Patent Publication Number: US-2004056151-A1

Title: High temperature resistant airfoil apparatus for a hypersonic space vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application is a divisional of U.S. patent application Ser. No. 09/703,947 filed on Nov. 1, 2000, presently allowed. The disclosure of which is incorporated herein by reference. 
    
    
     
       TECHNICAL FIELD  
       [0002] This invention relates to control surfaces for aerospacecraft, and more particularly to a ruddervator for an aerospacecraft incorporating a single piece, temperature resistant ceramic matrix composite shell secured over a composite structural member, wherein the structural member is adapted to be secured to a control element of the aerospacecraft.  
       BACKGROUND OF THE INVENTION  
       [0003] Current control surfaces for advanced aerospacecraft are formed by a carbon-based ceramic matrix composite (CMC) hot structure with conventional rib-stiffened structure and a mechanically fastened skin. The X-37 aerospacecraft presently in use uses a control surface termed a “ruddervator” with the above-described construction, and makes use of carbon/silicon carbide (C/SiC). This construction is shown in FIG. 1. The mechanically fastened upper skin  10  is secured by a high temperature metal, ceramic or ceramic composite fasteners at locations  12  to an integral C/SiC lower skin and substructure  14 . A C/SiC tail tip  16  is used to close the end of the ruddervator. A titanium spindle  16  is used to rotate the ruddervator as needed. Thermal protection system seals  18 ,  20  and ring  22  are used to help mount the ruddervator to the fuselage of the aerospacecraft.  
       [0004] The X-37 ruddervator approach described above uses an expensive 2800° F. CMC system in a 2400° F. “hot structure” application and uses an aircraft-like structural approach at the elevated temperature. The term “hot structure” refers to the temperature of the primary load-carrying structure, in this case the CMC and supports used at 2400° F. This construction reduces the service life of the fasteners. Additionally, carbon-based CMCs generally require complex and costly tooling, unique and expensive infiltration/furnace facilities, and fabrication cycles of six months or more. The use of new materials under development, such as oxide fibers/oxide-matrix based CMC (oxide-CMC), provide opportunities to design control surfaces in novel and more cost-effective ways including, but not limited to, maintaining internal supports and attachments below 600° F.  
       [0005] For present and planned reusable hypersonic vehicles there are also size constraints on control surfaces due to available volume which restrict the use of conventional, lower cost structure insulated with bonded tile thermal protection. The current solution is to use the CMC for control surface hot structure in areas which do not require their extreme high temperature properties. The result is high initial and recurring costs for these parts as well as weight penalties and high part counts. Without an order of magnitude reduction in thermal structure costs, commercial reusable access to space will be difficult, if not impossible, to achieve.  
       [0006] It is therefore a principal object of the present invention to provide a new construction for a ruddervator for an aerospacecraft which can be produced more inexpensively from a simpler fabrication process, and which has improved life and reliability over the conventional mechanically fastened upper skin-to-substructure approach presently in use for ruddervator applications.  
       [0007] It is another object of the present invention to provide a hybrid control surface for an aerospacecraft which can be manufactured more economically, which is simpler to repair, and which does not make use of typical mechanical fasteners to secure an upper skin to a substructure.  
       [0008] It is still another object of the present invention to provide a ruddervator for an aerospacecraft having a simplified design which requires significantly fewer independent component parts being needed in the construction of the ruddervator.  
       [0009] It is a further object of the present invention to provide a ruddervator for an aerospacecraft which can be constructed even more cost effectively, and which is reusable.  
       [0010] It is still another object of the present invention to provide a ruddervator for an aerospacecraft wherein the ruddervator employs a one piece, highly temperature resistant outer shell which is bonded to a composite structural member to provide a highly temperature resistant, lightweight and yet easy to manufacture assembly.  
       SUMMARY OF THE INVENTION  
       [0011] The above and other objects are provided by an airfoil in accordance with preferred embodiments of the present invention. The airfoil is specifically adapted to withstand the high temperatures encountered during hypersonic flight and is particularly suited for use as a ruddervator on an aerospacecraft.  
       [0012] The airfoil is comprised of a temperature resistant, ceramic matrix composite shell having an opening at one end and a hollowed out interior area. A structural member is inserted into the hollowed out interior area and bonded to an interior surface of the shell to form a structurally rigid airfoil assembly. A transition structure is secured to the structural member for interfacing the airfoil assembly to a control element of the space vehicle to permit the airfoil assembly to be controlled by the control element.  
       [0013] In one preferred embodiment the shell is comprised of a one-piece, oxide/oxide-based ceramic matrix composite (Oxide-CMC) shell. The structural member comprises a graphite composite structure having a graphite composite face sheet and a honeycomb core element.  
       [0014] During manufacture, the structural member is inserted into the hollowed out opening of the Oxide-CMC shell and is bonded thereto. The transition structure may be secured to the structural member either after the structural member is inserted into the shell or before attachment of the structural member to the shell. Finally, the transition structure is secured to a fuselage actuator spindle of the aerospacecraft.  
       [0015] The airfoil apparatus of the present invention thus forms a highly temperature resistant, easy to manufacture assembly. The assembly further reduces the cost and weight over present day ruddervator designs as a result of reducing the total number of parts required to form the ruddervator, in addition to providing a higher specific strength and stiffness of the materials used with the present invention. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016] The various advantages of the present invention will become apparent to one skilled in the art by reading the following specification and subjoined claims and by referencing the following drawings in which:  
     [0017]FIG. 1 is an exploded perspective view of a prior art ruddervator;  
     [0018]FIG. 2 is a side view of a ruddervator of an aerospacecraft in accordance with a preferred embodiment of the present invention;  
     [0019]FIG. 3 is a cross sectional end view of the ruddervator of FIG. 2 taken in accordance with section line  3 - 3  in FIG. 2; and  
     [0020]FIG. 4 is an enlarged view of a portion of the ruddervator of FIG. 3 corresponding to circled area  4  in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0021] Referring to FIG. 2, there is shown a ruddervator  100  for a hypersonic vehicle such as an aerospacecraft. The ruddervator  100  is secured to a fuselage actuator spindle  103  of an aerospacecraft  102  so as to be movable by the spindle.  
     [0022] With reference to FIGS.  2 - 4 , the ruddervator  100  includes a shell  104  and an internal structural member  106 . The shell  104  is formed as a single-piece, monolithic structure having a hollowed out portion  104   a . The shell  104  is comprised of a highly temperature resistant material, such as oxide/oxide-based ceramic matrix composite (Oxide-CMC) fabric  110  fused over a substrate of rigid ceramic foam insulation  108 . Preferably, a plurality of plies of Oxide-CMC fabric are incorporated, with the outer mold line (OML) ply being infused with a high-emissivity coating  110   a  such as reaction-cured glass (RCG). The high-emissivity coating provides plasma heating re-radiation outward to reduce internal temperatures within the ruddervator  100 .  
     [0023] With further reference to FIGS. 3 and 4, the structural member  106  comprises one or more graphite composite face sheets  112 , such as graphite/epoxy, and a honeycomb core  114 . The honeycomb core  114  may be formed from Nomex®, commercially available from E.l. du Pont de Nemours and Company. The hollowed out portion  104   a  is formed in the foam insulation  108  to permit the direct room temperature vulcanizing (RTV) adhesive bonding, as indicated at  116 , of the inner mold line  118  of the foam insulation onto the graphite face sheet  112  of the structural member  106 . The Oxide-CMC face sheets  110  provide a continuous shear flow around the OML of the ruddervator and, in concert with the directly bonded graphite composite structural member  106 , provide a quasi-torque box structure. The low thermal expansion property of the graphite minimizes the thermal mechanical stresses experienced due to thermal expansion differences between the ceramic foam insulation  108  and the structural member  106 .  
     [0024] With further reference to FIG. 2, the ruddervator  100  includes a torque box transition structure  120  for interfacing the ruddervator  100  to the fuselage actuator spindle  103  of the aerospacecraft  102 . The torque box transition structure  120  is sized to be slightly longer and wider than the graphite composite structural member  106  to provide space for fittings to attach the end of the member to the transition structure. The torque box transition structure  120  consists of a honeycomb sandwich torque box  122  with an access panel  124 . The access panel  124  allows access to the lower end of the graphite composite structural member  106  and also to machined internal fittings on the transition structure  120  to permit attachment of the ruddervator  100  to the actuator spindle. To limit the temperature of the transition structure  120  to a maximum temperature of about 250° F.-500° F., the transition structure is covered with a plurality of external, rigid insulation tiles  126 . Due to the lower temperatures at the base  100   a  of the ruddervator  100 , current or advanced tile systems can be utilized such as RCG and toughened uni-piece fibrous insulation (TUFI) coated alumina-enhanced thermal barrier (AETB) tile. The tile over the transition structure  120  is attached using standard RTV, strain isolation pads (SIP) and fillerbar (F/B). However, to provide access to the spindle axis panel  124  and the attachments for the graphite composite structural member  106 , specific tiles  126  are preferably bonded to intermediate carrier panels  129   a  and  129   b  and specific tiles have removable ceramic plugs  128  for access to carrier panel fasteners. The carrier panels are preferably fastened to inserts in the honeycomb sandwich transition structure  120  and thus permit easier removal of the tiles for damage replacement.  
     [0025] The ruddervator  100  also includes a serrated shape at a base portion  104   b  of the Oxide-CMC shell  104 . The tiles  126  at the interface of the Oxide-CMC shell  104  and the torque box transition structure  120  match the serrated end of the Oxide-CMC shell. The serrated interface minimizes direct high temperature plasma flow to the aft portions of the ruddervator  100 . It also permits direct access to the graphite composite structural member  106  when the transition structure  120  tile  126  are removed so that side attachments (not shown) between structural member  106  and structure  120  can be accessed.  
     [0026] It is an important advantage of the ruddervator  100  that the Oxide-CMC shell  104  is formed with the hollowed out interior area  104   a  defined by the outer mold line of the graphite composite structural member  106  in FIG. 3. This hollowed out interior area  104   a  allows a monolithic Oxide-CMC slab of material to be used to form the Oxide-CMC shell  104 , and allows insertion of the graphite composite structural member  106  into the hollowed out area and subsequent direct bonding of the shell to the structural member.  
     [0027] To further improve the structural integrity of the ruddervator  100 , the graphite composite structural member  106  preferably has a slight wedge shape, as indicated in FIG. 2, and also comprises open angles in all three dimensions, as indicated in FIG. 3. This feature permits air to escape during insertion of the structural member  106  into the hollowed out opening  104   a  of the Oxide-CMC shell  104  during manufacture to minimize the amount of air trapped in the RTV bond line  116  (FIG. 4). Excessive amounts of trapped air would reduce bond line strength to below acceptable levels. The wedge shape also permits pressure to be applied to the exposed end of the graphite composite structural member  106  and distributed throughout the Oxide-CMC shell  104  during bond curing to increase the adhesive joint strength and further reduce trapped air. The wedge shaped feature also provides an increasing thickness of CMC insulation between the outer mold line and inner mold line of the Oxide-CMC shell  104  to match the increasing temperature gradient to the ruddervator tip  100   b  (FIG. 2), while at the same time optimizing the ruddervator  100  overall structure to minimize weight. However, the individual panel surfaces creating the wedge shaped structural member  106  are all flat to minimize panel fabrication costs with the aerodynamic outer mold line insulation  108  cast directly into the Oxide-CMC.  
     [0028] The Oxide-CMC insulation  108  is further preferably uncoated at the open end (i.e. at serrated edge  104   b ) to allow the shell to vent during ascent. This reduces the risk of material fracture due to trapped air pressure differentials inside the insulation  108 .  
     [0029] The ruddervator  100  of the present invention provides an assembly of reduced cost and weight over present day ruddervator constructions by reducing the total number of parts required and by using higher specific strength and stiffness materials. The number of independent parts is significantly reduced by the use of the Oxide-CMC shell  104  and the single graphite composite structural member  106 , as compared with numerous built-up pieces of CMC hot structure and/or the traditional cold structure covered with numerous external tiles. The cost of the ruddervator  100  is also reduced by utilizing the reduced tooling complexities of Oxide-CMC over CMC fabrication processes, as well as the ability to implement more simple repair processes. Eliminating the need for oxidation protection coatings on the ruddervator also improves its life and reliability.  
     [0030] Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.