Apparatus

Abstract

A power source system for an electrically powered aircraft has a busbar which receives electrical power from a high energy density electrical power source 380 and receives and provides electrical power to a di-directional electrical power source 320. In use, a controller detects an aircraft propulsive power requirement, an aircraft flight characteristic, and a power source power characteristic, and controls the power outputs of the power sources based on these factors, and based on the power outputs of the power sources. Also claimed is a method of aircraft power management by detecting a propulsive power requirement, calculating and providing power from a high energy density power source and a bi-directional electrical power source, detecting a change in propulsive power requirement, then calculating and providing different power from either or both power sources. The method may also involve detecting a change in operating conditions. The high energy density power source may provide power from a fuel cell and a gas turbine, and provide less than a maximum propulsive power requirement. The bi-directional power source may be a battery, capacitor and / or super-capacitor.

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. Alongside or separately to green energy considerations there are also economic advantages that can be made from the system presented herein. Furthermore, the system presented herein may be used advantageously on non-commercial aircraft such as unmanned aircraft with reduced observation fuel types. Power systems in aircraft are designed to provide suitable power outputs at all stages of flight. The demands for power can change rapidly and therefore power systems are to account for both the demands and the change in demands as they arise. Furthermore, power output can vary and therefore it may be advantageous to account for these variations. 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 use of cryogenics leads to power density considerations alongside the consideration of providing power at controllably variable levels. The power sources discussed herein provide great control and flexibility over delivery of power to e.g. aircraft propulsion systems. The power sources discussed herein may be used with any propulsive vehicle. 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 power source system for an electrically powered aircraft, the system comprising: a high energy density power source arrangement for providing electrical power; a bi-directional electrical power source arrangement for providing and receiving electrical power; a busbar arranged to receive electrical power from the high energy density power source arrangement, and to receive electrical power from and provide electrical power to the bi-directional electrical power source arrangement, a controller arrangement arranged to: detect a flight characteristic of an aircraft; detect a power characteristic of the power source system; and, control a power output of the high energy density power source arrangement and the bi-directional electrical power source arrangement, wherein, the controller arrangement is further arranged, in use, to: detect a propulsive power requirement for an aircraft; detect a flight characteristic of an aircraft; detect a power characteristic of the power source system; control the power output of the high energy density power source arrangement and the bi-directional electrical power source arrangement based on the propulsive power requirement, flight characteristic of an aircraft and power characteristic of the power source system, the power output of the high energy density power source arrangement and the bi-directional electrical power source arrangement provided to the busbar. The term “high energy density” herein may relate to systems with energy densities in the region of around 500 Wh / kg or higher. Indeed, specific high energy density power source arrangements may have energy densities in the region of around 500-900 Wh / kg. High energy density power source arrangements may have energy densities in the region of around SOO-SOO Wh / kg. High energy density power source arrangements may have energy densities in the region of around 700 Wh / kg. Suitable high energy density power source arrangements have energy densities greater than modern batteries or super capacitors. The present power source system provides excellent energy provision for use in e.g. an electrically powered aircraft. The challenges associated with energy provision for aircraft include the variable requirements for energy during different stages of flight. In particular, the requirements for energy can change very quickly for example for taxi to take off and from climb to cruise. As such, the power source system herein accounts excellently for the power requirements of electrically powered aircraft. In this way, the present system accounts for the drawbacks of electrically powered aircraft wherein energy provision is not well suited to the changing demands of air travel while in examples enabling use of greener fuels such that the environmental impact of air travel is reduced. In particular, the present system uses a high energy outputting arrangement alongside a variable produce-receive energy arrangement. In this way, while the steady high energy output may be greater than the total power requirement at points and less than the total power requirement at other points, the difference (positive or negative) may be accounted for by the bi-directional power source arrangement. The present system utilises high energy density power source arrangement and bi-directional electrical power source arrangements. As noted above, the bi-directional electrical power source arrangement allows for energy to be provided when this is advantageous and for energy to be received and stored when this is advantageous. For example, during high energy need stages of flight (take off, climb, etc) the bi-directional electrical power source arrangement provides electrical energy for propulsion and during low energy need stages of flight (cruise, taxi, etc) the bi-directional electrical power source arrangement may not provide electrical energy. Further, during such low energy need stages of flight the bi-directional electrical power source arrangement may receive and store electrical energy. In other flight stages, there may be a varying power demand over a short response time. During such moments, it is advantageous to not alter the output from the high energy density power source arrangement, instead using the bi-directional electrical power source arrangement to handle whatever difference in power exists between that produced by the high energy density power source arrangement and that required from the system as a whole. Similarly, the present arrangement may prevent a stop-start cycle of the high energy density power source arrangement, instead using the flexibility in operation of the bi-directional electrical power source arrangement. The present power source system provides a novel power source system that is able to provide electrical energy for a propulsion arrangement in a manner that is reactive to the situation at the time of power provision. The system is flexible and therefore can provide a suitable power in response to a likely or known power need. For example, the system can detect a power requirement and control the power output required for the power requirement. Indeed, the output of power can be optimised between the high energy density power source arrangement and the bi-directional electrical power source arrangement. The present invention provides a far more robust and safe system that can use environmentally friendly fuels in comparison to modern systems. In examples, a maximum power output of the high energy density power source arrangement is less than a maximum propulsive power requirement. The maximum propulsive power requirement for an aircraft is likely to be in specific stages of flight. This may include take off and / or climb and / or emergency manoeuvres. In the arrangement disclosed herein, the high energy density power source arrangement alone is not capable of providing the full maximum propulsive power requirement of the aircraft in which, in use, the system is to be used. This is advantageous as aircraft (and indeed many other vehicles) are rarely operated at the maximum power output. Few cars regularly experience or are required by their users to operate at their maximum acceleration or top speeds. Aircraft spend far greater amounts of time at cruise than at take-off and climb. As such, it can be advantageous for sizing and overrating manufacturing and lifing of the high energy density power source arrangement to not be sufficient to provide the maximum power requirement. The high energy density power source arrangement may be smaller, cheaper, easier to manufacturer and have a longer service life by not needing to be capable of providing the maximum power output expected of the vehicle in which it is situated. In examples, the maximum propulsive power requirement is during at least one of: a take-off stage of flight; a climb stage of flight; and, during a safety event. This is a relevant consideration specifically for aircraft or the like. For automobiles, these requirements may occur during top acceleration or when travelling at maximum speed. In examples, the high energy density power source arrangement comprises at least one of: a fuel cell and a combustion turbine. Fuel cells are highly advantageous as being a high energy density power arrangement. Combustion turbines are also an example of a high energy density power arrangement. As such, the high energy density of the fuel for these power sources operate synergistically with the bi-directional electrical power source arrangement. Specifically, in aircraft, energy density has an impact on the amount of fuel that must be carried for a specific journey. Ideally, a light weight and highly energetic fuel is used. However, the present invention looks at that understanding alongside an aim to use environmentally friendly fuels. Fuel cells have a variable power output. This may not cause significant issues in some vehicles however in aircraft it is desirable to have a steady power output. Furthermore, the variance in power output from a fuel cell is relatively slow (in terms of time on an electrical scale). As such, the variance in power output from the fuel cell may be accounted for by the bi-directional electrical power source arrangement. Specifically, when the variable fuel cell power output is above expected power output, the bi-directional electrical power source arrangement can receive this excess energy from the fuel cell. Conversely, when the variable fuel cell power output is below expected power output, the bi-directional electrical power source arrangement can provide energy equivalent to the energy deficit from the fuel cell. Maintaining fuel cells at a constant output has been shown for aircraft to increase the fuel cell longevity and reduces deterioration mechanisms present in fuel cells. In this way, the bi-directional electrical power source arrangement provides a steady influence over the power output and, in the present arrangement, provides in combination with the high energy density power source arrangement a more consistent power output. Consistent here does not mean the same over the full use of the power source, rather for each stage of use a consistent output. In relation to an aircraft, the power output during climb will be different to that at cruise. Both stages (indeed all stages of transport whether for aircraft or other vehicles) benefit from a more consistent power output, though of course the total power output may be different for different stages of transport (e.g. take-off may require greater total power output than cruise). Indeed, in a preferred use of the present system, the high energy density power source arrangement operates at a steady state and the bi-directional electrical power source arrangement receives or provide energy based on whether the steady state is at a deficit or a surplus for the aircraft power requirements at any given time. This allows the high energy density power source arrangement to be tested, modelled, optimised, and sized for durability and performance at a predetermined stable power output while simultaneously allowing the larger system to provide additional power when required or receive and re-use surplus power when available. Accordingly, the bi-directional electrical power source arrangement allows for the high energy density power source arrangement to be set at providing a steady state power output while the bi-directional electrical power source arrangement accounts for variations in power demand. This is beneficial for the lifetime of e.g. any of fuel cells and gas turbines. The present invention may additional or alternatively use a gas turbine or the like. In examples, the bi-directional electrical power source arrangement comprises: a first bidirectional electrical power source arrangement; and, a second bi-directional electrical power source arrangement, wherein a difference between the power output of the high energy density power source arrangement and the propulsive power requirement of an aircraft is provided by at least one of the first bi-directional electrical power source arrangement and the second bidirectional electrical power source arrangement. Specifically, the arrangement of the bi-directional electrical power source arrangement is synergistic with the high energy density power source arrangement. The bi-directional electrical power source arrangement, in examples, has two portions one that provides a power output that may be used on a longer time scale than the other. The first bi-directional electrical power source arrangement provides a less-than-maximum power requirement output. I.e. the first bi-directional electrical power source arrangement is unable to provide all the power that could be required during use of the vehicle in which the system is arranged. This has similar sizing and manufacturing benefits as explained above in relation to the high energy density power source arrangement. The second bi-directional electrical power source arrangement provides a power source that can operate at specific moments to provide additional power when required. For example, the second bi-directional electrical power source arrangement may provide an emergency supply of energy which may be used in safety events such as when an error arises in the system and the emergency power is needed. Additionally or alternatively, the second bi-directional electrical power source arrangement may provide the additional power required on the maximum power output moments. For example, in an aircraft during take-off the second bi-directional electrical power source arrangement may provide the required additional power alongside the high energy density power source arrangement. As an illustrative example, the high energy density power source arrangement may provide 80% of the maximum power requirement and the bi-directional electrical power source arrangement may provide the remaining 20% via at least one of the first bi-directional electrical power source arrangement and the second bi-directional electrical power source arrangement. In the present arrangement, neither the first bi-directional electrical power source arrangement nor the high energy density power source arrangement can provide the maximum propulsive power requirement of the vehicle. Therefore, both power source arrangements are synergistic in achieving the required propulsion for a vehicle with each other. The delta power requirement may be an emergency additional power requirement which occurs outside of the standard propulsive power requirement. For example, under normal operation of an aircraft, a specific stage of flight has an associated propulsive power requirement. If an issue was to arise during this period, additional power may be required in a short time. For example, during cruise where significant turbulence is encountered a fast responding energy provision system is preferable to a slower responding energy provision system. As such, the delta-power requirement relates to a requirement above standard for that given stage of transit. This may relate to an emergency or safety issue or may relate to an unexpected change in travel conditions. The second bi-directional electrical power source arrangement may also be arranged to operate when there is a failure from one or more of the first bi-directional electrical power source arrangement and the high energy density power source arrangement. As such, the second bi-directional electrical power source arrangement may be arranged to provide a short term emergency power provision. In examples, the first bi-directional electrical power source arrangement comprises at least one battery, and the second bi-directional electrical power source arrangement comprises at least one of a capacitor and a super-capacitor. The bi-directional electrical power source arrangement may be or comprise a chemical energy store. The bi-directional electrical power source arrangement may be or comprise a controlled chemical-to-electrical energy converter, in that the bi-directional electrical power source arrangement may have a chemical energy store and an electrical converter for receiving chemical energy and converting to electrical. The bi-directional electrical power source arrangement may be or comprise a controlled bidirectional electro-chemical energy storage system, consisting of electro-chemical energy store and an electrical power converter for controlling power transfer from the storage system. The electrical power may subsequently be converted to kinetic energy such as propulsion or the like. The bi-directional electrical power source arrangement may also comprise an inertial storage system for storing energy as kinetic energy. The bi-directional arrangement can, in sum, store and provide energy on a variety of time scales, from short electrical timescales to longer kinetic timescales. A point to note is the advantageous use of both elements. Delivering high impulse power from a battery is feasible and may be performed as needed, however this may accelerate the aging of the battery. In contrast, delivery of high power loads from a flywheel does not age the flywheel. Therefore, the flywheel may be used to target high response rate ahead of the battery (in a similar way to a capacitor or super-capacitor). Acting together, the system can provide a suitable response for any situation. The output from a capacitor and / or super-capacitor is fast responding in comparison to the power output from a battery or the power output from a fuel cell arrangement. As such, the arrangement herein provides a power source system that can provide large amounts of electrical power over a long term scale (e.g. hours) and large amounts of electrical power over a short term scale (e.g. less than 10 seconds). The system can be well sized for the expected power requirements of transport of the vehicle in which the system is housed and the system is highly reactive and therefore robust against safety events and the like. In examples, the controller arrangement comprises at least one of: an altitude sensor; an airspeed sensor; an angle-of-attack sensor; a climb-rate sensor; an aircraft control surfaces sensor; a landing gear sensor; a user input module; a temperature sensor; a current sensor; a voltage sensor; a charge sensor; a fuel sensor; and, a component-state-of-health sensor, to detect a change in operating conditions. The controller arrangement is able to detect various characteristics of the vehicle and the stage of transit of the vehicle. The characteristics are used to ascertain the amount of propulsive energy that is required by the vehicle. For example, once the altitude sensors detects that the e.g. aircraft has reached cruise altitude the power requirement is likely to fall in comparison to the requirement for the climb to cruise altitude. In this way, the controller arrangement is able to detect a change in operating conditions (or a change that is to happen soon). Similarly, if the altitude sensor detects a sharp drop in altitude, this may imply a safety event has occurred and the controller may wish to pull power from a fast acting power source, such as a super-capacitor or the like. In this way, the change in operating conditions may be a failure in a system such that a power requirement arises. The electrical property sensors (current, voltage, charge) indicate an electrical health of the system, i.e. if current is not detected where it should be this implies the circuit has an issue. This can be used to ascertain an understanding of the available energy for conversion into electrical power for propulsion at any point and the viability of the system to provide that (e.g. current is at an expected level such that the charge can be delivered as expected). The system disclosed herein may be automatic or may be operated via a user and noting user input. Therefore a user input module allows a user to provide control instructions or the like for requesting additional power provision or the like. The state of health of the system may be the condition or lifetime of components within the system. As such, state of health may include the condition of electrical connections and joints and the like where weaknesses and / or breakages may occur. Estimated lifetime of components may be calculated or counted by the state of health sensor for components. In examples, the flight characteristic comprises at least one of: dormancy; start-up; stationary; taxi; take-off; initial climb; climb; cruise; descent; landing; go-around; emergency; and, shutdown. The propulsion generated from the operation of the power source system may be altitude driven. For example, during take-off and climb the altitude of the aircraft is low and the power demand is high. During cruise the altitude is high and the power demand is low (relative to other stages of flight). As such, it may be that altitude and velocity of vehicle can be used to ascertain the flight characteristics and therefore the power demand and therefore how best to supply that power demand. In examples, the controller arrangement comprises a data store, the controller arrangement being further arranged to store data in the data store representing a pre-determined flight path, wherein the data is labelled according to a flight characteristic. An understanding of the intending flight path can be used to ration energy from the energy stores according to flight need at specific flight times with an understanding of likely further energy requirements. Furthermore, an understanding of the intending flight path can be used to identify a deviation from the flight path and therefore the requirement for the application of energy from the energy stores to rectify the deviation. The deviation may be within acceptable limits from the flight path and therefore not require immediate rectification. A large deviation (beyond a predetermined deviation limit) may be actioned by application of energy from e.g. a battery or a super capacitor to rectify the deviation. In examples, the controller arrangement further comprises a flight path sensor arrangement arranged to detect at least one parameter associated with a flight path, the flight path sensor arrangement comprising at least one of: a weather sensor; a geographical sensor; and, an air temperature sensor, wherein the controller arrangement is further arranged to control the power output of the high energy density power source arrangement and the bi-directional electrical power source arrangement in response to signals from the flight path sensor arrangement. Weather sensors may be used to indicate when likely uses of the stored electrical energy or chemical energy will arise. Indications of incoming worsening weather can be used to provide energy to the bi-directional electrical power source arrangement ahead of needing to release energy from the bi-directional electrical power source arrangement during the bad weather impact. Advantageously, therefore, the high energy density power source and bi-directional electrical power source arrangements provide synergistic control over power output and power storing (in contrast to systems with output power only). In examples, the high energy density power source arrangement further comprises a cryogenic fuel arrangement arranged to provide fuel for power generation. The fuel used in the high energy density power source arrangement may be stored as a cryogenic fuel. Cryogenic fuels have high energy density (in comparison to carrying gases). Typically, in fuel cells etc, the cryogenicfuel is warmed up performing heat exchanger functions and then used in a gaseous form. This may be the case herein and this has further synergies with the efficient electrical conduct at lower temperatures. In accordance with some embodiments described herein, there is provided an at least partially electrically powered aircraft comprising the power source system of any of the above examples or embodiments. While the present power source system is applicable to any vehicles, it appears that particularly significant synergistic advantageous can be found with its implementation in aircraft. Such as use of green fuels to reduce environmental impact and the like alongside the significantly changing power requirements through the typical stages of flight experienced by an aircraft. In accordance with some embodiments described herein, there is provided a method of power management system in an aircraft, the method comprising: i) detecting a propulsive power requirement; ii) calculating and providing a power provision from a high energy density power source arrangement and a bi-directional electrical power source arrangement; iii) detecting a change in propulsive power requirement; iv) calculating and providing at least one of: a) different power provision from the high energy density power source arrangement; and, b) different power provision from the bi-directional electrical power source arrangement. The method herein is an advantageous application of the system disclosed above. In particular, this method allows for selective and controlled power provision from suitable power sources within the power source system. This may be the provision of a default level of power such as may be provided from a fuel cell arrangement in a hydrogen operated aircraft that is suitable for most flight stages alongside the selective power from a battery or a super capacitor for high need stages of flight such as take-off and climb. In examples, detecting a change in propulsive power requirement comprises at least one of: detecting a change in operating conditions; and, detecting a change in flight characteristic. In examples, providing a power provision from a high energy density power source arrangement comprises at least one of providing power provision from a fuel cell and providing power from a gas turbine. In examples, providing a power provision from a high energy density power source arrangement comprises providing power from a heat engine for example, but not exclusively, gas turbine or reciprocating internal combustion engines. In examples, providing a power provision from a bi-directional electrical power source arrangement comprises at least one of: providing power provision from a battery; providing power provision from a capacitor; and, providing power from a super-capacitor. In examples, providing a power provision from a high energy density power source arrangement comprises providing less than a maximum propulsive power requirement. 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 an arrangement of a propulsion system including a power source system according to an example of the present disclosure; Figure 2 shows a schematic view of an arrangement of a propulsion system including a power source system according to an example of the present disclosure; Figure 3 shows a schematic view of an arrangement of a propulsion system including a power source system according to an example of the present disclosure; Figure 4 shows a graph indicating specific energy against specific power for possible components of a power source system according to an example of the present disclosure; Figure 5 shows a flow diagram of a method according to an example of the present disclosure; and, Figure 6 shows a graph showing time along the x-axis against power output and input for possible components of a power source system according to an example of the present disclosure along the y-axis alongside throttle demand for a typical flight. 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 power sources for electrically powered aircraft. The power sources herein may be used with any vehicles however it is a particularly advantageous arrangement for use in 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, the propulsion in the aircraft in which the power sources disclosed herein are used 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 with a power source 100 according to an example of the present disclosure. The propulsion system comprises a power source 100, a fuel container 150 and a propulsion motor 160. The fuel container 150 may contain a fuel for use in the power source 100. In examples, the fuel container 150 may be part of the power source 100. In the example shown in Figure 1, the power source 100 comprises a high energy density power source arrangement 110 for providing electrical power. In examples, the high energy density power source arrangement 110 is at least one of a fuel cell and a combustion turbine. The high energy density power source arrangement 110 may be a fuel cell stack or a series of fuel cell stacks or the like. The power source 100 comprises a bi-directional electrical power source arrangement 120 for providing and receiving electrical power. The example shows bi-directional electrical power source arrangement 120 having two components. In examples, one component 122 may be a battery and the other component 124 may be a super-capacitor (or capacitor or the like). In either case, the bi-directional electrical power source arrangement 120 can be provided with electrical energy for storing as well as providing electrical energy. In contrast, the high energy density power source arrangement 110 only provides energy and does not receive energy. This is illustrated in Figure 1a by the arrow from the element 110 to the propulsion motor 160 and the arrows to and from the elements 122, 124 to (and from) the propulsion motor 160. Figure 1 shows a movement of fuel 102 from the fuel source 150 to the high energy density power source arrangement 110. In an example the high energy density power source arrangement 110 is a fuel cell stack and the fuel is hydrogen. The hydrogen may be stored in the tank 150 as a liquid cryogen. This fuel may perform advantageous heat exchanger functions in the aircraft prior to use in the fuel cell stacks. Where the high energy density power source arrangement 110 is a fuel cell stack, the power output from the fuel cell stack can be variable. However, in this arrangement herein, the main power output is from the high energy density power source arrangement 110. Therefore, the present arrangement has an issue that the main power output may be from a source that typically provides variable power output. It is preferable for a power output from the main power output element to be steady. In this arrangement, then, the combination of the high energy density power source arrangement 110 and the bi-directional electrical power source arrangement 120 provides this steady power output. Where the high energy density power source arrangement 110 is providing power and the power varies above and below the desired power output, the bidirectional electrical power source arrangement 120 receives energy or provides energy respectively. Specifically, where the high energy density power source arrangement 110 provides more power output than expected (due to variations in power output), this is received into the bi-directional electrical power source arrangement 120 (shown by the arrows 104 into and out of the elements 122, 124). Where the high energy density power source arrangement 110 provides less power output than expected (due to variations in power output), this power deficit is provided from the bi-directional electrical power source arrangement 120 to the propulsion motor 160. In this way, control over incoming and outgoing power to and from respectively the bi-directional electrical power source arrangement 120, the present arrangement is able to provide a reactive and extremely steady power output. In this way, power is not wasted (when power is over-generated the excess power is stored) and greener fuels can be used. I.e. accounting for the weaknesses of e.g. fuel cells allows fuel cells to be a more viable solution for thrust in aircraft, which has in turn large environmental benefits. The element 110 may be a combustion turbine or a series of combustion turbines. Suitable fuels for this example may include kerosene, sustainable aviation fuel, ammonia, hydrogen in gaseous and / or liquid form, liquid natural gas or the like, etc. This fuel may be maintained at a cryogenic temperature or at ambient temperatures. There is no requirement for the fuel to be of a specific temperature. The present invention may utilise a fuel cell that uses any fuel suitable for fuel cells, e.g. ammonia, hydrogen, methanol etc. (gas or liquid, or solid state storage). There is no requirement for the fuel to be a green fuel or the like though advantages can be obtained with the use of such a fuel. The present hybridization arrangement along with a gas turbine would work with any combustible gas or liquid fuel however it was stored. There is no requirement for the fuel to be cryogenic, though there are benefits that can be obtained via thermal exchange in the use of cryogenic fuels. Arrows 104 indicate the movement of electrical power. As noted above, electrical power may be sent from the high energy density power source arrangement 110 to the propulsion motor 160. Power may be sent and received by the elements of the bi-directional electrical power source arrangement 110. The elements 110 may be a fuel cell system or may be a combustion engine and an electrical generator or the like. The role of the high energy density power source arrangement 110 is to receive fuel and provide an electrical energy output. This can be provided to some form of electrical-to-kinetic converter 160 such as a propulsion motor. The power source system may also include (not shown) a busbar. The busbar may play the role of electrically connecting elements within the power source to elements beyond the power source. For example, the busbar may be arranged to receive electrical power from the high energy density power source arrangement, and to receive electrical power from and provide electrical power to the bi-directional electrical power source arrangement. There may additionally be a controller arrangement. The controller arrangement (not shown) may control operation and activation of elements within the power source system. For example, the controller may be arranged to detect a series of relevant characteristics (relevant to managing power loads) including power characteristics and travelling characteristics. In an example, power characteristics may relate to power output and / or the capability for providing output and / or the charge of a battery or super capacitor or the like. Travelling characteristics may include characteristics relevant to the vehicle in which the power source system is arranged, such as flight characteristics for aircraft. In broad terms, the controller arrangement is arranged to understand the needs of the vehicle and provide this from the power source system. Furthermore, the controller arrangement may be arranged to have an overview of the expected needs of the vehicle and therefore provide this over the course of the total travel length. Where the vehicle is anticipating a greater need later in the journey, the controller arrangement may look to ration power at an early stage of transport. Where the power source system is in an aircraft, the controller arrangement may include a stage of flight sensor arrangement and a diagnostics sensor arrangement for detecting power requirements alongside e.g. estimated remaining lifetime of components within the system. Referring now to Figure 2, there is shown a simple schematic view of a propulsion system with a power source 200 according to an example of the present disclosure. The propulsion system comprises a power source 200, a fuel container 250 and a propulsion motor 260. The fuel container 250 may contain a fuel for use in the power source 200. In examples, the fuel container 250 may be part of the power source 200. The numerals of Figure 2 correspond to the numerals of Figure 1, with the values increased by 100. In this way, features with the same or similar functions have the same numeral (increased by 100). For example, the fuel container 150 of Figure 1 has the same function (that of containing fuel for use in the system) as fuel container 250 of Figure 2. Corresponding to Figure 1, fuel movement is shown by arrow 202, electrical power transfer is shown by arrow 204 (which may be along one or more busbars or the like). Arrow 206 indicates transfer of torque along a shaft or the like. New features (for the example of Figure 2) include the fly wheel 270. The fly wheel 270 plays a role of receiving excess energy and stores this energy for use at a later point. The fly wheel 270 has a transient response similar to a super-capacitor but has an energy density lower than a battery. The fly wheel 270 provides a short term energy store and may be used to stabilise the speed of the motor 260 or the like. The flywheel receives shaft torque from and provides shaft torque to the propulsion motor. The propulsion motor 260 may be a propulsion motor and generator. The bi-directional electrical power source arrangement 220 includes a battery 222 in the example of Figure 2. The high energy density power source arrangement 210 may be a fuel cell system. The fuel container 250 may be a hydrogen tank or the like and provide hydrogen to a fuel cell system 210 for electrical power generation. This electrical power is conducted along one or more busbars 204 to the propulsion motor. Referring now to Figure 3, there is shown a simple schematic view of a propulsion system with a power source according to an example of the present disclosure. The propulsion system comprises a power source, a fuel container 350, a propulsion motor 360 and a combustion engine 380. The fuel container 350 may contain a fuel for use in the power source. In examples, the fuel container 350 may be part of the power source. The numerals of Figure 3 correspond to the numerals of Figure 2, with the values increased by 100. In this way, features with the same or similar functions have the same numeral (increased by 100). For example, the fuel container 250 of Figure 2 has the same function (that of containing fuel for use in the system) as fuel container 350 of Figure 3. The arrangement of Figure 3 differs to the arrangement of Figure 2 in a number of ways that will now be discussed. The fuel container 350 provides fuel to a combustion engine 380 which is providing torque to and may receive some torque from (double arrow 306) an electric motor 360. The combustion engine 380 is therefore a high energy density power source arrangement and also a bi-directional electrical power source arrangement. In this instance the combustion engine is primarily providing the role of the high energy density power source arrangement of the power source system. The bi-directional electrical power source arrangement 320 comprises a first bi-directional electrical power source arrangement 322 and a second bi-directional electrical power source arrangement 324. There is at least one busbar that electrically connects the propulsion motor 360 to the bi-directional electrical power source arrangement 320. Specifically, electrical energy can be passed to the propulsion motor 360 for conversion to kinetic energy and received from the propulsion motor 360 to store power in the bi-directional electrical power source arrangement 320. The fuel container 350 provides a fuel to combustion engine 380. The combustion engine 380 provides shaft torque 306 to the propulsion motor 360 and can receive shaft torque 306 from the propulsion motor 360. An advantage of returning torque to the engine 380 is transient stability due to the inertia of the engine 380. The arrangement may or may not include a gearbox for increased control. Referring now to Figure 4, there is shown a graph 400 indicating specific energy (in Wh / kg) against specific power (in kW / kg) for possible components of a power source system according to an example of the present disclosure. Specific energy is shown along the x-axis while specific power is shown along the y-axis. Specific components are shown on the graph for comparison. For example, line 402 indicates the behaviour of a super-capacitor. As indicative values, the super-capacitor may have a specific energy in the region of 3 Wh / kg and a specific power in the region of 1.8 kW / kg. As can also be seen the super capacitor has a very high efficiency (Q) of 0.95. The super capacitor has a capability to provide a very high output of electrical energy over a very short period of time (< 10s). As noted above, this allows for a rapid response to a change in a power requirement for the vehicle. In particular, the super capacitor is highly advantageous in instances of safety events and during high need over short periods of time (such as take-off for aircraft). Line 404 indicates the behaviour of a battery. As indicative values, the battery may have a specific energy in the region of 200 Wh / kg and a specific power in the region of 1.6 kW / kg. As can also be seen the battery has a high efficiency of 0.9. The battery has a capability to provide a high output of electrical energy over a reasonable period of time (on the scale of minutes). As noted above, this allows for a reasonable response to a change in a power requirement for the vehicle. In particular, the battery is highly advantageous in instances of longer term safety events and high need over medium periods of time. One can see that there is a synergy between the controlled electrical output of the supercapacitor and the controlled electrical output of the battery. By noting the conditions of the vehicle and the conditions of the power requirements, an optimum selection can be made as to which of the two elements is to be used to satisfy the power requirements. The controlled aspect of this invention is provided by a controller arrangement which will be discussed more below. Line 406 indicates the behaviour of a fuel cell system. As indicative values, the fuel cell system may have a specific energy in the region of 700 Wh / kg and a specific power in the region of 1.5 kW / kg. As can also be seen the fuel cell has an efficiency of 0.5. The fuel cell system has a capability to provide a good output of electrical energy over a reasonably long period of time (on the scale of minutes to hours). As noted above, this allows for the provision of a baseline power requirement for the vehicle. In particular, the fuel cell system is highly advantageous in the provision of the bulk of the power requirement for the vehicle allowing other elements (discussed above) to handle the changes from baseline power requirements. The fuel cell system (including the fuel cells and the fuel store) has a high specific energy and a high energy density. This means the fuel cell system is advantageously used as the main power provider in the power source discussed herein. Specifically, the fuel for the fuel cell can be maintained as a liquid cryogen and therefore a large amount of fuel can be carried for less volume, criteria which are very impactful to efficiency particularly in aircraft. On a broad level, the present invention may use a high energy density power source (such as a fuel cell arrangement) and a bi-directional power source (to provide and receive power). In practice, a high energy density that is sustainable that also provides bi-directional power functionality would be usable as per the invention herein. The changes in baseline power requirements discussed herein may relate to transient energy requirements. In particular, as can be readily understood, a safety event may cause a transient need for additional power over a short period of time. Based on an assessment of the issue and therefore an understanding of the transient power requirement, a response from at least one component of the bi-directional electrical power source arrangement can be provided. In particular, the component can be selected to provide power based on the suitability of that component for satisfying the power need. The energy requirements may be referred to herein as “transient energy needs” or “transients”. This language reflects the nature of the needs to be passing in time and therefore while a response is needed to satisfy the change in power requirement it is unlikely to be a permanent response. The bi-directional electrical power source arrangement disclosed herein may be an energy storage system (ESS) arranged within an aircraft. The high energy density power source arrangement disclosed herein may include any of a turbo-generator, low temperature fuel cell arrangement, and a high temperature fuel cell arrangement or the like. The high energy density power source arrangement as noted above provides the primary power source (i.e. is the main power source). In examples, the high energy density power source arrangement is operated throughout transit of the vehicle while the bi-directional electrical power source arrangement is used at select moments during the transit (specifically when the need arises). The use of the pair of arrangements provides a hybrid arrangement of permanent operation and temporary operation. High energy transients may come from propulsive power demand, for example a sudden surge in thrust may be needed in the event of an aborted landing or the like. Small energy transients may come from aircraft secondary load systems for example fluctuation in flight control surfaces or the like. Power transients are temporary changes in power requirement from the aircraft. This may stem from a number of features or stages of transit such as turbulence, changing temporary demands (such as cabin loads) or the like. During these moments, power from the high energy density power source arrangement remains steady and the difference is accounted for by the bi-directional electrical power source arrangement. Power transients can stem from propulsive power demand (which occurs over a longer time constant and at higher energy), and / or secondary demand (which occurs over a shorter time constant and at lower energy). For this reason, it may be advantageous to satisfy these two broad types of transients with one or more batteries for the longer time constant and higher energy and with one or more super-capacitors for the shorter time constant and lower energy transients. The present solution provides a suitable response to any transients using the bi-directional electrical power source. Together these arrangements can be used to provide a power delivery capability that is suitable for safe and robust flight. These can also be used to be compliant with present certification standards. The high energy density power source arrangement disclosed herein may be deemed to include a fuel container. For example, when considering the specific power and specific energy of the fuel cell arrangement (e.g. in Fig. 4) this includes consideration of the liquid cryogen fuel for use in the fuel cell. The high energy density power source arrangement is arranged to provide a maximum output that is judged to be less than an expected maximum power output for the full journey. In an example of an aircraft using a fuel cell arrangement, the fuel cell arrangement is sized to be able to provide sufficient power for taxi, cruise, and landing, however it is not sized to provide sufficient power for take-off and climb. During this power need, the bi-directional electrical power source arrangement provides temporary power assistance. This allows the fuel cell arrangement to be sized more efficiently and therefore weigh less. This leads to great efficiencies. The bi-directional electrical power source arrangement may be used, as noted above, in instances of high need. This may be expected high need, such as take-off and climb, and / or unexpected but detected by a controller arrangement. The controller arrangement may detect high need instances based on expected normal flight conditions. Deviations from such normal flight conditions may imply a need for greater power provision. This may be e.g. turbulence or the like. Furthermore, unexpected high need may arise from failure of e.g. a fuel cell within the fuel cell arrangement. This may lead to the high energy density power source arrangement providing a reduced power over the transit of the vehicle and therefore a requirement of power from the bi-directional electrical power source arrangement. In this instance, the battery would be preferable over the super-capacitor as a slower power delivery is preferred. In the instance of full loss of the high energy density power source arrangement, there may be a need for an emergency landing during which power from the super-capacitor may be rapidly delivered to stabilise the aircraft after the initial total loss of the high energy density power source arrangement. Subsequent power may be provided by a battery or the like during glide and landing phases. It is not envisaged that the battery would be able to provide full functioning on its own (i.e. without the fuel cell or gas turbine arrangement). This is because the sizing of the battery would be inefficient and would be well oversized for most use cases. As such, the proposal of the system herein is well sized and optimised both electrically and on a cost basis The battery may be seen as a power sharing component, in that the battery can share a main power load alongside a fuel cell arrangement where required. Accordingly, the super-capacitor (or capacitor) may be seen as a transient power component, in that the super-capacitor can provide a burst of power where it is required alongside (or separate from) the fuel cell arrangement. The transient power may be seen as a deviation of a power requirement from an expected power requirement at any given instance. This may be referred to as a delta power requirement above a maximum propulsive power requirement. The term “delta” being used as is standard to refer to a difference above a certain level. This may also or alternatively be referred to as a difference between the power output of the high energy density power source arrangement and the propulsive power requirement of an aircraft. The maximum expected propulsive output, in the case of an aircraft, may be take-off and climb. A transient power requirement may arise if turbulence is encountered during such phases or is a fuel cell (within a fuel cell arrangement) fails. In both cases, there is a transient need for additional power beyond that which was anticipated. The need is transient as once the high demand period is overcome, the fuel cell arrangement may be sized sufficiently to handle the remaining demand (e.g. the lesser demand experienced during cruise, descent and landing). The transient power requirement (or “delta power requirement”) may be higher than peak power offered by a battery and therefore a super-capacitor may be needed to provide the power. A super-capacitor can provide very high transient power (over a reasonably short period of time). This can be used as needed based on the assessment from the controller arrangement. The controller arrangement may include sensors and / or detectors or the like to ascertain various properties of the vehicle and the power source arrangement. The controller arrangement is able to detect movement and location properties of the vehicle alongside environmental properties relevant to the transit of the vehicle. Each of these can affect the expected power requirement of the vehicle over the next period of transit. As such, each of these can be factored in when ascertaining the power to provide for propulsion. The controller arrangement is able to detect electrical properties of the power source arrangement and mechanical properties of the propulsion system associated with the vehicle. In this way, the controller arrangement is able to understand the present capability of the power source arrangement to provide power as may be required based on the needs of the vehicle. For example, where a fuel cell arrangement has issues and is unable to provide power at an expected level, the controller arrangement may know this (based on detection of electrical properties of the power source) and therefore control power provision from the bi-directional electrical power source arrangement to account for the deficit in expected power from the high energy density power source arrangement. The controller arrangement may detect state-of-health of components within the vehicle for use in power provision calculations. For example, noting that a fuel cell arrangement or a propulsor is about to experience a mechanical fault (based on present performance or on expected lifetime) the power provision may be slightly rationed ahead of greater need for power after the fault has occurred. The controller arrangement may include a fuel sensor to track consumption and availability of fuel in the fuel container. Noting the fuel is below levels that can sustain propulsion for a required period of time (based on an understanding of the remaining journey length and power requirements), the controller arrangement can ration use of the fuel and switch to a hybrid power provision from both the high energy density power source arrangement and the bidirectional electrical power source arrangement. In the example of inclusion in an aircraft, flight characteristics may be useful in anticipating power requirement. Flight characteristics may be demarked by altitude ranges. It is reasonable to assume at low altitudes (and at the start of a journey) the aircraft will soon have a large requirement for power (for take-off and climb). As such, the controller can ready power release from the super-capacitor and / or battery where needed. At a high altitude, e.g. during cruise, power demands will reduce from climb and settle at a steady power requirement level. This may be satisfied by the fuel cell arrangement alone. Similarly, flight characteristics may be broken down by any of flight stage, altitude range, propulsion power requirement range, and any combination of these. The greater understanding of any one moment of a flight the more accurate an assessment of the power requirement from the power source can be made. Greater amounts of and greater fidelity of data for the controller arrangement leads to a more optimised performance. The controller arrangement may be able to store data for use in controlling power provision. E.g. the controller arrangement may include data of previous flight power characteristics that can be compared to the present flight and used to infer likely power requirements for the remainder of the flight. This data may also include e.g. flight data for the present flight and flight data for a recent flight between the same destinations. Precipitation, local air particulate and GPS data all provide a greater understanding of the flight conditions and therefore can be used in the calculation for best power provision. For example, when flying through high particulate containing air, it may be beneficial to reduce the power from the high energy density power source. The high energy density power source may use ambient air and therefore using air with high particulate matter may damage the power source. In such an instance it may be preferable to use a greater amount of energy from the bidirectional electrical power source arrangement. As such, the present system is able to account for and reduce the damage from flying through air with high levels of contaminants. The power source arrangement disclosed herein is particularly advantageous for use in aircraft by virtue of the use of greener fuels and the variance in power propulsion required during different stages of flight. Indeed, aircraft have greater weight and sizing concerns than other vehicles and therefore the advantages on these two criteria are more strongly felt for aircraft though still clearly beneficial for other vehicles. Referring now to Figure 5, a method 500 according to an example of the present disclosure is shown as a flow diagram. The method 500 has four steps 505, 510, 515, 520. In a first step 505, a propulsive power requirement is detected. As noted above, this may be during or prior to movement of a vehicle. In a second step 510, a power provision from a high energy density power source arrangement and a bi-directional electrical power source arrangement is calculated and provided. This calculation is based on the detection in step 505. These two steps together provide a suitable level of power provision for the vehicle in the present movement requirement from the vehicle. In a third step 515, a change in propulsive power is detected. This may be performed by a controller arrangement or the like. This may be due to a predetermined stage of movement or an unexpected deviation from said movement (such as an emergency acceleration or the like). 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 520, a calculation and provision of a different power provision occurs. This power provision may be at least one of different power provision from the high energy density power source arrangement, and different power provision from the bi-directional electrical power source arrangement. Referring now to Figure 6, there is shown a graph 600 showing the power output of components of a power source arrangement according to examples of the present disclosure. The graph 600 has 5 lines on the graph. The graph shows time along the x-axis. The graph shows power output on the y-axis. The graph is based on an aircraft journey. The journey includes take-off, climb, cruise, descent and landing. Line 602 shows the total power provided from the power source arrangement. Line 604 shows the throttle demand of the pilot operating the aircraft. Line 606 shows the power output from a fuel cell system. Une 608 shows the output / input for the battery. Where the line 608 goes below the x-axis this shows power being provided to the battery and the battery being charged from the excess output from the power source arrangement. Line 610 shows the output from and input to the super capacitor. As can be seen, initially, during take-off, the throttle demand 604 ramps up very quickly from zero (or a suitable idle state). During this time, the total power 602 is provided by an increasing output from the fuel cell system 606 and brief outputs from both the battery 608 and the super capacitor 610. The super capacitor 610 provides energy over a short period of time and then ceases to provide power. The super capacitor is later charged (where line 610 goes below the x-axis). Similarly, the battery 608 provides energy over a longer period of time but also eventually ceases to provide power. The battery 608 drops off as the throttle demand 604 and the total power 602 reach a similar level. The battery 608 then reduces the output of power and eventually receives power from the fuel cell system 606 once the throttle demand 604 has lessened (typically after take-off and towards and at cruise). The output of the fuel cell system 606 can be seen to provide the power output required for both total power 602 and for charging the super capacitor 610 and battery 608 during a middle portion of flight. As the demand 604 lessens the fuel cell system 606 lessens it output. At different stages of flight, throttle demand 602 varies and this can be accounted for by the fuel cell system 606 alongside intermittent assistance from the battery 608. As mentioned above, this is on a macro view. On a closer inspection, the fuel cell system 606 output is not so consistent and this can be added to (where required) or received (where required) by the battery 608. For example, where the output the fuel cell system 606 drops below expected output, this can be added to by output from the battery 608. Where the output the fuel cell system 606 is greater than expected output, this surplus energy can be routed to the battery 608 for charging the battery 608. This is therefore a highly robust and energy efficient system that also provides a very consistent power output. The present system also allows for boost power to be provided, e.g. from a super capacitor, in moments that require particularly high power output (such as take off for aircraft). The system proposed herein is flexible and reliable and has improved safety characteristics over modern systems. In this method therefore there is high responsiveness to a fault or change in propulsive requirements from any source. This provides for a robust and safe approach to changing power requirements in vehicles. In the above, the fuel containers may be tanks containing liquid cryogen or the like. These may be double walled fuel tanks with outer vacuum enclosures and inner hydrogen vessels with liquid fill and feed lines. These may have gas vent lines. These may have burst disks provided for vacuum space protection or the like.

Claims

1. A power source system for an electrically powered aircraft, the system comprising:a high energy density power source arrangement (110) for providing electrical power, 5 the high energy density power source arranged to operate at a steady state;a bi-directional electrical power source arrangement (120) for providing and receiving electrical power;a busbar arranged to receive electrical power from the high energy density power source arrangement, and to receive electrical power from and provide electrical power to the 10 bi-directional electrical power source arrangement,a controller arrangement arranged to:detect a flight characteristic of an aircraft;detect a power characteristic of the power source system; and,control a power output of the high energy density power source arrangement 15 and the bi-directional electrical power source arrangement,wherein,the controller arrangement is further arranged, in use, to:detect a propulsive power requirement for an aircraft;detect a flight characteristic of an aircraft;20 detect a power characteristic of the power source system; and,control the power output of the high energy density power source arrangement and the bi-directional electrical power source arrangement based on the propulsive power requirement, flight characteristic of an aircraft and power characteristic of the power source system,25 the power output of the high energy density power source arrangementand the bi-directional electrical power source arrangement to be provided to the busbar.

2. A power source system according to claim 1, wherein a maximum power output of the 30 high energy density power source arrangement is less than a maximum propulsive power requirement.

3. A power source system according to claim 2, wherein the maximum propulsive power requirement is during at least one of: a take-off stage of flight; a climb stage of flight; and, 35 during a safety event.

4. A power source system according to any preceding claim, wherein the high energy density power source arrangement comprises at least one of: a fuel cell and a combustion turbine.5 5. A power source system according to any preceding claim, whereinthe bi-directional electrical power source arrangement comprises:a first bi-directional electrical power source arrangement; and,a second bi-directional electrical power source arrangement, wherein a difference between the power output of the high energy density power source10 arrangement and the propulsive power requirement of an aircraft is provided by at least one of the first bi-directional electrical power source arrangement and the second bi-directional electrical power source arrangement.15202530356. A power source system according to claim 5,wherein the first bi-directional electrical power source arrangement comprises at least one battery, and,wherein the second bi-directional electrical power source arrangement comprises at least one of a capacitor and a super-capacitor.

7. A power source system according to any preceding claims, wherein the controller arrangement comprises at least one of:an altitude sensor;an airspeed sensor;an angle-of-attack sensor;a climb-rate sensor;an aircraft control surfaces sensor;a landing gear sensor;a user input module;a temperature sensor;a current sensor;a voltage sensor;a charge sensor;a fuel sensor; and,a component-state-of-health sensor,to detect a change in operating conditions.

8. A power source system according to any preceding claims, wherein the flight characteristic comprises at least one of:dormancy;start-up;5 stationary;taxi;take-off;initial climb;climb;10 cruise;descent;landing;go-around;emergency; and,15 shut-down.

9. A power source system according to any preceding claims, wherein the controller arrangement comprises a data store,the controller arrangement being further arranged to store data in the data store 20 representing a pre-determined flight path, wherein the data is labelled according to a flight characteristic.

10. A power source system according to any preceding claims, the controller arrangement further comprising a flight path sensor arrangement arranged to detect at least one parameter 25 associated with a flight path, the flight path sensor arrangement comprising at least one of:a weather sensor;a geographical sensor; and,an air temperature sensor,wherein the controller arrangement is further arranged to control the power output of30 the high energy density power source arrangement and the bi-directional electrical power source arrangement in response to signals from the flight path sensor arrangement.

11. A power source system according to any preceding claims, wherein the high energy density power source arrangement further comprises a cryogenic fuel arrangement arranged 35 to provide fuel for power generation.

12. An at least partially electrically powered aircraft comprising the power source system of any of claims 1-11.

13. A method of power management system in an aircraft, the method comprising:5 i) detecting (505) a propulsive power requirement;ii) calculating and providing (510) a power provision from a high energy density power source arrangement and a bi-directional electrical power source arrangement;iii) detecting (515) a change in propulsive power requirement;iv) calculating (520) and providing at least one of:10 a) different power provision from the high energy density power sourcearrangement; and,b) different power provision from the bi-directional electrical power source arrangementv) operating the high energy density power source arrangement at a steady state. 1514. A method according to claim 13, wherein detecting a change in propulsive power requirement comprises at least one of:detecting a change in operating conditions; and, detecting a change in flight characteristic.2015. A method according to claim 13 or 14, wherein providing a power provision from a high energy density power source arrangement comprises at least one of providing power provision from a fuel cell and providing power from a gas turbine.25 16. A method according to any of claims 13-15, wherein providing a power provision froma bi-directional electrical power source arrangement comprises at least one of: providing power provision from a battery; providing power provision from a capacitor; and, providing power from a super-capacitor.30 17. A method according to any of claims 13-16, wherein providing a power provision froma high energy density power source arrangement comprises providing less than a maximum propulsive power requirement.

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