A. The second end 512 is connected to the stringer 45 attached to the outer surface 4A. The connection position between the second end 512 and the stringer 45 in the front-rear direction of the inner tank 4 is a position where the center frame 43 is disposed in the inner tank cylinder 41.

The second strap 52 includes a main body portion similar to the first strap 51, a third end 521 attached to one end of the main body portion, and a fourth end 522 attached to the other end. The third end 521 on the rear end side of the second strap 52 is connected to the inner surface 3A of the outer tank 3. On the other hand, the fourth end 522 on the front end side of the second strap 52 is connected to the outer surface 4A of the inner tank 4. Specifically, the third end 521 is connected to the internal reinforcing frame 33 disposed at the rear end of the outer tank cylinder 31. The fourth end 522 is connected to the stringer 45 attached to the outer surface 4A. The connection position between the fourth end 522 and the stringer 45 in the front-rear direction of the inner tank 4 is a position where the center frame 43 is disposed in the inner tank cylinder 41.

In the first strap 51 and the second strap 52, the first end 511 and the third end 521 connected to the outer tank 3 are separated to an extent substantially corresponding to the front-back width of the outer tank cylinder 31. On the other hand, in the first strap 51 and the second strap 52, the second end 512 and the fourth end 522 connected to the inner tank 4 are in a positional relationship of being adjacent to each other in the front-rear direction. As a result, the first strap 51 and the second strap 52 are disposed in the vacuum layer VA in a posture having a V-shape with an inner angle being obtuse in a cross-sectional view of the hydrogen storage tank 2 in the front-rear direction. Further, the body portions of the first strap 51 and the second strap 52, and the first end 511, the second end 512, the third end 521, and the fourth end 522 are arranged in one cross section illustrated in FIG. 4.

Both the first end 511 and the third end 521 are connected to an internal reinforcing frame 33 which is a reinforcing member in the circumferential direction of the outer tank 3. Further, the second end 512 and the fourth end 522 are connected to the stringer 45 which is a reinforcing member in the longitudinal direction of the inner tank 4. That is, the first strap 51 and the second strap 52 transfers a load to the reinforcing member rather than the wall surface members of the outer tank 3 and the inner tank 4. Therefore, the inner tank 4 can be stably supported through the first strap 51 and the second strap 52. Further, it is possible to reduce the wall thickness of the wall surface member and reduce the weight thereof.

Note that, of the internal reinforcing frame 33 and the external reinforcing frame 34 that reinforce the outer tank 3, only a radial frame piece of the internal reinforcing frame 33 to which the first end 511 and the third end 521 are connected protrudes to the vacuum layer VA. Further, among the center frame 43, the end frame 44, and the stringer 45 that reinforce the inner tank 4, only the stringer 45 to which the second end 512 and the fourth end 522 are connected protrudes to the vacuum layer VA. In this manner, only the minimum necessary members for attaching the first end 511, the second end 512, the third end 521, and the fourth end 522 protrude to the vacuum layer VA. By this measure, outgas that can be released from the reinforcing material is suppressed, and a decrease in the degree of vacuum of the vacuum layer VA is suppressed. By reducing the surface area of the wall surface defining the vacuum layer VA as much as possible, the amount of outgas released from the wall surface can be suppressed.

With reference to FIGS. 5A to 5C, the attachment structure of the second end 512 and the fourth end 522 of the first strap 51 and the second strap 52 to the inner tank 4 will be described in detail. FIG. 5A is a perspective view illustrating an attachment portion of strap ends to the inner tank 4, FIG. 5B is a side view thereof, and FIG. 5C is a cross-sectional view taken along line VC-VC of FIG. 5B. A first holder 513 is attached to the second end 512 of the first strap 51. A second holder 523 is attached to the fourth end 522 of the second strap 52. Since the first holder 513 and the second holder 523 are the same members, the first holder 513 will be described here.

The second end 512 is thicker than the main body portion 510 of the first strap 51. The first holder 513 includes a clamping portion 514 and a pivoting portion 515. The clamping portion 514 clamps and holds the second end 512. The pivoting portion 515 is pivotally coupled to the stringer 45. The stringer 45 has a seat 451 protruding radially outward from the other portions at a center position in the front-rear direction. The position of the seat 451 is also the position where the stringer 45 intersects the center frame 43. The seat 451 protrudes radially outward from the other portion of the stringer 45. As illustrated in FIG. 5C, the stringer 45 includes a pair of left and right L-shaped frames.

A U-shaped support bracket 453 is held between the pair of L-shaped frames. A mounting bracket 452 is attached to a circumferentially outside of the inner tank 4 in the support bracket 453. The mounting bracket 452 includes an upright piece. The upright piece sandwiches the pivoting portion 515 of the first holder 513. A bolt hole is formed in the upright piece. A bolt hole is also formed in the pivoting portion 515. A bolt 45B passes through the bolt holes of the mounting bracket 452 and the pivoting portion 515 and is fastened. By attaching the first holder 513 to the seat 451 of the stringer 45 in this mode, the first strap 51 can pivot about the axis of the bolt 45B.

The second holder 523 is also attached to the seat 451 via the bolt 45B. Therefore, the second strap 52 can also pivot about the axis of the bolt 45B. The first end 511 and the third end 521 are also pivotally connected to the internal reinforcing frame 33 of the outer tank 3 via members similar to the first holder 513 and the bolt 45B. As described above, the first end 511 and the third end 521 are pivotally connected to the inner surface 3A of the outer tank 3, and the second end 512 and the fourth end 522 are pivotally connected to the outer surface 4A of the inner tank 4. As a result, the first strap 51 and the second strap 52 do not transfer a bending load. Therefore, it is possible to prevent a load that is not expected in design from being applied to the first strap 51 and the second strap 52 due to relative deformation between the inner tank 4 and the outer tank 3.

FIG. 6 is a cross-sectional view taken along the left-right direction of the hydrogen storage tank 2. Four rods 6 extend in the circumferential direction of the side peripheral surface of the inner tank 4. Each rod 6 connects the outer surface 4A, which is the side peripheral surface of the inner tank 4, and the inner surface 3A of the outer tank 3. Specifically, each rod 6 is disposed in the vacuum layer VA so as to extend in a tangential direction of the side peripheral surface of the inner tank cylinder 41 which is a cylindrical surface. Strictly speaking, since each rod 6 extends in the tangential direction, each rod 6 has a slight inclination with respect to the circumferential direction, but in the present specification, even such an extending mode is regarded as “extending in the circumferential direction”.

The four rods 6 are arranged on the upper, lower, left, and right sides of the inner tank 4 at intervals of 90 degrees in the circumferential direction of the inner tank cylinder 41. Among the four rods 6, two rods 6 arranged on the left and right of the inner tank 4 are defined as vertical rods 6A. The vertical rod 6A transfers an up-down direction load between the inner tank 4 and the outer tank 3. On the other hand, the two rods 6 disposed above and below the inner tank 4 are defined as lateral rods 6B. The lateral rod 6B transfers a left-right direction load between the inner tank 4 and the outer tank 3.

The four rods 6 have the same configuration and are attached to the outer tank 3 and the inner tank 4 in the same manner. The rods 6 each include a rod main body, a fifth end 61 on one end side of the rod main body, and a sixth end 62 on the other end side of the rod main body. The rod main body is a hollow rod having a circular cross section. It is desirable that the rod main body is formed by a material having low density and low thermal conductivity such as GFRP from the viewpoint of weight reduction and suppression of heat applied to the inner tank 4. When importance is placed on factors other than weight reduction and heat applied, the rod main body may be formed by a metal material such as stainless steel as a solid body.

The fifth end 61 is connected to the outer tank 3 via a mounting bracket 35. The sixth end 62 is connected to the inner tank 4 via a bracket 46. The mounting bracket 35 is fixed to the internal reinforcing frame 33 protruding from the inner surface 3A of the outer tank 3. In the present embodiment, the fifth end 61 of the rod 6 and the first end 511 of the first strap 51 or the third end 521 of the second strap 52 are connected to the internal reinforcing frame 33. The bracket 46 protrudes from the outer surface 4A at the arrangement position of the end frame 44 in the inner tank cylinder 41.

The fifth end 61 is connected to the mounting bracket 35 by pin connection using a bolt. Similarly, the sixth end 62 is connected to the bracket 46 by pin connection using a bolt. The pin connecting the fifth end 61 is disposed in a direction along an upper, lower, left, and right cross section of the inner tank 4 illustrated in FIG. 6. On the other hand, the pin connecting the sixth end 62 is disposed in a direction along a front and rear cross section of the inner tank 4 illustrated in FIG. 4. That is, the extending direction of a pin shaft of the fifth end 61 serving as a pivot is orthogonal to the extending direction of a pin shaft of the sixth end 62. In this manner, the fifth end 61 of the rod 6 is pivotally connected to the inner surface 3 A of the outer tank 3, and the sixth end 62 is pivotally connected to the outer surface 4A of the inner tank 4. As a result, the rod 6 does not subjected to a bending load and transfers only a tensile load or a compressive load. Therefore, it is possible to prevent a load that is not expected in design from being applied to the rod 6 due to relative deformation between the inner tank 4 and the outer tank 3.

[Behaviors of Support Structure of Inner Tank]

Behaviors of the support structure SU of the inner tank 4 during the operation of the hydrogen aircraft 1 will be described. FIG. 7A is a schematic view of a hydrogen storage tank 2A with a support mode of the inner tank 4 by the first strap 51 and the second strap 52 to which the idea of the present disclosure is applied. In the hydrogen storage tank 2A, the inner tank 4 is supported by the first strap 51 and the second strap 52. In the front-rear direction, which is the longitudinal direction of the inner tank 4, a distance between the first end 511 of the first strap 51 and the third end 521 of the second strap 52 connected to the outer tank 3 is defined as D1. Further, a distance between the second end 512 of the first strap 51 and the fourth end 522 of the second strap 52 connected to the inner tank 4 is defined as D2. In the present embodiment, D2 is set to be shorter than D1. That is, D1>D2 holds.

FIG. 7B is a schematic view of a hydrogen storage tank 2B having another support mode of the inner tank 4 by the first strap 51 and the second strap 52 according to the present embodiment. As illustrated in FIG. 4, the hydrogen storage tank 2B has a configuration in which the second end 512 and the fourth end 522 are adjacent to each other. That is, D2≈0 in this support mode.

Thermal expansion and thermal contraction may occur in the inner tank 4 due to a change in the storage amount of liquid hydrogen or the like. On the other hand, since the outer tank 3 is not in contact with liquid hydrogen, thermal expansion and thermal contraction of the outer tank 3 are smaller than those of the inner tank 4. Therefore, relative deformation may occur in which the size of the inner tank 4 greatly changes with respect to the outer tank 3. Further, the relative deformation may also occur due to pressure factors such as an external pressure of the hydrogen storage tank 2 and pressures of the inner tank 4 and the vacuum layer VA. Depending on the connection position of the first strap 51 and the second strap 52 with respect to the inner tank 4, the effect of expansion and contraction of the inner tank 4 may be added to the first strap 51 and the second strap 52.

FIG. 7C is a schematic view of a hydrogen storage tank 200 having a support mode of the inner tank by straps according to a comparative example. In the hydrogen storage tank 200, the distance D1≈0 between the first end 511 and the third end 521 connected to the outer tank 3. On the other hand, in the hydrogen storage tank 200, the distance D2 between the second end 512 and the fourth end 522 connected to the inner tank 4 has a considerable length in the longitudinal direction. In this case, the variation amount of D2 when the inner tank 4 expands and contracts increases. Therefore, an unexpected load is applied to the first strap 51 and the second strap 52, and damage such as buckling may occur in the first strap 51 and the second strap 52.

On the other hand, in the present embodiment, D1>D2 or D2≈0 is satisfied. That is, in the present embodiment, the second end 512 and the fourth end 522 are disposed at a relatively short distance. That is, the distance between two supporting points of the inner tank 4 is set shorter than that in the comparative example. Therefore, the variation amount of D2 when the inner tank 4 expands and contracts becomes small. Therefore, it is possible to suppress the influence of thermal expansion and contraction behaviors of the inner tank 4 on the support structure SU of the inner tank 4 including the first strap 51 and the second strap 52. In particular, in the support mode with D2≈0 illustrated in FIG. 7B, the influence of expansion and contraction on the inner tank 4 substantially does not reach the first strap 51 and the second strap 52. Note that, in the present embodiment, the case where the first strap 51 and the second strap 52 have the same length is exemplified, but the first strap 51 and the second strap 52 may have different lengths as long as the relationship of D1>D2 or D2≈0 is satisfied.

FIG. 8 is a cross-sectional view of the hydrogen storage tank 2 in the front-rear direction, illustrating a load bearing state of the first strap 51 and the second strap 52. The first strap 51 and the second strap 52 are arranged in an obtuse V-shaped posture with the center in the longitudinal direction of the inner tank 4 as a supporting point. Moreover, the main bodies of the first strap 51 and the second strap 52, and the first end 511, the second end 512, the third end 521, and the fourth end 522 are disposed in one plane.

When an inertia load that causes the inner tank 4 to move forward is generated as indicated by an arrow in FIG. 8, the second strap 52 positioned on the rear side transfers the load to the outer tank 3 as a tensile load. At this time, the first strap 51 on the front side is bent. On the other hand, when an inertia load that causes the inner tank 4 to move backward is generated, the first strap 51 positioned on the front side transfers the load to the outer tank 3 as a tensile load. At this time, the second strap 52 on the rear side is bent.

In this manner, the first strap 51 and the second strap 52 transfer the load of the inner tank 4 only by the in-plane tensile load. Thus, design consideration for buckling of the first strap 51 and the second strap 52 becomes unnecessary. Therefore, an elongated shape having a relatively small cross-sectional area can be selected as the first strap 51 and the second strap 52. In addition, as a material of the first strap 51 and the second strap 52, a strap having low density and low thermal conductivity such as GFRP can be employed instead of an expensive high-strength metal material. This also contributes to suppression of heat applied into the inner tank 4 through the first strap 51 and the second strap 52 and weight reduction of the support structure SU of the inner tank 4. Furthermore, since the first end 511, the second end 512, the third end 521, and the fourth end 522 are pivotally connected to the inner surface 3A of the outer tank 3 or the outer surface 4A of the inner tank 4, there is also an advantage that a bending load can be prevented from being applied to the first strap 51 and the second strap 52 due to relative deformation between the inner tank 4 and the outer tank 3.

FIG. 9 is a cross-sectional view of the hydrogen storage tank 2 in the left-right direction, illustrating a load bearing state of the rod 6. Among the four rods 6, the pair of vertical rods 6A arranged on the left and right of the inner tank 4 transfers an up-down load applied to the inner tank 4 to the outer tank 3. The pair of lateral rods 6B disposed above and below the inner tank 4 transfers a left-right load applied to the inner tank 4 to the outer tank 3. In general, the up-down load and the left-right load applied to the inner tank 4 by inertia are smaller than the front-rear load borne by the first strap 51 and the second strap 52. Therefore, in the present embodiment, the vertical rods 6A and the lateral rods 6B are arranged so as to transfer a compressive load in addition to a tensile load.

When a downward load is applied to the inner tank 4 as indicated by an arrow in FIG. 9, the vertical rods 6A transfer a tensile load to the outer tank 3. Further, when a right direction load is applied to the inner tank 4, the lateral rods 6B transfer a tensile load to the outer tank 3. Conversely, when loads in the upward direction and the leftward direction are applied to the inner tank 4, the vertical rods 6A and the lateral rods 6B transfer respective compressive loads to the outer tank 3. In view of such a load bearing state, it is desirable to use a member having a circular cross section that is resistant to buckling for the vertical rods 6A and the lateral rods 6B. In particular, when a hollow rod of GFRP is used for the vertical rods 6A and the lateral rods 6B, it is possible to contribute to suppression of heat input to the inner tank 4 through the vertical rods 6A and the lateral rods 6B and weight reduction of the support structure SU of the inner tank 4.

The vertical rods 6A and the lateral rods 6B extend in the tangential direction of the side peripheral surface of the inner tank cylinder 41 which is a cylindrical surface. Therefore, the structure constituting the wall surface of the inner tank 4 can easily receive the load transferred from the vertical rods 6A and the lateral rods 6B as a load in an in-plane direction. That is, since a load is not applied to the inner tank 4 in a direction intersecting the extending direction of the wall surface, a load that recesses the wall surface is hardly applied to the inner tank 4. Further, since the fifth end 61 and the sixth end 62 of 6A and 6B are pivotable, it is possible to prevent a bending load from being applied to the vertical rods 6A and the lateral rods 6B due to relative deformation between the inner tank 4 and the outer tank 3.

[Modification of Strap Support Structure]

FIG. 10A is a perspective view illustrating a first modification of the attachment portion of the end of the strap 5 to the inner tank 4. FIG. 10B is a side view of the attachment portion of FIG. 10A. In the first modification, an example in which the connection destination of the second end 512 and the fourth end 522 is a center frame 43A of the inner tank cylinder 41 will be described.

The center frame 43A includes a protruding piece 431 projecting radially outward from the outer surface of the center frame 43A. The protruding piece 431 is a plate-like projection formed integrally with the center frame 43A. Two bolt holes are drilled in the protruding piece 431 in the circumferential direction of the inner tank 4. Bolts 45B are respectively inserted into the two bolt holes and the bolt holes of the first holder 513 of the second end 512 and the second holder 523 of the fourth end 522. The first strap 51 and the second strap 52 are pivotable about the axis of the bolt 45B pivotally supported by the protruding piece 431.

FIG. 11A is a perspective view illustrating a second modification of the attachment portion of the end of the strap 5 to the inner tank 4. FIG. 11B is a side view of the attachment portion of FIG. 11A. In the second modification, an example will be described in which a fitting 47, which is a member separate from the reinforcing member of the inner tank cylinder 41 such as the stringer 45 and the center frame 43A, is used as the connection destination of the second end 512 and the fourth end 522.

The fitting 47 includes a seat 471 and a support plate 472. The seat 471 is a plate elongated in the front-rear direction. The seat 471 is welded along the outer surface of the inner tank cylinder 41. The support plate 472 protrudes radially outward of the inner tank 4 from the seat 471. Two bolt holes are drilled in the support plate 472 in the circumferential direction of the inner tank 4. Bolts 45B are respectively inserted into the two bolt holes and the bolt holes of the first holder 513 of the second end 512 and the second holder 523 of the fourth end 522. The first strap 51 and the second strap 52 are pivotable about the axis of the bolt 45B pivotally supported by the support plate 472.

[Heat Insulating Structure Between Inner Tank and Outer Tank]

FIG. 12 is a schematic view illustrating a heat insulating structure 7 disposed between the outer tank 3 and the inner tank 4. A vacuum layer VA is interposed between the inner surface 3A of the outer tank 3 and the outer surface 4A of the inner tank 4. Vacuum heat insulation provided by the vacuum layer VA suppresses heat conduction and convective heat transfer between the outer tank 3 and the inner tank 4. However, the vacuum layer VA alone cannot block radiant heat from the outer tank 3 to the inner tank 4. Further, it should be assumed that a situation occurs where the degree of vacuum of the vacuum layer VA is lowered for some reason. In view of these points, in the present embodiment, the heat insulating structure 7 is provided.

The heat insulating structure 7 includes a laminated heat insulating sheet layer 71, a vapor barrier layer 72, and a backup heat insulating layer 73. The laminated heat insulating sheet layer 71 is disposed so as to be separated from inner surface 3A of outer tank 3 by vacuum layer VA. The laminated heat insulating sheet layer 71 is formed by, for example, a multi-layer insulation (MLI) sheet. The MLI sheet formed by laminating a plastic film formed by depositing a metal such as aluminum and a spacer such as a glass fiber cloth in a multiple manner. The laminated heat insulating sheet layer 71 shields radiant heat from the outer tank 3 to the inner tank 4.

The vapor barrier layer 72 is disposed between the outer surface 4A of the inner tank 4 and the laminated heat insulating sheet layer 71. The backup heat insulating layer 73 is disposed between the vapor barrier layer 72 and the outer surface 4A of the inner tank 4. As the vapor barrier layer 72, for example, a laminate of a resin film and a metal sheet can be used. The backup heat insulating layer 73 is formed using, for example, a closed cell type foam material. The backup heat insulating layer 73 is provided so as to cover the outer surface 4A of the inner tank 4 including protrusions such as the stringer 45.

The laminated heat insulating sheet layer 71 functions exclusively as a heat insulating layer for radiant heat in a state where the degree of vacuum of the vacuum layer VA is maintained. However, for example, when air flows between the inner surface 3A of the outer tank 3 and the outer surface 4A of the inner tank 4 and impairs the vacuum layer VA, heat insulation performance is impaired. This is because the air serves as a heat carrier to transfer the heat retained in the outer tank 3 to the laminated heat insulating sheet layer 71. When such an event occurs, the backup heat insulating layer 73 exerts a heat insulating effect and protects the inner tank 4. The thickness of the backup heat insulating layer 73 is set to a thickness capable of suppressing generation of liquefied air on the outer surface 4A of the inner tank 4 and excessive boil-off of stored liquid hydrogen even when air flows into the vacuum layer VA.

The vapor barrier layer 72 suppresses penetration of outgas from the backup heat insulating layer 73 into the vacuum layer VA during normal operation. On the other hand, when the vacuum layer VA is damaged, the vapor barrier layer 72 suppresses inflow of air into the backup heat insulating layer 73. When any member is disposed in a vacuum environment, a gas adsorbed on the surface of the member, or a gas accumulated in the member flows out as an outgas. When the outgas enters the vacuum layer VA, the degree of vacuum of the vacuum layer VA decreases. The backup heat insulating layer 73 may allow outgas to flow out. The vapor barrier layer 72 blocks such outgas. Further, for example, when air that has entered between the inner surface 3A of the outer tank 3 and the outer surface 4A of the inner tank 4 comes into contact with the backup heat insulating layer 73, there is a possibility that the backup heat insulating layer 73 holds air therein. The vapor barrier layer 72 blocks contact between the backup heat insulating layer 73 and air.

By providing the heat insulating structure 7 as described above, the heat insulating property between the tanks can be secured by the vacuum layer VA and the laminated heat insulating sheet layer 71 during normal operation of the hydrogen storage tank 2. On the other hand, even if a situation occurs where the degree of vacuum of the vacuum layer VA drops for some reason, the backup heat insulating layer 73 is present, so that the heat insulating property can be maintained. Further, by interposing the vapor barrier layer 72, it is possible to prevent deterioration of the vacuum layer VA due to outgas and to protect the backup heat insulating layer 73.

[Divided Structure of Outer Tank]