Tensioned cord/tie attachment of antenna reflector to inflatable radial truss support structure

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.

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 '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 '451
 application, this inflation dependency problem is obviated by means of a
 hybrid antenna architecture, that effectively isolates the geometry of the
 antenna'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 '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.

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'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 '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'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 '451 application, it is
 preferred that the antenna'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'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 '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.