Abstract:
An inflatable structure is augmented with transverse frames and bracing cables to make a truss-like structure. This feature is adaptable for adding strength to a plain inflatable structure and to an inflatable structure forming a structural arch. It can also be incorporated into an inflatable wing.

Description:
BACKGROUND OF THE INVENTION 
     Inflatable structures, sold by applicant under the trademark AIRBEAM are characterized by low mass, low stowed volume for on-site deployment, overload tolerance and tailored strength and stiffness. Current applications use multiple deploy-strike cycles with inflation pressure maintained while in use. 
     The known inflatable structures are limited in size and load carrying by both manufacturing limitations and by material properties. This invention overcomes size limitations and improves strength and stiffness of very large inflatable structures. 
     The known inflatable structures are described in U.S. Pat. Nos. 5,421,128 and 5,735,083. A high bias angle that elongates under pressure provides high bending strength in these structures. This invention, having added external tension elements, provides an increased moment of inertia for even greater strength and stiffness for a given inflatable structure. This invention is applicable to, but not limited to structures for shelters, bridges, deployable wings, and space structures. 
     SUMMARY OF THE INVENTION 
     This invention uses external bracing tensioned by inflatable structures. The external tensile members are made of high modulus fibers and are spaced away from the central inflatable structure by transverse frames. The structure can be made rigid after deployment by unidirectional bundles of fibers to maximize compression performance after deployment. A truss can be made up of a central inflatable structure or member that is strengthened with external braces made of high modulus fibers spaced away from the central member by transverse frames. A structural member arch can be strengthened using a cable below the member and parallel to it at some distance with spoke-like linear attachments holding the member shape under loads that would tend to collapse the arch. A deployable wing with an inflatable member spar that also relies on span-wise tension in the skin of the wing for maintenance of shape, would operate under the same principle as the other externally braced inflatable structures of this invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an inflatable structure with three external tension cables. 
         FIG. 2  shows a cross section of the inflatable structure of  FIG. 1 . 
         FIG. 3  shows an arch with an inside strengthening cable. 
         FIG. 4  shows a cross-section of  FIG. 3 . 
         FIG. 5  shows an inflated wing. 
         FIG. 6  shows the inflatable structure with diagonal cables. 
     
    
    
     DETAILED DESCRIPTION 
     A truss-like structure is illustrated in  FIG. 1  and in  FIG. 2 , a cross section. The inflatable beam or member  1  comprises a bladder  4 , a braided restraint layer  5  and axial reinforcement straps  6 . The bladder  4  holds inflation gas, but has no structural function. The braided restraint layer  5  retains the gas pressure and provides shear and torsion resistance. The axial reinforcement straps  6  govern the inflatable structure&#39;s bending strength and stiffness. Transverse frames  2  restrain and align the bracing cables  3  at a distance from, and parallel to the central inflatable structure  1 . The end transverse frames  2 A provide tension to the bracing cables  3  at a distance from and parallel to the central inflatable structure  1 . The end transverse frames  2 A provide tension to the bracing cables  3  at a distance from and parallel to the central inflatable structure  1 . 
     The end transverse frames  2 A provide tension to the bracing cables  3  by the action of the central inflatable structure  1  tending to elongate when pressured. The axial reinforcement straps  6  are also tensioned by this action. A designer, by choosing materials with a particular elastic modulus, and by determining the amount of weight per unit length of each material, determines how much tension is carried in the bracing cables  3  compared to the tension carried in the axial reinforcement straps  6 , and, thus, tailors the structural properties of the truss-like externally braced structure. 
     Variations of this embodiment include trusses and beams, similar structures with more than three external cables and optional diagonal cables between transverse frames to increase shear and torsion stiffness and strength. 
     The various flexible elements of the truss example may be infused with a resin that is controllably hardened to create a permanently rigid structure that does not depend on the maintaining of the inflation pressure. This may be advantageous for very large structures for use in space that can be initially stowed in a small volume for launch. 
     An arched beam structure is illustrated in  FIGS. 3 and 4 . The inflatable component  7  is an inflatable beam comprising a gas-impermeable bladder  10 , a braided restraint layer  11  and one axial reinforcement strap  12 . The bladder  10  retains inflation gas, but has no structural function. The braided restraint layer  11  lends the structure the capability to retain high pressure, provides shear and torsion resistance, and can be curved during the manufacturing process without wrinkling. Transverse frames  9  restrain and align the bracing cable  8  at a distance from the central inflatable component  7 . Pivots  13  can be provided as part of the transverse frames  9  to reduce the size of the transverse frames  9  when the arched beam structure is deflated and folded for storage. 
     Inflating the inflatable component causes the axial reinforcement strap  12  and the bracing cable  8  to be tensioned. Tension is provided to the axial reinforcement strap  12  and to the bracing cable  8  by the action of the central inflatable structure  7  that elongates and straightens when pressurized. Such action, which the designer controls by choice of the various materials, material weight per unit length, inflatable component  7  diameter, and the offset distance of the bracing cable  8  from the inflatable component  7 , determines the strength and stiffness of the arched beam. 
     Compared to an un-braced inflatable structure, the arched beam of  FIG. 3  will have increased strength for downward loads, and little or no advantage for upward loads. Therefore, it would be beneficial for supporting structures subject to high snow loads, or for buried shelters as may be needed for lunar habitation. 
     Variations of the arched beam of  FIG. 3  include designs with multiple axial reinforcement straps  12  and/or multiple bracing cables for increasing strength in the direction perpendicular to the plane of the arch. 
     In  FIG. 6  the structure of  FIG. 1  ( 20 ) is reinforced with diagonal cables  21 . Such diagonal cables enhance the structure when the shear stiffness of the inflated member is not sufficient. 
     Another example of an externally braced inflatable structure is the membrane wing shown in  FIG. 5 . The inflatable spar  14  comprises a gas-impermeable bladder, a braided restraint layer, and axial reinforcement straps  15  previously described. The wing skin membrane  18  encloses the spar  14  and ribs  17  and provides the aerodynamic surface of the wing. The membrane  18  is attached to the tip rib  16  such that the action of the inflatable spar tending to elongate when pressurized creates tension in the membrane. A chord  19 , forming the trailing edge of the wing, is also tensioned by said action of the inflatable spar  14 , “span-wise”, which is necessary for controlling the aerodynamic shape of the membrane  18  between  16  and  17 . 
     In the wing example, the benefit of external bracing is not improved structural performance; it is the ability to control the distribution of tension into the wing skin membrane  18  for an aerodynamic benefit. 
     Variations of the inflatable wing example include additional inflatable elements to further improve membrane shape, the addition of cords or fibers to the membrane in order to tailor its modulus, and ribs that bend or have pivoting means in order to fold the wing flat for storage.