Patent Publication Number: US-2020283117-A1

Title: Auxiliary power unit enclosure and method of making the same

Description:
FIELD 
     The present disclosure is generally related to airframe components of an aircraft and, more particularly, to an auxiliary power unit enclosure that is integrated into the airframe of the aircraft. 
     BACKGROUND 
     An airframe of an aircraft is a mechanical structure that typically forms a fuselage, empennage and wings of the aircraft. The fuselage of most modern aircraft is constructed using a monocoque or a semi-monocoque type of airframe assembly. An auxiliary power unit (APU) is a device on the aircraft that provides energy for functions other than propulsion. As an example, the APU typically includes a gas turbine engine that operates to provide various power inputs when main propulsion engines are not operating and/or supplemental power to that generated during main engine operation. The APU is typically located within an isolated section of the fuselage that is segregated from other areas, commonly referred to as an APU compartment. The portion of the airframe forming the APU compartment must be capable of withstanding heat generated during normal operation of the APU and extreme temperatures resulting from a fault in the APU, such as a thermal event within the APU compartment. Thus, it is desirable to form an enclosure around the APU to protect the airframe from thermal effects of a thermal event and maintain an enclosure for a thermal suppression system. Conventional practice involves coupling insulation blankets to an interior of the airframe using support brackets. However, use of insulation blankets and associated support brackets increase the weight, cost, and production time of the aircraft. Accordingly, those skilled in the art continue with research and development efforts in the field of APU enclosures and, as such, apparatuses and methods, intended to address the above-identified concerns, would find utility. 
     SUMMARY 
     The following is a non-exhaustive list of examples, which may or may not be claimed, of the subject matter according to the present disclosure. 
     In an example, a disclosed auxiliary power unit enclosure of an aircraft includes a space frame configured to carry a load and defining an auxiliary power unit compartment. The space frame includes a plurality of frame elements coupled together at a plurality of nodes. The auxiliary power unit enclosure also includes a fairing coupled to and surrounding the space frame. 
     In an example, a disclosed fuselage of an aircraft includes an airframe and a space frame coupled to the airframe. The space frame is configured to carry a load and defines an auxiliary power unit compartment for stowing an auxiliary power unit of the aircraft. The space frame includes a plurality of frame elements coupled together at a plurality of nodes  110 . The fuselage also includes a fairing coupled to and surrounding the space frame. 
     In an example, a disclosed method of making an aircraft includes steps of: (1) coupling a plurality of annular frame elements and a plurality of longitudinal frame elements together at a plurality of nodes; (2) coupling a plurality of diagonal frame elements to at least one of the plurality of annular frame elements and the plurality of longitudinal frame elements proximate to the plurality of nodes to form a space frame; (3) coupling a fairing to the space frame; and (4) coupling the space frame to an airframe to form a fuselage of the aircraft. 
     Other examples of the disclosed fixtures, systems, and methods will become apparent from the following detailed description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of an example of an aircraft; 
         FIG. 2  is a schematic illustration of an example of the aircraft of  FIG. 1 ; 
         FIG. 3  is a schematic, perspective view of an example of a portion of a fuselage of the aircraft of  FIG. 1 ; 
         FIG. 4  is a schematic, perspective view of an example of an auxiliary power unit enclosure of the fuselage of  FIG. 1 ; 
         FIG. 5  is a schematic, perspective view of an example of a space frame of the auxiliary power unit enclosure of  FIG. 4 ; 
         FIG. 6  is a schematic, elevation view of an example of the auxiliary power unit enclosure; 
         FIG. 7  is a schematic, elevation view of an example of the space frame of the auxiliary power unit enclosure of  FIG. 6 ; 
         FIG. 8  is a schematic, elevation view of an example of a portion of the space frame; 
         FIG. 9  is a schematic, perspective view of an example of a portion of the auxiliary power unit enclosure; 
         FIG. 10  is a schematic, elevation view of an example of the auxiliary power unit enclosure of  FIG. 1 ; 
         FIG. 11  is a flow diagram of an example of a method of making an aircraft; and 
         FIG. 12  is a flow diagram of an example aircraft production and service methodology. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description refers to the accompanying drawings, which illustrate specific examples described by the present disclosure. Other examples having different structures and operations do not depart from the scope of the present disclosure. Like reference numerals may refer to the same feature, element, or component in the different drawings. 
     Illustrative, non-exhaustive examples, which may be, but are not necessarily, claimed, of the subject matter according the present disclosure are provided below. Reference herein to “example” means that one or more feature, structure, element, component, characteristic, and/or operational step described in connection with the example is included in at least one embodiment and/or implementation of the subject matter according to the present disclosure. Thus, the phrases “an example,” “another example,” “one or more examples,” and similar language throughout the present disclosure may, but do not necessarily, refer to the same example. Further, the subject matter characterizing any one example may, but does not necessarily, include the subject matter characterizing any other example. Moreover, the subject matter characterizing any one example may be, but is not necessarily, combined with the subject matter characterizing any other example. 
     Referring generally to  FIG. 1  and particularly to  FIGS. 2-10 , examples of an auxiliary power unit (APU) enclosure  100  of an aircraft  200  are disclosed. In an example, the APU enclosure  100  includes a space frame  102 . The space frame  102  is configured to carry and react to a load (i.e., provides a load path to distribute or transfer a load). The space frame  102  defines, or forms, an auxiliary power unit (APU) compartment  130 . The APU compartment  130  is configured to stow an auxiliary power unit (APU)  222  of the aircraft  200 . The space frame  102  includes a plurality of frame elements  124  coupled together at a plurality of nodes  110 . The APU enclosure  100  includes a fairing  104 . The fairing  104  is coupled to and surrounds the space frame  102 . 
     Referring to  FIGS. 1-3 , examples of a fuselage  150  of the aircraft  200  are disclosed. Generally, the APU enclosure  100  is integrated into and/or forms a structural component of the fuselage  150 . As used herein, a “structural component” refers to a load-bearing member or active structural element of an assembly that is configured to, or has the capacity to, carry and/or transfer a load. In an example, the fuselage  150  includes an airframe  202 . The airframe  202  is configured to carry a load (i.e., is a structural component of the fuselage  150 ). The fuselage  150  also includes the space frame  102 . The space frame  102  is coupled to the airframe  202 . The space frame  102  is configured to carry a load (i.e., is a structural component of the fuselage  150 ). The space frame  102  defines, or forms, the APU compartment  130  for stowing the APU  222  of the aircraft  200 . The space frame  102  includes the plurality of frame elements  124  coupled together at the plurality of nodes  110  ( FIG. 1 ). The fuselage  150  also includes the fairing  104 . The fairing  104  is coupled to and surrounds the space frame  102 . 
       FIG. 2  illustrates an example of the aircraft  200 . In the illustrative example, the aircraft  200  is a fixed-wing aircraft. The aircraft  200  includes the fuselage  150  and a pair of wings  220 . Generally, the airframe  202  forms the central structure of the fuselage  150  and the wings  220 . The aircraft  200  also includes a plurality of high-level systems  204 , such as, but not limited to, a propulsion system  208 , an electrical system  210 , a hydraulic system  212 , an environmental system  214 , and/or a communications system  216 . Any number of other systems may also be included. In an example, the APU  222  is a component of or provides power to one or more of the high-level systems  204 . 
     The fuselage  150  forms a main body of the aircraft  200  and defines an interior  206  configured to hold a crew, one or more passengers, cargo, and the high-level systems  204 . In the illustrative example, the fuselage  150  is an elongate, generally cylindrical central structure. The fuselage  150  includes a nose portion at a forward end and a tail portion at an aft end. In an example, the APU enclosure  100  is located proximate to or forms a part of the tail portion of the fuselage  150 . 
     As illustrated in  FIG. 2 , in an example of the aircraft  200 , the APU  222  is located within the APU enclosure  100  of the fuselage  150 . In an example, the APU  222  includes an internal combustion engine (not illustrated), such as a gas turbine engine, that operates to provide mechanical input to various components, such as an electrical generator (not illustrated). The APU enclosure  100  provides a containment structure capable of maintaining its strength in response to heat generated during normal operation of the APU  222  and extreme heat resulting from a thermal event within the APU compartment  130 , such as due to a fault in the APU  222  located within the APU enclosure  100 . 
       FIG. 3  illustrates a portion of the fuselage  150 . The APU enclosure  100  has a longitudinal axis  114 . Generally, the longitudinal axis  114  of the APU enclosure  100  is aligned with (e.g., coincident with) a longitudinal axis  218  of the airframe  202 . In one or more examples, the longitudinal axis  114  of the APU enclosure  100  defines a longitudinal axis of the space frame  102  and the fairing  104 . Throughout the present disclosure, the longitudinal axis  114  of the APU enclosure  100  may, but does not necessarily, refer to the longitudinal axis of the space frame  102  and/or the longitudinal axis of the fairing  104 . The APU enclosure  100  is coupled to an adjacent section of the airframe  202  to form a portion of the fuselage  150 . 
     As illustrated in  FIG. 3 , in an example, the space frame  102  includes, or takes the form of, a truss-like structure that has a closed cross-sectional shape, viewed along the longitudinal axis  114 , and that forms an open interior region at least partially defining the APU compartment  130 . Generally, the space frame  102  includes, or forms, a plurality of triangular units constructed with the plurality of frame elements  124  that are connected to each other at joints, referred to as the plurality of nodes  110 . Thus, external forces and reactions to those forces are considered to act only at the plurality of nodes  110  and result in forces in the plurality of frame elements  124  that are either tensile or compressive. Accordingly, the space frame  102  is an internal structure that provides load-carrying capacity for an associated portion (e.g., the tail portion) of the fuselage  150 . 
       FIG. 3  depicts a portion of the fairing  104  removed so that the space frame  102  is more clearly visible. In an example, the fairing  104  has a closed cross-sectional shape, viewed along the longitudinal axis  114 , that at partially surrounds the space frame  102 . Accordingly, the fairing  104  is an external structure that provides for a smooth outline for the associated portion of the fuselage  150  and reduces drag of the fuselage  150 . 
     As illustrated in  FIG. 3 , in an example, the airframe  202  is a semi-monocoque structure that includes a plurality of frames  228  and an outer skin  224  with a plurality of stringers  226  coupled to the outer skin  224 . The outer skin  224  and the plurality of stringers  226  are coupled to the plurality of frames  228 . In the semi-monocoque type airframe assembly, the plurality of frames  228  (also referred to as formers) establishes the shape of the fuselage  150  and loads are supported, at least in part, by the plurality of stringers  226  of the airframe  202 . In another example, the airframe  202  is a monocoque structure that includes the plurality of frames  228  and the outer skin  224 , which is coupled to the plurality of frames  228 . In the monocoque-type airframe assembly, the plurality of frames  228  establishes the shape of the fuselage  150  and loads are supported through the outer skin  224  of the airframe  202 . 
     Referring still to  FIG. 3 , the APU enclosure  100  is coupled to or is otherwise structurally integrated with the airframe  202  to form the fuselage  150  (only a section of the airframe  202  and the APU enclosure  100  are depicted in  FIG. 3  to illustrate a portion of the fuselage  150 ). In an example, one or more of the plurality of frame elements  124  of the space frame  102  of the APU enclosure  100  are coupled to or are otherwise structurally joined with one or more of the plurality of frames  228  and/or the plurality of stringers  226  of the airframe  202  to provide a load path between the APU enclosure  100  and the airframe  202 . In an example, the fairing  104  of the APU enclosure  100  is integrated with or is otherwise joined to the outer skin  224  of the airframe  202  to provide a smooth outline of the fuselage  150  and reduce drag. In an example, a firewall  230  is located between the airframe  202  and the APU enclosure  100 . 
     Referring to  FIG. 1 , in an example, the space frame  102  is formed of a first material  126  having a first critical temperature  132 . The fairing  104  is formed of a second material  128  having a second critical temperature  134 . The second critical temperature  134  is less than the first critical temperature  132 . As used herein, critical temperature refers to a temperature at which a material (e.g., a metallic material or composite material) exceeds its ultimate strength and ceases to provide sufficient load carrying capacity. In an example, the first material  126  does not exceed its ultimate strength in response to temperatures approximating the first critical temperature  132 . In an example, the second material  128  does not exceed its ultimate strength in response to temperatures approximating the second critical temperature  134 , however, may exceed its ultimate strength in response to temperatures approximating the first critical temperature  132 . This critical temperature and ultimate strength differential enables the space frame  102  to withstand a thermal event and maintain its load-carrying capacity in response to extreme temperatures within the APU compartment  130 , while the fairing  104  is permitted to structurally degrade in response to such temperatures. 
     In an example, the first critical temperature  132  is at least 500° F. (260° C.) and the second critical temperature  134  is below 500° F. (260° C.). In another example, the first critical temperature  132  is at least 600° F. (315° C.) and the second critical temperature  134  is below 600° F. (315° C.). In another example, the first critical temperature  132  is at least 700° F. (350° C.) and the second critical temperature  134  is below 700° F. (350° C.). In another example, the first critical temperature  132  is at least 800° F. (425° C.) and the second critical temperature  134  is below 800° F. (425° C.). In other examples, the first critical temperature  132  may be below 500° F. (260° C.) or above 800° F. (425° C.), depending on the composition of the first material  126 . 
     A typical thermal event located within the APU compartment  130  of the APU enclosure  100  may reach temperatures between approximately 500° F. (260° C.) and approximately 800° F. (425° C.) in regions proximate to the space frame  102  and the fairing  104 . During a thermal event, the space frame  102  (e.g., the plurality of frame elements  124 ) does not exceed its ultimate strength when experiencing temperatures between approximately 500° F. (260° C.) and approximately 800° F. (425° C.). As such, the space frame  102  maintains its load-carrying capacity and structural integrity when exposed to such extreme temperatures. The fairing  104  may exceed its ultimate strength and begin to plastically deform when exposed to such temperatures. However, since the fairing  104  does not serve as a primary structural (e.g., load-bearing) component of the fuselage  150 , plastic deformation, or even destruction, of the fairing  104  does not affect the structural integrity of the APU enclosure  100 . 
     Referring still to  FIG. 1 , in an example, the first material  126  of the space frame  102  includes a metallic material  136 . In an example, the metallic material  136  is titanium. Titanium provides a beneficial strength-to-weight ratio for aerospace applications. In another example, the metallic material  136  is carbon steel. In another example, the metallic material  136  is a corrosion resistant steel (CRES). In another example, the metallic material  136  is a metal matrix composite. In another example, the metallic material  136  is titanium silicon carbide (Ti 3 SiC 2 ). Other metals, metal alloys, or metallic composites are also contemplated for use as the metallic material  136  of the space frame  102 . 
     In an example, the second material  128  of the fairing  104  includes a composite material  138 . In an example, the composite material  138  is a fiber-reinforced polymer, such as a carbon fiber-reinforced polymer or a glass fiber-reinforced polymer (e.g., fiberglass). In another example, the second material  128  of the fairing  104  includes a metallic material, such as aluminum. 
     Referring to  FIGS. 4-10 , in an example, the plurality of frame elements  124  of the space frame  102  includes a plurality of annular frame elements  106  (also referred to individually as annular frame element  106 ). The plurality of annular frame elements  106  is spaced apart from each other along the longitudinal axis  114  ( FIG. 3 ) of the space frame  102 . Each one of the plurality of annular frame elements  106  circumscribes and is oriented approximately perpendicular to the longitudinal axis  114 . The plurality of annular frame elements  106  establishes the shape of at least a portion of the APU enclosure  100 . The plurality of frame elements  124  also includes a plurality of longitudinal frame elements  108  (also referred to individually as longitudinal frame element  108 ). The plurality of longitudinal frame elements  108  is coupled to the plurality of annular frame elements  106  at the plurality of nodes  110 . Each one of the plurality of longitudinal frame elements  108  extends along the longitudinal axis  114  and approximately perpendicularly intersects one or more of the plurality of annular frame elements  106 . The plurality of frame elements  124  also includes a plurality of diagonal frame elements  112  (also referred to individually as diagonal frame element  112 ). At least a portion of the plurality of diagonal frame elements  112  is coupled to at least one of the plurality of annular frame elements  106  and the plurality of longitudinal frame elements  108  proximate to the plurality of nodes  110 . For the purpose of the present disclosure, the term “node” refers to a central or connecting point or region at which at least two of the annular frame element  106 , the longitudinal frame element  108 , and/or the diagonal frame element  112  intersect or branch. Thus, external forces and reactions to those forces are considered to act only at the plurality of nodes  110  and result in forces in one or more of the plurality of annular frame elements  106 , the plurality of longitudinal frame elements  108 , and the plurality of diagonal frame elements  112  that are either tensile or compressive. The plurality of diagonal frame elements  112  provide for redundant load paths through the space frame  102 . 
     In an example, the plurality of annular frame elements  106  is formed of the first material  126  having the first critical temperature  132 . In an example, the plurality of longitudinal frame elements  108  is formed of the first material  126  having the first critical temperature  132 . In an example, the plurality of diagonal frame elements  112  is formed of the first material  126  having the first critical temperature  132 . In an example, at least one of the plurality of annular frame elements  106 , the plurality of longitudinal frame elements  108 , and the plurality of diagonal frame elements  112  are formed of the metallic material  136 , such as titanium. 
     Referring generally to  FIG. 3  and particularly to  FIGS. 4-7 , in an example, ends of one or more of the plurality of longitudinal frame elements  108  of the space frame  102  are coupled to one or more of the plurality of frames  228  and/or one or more of the plurality of stringers  226  of the airframe  202  ( FIG. 3 ) adjacent to the APU enclosure  100 , depending on the type of airframe assembly. This arrangement enables the APU enclosure  100  to be structurally integrated into the remaining portions of the fuselage  150  and enables loads to be distributed between the space frame  102  and the airframe  202  adjacent to the APU enclosure  100 . 
     Referring generally to  FIGS. 4-7  and particularly to  FIG. 8 , in an example, each one of the plurality of diagonal frame elements  112  extends between a diagonally opposed pair of the plurality of nodes  110  (depicted individually in  FIG. 8  as first node  110 A and second node  110 B and identified collectively as diagonally opposed pair of nodes  110 A,  110 B). The diagonally opposed pair of nodes  110 A,  110 B is formed at intersections of a longitudinally adjacent pair of the plurality of annular frame elements  106  (depicted individually in  FIG. 8  as first annular frame element  106 A and second annular frame element  106 B and identified collectively as longitudinally adjacent pair of annular frame elements  106 A,  106 B) and a radially adjacent pair of the plurality of longitudinal frame elements  108  (depicted individually in  FIG. 8  as first longitudinal frame element  108 A and second longitudinal frame element  108 B and identified collectively as radially adjacent pair of longitudinal frame elements  108 A,  108 B). As illustrated in  FIG. 8 , in an example, one end of the diagonal frame element  112  is coupled to at least one of the first annular frame element  106 A and the second longitudinal frame element  108 B proximate to the first node  110 A. An opposing end of the diagonal frame element  112  is coupled to at least one of the second annular frame element  106 B and the first longitudinal frame element  108 A proximate to the second node  110 B. This arrangement is repeated along the longitudinal axis  114  ( FIG. 3 ) of the space frame  102  to provide redundant load paths between each diagonally opposed pair of nodes  110 A,  110 B. 
     Referring generally to  FIGS. 4-8  and particularly to  FIG. 9 , in an example, an end  140  of the diagonal frame element  112  is coupled to the annular frame element  106  proximate to the node  110 . In another example, the end  140  of the diagonal frame element  112  is coupled to the longitudinal frame element  108  proximate to the node  110 . In another example, the end  140  of the diagonal frame element  112  is coupled to the annular frame element  106  and the longitudinal frame element  108  proximate to the node  110 . 
     The particular shape, dimensions, and angular orientation of each one of the diagonal frame elements  112  depends on various factors, such as the number of annular frame elements  106 , the number of longitudinal frame element  108 , the shape and dimensions of the space frame  102 , and other factors. In an example, one or more of the diagonal frame elements  112  includes one or more twists about its length. In an example, one or more of the diagonal frame elements  112  includes one or more bends along its length. 
     Referring to generally to  FIGS. 2 and 3  and particularly to  FIGS. 4 and 6 , in an example, the fairing  104  may include a plurality of stiffeners  116  and a plurality of skins  118  ( FIG. 4 ). The plurality of skins  118  is coupled to the plurality of stiffeners  116 . In an example, the plurality of stiffeners  116  is coupled to at least one of the plurality of annular frame elements  106 . In an example, the plurality of skins  118  is coupled to at least one of the plurality of annular frame elements  106 .  FIG. 4  depicts a portion of the plurality of stiffeners  116  coupled to a portion of the plurality of annular frame elements  106  and a portion of the plurality of skins  118  exploded from the plurality of stiffeners  116  and the plurality of annular frame elements  106 .  FIG. 6  depicts a portion of the plurality of stiffeners  116  coupled to a portion of the plurality of annular frame elements  106  and the plurality of skins  118  removed for clarity. In another example, the fairing  104  may include the plurality of skins  118  that are coupled to at least one of the annular frame elements  106  and/or at least one of the longitudinal frame elements  108 , such as illustrated in  FIG. 3 . 
     Referring generally to  FIG. 3  and particularly to  FIGS. 4 and 6 , in an example, ends of one or more of the plurality of stiffeners  116  of the fairing are coupled to one or more of the plurality of frames  228  and/or one or more of the plurality of stringers  226  of the airframe  202  ( FIG. 3 ) adjacent to the APU enclosure  100 , depending on the type of airframe assembly. Edges of one or more of the plurality of skins  118  are abutted with and joined to adjacent edges of the outer skin  224  of the airframe  202 . This arrangement enables the APU enclosure  100  to aerodynamically integrated into the remaining portions of the fuselage  150 . 
     In an example, the plurality of stiffeners  116  is formed of the second material  128  having the second critical temperature  134 . In an example, the plurality of skins  118  is formed of the second material  128  having the second critical temperature  134 . In example, at least one of the plurality of stiffeners  116  and the plurality of skins  118  is formed of the composite material  138 , such as the fiber-reinforced polymer, such as the carbon fiber-reinforced polymer or fiberglass. In an example, the skins  118  are formed of at least one sandwich panel, such as a honeycomb panel or foam core sandwich panel. 
     Referring to generally to  FIGS. 3-7  and particularly to  FIG. 10 , in an example, the APU enclosure  100  includes a door  122  ( FIG. 10 ). In an example, the space frame  102  includes a plurality of semi-annular frame elements  120  (also referred to individually as semi-annular frame element  120 ). The plurality of semi-annular frame elements  120  is spaced apart from each other and the plurality of annular frame elements  106  along the longitudinal axis  114  ( FIG. 3 ) of the space frame  102 . Each one of the plurality of semi-annular frame elements  120  circumscribes and is oriented approximately perpendicular to the longitudinal axis  114 . The plurality of semi-annular frame elements  120  establishes the shape of a portion of the APU enclosure  100 . An open region  142  ( FIGS. 4 and 10 ) formed by the semi-annular frame elements  120  is configured to accommodate the door  122 . The plurality of longitudinal frame elements  108  is coupled to the plurality of semi-annular frame elements  120  at the plurality of nodes  110 . Each one of the plurality of longitudinal frame elements  108  approximately perpendicularly intersects one or more of the plurality of semi-annular frame elements  120 . A portion of the plurality of diagonal frame elements  112  is coupled to at least one of the plurality of semi-annular frame elements  120  and the plurality of longitudinal frame elements  108  proximate to the plurality of nodes  110 . 
     As illustrated in  FIG. 10 , in an example, the door  122  is coupled to at least one of the plurality of semi-annular frame elements  120  and/or at least one of the plurality of longitudinal frame elements  108  that intersects the plurality of semi-annular frame elements  120  and that at least partially defines the open region  142  ( FIG. 4 ). The door  122  is moveable relative to the plurality of semi-annular frame elements  120  and/or the plurality of longitudinal frame elements  108  to which it is coupled. The door  122  provides access to the APU compartment  130  ( FIG. 3 ) and, thus, the APU  222  ( FIG. 2 ). 
     Referring to  FIGS. 4-7 , in an example, one or more of the plurality of annular frame elements  106  include a plurality of shear ties  144  (also referred to as individually as shear tie  144 ). The plurality of shear ties  144  is spaced apart from each other and extends about at least a portion of a perimeter of the annular frame element  106 . As best illustrated in  FIGS. 4 and 6 , an open region (commonly referred to as a mouse hole) formed between adjacent ones of the shear ties  144  accommodates one of the plurality of longitudinal frame elements  108  of the space frame  102  or one of the plurality of stiffeners  116  of the fairing  104 . Similarly, in an example, one or more of the plurality of semi-annular frame elements  120  include the plurality of shear ties  144  to accommodate ones of the plurality of longitudinal frame elements  108  of the space frame  102  and ones of the plurality of stiffeners  116  of the fairing  104 . 
     In an example, at least a portion of the plurality of shear ties  144  is formed of the first material  126  having the first critical temperature  132 . As an example, at least a portion of the plurality of shear ties  144  is formed of the metallic material  136 , such as titanium. In another example, at least a portion of the plurality of shear ties  144  is formed of the second material  128  having the second critical temperature  134 . As an example, at least a portion of the plurality of shear ties  144  is formed of the composite material  138 , such as the fiber-reinforced polymer, such as the carbon fiber-reinforced polymer. 
     Referring generally to  FIGS. 1-10  and particularly to  FIG. 11 , examples of a method  1000  of making the aircraft  200  are disclosed. The aircraft  200  ( FIGS. 1 and 2 ) manufactured according to the method  1000  includes the airframe  202  and the APU enclosure  100  that form at least a portion of the fuselage  150 . 
     In an example, the method  1000  includes a step of (block  1002 ) coupling the plurality of annular frame elements  106  and the plurality of longitudinal frame elements  108  together at the plurality of nodes  110 . The method  1000  includes a step of (block  1004 ) coupling the plurality of diagonal frame elements  112  to at least one of the plurality of annular frame elements  106  and the plurality of longitudinal frame elements  108  proximate to the plurality of nodes  110  to form the space frame  102 . 
     In an example, according to the method  1000 , the step of coupling the plurality of diagonal frame elements  112  to at least one of the plurality of annular frame elements  106  and the plurality of longitudinal frame elements  108  includes a step of (block  1006 ) coupling each one of the plurality of diagonal frame elements  112  between a diagonally opposed pair of the plurality of nodes  110  formed at intersections of a longitudinally adjacent pair of the plurality of annular frame elements  106  and a radially adjacent pair of the plurality of longitudinal frame elements  108 . According to the method  1000 , the step of coupling the plurality of diagonal frame elements  112  to at least one of the plurality of annular frame elements  106  and the plurality of longitudinal frame elements  108  includes a step of (block  1008 ) providing a plurality of redundant load paths. 
     In an example, the method  1000  includes a step of (block  1010 ) coupling the fairing  104  to the space frame  102 . The method  1000  includes a step of (block  1012 ) coupling the space frame  102  to the airframe  202  to form the fuselage  150  of the aircraft  200 . 
     In an example, the method  1000  includes a step of (block  1014 ) stowing the APU  222  of the aircraft  200  in the APU compartment  130  defined by the space frame  102 . According to the method  1000 , the space frame  102  is formed of the first material  126  having the first critical temperature  132 , the fairing  104  is formed of the second material  128  having the second critical temperature  134 , and the second critical temperature  134  is less than the first critical temperature  132 . 
     In an example, the method  1000  includes a step of (block  1016 ) not exceeding the ultimate strength of the first material  126  of the space frame  102  in response to a temperature up to the first critical temperature  132 . 
     Examples of the APU enclosure  100 , the fuselage  150 , the aircraft  200 , and the method  1000  may be used in the context of an aircraft manufacturing and service method  1100 , as shown in the flow diagram of  FIG. 12 . Aircraft applications of the disclosed examples may include manufacture, operation, and service of an aircraft that includes a fuselage compartment configured to stow an auxiliary power unit. While the examples of APU enclosure  100  and the method  1000  are described in connection with aerospace applications, the APU enclosure  100  and the method  1000  may find use in a variety of potential applications, particularly in the transportation industry for use with any type of vehicle that utilizes an auxiliary power unit. 
     As illustrated in  FIG. 12 , during pre-production, the method  1100  may include specification and design of the aircraft  200  (block  1102 ) and material procurement (block  1104 ). During production of the aircraft  200 , component and subassembly manufacturing (block  1106 ) and system integration (block  1108 ) of the aircraft  200  may take place. Thereafter, the aircraft  200  may go through certification and delivery (block  1110 ) to be placed in service (block  1112 ). The disclosed APU enclosure  100  and method  1000  may form a portion of component and subassembly manufacturing (block  1106 ) and/or system integration (block  1108 ). Routine maintenance and service (block  1114 ) may include modification, reconfiguration, refurbishment, etc. of one or more systems of the aircraft  200 . 
     Each of the processes of the method  1100  illustrated in  FIG. 12  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include, without limitation, any number of aircraft manufacturers and major-system subcontractors; a third party may include, without limitation, any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     Examples of the APU enclosure  100  and the method  1000  shown or described herein may be employed during any one or more of the stages of the manufacturing and service method  1100  shown in the flow diagram illustrated by  FIG. 12 . For example, components or subassemblies, such as the space frame  102  and the fairing  104  of the APU enclosure  100 , corresponding to component and subassembly manufacturing (block  1106 ) may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  200  is in service (block  1112 ). Also, one or more examples of the APU enclosure  100  and the method  1000  disclosed herein may be utilized during production stages (block  1108  and block  1110 ). Similarly, one or more examples of the APU enclosure  100  and the method  1000  disclosed herein may be utilized, for example and without limitation, while the aircraft  200  is in service (block  1112 ) and during maintenance and service (block  1114 ). 
     Although an aerospace example is shown, the principles disclosed herein may be applied to other industries, such as the automotive industry, the space industry, the construction industry, and other design and manufacturing industries. Accordingly, in addition to aircraft, the principles disclosed herein may apply to other vehicle structures (e.g., land vehicles, marine vehicles, space vehicles, etc.) and stand-alone structures. 
     As used herein, a system, apparatus, structure, article, element, component, or hardware “configured to” perform a specified function is indeed capable of performing the specified function without any alteration, rather than merely having potential to perform the specified function after further modification. In other words, the system, apparatus, structure, article, element, component, or hardware “configured to” perform a specified function is specifically selected, created, implemented, utilized, programmed, and/or designed for the purpose of performing the specified function. As used herein, “configured to” denotes existing characteristics of a system, apparatus, structure, article, element, component, or hardware that enable the system, apparatus, structure, article, element, component, or hardware to perform the specified function without further modification. For purposes of this disclosure, a system, apparatus, structure, article, element, component, or hardware described as being “configured to” perform a particular function may additionally or alternatively be described as being “adapted to” and/or as being “operative to” perform that function. 
     Unless otherwise indicated, the terms “first”, “second”, etc. are used herein merely as labels, and are not intended to impose ordinal, positional, or hierarchical requirements on the items to which these terms refer. Moreover, reference to a “second” item does not require or preclude the existence of lower-numbered item (e.g., a “first” item) and/or a higher-numbered item (e.g., a “third” item). 
     For the purpose of this disclosure, the terms “coupled,” “coupling,” and similar terms refer to two or more elements that are joined, linked, fastened, connected, put in communication, or otherwise associated (e.g., mechanically, electrically, fluidly, optically, electromagnetically) with one another. In various examples, the elements may be associated directly or indirectly. As an example, element A may be directly associated with element B. As another example, element A may be indirectly associated with element B, for example, via another element C. It will be understood that not all associations among the various disclosed elements are necessarily represented. Accordingly, couplings other than those depicted in the figures may also exist. 
     As used herein, the phrase “at least one of”, when used with a list of items, means different combinations of one or more of the listed items may be used and only one of each item in the list may be needed. For example, “at least one of item A, item B, and item C” may include, without limitation, item A or item A and item B. This example also may include item A, item B, and item C, or item B and item C. In other examples, “at least one of” may be, for example, without limitation, two of item A, one of item B, and ten of item C; four of item B and seven of item C; and other suitable combinations. 
     As used herein, the term “approximately” refers to or represents a condition that is close to, but not exactly, the stated condition that still performs the desired function or achieves the desired result. In an example, the term “approximately” refers to a condition that is within an acceptable predetermined tolerance or accuracy. In an example, the term “approximately” refers to a condition that is within 10% of the stated condition. However, the term “approximately” does not exclude a condition that is exactly the stated condition. Accordingly, the term “approximately” may be interpreted to mean exactly to or within a desired degree of accuracy. 
     In  FIG. 1 , referred to above, the blocks may represent elements, components, and/or portions thereof and lines, if any, connecting various elements and/or components may represent mechanical, electrical, fluid, optical, electromagnetic and other couplings and/or combinations thereof. Couplings other than those depicted in the block diagrams may also exist. One or more elements shown may be omitted from a particular example without departing from the scope of the present disclosure. Environmental elements, if any, are represented with dotted lines. Virtual (imaginary) elements may also be shown for clarity. Those skilled in the art will appreciate that some of the features illustrated in  FIG. 1 , may be combined in various ways without the need to include other features described in  FIG. 1 , other drawing figures, and/or the accompanying disclosure, even though such combination or combinations are not explicitly illustrated herein. Similarly, additional features not limited to the examples presented, may be combined with some or all of the features shown and described herein. 
     In  FIGS. 11 and 12 , referred to above, the blocks may represent operations and/or portions thereof and lines connecting the various blocks do not imply any particular order or dependency of the operations or portions thereof. It will be understood that not all dependencies among the various disclosed operations are necessarily represented.  FIGS. 11 and 12  and the accompanying disclosure describing the operations of the disclosed methods set forth herein should not be interpreted as necessarily determining a sequence in which the operations are to be performed. Rather, although one illustrative order is indicated, it is to be understood that the sequence of the operations may be modified when appropriate. Accordingly, modifications, additions and/or omissions may be made to the operations illustrated and certain operations may be performed in a different order or simultaneously. Additionally, those skilled in the art will appreciate that not all operations described need be performed. 
     Although various examples of the disclosed APU enclosure  100 , the fuselage  150 , the aircraft  200 , and the method  1000  have been shown and described, modifications may occur to those skilled in the art upon reading the specification. The present application includes such modifications and is limited only by the scope of the claims.