflated, the material is intended to assume and retain the desired antenna geometry. 
     Unfortunately, using the inflatable structure per se as the antenna surface creates several problems. First, the accuracy of the geometry of the antenna depends upon how faithfully the shape of the inflatable medium matches the antenna geometry, and also how well the shape of the inflatable medium can be maintained. Should there be (and there can expected to be) a change in the shape of the inflatable membrane, such as due to a change (most notably a decrease) in inflation pressure over time, the corresponding change in the contour of the inflatable structure will necessarily change the intended antenna profile, thereby impairing the energy gathering and focussing properties of the antenna. Although this inflation pressure decrease problem can ostensibly be addressed by the use of an auxiliary supply of inflation gas, it does not circumvent other causes of inflatable membrane distortion, such as, but not limited to, temperature and aging of the material, and particularly the fundamental ability of the inflated membrane to accurately produce the geometry of the antenna reflector. 
     In accordance with the invention described in the above-referenced &#39;451 application, this inflation dependency problem is obviated by means of a hybrid antenna architecture, that effectively isolates the geometry of the antenna&#39;s reflective surface from the contour of the inflatable support structure, while still using its support functionality to deploy the antenna. For this purpose, rather than make the reflective surface geometry of the antenna depend upon the ability to maintain a prescribed pressure, the inflated membrane is employed simply as a deployable ‘tensioning’ attachment surface. The inflatable tensioning membrane may support the tensioning tie/cord arrangement and the adjoining antenna surface either interiorly or exteriorly of the inflatable membrane. 
     FIG. 1 (which, except for the reference numerals corresponds to FIG. 2 of the &#39;451 application) is a cross-sectional view of an exterior support embodiment of this hybrid antenna architecture. The hybrid structure of FIG. 1 is taken through a plane that contains an axis of rotation AX. A generally parabolic reflective surface  10  of the antenna is made of a lightweight, reflective or electrically conductive and material, such as, but not limited to, gold-plated molybdenum wire or woven graphite fiber. This surface is also rotationally symmetric about the axis AX, passing though an antenna feed horn  12 . 
     The reflective surface  10  is attached by a tensioned cord and tie arrangement  20  to the exterior surface  31  of a generally toroidal or hoop-shaped inflatable support structure  30 , which is also rotationally symmetric about the axis AX. The inflatable support structure  30  for the tie and cord arrangement  20  is joined to a support base  40  (e.g., a spacecraft) by way of a rigid truss attachment structure  50 , that is formed of plurality of relatively stiff stabilizer struts or rods  51 , also rotationally symmetric about the axis AX. 
     The inflatable hoop  30  may comprise an inflatable laminate of multiple layers of sturdy flexible material, such as Mylar. For deployment, the hoop  30  may be inflated through a valve  32 , which may be located at or adjacent to its attachment to the truss  50 , or the hoop may contain a material that readily sublimes into a pressurizing gas, that fills the interior volume  33  of the hoop  30 . 
     The mesh reflector surface  10  is attached to the inflatable support structure  30  by means of tensionable ties  21  and cords  22  at perimeter attachment points  25 ,  27 , distributed around the exterior surface  31  of the inflated membrane  30 . This distribution of ties and cords is rotationally symmetric around the axis AX and is preferably made of a lightweight, thermally stable material, having a low coefficient of thermal expansion, such as woven graphite fiber. The hoop  30  is preferably inflated to a pressure greater than necessary to place the attachment cord and tie arrangement  20  at a minimum tension at which the reflective surface  10  acquires its intended shape. 
     This hybrid support structure enables the antenna surface to be maintained in a prescribed geometrical shape, that is independent of variations in the inflation pressure and shape of the hoop. Namely, the antenna is deployed and its geometry fully defined once the inflatable hoop is inflated to at least the extent necessary to place the attachment ties and cords at their prescribed tensions. Preferably, the inflation pressure is above a minimum value that will accommodate pressure variations (drops) that do not allow the hoop to deform to such a degree that would relax or deform the antenna from its intended geometry. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the configuration of the inflatable tensioning structure for supporting the tensioning tie/cord arrangement and the adjoining antenna surface exteriorly thereof is that of an inflated arrangement of radially extending ribs and posts, that form radial truss elements with components of the tie/cord arrangement. These ribs and posts are readily collapsible to a compact configuration, to facilitate stowage and deployment, particularly for spaceborne applications. The inflatable rib structure contains a plurality of generally segment-wise curvilinear ribs that extend radially from an antenna boom through which a boresight axis of rotation passes, and to which an antenna feed horn is affixed. 
     For enhanced stability and rigidity, either or both of the radially extending curvilinear rib segments and the posts may be embedded with or affixed to stiffening elements, such as graphite rods or the like, oriented parallel to the intended directions of deployment. Distal ends of the rib segments and distal and base ends of the posts are connected to a truss-forming arrangement of collapsible cords, and circumferential cord segments. These cords placed in tension by inflation of the ribs and act to stabilize the intended support geometry of the radial rib structure. 
     A reflective mesh surface is attached to the distal ends of the radial rib segments by a collapsible arrangement of tensionable ties and a set of radially extending backing cords. The backing cords are connected by tensioning ties to a plurality of attachment points distributed along the radial rib segments. Since each of the reflective mesh and its attachment ties and cords are collapsible, the entire antenna reflective surface and its associated tensioned attachment structure can be readily furled together with the inflatable radial structure in their non-deployed, stowed state. Each of these respective components of the support structure and the reflective surface readily unfurls into a predetermined geometry, highly stable reflector structure, once the ribs and posts of the radial support structure are fully inflated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic cross-sectional illustration of an architecture of the invention described in the above-referenced &#39;451 application; 
     FIG. 2 is a diagrammatic side view of an inflated radial, truss-configured antenna support structure of the present invention; 
     FIG. 3 is a diagrammatic perspective front view of the inflated radial, truss-configured antenna support structure of FIG. 2; and 
     FIG. 4 is a diagrammatic perspective rear view of the inflated radial, truss-configured antenna support structure of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     Attention is now directed to FIG. 2, which is a diagrammatic side view of an inflated radial, truss-configured antenna support structure of the present invention, taken through a plane containing a (boresight) axis of rotation  101 . Axis  101  passes though a generally cylindrical boom  103 , to which an antenna feed horn  104  is affixed. A collapsible, generally parabolic, energy reflective surface  110  is supported by an associated radially, extending inflatable radial rib structure  120 , that is rotationally symmetric about the axis  101 . 
     For purposes of providing a non-limiting illustrative example, the reflective antenna surface  110  may comprise a relatively lightweight mesh, gold-plate molybdenum wire mesh, that readily reflects electromagnetic or solar energy. It may also comprise other materials, such as one that it is highly thermally stable, for example, woven graphite fiber. The strands of the reflective mesh of the reflector surface  110  have a weave tow and pitch that are selected in accordance with the physical parameters of the antenna&#39;s intended deployment. It should also be noted that the reflective surface may be used to reflect other forms of energy, such as, but not limited to, acoustic waves. 
     The inflatable medium of the radially, extending rib structure  120  may comprise a laminate of multiple layers of a sturdy material, that is effectively transparent to energy in the spectrum of interest. For electromagnetic and solar energy applications, a material such as Mylar may be used. Each of the ribs may be configured of a plurality of rib segments  121  that extend radially in a generally segment-wise curvilinear from a base  122  through which axis  101  passes. 
     Projecting generally orthogonally from a plurality of radially spaced apart locations  123  along each rib segment  121  are respective posts  124 . Posts  124  are integrated as part of the radial ribs and are therefore inflated during the inflation of the ribs. This radial rib and post configuration readily allows the rib segments and posts to collapse radially (in an accordion fashion), or they may be folded. When not inflated, the rib structure  120  may be stowed radially around the boom  103 . 
     For enhanced stability and rigidity, the membrane material of either or both of the radially extending curvilinear rib segments  121  and the posts  124  thereof may be embedded with or affixed to lightweight stiffening elements, such as graphite rods or the like, that are oriented parallel to the intended directions of deployment, as shown at  125  and  126 . Distal ends  127  of the rib segments  121 , and respective distal and base ends  128  and  129  of the posts  124  are connected with a truss-forming arrangement of collapsible cords  130 , and circumferential cord segments  132 , that are placed in tension by and are operative to stabilize the intended support geometry of the radial rib structure  120  upon its inflation. 
     The rib structure  120  may be inflated by way of an fluid inflation port  140  installed at or in the vicinity of the axis  101 . Also, a pressure regulator valve coupled with an auxiliary supply of inflation gas may be coupled to port  140  for maintaining the pressure and thereby the desired ‘stiffness’ of the inflatable rib structure. Alternatively, the ribs may contain a material (such as mercuric oxide powder, as a non-limiting example) that readily sublimes into a pressurizing gas, filling the interior volume of the truss, thereby causing it to expand from an initially compactly furled or collapsed (stowed) state to the fully deployed state shown in FIGS. 2-4. 
     Like the inflatable support structures described in the &#39;451 application, the inflatable radial rib and truss antenna architecture of the present invention effectively isolates the geometry of the reflective surface  110  of the antenna from the contour of the inflatable support structure  120 , while still using the support functionality of the inflatable truss to deploy the antenna&#39;s reflective surface  110  to its intended (e.g., parabolic) geometry. 
     For this purpose, the reflective mesh surface  110  is attached to the distal ends  127  of the radial rib segments  121  by a collapsible arrangement  150  of tensionable ties  151 , and to a set of radially extending backing cords  152 . The backing cords  152  are connected by tensioning ties  153  to a plurality of attachment points  154  distributed along the rib segments  121 . Like the other components of the support structure of the invention, these tensionable ties and cords are also preferably made of a lightweight, thermally stable material, such as woven graphite fiber. 
     With each of the reflective (mesh) structure  110  and its associated attachment ties and cords  150  being collapsible, the entire antenna reflective surface and its associated tensioned attachment structure can be readily furled together with the inflatable radial structure  120  in their non-deployed, stowed state. Each of these respective components of the support structure and the reflective surface readily unfurls into a predetermined geometry, highly stable reflector structure, once the ribs and posts of the radial support structure are fully inflated. 
     As in the inflatable structure described in the &#39;451 application, it is preferred that the antenna&#39;s radial support structure  120  be inflated to a pressure that is greater than necessary to place the cord and tie arrangement  150  in tension and cause the reflector structure (mesh)  110  to acquire its intended geometry. Such an elevated pressure will not only maintain the support membrane  120  inflated, but will accommodate pressure variations (drops) therein, that do not permit the inflated support membrane to deform to such a degree as to relax the tension in the reflector&#39;s attachment ties and cords, so that the reflective surface  110  will retain its intended deployed shape. 
     As will be appreciated from the foregoing description, the above discussed geometry dependency shortcoming of conventional inflated antenna structures is effectively remedied by the radially configured hybrid antenna architecture of the present invention, which like the inflatable support structure of the &#39;451 application, essentially isolates the reflective surface of the antenna from the contour of the inflatable support structure, while still using the support functionality of the inflatable truss to deploy the antenna and stably maintain its reflective surface in an intended energy directing geometry. 
     While we have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as are known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.