Abstract:
A method of manufacturing a duct is disclosed that is suitable for aerospace applications, for example. The method includes the steps of providing a longitudinally extending duct having a first wall thickness at an end portion and a second wall thickness at a portion that is less than the first wall thickness. The duct is bent at the portion to a desired shape to provide a tube having at least one bend located within a distance of one duct diameter from the first wall thickness.

Description:
[0001]    This invention was made with government support from the National Aeronautics and Space Administration under Contract No.: NNM06AB13C. The government may have certain rights to this invention pursuant to Contract No. NNM06AB13C awarded by the National Aeronautics and Space Administration. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates to a method of manufacturing a thin walled duct with a bend. This disclosure also relates to an engine having one or more thin walled ducts with bends. 
         [0003]    Thin walled ducts or tubes are used in a wide variety of applications. Typically, a duct having a uniform wall thickness is bent to the desired shape. The wall thickness of the duct is selected based upon the most highly stressed area of the duct. Such an approach to duct design results in ducts having a thicker than necessary wall thickness along much of the length of the duct. 
         [0004]    In aerospace applications, for example, ducts are used frequently. Precise alignment is required between the ends of the ducts and the adjoining components to which the ends are attached. Thus, flow lined pressure compensating bellows, which are heavy and costly, are often used to join the duct ends to their adjoining components. The use of compensating bellows may be avoided by increasing the wall thickness of the duct, along its entire length, which enables the ends to be welded to attaching flanges that then can be secured to the adjoining components, however, this approach increases the overall stiffness of the duct. Most aerospace architecture applications are, in part, centered around compactness which yield benefits in the form of reduced vehicle weight and improved aerodynamics. More compact duct arrangements help reduce vehicle engine envelope and thereby reduce overall vehicle gross weight. Variable wall thickness ducts increase ability to compact engine systems within smaller envelopes. 
         [0005]    Chemical milling has been used to thin the wall thickness of bent ducts in aerospace applications. Typically, a uniform thickness duct is bent to the desired shape. The duct is masked around the desirably thick areas, and then chemicals are applied to the unmasked areas to chemically remove some of the wall thickness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The disclosure can be further understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0007]      FIG. 1A  is a schematic of an example duct prior to manufacturing according to the disclosed method. 
           [0008]      FIG. 1B  is a schematic of an example mechanical working process used to further manufacture the duct shown in  FIG. 1A . 
           [0009]      FIG. 1C  is a schematic of the duct with a spacer in an uncompressed state prior to bending. 
           [0010]      FIG. 1D  is a schematic of the duct with the spacer in a compressed state prior to bending. 
           [0011]      FIG. 1E  is a schematic of an example bending process used to further manufacture the duct shown in  FIG. 1B . 
           [0012]      FIG. 1F  is a schematic of the duct illustrated in  FIG. 1E  joined to a flange. 
           [0013]      FIG. 2  is an example rocket engine incorporating ducts manufactured according to the disclosed method. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Referring to  FIGS. 1A-1D , a manufacturing method is illustrated in which a duct having a variable wall thickness is formed from a straight preform having a uniform wall thickness. 
         [0015]    A duct  10  is shown in  FIG. 1A  that includes a wall  12  having a generally uniform thickness defined by an inner diameter  18  and an outer diameter  20 . The duct  10  may be constructed from nickel, titanium, aluminum, steel or alloys thereof, for example. The duct  10  extends linearly between ends  14  along a length  16 . In one example, the duct  10  is provided by a seamless tube formed, for example, by flow forming, although other processes may be used to provide the tubular blank. The duct  10  may be formed by forging, for example. In one example, a cup-shaped structure may be forged having a cylindrical wall thickness of about a half an inch (12.7 mm) and an end wall of about of about a quarter inch (6.4 mm). 
         [0016]    The duct  10  is mechanically worked, as illustrated in  FIG. 1B . In one example, a machine  22  includes a mandrel  24  that supports the duct  10 . A tool  26  engages an exterior surface of the duct  10 , working from the end  14 , to provide a first wall thickness  30  at an end portion  30  at the end  14 . As the tool  26  moves along the exterior surface, material is displaced along a portion  32  of the duct  10  to provide a second wall thickness  34  that is less than the first wall thickness  30 . In one example, the duct  10  has a “thin” wall with an initial outer diameter to wall thickness ratio of, for example, 40. Unlike a chemical milling process, the displaced exterior is not structurally impacted by the chemicals causing, for example, intergranular attack. In one example, the second wall thickness  34  is about half that of the first wall thickness  30 . Although the end portions  28  are illustrated as the circumferential areas having the thicker wall thickness, it should be understood that other circumferential areas of the duct  10  may have thicker walls to provide localized strengthening. 
         [0017]    Example mechanical working processes include flow forming, turning and grinding in which material is plastically deformed and/or removed from the exterior surface of the duct  10 . Flow forming produces a smooth, wavy surface, which may in some cases have subtle surface ripples or waves. It also should be understood that the interior surface may be deformed to provide the variable wall thickness described above. 
         [0018]    In the example, the end portions  28  are generally uniformly cylindrical. However, it should be understood than the end portions  28  may become ovalized from the bending operation, but if this occurs the ends will typically undergo a rounding operation. A transition  36  adjoins the end portion  28  and the portion  32  such that an abrupt step is avoided, which may be a byproduct of a given mechanical working process. Transition  36  is structurally beneficial as it mitigates the occurrence of undesirable stress concentrations from developing in the duct during engine operation. 
         [0019]    A bending process is employed to produce a bend in the area of the portion  32 . A spacer  39  is provided about the portion  32  prior to bending the duct  10  to the desired shape, as shown in  FIG. 1C . In the example, the spacer  39  provides a diameter that is larger than the diameter of the end  28  in an uncompressed state. In one example, the spacer  39  is constructed from a soft, conformable PTFE sheet of material, such as GORE-TEX, wrapped about the portion  32 . In an example shown in  FIG. 1E , a bending machine  38  includes the fixtures  40 , which are used to clamp and bend the duct  10 . A mandrel  42 , such as a ball mandrel, is arranged inside the duct  10  to maintain the inner diameter  18  (shown in  FIG. 1A ) during bending. The spacer  39  maintains the outer diameter of the portion  32 , and is removed and discarded when then bending operation has been complete. The bends may be performed iteratively to avoid wrinkles, if needed. Although only one bend is shown, the duct  10  may include more than one bend. 
         [0020]    Referring to  FIG. 1D , the spacer  39  is sized such that when compressed by fixtures  40  during bending the spacer  39  fills and supports the portion  32  in its compressed state. Referring to  FIG. 1F , the variable wall sections of the duct may be closed-coupled to the tangency point (indicated by the dashed lines) of the duct bend radius (extending between the intrados  48  and extrados  50 ), as indicated by distance  56 . The distance  56  is defined as the distance from the tangency point to the location where the second wall thickness  34  transitions to the first wall thickness  30 . In one example, the distance  56  is within one duct outer diameter or less, although it should be understood that this disclosure is also intended to include distances  56  of greater than one duct outer diameter. The ends portions  28  may be trimmed or squared up after the bending process to prepare for further processing. In one example, the tube is made with several inches of straight after the bend. The tube is subsequently trimmed leaving, for example, a minimum of four times the first wall thickness  30 . 
         [0021]    The thicker first wall thickness  30  provides strength in desired circumferential areas, while the thinner second wall thickness  34  provides a smaller cross section to reduce stiffness and/or eliminate unneeded weight. For example, the end portion  28  provides sufficient structure to accommodate the heat produced when securing to a flange  52  to the duct  10  by a weld bead  54 . The end portion  28  has an end portion width  44  that is approximately four times the first wall thickness  30 , for example, which sufficiently accommodates the heat when applying the weld bead  54 . The transition  36  extends a transition width  46  that is three times the first wall thickness  30  in one example. The contour of the transition  36  is based upon the forming process and profile of the tool  26 , for example. 
         [0022]    The intrados (inner radius)  48  may be slightly thicker and the extrados (outer radius)  50  slightly thinner than the second wall thickness  34  from the bending process. The inner diameter  18  has a circular cross-section and is uniform in its dimensions and without wrinkles. It is desirable to provide the distance  56  adjoining the transition  36  and any bends for at least the reasons described above. 
         [0023]    A rocket engine  58  is illustrated in  FIG. 2  and includes several ducts  60 ,  66 ,  72  with flanges secured to the ducts&#39; opposite ends similar to the arrangement shown in  FIG. 1F . The ducts fluidly connect first and second components to one another. 
         [0024]    A fuel turbopump discharge duct  60  fluidly connects a fuel turbopump  62  to a main fuel valve  64 . An oxidizer turbopump discharge duct  66  fluidly connects an oxidizer turbopump  68  to a main oxidizer valve  70 . A nozzle coolant discharge duct  72  fluidly connects a nozzle  74  to an injector mixer  76 . The thinner portions of the duct are less stiff than the thicker portions to which the flanges are secured. This reduced stiffness desirably reduces the loads and stresses imparted to the components to which the ducts are secured. 
         [0025]    Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.