Hydrogen pressurization of a tank with a liquid stored therein

Abstract

A system includes a fuel tank configured to store a fuel in liquid form, and a fuel pressurization subsystem configured to pressurize the fuel tank. The subsystem includes a hydrogen vessel configured to store hydrogen pressurant in gaseous form, a flow controller configured to receive hydrogen pressurant from the hydrogen vessel, and a pressurant conduit between the flow controller and an ullage space of the fuel tank. The flow controller is configured to selectively pass at least a first portion of the hydrogen pressurant received from the hydrogen vessel to the pressurant conduit for transport to the ullage space of the fuel tank. A related method is also disclosed.

Description

07338-2502HYDROGEN PRESSURIZATION OF A TANK WITH A LIQUID STORED THEREINCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 717,747, filed on November 7, 2024, the contents of which are incorporated by reference herein in their entirety.TECHNICAL FIELD

[0002] The present disclosure generally relates to a system and method for pressurizing a tank having a liquid stored therein. The present disclosure more particularly relates to a system and method for pressurizing a tank having a liquid stored therein using hydrogen gas sourced from a separate storage vessel.BACKGROUND

[0003] Referring to FIGS. 1 and 2, it is known to provide a system 110 (e.g., a rocket, a missile, a spacecraft, an aircraft, etc.) that includes a liquid fuel 118 (e.g., a cryogenic liquid fuel, a liquid fuel stored at a same temperature as an ambient environment, etc.) and a liquid oxidizer 134 (e.g., a cryogenic liquid oxidizer) that are expelled from respective tanks 114, 128 and fed to an engine 130 (e.g., a rocket engine) that includes at least one powerhead 138, at least one combustion chamber 140, and at least one nozzle 142. The powerhead 138 includes machinery (e.g., at least one fuel pump 144 and at least one oxidizer pump 146) that pumps the fuel 118 and the oxidizer 134 to higher pressures and delivers them to the combustion chamber 140. In the combustion chamber 140, a mixture of the fuel 118 and the oxidizer 134 is combusted to generate a combustion gas flow that is exhausted from the engine 130 via the nozzle 142 to produce thrust.

[0004] During operation of the engine 130, it is critically important to maintain an acceptable Net Positive Suction Head (NPSH) margin at the inlet 152, 154 of each pump 144, 146 included in the powerhead 138 of the engine 130. NPSH margin refers to the difference between the NPSH Available (NPSHa) and the NPSH Required (NPSHr). NPSHa is the total absolute head available at the pump inlet152, 154 minus the vapor pressure head of the liquid that is being pumped. NPSHr is the minimum NPSHa value required to avoid cavitation and ensure proper operation of the pump 144, 146. For each pump 144, 146 to operate without cavitation, the NPSHa must be greater than the NPSHr by a safe margin. If the NPSHa is less than the NPSHr, then the low pressure at the respective pump inlet 152, 154 will cause the liquid to boil at the inlet, which will cause vapor bubbles that implode violently inside the pump 144, 146 and thereby cause damage and reduce performance.

[0005] The cold temperatures associated with cryogenic liquid propellants (e.g., fuel, oxidizer) means that such propellants will warm up due to heat transfer from ambient surroundings during storage and / or during operation of the engine. This limits the amount of time that each propellant tank may be pressurized before its contents must be consumed, and limits the number of times the engine may be started, before the respective NPSH margins at the pump inlets become unacceptably low.

[0006] To maintain an acceptable NPSH margin and avoid cavitation, it is known to maintain a liquid propellant in a predetermined subcooled thermodynamic state. Subcooling (also known as “subchilling”) involves lowering the vapor pressure of the liquid propellant below (or further below) its normal boiling point. Subcooling results in a denser liquid that is easier to feed into a pump inlet without vapor formation.

[0007] It is known to subcool a liquid propellant (e.g., a cryogenic liquid propellant, a liquid propellant stored at a same temperature as an ambient environment, etc.) using autogenous pressurization. In such systems, a portion of a liquid propellant that has been heated and transitioned to a gaseous state is fed back into the liquid propellant tank from which it was sourced. The heating of the liquid propellant can be achieved using various types of heaters (e.g., a gas generator, a heat exchanger, an electric heater, etc.). The heated gaseous propellant is selectively fed back into an ullage space of the tank to control at least a pressure within the tank and thereby maintain a predetermined NPSH margin at an inlet of a pump that receives the liquid propellant.

[0008] Referring to FIG. 1 , for example, the illustrated system 110 uses autogenous pressurization to pressurize the main fuel tank 114 and the main oxidizer tank 128. The system 110 includes a fuel pressurization subsystem 116 for pressurizing the fuel tank 114 having a liquid fuel 118 (e.g., cryogenic LNG, cryogenic liquid methane, rocket-grade Kerosene such as RP-1 , RP-2, etc.) stored therein, and an oxidizer pressurization subsystem 132 for pressurizing the oxidizer tank 128 having a liquid oxidizer 134 (e.g., cryogenic liquid oxygen (LOX)) stored therein. In this representative embodiment of a prior art system, the fuel tank 114 and the oxidizer tank 128 are arranged such that a common bulkhead 156 defines both the aft end of the oxidizer tank 128 and the forward end of the fuel tank 114. The liquid fuel 118 is expelled from the fuel tank 114 and fed to the engine 130, and the liquid oxidizer 134 is expelled from the oxidizer tank 128 and fed to the engine 130 (e.g., via an oxidizer supply line 136 that passes through the fuel tank 114). The powerhead 138 of the engine 130 includes a fuel pump 144 configured to pump the liquid fuel 118 from the fuel tank 114 to a higher pressure, and an oxidizer pump 146 configured to pump the liquid oxidizer 134 from the oxidizer tank 128 to a higher pressure. The fuel pressurization subsystem 116 includes: a fuel heater 148 configured to heat fuel received from the fuel pump 144 and thereby transition at least a portion of the fuel from a liquid state to a gaseous state; a fuel pressurantflow controller 122 configured to receive a portion of the gaseous fuel flowing from the fuel heater 148 to the combustion chamber 140; and a fuel pressurant conduit 124 between the fuel pressurant flow controller 122 and the ullage space 126 of the fuel tank 114. The fuel pressurant flow controller 122 is configured to selectively pass gaseous fuel received from the fuel heater 148 to the fuel pressurant conduit 124 for transport to the ullage space 126 of the fuel tank 114. Similarly, the oxidizer pressurization subsystem 132 includes: an oxidizer heater 150 configured to heat oxidizer received from the oxidizer pump 146 and thereby transition the oxidizer 134 from a liquid state to a gaseous state; an oxidizer pressurant flow controller 184 configured to receive a portion of the gaseous oxidizer flowing from the oxidizer heater 150 to the combustion chamber 140; and an oxidizer pressurant conduit 186 between the oxidizer pressurant flow controller 184 and the ullage space 188 of the oxidizer tank 128. The oxidizer pressurant flow controller 184 is configured to selectively passgaseous oxidizer received from the oxidizer heater 150 to the oxidizer pressurant conduit 186 for transport to the ullage space 188 of the oxidizer tank 128. The fuel heater 148 and the oxidizer heater 150 are each in the form of a heat exchanger with at least one fluid conduit that allows for transfer of heat from the nozzle 142, the combustion chamber 140, and / or another heat source within the system 110 to the respective propellant flowing through the fluid conduit. The fuel pressurant flow controller 122 and the oxidizer pressurant flow controller 184 each include one or more flow components (e.g., one or more conduits, valves, regulators, vents, orifices, sensors, transducers, flow meters, pressure meters, diffusers, etc.).

[0009] There are multiple problems associated with such autogenous pressurization systems. Notably, the gaseous propellant delivered to the ullage space of the tank for pressurization (the “pressurant gas”) will not be in equilibrium with the liquid propellant remaining within the tank, which leads to condensation of the gaseous propellant back into the liquid phase. This condensing of the pressurant gas in the ullage space reduces the pressure of the ullage space and adds energy to the liquid propellant in the tank. Both have the effect of reducing the NPSH margin, which places the pump at risk of cavitation. To counteract these effects, there is a need for constant replenishment of the pressurant gas to maintain a predetermined tank pressure.

[0010] Another problem with autogenous pressurization systems is that the gaseous propellant used for pressurization will condense after it contacts other cold components of the vehicle. This effect is exaggerated in cases where the pressurant gas is in contact with a component of the vehicle that is below the saturation temperature of the pressurant gas at the given pressure, driving condensation.Some vehicles combat this issue with insulation schemes between the pressurant gas and colder components, but these insulation schemes are their own engineering difficulties in themselves. Moreover, the method by which the liquid propellant is heated and transitioned to the gaseous state is not always straightforward and presents its own engineering difficulties, such as heat exchanger design and packaging. Solving these engineering difficulties is often an undesirable secondary task.

[0011] Another problem with autogenous pressurization systems is that the method by which the liquid propellant is heated and transitioned to the gaseous state may not always be available in all phases of operation of the vehicle. In common rocket applications, for example, a component of the engine typically functions as the heater that transitions the liquid propellant to the gaseous state for use as a pressurant gas, and thus pressurant gas is only available during operation of the engine. It is often desirable to deconflict such heating needs from engine operation, such as during pre-flight operations when engines are not operating.

[0012] It is also known to subcool a liquid propellant using exogenous pressurization. For example, it is known to subcool a liquid propellant stored in a tank using helium pressurant gas that is sourced from a separate storage vessel. Referring to FIG. 2, for example, the system 110 again includes a fuel pressurization subsystem 116 for pressurizing the main fuel tank 114 having a liquid fuel 118 stored therein, and an oxidizer pressurization subsystem 132 for pressurizing the main oxidizer tank 128 having a liquid oxidizer 134 stored therein. The oxidizer pressurization subsystem 132 in FIG. 2 is substantially identical to the oxidizer pressurization subsystem 132 of FIG. 1 , but the fuel pressurization subsystem 116 in FIG. 2 differs in that it uses helium pressurant gas (GHe) sourced from a separate storage vessel 190 to pressurize (and thereby subcool) the liquid fuel 118 stored in the main fuel tank 114. In this representative embodiment of a prior art system, the helium pressurant gas is stored in at least one composite overwrapped pressure vessel (COPV) (hereinafter the “helium vessel 190”) that is positioned within the cavity of the fuel tank 114 such that it is at least partially immersed in the cryogen liquid fuel 118. The helium vessel 190 stores the helium pressurant gas at a pressure (e.g., 40-50 MPa) that is one or more orders of magnitude higher than the pressure within the cavity of the fuel tank 114. The fuel pressurization subsystem 116 includes: the helium vessel 190; a helium heater 192 configured to heat helium pressurant gas received from the helium vessel 190; a helium pressurant flow controller 122 configured to receive heated helium pressurant gas from the helium heater 192; and a helium pressurant conduit 124 between the helium pressurant flow controller 122 and the ullage space 126 of the fuel tank 114. The helium pressurant flow controller 122 is configured toselectively pass heated helium pressurant received from the helium heater 192 to the helium pressurant conduit 124 for transport to the ullage space 126 of the fuel tank 114. The helium heater 192 is in the form of a heat exchanger with at least one fluid conduit that allows for transfer of heat from the nozzle 142 and / or the combustion chamber 140 to the helium pressurant gas flowing through the fluid conduit. The helium pressurant flow controller 122 includes one or more flow components (e.g., one or more conduits, valves, regulators, vents, orifices, sensors, transducers, flow meters pressure meters, diffusers, etc.).

[0013] Helium is inert, has a low molecular weight, and remains gaseous at cryogenic temperatures. This means it will not condense and react with commonly used cryogenic liquid propellants, which is advantageous. However, there are problems associated with pressurization systems that use helium as an exogenous pressurant gas. Notably, helium gas is in limited supply and therefore costly to purchase. This makes it impractical to use cold helium gas (e.g., cryogenic helium gas) as a pressurant because the increased density of the helium would significantly drive up the mass and therefore the cost of the helium required to reach a target pressure. The resulting need to use heated helium propellant gas requires the design and production of the helium heater, which presents its own engineering difficulties. Solving these engineering difficulties is often an undesirable secondary task.

[0014] Another problem is that the heated helium pressurant gas may cool after it is delivered to the ullage space 126 of the fuel tank 114, which may cause the pressure of the ullage space 126 to fall and thereby reduce the NPSH margin. On the other hand, the introduction of the heated helium pressurant gas into the ullage space 126 of the fuel tank 114 may cause the liquid fuel to vaporize as the liquid chases vapor pressure equilibrium, which may cause the pressure of the ullage space 126 to rise and thereby increase the NPSH margin. A rapid interaction of the heated helium pressurant gas and the liquid fuel 118 in the fuel tank 114 presents a risk of rapid depressurization from chilling of the helium or over-pressurization from evaporation of fuel at the interface of the gas and liquid species. If the pressure is too low, NPSH will be insufficient for the pump 144. If the pressure is too high, structuralissues may arise due to the pressure rating of the tanks, ducts, and other fluid components.

[0015] Another problem is that the heated helium pressurant gas delivered to the ullage space 126 of the fuel tank 114 will heat the common bulkhead 156 that separates the fuel tank 114, and will heat the oxidizer supply line 136 that extends from the common bulkhead 156 through the cavity of the fuel tank 114, and will therefore cause unwanted heating of the oxidizer 134 in the oxidizer tank 128 and the oxidizer supply line 136. To prevent issues associated with such heating, it is necessary to provide insulation at the common bulkhead 156 and oxidizer supply line 156. Again, these insulation schemes present their own engineering difficulties in themselves. Solving these engineering difficulties is often an undesirable secondary task.

[0016] Launch vehicles are typically constructed with the minimum structure and complexity since any mass addition to the vehicle results in a 1 :1 reduction in payload capacity. Accordingly, in designing any propellant tank pressurization system for a launch vehicle, the dry structure mass and pressurant mass must be reduced as much as practical, while simultaneously maintaining control of the pressure within the propellant tanks throughout use of the propellant.

[0017] Aspects of the present invention are directed to these and other problems.SUMMARY

[0018] According to an aspect of the present invention, a system includes a fuel tank configured to store a fuel in liquid form, and a fuel pressurization subsystem configured to pressurize the fuel tank. The fuel pressurization subsystem includes a hydrogen vessel configured to store hydrogen pressurant in gaseous form, a flow controller configured to receive hydrogen pressurant from the hydrogen vessel, and a pressurant conduit between the flow controller and an ullage space of the fuel tank. The flow controller is configured to selectively pass at least a first portion of the hydrogen pressurant received from the hydrogen vessel to the pressurant conduit for transport to the ullage space of the fuel tank.

[0019] According to another aspect of the present invention, a method includes the steps of: filling a fuel tank with a fuel in liquid form such that the fuel tank includes a liquid-filled space and an ullage space; storing a hydrogen pressurant gas in a hydrogen vessel such that the hydrogen pressurant gas is physically separate from the fuel in the fuel tank; determining that a pressure magnitude is below a predetermined nominal pressure magnitude; and, in response to the determination that the pressure magnitude is below the predetermined nominal pressure magnitude, increasing the pressure magnitude toward the predetermined nominal pressure magnitude by selectively passing at least a first portion of the hydrogen pressurant gas from the hydrogen vessel to the ullage space of the fuel tank.

[0020] In addition to, or as an alternative to, one or more of the features described above, further aspects of the present invention can include one or more of the following features, individually or in combination:- the fuel tank is configured to store the fuel in cryogenic liquid form;- the fuel is liquid natural gas;- the fuel is liquid methane;- the fuel tank is configured to store the fuel in liquid form at a same temperature as an ambient environment;- the fuel is a rocket-grade Kerosene;- the hydrogen vessel, the flow controller, and the pressurant conduit are configured such that the at least a first portion of the hydrogen pressurant is not intentionally heated during transport from the hydrogen vessel to the ullage space of the fuel tank via the flow controller and the pressurant conduit;- the system further includes a fluid heater, and the hydrogen vessel, the flow controller, and the pressurant conduit are configured such that the at least a first portion of the hydrogen pressurant is not in thermal communication with the fluid heater during transport from the hydrogen vessel to the ullage space of the fuel tank via the flow controller and the pressurant conduit;- the fluid heater is at least one of a heat exchanger, a gas generator, and an electric heater;- the fuel tank includes a fuel cavity in which the fuel is stored, the hydrogen vessel includes a hydrogen cavity in which the hydrogen pressurant is stored, the hydrogen vessel is positioned within the fuel cavity of the fuel tank, and the fuel tank and the hydrogen vessel are configured such that the fuel stored in the fuel cavity is physically separate from the hydrogen pressurant stored in the hydrogen cavity;- the hydrogen vessel is a composite overwrapped pressure vessel (COPV);- the fuel tank includes a top wall, a bottom wall, and a sidewall extending between the top wall and the bottom wall, and the top wall, the bottom wall, and the sidewall of the fuel tank collectively define the fuel cavity;- the fuel tank is transitionable between a filled state, in which the fuel cavity of the fuel tank stores a maximum volume of the fuel in liquid form, and a depleted state, in which the fuel cavity of the fuel tank stores a minimum volume of the fuel in liquid form, in the filled state of the fuel tank a liquid surface of the fuel is at a first distance from a top wall of the fuel tank, in the depleted state of the fuel tank the liquid surface of the fuel is at a second distance from the top wall of the fuel tank, and the second distance is greater than the first distance;- the hydrogen vessel is positioned within the fuel cavity of the fuel tank such that a top of the hydrogen vessel is at a third distance from the top wall of the fuel tank, and the third distance is greater than the first distance;- the third distance is greater than the second distance;- the hydrogen vessel is positioned within the fuel cavity of the fuel tank such that, in the filled state of the fuel tank, the hydrogen vessel is at least partially immersed in the fuel;- the hydrogen vessel is positioned within the fuel cavity of the fuel tank such that, in the filled state of the fuel tank, the hydrogen vessel is fully immersed in the fuel;- the hydrogen vessel is positioned within the fuel cavity of the fuel tank such that, in the depleted state of the fuel tank, the hydrogen vessel is at least partially immersed in the fuel;- the hydrogen vessel is positioned within the fuel cavity of the fuel tank such that, in the depleted state of the fuel tank, the hydrogen vessel is fully immersed in the fuel;- the pressurant conduit has a pressurant conduit inlet in fluid communication with the flow controller and a pressurant conduit outlet positioned within the fuel cavity of the fuel tank, and the pressurant conduit outlet is positioned within the fuel cavity of the fuel tank at a fourth distance from the top wall of the fuel tank, and the fourth distance is less than the first distance;- the fuel pressurization subsystem further includes a diffuser at the pressurant conduit outlet;- at least a portion of the pressurant conduit is positioned outside of the fuel cavity of the fuel tank;- at least a portion of the flow controller is positioned outside of the fuel cavity of the fuel tank;- the pressurant conduit is a first pressurant conduit and the fuel pressurization subsystem further includes a second pressurant conduit between the flow controller and a liquid-filled space of the fuel tank, the flow controller is configured to selectively pass the first portion of the hydrogen pressurant received from the hydrogen vessel to the first pressurant conduit for transport to the ullage space of the fuel tank, and the flow controller is configured to selectively pass a second portion of the hydrogen pressurant received from the hydrogen vessel to the second pressurant conduit for transport to the liquid-filled space of the fuel tank;- the flow controller is configured to selectively control at least one flow parameter of the first portion of the hydrogen pressurant passed to the first pressurant conduit for transport to the ullage space of the fuel tank, and the flow controller is configured to selectively control at least one flow parameter of the second portion of the hydrogen pressurant passed to the second pressurant conduit for transport to the liquid-filled space of the fuel tank;- the flow controller is configured to selectively control the at least one flow parameter of the first portion of the hydrogen pressurant independent of the at least one flow parameter of the second portion of the hydrogen pressurant, and vice versa;- the at least one flow parameter of the first portion of the hydrogen pressurant is a first flow rate, and the at least one flow parameter of the second portion of the hydrogen pressurant is a second flow rate;- the second pressurant conduit has a second pressurant conduit inlet in fluid communication with the flow controller and a second pressurant conduit outlet positioned within the fuel cavity of the fuel tank;- the second pressurant conduit outlet is positioned within the fuel cavity of the fuel tank such that, in the depleted state of the fuel tank, the second pressurant conduit outlet is fully immersed in the fuel;- the second pressurant conduit outlet is positioned within the fuel cavity of the fuel tank at a fifth distance from the top wall of the fuel tank, and the fifth distance is greater than the second distance;- the fuel pressurization subsystem further includes a second diffuser at the second pressurant conduit outlet;- at least a portion of the second pressurant conduit is positioned outside of the fuel cavity of the fuel tank;- the flow controller includes a pressure sensor configured to measure a pressure magnitude within the system, and a processor configured to determine that the pressure magnitude is below a predetermined nominal pressure magnitude;- the flow controller is configured to selectively pass the at least the first portion of the hydrogen pressurant to the pressurant conduit in response to the determination that the pressure magnitude is below the predetermined nominal pressure magnitude;- the pressure magnitude is a pressure magnitude in the ullage space of the fuel tank.- the pressure magnitude is a pressure magnitude in a liquid-filled space of the fuel tank;- the system further includes a fuel pump configured to receive fuel from the fuel tank and output a pressurized fuel flow, and the pressure magnitude is a pressure magnitude at an inlet of the fuel pump;- the system is a fuel storage system;- the system is a fuel transport ground vehicle;- the system is one of a rocket, a missile, a spacecraft, and an aircraft;- the system further includes an oxidizer tank configured to store an oxidizer in liquid form, and an engine including a fuel pump configured to receive fuel from thefuel tank and output a pressurized fuel flow, an oxidizer pump configured to receive oxidizerfrom the oxidizer tank and output a pressurized oxidizer flow, a combustion chamber configured to combust a mixture of the pressurized fuel flow and the pressurized oxidizer flow to generate a combustion gas flow, and a nozzle configured to exhaust the combustion gas flow from the combustion chamber to generate thrust;- the oxidizer is liquid oxygen (LOX);- the engine includes a fluid heater, and the hydrogen vessel, the flow controller, and the pressurant conduit are configured such that the hydrogen pressurant is not in thermal communication with the fluid heater during transport from the hydrogen vessel to the ullage space of the fuel tank via the flow controller and the pressurant conduit;- the fluid heater is at least one of a heat exchanger, a gas generator, and an electric heater;- the fuel tank defines a fuel cavity in which the fuel is stored and the oxidizer tank defines an oxidizer cavity in which the oxidizer is stored, and the fuel tank and the oxidizer tank are arranged in a common bulkhead configuration in which a first side of a common bulkhead wall partially defines the fuel cavity of the fuel tank and an opposing second side of the common bulkhead wall partially defines the oxidizer cavity of the oxidizer tank;- the common bulkhead wall is not insulated;- the step of increasing the pressure magnitude toward the predetermined nominal pressure magnitude does not involve intentionally heating the at least a first portion of the hydrogen pressurant gas;- the step of storing the hydrogen pressurant gas in the hydrogen vessel involves positioning the hydrogen vessel within the fuel tank such that, in a filled state of the fuel tank, the hydrogen vessel is at least partially immersed in the fuel in the fuel tank.- the step of increasing the pressure magnitude toward the predetermined nominal pressure magnitude involves selectively and independently passing the first portion of the hydrogen pressurant gas from the hydrogen vessel to the ullage space of the fuel tank and selectively and independently passing a second portion of thehydrogen pressurant gas from the hydrogen vessel to the liquid-filled space of the fuel tank; and-the step of increasing the pressure magnitude toward the predetermined nominal pressure magnitude maintains a predetermined subcooled state of the fuel in the fuel tank.

[0021] These and other aspects of the present invention will become apparent in light of the drawings and detailed description provided below.BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 schematically illustrates a representative embodiment of a prior art system in which liquid propellant tanks are pressurized by feeding autogenous pressurant gases into the respective propellant tanks.

[0023] FIG. 2 schematically illustrates a representative embodiment of a prior art system in which a liquid oxidizer tank is pressurized by feeding autogenous pressurant gas into the oxidizer tank, and a liquid fuel tank is pressurized by feeding exogenous helium gas from a separate storage vessel into the fuel tank after heating of the helium gas.

[0024] FIG. 3 schematically illustrates an embodiment of the present system in which a liquid fuel tank is pressurized by feeding exogenous hydrogen gas from a separate storage vessel into the fuel tank.

[0025] FIG. 4 schematically illustrates the fuel tank of FIG. 3 in a filled state.

[0026] FIG. 5 schematically illustrates the fuel tank of FIG. 3 in a depleted state.

[0027] FIG. 6 schematically illustrates the flow controller included in the fuel pressurization subsystem of FIG. 3.

[0028] FIG. 7 schematically illustrates the first bang-bang controller included in the flow controller of FIG. 6.

[0029] FIG. 8 illustrates an embodiment of the present method.DETAILED DESCRIPTION

[0030] The present disclosure describes a system 10 (FIG. 3) and a related method 12 (FIG. 8).

[0031] Referring to FIG. 3, the system 10 includes a fuel tank 14 and a fuel pressurization subsystem 16. The fuel tank 14 is configured to store a fuel 18 in liquid form (e.g., cryogenic liquid natural gas (LNG), cryogenic liquid methane (CH4), rocket-grade Kerosene such as RP-1 , RP-2, or another fuel stored at a same temperature as an ambient environment, etc.), and a fuel pressurization subsystem 16 configured to pressurize the fuel tank 14. The fuel pressurization subsystem 16 includes at least one hydrogen vessel 20 configured to store hydrogen pressurant in gaseous form (GH2), at least one flow controller 22 configured to receive hydrogen pressurant from the hydrogen vessel 20, and at least one pressurant conduit 24 (e.g., tubing, plumbing, etc.) between the flow controller 22 and an ullage space 26 of the fuel tank 14. The flow controller 22 is configured to selectively pass at least a first portion of the hydrogen pressurant received from the hydrogen vessel 20 to the pressurant conduit 24 for transport to the ullage space 26 of the fuel tank 14. In some embodiments, the flow controller 22 is configured to selectively control at least one flow parameter (e.g., flow rate, flow pressure, etc.) of the hydrogen pressurant passed to the pressurant conduit 24 for transport to the ullage space 26 of the fuel tank 14.

[0032] The system 10 can take various forms. In some embodiments, the system 10 is a rocket (e.g., a multi-stage rocket, a single-stage-to- orbit (SSTO) rocket, an upper stage rocket, a booster rocket, etc.), a missile, a spacecraft, an aircraft, or another vehicle designed for travel (e.g., flight) up to at least supersonic speeds (e.g., supersonic speeds, hypersonic speeds, re-entry speeds, etc.) in atmospheric, sub-orbital, orbital, extraterrestrial, and / or outer space environments. In some such embodiments, including the illustrated embodiment, the system 10 further includes at least one oxidizer tank 28, at least one engine 30 (e.g. , a rocket engine), and at least one oxidizer pressurization subsystem 32. In such embodiments, the oxidizer tank 28 is configured to store an oxidizer 34 in liquid form (e.g., cryogenic liquid oxygen(LOX)). The liquid fuel 18 is expelled from the fuel tank 14 and fed to the engine 30, and the liquid oxidizer 34 is expelled from the oxidizer tank 28 and fed to the engine 30 (e.g., via an oxidizer supply line 36 that passes through the fuel tank 14). The engine 30 includes at least one powerhead 38, at least one combustion chamber 40, and at least one nozzle 42. The powerhead 38 includes machinery (e.g., at least one fuel pump 44 and at least one oxidizer pump 46) that pumps the fuel 18 and the oxidizer 34 to higher pressures and delivers them to the combustion chamber 40. In the illustrated embodiment, the engine 30 is an expander cycle engine that includes a fuel heat exchanger 48 configured to transfer heat from the nozzle 42 and / or the combustion chamber 40 to the pressurized fuel 18 before it is delivered to the combustion chamber 40, and an oxidizer heat exchanger 50 configured to transfer heat from the nozzle 42 and / or the combustion chamber 40 to the pressurized oxidizer 34 before it is delivered to the combustion chamber 40. In the combustion chamber 40, a mixture of the pressurized fuel 18 and the pressurized oxidizer 34 is combusted to generate a combustion gas flow that is exhausted from the engine 30 via the nozzle 42 to produce thrust. During operation of the engine 30, it is critically important to maintain an acceptable NPSH margin at the respective inlets 52, 54 of the pumps 44, 46 included in the powerhead 38. In such embodiments, the fuel pressurization subsystem 16 is configured to selectively pressurize the fuel tank 14 (and thus the liquid fuel 18 stored in the fuel tank 14) using hydrogen pressurant received from the hydrogen vessel 20 for maintaining a predetermined subcooled state of the fuel 18. In the predetermined subcooled state, the fuel 18 has one or more thermodynamic parameters (e.g., pressure, temperature, etc.) that achieve an NPSH margin at the inlet 52 of the fuel pump 44 that is within a range of acceptable NPSH margins for the fuel pump 44. In some such embodiments, the oxidizer pressurization subsystem 32 is substantially similar to the one included in the prior art systems of FIGS. 1 and 2. That is, the oxidizer pressurization subsystem 32 may include an oxidizer pressurant flow controller 84 that receives a portion of the oxidizer 34 flowing from the oxidizer heat exchanger 50 to the combustion chamber 40, an oxidizer pressurant conduit 86 between the oxidizer pressurant flow controller 84 and the ullage space 88 of the oxidizer tank 28, and the oxidizer pressurant flow controller 84 may be configured to selectively pass gaseous oxidizer received fromthe oxidizer heater 50 to the oxidizer pressurant conduit 86 fortransport to the ullage space 88 of the oxidizer tank 28.

[0033] In other embodiments not shown in the drawings, the system 10 is a ground- based fuel storage facility, a ground vehicle for transporting fuel (e.g., a fuel tanker truck), or the system 10 has another form factor. In some such embodiments, the system 10 does not include an oxidizer tank, an engine, or an oxidizer pressurization subsystem, for example.

[0034] In other embodiments not shown in the drawings, the fuel pressurization subsystem 16 is configured to selectively pressurize the fuel tank 14 (and thus the liquid fuel 18 stored in the fuel tank 14) using hydrogen pressurant received from the hydrogen vessel 20 for providing a motive force that moves liquid fuel 18 in the fuel tank 14 from a first position to a second position. In some embodiments, the first and second positions are different positions within the fuel cavity of the fuel tank 14. In other embodiments, the first position is the fuel cavity of the fuel tank 14 and the second position is a fuel cavity of a second fuel tank.

[0035] Referring to FIG. 3, the fuel tank 14 includes a fuel cavity in which the fuel 18 is stored, and the hydrogen vessel 20 includes a hydrogen cavity in which the hydrogen pressurant (GH2) is stored. In some embodiments, including the illustrated embodiment, the hydrogen vessel 20 is positioned within the fuel cavity of the fuel tank 14. In such embodiments, the fuel tank 14 and the hydrogen vessel 20 are configured such that the fuel 18 stored in the fuel cavity of the fuel tank 14 is physically separate from the hydrogen pressurant stored in the hydrogen cavity of the hydrogen vessel 20. In other embodiments not shown in the drawings, the hydrogen vessel 20 is positioned at least partially outside of the fuel cavity of the fuel tank 14.

[0036] Referring to FIG. 3, in the illustrated embodiment, the fuel tank 14 includes a top wall 56, a bottom wall 58, and a sidewall 60 extending between the top wall 56 and the bottom wall 58. In the illustrated embodiment, the top wall 56, the bottom wall 58, and the sidewall 60 of the fuel tank 14 are sheet metal components that collectively define the fuel cavity in which the fuel 18 is stored. In the illustratedembodiment, the top wall 56 and / or the bottom wall 58 of the fuel tank 14 are manufactured using the techniques disclosed in the commonly-assigned International Patent Application No. PCT / US2023 / 020269, filed April 27, 2023, the contents of which are incorporated by reference herein in its entirety. In other embodiments, the top wall 56 and / or the bottom wall 58 of the fuel tank 14 can be milled workpieces, forged workpieces, billet workpieces, composites, or other materials. In the illustrated embodiment, the fuel tank 14 and the oxidizer tank 28 are arranged in a common bulkhead configuration. That is, the top wall 56 of the fuel tank 14 is a common bulkhead wall with a bottom side that partially defines the fuel cavity of the fuel tank 14 and an opposing top side that partially defines the oxidizer cavity of the oxidizer tank 28 in which the oxidizer 34 is stored. In the illustrated embodiment, the top wall 56 of the fuel tank 14 (i.e., the common bulkhead wall) is a sheet metal component that is not insulated.

[0037] Referring to FIGS. 4 and 5, in the illustrated embodiment, the fuel tank 14 is transitionable between a filled state (FIG. 4), in which the fuel cavity of the fuel tank 14 stores a maximum volume of the fuel 18 in liquid form (e.g., cryogenic liquid form), and a depleted state (FIG. 5), in which the fuel cavity of the fuel tank 14 stores a minimum volume of the fuel 18 in liquid form (e.g., cryogenic liquid form). In the filled state (FIG. 4), the fuel tank 14 includes a liquid-filled space and an ullage space 26. A liquid surface of the fuel 18 defines a transition between the liquid-filled space and the ullage space 26 of the fuel tank 14. In the filled state of the fuel tank 14 (FIG. 4), the liquid surface of the fuel 18 is at a first distance D1 from the top wall 56 of the fuel tank 14. In the depleted state of the fuel tank 14 (FIG. 5), the liquid surface of the fuel 18 is at a second distance D2 from the top wall 56 of the fuel tank 14. The second distance D2 is greater than the first distance D1.

[0038] In some embodiments, including the illustrated embodiment, the hydrogen vessel 20 is a composite overwrapped pressure vessel (COPV) that stores the hydrogen pressurant gas at a pressure (e.g., 40-50 MPa) that is one or more orders of magnitude higher than the pressure within the fuel cavity of the fuel tank 14. In other embodiments, the hydrogen vessel 20 is another type of pressure vessel that stores the hydrogen pressurant gas at a pressure that is higher (e.g., higher by oneor more orders of magnitude) than the pressure within the fuel cavity of the fuel tank 14.

[0039] In some embodiments, the hydrogen vessel 20 is positioned within the fuel cavity of the fuel tank 14 such that, in the filled state of the fuel tank 14 (FIG. 4), the hydrogen vessel 20 is at least partially immersed in the liquid fuel 18. In the illustrated embodiment, the hydrogen vessel 20 is positioned within the fuel cavity of the fuel tank 14 such that, in the filled state of the fuel tank 14 (FIG. 4), the hydrogen vessel 20 is fully immersed in the fuel 18. In this embodiment, the top of the hydrogen vessel 20 is at a third distance D3 from the top wall 56 of the fuel tank 14, and the third distance D3 is greater than the first distance D1 (i.e., the distance from the liquid surface of the fuel 18 to the top wall 56 of the fuel tank 14 in the filled state). In some embodiments not shown in the drawings, the hydrogen vessel 20 is positioned within the fuel cavity of the fuel tank 14 such that, in the depleted state of the fuel tank 14 (FIG. 5), the hydrogen vessel 20 is partially immersed or fully immersed in the liquid fuel 18.

[0040] Referring to FIG. 3, in the illustrated embodiment, the pressurant conduit 24 has an inlet in fluid communication with the flow controller 22 and an outlet positioned within the fuel cavity of the fuel tank 14. The pressurant conduit 24 passes through a first opening in the sidewall 60 of the fuel tank 14. In some embodiments, including the illustrated embodiment, at least a portion of the pressurant conduit 24 is positioned outside of the fuel cavity of the fuel tank 14. In the illustrated embodiment, the outlet of the pressurant conduit 24 is positioned within the fuel cavity of the fuel tank 14 such that it (the outlet) is always in the ullage space 26 of the fuel tank 14 (i.e., the outlet is not immersed in the liquid fuel 18 in the filled state (FIG. 4) or the depleted state (FIG. 5) of the fuel tank 14). Referring to FIG. 4, the outlet of the pressurant conduit 24 is positioned within the fuel cavity of the fuel tank 14 such that it is at a fourth distance D4 from the top wall 56 of the fuel tank 14, and the fourth distance D4 is less than the first distance D1 (i.e., the distance from the liquid surface of the fuel 18 to the top wall 56 of the fuel tank 14 in the filled state).

[0041] In some embodiments, including the illustrated embodiment, the pressurant conduit 24 is a first pressurant conduit and the fuel pressurization subsystem 16 further includes a second pressurant conduit 62 between the flow controller 22 and the liquid-filled space of the fuel tank 14. In such embodiments, the flow controller 22 is configured to selectively and independently pass a first portion of the hydrogen pressurant received from the hydrogen vessel 20 to the first pressurant conduit 24 for transport to the ullage space 26 of the fuel tank 14, and selectively and independently pass a second portion of the hydrogen pressurant received from the hydrogen vessel 20 to the second pressurant conduit 62 for transport to the liquid- filled space of the fuel tank 14. In some embodiments, the flow controller 22 is configured to selectively and independently control at least one flow parameter (e.g., flow rate, flow pressure, etc.) of the first portion of the hydrogen pressurant passed to the first pressurant conduit 24 for transport to the ullage space 26 of the fuel tank 14, and selectively and independently control at least one flow parameter (e.g., flow rate, flow pressure, etc.) of the second portion of the hydrogen pressurant passed to the second pressurant conduit 62 for transport to the liquid-filled space of the fuel tank 14.

[0042] Referring to FIG. 3, in the illustrated embodiment, the second pressurant conduit 62 has an inlet in fluid communication with the flow controller 22 and an outlet positioned within the fuel cavity of the fuel tank 14. The second pressurant conduit 62 passes through a second opening in the sidewall 60 of the fuel tank 14. The flow controller 22 and the respective inlets of the first and second pressurant conduits 24, 62 are positioned outside of the fuel cavity of the fuel tank 14, and the respective outlets of the first and second pressurant conduits 24, 62 are positioned inside the fuel cavity of the fuel tank 14. The fuel pressurization subsystem 16 further includes a first diffuser 64 at the outlet of the first pressurant conduit 24, and a second diffuser 66 at the outlet of the second pressurant conduit 62.

[0043] Referring to FIGS. 4 and 5, in the illustrated embodiment, the outlet of the second pressurant conduit 62 is positioned within the fuel cavity of the fuel tank 14 such that it is immersed in the liquid fuel 18 when the fuel tank 14 is in both the filled state (FIG. 4) and the depleted state (FIG. 5). In the illustrated embodiment, theoutlet of the second pressurant conduit 62 is positioned within the fuel cavity of the fuel tank 14 at a fifth distance D5 from the top wall 56 of the fuel tank 14, and the fifth distance D5 is greater than the second distance D2 (i.e., the distance from the liquid surface of the fuel 18 to the top wall 56 of the fuel tank 14 in the depleted state).

[0044] The flow controller 22 includes one or more flow components (e.g., one or more conduits, valves, regulators, vents, orifices, sensors, computers, transducers, flow meters, pressure meters, diffusers, etc.) configured to perform the abovedescribed functionality. In some embodiments, the flow controller 22, and / or one or more components thereof, is configured to receive and / or process signals using analog and / or digital hardware (e.g., counters, switches, logic devices, memory devices, programmable processors, non-transitory computer-readable storage media), software, firmware, or a combination thereof. In some embodiments, the flow controller 22, and / or one or more components thereof, can perform one or more of the functions described herein by executing software, which can be stored, for example, in a non-transitory computer-readable storage medium. In some embodiments, the flow controller 22 includes at least one pressure sensor configured to measure a pressure magnitude within the system 10 (e.g., a pressure in the ullage space 26 of the tank 14, a pressure in the liquid-filled space of the tank 14, a pressure at the inlet 52 of the fuel pump 44, etc.), and at least one processor configured to determine that the pressure magnitude is below a predetermined nominal pressure magnitude. In some such embodiments, the flow controller 22 is configured such that, in response to the determination that the pressure magnitude is below the predetermined nominal pressure magnitude, the flow controller 22 selectively passes hydrogen pressurant to the fuel tank 14 via the one or more pressurant conduits 24, 62 to thereby increase the pressure magnitude toward the predetermined nominal pressure.

[0045] Referring to FIG. 6, in the illustrated embodiment, the flow controller 22 includes at least: a hydrogen supply line 68 configured to receive hydrogen pressurant from the hydrogen vessel 20; a three-way control valve 70 configured to selectively pass respective portions of the hydrogen pressurant from the hydrogen supply line 68 to the first and second hydrogen pressurant conduits 24, 62;respective bang-bang controllers 72, 74 for independently and selectively controlling at least the flow rate and the flow pressure of the respective portions of the hydrogen pressurant transported to the fuel tank 14 via the first and second hydrogen pressurant conduits 24, 62; a pressure sensor 75 configured to generate pressure signals 77 indicative of a pressure magnitude within the fuel tank 14; and a processor 79 configured to use the pressure signals 77 received from the pressure sensor 75 to determine that a pressure magnitude in the system 10 (e.g. , a pressure in the ullage space 26 of the fuel tank 14, a pressure in the liquid-filled space of the tank 14, a pressure at the inlet 52 of the fuel pump 44, etc.) is below a predetermined nominal pressure magnitude, and configured to generate and transmit respective control signals 81 , 83, 85 to the three-way control valve 70 and the bang-bang controllers 72, 74 for selectively controlling the operation of those flow components. Referring to FIG. 7, each bang-bang controller 72, 74 includes a plurality of valves and pressure regulators connected via a plurality of conduits. Although not shown in FIG. 6, the pressure sensor 75 of the flow controller 22 is positioned within the ullage space 26 of the fuel tank 14. In other embodiments, the pressure sensor 75 of the flow controller 22 is positioned in the liquid-filled space of the fuel tank 14, at the inlet 52 of the fuel pump 44, or at another position within the system 10. In the illustrated embodiment, the various flow components of the flow controller 22 are automatically controllable in response to signals received via wired and / or wireless connections.

[0046] In some embodiments, including the illustrated embodiment, the hydrogen vessel 20, the flow controller 22, and the one or more pressurant conduits 24, 62 are configured such that the hydrogen pressurant is not intentionally heated during transport from the hydrogen vessel 20 to the fuel cavity of the fuel tank 14 via the hydrogen vessel 20, the flow controller 22, and the one or more pressurant conduits 24, 62. The present system 10 therefore differs from prior art systems (see, e.g., FIG. 2) in which exogenous helium pressurant gas is intentionally heated before being passed to the ullage space of the fuel tank for pressurization of the fuel tank.

[0047] In some embodiments, the system 10 further includes a fluid heater (e.g., a heat exchanger, a gas generator, an electric heater), but the hydrogen vessel 20, theflow controller 22, and the one or more pressurant conduits 24, 62 are configured such that the hydrogen pressurant is not in thermal communication with the fluid heater during transport from the hydrogen vessel 20 to the fuel tank 14. Referring to FIG. 3, in the illustrated embodiment, the system 10 includes an engine 30 having a fuel heat exchanger 48 and an oxidizer heat exchanger 50 that function as fluid heaters, as described above. In this embodiment, the hydrogen vessel 20, the flow controller 22, and the first and second pressurant conduits 24, 62 are configured such that the hydrogen pressurant is not in thermal communication with the fuel heat exchanger 48 or the oxidizer heat exchanger 50 during transport from the hydrogen vessel 20 to the fuel cavity of the fuel tank 14 via the hydrogen vessel 20, the flow controller 22, and the one or more pressurant conduits 24, 62.

[0048] Referring to FIG. 8, the present method includes the steps of: filling the fuel tank 14 with the fuel 18 in liquid form (e.g., cryogenic liquid form) such that the fuel tank 14 includes a liquid-filled space and an ullage space 26 (step 76); storing a hydrogen pressurant gas in a hydrogen vessel 20 such that the hydrogen pressurant gas is physically separate from the fuel in the fuel tank 14 (step 78); determining that a pressure magnitude in the system 10 (e.g., a pressure in the ullage space 26 of the fuel tank 14, a pressure in the liquid-filled space of the tank 14, a pressure at the inlet 52 of the fuel pump 44, etc.) is below a predetermined nominal pressure magnitude (step 80); and, in response to the determination that the pressure magnitude is below the predetermined nominal pressure magnitude, increasing the pressure magnitude toward the predetermined nominal pressure magnitude by selectively passing at least a first portion of the hydrogen pressurant gas from the hydrogen vessel 20 to the ullage space 26 of the fuel tank 14 (step 82).

[0049] In some embodiments, the step of storing the hydrogen pressurant gas in the hydrogen vessel (step 78) involves positioning the hydrogen vessel 20 within the fuel tank 14.

[0050] In some embodiments, the step of increasing the pressure magnitude toward the predetermined nominal pressure magnitude (step 82) does not involve intentionally heating the at least a first portion of the hydrogen pressurant gas.

[0051] In some embodiments, the step of increasing the pressure magnitude toward the predetermined nominal pressure magnitude (step 82) involves selectively and independently passing the first portion of the hydrogen pressurant gas from the hydrogen vessel 20 to the ullage space 26 of the fuel tank 14 (step 82a) and selectively and independently passing a second portion of the hydrogen pressurant gas from the hydrogen vessel to the liquid-filled space of the fuel tank 14 (step 82b).

[0052] In some embodiments, the step of increasing the pressure magnitude toward the predetermined nominal pressure magnitude (step 82) maintains a predetermined subcooled state of the fuel in the fuel tank 14.

[0053] The present system 10 and method 12 offer many advantages over prior art systems and methods that use autogenous pressurization and / or exogenous pressurization with helium pressurant gas, including the following advantages:- Hydrogen is a non-condensable gas at the common storage temperatures and pressures of cryogenic liquid fuels (e.g., LNG, liquid methane, etc.). Use of hydrogen as a pressurant therefore allows the gas pressurant and liquid fuel to co-exist in thermal equilibrium, allowing the elimination of heat transfer between the gas pressurant and stored liquid fuel.- Given the possibility of thermal equilibrium, it is possible to design the present system with very deterministic gas densities, given that heat transfer can be eliminated from consideration in calculations and system sizing. This is desirable for the system designer as they attempt to size a pressurant storage system.- Like helium, hydrogen gas in the ullage space of the fuel tank will cause some liquid fuel to evaporate as the liquid fuel chases vapor pressure equilibrium. This evaporated fuel will further increase the ullage pressure and cool the bulk liquid temperature. This effect increases the NPSH margin.- The present system will have a longer time before the NPSH margin decreases due to ambient heating. In embodiments in which the systemincludes an engine, the engine will be able to start multiple times and execute more complex missions than was possible in prior art systems.- Hydrogen gas is readily available and inexpensive compared to helium.- Hydrogen does not liquify when stored at many cryogenic liquid fuel temperatures, even when stored at high pressure.- The Joule-Thompson ratio of hydrogen allows it to not significantly change temperatures as the gas is expanded from high pressure to low pressure.- Hydrogen is the lightest gas in the universe, meaning for a given volume, temperature, and pressure, hydrogen gas will be the lightest gas of any given selection of gases.- Using hydrogen as the pressurant gas may simplify the design of the system by removing the need for many components, such as fluid heaters (e.g., heat exchangers), pressurant and vent valves, and / or interfaces with propulsion systems.- After the fuel tank is transitioned from the filled state to the depleted state, any remaining hydrogen pressurant gas may be collected and reused for future tank pressurization.- After the fuel tank is transitioned from the filled state to the depleted state, any remaining hydrogen pressurant gas may be used for some other purpose such as feedstock for electricity generation with a fuel cell or water production.- The present system eliminates the need for fluid heaters to heat the hydrogen pressurant before it is fed into the fuel tank. In embodiments in which the system includes an engine, the system therefore allows for pressurization of the fuel tank at times when the engine is not operating (e.g., during pre-flight operations) due to elimination of the fluid heaters.- The “cold” hydrogen pressurant introduced into the ullage space of the fuel tank will not change density (temperature) when rapid interaction of liquid and gas occurs, and it drastically reduces heating of the common bulkhead walland the oxidizer supply line, which can eliminate the need to insulate these parts to prevent unwanted heating of the oxidizer.- The “cold” hydrogen pressurant introduced in the liquid-filled space of the fuel tank 14 will cause the hydrogen pressurant to “bubble” up through the liquid fuel, which serves to create pockets of low partial-pressure gas to which the liquid fuel will evaporate, further chilling the liquid fuel and maintaining subcooling.

[0054] While several embodiments have been disclosed, it will be apparent to those having ordinary skill in the art that aspects of the present invention include many more embodiments. Accordingly, aspects of the present invention are not to be restricted except in light of the attached claims and their equivalents. It will also be apparent to those of ordinary skill in the art that variations and modifications can be made without departing from the true scope of the present disclosure. For example, in some instances, one or more features disclosed in connection with one embodiment can be used alone or in combination with one or more features of one or more other embodiments.

Claims

What is claimed is:

1. A system, comprising: a fuel tank configured to store a fuel in liquid form; and a fuel pressurization subsystem configured to pressurize the fuel tank, the fuel pressurization subsystem including: a hydrogen vessel configured to store hydrogen pressurant in gaseous form; a flow controller configured to receive hydrogen pressurant from the hydrogen vessel; and a pressurant conduit between the flow controller and an ullage space of the fuel tank; wherein the flow controller is configured to selectively pass at least a first portion of the hydrogen pressurant received from the hydrogen vessel to the pressurant conduit for transport to the ullage space of the fuel tank.

2. The system of claim 1 , wherein the fuel tank is configured to store the fuel in cryogenic liquid form.

3. The system of claim 2, wherein the fuel is liquid natural gas.

4. The system of claim 2, wherein the fuel is liquid methane.

5. The system of claim 1 , wherein the fuel tank is configured to store the fuel in liquid form at a same temperature as an ambient environment.

6. The system of claim 1 , wherein the fuel is a rocket-grade Kerosene.

7. The system of claim 1 , wherein the hydrogen vessel, the flow controller, and the pressurant conduit are configured such that the at least a first portion of the hydrogen pressurant is not intentionally heated during transport from the hydrogenvessel to the ullage space of the fuel tank via the flow controller and the pressurant conduit.

8. The system of claim 1 , further comprising a fluid heater; and wherein the hydrogen vessel, the flow controller, and the pressurant conduit are configured such that the at least a first portion of the hydrogen pressurant is not in thermal communication with the fluid heater during transport from the hydrogen vessel to the ullage space of the fuel tank via the flow controller and the pressurant conduit.

9. The system of claim 8, wherein the fluid heater is at least one of a heat exchanger, a gas generator, and an electric heater.

10. The system of claim 1 , wherein the fuel tank includes a fuel cavity in which the fuel is stored; wherein the hydrogen vessel includes a hydrogen cavity in which the hydrogen pressurant is stored; wherein the hydrogen vessel is positioned within the fuel cavity of the fuel tank; and wherein the fuel tank and the hydrogen vessel are configured such that the fuel stored in the fuel cavity is physically separate from the hydrogen pressurant stored in the hydrogen cavity.11 . The system of claim 10, wherein the hydrogen vessel is a composite overwrapped pressure vessel (COPV).

12. The system of claim 10, wherein the fuel tank includes a top wall, a bottom wall, and a sidewall extending between the top wall and the bottom wall; and wherein the top wall, the bottom wall, and the sidewall of the fuel tank collectively define the fuel cavity.

13. The system of claim 10, wherein the fuel tank is transitionable between a filled state, in which the fuel cavity of the fuel tank stores a maximum volume of the fuel in liquid form, and a depleted state, in which the fuel cavity of the fuel tank stores a minimum volume of the fuel in liquid form; wherein, in the filled state of the fuel tank, a liquid surface of the fuel is at a first distance from a top wall of the fuel tank; wherein, in the depleted state of the fuel tank, the liquid surface of the fuel is at a second distance from the top wall of the fuel tank; and wherein the second distance is greater than the first distance.

14. The system of claim 13, wherein the hydrogen vessel is positioned within the fuel cavity of the fuel tank such that a top of the hydrogen vessel is at a third distance from the top wall of the fuel tank; and wherein the third distance is greater than the first distance.

15. The system of claim 14, wherein the third distance is greater than the second distance.

16. The system of claim 13, wherein the hydrogen vessel is positioned within the fuel cavity of the fuel tank such that, in the filled state of the fuel tank, the hydrogen vessel is at least partially immersed in the fuel.

17. The system of claim 16, wherein the hydrogen vessel is positioned within the fuel cavity of the fuel tank such that, in the filled state of the fuel tank, the hydrogen vessel is fully immersed in the fuel.

18. The system of claim 17, wherein the hydrogen vessel is positioned within the fuel cavity of the fuel tank such that, in the depleted state of the fuel tank, the hydrogen vessel is at least partially immersed in the fuel.

19. The system of claim 18, wherein the hydrogen vessel is positioned within the fuel cavity of the fuel tank such that, in the depleted state of the fuel tank, the hydrogen vessel is fully immersed in the fuel.

20. The system of claim 13, wherein the pressurant conduit has a pressurant conduit inlet in fluid communication with the flow controller and a pressurant conduit outlet positioned within the fuel cavity of the fuel tank; wherein the pressurant conduit outlet is positioned within the fuel cavity of the fuel tank at a fourth distance from the top wall of the fuel tank; and wherein the fourth distance is less than the first distance.21 . The system of claim 20, wherein the fuel pressurization subsystem further includes a diffuser at the pressurant conduit outlet.

22. The system of claim 20, wherein at least a portion of the pressurant conduit is positioned outside of the fuel cavity of the fuel tank.

23. The system of claim 20, wherein at least a portion of the flow controller is positioned outside of the fuel cavity of the fuel tank.

24. The system of claim 13, wherein the pressurant conduit is a first pressurant conduit and the fuel pressurization subsystem further includes a second pressurant conduit between the flow controller and a liquid-filled space of the fuel tank; wherein the flow controller is configured to selectively pass the first portion of the hydrogen pressurant received from the hydrogen vessel to the first pressurant conduit for transport to the ullage space of the fuel tank; and wherein the flow controller is configured to selectively pass a second portion of the hydrogen pressurant received from the hydrogen vessel to the second pressurant conduit for transport to the liquid-filled space of the fuel tank.

25. The system of claim 24, wherein the flow controller is configured to selectively control at least one flow parameter of the first portion of the hydrogen pressurantpassed to the first pressurant conduit for transport to the ullage space of the fuel tank; and wherein the flow controller is configured to selectively control at least one flow parameter of the second portion of the hydrogen pressurant passed to the second pressurant conduit for transport to the liquid-filled space of the fuel tank.

26. The system of claim 25, wherein the flow controller is configured to selectively control the at least one flow parameter of the first portion of the hydrogen pressurant independent of the at least one flow parameter of the second portion of the hydrogen pressurant, and vice versa.

27. The system of claim 25, wherein the at least one flow parameter of the first portion of the hydrogen pressurant is a first flow rate, and the at least one flow parameter of the second portion of the hydrogen pressurant is a second flow rate.

28. The system of claim 24, wherein the second pressurant conduit has a second pressurant conduit inlet in fluid communication with the flow controller and a second pressurant conduit outlet positioned within the fuel cavity of the fuel tank.

29. The system of claim 28, wherein the second pressurant conduit outlet is positioned within the fuel cavity of the fuel tank such that, in the depleted state of the fuel tank, the second pressurant conduit outlet is fully immersed in the fuel.

30. The system of claim 28, wherein the second pressurant conduit outlet is positioned within the fuel cavity of the fuel tank at a fifth distance from the top wall of the fuel tank; and wherein the fifth distance is greater than the second distance.31 . The system of claim 28, wherein the fuel pressurization subsystem further includes a second diffuser at the second pressurant conduit outlet.

32. The system of claim 24, wherein at least a portion of the second pressurant conduit is positioned outside of the fuel cavity of the fuel tank.

33. The system of claim 1 , wherein the flow controller includes a pressure sensor configured to measure a pressure magnitude within the system, and a processor configured to determine that the pressure magnitude is below a predetermined nominal pressure magnitude.

34. The system of claim 33, wherein the flow controller is configured to selectively pass the at least the first portion of the hydrogen pressurant to the pressurant conduit in response to the determination that the pressure magnitude is below the predetermined nominal pressure magnitude.

35. The system of claim 34, wherein the pressure magnitude is a pressure magnitude in the ullage space of the fuel tank.

36. The system of claim 34, wherein the pressure magnitude is a pressure magnitude in a liquid-filled space of the fuel tank.

37. The system of claim 34, further comprising a fuel pump configured to receive fuel from the fuel tank and output a pressurized fuel flow; wherein the pressure magnitude is a pressure magnitude at an inlet of the fuel pump.

38. The system of claim 1 , wherein the system is a fuel storage system.

39. The system of claim 1 , wherein the system is a fuel transport ground vehicle.

40. The system of claim 1 , wherein the system is one of a rocket, a missile, a spacecraft, and an aircraft.41 . The system of claim 1 , further comprising: an oxidizer tank configured to store an oxidizer in liquid form; an engine including: a fuel pump configured to receive fuel from the fuel tank and output a pressurized fuel flow; an oxidizer pump configured to receive oxidizer from the oxidizer tank and output a pressurized oxidizer flow; a combustion chamber configured to combust a mixture of the pressurized fuel flow and the pressurized oxidizer flow to generate a combustion gas flow; and a nozzle configured to exhaust the combustion gas flow from the combustion chamber to generate thrust.

42. The system of claim 41 , wherein the oxidizer is liquid oxygen (LOX).

43. The system of claim 41 , wherein the engine includes a fluid heater; and wherein the hydrogen vessel, the flow controller, and the pressurant conduit are configured such that the hydrogen pressurant is not in thermal communication with the fluid heater during transport from the hydrogen vessel to the ullage space of the fuel tank via the flow controller and the pressurant conduit.

44. The system of claim 43, wherein the fluid heater is at least one of a heat exchanger, a gas generator, and an electric heater.

45. The system of claim 41 , wherein the fuel tank defines a fuel cavity in which the fuel is stored and the oxidizer tank defines an oxidizer cavity in which the oxidizer is stored; and wherein the fuel tank and the oxidizer tank are arranged in a common bulkhead configuration in which a first side of a common bulkhead wall partially defines the fuel cavity of the fuel tank and an opposing second side of the common bulkhead wall partially defines the oxidizer cavity of the oxidizer tank.

46. The system of claim 45, wherein the common bulkhead wall is not insulated.

47. A method, comprising: filling a fuel tank with a fuel in liquid form such that the fuel tank includes a liquid-filled space and an ullage space; storing a hydrogen pressurant gas in a hydrogen vessel such that the hydrogen pressurant gas is physically separate from the fuel in the fuel tank; determining that a pressure magnitude is below a predetermined nominal pressure magnitude; in response to the determination that the pressure magnitude is below the predetermined nominal pressure magnitude, increasing the pressure magnitude toward the predetermined nominal pressure magnitude by selectively passing at least a first portion of the hydrogen pressurant gas from the hydrogen vessel to the ullage space of the fuel tank.

48. The method of claim 47, wherein the fuel is in cryogenic liquid form.

49. The method of claim 48, wherein the fuel is liquid natural gas.

50. The method of claim 48, wherein the fuel is liquid methane.51 . The method of claim 47, wherein the fuel tank is configured to store the fuel in liquid form at a same temperature as an ambient environment.

52. The method of claim 47, wherein the fuel is a rocket-grade Kerosene.

53. The method of claim 47, wherein the step of increasing the pressure magnitude toward the predetermined nominal pressure magnitude does not involve intentionally heating the at least a first portion of the hydrogen pressurant gas.

54. The method of claim 47, wherein the step of storing the hydrogen pressurant gas in the hydrogen vessel involves positioning the hydrogen vessel within the fueltank such that, in a filled state of the fuel tank, the hydrogen vessel is at least partially immersed in the fuel in the fuel tank.

55. The method of claim 54, wherein the hydrogen vessel is a composite overwrapped pressure vessel (COPV).

56. The method of claim 47, wherein the step of increasing the pressure magnitude toward the predetermined nominal pressure magnitude involves selectively and independently passing the first portion of the hydrogen pressurant gas from the hydrogen vessel to the ullage space of the fuel tank, and selectively and independently passing a second portion of the hydrogen pressurant gas from the hydrogen vessel to the liquid-filled space of the fuel tank.

57. The method of claim 47, wherein the step of increasing the pressure magnitude toward the predetermined nominal pressure magnitude maintains a predetermined subcooled state of the fuel in the fuel tank.

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