Apparatus

Abstract

The present invention relates to a propulsion system 100 for an aircraft, the system comprising: a first cryogenic fuel propulsion system 110; and, a second cryogenic fuel propulsion system 120. The f

Description

Technical Field The present invention is concerned with electrical propulsion systems and the configuration and arrangement of fuel provision for electrical propulsion systems within aircrafts. There are several alternative power sources options (that are much more environmentally friendly than typical combustion of fossil fuels) but these are not widespread. Moreover, such systems are not at present used in commercial-sized aircraft (e.g. 48 or 96 passenger aircraft and / or CS-25 related aircraft). The system used herein is directed toward use of green energy options within commercial-sized aircraft. The system used herein may be directed toward use of green energy options with any sized aircraft, e.g. CS-23 vehicles or rotorcraft. Power systems in aircraft are designed to include fault mitigation. In particular, for developing technologies high levels of safety are to be shown prior to widespread use. As such, the system disclosed herein is directed towards improved safety of operation for fuel provision for electrical propulsion systems within aircrafts. The system herein is safe, robust and more environmentally-friendly than modern systems, and more specifically tailored to the system integration needs of cryogenic fuel systems. The use of cryogenic fuel as the primary fuel source for an aircraft is an area of cutting-edge research; conventional fuel system architectures are not sufficient to deal with the new complexities involved. Cryogenic fuel in particular presents many advantages over contemporary fuels, not least that they are more environmentally-friendly and provide a significant heat sink. However, the combination of cryogenics and fuel system engineering presents challenges that the present invention seeks to overcome. Electrical propulsion systems have many benefits over combustion propulsion systems, particularly in relation to chemical emissions and the like. It is widely seen that electrical propulsion systems may render transport as more viable in a long term perspective. Summary of the Invention Aspects of the invention are set out in the accompanying claims. In accordance with some embodiments described herein, there is provided a propulsion system for an electrically powered aircraft, the system comprising: a first cryogenic fuel propulsion system; and, a second cryogenic fuel propulsion system; the first cryogenic fuel propulsion system comprising: a first cryogenic fuel source; a first propulsion source arranged to generate propulsion from a cryogenic fuel; a first flow path for transporting cryogen from the first cryogenic fuel source to the first propulsion source; the second cryogenic fuel propulsion system comprising: a second cryogenic fuel source; a second propulsion source arranged to generate propulsion from a cryogenic fuel; a second flow path for transporting cryogen from the second cryogenic fuel source to the second propulsion source; the system further comprising: a first conduit, arranged to transport fluid cryogen from the first cryogenic fuel propulsion system to the second cryogenic fuel propulsion system, a second conduit, arranged to transport fluid cryogen from the second cryogenic fuel propulsion system to the first cryogenic fuel propulsion system. The present propulsion system provides excellent redundancy in response to a need for providing cryogenic fuel around the system. For example, this may occur in response to a fault or during a safety event or the like. In particular, the present system provides multiple pathways within and between cryogenic fuel propulsion systems. That the system has two cryogenic fuel propulsion systems provides a first level of redundancy, however the present system takes this advantage further by introducing redundant pathways from one cryogenic fuel propulsion system to the other cryogenic fuel propulsion system. In this way, if an issue is experienced by one cryogenic fuel propulsion system, the other is able to provide some form of support for this issue. Support may be in the form of provision of fuel which was intended for use in the non-affected system or in the provision of additional propulsion output from the non-affected system to account for a loss in propulsion output from the affected system. In particular, there is a conduit from each cryogenic fuel propulsion system to the other. In particular, one conduit is arranged to provide cryogen from one system to the other and another conduit to perform the opposite. In the above, the first conduit may provide cryogen from the first system to the second system and the second conduit may provide cryogen from the second system to the first system. In this way, transport of cryogen through certain conduits allows for the system as a whole to overcome issues that may affect transportation of the vehicle in which the propulsion system is used. In modern systems, such arrangements are not present. In particular, where the vehicles are large commercial aircraft, the present system is not used. Most modern aircraft use less environmentally friendly fuel and therefore do not require the solution proposed herein. In contrast, the present system advantageously provides high level of redundancy for operation alongside the use of environmentally friendly fuel. As a result, the system is quick to react in safety events and is able to provide smooth and reliable transportation with reduced environmental effect over modern systems. The present system also may be used with environmentally friendly fuels, such as liquid hydrogen or the like. The present system may be used with liquid methane or liquefied natural gas. The present propulsion system provides a novel cryogenic fuel propulsion system that segregates the first and second cryogenic fuel propulsion systems to ensure a failure in a component of one does not cascade to a total failure of both; in contrast, by both segregating the first and second cryogenic fuel propulsion systems and providing the herein described redundancy measures, the present invention provides a far more robust and safe system. In examples, the first conduit is arranged to transport gaseous cryogen from the first cryogenic fuel propulsion system to the second cryogenic fuel propulsion system. Gaseous cryogen may be held or produced in either cryogenic fuel propulsion system. The gaseous cryogen may be gaseous hydrogen or gaseous methane or the like. The gaseous cryogen may be provided to the other cryogenic fuel propulsion system for use in provision of propulsion. In an example, the provision of gaseous cryogen in the second cryogenic fuel propulsion system fails (or is reduced). To allow for normal functioning (or functioning that is sufficient), the first cryogenic fuel propulsion system provides gaseous cryogen to the second cryogenic fuel propulsion system. In this way, while the second cryogenic fuel propulsion system may be experiencing an issue, this can be handled by the whole system and normal functioning can be maintained. Gaseous cryogen as used herein refers to cryogenic fuel from cryogenic fuel sources that has transitioned from a liquid state into a gaseous state. Such gaseous cryogen may be in a cryogenic state or a non-cryogenic state depending on the desired heat provision, which is in part based on the design and control of heat exchangers. Indeed, it may be favourable that the gaseous cryogen is at a non-cryogenic temperature, both due to improved control of transfer of a non-cryogen gas compared to a cryogenic gas, and due to cryogenic fuel for a fuel cell or combustor arrangement preferentially being at a temperature above cryogenic temperatures. However, the present invention also encompasses the supply and transferal of gaseous cryogen at cryogenic temperatures. In examples, the first conduit is arranged to transport a gaseous cryogen from the first cryogenic fuel propulsion system to the second propulsion source. When an issue is experienced, to allow for normal functioning (or functioning that is sufficient), the first cryogenic fuel propulsion system may provide gaseous cryogen directly to the propulsion source of the second cryogenic fuel propulsion system. In this way, while the second cryogenic fuel source may be experiencing an issue in cryogen release or delivery or the like, this can be handled by the whole system and normal functioning can be maintained. In a specific example, it may be that the gaseous cryogen is gaseous hydrogen and this is provided to one or more fuel cells of the second cryogenic fuel propulsion system. These fuel cells are able to continue providing power for the propulsor of the second cryogenic fuel propulsion system such that the overall system continues to provide sufficient propulsion for the aircraft in which the system is located. Alternatively, the gaseous hydrogen is provided to one or more combustors of the second cryogenic fuel propulsion system, which likewise are able to continue providing power for the propulsor. In examples, the second conduit is arranged to transport gaseous cryogen from the second cryogenic fuel propulsion system to the first cryogenic fuel propulsion system. In examples, the second conduit is arranged to transport a gaseous cryogen from the second cryogenic fuel propulsion system to the first propulsion source. Similar advantageous effects described above are provided for these preferred features. In examples, the system further comprises a third conduit and a fourth conduit, wherein the third conduit is arranged to transport liquid cryogen from the first cryogenic fuel propulsion system to the second cryogenic fuel propulsion system, and wherein the fourth conduit is arranged to transport liquid cryogen from the second cryogenic fuel propulsion system to the first cryogenic fuel propulsion system. In such an arrangement, there is provided redundant conduits for both liquid transfer and gaseous transfer from cryogenic fuel propulsion system to cryogenic fuel propulsion system. As such, the arrangement is able to account for issues encountered that may block either or both liquid and gaseous resources. Additionally, the present arrangement has greater freedom to overcome faults by virtue of having two distinctly separate and functionally dissimilar redundancy measures, i.e. the gaseous transport and liquid transport. In this way, the system has even greater resilience against issues such as faults in the system. The system is highly robust and additionally highly safe for users. In examples, the third conduit is arranged to transport liquid cryogen from the first cryogenic fuel source to the second cryogenic fuel source, and wherein the fourth conduit is arranged to transport liquid cryogen from the second cryogenic fuel source to the first cryogenic fuel source. By providing liquid cryogen from one system into the fuel source of the other system, one side can fuel the other and allow for normal (or lessened but acceptable) functioning of both sides. The fuel source may be a tank or the like for storing liquid cryogen. If an issue is encountered, the fuel source may need to be provided with additional liquid cryogen. This arrangement allows for that. This arrangement therefore improves the robustness of the system against a greater array of problems that may arise during operation. In examples, the first conduit is arranged, in use, to transport a gaseous cryogen during predetermined stages of flight. In examples, the second conduit is arranged, in use, to transport a gaseous cryogen during predetermined stages of flight. Predetermined stages of flight may include predicted stages of flight where a cryogen is to be transported through the conduit. This may include any as discussed herein, including diagnostics-based variations in conditions that may lead to a circumvention of failure, prevention of failure or mitigation of failure. This may include e.g. a single propulsor failure, where more propulsion is then provided from other propulsors. This may include fuel contamination, wherein the fuel from a non-contaminated source may be used with the propulsion source associated with the contaminated source. This may allow those propulsion sources to continue operating. Predetermined stages of flight may also include a failure such that the level of fuel that can be supplied to one cryogenic fuel propulsion system is not sufficient without provision of further fuel from the other cryogenic fuel propulsion system. Predetermined stages of flight may include reactive stages of flight where a cryogen is to be transported through the conduit. This may include, for example, when a fault arises in one of the cryogenic fuel propulsion systems and actions for remedying the issue are required such as provision of liquid or gaseous cryogen into the system with a fault. Predetermined stages of flight may be during fuel faults or during scheduled redistribution of weight or the like for additional thrust or stability or, in general, improved flight characteristics. Other predetermined stages of flight may be failure of a propulsor or any failure that leads to a need for transfer of fuel. Another example of a predetermined stage of flight may be based in part on the life expectancy of a tank - in other words, where a tank may be reaching the last period of its predetermined life expectancy, the amount of cryogen stored during a flight envelope may need to be actively managed with more fidelity. In examples, this may include: maintaining the tank at a higher or lower pressure; maintaining the tank at a higher or lower temperature; providing cryogen from the tank at a higher or lower rate; and maintaining the cryogen capacity above or below a certain amount. In examples, predetermined stages of flight include: a safety event; an output power failure; an output power reduction; a reduction in capability from the system; ... External effects may also render advantageous the movement or cessation of provision of fuel from fuel sources. For example, where suspected or real damage has occurred from an external source, a fuel source performance may degrade significantly. In such events, the fuel source may need to provide all fuel to another fuel source for storing and using or may need to be cut off from the rest of the system via for example vents and valves and the like. In examples, the first cryogenic fuel propulsion system further comprises a first gaseous store for storing gaseous cryogen, wherein the first conduit is arranged to transport a gaseous cryogen from the first gaseous store to the second cryogenic fuel propulsion system. In examples, the second cryogenic fuel propulsion system further comprises a second gaseous store for storing gaseous cryogen, wherein the second conduit is arranged to transport a gaseous cryogen from the second gaseous store to the first cryogenic fuel propulsion system. Gaseous cryogen stores are useful in situations wherein the gas may be held or conditioned prior to use. The stores may also be useful to provide a reservoir of gaseous cryogen from where such gaseous cryogen can be provided through the conduits to areas of need such as propulsion sources or the like. In examples, the system further comprises a controller arrangement arranged to: detect a change in operating state; and, activate cryogen transportation via at least one of the first conduit and the second conduit based on the change in operating state. A change in operating state may occur at a predetermined stage of flight or may occur outside of such a stage of flight. A change in operating state may include any of change of flight stage (e.g. from take-off to climb to cruise etc.). The change in operating state may be a change in the amount of propulsion provided to the system as a whole or the amount of propulsion provided by each cryogenic fuel propulsion system in the overall system. The change may be a change in the flow or delivery of cryogen to elements within the overall system. The change may be a change in the pressure in any part of the overall system, in examples a change in the pressure in the cryogenic fuel sources. Any of these changes may be noted and may advantageously be mitigated or resolved by providing cryogen transportation through the system. In examples, the first conduit is further arranged to transport a gaseous cryogen from the first cryogenic fuel propulsion system to the first cryogenic fuel source, the second conduit is further arranged to transport a gaseous cryogen from the second cryogenic fuel propulsion system to the second cryogenic fuel source, wherein the first cryogenic fuel propulsion system further comprises a first controllable repressurisation valve, wherein in an open state the first controllable repressurisation valve provides fluid communication from the first conduit to the second cryogenic fuel source, and, wherein the second cryogenic fuel propulsion system further comprises a second controllable repressurisation valve, wherein in an open state the second controllable repressurisation valve provides fluid communication from the second conduit to the first cryogenic fuel source. The arrangement may be able to account for difficulties in repressurising the cryogenic fuel sources. In particular, the cryogenic fuel sources may more easily distribute cryogen from the fuel sources where there is gaseous cryogen provided back to the cryogenic fuel sources so that there is no pressure differential build up during use. As such, where there are difficulties in repressurising due to e.g. a fault or the like, the system may provide gaseous cryogen from one of the cryogenic fuel propulsion systems to the other. In this way, both cryogenic fuel sources may be provided with gaseous cryogen from one cryogenic fuel source. In accordance with some embodiments described herein, there is provided an at least partially electrically powered aircraft comprising the propulsion system of any of the above embodiments and examples. Such an aircraft may comprise fuel cells as well as e.g. a gas turbine power generator. Combined power propulsion may be advantageous for power redundancy. Similarly, combined power propulsion may provide advantageous thrust benefits considering differing requirements of thrust at different stages of flight. The propulsion system disclosed herein is particularly advantageous within an aircraft, however the propulsion system may be used in any vehicle. In accordance with some embodiments described herein, there is provided a method of power management system in an aircraft, the method comprising: (i) providing a first conduit for transporting a cryogen from a first cryogenic fuel propulsion system to a second cryogenic fuel propulsion system; (ii) providing a second conduit for transporting a cryogen from a second cryogenic fuel propulsion system to a first cryogenic fuel propulsion system; (iii) detecting a change in operating conditions; (iv) providing at least one of: (a) cryogen to the second cryogenic fuel propulsion system from the first cryogenic fuel propulsion system via the first conduit; and, (b) cryogen to the first cryogenic fuel propulsion system from the second cryogenic fuel propulsion system via the second conduit. With this use, the method provides a robust and quickly reactive approach for fault mitigation. This method provides improvement over previous systems as per the above discussion particularly in light of the ability to respond quickly in fault mitigation and in use of green fuels. This therefore improves the overall safety of the system and the methods disclosed over modern versions. In examples, providing cryogen via the first conduit and / or the second conduit comprises providing gaseous cryogen via the first conduit and / or the second conduit, and providing cryogen from a first cryogenic fuel propulsion system to a second cryogenic fuel propulsion system comprises providing cryogen from the first cryogenic fuel propulsion system to a second propulsion source, and providing cryogen from a second cryogenic fuel propulsion system to a first cryogenic fuel propulsion system comprises providing cryogen from the second cryogenic fuel propulsion system to a first propulsion source. In examples, detecting a change in operating conditions comprises detecting occurrence of at least one of: a predetermined stage of flight; a safety event; an output power failure; an output power reduction; and a reduction in capability from the system. Other examples include considering a ballast and weight distribution within the aircraft. This may benefit from a 5 redistribution of fuel within the larger system. Further examples include flight management events such as stability and tank lifetime (and mitigating against e.g. impending failure as the lifetime of tanks approach expected end of lifetime). In examples, the method further comprises providing a first controllable repressurisation valve 10 to control fluid communication from the first conduit to the second cryogenic fuel source; providing a second controllable repressurisation valve to control fluid communication from the second conduit to the first cryogenic fuel source; and in response to detecting a change in operating conditions, actuating one of the first controllable repressurisation valve and the second controllable repressurisation valve. 15 Brief Description of the Drawings One or more embodiments of the invention will now be described, by way of example only, and with reference to the following figures in which: Figure 1 shows a schematic view of a propulsion system according to an example of the present disclosure; Figure 2 shows a schematic view of a portion of a propulsion system according to an example of the present disclosure; Figure 3 shows a schematic view of a portion of a propulsion system according to an example of the present disclosure; Figure 4 shows a schematic view of a propulsion system according to an example of the present disclosure; Figure 5 shows a schematic view of a propulsion system according to an example of the present disclosure; and, Figure 6 shows a flow diagram of a method according to an example of the present disclosure. Any reference to prior art documents in this specification is not to be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field. As used in this specification, the words “comprises”, “comprising”, and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean “including, but not limited to”. The invention is further described with reference to the following examples. It will be appreciated that the invention as claimed is not intended to be limited in any way by these examples. It will also be recognised that the invention covers not only individual embodiments but also combination of the embodiments described herein. The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and / or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the spirit and scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc, other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future. Detailed Description An invention described herein relates to propulsion systems for electrically powered aircraft. A particular use for this invention may be in an aircraft with an electrically drivable motor or a drivable motor that is at least partially electrically driven. For example, a propulsion source in the arrangements discussed may be fully or partially electrically powered. Partially powered aircraft may use thrust provided in part by electrical means and in part by combustion means. This invention may be used in a fully or partially combustion powered aircraft. The electrical and combustion aspects may be provided by one or by a few fuels. Figure 1 shows a simple schematic view of a propulsion system 100. The propulsion system 100 may be for an electrically powered aircraft. The system 100 comprises a first cryogenic fuel propulsion system 110 and a second cryogenic fuel propulsion system 120. The first cryogenic fuel propulsion system 110 comprises a first cryogenic fuel source 112. The cryogenic fuel source 112 may be tank holding liquid cryogen or the like. The liquid cryogen may be liquid hydrogen, liquefied natural gas and / or liquid methane to name only a few. The first cryogenic fuel propulsion system 110 comprises a first propulsion source 114 arranged to generate propulsion from a cryogenic fuel. The propulsion source 114 may be a converter of chemical energy to kinetic energy. The propulsion source 114 may be a fuel cell and propulsor arrangement that is capable of providing electrical energy from chemical energy for conversion into kinetic energy. The propulsion source 114 may be a gas turbine and propulsor arrangement that combusts chemical energy into kinetic energy. As used herein, a reference to fuel cell may be a reference to more than one fuel cell or a fuel cell stack or the like. The first cryogenic fuel propulsion system 110 comprises a first flow path 113 for transporting cryogen from the first cryogenic fuel source 112 to the first propulsion source 114. The flow path 113 is able to transport cryogen within and around the first cryogenic fuel propulsion system 110. The second cryogenic fuel propulsion system 120 comprises a second cryogenic fuel source 122 and a second propulsion source 124 arranged to generate propulsion from a cryogenic fuel. These may be similar to the fuel source 112 and propulsion source 114. These elements perform the same function at the least. In that, the fuel sources provide a source of fuel for conversion into propulsion by the propulsion sources. That the system 100 has two cryogenic fuel propulsion systems provides good redundancy from a safety perspective. If one cryogenic fuel propulsion system fails, the other can be used. This system 100 goes further than that however and provides for cross linking of the two systems 110, 120 for a more intricate sharing of resources. This allows better use of the fuel from one system if the other requires additional fuel or if the first system can no longer use the fuel due to faults or the like. In particular, the propulsion system 100 further comprises a first conduit 115 and a second conduit 126. The first conduit 115 is arranged to transport fluid cryogen from the first cryogenic fuel propulsion system 110 to the second cryogenic fuel propulsion system 120. The second conduit 126 is arranged to transport a fluid cryogen from the second cryogenic fuel propulsion system 120 to the first cryogenic fuel propulsion system 110. As shown, the first conduit 115 is arranged to provide fluid communication between the two systems 110, 120. By providing this link, the first conduit 115 is able to provide cryogen from the first system 110 to the second system 120. This arrangement provides a redundancy of fuel resource provision. This may be desirable in handling safety or other similar fuel provision issues. Furthermore, as can be seen in Figure 1, the conduit 115 is illustrative of conduits that can be installed to take cryogen from one side of the system 100 to the other (by “sides” reference to the two systems 110, 120 is meant). The first conduit 115 may be arranged to provide fluid communication between any of the features of the first system 110 to any of the features of the second system 120. For example, conduits 115a and 115b are arranged to provide fluid communication for gaseous cryogen between the two systems 110, 120: the former 115a substantially after a gaseous buffer store 119; the latter 115b substantially before the gaseous buffer store 119. The second conduit 126 is shown to link the second system 120 to the first system 110 such that the fuel from the second system 120 can be provided to the first system 110 e.g. should the first system experience problems in fuel delivery or transportation. The system 100 further comprises heat exchangers 117, 127 for exchanging heat within the system. The heat exchangers may be used to warm up liquid cryogen into gaseous cryogen. In use, the fuel store 112 which may be a tank or the like may store liquid cryogen. This flows along the first flow path to the heat exchangers 117 for thermal energy transfer. The liquid cryogen is heated to gaseous form. The liquid cryogen is heated such that it undergoes phase transition into gaseous form - additional heat may be provided so as to superheat the gaseous cryogen. We may refer to the gaseous form as “gaseous cryogen” though the temperature may be ambient or the like. The gaseous form cryogen is then passed to the gaseous buffer store 119. From the gaseous buffer store 119 the gaseous cryogen flows along the first flow path to the propulsion sources 114. As mentioned above, the propulsion sources 114 may be a series of fuel cells or fuel cell stacks. The propulsion sources 114 may include propulsors or the like. The propulsion sources 114 and propulsion sources 124 are segregated from one another such that they receive fuel from the first and second cryogenic fuel propulsion systems 110, 120 respectively. In this example, the cryogen is therefore passed through the first cryogenic fuel propulsion system 110 to ultimately provide propulsion from a cryogenic fuel. The same arrangement can be seen in the second cryogenic fuel propulsion system 120. The cryogenic fuel is stored in second cryogenic fuel store 122. This flows along the second flow path to the heat exchangers 127 for conversion into gaseous cryogen. The gaseous cryogen may be provided to and stored in a gaseous fuel buffer store 129 prior to being passed to the propulsion source 124. The first and second conduits therefore provide resilience against problems being encountered by the system 100. By enabling passing of fuel from one side of the arrangement to the other, the system 100 has a series of flexible solutions for overcoming errors that arise in different portions of the systems 110, 120. For example, where an issue arises that the tank 122 experiences a fault such as a blockage in the pipe from freezing, the tank 112 can provide liquid cryogen via a conduit to the heat exchangers 127 of the second system 120. In an example, where an issue arises that the tank 122 can no longer provide the required rate of cryogen to the second cryogenic fuel propulsion system 120 (e.g. a blockage in the pipe from freezing, or failure of the tank itself), the tank 112 can provide cryogen via the first conduit 115 to second system 120, such that propulsion source 124 can remain at least partially operational. In this example, the heat exchangers 127 could be circumvented - this may place more load on the heat exchangers 117 of the first system 110 to provide sufficient heat to the cryogen, however this is favourable to the total failure of second system 120. For example, where an issue arises in the gaseous buffer store 119 of the first system 110 (such as a loss of pressure or the like) such that gaseous cryogen cannot be provided to the fuel cells 114, the gaseous buffer store 129 of the second system 120 can provide gaseous cryogen via the second conduit 126 to the fuel cells 114 of the first system 110. For example, where an issue arises that one or more fuel cells within a first propulsion source 114 ceases to function (for example due to mechanical fault), the remaining fuel cells from that propulsion source 114 may require an increased delivery of gaseous cryogen. This may occur by augmenting the delivery of gaseous cryogen from the first system 110 by additional delivery of gaseous cryogen from the second system 120. In this way, a variety of different issues may be overcome by the present arrangement. All such examples are applicable to both sides, i.e. from the first system 110 to the second system 120, and vice versa. A conduit (or more than one conduit) may carry cryogen from the first cryogenic fuel propulsion system 110 to anywhere in the second cryogenic fuel propulsion system 120 as is deemed useful. This may be into the tank 122 of the second cryogenic fuel propulsion system 120. This allows the cryogen to be redistributed from tank to tank where this may be beneficial for weight reasons. This may be into the gaseous buffer store 129 of the second system 120. This system 100 is therefore robust against a tank faults or buffer store faults in either the first cryogenic fuel propulsion system 110 or the second cryogenic fuel propulsion system 120. If the tank or buffer store of one system (e.g. the first system 110) has an issue that would lose the fuel therein, the fuel can be preferentially moved into and stored in the tank or buffer store of the other system (e.g. the second system 120). In this way, there is not a great loss of fuel when one of the e.g. storage components for cryogenic fuel experiences a fault. The system 100 is therefore highly robust against issues that may affect functioning of elements within the system 100. As noted above, cryogen may be provided through the first and second conduits at predetermined stages of flight, such as during fault mitigation or during weight redistribution. Other examples include where there has been a failure in one of the cryogenic fuel propulsion systems. When a full systems fails, there is a need to move cryogenic fuel into the other system (or one of the other multiple systems) for propulsion. Where a failure is partial, there may be a redistribution of fuel and operation of the propulsion sources that accounts for the partial failure. For example, whereas while in normal operation both cryogenic fuel propulsion systems provide 50% propulsion of the total propulsion, in partial failure the partially-failed system may provide 40% while the fully operational system may provide 60% propulsion or the like. This may be referred to asymmetric power or propulsion output. Different failures (or safety events or the like) may use different solutions. Where there is a liquid tank failure, the liquid from that tank may be moved to other tanks, where there is a gaseous tank failure, the gas from that store may be moved to other locations or stores. Where a busbar in the system is not providing enough power, there may a rearrangement (via actuating valves and the like) for additional gaseous fuel to be provided to e.g. fuel cells for that busbar. This will increase the power output on the busbar (up to a point). Referring now to Figure 2, there is shown a portion 200 of a propulsion system according to an example of the present disclosure. The portion 200 does not show the second cryogenic fuel propulsion system (as shown in Figure 1). Rather the portion 200 shows a schematic first cryogenic fuel propulsion system. The cryogenic fuel propulsion system 200 of Figure 2 comprises a cryogenic fuel source 210 that, in use, contains a cryogenic resource. The arrows of Figure 2 show the movement of cryogen fuel through the system 200. The fuel source 210 is linked to the heat exchanger arrangement 220. The heat exchanger arrangement 220 heats the cryogenic liquid and provides a gaseous cryogen as noted for Figure 1. The cryogen flows from the heat exchanger arrangement 220 to the interface 230. The interface 230 may be a delivery mechanism for providing gaseous cryogen to at least the first conduit (or the second conduit when corresponding to the second system 120 of Figure 1) and propulsion source. The interface 230 may be include a manifold or the like. As stated above, the interface 230 is connected to a propulsion source arrangement 240. The propulsion source arrangement 240 may comprise an arrangement of fuel cells and a prime mover arrangement, wherein the fuel cells provide electrical power to the prime mover arrangement. The prime mover arrangement may be a propulsor or series of propulsors. In this example, the interface 230 is a fuel cell interface 230 arranged to interface with the fuel cells of the propulsion source arrangement 240. In examples, the propulsion source arrangement 240 comprises a prime mover arrangement 240 arranged to convert chemical energy into kinetic energy for propulsion for the vehicle, i.e. without fuel cells. In this example, the prime mover arrangement may be a combustor or series of combustors coupled to a propulsor. Referring now to Figure 3, there is shown a portion 300 of a propulsion system according to an example of the present disclosure. The portion 300 does not show the second cryogenic fuel propulsion system (as shown in Figure 1). Rather the portion 300 shows a schematic first cryogenic fuel propulsion system 300. The arrangement of Figure 3 includes a number of the same features as per Figure 2 with reference numerals increased by 100. For example element 310 of Figure 3 is a cryogenic fuel source containing, in use, a cryogenic resource. This is the same as for element 210 of Figure 2. These features may not be discussed in detail for expediency. The arrangement of Figure 3 further includes a manifold 312 for distributing the cryogenic resource to the heat exchanger arrangement 320. The heat exchanger arrangement 320 includes segregated heat exchangers 320a, 320b, 320c. Each is arranged to provide heat exchange functions such that the liquid cryogen from the source 310 can be turned into a gaseous cryogen for use in later components of the system 300. Figure 3 shows three such segregated heat exchangers 320a, 320b, 320c; Figure 1 shows four segregated heat exchangers (within heat exchanger 117) - these are examples, and any number of segregated heat exchangers may be used. Alternatively, only one heat exchanger may be used. From these segregated heat exchangers 320a, 320b, 320c gaseous cryogen is provided to the interface 330 and the propulsion source arrangement 340 as per Figure 2. Referring now to Figure 4, there is shown a propulsion system 400 according to an example of the present disclosure. The system 400 has a first cryogenic fuel propulsion system 400a and a second cryogenic fuel propulsion system 400b. The arrangement of Figure 4 includes a number of the same features as per Figure 3 with reference numerals increased by 100. For example elements 410a, 410b of Figure 4 are cryogenic fuel sources containing, in use, a cryogenic resource. This is the same as for element 310 of Figure 3. These features may not be discussed in detail for expediency. The arrangement of Figure 4 shows two systems 400a, 400b with cryogenic fuel sources 410a, 41 Ob, heat exchanger arrangements 420a, 420b, interfaces 430a, 430b, and propulsion source arrangements 440a, 400b. Figure 4 also shows a series of cross links in the system 400. The system 400 has a first liquid interlink 450a and a second liquid interlink 450b. These allow liquid transfer between the two systems. Specifically, first liquid interlink 450a allows liquid from the cryogenic source 410a of the first cryogenic fuel propulsion system 400a to be provided to the cryogenic source 410b of the second cryogenic fuel propulsion system 400b. Likewise, the second liquid interlink 450b allows liquid from the cryogenic source 410b of the second cryogenic fuel propulsion system 400b to be provided to the cryogenic source 410a of the first cryogenic fuel propulsion system 400a. Each of the interlinks are shown with liquid interlink valves 452a, 452b. The interlinks may be conduits or the like. Figure 4 also shows a gaseous interlink 460a from the first system 400a to the second system 400b and vice versa for gaseous interlink 460b. These also have valves 462a, 462b respectively. The gaseous interlinks may provide gaseous cryogen from the system interfaces 430a, 430b to each other. While Figure 4 does not show them, the gaseous interlinks 460a, 460b may further include repressurisation interlinks connected to the cryogenic fuel sources 410a, 410b. Figure 4 shows an example arrangement that has high communicability between the two systems 400a, 400b. This allows the two systems to operate in tandem to overcome isolated issues within the system 400 as a whole. The two systems 400a, 400b can be cooperative to share the burden of overcoming problems to components within the overall system 400. This allows the system to be resilient against problems, issues, safety events or the like and to provide a high level of service throughout. Referring now to Figure 5, there is shown a propulsion system 500 according to an example of the present disclosure. The system 500 has a first cryogenic fuel propulsion system 510 and a second cryogenic fuel propulsion system 520. The arrangement of Figure 5 is similar in part to the arrangement of Figure 1. Numerals for similar or identical elements are increased by 400. In that, fuel source 512 in Figure 5 is the same as fuel source 112 of Figure 1. Similar or identical elements will not be discussed in detail for expediency. Specifically, Figure 5 shows repressurisation paths shown in yellow (portions on the path are indicated at 528a and 528b) and in red (a portion on the path is indicated at 519). Specifically, in Figure 5, an example is shown wherein the repressurisation path in red is not functioning due to 518a having failed, therefore the repressurisation path in yellow is provided gaseous cryogen to both tanks for repressurising the two tanks to improve delivery of liquid cryogen from the tanks. Figure 5 further shows a series of repressurisation valves for allowing repressuring of the tanks from gaseous cryogen. Repressuring the tanks 512, 522 is advantageous to encourage release of liquid cryogen from the tanks. Without repressurisation, pumping liquid from the tanks 512, 522 will tend to reduce their pressure until it becomes difficult or impossible to withdraw the fuel for consumption. In the arrangement of Figure 5, gaseous cryogen is fed back to the tanks 512, 522 to prevent a significant pressure drop arising in the tanks 512, 522 and therefore negatively impacting liquid flow from the tanks 512, 522. In particular, the system 500 shows a first system 510 controllable repressurisation valve 518a and a second system 520 controllable repressurisation valve 528a. When these are open, the system provides to its own respective tank a gaseous cryogen to assist in repressurisation. The system 500 shows a first system 510 controllable repressurisation valve 518b and a second system 520 controllable repressurisation valve 528b. When these are open, the system can provide to the tank of the other system a gaseous cryogen to assist in repressurisation. As such, the valves allow for repressurisation to occur from one system to itself or from that system to the other. In this way, where e.g. the first system 510 encounters a fault in the transport of or creation of gaseous cryogen, the second system 520 can provide a gaseous cryogen into the tank 512 of the first system 510. In this way, incident cascades can be prevented. The controllable valves may be controlled by a controller arrangement. The controller arrangement may be used to detect changes in circumstances, such as a change in pressure in the cryogenic fuel propulsion systems, and then control the actuation of the valves. In examples, the pressure in a first cryogen source is detected as being lower than preferred. The controller arrangement may then detect a further issue in repressurisation of the first tank. In an example, the controller may detect the failure of the controllable repressurisation valve 518a, and in response control the actuation of the corresponding controllable repressurisation valve 528b to ensure that the first tank 512 is pressurised to permit continued pumping of fuel as required. For example, the valves prevent the following issues. Where a system (e.g. the first 510) experiences an issue in the heat exchangers, the first system 510 will not generate sufficient gaseous cryogen to provide repressurisation to the tank 512. At this point, the tank 512 will find provision of liquid cryogen from the tank 512 increasingly difficult and eventually the system 510 will not be able to provide any liquid whatsoever from the tank 512. At this point, the whole first system 510 cannot provide any function. Furthermore, the system may require venting of fuel to avoid tank over-pressurisation from liquid cryogen boil off. Instead, in the present arrangement, when the first system 510 does not generate sufficient gaseous cryogen to provide repressurisation to the tank 512, gaseous cryogen can be provided via actuating of the controllable repressurisation valve to provide fluid communication from the gaseous cryogen of the second system 520 to the tank 512 of the first system 510. While the liquid from the tank 512 may not be functional in the first system 510 (if, as above, the heat exchangers 517 are faulty), this arrangement allows the liquid from the first tank 512 to be provided into the tank 522 of the second system 520 and / or used in the second system 520. In this way, the primary issue can be circumvented by intelligent re-routing of resources via the conduits of the system 500, in this case, liquid can be prevented from going to the faulty heat exchangers 517 but can be passed to the tank 522 and provided to the functioning heat exchangers 527. In another example, where the controllable repressurisation valve 518a fails and the first system 510 can no longer repressurise itself (i.e. repressurise tank 512), the controllable repressurisation valve 528b may be opened so that the second system 520 can perform repressurisation of the tank 512. The system disclosed herein is highly reactive and high robust against component failure. The present system prevents failure cascades and maintains a high level of service throughout operation. Gaseous cryogen may be provided from any location on either system to the other where desired. For example, the gaseous cryogen in the buffer stores 519, 529 may be rerouted to the other system such as into the tank or into the fuel cells therein. In examples, gaseous buffers or gaseous conduits may be interlinked to provide additional redundancy in the event of loss of liquid cryogen supply to the fuel heating system linked to any of the cryogen fuel sources. In a further example, the buffer store 519 may be interlinked to the buffer store 529 (and vice versa) such that gaseous cryogen may be rerouted there between. It may be favourable to include a check valve (non-return valve) substantially immediately prior to the buffer stores 519, 529, such that conduits between buffer stores 519, 529 permit both the transferal of gaseous cryogen and pressure management in the buffer stores. In each of these examples, the conduits may be surrounded (not shown) by a vacuum or the like. This improves the prevention of external warming thermal energy from impinging on the conduits which are being maintained in a low temperature. Referring now to Figure 6, a method 600 according to an example of the present disclosure is shown as a flow diagram. The method 600 has four steps 605, 610, 615, 620. In a first step 605, a first conduit is provided for transporting a cryogen from a first cryogenic fuel propulsion system to a second cryogenic fuel propulsion system. As noted above, this may be during fault stages or during weight distribution or the like. In a second step 610, a second conduit is provided for transporting a cryogen from a second cryogenic fuel propulsion system to a first cryogenic fuel propulsion system. As noted above, this may be during fault stages or during weight distribution or the like. These two steps together provide a great level of redundancy for the vehicle. This is useful in many vehicles but is particularly advantageous in aircraft. In a third step 615, a change in operating conditions is detected. This may be performed by a controller arrangement or the like. This may be a predetermined stage of flight. A change may relate to a change in the thrust, thrust demand, climb rate, altitude, flow of cryogen or operational state of any of the components of the systems discussed herein. In a fourth step 620, at least one of the following steps occurs: provision of cryogen to the second cryogenic fuel propulsion system from the first cryogenic fuel propulsion system via the first conduit; and provision of cryogen to the first cryogenic fuel propulsion system from the second cryogenic fuel propulsion system via the second conduit. In this method therefore there is high redundancy and high responsiveness to a fault or the like 5 where cryogen is advantageously moved throughout a propulsion system. In the above, the cryogenic sources may be tanks containing liquid cryogen or the like. These may be double walled fuel tanks with outer vacuum enclosures and inner hydrogen vessels in liquid fill and feed lines. These may have gas vent lines. These may have burst disks provided 10 for vacuum space protection or the like. In the above, there may be pressure relief valves as well as flow control valves. These may have specific conduits (not discussed above) to allow for release or redistribution of gaseous cryogen or the like.

Claims

1. A propulsion system for an aircraft, the system comprising: a first cryogenic fuel propulsion system; and,5 a second cryogenic fuel propulsion system;the first cryogenic fuel propulsion system comprising:a first cryogenic fuel source;a first propulsion source arranged to generate propulsion from a cryogenic fuel;a first flow path for transporting cryogen from the first cryogenic fuel source to 10 the first propulsion source;the second cryogenic fuel propulsion system comprising:a second cryogenic fuel source;a second propulsion source arranged to generate propulsion from a cryogenic fuel;15 a second flow path for transporting cryogen from the second cryogenic fuelsource to the second propulsion source;the system further comprising:a first conduit, arranged to transport fluid cryogen from the first cryogenic fuel propulsion system to the second cryogenic fuel propulsion system,20 a second conduit, arranged to transport fluid cryogen from the second cryogenic fuelpropulsion system to the first cryogenic fuel propulsion system.

2. A propulsion system according to claim 1, wherein the first conduit is arranged to transport gaseous cryogen from the first cryogenic fuel propulsion system to the second 25 cryogenic fuel propulsion system.

3. A propulsion system according to claim 2, wherein the first conduit is arranged to transport a gaseous cryogen from the first cryogenic fuel propulsion system to the second propulsion source.

304. A propulsion system according to any preceding claim, wherein the second conduit is arranged to transport gaseous cryogen from the second cryogenic fuel propulsion system to the first cryogenic fuel propulsion system.35 5. A propulsion system according to claim 4, wherein the second conduit is arranged totransport a gaseous cryogen from the second cryogenic fuel propulsion system to the first propulsion source.

6. A propulsion system according to any preceding claim, further comprising a third conduit and a fourth conduit,wherein the third conduit is arranged to transport liquid cryogen from the first cryogenic 5 fuel propulsion system to the second cryogenic fuel propulsion system, andwherein the fourth conduit is arranged to transport liquid cryogen from the second cryogenic fuel propulsion system to the first cryogenic fuel propulsion system.

7. A propulsion system according to claim 6, wherein the third conduit is arranged to 10 transport liquid cryogen from the first cryogenic fuel source to the second cryogenic fuel source, andwherein the fourth conduit is arranged to transport liquid cryogen from the second cryogenic fuel source to the first cryogenic fuel source.15 8. A propulsion system according to any preceding claim, wherein the first conduit isarranged, in use, to transport a gaseous cryogen during predetermined stages of flight.

9. A propulsion system according to any preceding claim, wherein the second conduit is arranged, in use, to transport a gaseous cryogen during predetermined stages of flight.

10. A propulsion system according to claim 8 or 9, wherein predetermined stages of flight include: a safety event; an output power failure; an output power reduction; and, a reduction incapability from the system.25 11. A propulsion system according to any preceding claim, the first cryogenic fuelpropulsion system further comprising:a first gaseous store for storing gaseous cryogen, wherein the first conduit is arranged to transport a gaseous cryogen from the first gaseous store to the second cryogenic fuel propulsion system.3012. A propulsion system according to any preceding claim, the second cryogenic fuel propulsion system further comprising:a second gaseous store for storing gaseous cryogen, wherein the second conduit is arranged to transport a gaseous cryogen from the second gaseous store to the first cryogenic 35 fuel propulsion system.

13. A propulsion system according to any preceding claim further comprising:1015xt CM2530a controller arrangement arranged to:detect a change in operating state; and,activate cryogen transportation via at least one of the first conduit and the second conduit based on the change in operating state.

14. A propulsion system according to any preceding claim, whereinthe first conduit is further arranged to transport a gaseous cryogen from the first cryogenic fuel propulsion system to the first cryogenic fuel source,the second conduit is further arranged to transport a gaseous cryogen from the second cryogenic fuel propulsion system to the second cryogenic fuel source,wherein the first cryogenic fuel propulsion system further comprises a first controllable repressurisation valve,wherein in an open state the first controllable repressurisation valve provides fluid communication from the first conduit to the second cryogenic fuel source, and,wherein the second cryogenic fuel propulsion system further comprises a second controllable repressurisation valve,wherein in an open state the second controllable repressurisation valve provides fluid communication from the second conduit to the first cryogenic fuel source.

15. An aircraft comprising the propulsion system of any of claims 1-14.

16. A method of power management system in an aircraft, the method comprising:i) providing a first conduit for transporting a cryogen from a first cryogenic fuel propulsion system to a second cryogenic fuel propulsion system;ii) providing a second conduit for transporting a cryogen from a second cryogenic fuel propulsion system to a first cryogenic fuel propulsion system;iii) detecting a change in operating conditions;iv) providing at least one of:a) cryogen to the second cryogenic fuel propulsion system from the first cryogenic fuel propulsion system via the first conduit; and,b) cryogen to the first cryogenic fuel propulsion system from the second cryogenic fuel propulsion system via the second conduit.

17. A method according to claim 16,35 wherein providing cryogen via the first conduit and / or the second conduitcomprises providing gaseous cryogen via the first conduit and / or the second conduit, andwherein providing cryogen from a first cryogenic fuel propulsion system to a second cryogenic fuel propulsion system comprises providing cryogen from the first cryogenic fuel propulsion system to a second propulsion source, andwherein providing cryogen from a second cryogenic fuel propulsion system to5 a first cryogenic fuel propulsion system comprises providing cryogen from the second cryogenic fuel propulsion system to a first propulsion source.

18. A method according to claim 16 or 17, wherein detecting a change in operating conditions comprises detecting occurrence of at least one of:10 a predetermined stage of flight; a safety event; an output power failure; an output powerreduction; and, a reduction in capability from the system.1519. A method according to any of claims 16 to 18, further comprising:providing a first controllable repressurisation valve to control fluid communication from the first conduit to the second cryogenic fuel source;providing a second controllable repressurisation valve to control fluid communication from the second conduit to the first cryogenic fuel source; andin response to detecting a change in operating conditions, actuating one of the first controllable repressurisation valve and the second controllable repressurisation valve.2020.253035A fuel supply system for an aircraft propulsion system comprising:a first cryogenic fuel system; and,a second cryogenic fuel system;the first cryogenic system comprising:a first cryogenic fuel source;a first flow path for transporting cryogen from the first cryogenic fuel sourcewithin the first cryogenic system;the second cryogenic fuel system comprising:a second cryogenic fuel source;a second flow path for transporting cryogen from the second cryogenic fuel source within the second cryogenic system;the system further comprising:a first conduit, arranged to transport fluid cryogen from the first cryogenic fuel system to the second cryogenic fuel system,a second conduit, arranged to transport fluid cryogen from the second cryogenic fuel system to the first cryogenic fuel system.

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