Abstract:
Methods and apparatus are provided for a tubular composite structure with particular application to the design and assembly of space vehicles. The apparatus, designated herein as an X-strut, comprises alternately crossing tow-placed layers of parallel strands of a composite material, to form top and bottom intersecting faces. Alternately crossing strand layers are built up from the bottom faces to form the side walls of the tubular composite structure. The top and bottom faces and the side walls are configured to form intersecting tubular members in an X-shape. The layers of strands run continuously through the intersecting tubular members to form an integral joint, without the need for adhesive bonding or mechanical fasteners.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to the design and manufacture of spaceframe structures, and more particularly relates to the design and manufacture of upper stage intertank structures for space vehicles. 
     BACKGROUND OF THE INVENTION 
     Prior art designs for certain structural components of space vehicles, such as the upper stage intertank structure of the Boeing Delta III and Delta IV launch vehicles, have been configured with X-shaped structural elements, referred to hereinafter as X-panels. One such X-panel  100  is shown in FIG.  1 . This type of X-panel has been fabricated from either aluminum or a composite material, but studies have indicated that a composite material design allows meeting structural performance requirements with less weight and lower costs, in comparison to aluminum. 
     An existing X-panel  100  generally incorporates two full-length adhesively bonded joints between two molded panel halves, plus numerous mechanical fasteners  102 . As such, the X-panel design is labor-intensive, and relatively costly to manufacture. Moreover, the load path eccentricity  101  at the intersection of the two legs of the “X” (X-panel members  103 ,  104 ) typically requires local reinforcement with an associated weight penalty. In addition, the cross-section of the X-panel design, as shown in FIG. 2, frequently includes a central web  105 , which also adds unwanted weight to the structure. 
     Accordingly, it is desirable to modify the design and fabrication of the aforementioned X-shaped structural element (X-panel  100 ) to reduce the associated labor and manufacturing costs, and to increase the structural efficiency of the intersection joint  101 . In addition, it is desirable to modify the design of the X-panel  100  to reduce its weight contribution to the upper stage intertank, or any other structural component, of a space vehicle assembly. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
     BRIEF SUMMARY OF THE INVENTION 
     An apparatus is provided for a tubular composite structure, which requires no adhesively bonded or mechanically fastened joints. The apparatus comprises intersecting tubular struts, each strut having a top face connected to a bottom face by side walls. The tows (bundles of reinforcing fibers) comprising the intersecting side walls of the struts are interwoven with each other to form a seamless integral joint between the struts. This technique can be used to form intersecting composite tubes of any cross sectional shape, and is not restricted to the rectangular cross sections shown in the accompanying drawings. 
     A method is provided for fabricating the tubular composite structure. The method comprises the following steps: 
     a) layering parallel strands of a composite material in an alternately crossing orientation to form intersecting bottom faces of the tubular composite structure; 
     b) building up from the intersecting bottom faces, strand layers of the composite material in an alternately crossing orientation to form intersecting side walls of the tubular composite structure; 
     c) layering parallel strands of the composite material in an alternately crossing orientation to form intersecting top faces of the tubular composite structure, the intersecting top faces corresponding to the configuration of the intersecting bottom faces and the intersecting side walls; 
     d) oven curing the composite material, using a combination of external and internal tooling to control the tube configuration during cross-linking of the composite matrix material; 
     wherein the layers of parallel strands forming intersecting bottom and intersecting top faces, and the layers of strands forming intersecting side walls, run continuously through the tubular composite structure to form a seamless integral intersection joint. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
     FIG. 1 depicts a prior art X-panel; 
     FIG. 2 illustrates the cross-section of a prior art X-panel; 
     FIG. 3 is a depiction of an exemplary X-strut; 
     FIG. 4 shows an exemplary tow-placement process; 
     FIG. 5 is a sketch of crossing tows; 
     FIG. 6 is a cut-away of an X-strut intersection; 
     FIG. 7 is a perspective view of an exemplary prototype composite strut; 
     FIG. 8 shows a comparison between an exemplary X-panel cross-section and an exemplary X-strut cross-section; and 
     FIG. 9 illustrates an exemplary embodiment of an intertank structure incorporating X-struts. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
     In order to achieve the desired objectives of decreased manufacturing costs and reduced weight of the prior art X-shaped structural element (X-panel  100 ), as well as reducing weight by increasing the structural efficiency of the intersection joint  101 , a novel design approach has been applied to the fabrication technique of this element. For clarity, the inventive X-shaped structural element will hereinafter be referred to as an X-strut, to distinguish it from the prior art X-panel. 
     An exemplary embodiment of an X-strut  200  is shown in FIG.  3 . The strut cross-section is in the form of a rectangular tube near the center  201 , with flattened ends  202  to provide for single lap joints. Each leg is bifurcated to leave an opening  204  for withdrawing a tooling bladder (not shown). Holes  205 , for shear joints between X-struts and intertank skirts (not shown), are drilled on assembly for maximum load carrying capability. 
     A process used for fabricating an exemplary X-strut  200  is based on the conventional “tow-placement” technique, which involves laying down strands of a composite material with a tow placement machine, such as the Viper Fiber Placement Systems, made by Cincinnati Machine. These strands (tows) are typically about ⅛ inch wide, and approximately 0.0055 to 0.011 inch thick. The tows can be reformed to different shapes and proportions during cure by the pressures applied by supporting tooling. 
     In an exemplary embodiment, as depicted in FIG. 4, parallel strands  301  of a composite material are tow-placed to form layers, in a crossing orientation, which become bottom tube faces  301 ′. Crossing strands  302  of the composite material are then alternately layered at the crossing intersections  303  to form intersecting tube sidewalls  302 ′. Finally, alternating layers of parallel strands are tow-placed on supporting tooling (not shown), to form the top tube faces  304 . 
     The tow-placed bottom and top faces of parallel strand layers  301 ′,  304  and the tow-placed strand layered side walls  302 ′ are formed into intersecting tubes, with continuous fibers running through the intersection to form an integral joint  300 . Importantly, this technique provides continuous load paths through the intersection  300  (equivalent to intersection  201  of X-strut  200 ), and eliminates the need for a bonded or mechanically fastened joint, as in the prior art X-panel  100 . As such, the integral node X-strut intersection  201 / 300  has straighter load paths, allowing the applied load to be carried with less material than is required with the indirect load paths in the connected X-panel  100  joint  101 . 
     When the X-strut  200  configuration is complete, the composite material is cured, using rigid external tooling in combination with internal tooling. In accordance with an exemplary embodiment, the external tooling comprises a two-part mold, which encloses the uncured X-strut and the internal tooling. The entire assembly is placed into an oven for the curing process. The internal tooling may be a removable type, such as an inflatable bladder, or may be a washout mandrel of a variety of materials. Additional internal tooling may be light enough to be left in place; e.g., tooling foam. 
     For further clarity, a more detailed sketch of crossing tows  302  is shown in FIG. 5, and a cut-away of the continuous load path intersection is shown in FIG. 6, with the side walls  302 ′ intersecting at the center  201 / 300 . The tooling for this process is specially designed to accommodate the tow spread at the crossing point  201 / 300 , due to compaction. FIG. 7 is a perspective view of a prototype unitized composite strut, fabricated in accordance with the process described above. 
     An exemplary composite material for the X-strut  200  design is uniaxial IM7/977-2 graphite/epoxy prepreg tow. In one embodiment, this material is an intermediate modulus IM7 fiber in a highly oriented, 80% axial fiber layup in the X-strut  200 , as contrasted to the typical hand layup of woven fabric, using G30-500 lower modulus fibers, in a quasi-isotropic layup in the prior art X-panel  100 . Illustratively, an X-strut  200  design uses one ply of cloth in a +45 degree orientation, as the inner and outer ply of the layup. The remainder of the wall is tow-placed, with all tow-placed fibers being axial. The percentage of off-axis fibers can be controlled by appropriate selection of the fabric thickness. As an example, using readily available 0.011 in. thick fabric, the axial fiber volume fraction for an X-strut  200  cross-section might be approximately 75%. 
     The aforementioned material choices for the X-strut  200  design provide significant performance improvements over the prior art X-panel  100  design. Comparative analysis indicates that the X-strut  200  design increases axial specific stiffness by an approximate ratio of 1.9 to 1, and also increases the axial specific tensile strength by an approximate ratio of 3.5 to 1. 
     In an exemplary embodiment, as shown in FIG. 8, the X-strut  200  cross-section is rectangular in shape, with approximate dimensions of A=4 to 5 inches, and B=4 to 4½ inches. The X-strut  200  design eliminates the central web  105  of the X-panel  100  design (see FIGS.  2  and  8 ), thus achieving a desired reduction in weight. Moreover, the X-strut  200  design can be implemented with automated material placement, and with no structural joints to assemble, as compared to the labor intensive hand layup, with two full length bonded joints, plus  36  anti-peel mechanical fasteners, of the X-panel  100  design. As such, the X-strut  200  design enables a significant reduction in manufacturing costs. 
     FIG. 9 shows one implementation of an intertank structure  400 , which incorporates the inventive X-strut  200  elements between a forward skirt  401  and an aft skirt  402 . Also shown in FIG. 9 is an exemplary mounting arrangement of helium bottles  403 , which are mounted to the X-struts  200  aft of the strut intersection. While this embodiment includes twelve X-struts, each having a single intersection  201 , alternate embodiments may vary the number of X-struts, as well as extending the concept to tubular spaceframe elements with multiple intersections. 
     To summarize, the inventive X-strut  200  design described herein achieves the desired objectives of weight reduction and decreased manufacturing costs. In fact, current estimates indicate the X-strut  200  design to be on the order of 26% lighter and 46% less expensive than a corresponding X-panel  100  design. Moreover, the X-strut  200  design permits the assembly of intersecting beam columns without a discrete joint. Using the continuous fiber technique described above, the structural elements “flow” into each other, and no bonds or fasteners are required. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.