Cryogenic tank for an aircraft with hydraulic actuators

Abstract

The invention relates to a vehicle, in particular an aircraft or a space vehicle for a subsonic, supersonic or hypersonic flight. The vehicle comprises an airframe (1), a fuel tank (3) and hydraulic actuators (6). The fuel tank (3) is attached to the airframe (1) via the hydraulic actuators (6) and the hydraulic actuators (6) are adapted to transfer forces between the airframe (1) and the fuel tank (3).

Description

[0001] Cryogenic tank for an aircra t with hydraulic actuators

[0002] Technical Field

[0003] The invention relates to a vehicle , in particular an aircraft or a space vehicle for a subsonic, supersonic or hypersonic flight . The vehicle comprises an airframe , a fuel tank and hydraulic actuators .

[0004] Background Art

[0005] Liquid hydrogen can be used as fuel for operating aircrafts , in particular space vehicles . To exist as a liquid, hydrogen must be cooled below its critical point of 33 Kelvin . Usually, liquid hydrogen cools the noz zle and other parts of the aircraft before being mixed with the oxidi zer - usually liquid oxygen - to be burned .

[0006] Liquid hydrogen requires cryogenic storage technology such as special thermally insulated containers and requires special handling common to all cryogenic fuels . Even with thermally insulated containers it is di f ficult to keep such a low temperature , and the hydrogen will gradually leak away .

[0007] Disclosure of the Invention

[0008] The problem to be solved by the present invention is to provide a vehicle , in particular an aircraft , with a tank for cryogenic fuel stabili zing the aircraft structure in an ef fective way .

[0009] The problem is solved by the subj ect of the independent claim . According to this , a vehicle , in particular an aircraft or a space vehicle for a subsonic, supersonic or hypersonic flight , comprises an airframe , a fuel tank and hydraulic actuators . The fuel tank is attached to the airframe via the hydraulic actuators . The hydraulic actuators are adapted to trans fer forces between the airframe and the fuel tank . In particular, the hydraulic actuators are pistons in cylinders .

[0010] The fuel tank can be filled with cryogenic fuel , in particular with liquid hydrogen, methanol , methane or ammonia . Furthermore , the fuel tank can be made of di f ferent materials such as titanium, aluminium, stainless steel alloys or composites .

[0011] This setting has the advantage that not only the airframe , but also the fuel tank carries structural loads of the vehicle . Using hydraulic actuators for trans ferring forces between the airframe and the fuel tank allows to compensate thermal expansion or thermal shrinking of the fuel tank due to temperature variation . In particular in case of cryogenic fuel , the fuel tank will be exposed di f ferent thermal conditions depending on the filling level of the fuel tank . The hydraulic actuators can provide a rigid connection under di f ferent thermal conditions to transmit force loads .

[0012] Advantageously, the vehicle comprises a control system for controlling the hydraulic actuators , adapted such that the hydraulic actuators can be controlled in a constant force mode and / or in a constant position mode . I f the hydraulic actuators can be controlled in both the constant force mode and the constant position mode , the control system selects one of both modes based on di f ferent operations and mission phases .

[0013] In particular, the hydraulic actuators are controlled in constant force mode during ground operations of filling or emptying the fuel tank with cryogenic fuel . As the fuel tank is fuelled up before flight , the fuel tank shrinks due to a temperature decrease of 200- 300 Kelvin . The oil pressure inside the hydraulic actuators is controlled to provide a constant force acting on the airframe and the fuel tank . This makes the length of the hydraulic actuators adapt to the length of the fuel tank while maintaining constant forces on the fuel tank . In constant force mode , hydraulic actuators extend as the fuel tank shrinks , maintaining constant compressive force on the fuel tank and airframe structure .

[0014] As the fuel tank is emptied on ground, the fuel tank expands due to a temperature increase i f the fuel tank is emptied with cryogenic fuel . In constant force mode , the hydraulic actuators retract as the fuel tank expands , maintaining constant compressive force on the fuel tank and airframe structure .

[0015] Advantageously, the hydraulic actuators are controlled in constant position mode during flight . During flight , the dimensions of the fuel tank are approximately constant , as far as its cryogenic temperature is maintained . However, aerodynamic loads on the aircraft are producing variable structural loads which have to be transmitted along the length of the aircraft . These include axial compression, longitudinal bending moments , torsion, and "noisy" loads like engine vibrations and flutter . Transmitting these loads is best done with a rigid structure . The hydraulic actuators can be controlled to behave as such, by having them maintain a fixed position . This requires hydraulic actuator position feedback control to control the oil pressure inside the hydraulic actuators . Making the actuators maintain their position precisely, transmits axial loads between airframe and fuel tank through the hydraulic actuators as a rigid structure .

[0016] In particular, the control system is adapted to change the constant position of the hydraulic actuator during the constant position mode from constant position A to constant position B . Advantageously, the control is adapted to change the constant position dependent on a measured force trans ferred via the hydraulic actuator or dependent on a measured temperature of the fuel tank .

[0017] The fuel tank may be warming up slightly during flight , e . g . , i f the fuel tank is getting close to empty . This expansion is compensated by deliberate changing of the constant position of the hydraulic actuator . The required position change can be based on tank temperature measurements , or more elegantly by the monitoring of force through the hydraulic actuator . Fuel tank expansion is observable as a global force increase on the hydraulic actuators , superposed on the aerodynamic loads being transmitted through the airframe and fuel tank in flight operation .

[0018] In an advantageous embodiment , the control system comprises a first pump adapted to feed the hydraulic actuators during shrinkage of the fuel tank in constant force mode , and / or a pressure relief valve to drain liquid from the hydraulic actuators during expansion of the fuel tank in constant force mode .

[0019] In particular, the control system comprises an auxiliary hydraulic circuit with an auxiliary actuator . A position change of one of the hydraulic actuators during constant position mode generates a pressure change , in particular a pressure drop, in the auxiliary hydraulic circuit displacing the auxiliary actuator such that the displacement of the auxiliary actuator activates a second pump feeding the hydraulic actuator . Such a system provides a high level of reliability . However, backup system can be foreseen to mitigate possible failure risks .

[0020] Advantageously, a thermal insulation, in particular foam or fibrous material , is arranged between the airframe and the fuel tank . In particular the thermal insulation is compressed between the fuel tank and the airframe . The insulation can be a combination of closed and open cell foam, such that part of the foam is rigid and part of the foam is flexible . Common cryogenic insulator materials are PU or EPS . The flexible foam is compressed between the tank and the fuselage skin . This arrangement supports the tank radially .

[0021] In particular, the thermal insulation comprises a plurality of deformable elements , in particular stringers or corrugated sheets , supporting and / or sti f fening the structure of the thermal insulation . Such elements support the force trans fer between airframe and fuel tank .

[0022] In an advantageous embodiment , the fuel tank has an axially symmetrical shape , in particular a cylindrical shape . Such an elongated form fits into a fuselage of an aircraft and has a very small surf ace / volume ratio in order to reduce thermal trans fer into and out of the fuel tank .

[0023] In particular, the thermal insulation has an annular shape for supporting the fuel tank radially . The thermal insulation surrounds the axially symmetrical fuel tank .

[0024] In particular, hydraulic actuators are arranged to trans fer forces between the airframe and the fuel tank in axial , radial and tangential direction with respect to the cylindrical shape of the fuel tank .

[0025] In case of a fuel tank with an elongated, cylindrical shape , the hydraulic actuators are preferably arranged to trans fer forces between the airframe and the fuel tank in axial direction with respect to the elongated shape of the fuel tank . The expansion in axial direction is the biggest why hydraulic actuators are preferably used to compensate the axial expansion of the fuel tank .

[0026] Advantageously, heat insulating pads , in particular ceramic pads , are arranged between the hydraulic actuators and the fuel tank for reducing heat trans fer between the fuel tank and the hydraulic actuators . This is an additional measure to reduce thermal trans fer between the airframe and the fuel tank .

[0027] In particular, the fuel tank is arranged inside the fuselage . Forces can be trans ferred between the fuselage and the fuel tank via the hydraulic actuators .

[0028] Other advantageous embodiments are listed in the dependent claims as well as in the description below . Brief Description of the Drawings

[0029] The invention will be better understood and obj ects other than those set forth above will become apparent from the following detailed description thereof . Such description makes reference to the annexed drawings , wherein :

[0030] Fig . 1 shows an aircraft with an airframe and a fuel tank arranged inside the fuselage ;

[0031] Fig . 2 shows a schematic sketch of the fuel tank arranged inside the fuselage , wherein hydraulic actuators trans fer forces between the fuselage and the fuel tank;

[0032] Fig . 3 shows a schematic sketch of a control system for controlling the hydraulic actuators in constant force mode ;

[0033] Fig . 4 shows a schematic sketch of a first control system for controlling the hydraulic actuators in constant position mode ;

[0034] Fig . 5 shows a second control system for controlling the hydraulic actuators in constant position mode , wherein the second control system is an alternative control system compared to the first control system of Fig . 4 ; and

[0035] Fig . 6 shows a third control system for controlling the hydraulic actuators in constant position mode , wherein the third control system is an alternative control system compared to the first and second control system .

[0036] Modes for Carrying Out the Invention

[0037] Fig . 1 shows an aircraft with an airframe 1 .

[0038] The airframe 1 is the internal load bearing structure . It is a structural assembly, typically made from frames , stringers , spars , ribs , and panels , which are typically machined or formed from sheet metal . The aircraft ' s main body is the fuselage 2 . The fuselage 2 usually contains cargo , passengers , and a control system for controlling the aircraft . In case of the aircraft of Fig . 1 , the fuselage 2 contains the fuel tank 3 . The fuel tank 3 can be made of di f ferent materials such as titanium, aluminium, stainless steel alloys or composites . An exemplary tank could have a filling volume of 150 m3. The aircraft is powered with hydrogen, a cryogenic fuel . Hydrogen is stored in the fuel tank 3 in a liquid state . To exist as liquid hydrogen, it has to be cooled below its critical point of 33 Kelvin . For it to be in a fully liquid state at atmospheric pressure , hydrogen needs to be cooled below 20 Kelvin .

[0039] Fig . 2 shows a schematic view of the fuel tank 3 arranged inside the fuselage 2 . The fuel tank 3 is mainly cylindrical . An annular piece of thermal insulation 4 surrounds the fuel tank 3 . The thermal insulation 4 can be made of foam or a fibrous material , e . g . , PU or EPS . The thermal insulation 4 is sandwiched and slightly compressed between the fuel tank 3 and the inner skin of the fuselage 2 . This arrangement supports the fuel tank 2 radially . The radial support can be enhanced with deformable elements 5 .

[0040] At one longitudinal end of the fuel tank 3 , the fuel tank 3 is connected with the fuselage 2 via movable hydraulic actuators 6 , which are pistons in cylinders . These hydraulic actuators 6 are situated between a load transmitting external frame 7 of the fuel tank 3 and main frames 8 of the fuselage 2 . Ceramic insulating pads 9 are arranged between the hydraulic actuators 6 and the external frame 7 of the fuel tank 3 . These ceramic insulating pads 9 reduce heat bridging and prevent oil freezing inside the hydraulic actuators 6 . The external frame 7 is part of the fuel tank 3 and can expand and shrink . At the other longitudinal end of the fuel tank 3 , a load transmitting external frame 7 of the fuel tank 3 is connected with main frames 8 of the fuselage 2 only with heat insulating ceramic pads 9 . Hydraulic actuators are not arranged at the other longitudinal end of the fuel tank 3 .

[0041] Longitudinal sti f feners can be installed on the inner skin of the fuselage , to ensure the skin "tube" does not buckle . Aerodynamic structural loads are transmitted between the main frames 8 of the fuselage 2 at the one longitudinal end of the fuel tank 3 and the main frames 8 of the fuselage 2 at the other longitudinal end of the fuel tank 3 by using the fuel tank 3 as a load path . Axial loads , due to direct compression, and more importantly due to bending moments acting on the fuselage 2 , are transmitted through the hydraulic actuators 6 .

[0042] The hydraulic actuators 6 are meant to compensate for thermal expansion and shrinking of the fuel tank 3 filled with cryogenic hydrogen . When expanding or shrinking, the fuel tank 3 slides along the thermal insulation 4 . To facilitate this , low friction liners 10 like Teflon are arranged between the fuel tank 3 and the thermal insultation 4 . In addition, the thermal insulation 4 is segmented in parts to avoid developing stresses and cracking .

[0043] Fig . 2 shows only two hydraulic actuators 6 . In an alternative embodiment , four or more hydraulic actuators 6 could be used to trans fer forces between the fuselage 2 and the fuel tank 3 . In this case , the hydraulic actuators 6 could be clocked around the external frame 7 .

[0044] As already described above , the hydraulic actuators 6 can be controlled in a constant force mode and in a constant position mode . Fig . 3 illustrates a possible control system for the constant force mode and Fig . 4 illustrates a possible control system for the constant position mode .

[0045] The hydraulic actuators 6 are operated in constant force mode during filling up or during emptying the fuel tank 3 on ground . While filling up, the fuel tank 3 shrinks due to a temperature decrease of 200-300 Kelvin . The hydraulic actuators 6 are controlled to maintain a constant oil pressure and a constant compressive force on the fuel tank 3 .

[0046] The control system for the constant force mode shown in Fig . 3 is a pure hydromechanical loop . It comprises an accumulator 11 for accumulating the oil , an external hydraulic liquid pump 12 , a pressure switch 13 , a pressure relief valve 14 and a check valve 15 . During shrinkage of the fuel tank 3 , the hydraulic actuators 6 expand and the external hydraulic liquid pump 12 feeds the hydraulic actuators 6 with oil from the accumulator 11 . The feed flowrate is controlled via the pressure switch 13 . The check valve 15 isolates the external hydraulic liquid pump 12 from backflows from the circuits .

[0047] When emptying the fuel tank 3 after landing on ground, the fuel tank 3 will warm up and expand . The hydraulic actuators 6 are controlled in the same way, to provide constant compressive force . This makes them retract as the fuel tank 3 expands . This control is implemented via the pressure relief valve 14 . The expansion of the fuel tank 3 increases the pressure in the chamber of the hydraulic actuators 6 and triggers the pressure relief valve 14 once it exceeds the set pressure , relieving in this way the pressure in the chamber of the hydraulic actuators 6 .

[0048] On the ground, hydraulic actuators 6 are controlled to maintain constant oil pressure and constant compressor force on the fuel tank 3 to adapt to the length of the fuel tank 3 . In flight , there is another situation . The fuel tank 3 does not shrink or expand as far as the temperature of the cryogenic fuel is maintained . Aerodynamic loads on the aircraft have to be transmitted along the length of the aircraft . For this , the hydraulic actuators 6 are controlled in constant position mode to provide a rigid structure . The hydraulic actuators 6 are kept in a fixed position . Axial loads can be transmitted via the fuel tank 3 and the hydraulic actuators 6 .

[0049] The constant position control can be implemented as shown in Fig . 4 . The hydraulic system comprises two flow control valves 16a and 16b, in particular throttle valves , controlling the flow rate of the oil circulating in the auxiliary hydraulic circuit 18 . The flow control valve 16a is attached to a piston of the hydraulic actuator 6 . The flow control valve 16b is set as reference pressure drop for position definition . Depending on the piston position, the flow control valve 16a imposes a pressure drop in the auxiliary hydraulic circuit 18 powered by pump 19 . Di f ferences in the piston position leads to a di f ference in the pressure drop of control valve 16a . This af fects a displacement of an auxiliary actuator 20 arranged in the auxiliary hydraulic circuit 18 . The auxiliary actuator 20 comprises two separate chambers 20a and 20b . In Fig . 4 , the auxiliary actuator 20 displaces in vertical direction and opens either valve 21a or valve 21b arranged in the main hydraulic circuit 17 .

[0050] For example , i f the force acting on the hydraulic actuator 6 increases , its rod moves and the corresponding flow control valve 16a decreases the pressure drop in the auxiliary hydraulic circuit 18 . This af fects that the pressure inside the first chamber 20a decreases and is lower than the pressure inside the second chamber 20b . The auxiliary actuator 20 displaces upwards in direction 22a and opens valve 21a in the main hydraulic circuit 17 . Pump 22 feeds the hydraulic actuator 6 with oil from the accumulator 23 triggering in this way the hydraulic actuator 6 to follow the correct position . Similarly, i f the force acting on the hydraulic actuator 6 decreases , its rod moves and sets a higher pressure drop in valve 16a, increasing the pressure in chamber 20a and auxiliary actuator moves downward in direction 22b and triggers valve 21b, releasing the pressure inside actuator 6 and correcting in this way its position .

[0051] Fig . 5 shows a second control system for controlling the hydraulic actuators in constant position mode . The second control system is an alternative control system . In contrast to the first control system of Fig . 4 , the second control system comprises two separate accumulators 23 .

[0052] Fig . 6 shows a third control system, which is another alternative control system for controlling the hydraulic actuators . It comprises two hydraulic actuators 6a and 6b, wherein the pistons of the hydraulic actuators are attached to the control valves 16a and 16b .

[0053] I f the force acting on the hydraulic actuator 6a increases , its rod moves and the corresponding flow control valve 16a decreases the pressure drop . The auxiliary actuator 20 displaces upwards and opens valve 21a . Pump 22 feeds the hydraulic actuator 6a with oil from accumulator 23 .

[0054] I f the force acting on the hydraulic actuator 6b increases , the control valve 16b decreases the pressure drop . The auxiliary actuator 20 displaces downwards and opens valve 21b . Pump 22 fees the hydraulic actuator 6b with oil from accumulator 23 .

[0055] During constant position mode , the control system can change the constant position of the hydraulic actuators 6 from a first constant position A to a second constant position B . This is triggered by a temperature change of the fuel inside the fuel tank 3 or a changed constant force at the hydraulic actuator 6 . The temperature of the fuel inside the fuel tank 3 is measured by a temperature sensor 31 and the change of the constant force at the hydraulic actuator is measured by a force meter 32 .

[0056] The proposed concept per definition is a safety relevant system that implies relevant certi fication requirements in order to be commissioned in a commercial service . So as to achieve such a certi fication, the system should provide important levels of reliability . To this aim several back-up systems are foreseen to mitigate possible failures risks : - Redundant separated hydraulic circuits and actuators .

[0057] - Inclusion of an auxiliary accumulator to provide additional pressuri zation in case of leakage in the main hydraulic circuit . - Mechanical tube-pin connections that engage upon complete actuator failure .

[0058] - Proposed superior reliability of purely hydromechanical control system vs . electronic control system .

Claims

Claims1. Vehicle, in particular an aircraft or a space vehicle for a subsonic, supersonic or hypersonic flight, comprising- an airframe (1) ,- a fuel tank ( 3 ) ,- actuators (6) , wherein the actuators (6) are hydraulic or electromechanical actuators, characterized in that the fuel tank (3) is attached to the airframe (1) via the actuators (6) and the actuators (6) are adapted to transfer forces between the airframe (1) and the fuel tank (3) .

2. Vehicle according to claim 1, comprising a control system for controlling the actuators (6) , adapted such that the actuators (6) can be controlled in a constant force mode and / or in a constant position mode.

3. Vehicle according to claim 2, wherein the actuators (6) are controlled in constant force mode- during ground operations of filling the fuel tank ( 3 ) , or- during ground operations of emptying the fuel tank (3) .

4. Vehicle according to claim 2 or 3, wherein the actuators (6) are controlled in constant position mode during flight.

5. Vehicle according to any one of the claims 2 to 4, wherein the control system is adapted to change the constant position of the actuator (6) during the constant position mode from constant position A to constant position B.

6. Vehicle according to claim 5, wherein the control system is adapted to change the constant position dependent on a measured force transferred via the actuator (6) or dependent on a measured temperature of the fuel tank.

7. Vehicle according to any one of the claims 2 to 6, wherein the control system comprises- a first pump (12) adapted to feed the actuators (6) during shrinkage of the fuel tank (3) in constant force mode, and / or- a pressure relief valve (14) to drain liquid from the actuators (6) during expansion of the fuel tank (3) in constant force mode.

8. Vehicle according to any one of the claims 2 to 7, wherein the control system comprises an auxiliary hydraulic circuit (18) with an auxiliary actuator (20) , wherein a position change of one of the actuators (6) during constant position mode generates a pressure change, in particular a pressure drop, in the auxiliary hydraulic circuit (18) displacing the auxiliary actuator (20) such that the displacement of the auxiliary actuator (20) activates a second pump (22) feeding the actuator(6) .

9. Vehicle according to any one of the preceding claims, wherein a thermal insulation (4) , in particular foam or fibrous material, is arranged between the airframe (1) and the fuel tank (3) , in particular wherein the thermal insulation (4) is compressed between the fuel tank (3) and the airframe ( 1 ) .

10. Vehicle according to claim 9, wherein the thermal insulation (4) comprises a plurality of deformable elements (5) , in particular stringers or corrugatedsheets, supporting and / or stiffening the structure of the thermal insulation (4) .

11. Vehicle according to any one of the preceding claims, wherein the fuel tank (3) has an axially symmetrical shape, in particular an elongated shape, in particular a cylindrical shape.

12. Vehicle according to claim 9 and 11, wherein the thermal insulation (4) has an annular shape for supporting the fuel tank (3) radially.

13. Vehicle according to any one of the claims 11 to 12, wherein actuators (6) are at least arranged to transfer forces between the airframe (1) and the fuel tank (3) in axial direction with respect to the elongated, in particular cylindrical, shape of the fuel tank ( 3 ) .

14. Vehicle according to any one of the claims 11 to 13, wherein actuators (6) are arranged to transfer forces between the airframe (1) and the fuel tank (3) in axial, radial and tangential direction with respect to the elongated, in particular cylindrical, shape of the fuel tank (3) .

15. Vehicle according to any of the preceding claims, wherein heat insulating pads (9) , in particular ceramic pads, are arranged between the actuators () 6 ant the fuel tank (3) for reducing heat transfer between the fuel tank (3) and the actuators (6) .

16. Vehicle according to any one of the preceding claims, wherein the fuel tank (6) is arranged inside the fuselage (2) and the actuators (6) are adapted to transfer forces between the fuselage (2) and the fuel tank ( 3 ) .

17. Vehicle according to any one of the preceding claims, wherein the fuel tank (3) is filled with cryogenic fuel, in particular with liquid hydrogen, meth- anol, methane or ammonia.

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