Patent Publication Number: US-2019185129-A1

Title: Pressure Bulkhead For A Pressurized Cabin Of An Aerospace Craft, And An Aerospace Craft

Description:
FIELD OF THE INVENTION 
     The invention relates to a pressure bulkhead for a pressurized cabin of an aerospace craft, and to an aerospace craft. 
     BACKGROUND OF THE INVENTION 
     Aerospace craft, e.g. passenger aircraft, fly at altitudes at which the air pressure is significantly lower than at ground level. Those areas in which the crew and passengers are accommodated are therefore designed as pressurized cabins. In pressurized cabins, a cabin pressure corresponding approximately to the air pressure at the earth&#39;s surface is applied. These pressurized cabins extend along the aircraft and are sealed off at the rear of the aircraft by means of a pressure bulkhead, which can extend over virtually the entire fuselage diameter in the transverse direction relative to the direction of flight. Pressure bulkheads therefore have a large area against which the pressure prevailing in the pressurized cabin acts against the external, lower pressure. They must therefore satisfy special stability and safety requirements. 
     If pressure bulkheads are to be designed in a material- and weight-saving manner, they typically have a curvature which projects into the pressurized cabin. However, this reduces the space available in the pressurized cabin. 
     For reasons of stability, more material is required for pressure bulkheads of flat design, and therefore they have a high weight and are of very thick construction. 
     A flat pressure bulkhead is known from DE 10 2006 029 231 B4, for example. In this case, it has a main bulkhead section and a frame, which carries the main bulkhead section and connects it to the aerospace craft. Here, the main bulkhead section can be designed to have an approximately flat configuration in a no-load state. 
     As regards weight-saving components, sandwich elements with auxetic, three-dimensional open lattice cores are known from “lightweightdesign—Die Fachzeitschrift für den Leichtbau bewegter Massen”, Obrecht et al., May 2011, page 37-42, said elements providing protection against impinging objects. 
     BRIEF SUMMARY OF THE INVENTION 
     An aspect of the invention may provide an improved pressure bulkhead. 
     Here, a pressure bulkhead for a pressurized cabin of an aerospace craft is provided, wherein the pressure bulkhead comprises: a pressure wall and a frame for connecting the pressure wall to the aerospace craft; wherein the frame is connected to the pressure wall; wherein the pressure wall has: a core layer; and a first covering layer and a second covering layer; wherein the core layer is arranged between the first covering layer and the second covering layer; and wherein the core layer comprises an auxetic foam. 
     An auxetic foam is understood to mean a foam which has auxetic properties. Auxetic foam has the property that its Poisson ratio is negative, wherein the Poisson ratio indicates the ratio of a deformation in a direction of load to a deformation transversely to the direction of load. In this context, a negative ratio means that, when the auxetic foam is pulled apart in one direction, it likewise expands in the direction transverse thereto. A non-auxetic material would contract in this direction. Moreover, this means that, when the auxetic material is compressed, the material contracts transversely to the compression. The material becomes more dense than conventional material when it is compressed. This increases the bending stiffness of auxetic material. The bending stiffness of the auxetic core layer is dependent on the Poisson ratio. When the Poisson ratio is close to −1, there is a rapid increase in bending stiffness. There is therefore a much smaller bending deformation in the auxetic core layer due to a pressure difference between the cabin and the environment than in the case of a conventional material. Furthermore, the reduced bending deformation causes a reduction in the tension forces at the edges of the pressure wall. By virtue of this reduced bending deformation and the reduced tension forces, the pressure wall and hence the pressure bulkhead can be designed to be thinner and lighter than in the prior art. As a result, the space required by the pressure bulkhead is reduced, and therefore more space is available for the passengers and/or the crew when it is installed in an aerospace craft. The increased stiffness raises the natural frequency of the pressure wall further, and therefore the pressure bulkhead is less sensitive to vibration and resonance. In this way, a pressure bulkhead is made provided which has increased stiffness, thus ensuring that smaller bending deformations and lower stresses occur and hence that costs can be saved, and that the volume of the pressurized cabin which can be made available to the crew and passengers is maximized. 
     Furthermore, it is advantageously envisaged that the auxetic foam has a Poisson ratio in a range of from −0.5 to −1, preferably in a range of from −0.85 to −1, more preferably of −1. 
     In this range, the auxetic foam has the most favourable properties for use in a pressure bulkhead. The stiffness of the auxetic foam with a Poisson ratio in this range leads to low bending deformation with a low outlay on materials for the pressure differences that prevail in the aerospace sector. 
     It is furthermore advantageous if the core layer has two parallel surfaces facing away from one another; and if one of the two parallel surfaces facing away from one another is arranged on the first covering layer and the other of the two parallel surfaces is arranged on the second covering layer. 
     It is expedient here if the two parallel surfaces facing away from one another have a spacing in a range of from 50 mm to 200 mm, preferably from 75 mm to 150 mm, more preferably from 90 to 100 mm, most preferably a spacing of 96 mm. 
     It is advantageous if the first and the second covering layer are flat. 
     Thus, the pressure bulkhead is of flat design and therefore requires only a small amount of space in the pressurized cabin for installation, maximizing the volume of the pressurized cabin which can be made available to the crew and passengers. 
     It is furthermore advantageously possible for the first covering layer and the second covering layer to comprise aluminium or carbon-fibre-reinforced plastic (CFRP). 
     Aluminium can be processed and repaired more cheaply and more easily than steel or other conventional materials. CFRP is even lighter and has even greater strength and stiffness. Moreover, CFRP is more resistant to erosion and fatigue. 
     It is advantageous if the first covering layer and the second covering layer have a thickness in a direction away from the core layer in a range of from 0.5 mm to 6 mm, preferably from 1 mm to 3 mm, more preferably a thickness of 2 mm. 
     The pressure bulkhead can furthermore advantageously have a diameter in a range of from 500 mm to 6000 mm, preferably in a range of from 1500 mm to 4500 mm, more preferably from 1750 mm to 2250 mm, most preferably of 2000 mm. In this case, the pressure bulkhead is designed in such a way that it fits into the structure of the aerospace craft. 
     An aerospace craft is furthermore provided, wherein the aerospace craft comprises: a pressurized cabin having a rear section; and a pressure bulkhead according to the description presented above; wherein the pressure bulkhead is connected in the rear section to the pressurized cabin and forms a rear wall of the pressurized cabin. 
     The advantages and developments of the aerospace craft will emerge analogously from the description of the pressure bulkhead. In this respect therefore attention is drawn to the description presented above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described below with reference to an illustrative embodiment by means of the attached drawing. In the drawing: 
         FIG. 1  shows a schematic illustration of a pressure bulkhead; 
         FIG. 2  shows a schematic illustration of a pressure wall under pressure; 
         FIGS. 3 a , 3 b    show schematic illustrations of an auxetic lattice structure; 
         FIGS. 4 a , 4 b    show schematic illustrations of an intergranular auxetic structure; and 
         FIG. 5  shows a schematic illustration of an aircraft having a pressure bulkhead at the rear. 
     
    
    
     DETAILED DESCRIPTION 
     The pressure bulkhead is denoted overall below by the reference sign  10 , as illustrated in  FIG. 1 . 
     The pressure bulkhead  10  has a frame  12  and a pressure wall  14 . The pressure wall  14  is inserted into the frame  12 . The function of the frame  12  is to connect the pressure wall  14  to an aerospace craft  28 . Here, the frame  12  is designed for connection to the aerospace craft  28  over its entire circumference. 
     The pressure wall  14  is of sheet-like design and fills the frame  12 . No pressure equalization can therefore take place between the pressure wall  14  and the frame  12  or through the pressure wall  14  or the frame  12 . In this case, the pressure bulkhead  10  has a diameter in a range of from 500 mm to 6000 mm, preferably in a range of from 1500 mm to 4500 mm, more preferably from 1750 mm to 2250 mm, most preferably of 2000 mm. 
     According to  FIG. 2 , the pressure wall  14  has a core layer  18 , which is enclosed on one side by a first covering layer  16  and on the opposite side by a second covering layer  17 . The first covering layer and the second covering layer  16 ,  17  have an extent in a direction away from the core layer  18 , i.e. a thickness, in a range of from 0.5 mm to 6 mm, preferably from 1 mm to 3 mm, more preferably a thickness of 2 mm. 
     Here, the first covering layer  16 , the core layer  18  and the second covering layer  17  form a sandwich structure, wherein the first covering layer  16  is connected to a first surface  13  of the core layer  18 . The second covering layer  17  is connected to a second surface  11 , which is parallel to the first surface  13 . In this case, the first surface  13  faces away from the second surface  11 . The spacing between the first and the second surface  11 ,  13  can be in a range of from 50 mm to 200 mm, preferably from 75 mm to 150 mm, more preferably from 90 to 100 mm, most preferably a spacing of 96 mm. 
     The core layer  18  furthermore has an auxetic foam, wherein the auxetic foam has a Poisson ratio in a range of from −0.5 to −1, preferably in a range of from −0.85 to −1, more preferably of −1. In this range, the auxetic foam has the most favourable properties for use in a pressure bulkhead. 
     As illustrated in  FIG. 2 , the bending stiffness of the core layer  18  is furthermore increased when it is compressed in direction  24 . 
     During a flight through the troposphere, for example, the pressure bulkhead  10 , which is used to seal a pressurized cabin  32 , as illustrated in  FIG. 5 , is therefore subjected to a pressure difference between the pressure in the pressurized cabin  32  and the pressure of the troposphere. The cabin pressure in the pressurized cabin  32  will therefore exert a force on the pressure wall  14 . As a result, the core layer  18  is compressed, wherein the bending stiffness of the core layer  18  increases. 
     The increase in the bending stiffness has the effect that bending deformation of the pressure wall  14  due to the pressure difference is less than with conventional pressure bulkheads. By virtue of the reduced bending deformation, lower deformation forces occur at the edges of the pressure wall  14  or at the joint between the pressure wall  14  and the frame  12  and the joint with the aerospace craft  28 . At a Poisson ratio of −1, the bending stiffness is increased to such an extent that no bending deformation occurs. 
     The pressure bulkhead  10  can therefore be of flat and thin design, thus maximizing the space in the pressurized cabin  32 . 
     Furthermore, in one illustrative embodiment, at a pressure difference of 1 bar between the two sides of the pressure bulkhead  10 , at a diameter of the pressure bulkhead  10  of 2000 mm and at a Poisson ratio of the auxetic foam of −0.85, a thickness of the core layer  18  of 96 mm and a thickness of a first covering layer  16  and of a second covering layer  17  made from aluminium of 2 mm can be chosen. The total thickness of the pressure wall of the pressure bulkhead is therefore just 100 mm. 
     Furthermore, the bending deformation of the centre  19  of the pressure wall  14  can thereby be reduced by about 50% in comparison with conventional flat pressure bulkheads. Furthermore, the deformation forces at the edges of the pressure wall  14  can be reduced by 50% in comparison with conventional flat pressure bulkheads. By virtue of the increased stiffness, the natural frequency of the pressure bulkhead  10  is also increased by about 40%. Owing to the lower forces and increased natural frequency, the pressure bulkhead  10  is more stable than conventional pressure bulkheads and is subject to less vibration, as a result of which fatigue phenomena of the material are reduced and also less material is required overall. Furthermore, service intervals can thereby be lengthened, and weight is saved. 
     The property of auxetic materials is described in greater detail below with reference to  FIGS. 3 a  and 3 b   . Here,  FIGS. 3 a  and 3 b    show an auxetic open lattice structure  34 . 
     The auxetic open lattice structure  34  has first members  36 , which are connected to one another by second members  38 . In this case, the second members  38  are connected in articulated fashion to the first members  36 . The first members  36  and the second members  38  have a constant length. Furthermore, the second members  38  can be pivoted at the joint with the first members  36 . That is to say that the angle between the second members  38  and the first members  36  can be changed. 
     The first members  36  form a plurality of rows  33 ,  35 . In this arrangement, the first members  36 , which form a row  33 , are each connected to one another by two second members  38 . The first members  36  which are arranged in different adjacent rows  33  and  35  are each connected to one another by a single second member  38 . According to  FIG. 3 a   , the rows  33 ,  35  of the first members  36  overlap in this arrangement, with the result that the second members  38  enclose a small angle with the first members  36  connected to them. 
     According to  FIG. 3 b   , the auxetic open lattice structure  34  is pulled apart in directions  44  and  45 . As a result, the auxetic open lattice structure  34  unfolds, wherein the angle between the second members  38  and the first members  36  increases. This has the effect that the auxetic open lattice structure  34  expands in directions  41  and  42 . 
     Conversely, this means that the auxetic open lattice structure  34  acts in the direction opposite to directions  44  and  45 , with the result that the angles between the first members  36  and the second members  38  are reduced. As a result, the overlap between the rows  33 ,  35  of the first members  36  is increased. This has the effect that the auxetic open lattice structure  34  contracts in the directions opposite to directions  41  and  42 . 
     The two effects described above are likewise achieved if the forces are applied with an offset of 90°, i.e. in directions  41 ,  42 . It is furthermore sufficient if one component of a force acts in these directions. 
     A two-dimensional auxetic open lattice structure is described in each of  FIGS. 3 a  and 3 b   . The effect described above is similar for three-dimensional auxetic open lattice structures. 
     An intergranular auxetic structure  46  is illustrated in  FIGS. 4 a    and  4   b.    
     The intergranular auxetic structure  46  comprises granules  48  which are connected to one another at the corners or edges  50  thereof. At the same time, the connection between the granules  48  is of articulated design, i.e. the granules  48  can be pivoted relative to one another about the corners or edges  50 . Furthermore, the granules  48  in this example are designed to have a rigid shape, and therefore their shape is not changed by a force on the intergranular auxetic structure  46 . 
     In  FIGS. 4 a  and 4 b   , the granules  48  are illustrated two-dimensionally as squares. In this case, the granules  48  are connected to one another in an articulated fashion via the corners  50 . However, this does not exclude the possibility of making the granules  48  three-dimensional and connecting them to one another via additional corners or edges  50 . Furthermore, the granules  48  merely stand symbolically for any granular structure of a foam, and therefore the individual elements of the foam are not restricted to square structures. 
     In  FIG. 4 a   , the granules  48  are pivoted by only a small angle relative to the horizontal or to the vertical. In this case, the angles  60  and  62  between the granules  48  are smaller than 90°. Furthermore, the angles  64  and  66  are greater than 90°. The spaces between the granules  48  are therefore of diamond-shaped configuration. 
     In  FIG. 4 b   , forces are exerted on the intergranular structure  46  in directions  52  and  54 . By virtue of the connection at the corners or edges  50  of the granules  48 , these forces bring about a further rotation of the granules  48 . This increases the angles  60  and  62  between the granules  48  to an angle of 90°. Furthermore, the angles  64  and  66  decrease to 90°. The spaces between the granules  48  are now of square configuration. 
     Owing to the rotation of the granules  48 , the intergranular auxetic structure  46  is pushed apart in directions  56  and  58 . If, therefore, the granules  48  are moved away from one another in a direction  52  and/or  54 , this has the effect of moving the granules  48  apart in direction  56  and/or  58  transversely thereto. 
     The same effect is therefore achieved if the forces act on the granules  48  in directions  56  and  58 . In this case, forces act in such a way that the granules  48  also move away from one another in directions  52  and  54 . 
     Starting from  FIG. 4 a   , forces which act in the directions opposite to directions  52  and  54  bring about a reduction in the angles  60  and  62  and an increase in the angles  66  and  64 . As a result, the spaces between the granules  48  are elongated, and the granules  48  are thereby pushed closer together. This likewise brings about a reduction in the extent of the intergranular auxetic structure  46  in directions  56  and  58 . 
     In  FIG. 5 , an aircraft  28  is illustrated as an example of an aerospace craft. In this case, there is no intention to exclude the possibility that the aerospace craft can also be a spaceship, e.g. a space shuttle or a space capsule. 
     The aircraft  28  has a pressurized cabin  30 . The pressurized cabin  30  serves to accommodate passengers and crew. Furthermore, the pressurized cabin  30  serves to maintain a cabin pressure corresponding to atmospheric pressure at the surface of the earth in an environment which has a lower atmospheric pressure. This can be in the troposphere or even in interstellar space, for example. 
     The pressurized cabin  30  furthermore has a rear section  32 , which is arranged at the rear of the aircraft  28 . The pressurized cabin  30  is sealed off at this point by the pressure bulkhead  10 . That is to say that the pressure bulkhead  10  maintains the cabin pressure prevailing in the pressurized cabin  30  relative to the environment. The pressure bulkhead  10  is therefore connected airtightly to the pressurized cabin  30  and likewise seals off the pressurized cabin  30  airtightly relative to the environment. 
     In this arrangement, the pressure bulkhead  10  forms a rear wall of the pressurized cabin  30 . Here, the first and the second covering layer  16 ,  17  of the pressure bulkhead  10  are of virtually flat construction when not subject to any external forces. Therefore, the pressure bulkhead  10  takes up only a small amount of space in the pressurized cabin  30 , thus enabling a maximum amount of space to be made available to the passengers and the crew in the pressurized cabin  30 . This space can be used to provide additional passenger seats or additional stowage space for freight or other objects. 
     When the cabin pressure in the pressurized cabin  30  is higher than in the environment of the aircraft  28 , the pressure wall  14  of the pressure bulkhead  10  arches in the direction of the rear of the aircraft  28 . In this case, the centre  19  of the pressure wall  14  of the pressure bulkhead  10  is deflected to the greatest extent from the overall structure of the pressure bulkhead  10 . In this case, the forces which arise at the edges of the pressure bulkhead  10  also act on the frame structure of the aircraft  28 . 
     The core layer  18 , which comprises an auxetic foam, keeps the bending deformation of the centre  19  of the pressure wall  14  small, and it is therefore likewise possible to keep the forces at the edges of the pressure bulkhead  10  and hence also on the frame structure of the aircraft  28  small. As a result, the overall structure of the aircraft  28  is subjected to weaker forces, and therefore fatigue of the materials of the frame structure of the aircraft  28  and of the pressure bulkhead  10  occurs later than when using a conventional pressure bulkhead  10 . 
     In another embodiment (not shown), the first covering layer  16  and the second covering layer  17  together form a lens shape when they enclose the core layer  18 . In this embodiment, the first and the second surface  11 ,  13  of the core layer  18  can be of convex design. In this form, the pressure wall  10  has a thickened shape in the centre  19 , and therefore the pressure wall  10  is of more stable design at this point by virtue of the increased thickness than at the edge thereof. 
     While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.