FIELD OF THE INVENTION

[0001] An aircraft comprising a first and second fuel, an aircraft comprising a fuel control system, an aircraft having a heat exchanger system, and a method of operating an aircraft heat exchanger system.

BACKGROUND OF THE INVENTION

[0002] Since the dawn of manned powered flight, aircraft have used mineral derived liquid petroleum as an energy storage media, with some small scale exceptions:

- Electrical battery powered flight was first demonstrated in 1973 by Fred Militky and Heino Brditschka, who converted a Brditschka HB-3 motor glider into an electric aircraft (the Militky MB-E1).

- Electrical Solar powered flight first took place in 1979, with the flight of the Mauro Solar Riser.

- Bio Kerosene powered flight was demonstrated in 2007 by Green Flight International Inc. using an Aero L39 Albatros.

- Hydrogen powered flight was first demonstrated in 1955 using a B57 Canberra as part of NACA project Bee.

- Liquid Natural Gas powered flight was first demonstrated in 1988 using a Tupolev-155.

- Nuclear powered flight was first demonstrated in 1961 using a Tupolev- 95LAL.

[0003] A number of the above examples used combinations of different energy storage media. In particular, for the hydrogen powered flight, the Liquid Natural Gas powered flight and the nuclear powered flight, kerosene was used as the power source for the majority of each flight.

[0004] Manned rocket aircraft have been flown using fuels including hydrazine, hydrogen peroxide, ethanol, and ammonia. However; in each case an oxidizer has also been required. SUMMARY OF THE INVENTION

[0005] A first aspect of the invention provides an aircraft comprising: one or more main engines; a first fuel in a first fuel tank configured to be fed towards at least one of the one or more main engines through a first fuel line; and a second fuel different to the first fuel in a second fuel tank configured to be fed towards at least one of the one or more main engines through a second fuel line; the first fuel being anhydrous ammonia and the second fuel being a fuel other than anhydrous ammonia.

[0006] The aircraft may comprise a port wing and a starboard wing. The first fuel tank and the second fuel tank may be located in the port wing and/or the first fuel tank and the second fuel tank may be located in the starboard wing.

[0007] The second fuel tank may be located in the port wing and/or starboard wing and the first fuel tank may be located outside the port wing and starboard wing.

[0008] The one or more main engines may comprise two inner engines and two outer engines located on the port and/or starboard wing, the inner engines being inboard of the outer engines in the spanwise direction of the aircraft, wherein the first fuel is fed towards the inner engines and the second fuel is fed towards the outer engines.

[0009] The first fuel may be configured to flow to a first of the one or more main engines, and the second fuel may be configured to flow to a second of the one or more main engines.

[0010] The aircraft may comprise a fuel control system configured to control the flow of the first fuel through the first fuel line and to control the flow of the second fuel through the second fuel line, wherein a flow rate of the first fuel with respect to a flow rate of the second fuel varies according to a flight phase of the aircraft.

[0011] The flow rate of the first fuel may be greater than the flow rate of the second fuel in a first flight phase, and wherein the first flight phase is one or more of cruise, descent, approach, taxiing and ground manoeuvres.

[0012] The flow rate of the second fuel may be greater than the flow rate of the first fuel in a second flight phase, and wherein the second flight phase is one or more of takeoff and climb. [0013] The fuel control system may be configured to substantially prevent flow of the second fuel in the first flight phase.

[0014] The fuel control system may be configured to substantially prevent flow of the first fuel in the second flight phase.

[0015] The first fuel may be a liquid and/or the second fuel may be a liquid.

[0016] The second fuel may be hydrocarbon based aviation fuel.

[0017] The second fuel may have a density of between 775 kg/m3 and 840 kg/m3.

[0018] The second fuel may comprise sustainable aviation fuel and/or mineral kerosene.

[0019] The first fuel tank may comprise a controllable venting system configured to control the flow of air into and out of the first fuel tank and/or a thermal insulation layer configured to thermally insulate the fuel tank.

[0020] The first fuel line may be configured to feed a first portion of the anhydrous ammonia to an ammonia cracking device, and wherein the ammonia cracking device is configured to split the first portion of the anhydrous ammonia into hydrogen and nitrogen.

[0021] The hydrogen may be arranged to be fed to one or more of the one or more engines to power the one or more engines.

[0022] A second portion of the anhydrous ammonia may be fed with the hydrogen to the one or more engines to power the one or more engines.

[0023] The hydrogen may be fed to a hydrogen fuel cell.

[0024] The hydrogen fuel cell may power a first of the one or more main engines and the second fuel may power a second of the one or more main engines.

[0025] At least one of the one or more main engines may comprise a propeller.

[0026] The aircraft may comprise a third fuel configured to feed one or more of the main engines.

[0027] A flow rate of the third fuel with respect to the flow rate of the first fuel and/or second fuel may vary according to the flight phase of the aircraft. [0028] A third aspect of the invention provides a method of controlling fuel flow to one or more main engines of an aircraft, comprising: varying a flow rate of a first fuel to at least one of the one or more main engine according to a first flight phase of the aircraft; and varying a flow rate of the second fuel to at least one of the one or more main engines according to a second flight phase of the aircraft, wherein the first fuel is anhydrous ammonia and the second fuel is a fuel other than anhydrous ammonia.

[0029] The method may further comprise increasing the flow rate of the first fuel above the flow rate of the second fuel in a first flight phase, wherein the first flight phase is one or more of cruise, descent, approach, taxiing and ground manoeuvres.

[0030] The method may further comprise increasing the flow rate of the second fuel above the flow rate of the first fuel in a second flight phase, wherein the second flight phase is one or more of take-off and climb.

[0031] A fourth aspect of the invention provides a dual -fuel tank for an aircraft, the dual-fuel tank configured to selectively contain anhydrous ammonia and kerosene, comprising: a controllable venting system adapted to control the flow of air into and out of the dual-fuel tank, the controllable venting system configured such that: when the dual-fuel tank comprises kerosene, the controllable venting system allows air to enter and exit the dual-fuel tank; and when the dual-fuel tank comprises anhydrous ammonia, the controllable venting system allows air to flow into the dual-fuel tank and substantially prevents air exiting the dual-fuel tank.

[0032] A fifth aspect of the invention provides a method of refuelling an aircraft, comprising: fuelling an aircraft to deposit a first fuel in a fuel tank of the aircraft; emptying the first fuel from the fuel tank of the aircraft; and refuelling the aircraft to deposit a second fuel in the fuel tank of the aircraft, wherein the first fuel or second fuel is anhydrous ammonia, and the other of the first or second fuels is kerosene.

[0033] The aircraft may comprise a plurality of fuel tanks, and the method comprise: fuelling the aircraft to deposit the first fuel in each of the plurality of fuel tanks; emptying the first fuel from the plurality of fuel tanks; and refuelling the aircraft to deposit the second fuel in the plurality of fuel tanks.

[0034] The aircraft may comprise a plurality of fuel tanks, and the method comprise: fuelling the aircraft to deposit the first fuel in a first set of the plurality of fuel tanks and deposit a second fuel in a second set of the plurality of fuel tanks; emptying the first fuel from at least one of the first set of the plurality of fuel tanks to provide at least one empty fuel tank; and refuelling the aircraft to deposit the second fuel in the at least one empty fuel tank.

[0035] A sixth aspect of the invention provides an aircraft having a heat exchanger system, the heat exchanger system comprising: a first heat exchanger unit located adjacent a hot air outlet of an aircraft engine; a second heat exchanger unit located adjacent an inlet of the aircraft engine; an aircraft fuel tank containing a fuel; a heat exchanger line connecting the second heat exchanger unit to the fuel tank via the first heat exchanger unit, wherein the heat exchanger system is configured to allow fuel to flow from the fuel tank through the heat exchanger line so as to transfer heat from the hot air outlet to air input to the inlet.

[0036] The heat exchanger may be located adjacent an air intake of the aircraft engine so as to transfer heat from hot air output from the hot air outlet to the air input to the air intake.

[0037] The heat exchanger may be located adjacent a fuel line of a fuel inlet, so as to warm up a fuel in the fuel line prior to the fuel exiting the fuel inlet.

[0038] The fuel may be at a temperature below an ambient temperature of the air surrounding the aircraft.

[0039] The fuel may be at a temperature below 0 degrees Celsius.

[0040] The fuel may be a liquid.

[0041] The fuel may be a non-cryogenic fuel.

[0042] The fuel may comprise mineral kerosene and/or sustainable aviation fuel.

[0043] The fuel may be anhydrous ammonia.

[0044] The aircraft engine may be configured to be powered by anhydrous ammonia.

[0045] The aircraft may comprise a wing, wherein the fuel tank is located in the wing.

[0046] A seventh aspect of the invention provides a method of operating an aircraft heat exchanger system, comprising: chilling a non-cryogenic aviation fuel; adding the non- cryogenic aviation fuel to a fuel tank of an aircraft; and connecting the fuel tank to the heat exchanger system of the aircraft of the third aspect, and circulating the fuel through the heat exchanger system to transfer heat through the heat exchanger system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0048] Figure l is a schematic perspective view of a first example aircraft according to the invention;

[0049] Figure 2 is a schematic plan view of a second example aircraft according to the invention;

[0050] Figure 3 is a schematic plan view of a third example aircraft according to the invention;

[0051] Figure 4 shows a schematic view of the in-wing fuel tanks of the aircraft;

[0052] Figure 5 shows part of a fuel system connecting the fuel tanks to a common aircraft engine;

[0053] Figure 6 shows part of a fuel system connecting each of the fuel tanks to a respective aircraft engine;

[0054] Figure 7 shows part of a fuel system connecting the fuel tanks to a common aircraft engine having a propeller;

[0055] Figure 8 shows of a fuel system connecting each of the fuel tanks to a respective aircraft engine each having a propeller;

[0056] Figure 9 shows a schematic plan view of a fourth example aircraft according to the invention;

[0057] Figure 10 shows a flight phase diagram;

[0058] Figure 11 shows a flight phase diagram representative of the relative fuel usage in each flight phase;

[0059] Figure 12 shows a schematic view of the in-wing fuel tanks of the aircraft;

[0060] Figure 13 shows a first example of heat exchanger system of an aircraft engine; [0061] Figure 14 shows a second example of heat exchanger system of an aircraft engine.

DETAILED DESCRIPTION OF EMBODIMENT(S)

[0062] The invention takes advantage of the fact that kerosene and anhydrous ammonia have similar densities. Aircraft according to the invention are configured to be fuelled by a combination of kerosene and anhydrous ammonia. Aircraft according to the invention may be configured to store kerosene and anhydrous ammonia in a common fuel tank, and/or may be configured to burn both kerosene and anhydrous ammonia in a gas turbine. The combustion of ammonia in a gas turbine has been demonstrated and is described, for example, in Hayakawa et. al. (2018). ‘Two Stage Ammonia Combustion in a Gas Turbine like Combustor for Simultaneous NO and Unbumt Ammonia Reductions’, presented at AIChE Annual Meeting, 2018.

[0063] A first embodiment of the invention is shown in Figure 1. Figure 1 shows an aircraft 1 configured to be fuelled by either kerosene (either mineral or sustainable aviation fuel (SAF)), anhydrous ammonia, or a combination of both kerosene and anhydrous ammonia. The illustrated aircraft 1 is a single aisle passenger aircraft. The aircraft comprises a fuselage 11 and a pair of wings 12a, 12b. The wings 12a, 12b have a substantially conventional design and comprise conventional in-wing fuel tanks (not visible). The fuselage 11 has a conventional design. A gas turbine engine 13a, 13b is mounted on each of the wings 12a, 12b. Each gas turbine engine 13a, 13b is configured to burn kerosene, anhydrous ammonia, or any mixture of kerosene and anhydrous ammonia.

[0064] The aircraft 1 has an enlarged fuel storage capacity compared to a conventional aircraft of a similar type. This may be achieved in any manner known in the art, but in the illustrated example additional fuel storage pods 14 are provided on the wings 12a, 12b. Each fuel storage pod 14 may be (but need not be) in direct communication with one or more of the in-wing tanks. Each fuel tank of the aircraft 1 may be used to store kerosene, anhydrous ammonia, or any mixture of kerosene and anhydrous ammonia. This advantageously enables the aircraft 1 to operate between locations where anhydrous ammonia is available and locations where only kerosene is available. When refuelling the aircraft 1, the uptake of either fuel type can be tailored to the specific range demands of the operator.

[0065] A second embodiment of the invention is shown in Figure 2. Figure 2 shows a different aircraft 2 which is configured to be fuelled by either kerosene (either mineral or sustainable aviation fuel (SAF)), anhydrous ammonia, or a combination of both kerosene and anhydrous ammonia. The illustrated aircraft 2 is a long-range passenger aircraft. The aircraft comprises a fuselage 21 and a pair of wings 22a, 22b. The wings 22a, 22b have a substantially conventional design and comprise conventional in-wing fuel tanks 24a, 24b. The in-wing tanks 24a, 24b are each configured to store kerosene. Storing kerosene in the in-wing tanks maintains conventional loading and balance requirements of the aircraft 2.

[0066] The fuselage 21 has a conventional design and comprises a centre fuel tank 25. In some examples the centre tank 25 comprises multiple tanks. The centre tank or tanks 25 is/are pressurised and is/are mounted in the cargo bay of the aircraft 2. The capacity of the centre tank or tanks 25 is larger than the capacity of conventional cargo-bay- mounted centre tanks currently used by long-range passenger aircraft. The centre tank or tanks 25 is/are configured to store anhydrous ammonia. The centre tank(s) 25 may be permanently mounted in the cargo bay. Alternatively, the centre tank(s) 25 could be configured to be routinely removable from the aircraft 2. This enables the centre tank(s) 25 to be filled off-board the aircraft and then loaded onto the aircraft 2 in a full condition. In such examples the or each removable tank 25 could have the size and configuration of a unit load device (ULD). The number of such removable centre tanks 25 which are loaded onto the aircraft 2 may tailored according to the nature of the particular flight to be operated by the aircraft 2. [0067] Two gas turbine engines 23a, 23b, 26a, 26b are mounted on each of the wings 22a, 22b. The inboard pair of engines 23a, 23b are each configured to bum kerosene, and are therefore each configured to receive fuel from the corresponding in-wing tank 24a, 24b. The outboard pair of engines 26a, 26b are each configured to burn anhydrous ammonia, and are therefore each configured to receive fuel from the centre tank 25. Alternative arrangements are possible in which the inboard engines 23a, 23b are configured to burn anhydrous ammonia and to receive fuel from the centre tank 25, and the outboard engines 26a, 26b are configured to burn kerosene and to receive fuel from the in-wing tanks 24a, 24b. Separate fuel feed systems are provided for ammonia (connecting the centre tank 25 to the inboard engines 23a, 23b) and for kerosene (connecting the in-wing tanks 24a, 24b to the outboard engines 26a, 26b).

[0068] It is generally expected that all four engines 23a, 23b, 26a, 26b of the aircraft 2 will be used at all times during operation of the aircraft 2. However; the inboard engines 23a, 23b and outboard engines 26a, 26b are separately controllable, such that the relative proportions of kerosene and ammonia being used at a given time during operation of the aircraft can be altered. For example, it may be advantageous to operate the engines 23a, 23b, 26a, 26b such that during the cruise phase of flight, the aircraft 2 is mostly or entirely using ammonia. However; during the taxi, climb and descent phases it may be necessary or desirable to increase the proportion of kerosene being used.

[0069] Furthermore, some of the kerosene stored on the aircraft may be designated as reserve fuel. This means that, if the aircraft needs to divert or otherwise deviate from its flight plan in a manner that requires more fuel than expected, the amount of power provided by the kerosene-burning engines 23a, 23b can be increased such that the aircraft is mostly or entirely fuelled by kerosene. [0070] Figure 3 shows a third example of an aircraft 101. The aircraft 101 may have any number of main engines 104a, 104b, for example one, two or three. The example shown in Figure 3 shows the aircraft 101 having four main engines 104a, 104b, two on each of the port and starboard wings 102, 103. The wings 102, 103 extend from a fuselage 106. Each wing 102, 102 may comprise an inner engine 104a and an outer engine 104b, with the inner engines 104a located inboard of the outer engines 104b in the spanwise direction of the aircraft 101 (the spanwise direction being perpendicular to the longitudinal axis of the aircraft 101). The main engines 104a, 104b may be gas turbine engines.

[0071] Each wing 102, 103 of the aircraft 101 may comprise a first fuel tank I l la comprising a first fuel, and a second fuel tank 111b comprising a second fuel. The first fuel is anhydrous ammonia and the second fuel is a fuel other than anhydrous ammonia.

[0072] The first and second fuels may both be liquids. The second fuel may be a hydrocarbon based aviation fuel. The second fuel may be sustainable aviation fuel and/or kerosene. The first and second fuels may have similar densities. For example, liquid anhydrous ammonia has a density of approximately 730 kg/m3, whilst Jet a-1 fuels have a density of 775 kg/m3 to 840 kg/m3. Jet a-1 fuels are used in conventional aircraft 101, and so the replacement of the jet a-1 fuel in one of the fuel tanks I l la, 111b with liquid anhydrous ammonia may be achievable without the need for additional reinforcement and/or structural reconfiguration of the aircraft 101, and in particular without additional reinforcement and/or structural reconfiguration to the wings 102, 103.

[0073] The first fuel tank 11 la may be an inner fuel tank and the second fuel tank 111b may be an outer fuel tank, as shown in Figure 3. In this case, the first fuel in the first fuel tank I l la may be configured to be fed towards the inner engine 104a of each respective wing 102, 103. Similarly, the second fuel in the second fuel tank 111b may be configured to be fed towards the outer engine 104b of each respective wing 102, 103. [0074] Alternatively, the first fuel tank I l la may be an outer fuel tank and the second fuel tank 111b may be an inner fuel tank. In this case, the first fuel in a first fuel tank I l la may be configured to be fed towards the outer engine 104b of each respective wing 102, 103. Similarly, the second fuel in the second fuel tank 111b may be configured to be fed towards the inner engine 104a of each respective wing 102, 103.

[0075] In examples in which the fuels have similar densities, the first fuel tank I l la may be located in one of the port and starboard wings 102, 103 and not in the other of the wings 102, 103. Similarly, the second fuel tank 111b may be located in the other of the port and starboard wings 102, 103 to the first fuel tank I l la and not in the wing 102, 103 where the first fuel tank 11 la is located.

[0076] A conventional fuel tank comprising hydrocarbon-based aviation fuels may include vents that allow air into the tank to displace used fuel. For example, the outer fuel tank 111b shown in Figure 4 comprises a vent 112 towards a spanwise end of the outer fuel tank 111b. The vent 112 may allow changes in atmospheric pressure and temperature inside the fuel tank 11 lb.

[0077] In contrast, a fuel tank configured to store anhydrous ammonia may not include a vent. The fuel tank may be sealed and controllably vented and/or thermally insulated. A sealed and controllably vented fuel tank prevents evaporated anhydrous ammonia from escaping. Thermally insulating the fuel tank may further reduce temperature changes of the anhydrous ammonia fuel, for example it may substantially prevent the anhydrous ammonia from evaporating. The inner fuel tank I l la shown in Figure 4 is sealed and controllably vented, and thermally insulated. The thermal insulation 115 may be any suitable thermal barrier known in the art.

[0078] It will be understood that an aircraft 101 comprising two or more fuels, such as shown in the examples of Figures 1 to 4, may utilise the fuels in any suitable manner.

[0079] In some examples, the fuels in the fuel tanks I l la, 111b may flow to a common main engine. Figure 5 shows an example in which the fuel in the inner fuel tank I l la and the outer fuel tank 111b are fed to a common main engine 104. One of the inner and the outer fuel tanks I l la, 111b comprises anhydrous ammonia, whilst the other of the inner and the outer fuel tanks I l la, 111b comprises a second fuel that is not anhydrous ammonia. [0080] Each fuel line 113a, 113b may comprise a respective valve 114a, 114b to selectively control the flow of fuel to the main engine 104 from each of the inner and outer fuel tanks I l la, 111b. The flow of fuel through the respective fuel lines 113a, 113b may be controlled such that when a fuel flows from one of the fuel tanks I l la, 111b flow is substantially prevented from flowing through the other of the respective fuel tanks I l la, 111b. Alternatively, or in addition, the fuel lines 113a, 113b may be configured to control relative proportions of the two fuels flowing from each of the fuel tanks I l la, 111b. The advantages of varying the relative quantities of two or more fuels will be discussed in further detail below, in particular in relation to Figures 10 and 11.

[0081] In some examples, the fuels in the fuel tanks may flow to separate main engines. Figure 6 shows an example in which the fuel in the inner fuel tank I l la is fed to an inner main engine 104a through a first fuel line 113a and the fuel in the outer fuel tank 111b is fed to an outer main engine 104b through a second fuel line 113b. One of the inner and the outer fuel tanks I l la, 111b comprises anhydrous ammonia, whilst the other of the inner and the outer fuel tanks I l la, 111b comprises a second fuel that is not anhydrous ammonia.

[0082] Each fuel line 113a, 113b may comprise a respective valve 114a, 114b to selectively control the flow of fuel to the respective main engine 104a, 104b. The flow of fuel through the respective fuel lines 113a, 113b may be controlled such that when a fuel flows from one of the fuel tanks I l la, 111b flow is substantially prevented from flowing through the other of the respective fuel tanks I l la, 11 lb. Alternatively, or in addition, the fuel lines 113a, 113b may be configured to control relative proportions of the two fuels flowing from each of the fuel tanks I l la, 11 lb.

[0083] Whilst Figures 5 and 6 are described in relation to an inner and an outer fuel tank I l la, 11 lb, it will be understood that the fuel may be arranged to flow from any suitable fuel source. For example, a fuel pod 14 (See Figure 1) or a fuel tank 25 located in the fuselage 106 (See Figure 2). [0084] It will be understood that the engines 104, 104a, 104b may be any suitable engines. For example, one or more of the engines may comprise a propeller. Figures 7 and 8 show examples in which the main engines 104’, 104a’, 104b’ comprise a propeller 121. The propellers 121 may be powered by any suitable means, for example a gas turbine or fuel cell 126.

[0085] As previously discussed, the fuels may be stored in any suitable configuration on the aircraft 101. There may be any number of fuels. Figure 9 shows an example in which each wing 102, 103 of the aircraft 101 includes a first (inner) fuel tank I l la and a second (outer) fuel tank 111b, and wherein a third fuel tank 111c is located in the fuselage 106.

[0086] The first, second and third fuel tanks I l la, 111b, 111c may include different fuels, such that the aircraft 101 comprises a third fuel. For example, one of the first, second and third fuel tanks I l la, 111b, 111c may comprise anhydrous ammonia whilst the other of the first, second and third fuel tanks I l la, 111b, 111c may each comprise a respective second and third fuel source.

[0087] It will be understood that the fuel tanks I l la, 111b 111c may be modular, such that each fuel tank I l la, 111b, 111c comprises multiple smaller fuel tanks.

[0088] The fuel may be any suitable fuel known in the art. For example, a hydrocarbon based fuel such as Jet-A, Jet A-l, JP-5 or JP-8 grade fuels. The fuel may be kerosene (mineral or SAF). The fuel may be liquid hydrogen or gaseous hydrogen. The fuel may be liquid natural gas. The fuel may be used to power the aircraft 101 directly, or may be used to supply power indirectly (for example via a fuel cell).

[0089] There are many advantages to having two or more fuels able to feed (directly or indirectly) the main engines 104, 104a, 104b of the aircraft 101. The aircraft 101 may be operable with either of the fuels, such that a fuel can be selected based on local fuel supplies and prices. The aircraft 101 may be operable so as to vary the relative utilisation of each fuel in different flight phases, or in different locations, based on operational requirements, relative fuel quantities, aircraft balancing, as well as environmental considerations, as will be discussed below. [0090] The aircraft 101 may comprise a fuel control system configured to control the flow of a first fuel through the first fuel line 113a and to control the flow of a second fuel through the second fuel line 113b. A third fuel line (not shown) may be supplied to control flow of a third fuel as required. The first fuel is anhydrous ammonia, whilst the second fuel is a fuel other than anhydrous ammonia. The fuel control system controls the flow of the first and second fuels so as to vary the flow rate of the first fuel with respect to the flow rate of the second fuel. The fuel control system may vary the respective flow rates according to a flight phase of the aircraft.

[0091] Figure 10 shows an example of a flight phase diagram showing six different flight phases: taxiing/ground manoeuvres 151, take-off 152, climb 153, cruise 154, descent 155, approach/landing 156. Figure 10 is representative of the flight time 157 against the aircraft altitude 158. Figure 11 shows an example of the flow rate 159 of the anhydrous ammonia (line 161a) and the second fuel (line 161b) in each of the flight phases 151, 152, 153, 154, 155, 156. The flow rate of anhydrous ammonia may be greater than the flow rate of the second fuel in a particular flight phase, or the flow rate of the second fuel may be greater than the flow rate of anhydrous ammonia, depending on the operational requirements of the aircraft 101.

[0092] In some examples, the flow rate of anhydrous ammonia may increase/decrease whilst the flow rate of the second fuel increases/decreases. Alternatively, the flow rate of anhydrous ammonia/second fuel may be substantially prevented before the other of the anhydrous ammonia/second fuel is increased. For example, the engine 104 may be operable on each fuel independently such that the flow of one fuel is prevented before the flow of the other fuel begins.

[0093] The relative utilisation of each fuel may be determined based any of a number of factors (such as those mentioned above), however anhydrous ammonia has a relatively low energy density compared to kerosene. For example, anhydrous ammonia has an energy density of approximately 20-30% of kerosene-based fuels. The relatively low energy density of anhydrous ammonia compared to kerosene, and many other hydrocarbon based fuels, may make alternative fuels to anhydrous ammonia a more favourable fuel to use during flight phases requiring higher power (e.g. take-off and climb). [0094] In contrast, anhydrous ammonia is a non-carbon based fuel and therefore anhydrous ammonia may be an attractive fuel to burn during flight phases requiring lower power (e.g. taxiing, ground manoeuvres, cruise, descent, approach and landing). This can reduce the carbon emissions emitted during a flight.

[0095] In the example shown in Figure 11, the aircraft 101 initially operates substantially only on anhydrous ammonia during ground manoeuvres and taxiing. Ground manoeuvres and taxiing generally require low power, and therefore powering the aircraft with only anhydrous ammonia may be sufficient.

[0096] During take-off 152 and climb 153 the power requirements of the aircraft 101 increase. As a result, the flow rate of the second fuel (e.g. a hydrocarbon-based fuel) is increased. As shown in Figure 11, the flow rate of anhydrous ammonia may also be decreased or substantially prevented from flowing. Although alternatively the flow rate of anhydrous ammonia may remain substantially the same or increase.

[0097] Once the aircraft 101 reaches a steady altitude, i.e. cruise 154, the power requirements of the aircraft will typically decrease with respect to the power required for take-off 152 and climb 153. Accordingly, the flow rate of the second fuel may be decreased, or substantially prevented from flowing, and the flow rate of anhydrous ammonia may be increased. This reduces the carbon emissions from the aircraft.

[0098] Similarly, during descent of the aircraft 155 the power requirements may be relatively low such that the flow rate of anhydrous ammonia is greater than a flow rate of the second fuel.

[0099] Figure 11 shows the flow rate of anhydrous ammonia being greater than the flow rate of the second fuel in the approach and landing phase 156 of the flight. However, the relative flow rate of the second fuel is shown to be slightly increased with respect to the flow rate during cruise. This may be necessary in the event of an aborted landing or other operational requirement; such that full engine utilisation is quickly available if required.

[0100] Upon the ground, i.e. during ground manoeuvres and taxiing, the aircraft 151 may operate substantially only on anhydrous ammonia. [0101] It will be understood that reference to substantially above refers to the small flow rates that may still be required. For example to maintain an engine in idle, or to power other components of the aircraft such as onboard electronics or an auxiliary power unit. Alternatively, the flow rate may be zero.

[0102] In some examples, the aircraft 101 may comprise a third fuel in a third fuel tank (See e.g. Figure 9). In this case, the flow rate of the third fuel with respect to the flow rate of the first fuel and/or second fuel may be varied. For example, the flow rate may be varied according to the flight phase 151, 152, 153, 154, 155, 156 of the aircraft 101.

[0103] An aircraft 101 operable using two or more fuels provides flexibility. For example, if one of the fuels is not available at a refueling location, the aircraft 101 is still operable on alterative fuels.

[0104] In some examples, the fuel tanks may be configurable to store multiple fuels, either at the same time or alternately. For example, mineral based kerosene and SAF may be combinable in a single fuel tank I l la, 111b, 111c, whilst a single fuel tank l l la, 11 lb, 111c may be able to hold a first fuel (e.g. a hydrocarbon based fuel) for a first period, and hold a second fuel (e.g. anhydrous ammonia) for a second period after the first period. This may improve the operability of the aircraft 101 in different situations and according to fuel availability. In examples in which a fuel tank I l la, l llb, 111c is configured to switch between fuel sources, a vent 112a of the fuel tank I l la, 111b, 111c may be selectively covered or otherwise controllable so as to selectively allow fuel vapour to escape through the vent 112a or the prevent fuel vapour from escaping through the vent 112a whilst allowing air to enter through the vent to replace used fuel.

[0105] Figure 12 shows an example in which the first fuel tank I l la and second fuel tank 11 lb are dual-fuel tanks configured to selectively contain anhydrous ammonia and kerosene. Each fuel tank I l la, 111b may be sealed and controllably vented by a controllable venting system 112a. Selectively controlling the venting to and from the fuel tank prevents evaporated anhydrous ammonia from escaping from the fuel tanks I l la, 11 lb. The controllable venting system 112a may be arranged to allow air to flow into the fuel tanks I l la, 111b when the respective fuel tank I l la, 111b contains kerosene, and to allow air flow into the fuel tanks I l la, 111b whilst substantially preventing air exiting the fuel tanks I l la, 111b when the respective fuel tank I l la, 111b contains anhydrous ammonia. It will be understood that whilst Figure 12 shows the controllable venting system 112a comprises a single vent, the controllable venting system may comprise any number of vents each configured to function similarly, or differently, as required. For example, the controllable venting system 112a may comprise a first vent for allowing air to enter each fuel tank I l la, 111b, and a second vent to selectively allow air to exit each fuel tank I l la, 11 lb.

[0106] As shown in Figure 12, the dual-fuel tanks I l la, 111b may also be thermally insulated with thermal insulation 115. Thermally insulating the fuel tank may further reduce temperature changes of the anhydrous ammonia fuel, for example it may substantially prevent the anhydrous ammonia from evaporating. The thermal insulation 115 may be any suitable thermal barrier known in the art. Alternatively, the anhydrous ammonia may be cooled by other means, such as refrigeration or other means known in the art. As shown in the above examples, the fuel may be located in any suitable location on the aircraft 101. For example, the fuel may be stored in wing tanks 24a, 24b, I l la, 11 lb, or stored in centre tanks 25, 111c in the fuselage 106, or fuel storage pods 14 or any suitable location. Typically, it will be desirable to distribute the fuel so as to balance the weight of the aircraft 101. The fuel may be arranged symmetrically. For example, such that a first fuel is located in the inner fuel tanks 11 la of each wing 102, 103 and a second fuel is located in the outer fuel tanks 11 lb of each wing 102, 103. [0107] In some cases, there may be benefits in heating up the fuel prior to combustion and/or heating up the air prior to entry into the intake of the engine 104, 104a, 104b. In many cases, aviation fuel is kept at very low temperatures such that steps are required to prevent the fuel from crystallizing or freezing. Aviation fuel can also be an effective heat exchanger fluid.

[0108] According to a further example shown in Figure 13, there is provided a heat exchanger system of an aircraft engine 104c. The aircraft engine 104c may be a turbojet engine having a compressor section 122, a combustor section 123, a turbine section 124, and an exhaust section 125. The heat exchanger system comprises a first heat exchanger unit 141 and a second heat exchanger unit 142. The first heat exchanger unit 141 is located adjacent a hot air outlet of the aircraft engine 104c (in this example the exhaust 125 of the aircraft engine 104c). The second heat exchanger unit 142 is located adjacent an inlet of the aircraft engine 104c (in this case an intake of the aircraft engine 104c).

[0109] A heat exchanger line 143 connects the first heat exchanger unit 141 to a fuel tank 111. The fuel tank 111 contains fuel that is configured to flow from the fuel tank 111 to the first heat exchanger unit 141. The first heat exchanger unit 141 transfers heat from the hot air outlet of the aircraft engine 104c to the fuel, thereby resulting in the local fuel temperature increasing. The first heat exchanger unit 141 is connected to the second heat exchanger unit 142 by the heat exchanger line 143 such that the heated fuel is able to flow to the second heat exchanger unit 142.

[0110] The second heat exchanger unit 142 is located adjacent the air intake of the aircraft engine 104c and is thereby able to heat up the air input to the air intake. The heat exchanger line 143 connects from the second heat exchanger unit 142 to the fuel tank 111, such that the fuel can flow back to the fuel tank 111.

[0111] In this way, the heat exchanger system is configured to allow fuel to flow from the fuel tank 111 through the heat exchanger line 143 so as to transfer heat from a hot air outlet (e.g. an exhaust 125 of an aircraft engine 104c) to air input to an inlet (e.g. an intake of an aircraft engine 104c). The first heat exchanger unit 141 may be located in the flow of the hot air from the hot air outlet, or may be configured to conduct heat from components surrounding the outlet. [0112] In some examples, the fuel circulated through the heat exchanger system may also be arranged to fuel the aircraft 104c through a fuel line 113c connected to the fuel tank 111, as shown in Figure 13.

[0113] In an alternative example show in Figure 14 the first heat exchanger unit 141 is located adjacent an exhaust of the aircraft engine 104c (although any hot air outlet of an aircraft engine 104c may be utilised), whilst the second heat exchanger unit 142 is located adjacent a fuel line 113c. In the example shown in Figure 14 the fuel line 113c extends through the second heat exchanger unit 142 (as indicated by dotted line 113c’). In this way, the temperature of the fuel in the fuel line 113c may be increased. This can be particularly beneficial in certain situations to promote or ease combustion.

[0114] The increased temperature of the fuel may aid in catalytic reactions. For example, Figure 14 shows the fuel line 113c extending from the second heat exchanger unit 142 to a catalyst 145. The fuel may be anhydrous ammonia, and the catalyst may be configured to split the anhydrous ammonia into nitrogen and hydrogen. The resultant components from the catalyst 145 may then be conveyed to the aircraft engine 104c to be combusted. Providing a catalyst 145 that splits the anhydrous ammonia to produce hydrogen may provide many benefits, such as an onboard supply of hydrogen that nullifies the requirement to provide a hydrogen liquefaction facility at the airport for providing hydrogen to the aircraft 101.

[0115] The aircraft 101 may be powered by anhydrous ammonia directly or indirectly. The aircraft engines 104, 104’, 104a, 104a’, 104b, 104b’ may be powered by the hydrogen alone or in combination with another fuel. For example, the hydrogen may be fed to power one or more of the aircraft engines 104, 104’, 104a, 104a’, 104b, 104b’. The hydrogen may be fed with a portion of the anhydrous ammonia to one of the aircraft engines 104, 104’, 104a, 104a’, 104b, 104b’. [0116] The hydrogen may be fed to a hydrogen fuel cell, and wherein the hydrogen fuel cell powers the aircraft 101. The hydrogen fuel cell may power a first engine 104, 104’, 104a, 104a’, 104b, 104b’ of the aircraft 101, and a second fuel (e.g. kerosene) may power a second engine 104, 104’, 104a, 104a’, 104b, 104b’ of the aircraft 101. Supplying hydrogen to a hydrogen fuel cell 124 converted from anhydrous ammonia may allow the weight of storage of liquid or gas hydrogen to be offset, for example the weight of a high pressure cylinder to store gaseous hydrogen.

[0117] In addition, one of the fuels may power other components of the aircraft 101, such as onboard electronics or an auxiliary power unit.

[0118] As shown in Figures 13 and 14, the fuel in the fuel tank 111 acts as a heat exchanger fluid in the heat exchanger system. The fuel in the fuel tank 111 may additionally feed the aircraft engine 104c, and/or the fuel in the fuel tank 111 may feed a different aircraft engine of the aircraft 101. Using the fuel as a heat exchange fluid means that a separate heat exchange fluid, and accompanying system, is no longer required. This can reduce aircraft weight and system complexity. The fuel tank 111 also provides a large body of fluid to minimise any increase in temperature of the fuel due to it acting as a heat transfer fluid.

[0119] The fuel may be a hydrocarbon-based aviation fuel, such as kerosene (either mineral or SAF), however the heat exchange properties are increased when the fuel has a high specific heat capacity. Anhydrous ammonia has a high specific heat capacity (approximately 4.6-6.74 KJ/Kg K) compared to kerosene (approximately 2.01 KJ/Kg K), and therefore is a preferred option as the heat exchanger fluid.

[0120] The aircraft engine 104, 104a, 104b, 104c may be configured to be powered by anhydrous ammonia. A heat exchanger system arranged on an aircraft engine 104, 104a, 104b, 104c powered by anhydrous ammonia may be particularly advantageous as the intake air temperature can be increased, thereby aiding combustion of the anhydrous ammonia.

[0121] The fuel is able to operate as a more effective heat exchanger fluid if it is kept at lower temperatures. For example, the fuel may be at a temperature below an ambient temperature of the air surrounding the aircraft 101. The fuel may be at a temperature below 0 degrees Celsius. [0122] In some examples, the fuel may be located in a wing 102, 103 of the aircraft 101. Each wing 102, 103 has a large volume to surface area ratio and may be a good place to maintain the fuel at a low temperature due to the low air temperatures encountered at high altitudes.

[0123] In some instances, there may be benefits in pre-chilling the aviation fuel prior to putting it in the aircraft 101. For example, the fuel may be chilled to improve the performance of the fuel as a heat exchanger fluid. The fuel may be any suitable fuel, such as kerosene or anhydrous ammonia.

[0124] In certain situations (such as emergencies) an aircraft may jettison fuel in order to rapidly reduce the aircraft’s weight. A potential advantage to the use of anhydrous ammonia as a fuel is that anhydrous ammonia is water soluble, such that jettisoning anhydrous ammonia may be less environmentally impactful than jettisoning kerosene. In some situations, an aircraft comprising anhydrous ammonia and a second fuel may be able to retain the second fuel whilst jettisoning only the anhydrous ammonia fuel.

[0125] Although the invention has been described above mainly in the context of a fixed-wing aircraft application, it may also be advantageously applied to various other applications, including but not limited to applications on vehicles such as helicopters, drones, trains, automobiles and spacecraft.