Patent Publication Number: US-2022220924-A1

Title: Engine

Description:
FIELD 
     The present disclosure relates to engines, in particular precooled engines, such as of the type which may be used in aerospace applications, and to a method of operating such an engine. 
     The disclosure also relates to an aircraft, flying machine or aerospace vehicle comprising such an engine. 
     BACKGROUND 
     Efficient air-breathing propulsion for flight from low speed to high Mach (e.g. above Mach 2.5) can require the integration of several engine types (for example, an air-breathing engine and a rocket engine, or a turbomachinery based engine and a Ramjet) into a single system, with carefully managed transitions between the various modes of operation provided by the different engine types, as thrust falls in one mode while thrust in another mode rises. The Mach number at which these transitions between different modes of operation occurs is largely a result of temperature limitations. For example, where a system is configured to operate using turbomachinery and a Ramjet, typically there will be a transition from turbomachinery to Ramjet operation at a flight Mach where neither mode is optimised. This is because if, for example, material temperature limits in the turbomachinery are hit early and throttling thus occurs at lower supersonic speeds, well before the Ramjet kicks in to produce good thrust performance, this results in a gap/deficiency in overall thrust performance, in which the propulsion system struggles to provide sufficient thrust to continue acceleration. If the temperature of the intake air entering the engine could be controlled (i.e. reduced below stagnation), then the transition point for a given engine mode could be at higher Mach. This would allow the various engine modes to be better optimised, and for greater flexibility of operation through the transition speeds. It could even allow for some engine modes to be removed completely. One means of controlling the air temperature entering the engine is by precooling. 
     US2015/0275762 describes a cooling system for a turbine engine including a heat exchanger in fluid communication with a first fluid inlet stream and disposed upstream and in fluid communication with a core engine. The heat exchanger is operative to cool the first fluid inlet stream. The heat exchanger includes a heat exchanger inlet for input of a heat exchanging medium for exchange of heat from the first fluid inlet stream to the heat exchanging medium. The heat exchanger further includes a heat exchanger outlet for discharge of a heated output stream into one of a turbine of a downstream engine, an augmentor or a combustor of the core engine. The heated output stream provides an additional flow to the downstream engine. A turbine engine including the cooling system is also described. 
     WO2015052469 describes a heat exchanger which may be used in an engine, such as a vehicle engine for an aircraft or orbital launch vehicle. The heat exchanger may be configured as generally drum-shaped with a multitude of spiral sections, each containing numerous small diameter tubes. The spiral sections may spiral inside one another. The heat exchanger may include a support structure with a plurality of mutually axially spaced hoop supports, and may incorporate an intermediate header. The heat exchanger may incorporate recycling of methanol or other antifreeze used to prevent blocking of the heat exchanger due to frost or ice formation. 
     GB2238080 describes a hybrid propulsion system for a transatmospheric or hypersonic cruise vehicle comprising a pre-cooled air-breathing jet engine and a rocket motor. Previously ice formed on the inlet cooling heat exchanger at altitudes where there is significant humidity sufficiently to deleteriously affect jet thrust. The rocket motor is therefore operated temporarily to compensate for the loss of jet thrust when icing is present and during defrost. The rocket motor is mounted co-axially with the jet pipe which exhausts through an annular orifice into a common propelling nozzle. The arrangement avoids transverse thrust vectors as the relative engine thrusts vary which would cause the vehicle to pitch or yaw. The air inlet heat exchange is intermittently by-passed via a line to allow deicing. 
     The present disclosure seeks to alleviate, at least to a certain degree, the problems and/or address at least to a certain extent, the difficulties associated with the prior art. 
     SUMMARY 
     According to a first aspect of the disclosure, there is provided an engine, comprising: 
     an air intake arrangement configured to receive air; 
     a heat exchanger arrangement arranged downstream of the air intake arrangement, configured to cool the air, and comprising a plurality of heat exchanger modules; and one or more turbomachinery components configured to receive cooled air from the heat exchanger arrangement; 
     wherein: 
     the plurality of heat exchanger modules are arranged to be generally centred on and to be arranged along a longitudinal axis of the engine; 
     and 
     at least one of the plurality of heat exchanger modules is arranged to at least partially overlap with at least one other one of the plurality of heat exchanger modules relative to the longitudinal axis of the engine. 
     Advantageously, such an engine may provide the ability to reduce the temperature of air entering the engine. Additionally, the transitions between different modes of operation of the engine may be better managed. In particular, the transition points for a given engine mode may be at a higher Mach. As such, different modes of operating the engine may be optimised, and there may be greater flexibility of operation through the transition speeds between different engine modes. The engine modes may even be removed completely. In addition, such an engine may provide for improved hot day performance (e.g. performance when the environmental conditions are at a relatively hot temperature compared with a typical environmental temperature) of the one or more turbomachinery components. Furthermore, the heat exchanger modules may advantageously allow a proportion of the air to pass through each of the heat exchanger modules and be cooled by a heat transfer medium flowing through the heat exchanger modules. 
     At least one of the plurality of heat exchanger modules may be arranged to partially overlap with at least one other one of the plurality of heat exchanger modules relative to the longitudinal axis of the engine. A respective portion of at least one of the plurality of heat exchanger modules may be arranged to extend beyond or over, or overhang from, a respective end of an adjacent one of the other heat exchanger modules along the longitudinal axis of the engine, to provide the partial overlap. 
     The modular structure of the heat exchanger arrangement provides that the engine may be configured such that the flow of a heat transfer medium in one or more of the heat exchanger modules may be modulated or completely turned off separately (i.e. independently from the flow of a heat transfer medium in the other heat exchanger modules). Additionally, the heat exchanger modules of the engine may provide that a proportion of the air may pass through each heat exchanger module and be cooled by a heat transfer medium in the heat exchanger arrangement. Advantageously, this may further provide for improved optimisation, flexibility and control of the temperature profile of the air flowing through the engine. 
     The overlap of the plurality of heat exchanger modules relative to the longitudinal axis of the engine may advantageously provide for a reduced overall length of the heat exchanger arrangement and hence of the engine, and may be particularly suitable where the angle of air flow through the heat exchanger arrangement has an axial element and is not desired to be purely radial. Said overlap may be chosen so as to optimise the radial temperature distribution of the flow in the heat exchanger arrangement and to balance this with minimising any unwanted aerodynamic effects. For example, said overlap may be chosen to be partial only (i.e. for the heat exchanger modules not to fully overlap with one another relative to the longitudinal axis of the engine). 
     Furthermore, precooling the air entering the engine reduces the temperature operating range over which the one or more turbomachinery components must operate. This may advantageously provide, for example, for a decrease in the extent to which the tip clearances of a compression system of the one or more turbomachinery components must be managed over the temperature operating range of the one or more turbomachinery components. This may provide for improved tip clearance losses in the one of more turbomachinery components and corresponding engine handling improvements, thus resulting in an increase in overall mission efficiency (fuel burn). 
     Accordingly, such an engine may provide for the careful management of the transitions between various modes of operation of the engine, for an increase in the maximum flight Mach number of the operation of the one or more turbomachinery components, and for a reduction in the impact of transitions between different engine modes on a mission profile, thus increasing the maximum operational flight speed of the engine. Additionally, such an engine may allow for a Ramjet system to be included in the engine that has a reduced cross-sectional area, which may provide for better airframe integration. 
     The plurality of heat exchanger modules are arranged to be generally centred on the longitudinal axis of the engine. This may mean, for example, that each of the heat exchanger modules has a centre (e.g. if one of the heat exchanger modules is approximately cylindrical, then its centre is determined by the radial centre of the circular face of said cylindrical heat exchanger module) that is arranged to be generally coincident with the longitudinal axis of the engine. 
     The plurality of heat exchanger modules are arranged along the longitudinal axis of the engine. This may mean, for example, that a first one of the heat exchanger modules may be arranged at a position along the longitudinal axis of the engine that is proximate to the air intake arrangement and is upstream of all of the other heat exchanger modules. A last one of the heat exchanger modules may be arranged at a position along the longitudinal axis of the engine that is proximate to the one or more turbomachinery components and is downstream of all of the other heat exchanger modules. If the plurality of heat exchanger modules comprises more than two heat exchanger modules, then the remaining heat exchanger modules may be arranged along the longitudinal axis of the engine between the first heat exchanger module and the last heat exchanger module. 
     The intake arrangement may be configured for the flow of subsonic, supersonic and/or hypersonic air. 
     The one or more turbomachinery components may comprise a compressor. The one or more turbomachinery components may further comprise a turbine configured to drive the compressor. 
     The engine may further comprise an air-breathing combustion chamber configured to receive compressed air from the compressor and for the combustion of air and fuel. 
     The engine may further comprise an augmentation system such as a reheat or afterburner system. 
     The engine may further comprise one or more nozzles, such as one or more variable area nozzles. 
     The engine may further comprise an exhaust. 
     The engine may comprise an air-breathing jet engine, wherein the one or more turbomachinery components may comprise a compressor and a turbine. 
     The engine may further comprise a Ramjet for high Mach (e.g. above Mach 2.5) operation of the engine. The Ramjet may be configured to be supplied by air that bypasses the one or more turbomachinery components and that optionally may also bypass the heat exchanger arrangement. Advantageously, the Ramjet may be used as the main thrust source. The Ramjet may be run jointly with the one or more turbomachinery components during a mode transition of the engine. Advantageously, the heat exchanger arrangement may be used when the one or more turbomachinery components reach a material temperature limit, such as in a compression system of the one or more turbomachinery components, as the flight speed of the engine increases during operation of the engine. This may advantageously raise the maximum operational flight speed of the engine at which the one or more turbomachinery components could be operated. 
     The heat exchanger arrangement may comprise an inlet and an outlet. The inlet of the heat exchanger arrangement may be configured to be supplied with a heat transfer medium by an inlet manifold, and the outlet of the heat exchanger arrangement may be configured to supply the heat transfer medium to an outlet manifold. The inlet manifold and/or the outlet manifold may be cylindrical or longitudinal in shape. 
     The heat transfer medium may be any fuel type, any gas, any liquid, and/or any cryogenic fluid, for example, the heat transfer medium may be helium. 
     The heat transfer medium may comprise a fuel, or the heat transfer medium may comprise a fluid that is different to and separate to (i.e. fluidly isolated from) a fuel used in the engine. 
     The heat transfer medium may comprise a fluid that is different to and separate to (i.e. fluidly isolated from) a fuel used in the engine, and the engine may further comprise a second heat exchanger arrangement configured to provide for heat transfer between said heat transfer medium and said fuel. 
     The fuel may comprise hydrogen. 
     The fuel may be provided in liquid form or cryogenic form, for example, the fuel may comprise liquid hydrogen. 
     One or more of each of the plurality of heat exchanger modules may be annular in shape. 
     One or more of each of the plurality of heat exchanger modules may be at least partially substantially cylindrical or conical in shape, or may be any other substantially annular shape centred on the longitudinal axis of the engine. Advantageously, a substantially cylindrical heat exchanger module may provide for a simple construction of said heat exchanger module. A substantially conical heat exchanger module may provide for a slimmer engine installation where diameter is at a premium, and may also provide for protection from debris and objects, thus aiding the compactness and effectiveness of the heat exchanger arrangement in the engine. 
     Each of the plurality of heat exchanger modules may have a respective inner diameter and a respective outer diameter. 
     The plurality of heat exchanger modules may be sized such that they get progressively larger or smaller in diameter along the length of the longitudinal axis of the engine. In other words, the respective inner diameters of each of the plurality of heat exchanger modules may be chosen such that a first one of the plurality of heat exchangers arranged proximate the air intake arrangement has a smaller inner diameter than all of the other plurality of heat exchanger modules; a last one of the plurality of heat exchangers arranged proximate the one or more turbomachinery components has a larger inner diameter than all of the other plurality of heat exchanger modules; and if the plurality of heat exchanger modules comprises more than two heat exchanger modules, then the respective inner diameters of the remaining plurality of heat exchanger modules arranged between the first and last heat exchanger modules gradually increase relative to the downstream direction of the engine. Similarly, in other words, the respective outer diameters of each of the plurality of heat exchanger modules may be chosen such that a first one of the plurality of heat exchangers arranged proximate the air intake arrangement has a smaller outer diameter than all of the other plurality of heat exchanger modules; a last one of the plurality of heat exchangers arranged proximate the one or more turbomachinery components has a larger outer diameter than all of the other plurality of heat exchanger modules; and if the plurality of heat exchanger modules comprises more than two heat exchanger modules, then the respective outer diameters of the remaining plurality of heat exchanger modules arranged between the first and last heat exchanger modules gradually increase relative to the downstream direction of the engine. 
     The one or more turbomachinery components may comprise an engine fan case having an outer diameter. At least one of the plurality of heat exchanger modules may be sized and positioned such that the inner diameter of said at least one of the plurality of heat exchanger modules is in line (i.e. substantially coincident with) the outer diameter of the engine fan case, relative to the radial direction of the engine. Advantageously, this may minimise blockage to the one or more turbomachinery components. The precise inner and outer diameters of the plurality of heat exchanger modules may be chosen according to the aerodynamic considerations of a particular application. 
     At least one of the plurality of heat exchanger modules may have a different inner and/or outer diameter than at least one other one of the plurality of heat exchanger modules. 
     The longitudinal axis of the engine may be at least partially curved, e.g. there may be some degree of convolution in the air intake arrangement. 
     The plurality of heat exchanger modules may be orientated along the longitudinal axis of the engine. Advantageously, this may provide that the heat exchanger arrangement may be closely integrated with the air intake arrangement. The use of a plurality of modular heat exchanger modules may help facilitate this integration by providing for the careful arrangement of the axial positioning, orientation and/or size of the plurality of heat exchanger modules along the longitudinal axis of the engine. Furthermore, the plurality of heat exchanger modules may be positioned and orientated along the longitudinal axis of the engine such that there may be provided complete protection from debris and particles that follow a mostly ballistic trajectory (rather than being carried along aerodynamic streamlines, for example). 
     Each of the plurality of heat exchanger modules has a respective length. 
     At least one of the plurality of heat exchanger modules may be arranged such that its respective length at least partially overlaps with the respective length of at least one other one of the plurality of heat exchanger modules, relative to the longitudinal axis of the engine. For example, at least one of the plurality of heat exchanger modules may be arranged such that at least approximately one sixth, one fifth, one quarter, one third or one half of the length of said at least one of the plurality of heat exchanger modules overlaps with the respective length of at least one other one of the plurality of heat exchanger modules, relative to the longitudinal axis of the engine. 
     At least one of the heat exchanger modules may have a different length to at least one other one of the plurality of heat exchanger modules. For example, at least one of the heat exchanger modules may have a length that is approximately one half of the length of at least one other one of the plurality of heat exchanger modules. Advantageously, the lengths of one or more of the heat exchanger modules may be chosen to be different to one another to provide an additional means to control the air flow through each of the heat exchanger modules. 
     Each of the plurality of heat exchanger modules may comprise a plurality of tubes for the flow of a heat transfer medium in heat exchange with the air. 
     The heat exchanger arrangement may comprise three or more heat exchanger modules, for example, three, four, five, six, or any other number of heat exchanger modules. 
     The plurality of heat exchanger modules may be arranged to be in fluid communication with one another. 
     The plurality of heat exchanger modules may be arranged in series and/or in parallel relative to the longitudinal axis of the engine. 
     Each of the plurality of heat exchanger modules may comprise a respective inlet for receiving air and a respective outlet for expelling air. The respective inlets of each of the plurality of heat exchanger modules may each be in fluid communication with a respective inlet manifold of each of the plurality of heat exchanger modules. The respective outlets of each of the plurality of heat exchanger modules may each be in fluid communication with a respective outlet manifold of each of the plurality of heat exchanger modules. The inlet manifold of one of the heat exchanger modules may be common with (e.g. directly connected to and/or in direct fluid communication with) the inlet manifold of the heat exchanger arrangement, and/or the outlet manifold of one of the heat exchanger modules may be common with (e.g. directly connected to and/or in direct fluid communication with) the outlet manifold of the heat exchanger arrangement. 
     One or more of the respective inlets and outlets of each of the plurality of heat exchanger modules may be overlapping (i.e. generally coincident along the longitudinal axis of the engine) with one another, and/or one or more of the respective inlets and outlets of each of the plurality of heat exchanger modules may be spaced apart from one another along the longitudinal axis of the engine. 
     The heat exchanger arrangement may be configured such that air is configured to flow into and through the heat exchanger arrangement substantially along the radial direction of the engine, i.e. to flow substantially perpendicular to the longitudinal axis of the engine. 
     The heat exchanger arrangement may be configured such that air is configured to flow into and through the heat exchanger arrangement having a component that is substantially parallel to the radial direction of the engine, and/or a component that is substantially parallel to the longitudinal axis of the engine. 
     The heat exchanger arrangement may be configured such that air is configured to flow around the plurality of tubes as it flows through the heat exchanger arrangement. 
     The plurality of tubes may be arranged to be each wound in a respective path that gradually widens or tightens about the longitudinal axis of the engine and each spaced apart from one another in rows along the longitudinal axis of the engine. For example, the plurality of tubes may be arranged in involute spiral paths, which may extend circumferentially about the longitudinal axis of the engine. 
     The plurality of tubes may be each connected at a first end thereof to an inlet header and at a second end thereof to an outlet header. The inlet header and the outlet header may each be arranged to extend substantially parallel to the longitudinal axis of the engine. The inlet header may be arranged to be in fluid communication with the inlet of the heat exchanger arrangement and the outlet header may be arranged to be in fluid communication with the outlet of the heat exchanger arrangement. 
     The plurality of tubes may be arranged relative to one another (e.g. spaced apart from and positioned relative to one another) such that during use of the engine, air may flow around the plurality of tubes in a perpendicular, or parallel, and/or combined direction. The plurality of tubes may be arranged to be straight and/or curved. For example, the plurality of tubes may be arranged to be curved at a fixed radius, curved with a varying radius, and/or in an involute spiral. The plurality of tubes may be arranged in-line relative to one another, and/or in a symmetrical and/or asymmetrical and/or random staggered arrangement, and/or in a random arrangement. 
     The plurality of tubes in at least one of the plurality of heat exchanger modules may be spaced apart and positioned relative to one another such that said one of the plurality of heat exchanger modules is configured to provide an angle to a bulk flow of the air flowing through the heat exchanger arrangement. Advantageously, the plurality of tubes may thus be arranged so as to provide an angle to a bulk flow of the air flowing through the heat exchanger arrangement. The spacing and/or the positioning of the plurality of tubes relative to one another may be chosen to advantageously use asymmetry to avoid co-operative forcing and resonance due to aero-elastic phenomena between the heat exchanger arrangement and the one or more turbomachinery components, and to help control aerodynamic effects such as pressure distortion. This may be achieved by, for example, randomised tube spacing (e.g. allowing large tolerances in the spacing of the tubes), or by systematic and selective variations in the spacing of the plurality of tubes relative to one another. In particular, the precise arrangement (e.g. the relative positioning and/or spacing) of the plurality of tubes relative to one another may be chosen according to a particular application such that: the air flow may exit the heat exchanger arrangement in a condition that is acceptable to the downstream one or more turbomachinery components without either the heat exchanger arrangement or the one or more turbomachinery components producing detrimental effects on each other; and such that allowance is made for thermal expansion by any unwinding of the plurality of tubes sliding relative to each other, which may occur if the tubes are arranged in involute spiral paths, for example. 
     The plurality of tubes in at least one of the plurality of heat exchanger modules may be arranged relative to one another (e.g. spaced apart from and positioned relative to one another) in a different arrangement to the plurality of tubes in at least one other one of the plurality of heat exchanger modules. Advantageously, this may provide that greater cooling is provided to one part of the air flow than the rest of the air flow through the engine. This may also provide that the surface area through which the air flows through the heat exchanger arrangement is increased, and may also provide for a slimmer engine installation. 
     The engine may further comprise an outer ducting and/or an inner centre structure configured to direct the air to flow from the air intake arrangement into the heat exchanger arrangement. 
     The engine may further comprise an outer case that forms a seal with the outer ducting. 
     The inner centre structure may comprise one or more movable sections. The one or more movable sections may be configured to be selectively moved to allow for the direct flow of air from the air intake arrangement to the one or more turbomachinery components, by bypassing the heat exchanger arrangement. The one or more movable sections may be configured to move by rotation, translation, a hinged means, and/or any other suitable mechanical means. Advantageously, the one or more movable sections of the inner centre structure may be moved to cause the air to bypass the heat exchanger arrangement. In other words, the one or more movable sections of the inner centre structure may provide for the direct flow of air through to the one or more turbomachinery components, thus bypassing the heat exchanger arrangement. This may be desirable when the engine is operating at low Mach or high corrected flows. In addition, the one or more movable sections may advantageously provide that the engine may be configured such that the flow of a heat transfer medium in one or more of the heat exchanger modules may be modulated or completely turned off separately (i.e. independently from the flow of a heat transfer medium in the other heat exchanger modules). Advantageously, this may provide for improved optimisation, flexibility and control of the temperature profile of the air flowing through the engine, by balancing pressure losses and temperature drops through the heat exchanger modules. 
     The engine may further comprise a plurality of turning vanes configured to direct the flow of air into and out of the heat exchanger arrangement. Advantageously, this may provide for the air flow to be directed onto and out of the heat exchanger arrangement. 
     The engine may further comprise a separator duct arranged on the outside of the heat exchanger arrangement. Advantageously, the separator duct may prevent debris and large particles that may be ingested into the air intake arrangement, from impacting on the heat exchanger arrangement. 
     The engine may further comprise one or more support structures arranged inside (i.e. proximate the inner diameter of), and/or outside (i.e. proximate the outer diameter of) at least one of the plurality of heat exchanger modules. Advantageously, the one or more support structures may help maintain the structural integrity of said at least one of the plurality of heat exchanger modules. 
     The engine may further comprise a bypass arrangement configured to prevent the air received by the air intake arrangement from flowing through at least one of the plurality of heat exchanger modules. Advantageously, this may provide for improved optimisation of the engine. Pressure losses through the heat exchanger arrangement and temperature drop requirements in the engine may be balanced. Additionally, pressure losses in the engine may be minimised when air cooling is not important (e.g. at low operating flight speeds of the engine). 
     Optionally, the at least one or more turbomachinery components comprises a compressor, and the engine further comprises an air-breathing combustion chamber configured to receive compressed air from the compressor and for the combustion of air and fuel; 
     wherein the heat exchanger is configured to be selectively operated in a first operating mode which corresponds with a first operating flight speed of the engine, and a second operating mode which corresponds with a second operating flight speed of the engine that is lower than the first operating flight speed of the engine; 
     wherein in the first operating mode of the heat exchanger arrangement, the heat transfer medium is configured to heat the fuel before it is delivered to the air-breathing combustion chamber; and 
     wherein in the second operating mode of the heat exchanger arrangement, the heat transfer medium is configured to cool the fuel before it is delivered to the air-breathing combustion chamber. 
     Advantageously, such an engine may provide for improved management of the temperature of a fuel. In the first operating mode of the heat exchanger arrangement, which may correspond with a high operating flight speed of the engine, for example, a short duration hypersonic “dash” mode or cruise at high Mach, a portion of the heat transfer medium may be used to heat the fuel. The first operating mode of the heat exchanger arrangement is particularly advantageous because it may provide that during high speed flight, for example during a short duration hypersonic “dash” mode, during acceleration to high speed, heat removed into the heat transfer medium by cooling the incoming air may be dissipated (i.e. dumped) in the fuel. The fuel may represent a heat reservoir facilitating further cooling of the air. In other words, the fuel may provide a good medium for dumping heat out from the heat transfer medium, and dumping heat from the heated heat transfer medium into the fuel may thus help cool the air. 
     In the second operating mode of the heat exchanger arrangement, which may correspond with a low operating flight speed of the engine, for example, a long duration supersonic or subsonic cruise, the heat transfer medium may be used to cool the fuel. The second operating mode of the heat exchanger arrangement is thus particularly advantageous because it may provide that during low flight speed, for example during a long duration supersonic or subsonic cruise, heat dumped into the fuel from the heat transfer medium during the first operating mode of the heat exchanger arrangement may be transferred back into the heat transfer medium. Advantageously, this may allow for the operation of the heat exchanger arrangement in the first operating mode to be potentially repeated. In other words, without the second operating mode of the heat exchanger arrangement, it might not otherwise be possible to keep on using the first operating mode of the heat exchanger arrangement, because the fuel may ultimately reach a temperature limit. 
     By using both the first and second operating modes of the heat exchanger to run the cycle of the engine forwards and in reverse respectively, the heat transfer medium may effectively be used as both a heat source and a heat sink for the fuel. Specifically, the temperature of the fuel may need to be managed in order to effectively use the fuel as a heat sink for helping the heat transfer medium to more effectively cool the air. In the first operating mode of the heat exchanger arrangement, the fuel may provide a good medium for heat from the heat transfer medium to be dumped into, to help further cool the air, and in the second operating mode of the heat exchanger arrangement, the fuel may be cooled down so that the first operating mode may be repeated/used again. 
     In the second operating mode of the heat exchanger arrangement, the heat exchanger arrangement may be configured to operate in a reverse direction to the first operating mode of the heat exchanger arrangement. 
     The engine may further comprise a fuel tank for the storage of the fuel. 
     The engine may further comprise a first flow path for the flow of the heat transfer medium and extending between and in fluid communication with the outlet of the heat exchanger arrangement and a point in the engine downstream of the one or more turbomachinery components, such that in the first operating mode of the heat exchanger arrangement, a portion of the heat transfer medium may be used for re-heating and/or a Ramjet mode of operating the engine. 
     The engine may further comprise a second flow path for the flow of the heat transfer medium and extending between and in fluid communication with the outlet of the heat exchanger arrangement and an inlet of the fuel tank, such that in the first and second operating modes of the heat exchanger arrangement, a portion of the heat transfer medium may be placed in heat exchange with the fuel in the fuel tank. 
     The engine may further comprise a third flow path for the flow of the heat transfer medium and extending between and in fluid communication with an outlet of the fuel tank and the inlet of the heat exchanger arrangement, such that in the first and second operating modes of the heat exchanger arrangement, the portion of the heat transfer medium that is placed in heat exchange with the fuel in the fuel tank may then be configured to re-enter the heat exchanger arrangement. 
     Advantageously, in the first operating mode of the heat exchanger arrangement, hot heat transfer medium which exits the heat exchanger arrangement after it has been used to cool the air flowing through the heat exchanger arrangement may then be used to heat the fuel in the fuel tank. Cold heat transfer medium (which has been cooled as a result of heating the fuel) exiting the fuel tank may then be fed back into the heat transfer arrangement where it may be used to cool the air flowing through the heat exchanger arrangement. In addition, advantageously, in the second operating mode of the heat exchanger arrangement, heat transfer medium which exits the heat exchanger arrangement after it has been used to warm the air flowing through the heat exchanger arrangement may be used to cool the fuel in the fuel tank. Hot heat transfer medium (which has been heated as a result of cooling the fuel) exiting the fuel tank may then be fed back into the heat exchanger arrangement where it may be used to warm the air flowing through the heat exchanger arrangement. 
     According to a second aspect of the disclosure, there is provided an engine, comprising: an air intake arrangement configured to receive air; 
     a heat exchanger arrangement arranged downstream of the air intake arrangement and configured to cool the air; and 
     one or more turbomachinery components configured to receive cooled air from the heat exchanger arrangement; 
     wherein the heat exchange arrangement is at least partially substantially conical in shape. 
     Advantageously, such an engine may provide a slimmer engine and may be particularly suitable for an installation where diameter is at a premium. The length of the heat exchanger arrangement may be chosen so as to optimise the length and diameter of the engine. The conical shape of the heat exchanger arrangement may also provide for protection from debris and objects, thus aiding the compactness and effectiveness of the heat exchanger arrangement in the engine. 
     Additionally, such an engine may provide the ability to reduce the temperature of air entering the engine. Additionally, the transitions between different modes of operation of the engine may be better managed. In particular, the transition points for a given engine mode may be at a higher Mach. As such, different modes of operating the engine may be optimised, and there may be greater flexibility of operation through the transition speeds between different engine modes. The engine modes may even be removed completely. In addition, such an engine may provide for improved hot day performance (e.g. performance when the environmental conditions are at a relatively hot temperature compared with a typical environmental temperature) of the one or more turbomachinery components. 
     Furthermore, precooling the air entering the engine reduces the temperature operating range over which the one or more turbomachinery components must operate. This may advantageously provide, for example, for a decrease in the extent to which the tip clearances of a compression system of the one or more turbomachinery components must be managed over the temperature operating range of the one or more turbomachinery components. This may provide for improved tip clearance losses in the one of more turbomachinery components and corresponding engine handling improvements, thus resulting in an increase in overall mission efficiency (fuel burn). 
     Accordingly, such an engine may provide for the careful management of the transitions between various modes of operation of the engine, for an increase in the maximum flight Mach number of the operation of the one or more turbomachinery components, and for a reduction in the impact of transitions between different engine modes on a mission profile, thus increasing the maximum operational flight speed of the engine. Additionally, such an engine may allow for a Ramjet system to be included in the engine that has a reduced cross-sectional area, which may provide for better airframe integration. 
     The heat exchanger arrangement may comprise an inlet and an outlet. The inlet and the outlet of the heat exchanger may be spaced apart from one another relative to a radial direction of the engine that is substantially parallel to a longitudinal axis of the engine. 
     The heat exchanger arrangement may comprise a plurality of heat exchanger modules that are each at least partially substantially conical in shape. 
     Each of the plurality of heat exchanger modules may comprise a respective inlet and a respective outlet. The plurality of heat exchanger modules may be in fluid communication with one another. The plurality of heat exchanger modules may be arranged in series and/or parallel relative to a longitudinal axis of the engine. Advantageously, this may allow for a shorter engine installation. 
     The plurality of heat exchanger modules may be arranged in series or in parallel relative to a longitudinal axis of the engine. The longitudinal axis of the engine may be at least partially curved, e.g. there may be some degree of convolution in the air intake arrangement. 
     Each of the plurality of heat exchanger modules may comprise a plurality of tubes for the flow of a heat transfer medium in heat exchange with the air. 
     Optionally, the plurality of tubes in at least one of the plurality of heat exchanger modules are spaced apart and positioned relative to one another such that said one of the plurality of heat exchanger modules is configured to provide an angle to a bulk flow of the air flowing through the heat exchanger arrangement. 
     The engine may further comprise a bypass arrangement configured to prevent the air received by the air intake arrangement from flowing through at least one of the plurality of heat exchanger modules. 
     Optionally, the at least one or more turbomachinery components comprises a compressor, and the engine further comprises an air-breathing combustion chamber configured to receive compressed air from the compressor and for the combustion of air and fuel; 
     wherein the heat exchanger is configured to be selectively operated in a first operating mode which corresponds with a first operating flight speed of the engine, and a second operating mode which corresponds with a second operating flight speed of the engine that is lower than the first operating flight speed of the engine; 
     wherein in the first operating mode of the heat exchanger arrangement, the heat transfer medium is configured to heat the fuel before it is delivered to the air-breathing combustion chamber; and 
     wherein in the second operating mode of the heat exchanger arrangement, the heat transfer medium is configured to cool the fuel before it is delivered to the air-breathing combustion chamber. 
     Advantageously, such an engine may provide for improved management of the temperature of a fuel. In the first operating mode of the heat exchanger arrangement, which may correspond with a high operating flight speed of the engine, for example, a short duration hypersonic “dash” mode or cruise at high Mach, a portion of the heat transfer medium may be used to heat the fuel. The first operating mode of the heat exchanger arrangement is particularly advantageous because it may provide that during high speed flight, for example during a short duration hypersonic “dash” mode, during acceleration to high speed, heat removed into the heat transfer medium by cooling the incoming air may be dissipated (i.e. dumped) in the fuel. The fuel may represent a heat reservoir facilitating further cooling of the air. In other words, the fuel may provide a good medium for dumping heat out from the heat transfer medium, and dumping heat from the heated heat transfer medium into the fuel thus may help cool the air. 
     In the second operating mode of the heat exchanger arrangement, which may correspond with a low operating flight speed of the engine, for example, a long duration supersonic or subsonic cruise, the heat transfer medium may be used to cool the fuel. The second operating mode of the heat exchanger arrangement is thus particularly advantageous because it may provide that during low flight speed, for example during a long duration supersonic or subsonic cruise, heat dumped into the fuel from the heat transfer medium during the first operating mode of the heat exchanger arrangement may be transferred back into the heat transfer medium. Advantageously, this may allow for the operation of the heat exchanger arrangement in the first operating mode to be potentially repeated. In other words, without the second operating mode of the heat exchanger arrangement, it might not otherwise be possible to keep on using the first operating mode of the heat exchanger arrangement, because the fuel may ultimately reach a temperature limit. 
     By using both the first and second operating modes of the heat exchanger to run the cycle of the engine forwards and in reverse respectively, the heat transfer medium may effectively be used as both a heat source and a heat sink for the fuel. Specifically, the temperature of the fuel may need to be managed in order to effectively use the fuel as a heat sink for helping the heat transfer medium to more effectively cool the air. In the first operating mode of the heat exchanger arrangement, the fuel may provide a good medium for heat from the heat transfer medium to be dumped into, to help further cool the air, and in the second operating mode of the heat exchanger arrangement, the fuel may be cooled down so that the first operating mode may be repeated/used again. 
     In the second operating mode of the heat exchanger arrangement, the heat exchanger arrangement may be configured to operate in a reverse direction to in the first operating mode of the heat exchanger arrangement. 
     The engine may further comprise a fuel tank for the storage of the fuel. 
     The engine may further comprise a first flow path for the flow of the heat transfer medium and extending between and in fluid communication with the outlet of the heat exchanger arrangement and a point in the engine downstream of the one or more turbomachinery components, such that in the first operating mode of the heat exchanger arrangement, a portion of the heat transfer medium may be used for re-heating and/or a Ramjet mode of operating the engine. 
     The engine may further comprise a second flow path for the flow of the heat transfer medium and extending between and in fluid communication with the outlet of the heat exchanger arrangement and an inlet of the fuel tank, such that in the first and second operating modes of the heat exchanger arrangement, a portion of the heat transfer medium may be placed in heat exchange with the fuel in the fuel tank. 
     The engine may further comprise a third flow path for the flow of the heat transfer medium and extending between and in fluid communication with an outlet of the fuel tank and the inlet of the heat exchanger arrangement, such that in the first and second operating modes of the heat exchanger arrangement, the portion of the heat transfer medium that is placed in heat exchange with the fuel in the fuel tank may then be configured to re-enter the heat exchanger arrangement. 
     Advantageously, in the first operating mode of the heat exchanger arrangement, hot heat transfer medium which exits the heat exchanger arrangement after it has been used to cool the air flowing through the heat exchanger arrangement may then be used to heat the fuel in the fuel tank. Cold heat transfer medium (which has been cooled as a result of heating the fuel) exiting the fuel tank may then be fed back into the heat transfer arrangement where it may be used to cool the air flowing through the heat exchanger arrangement. In addition, advantageously, in the second operating mode of the heat exchanger arrangement, heat transfer medium which exits the heat exchanger arrangement after it has been used to warm the air flowing through the heat exchanger arrangement may be used to cool the fuel in the fuel tank. Hot heat transfer medium (which has been heated as a result of cooling the fuel) exiting the fuel tank may then be fed back into the heat exchanger arrangement where it may be used to warm the air flowing through the heat exchanger arrangement. 
     It is to be understood that the engine according to the second aspect of the disclosure may further comprise any one or more of the optional features recited above in relation to the engine according to the first aspect of the disclosure. These are not recited herein in relation to the engine according to the second aspect of the disclosure, purely for the sake of conciseness. 
     According to a third aspect of the disclosure, there is provided an engine, comprising: an air intake arrangement configured to receive air; 
     a fuel tank configured to store a fuel for being supplied to an air-breathing combustion chamber; 
     a heat exchanger arrangement arranged downstream of the air intake arrangement and configured to cool the air by providing for heat exchange between the air and a heat transfer medium; and 
     a compressor arranged downstream of the heat exchanger arrangement and configured to supply compressed air to the air-breathing combustion chamber for the combustion of the air and the fuel; 
     wherein the heat exchanger arrangement is configured to be selectively operated in a first operating mode which corresponds with a first operating flight speed of the engine, and a second operating mode which corresponds with a second operating flight speed of the engine that is lower than the first operating flight speed of the engine; 
     wherein in the first operating mode of the heat exchanger arrangement, the heat transfer medium is configured to heat the fuel before the fuel is delivered to the air-breathing combustion chamber; and 
     wherein in the second operating mode of the heat exchanger arrangement, the heat transfer medium is configured to cool the fuel before the fuel is delivered to the air-breathing combustion chamber. 
     Advantageously, such an engine may provide for improved management of the temperature of a fuel. In the first operating mode of the heat exchanger arrangement, which may correspond with a high operating flight speed of the engine, for example, a short duration hypersonic “dash” mode or cruise at high Mach, a portion of the heat transfer medium may be used to heat the fuel. The first operating mode of the heat exchanger arrangement is particularly advantageous because it may provide that during high speed flight, for example during a short duration hypersonic “dash” mode, during acceleration to high speed, heat removed into the heat transfer medium by cooling the incoming air may be dissipated (i.e. dumped) in the fuel. The fuel may represent a heat reservoir facilitating further cooling of the air. In other words, the fuel may provide a good medium for dumping heat out from the heat transfer medium, and dumping heat from the heated heat transfer medium into the fuel thus may help cool the air. 
     In the second operating mode of the heat exchanger arrangement, which may correspond with a low operating flight speed of the engine, for example, a long duration supersonic or subsonic cruise, the heat transfer medium may be used to cool the fuel. The second operating mode of the heat exchanger arrangement is thus particularly advantageous because it may provide that during low speed operation of the engine, for example during a long duration supersonic or subsonic cruise, heat dumped into the fuel from the heat transfer medium during the first operating mode of the heat exchanger arrangement may be transferred back into the heat transfer medium. Advantageously, this may allow for the operation of the heat exchanger arrangement in the first operating mode to be potentially repeated. In other words, without the second operating mode of the heat exchanger arrangement, it might not otherwise be possible to keep on using the first operating mode of the heat exchanger arrangement, because the fuel may ultimately reach a temperature limit. 
     By using both the first and second operating modes of the heat exchanger to run the cycle of the engine forwards and in reverse respectively, the heat transfer medium may effectively be used as both a heat source and a heat sink for the fuel. Specifically, the temperature of the fuel needs to be managed in order to effectively use the fuel as a heat sink for helping the heat transfer medium to more effectively cool the air. In the first operating mode of the heat exchanger arrangement, the fuel may provide a good medium for heat from the heat transfer medium to be dumped into, to help further cool the air, and in the second operating mode of the heat exchanger arrangement, the fuel may be cooled down so that the first operating mode may be repeated/used again. 
     In the second operating mode of the heat exchanger arrangement, the heat exchanger arrangement may be configured to operate in a reverse direction to in the first operating mode of the heat exchanger arrangement. 
     The engine may further comprise a first flow path for the flow of the heat transfer medium and extending between and in fluid communication with the outlet of the heat exchanger arrangement and a point in the engine downstream of the one or more turbomachinery components, such that in the first operating mode of the heat exchanger arrangement, a portion of the heat transfer medium may be used for re-heating and/or a Ramjet mode of operating the engine. 
     The engine may further comprise a second flow path for the flow of the heat transfer medium and extending between and in fluid communication with the outlet of the heat exchanger arrangement and an inlet of the fuel tank, such that in the first and second operating modes of the heat exchanger arrangement, a portion of the heat transfer medium may be placed in heat exchange with the fuel in the fuel tank. 
     The engine may further comprise a third flow path for the flow of the heat transfer medium and extending between and in fluid communication with an outlet of the fuel tank and the inlet of the heat exchanger arrangement, such that in the first and second operating modes of the heat exchanger arrangement, the portion of the heat transfer medium that is placed in heat exchange with the fuel in the fuel tank may then be configured to re-enter the heat exchanger arrangement. 
     Advantageously, in the first operating mode of the heat exchanger arrangement, hot heat transfer medium which exits the heat exchanger arrangement after it has been used to cool the air flowing through the heat exchanger arrangement may then be used to heat the fuel in the fuel tank. Cold heat transfer medium (which has been cooled as a result of heating the fuel) exiting the fuel tank may then be fed back into the heat transfer arrangement where it may be used to cool the air flowing through the heat exchanger arrangement. In addition, advantageously, in the second operating mode of the heat exchanger arrangement, heat transfer medium which exits the heat exchanger arrangement after it has been used to warm the air flowing through the heat exchanger arrangement may be used to cool the fuel in the fuel tank. Hot heat transfer medium (which has been heated as a result of cooling the fuel) exiting the fuel tank may then be fed back into the heat exchanger arrangement where it may be used to warm the air flowing through the heat exchanger arrangement. 
     The heat exchanger arrangement may comprise a plurality of heat exchanger modules. 
     It is to be understood that the engine according to the third aspect of the disclosure may further comprise any one or more of the optional features recited above in relation to the engines according to the first and second aspects of the disclosure. These are not recited herein in relation to the engine according to the third aspect of the disclosure, purely for the sake of conciseness. 
     According to a fourth aspect of the disclosure, there is provided a method of operating an engine according to the third aspect of the disclosure, the method comprising: 
     suppling air to the air intake arrangement; 
     causing the air to flow through the air intake arrangement, the heat exchanger arrangement, the compressor and the air-breathing combustion chamber; 
     supplying heat transfer medium to the heat exchanger arrangement; 
     causing the heat transfer medium to flow through the heat exchanger arrangement; 
     supplying fuel to the fuel tank; and 
     causing the fuel to flow from the fuel tank to the air-breathing combustion chamber; 
     wherein the method further comprises operating the heat exchanger arrangement in the first operating mode when the engine is operating at the first operating flight speed, and operating the heat exchanger arrangement in the second operating mode when the engine is operating at the second operating flight speed. 
     According to a fifth aspect of the disclosure, there is provided an aircraft, flying machine or aerospace vehicle comprising an engine according to any one of the first, second or third aspects of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be carried out in various ways and embodiments of the disclosure will now be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  (prior art) shows a cross-sectional schematic view of a precooled engine; 
         FIG. 2  shows a cross-sectional schematic view of a precooled engine having a single cylindrical heat exchanger module; 
         FIG. 3A  shows a cross-sectional schematic view of coolant tubes of a heat exchanger in an in-line arrangement; 
         FIG. 3B  shows a cross-sectional schematic view of coolant tubes of a heat exchanger in a symmetrically staggered arrangement; 
         FIG. 3C  shows a cross-sectional schematic view of coolant tubes of a heat exchanger in a staggered arrangement; 
         FIG. 4  shows another cross-sectional schematic view of the precooled engine of  FIG. 2 ; 
         FIG. 5  shows a cross-sectional schematic view of a precooled engine having three cylindrical modular heat exchanger modules arranged in series; 
         FIG. 6  shows a cross-sectional schematic view of a precooled engine having three cylindrical radially spaced modular heat exchanger modules arranged in series; 
         FIG. 7  shows a cross-sectional schematic view of a precooled engine having three cylindrical radially spaced and longitudinally overlapping modular heat exchangers arranged in series; 
         FIG. 8  shows a cross-sectional schematic view of a precooled engine having a single conical heat exchanger module; 
         FIG. 9  shows a cross-sectional schematic view of a precooled engine having a single conical heat exchanger module; 
         FIG. 10  shows a cross-sectional schematic view of a precooled engine having two conical modular heat exchanger modules arranged in series; 
         FIG. 11  shows a cross-sectional schematic view of a precooled engine having two conical modular heat exchanger modules arranged in series; 
         FIGS. 12A to 12C  show cross-sectional schematic views of precooled engines each having three conical radially spaced modular heat exchangers arranged in series orientated along the curved centreline of the engine air intake; 
         FIG. 12D  shows a cross-sectional schematic view of a precooled engine having three cylindrical radially spaced modular heat exchangers arranged in series orientated along the curved centreline of the engine air intake; 
         FIG. 13  shows a schematic diagram of an engine cycle operating at a high flight speed wherein the heat transfer medium heats the fuel; 
         FIG. 14  shows a schematic diagram of an engine cycle operating at a low flight speed wherein the heat transfer medium cools the fuel; 
         FIG. 15A  shows a schematic diagram of an engine cycle operating at a high flight speed wherein the heat transfer medium is a fuel and the fuel is heated through heat transfer with the air; 
         FIG. 15B  shows a schematic diagram of an engine cycle operating at a low flight speed wherein the heat transfer medium is a fuel and the fuel is cooled through heat transfer with the air; 
         FIG. 16A  shows a schematic diagram of an engine cycle operating at a high flight speed wherein the heat transfer medium heats the fuel; and 
         FIG. 16B  shows a schematic diagram of an engine cycle operating at a low flight speed wherein the heat transfer medium cools the fuel. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  (prior art) shows a cross-sectional schematic view of a precooled turbomachinery based engine  1  having a supersonic/hypersonic air intake arrangement  2 . Arranged downstream of the air intake arrangement  2  is a cylindrical heat exchanger  3  configured to cool air received by the air intake arrangement  2 . Air cooled by the heat exchanger  3  is then configured to flow to turbomachinery components  4 , before flowing downstream to an augmentation system  5  (reheat/afterburner), a variable area nozzle  6  and then an exhaust  7 . The heat exchanger  3  is positioned in close proximity to the turbomachinery components  4  such that the two may be considered to be close-coupled, i.e. the aerodynamic flow field from the heat exchanger  3  influences the turbomachinery components  4 , and the turbomachinery components  4  can have an upstream aerodynamic influence on the heat exchanger  3 . For high Mach (e.g. above Mach 2.5) operation, the engine  1  also includes a Ramjet, supplied by air that bypasses the turbomachinery components  4 , and that can also optionally bypass the heat exchanger  3 . At high Mach (e.g. above Mach 2.5), the Ramjet will be the main thrust source. The Ramjet can be run jointly with the turbomachinery components  4  during mode transition. The heat exchanger  3  can be used when the turbomachinery components  4  reach a material temperature limit, typically in a compression system  4   a , as flight speed of the engine  1  increases. This would be in order to raise the maximum operational flight speed of the engine  1  at which the turbomachinery components  4  could be used. 
     Alternative heat exchanger arrangements which may be used in place of the heat exchanger  3  shall now be discussed, with reference to the cross-sectional schematic views of  FIGS. 2 to 12D . The exemplary engines described in relation to  FIGS. 2 to 12D  shall be discussed with a focus on their respective heat exchanger arrangements. It is to be understood that the engines described below may additionally include any one or more of the features of the engine  1  shown in  FIG. 1 , and may also include one or more optional additional and/or alternative features. 
       FIG. 2  shows a cross-sectional schematic view of an exemplary alternative precooled turbomachinery based engine  101 , for example an air-breathing jet engine, having an air intake arrangement  102 . Arranged downstream of the air intake arrangement  102  is a heat exchanger arrangement  103  configured to cool air received by the air intake arrangement  102 . Air cooled by the heat exchanger arrangement  103  is then configured to flow to turbomachinery components  104  (e.g. a compressor and a turbine, as in a typical jet engine). The turbomachinery based engine  101  may also include an augmentation system (reheat/afterburner), a variable area nozzle, an exhaust, and/or a Ramjet, which are similar to those as described above in relation to  FIG. 1 , but which are not shown in  FIG. 2 . 
     For example, the engine  101  may further comprise a Ramjet (not shown), supplied by air that bypasses the turbomachinery components  104 , and that can also optionally bypass the heat exchanger arrangement  103 . At high Mach (e.g. above Mach 2.5) the Ramjet will be the main thrust source. The Ramjet can be run jointly with the turbomachinery components  104  during mode transition. The heat exchanger  103  can be used when the turbomachinery components  104  reach a material temperature limit, typically in a compression system (not shown) of the turbomachinery components  104 , as the flight speed of the engine  101  increases. This would be in order to raise the maximum operational flight speed of the engine  101  at which the turbomachinery components  104  could be used. 
     In the example shown in  FIG. 2 , the heat exchanger arrangement  103  comprises a first heat exchanger module  103   a  that is centred about a longitudinal axis  113  of the engine  101  ( FIG. 2  is a cross-sectional schematic view showing only half of the engine  101 ). In the example, the first heat exchanger module  103   a  is substantially cylindrical in shape, though it is also envisaged that the first heat exchanger module may alternatively be substantially conical (see  FIG. 8 ), or any other suitable shape. A heat transfer medium (not shown) in heat exchange with the air is configured to flow within the heat exchanger module  103   a , in through an inlet  105  of the heat exchanger arrangement  103  and out through an outlet  106  of the heat exchanger arrangement  103 . The inlet  105  is supplied with the heat transfer medium by an inlet manifold  107 , and the outlet  106  is in fluid communication with an outlet manifold  108 . In the example shown, the inlet and outlet manifolds  107 ,  108  are both cylindrical, but it is also envisaged that the inlet and outlet manifolds  107  and  108  may be any other shape, for example they may be longitudinal manifolds on either side of the heat exchanger arrangement  103 . The heat transfer medium can be, for example, any fuel type, any gas, any liquid, or any cryogenic fluid, such as helium. 
     The first heat exchanger module  103   a  has an inner diameter  115  and an outer diameter  116 . The first heat exchanger module  103   a  is sized and positioned such that the inner diameter  115  is approximately in line with an outer diameter  117  of an engine fan case  118  of the turbomachinery components  104 . Advantageously, this may minimise blockage to the turbomachinery components  104 . The precise inner and outer diameters  115 ,  116  of the heat exchanger module  103   a  may be chosen according to the aerodynamic considerations of a particular application. 
     The air is configured to flow into and through the heat exchanger arrangement  103  substantially along a radial direction  114  of the engine  101 , that is perpendicular to the longitudinal axis  113  of the engine  101 . In particular, outer ducting  109  and an inner centre structure  110  of the engine  101  are configured to direct the air to flow from the air intake arrangement  102  into the heat exchanger arrangement  103 . The outer ducting  109  forms a seal with a case  119  of the engine  101 . The inner centre structure  110  comprises one or more movable sections  111 . The one or more movable sections  111  are configured to be selectively moved to allow for the direct flow of air through from the air intake arrangement  102  to the turbomachinery components  104 , thus bypassing the heat exchanger arrangement  103 . For example, the one or more movable sections  111  may be moved to cause the air to bypass the heat exchanger arrangement  103  when the engine  101  is operating at low Mach or high corrected flows. The one or more movable sections  111  may be configured to move by rotation, translation, a hinged petal arrangement, and/or any other suitable mechanical means. The engine  101  also includes a plurality of turning vanes (not shown) configured to direct the flow of air into and out of the heat exchanger arrangement  103 . The engine  101  also includes a separator duct (not shown), arranged on the outside of the heat exchanger arrangement  103  (i.e. proximate the outer diameter  116  of the heat exchanger module  103   a ) to prevent debris and large particles that may be ingested into the intake, from impacting on the heat exchanger arrangement  103 . It is envisaged that the separator duct could be sized to be large enough to allow a complete engine bypass. 
     With reference to the cross-sectional schematic views of  FIGS. 3A to 3C , the first heat exchanger module  103   a  comprises a plurality of small diameter thin-walled tubes  120  through which the heat transfer medium is configured to flow. The tubes  120  are shown in cross-section in  FIGS. 3A to 3C . The air is configured to flow around the plurality of tubes as it flows through the first heat exchanger module  103   a , as depicted by the arrows in  FIGS. 3A to 3C . 
     In an example, the tubes  120  are arranged in involute spiral paths, which extend circumferentially about the longitudinal  113  of the engine, i.e. in a plane that is perpendicular to the page in  FIG. 2 . In particular, the tubes  120  are each wound in a respective path that gradually widens or tightens about the longitudinal axis  113  of the engine  101  and are each spaced apart from one another in rows along the longitudinal axis  113  of the engine  101 . The tubes  120  are connected at a first end thereof to an inlet header (not shown) and at a second end thereof to an outlet header (not shown). The inlet header and the outlet header each extends substantially parallel to the longitudinal axis  113  of the engine  101 . The inlet header is arranged to be in fluid communication with the inlet  105  and the outlet header is arranged to be in fluid communication with the outlet  106 . Though, it is also envisaged that the tubes  120  may be arranged and/or positioned in any other path and/or orientation. 
     The tubes  120  are arranged relative to one another such that air flows around the tubes  120  in a perpendicular, or parallel, or combined direction. Physically, the tubes in the first heat exchanger module  103   a  can be arranged to be straight, simply curved at a fixed radius, curved with a varying radius, and/or an involute spiral. In such arrangements, the air may flow over multiple tubes. In the airflow direction for these arrangements, the tubes  120  can be arranged in-line (see  FIG. 3A ), symmetrical staggered (see  FIG. 3B ), or staggered (see  FIG. 3C ), in such a way as to provide an angle to the bulk flow of the air. In these and other examples, the spacing and the positioning of the tubes  120  relative to one another can be chosen to use asymmetry to avoid co-operative forcing and resonance due to aero-elastic phenomena between the heat exchanger arrangement  103  and the closely-coupled turbomachinery components  104 . This can be achieved by randomised tube spacing (e.g. allowing large tolerances in the spacing of the tubes  120 ), or by systematic and selective variations in the spacing of the tubes  120 . In particular, the precise arrangement (e.g. the relative positioning and/or spacing) of the tubes  120  relative to one another may be chosen according to a particular application such that: the air flow exits the heat exchanger arrangement  103  in a condition that is acceptable to the downstream turbomachinery components  104  without either the heat exchanger  103  or the turbomachinery components  104  producing detrimental effects on each other; and such that allowance is made for thermal expansion by any unwinding of tubes  120  sliding relative to each other, which may occur if the tubes  120  are arranged in involute spiral paths, for example, like those described above and/or in WO2015052469. 
     In the example shown in  FIG. 2 , a squirrel-cage/drum-like structure  112  is arranged inside the first heat exchanger module  103   a  (i.e. proximate the inner diameter  115  of the heat exchanger module  103   a ) to maintain structural integrity of the first heat exchanger module  103   a . Though, it is also envisaged that one or more squirrel-cage/drum-like structures may alternatively be arranged outside, or both inside and outside the heat exchanger module  103   a.    
     Precooling the air using the heat exchanger arrangement  103  before it enters the turbomachinery components  104  is advantageous because it raises the maximum flight Mach number of the operation of the turbomachinery components  104  and thus reduces the impact of transition between engine modes on a mission profile. Additionally, it reduces the cross-sectional area of a Ramjet system within the engine  101 , which provides for better airframe integration. It also provides for improved hot day performance (e.g. performance when the environmental conditions are at a relatively hot temperature compared with a typical environmental temperature) of the turbomachinery components  104 . 
     To assist with the following discussion,  FIG. 4  shows a simplified cross-sectional schematic view of the engine  101  of  FIG. 2 . The inlet and outlet manifolds  107 ,  108  and the one or more movable sections  111  are not shown in  FIG. 4 . This is because the focus of the following discussion, in relation to  FIGS. 5 to 12D , shall be alternative structural configurations of the heat exchanger arrangement  103 . 
     With reference to the cross-sectional schematic views of  FIGS. 5 to 12D , alternative structural configurations of the heat exchanger arrangement  103  shall now be described. In  FIGS. 5 to 12D , like reference numerals are used to denote alike elements, for example, the longitudinal axis  113  and the air intake arrangement  102 . Similar reference numerals increasing in value are used to denote similar elements, for example, heat exchanger arrangements  103 ,  203 ,  403  etc. It is to be understood that other than their internal structural configurations and shapes, the alternative exemplary heat exchanger arrangements  203 ,  403 ,  503  etc. shall be considered to be similar to the heat exchanger arrangement  103 . For example, the heat exchanger arrangements  203 ,  403 ,  503  etc. shall be considered to be functionally and positionally related to the other components such as the air intake arrangement  102  and the turbomachinery components  104  in the same way as the heat exchanger arrangement  103 . 
     In the example shown in  FIG. 5 , the heat exchanger arrangement  203  comprises a first heat exchanger module  203   a , a second heat exchanger module  203   b  and a third heat exchanger module  203   c , that are each substantially identical in structure to the first heat exchanger module  103   a . Like the first heat exchanger module  103   a  in the example of  FIG. 2 , each of the first, second and third heat exchanger modules  203   a - c  comprises a plurality of small diameter thin-walled tubes running from a respective inlet  203   ai ,  203   bi ,  203   ci  to a respective outlet  203   ao ,  203   bo ,  203   co , through which the heat transfer medium is configured to flow. Although there are three heat exchanger modules shown in the example of  FIG. 5 , it is also envisaged that the heat exchanger arrangement  203  may alternatively comprise any number of a plurality of heat exchanger modules. 
     The first, second and third heat exchanger modules  203   a - c  are arranged to be in fluid communication with one another and are arranged in series relative to the longitudinal axis  113  of the engine  101 . In other words, the second heat exchanger module  203   b  is arranged downstream of the first heat exchanger module  203   a , and the third heat exchanger module  203   c  is arranged downstream of the second heat exchanger module  203   b . Though, it is also envisaged that one or more of the first, second and third heat exchanger modules  203   a - c  may alternatively be arranged in parallel relative to the longitudinal axis  113  of the engine  101 . 
     The heat transfer medium in heat exchange with the air is configured to flow within the heat exchanger arrangement  203  through all of the first, second and third heat exchanger modules  203   a - c  by entering each module in through its respective inlet  203   ai ,  203   bi ,  203   ci , and then exiting each module out through its respective outlet  203   ao ,  203   bo ,  203   co.    
     Each of the inlets  203   ai ,  203   bi ,  203   ci  and the outlets  203   ao ,  203   bo  and  203   co  of the first, second and third heat exchanger modules  203   a - c  is in fluid communication with a respective inlet or outlet manifold (not shown). This is akin to how in the example shown in  FIG. 2 , the inlet  105  is supplied with the heat transfer medium by an inlet manifold  107  and the outlet  106  is in fluid communication with an outlet manifold  108 . 
     The outlet  203   ao  and the inlet  203   bi , and the outlet  203   bo  and the inlet  203   ci  are shown to be overlapping (i.e. generally coincident along the longitudinal axis  113  of the engine  101 ) in the example of  FIG. 5 . Though it is also envisaged that the outlet  203   ao  and the inlet  203   bi , and/or the outlet  203   bo  and the inlet  203   ci  may also be spaced apart from one another along the longitudinal axis  113  of the engine  101  (for example as in the example shown in  FIG. 6 ). 
     Referring back to  FIG. 5 , the modular structure of the heat exchanger arrangement  203  provides that the engine  101  may be configured such that the flow of the first fluid in one or more of the heat exchanger modules  203   a - c  can be modulated or completely turned off separately (i.e. independently from the flow of the first fluid in the other heat exchanger modules). Advantageously, this can provide for improved optimisation, flexibility and control of the temperature profile of the air. 
     In another example shown in  FIG. 6 , the heat exchanger  303  comprises first, second and third heat exchanger modules  303   a - c  that are substantially identical in structure to the first heat exchanger module  103   a , and each having an inner diameter  315   a - c  respectively, and an outer diameter  316   a - c  respectively. The first, second and third heat exchanger modules  303   a - c  are sized such that they get progressively larger in diameter. In other words, the inner diameter  315   b  is larger than the inner diameter  315   a  but smaller than the inner diameter  315   c , and the outer diameter  316   b  is larger than the outer diameter  316   a  but smaller than the outer diameter  316   c . In this example, the first, second and third heat exchanger modules  303   a - c  may therefore be referred to as being spaced apart from one another relative to the radial direction  114  of the engine  101 . The structural configuration of the heat exchanger arrangement  303  may allow a proportion of the air to pass through each and be cooled by the heat transfer medium flowing through the first, second and third heat exchanger modules  303   a - c . The radially spaced modular structure of the heat exchanger arrangement  303  provides that the engine  101  may be configured such that the flow of the first fluid in one or more of the heat exchanger modules  303   a - c  can be modulated or completely turned off separately (i.e. independently from the flow of the first fluid in the other heat exchanger modules). This could be provided for by a plurality of movable sections  111  (as described above in relation to  FIG. 2 ), and/or by fluidic control, for example. Advantageously, this can provide for improved optimisation, flexibility and control of the temperature profile of the air, by balancing pressure losses and temperature drops through the heat exchanger modules  303   a - c.    
     In the example shown in  FIG. 6 , the first, second and third heat exchanger modules  303   a - c  each has a respective equal length  330   a - c . Though, it is also envisaged that one or more of the first, second and third heat exchanger modules  303   a - c  could have a different length to the other(s) of the heat exchanger modules  303   a - c . The lengths  330   a - c  of the heat exchanger modules  303   a - c  may be chosen to be different to one another to provide an additional means to control the air flow through each of the heat exchanger modules  303   a - c.    
     The spacings and positionings of the tubes  120  in each of the heat exchanger modules  103   a - c  can be chosen to be different to in the other heat exchanger modules  303   a - c . Although there are three heat exchanger modules shown in the example of  FIG. 6 , it is also envisaged that the heat exchanger arrangement  303  may alternatively comprise any number of a plurality of heat exchanger modules. 
     The example shown in  FIG. 7  differs from that shown in  FIG. 6  in that the first, second and third heat exchanger modules  403   a - c  of the heat exchanger arrangement  403  are arranged along the longitudinal axis  113  of the engine  101  such that their respective lengths  430   a - c  at least partially overlap by distances  431  and  432 . In particular, as shown in  FIG. 7 , the length  430   b  of the second heat exchanger module  403   b  is arranged to partially overlap with the length  430   a  of the first heat exchanger module  403   a  at one end thereof, and at the other end thereof to overlap with the length  430   c  of the third heat exchanger module  430   c . In this example, the first, second and third heat exchanger modules  403   a - c  may therefore be referred to as being partially overlapping with one another along the longitudinal axis  113  of the engine  101 , as well as being spaced apart from one another relative to the radial direction  114  of the engine  101 . This longitudinally overlapping arrangement advantageously provides for a reduced overall length of the heat exchanger arrangement  403  and hence of the engine  101 , and may be particularly suitable where the angle of air flow through the heat exchanger arrangement  403  has an axial element and is not desired to be purely radial. Although there are three heat exchanger modules shown in the example of  FIG. 7 , it is also envisaged that the heat exchanger arrangement  203  may alternatively comprise any number of a plurality of heat exchanger modules. In the example shown in  FIG. 7 , the length  430   b  of the second heat exchanger module  403   b  is arranged to overlap the length  403   a  of the first heat exchanger module  403   a  and the length  403   c  of the third heat exchanger module  403   c  each by approximately one third of the length  430   b  of the second heat exchanger module  403   b , relative to the longitudinal axis  113  of the engine  101 . Though, it is also envisaged that the first, second and third heat exchanger modules  403   a - c  of the heat exchanger arrangement  103  may be arranged such that their respective lengths  430   a - c  at least partially overlap relative to the longitudinal axis  113  of the engine  101  by any other amount, for example by approximately one sixth, one fifth, one quarter or one half of the length of one of said lengths  430   a - c.    
     An alternative heat exchanger arrangement  503  comprising a single heat exchanger module  503   a  (similar to the arrangements shown in  FIGS. 2 and 4 ) is shown in  FIG. 8 . In this example, the heat exchanger arrangement  503  is substantially conical in shape. In other words, the inlet  505  and the outlet  506  are spaced apart from one another along the radial direction  114  of the engine  101 . The conically shaped heat exchanger arrangement  503  can advantageously provide a slimmer installation and may be particularly suitable for an installation where diameter is at a premium. 
     A similar example is shown in  FIG. 9 , in which the heat exchanger arrangement  603  has a shorter length than that shown in the example of  FIG. 8 . The length of a conical heat exchanger arrangement  503 ,  603  may be chosen so as to optimise the length and diameter of the engine  101 . 
     The heat exchanger arrangement  703  shown in  FIG. 10  comprises a first heat exchanger module  703   a  and a second heat exchanger module  703   b , each having a respective inlet  703   ai ,  703   bi , and a respective outlet  703   ao ,  703   bo . Similarly to the arrangement shown in  FIG. 5 , the first and second heat exchanger modules  703   a  and  703   b  are arranged to be in fluid communication with one another and are arranged in series relative to the longitudinal axis  113  of the engine  101 . In other words, the second heat exchanger module  703   b  is arranged downstream of the first heat exchanger module  703   a . Though, it is also envisaged that the first and second heat exchanger modules  703   a  and  703   b  may alternatively be arranged in parallel. Advantageously, this may allow for a shorter installation. In the example, one of the first and second heat exchanger modules  703   a  and  703   b  has a different arrangement of the tubes  120  inside said module compared with the other of the first and second heat exchanger modules  703   a  and  703   b . Advantageously, this may provide that greater cooling is provided to one part of the air flow than the rest of the air flow. 
     In the example shown in  FIG. 10 , the first heat exchanger module  703   a  has a length  730   a  that is shorter than a length  730   b  of the second heat exchanger module  703   b . In the example, the length  730   a  is approximately half of the length  730   b , though it is also envisaged that the lengths  730   a  and  730   b  be also be any other relative lengths. It is also envisaged that alternatively the length  730   a  of the first heat exchanger module  703   a  may be longer than the length  730   b  of the second heat exchanger module  703   b , or the first and second heat exchanger modules  703   a ,  703   b  may be approximately equal in length, as in the example shown in  FIG. 11 , wherein the first heat exchanger module  803   a  has a length  830   a  that is equal to a length  830   b  of the second heat exchanger module  803   b . Although there are two heat exchanger modules shown in the examples of  FIGS. 10 and 11 , it is also envisaged that the heat exchanger arrangements  703  and  803  may alternatively comprise any number of a plurality of heat exchanger modules. 
     In the exemplary arrangements shown in  FIGS. 12A to 12C , the heat exchanger arrangement  903  comprises first, second and third heat exchanger modules  903   a - c  that are each substantially conical in shape and are arranged in series along the longitudinal axis  113  of the engine  101 . In the example shown in  FIG. 12D , the heat exchanger arrangement  1003  comprises first, second and third heat exchanger modules  1003   a - c  that are each substantially cylindrical in shape and are arranged in series along the longitudinal axis  113  of the engine  101 . 
     In the examples shown in  FIGS. 12A to 12D , the longitudinal axis  113  of the engine  101  is curved, i.e. there is some degree of convolution in the air intake arrangement  102 . It is desirable in many applications for the heat exchanger arrangement  903  to be closely integrated with the air intake arrangement  102 . The use of multiple modular heat exchanger modules such as the modules  903   a - c  or  1003   a - c  can facilitate this integration by careful arrangement of the axial positioning, orientation and size of the heat exchanger modules. Furthermore, multiple modular heat exchanger modules may be positioned and orientated such that there is complete protection from debris and particles that follow a mostly ballistic trajectory (rather than being carried along aerodynamic streamlines). 
     According to the above, and with reference to the exemplary heat exchanger arrangements  103 ,  203 ,  303 ,  403 ,  503 ,  603 ,  703 ,  803 ,  903  and  1003  as shown in  FIGS. 2 and 4 to 12D , it is to be understood that a heat exchanger arrangement according to the present disclosure may comprise any combination of the following features:
         (a) the heat exchanger arrangement may comprise any number of one or more individual heat exchanger modules;   (b) the one or more heat exchanger modules may be arranged either in series or in parallel relative to the longitudinal axis of the engine;   (c) the one or more heat exchanger modules may be either substantially cylindrical or conical in shape;   (d) the respective lengths of the one or more heat exchanger modules may be the same or different;   (e) the respective spacing and positioning of the tubes inside each of the one or more heat exchanger modules may be different in each of the heat exchanger modules;   (f) the lengths of one or more of the heat exchanger modules may be at least partially overlapping along the longitudinal axis of the engine and/or   (g) the longitudinal axis of the engine may be either straight or curved (i.e. convoluted).       

     Advantageously, any of the heat exchanger arrangements as described above may be used to help manage the temperature of a fuel tank.  FIGS. 13 and 14  show schematic cycle diagrams in which the heat exchanger arrangement  103  (though it is also envisaged that any of the alternative heat exchangers  203 ,  303 ,  403 , etc. described above could be employed instead of the heat exchanger arrangement  103 ) is used to cool the air which enters the air intake arrangement  102 . The engine  101  further includes a fuel tank  40  that is configured to store a fuel (not shown) that can be combusted in an air-breathing combustion chamber (not shown) that is supplied with air compressed by a compressor of the turbomachinery components  104 . 
       FIG. 13  represents a first operating mode of the heat exchanger arrangement  103 , which corresponds with a high operating flight speed of the engine  101 , for example, a short duration hypersonic “dash” mode. In the first operating mode of the heat exchanger arrangement  103 , a portion of the heat transfer medium is configured to flow out from the outlet  106  of the heat exchanger arrangement  103  along a first flow path  43 , to be fed downstream of the turbomachinery components  104  to be used for re-heating and/or a Ramjet mode of operating the engine  101 . Also in the first operating mode, a portion of the heat transfer medium is configured to flow out from the outlet  106  along a second flow path  44 , into an inlet  41  of the fuel tank  40 , before then being placed in heat exchange with the fuel in the fuel tank  40 . The heat transfer medium is then configured to exit the fuel tank  40  via an outlet  42 , and then to flow along a third flow path  45  towards the inlet  105  where it re-enters the heat exchanger arrangement  103 . 
     In this manner, in the first operating mode of the heat exchanger arrangement  103 , hot heat transfer medium which exits the heat exchanger arrangement  103  after it has been used to cool the air flowing through the heat exchanger arrangement  103  is used to heat the fuel in the fuel tank  40 . Cold heat transfer medium (which has been cooled as a result of heating the fuel) exiting the fuel tank  40  is then fed back into the heat exchanger arrangement  103  where it is used to cool the air flowing through the heat exchanger arrangement  103 . This mode of operation of the heat exchanger arrangement  103  is thus particularly advantageous because it provides that during high speed operation of the engine  101 , for example during a short duration hypersonic “dash” mode, during acceleration to high speed, heat removed into the heat transfer medium by cooling the incoming air can be dissipated (i.e. dumped) in the fuel tank  40 . The fuel volume in the fuel tank  40  thus represents a heat reservoir facilitating further cooling of the air. In other words, the fuel provides a good medium for dumping heat out from the heat transfer medium, and dumping heat from the heated heat transfer medium into the fuel thus helps cool the air. 
     Turning now to  FIG. 14 , in a second operating mode of the heat exchanger arrangement  103 , which corresponds with a low operating flight speed of the engine  101 , for example, a long duration supersonic or subsonic cruise, the heat transfer medium can instead be used to cool the fuel. In the second operating mode of the heat transfer arrangement  103 , the first fluid is configured to flow out from the outlet  106  of the heat exchanger arrangement  103  along the second flow path  44 , into the inlet  41  of the fuel tank  40 . The first fluid is then configured to exit the fuel tank  40  via the outlet  42 , and then to flow along the third flow path  45  towards the inlet  105  where it re-enters the heat exchanger arrangement  103 . 
     In this manner, in the second operating mode of the heat exchanger arrangement  103 , heat transfer medium which exits the heat exchanger arrangement  103  after it has been used to warm the air flowing through the heat exchanger arrangement  103  is used to cool the fuel in the fuel tank  40 . Hot heat transfer medium (which has been heated as a result of cooling the fuel) exiting the fuel tank  40  is then fed back into the heat exchanger arrangement  103  where it is used to warm the air flowing through the heat exchanger  103 . The second operating mode of the heat exchanger arrangement  103  is thus particularly advantageous because it provides that during low speed operation of the engine  101 , for example during a long duration supersonic or subsonic cruise, heat dumped into the fuel from the heat transfer medium during the first operating mode of the heat exchanger arrangement  103  can be transferred back into the heat transfer medium. This allows for the operation of the heat exchanger arrangement  103  in the first operating mode to be potentially repeated. In other words, without the second operating mode of the heat exchanger arrangement  103 , it might not otherwise be possible to keep on using the first operating mode of the heat exchanger arrangement  103 , because the fuel would ultimately reach a temperature limit. By using both the first and second operating modes of the heat exchanger  103  to run the cycle forwards ( FIG. 13 ) and in reverse ( 14 ), the heat transfer medium can effectively be used as both a heat source and a heat sink for the fuel. Specifically, the temperature of the fuel needs to be managed in order to effectively use the fuel as a heat sink for helping the heat transfer medium to more effectively cool the air. In the first operating mode of the heat exchanger arrangement  103 , the fuel provides a good medium for heat from the heat transfer medium to be dumped into, to help further cool the air, and in the second operating mode of the heat exchanger arrangement  103 , the fuel is cooled down so that the first operating mode can be repeated/used again. 
       FIG. 15A  shows an alternative exemplary first operating mode of the heat exchanger arrangement  103 , which corresponds with a high operating flight speed of the engine  101 , for example, a short duration hypersonic “dash” mode, in which the fuel is used as the heat transfer medium. In the first operating mode example shown in  FIG. 15A , the fuel is heated, by being in heat exchange with the air. In the first operating mode of the heat exchanger arrangement  103  shown in the example, a portion of the heat transfer medium (i.e. the fuel) is configured to flow out from the outlet  106  of the heat exchanger arrangement  103  along a first flow path  43 , to be fed downstream of the turbomachinery components  104  to be used for re-heating and/or a Ramjet mode of operating the engine  101 . A portion of the heat transfer medium (i.e. the fuel) is also configured to flow out from the outlet  106  of the heat exchanger arrangement  103  along a fourth flow path  46 , to be fed into the one or more turbomachinery components  104 . Also in the first operating mode, a portion of the heat transfer medium (i.e. the fuel) is configured to flow out from the outlet  106  along a second flow path  44 , into an inlet  41  of the fuel tank  40 . The fuel is then configured to exit the fuel tank  40  via an outlet  42 , and then to flow along a third flow path  45  towards the inlet  105  where it re-enters the heat exchanger arrangement  103 . 
     Turning now to  FIG. 15B , in a second operating mode of the heat exchanger arrangement  103  shown in  FIG. 15A , which corresponds with a low operating flight speed of the engine  101 , for example, a long duration supersonic or subsonic cruise, the fuel is cooled, by being in heat exchange with the air. In the second operating mode of the heat exchanger arrangement  103  show in the example of  FIG. 15B , the heat transfer medium (i.e. the fuel) is configured to flow out from the outlet  106  of the heat exchanger arrangement  103  along the second flow path  44 , into the inlet  41  of the fuel tank  40 . A portion of the heat transfer medium (i.e. the fuel) is configured to flow out from the fuel tank  40  along a fifth flow path  47  to be fed into the one or more turbomachinery components  104 , and another portion of the heat transfer medium (i.e. the fuel) is configured to flow out from the fuel tank  40  along a sixth flow path  48  to be fed downstream of the one or more turbomachinery components  104  to be used for re-heating and/or a Ramjet mode of operating the engine  101 . The heat transfer medium (i.e. the fuel) is, after some of the heat from the fuel tank  40  has been dumped by feeding it to the one or more turbomachinery components  104  and downstream of the turbomachinery components for re-heating and/or a Ramjet mode, then configured to exit the fuel tank  40  via the outlet  42 , and then to flow along the third flow path  45  towards the inlet  105  where it re-enters the heat exchanger arrangement  103 . 
     In this manner, in the example shown in  FIGS. 15A and 15B , in the first operating mode of the heat exchanger arrangement  103 , hot fuel which exits the heat exchanger arrangement  103  after it has been used to cool the air flowing through the heat exchanger arrangement  103  is used to heat the rest of the fuel in the fuel tank  40 . Fuel exiting the fuel tank  40  is then fed back into the heat exchanger arrangement  103  where it is used to cool the air flowing through the heat exchanger arrangement  103 . This mode of operation of the heat exchanger arrangement  103  is thus particularly advantageous because it provides that during high speed operation of the engine  101 , for example during a short duration hypersonic “dash” mode, during acceleration to high speed, heat removed into the fuel by cooling the incoming air can be dissipated (i.e. dumped) in the fuel tank  40 . The fuel volume in the fuel tank  40  thus represents a heat reservoir facilitating further cooling of the air. In other words, the fuel tank  40  provides a good medium for dumping heat out from the heat transfer medium, and dumping heat from the fuel into the fuel tank  40  thus helps cool the air. Then, in the second operating mode of the heat exchanger arrangement  103 , the fuel is cooled down so that the first operating mode can be repeated/used again. 
       FIG. 16A  shows another alternative exemplary first operating mode of the heat exchanger arrangement  103 , which corresponds with a high operating flight speed of the engine  101 , for example, a short duration hypersonic “dash” mode, in which a fluid that is separate to and different to the fuel in the fuel tank  40  is used as the heat transfer medium. In the example shown in  FIGS. 16A and 16B , the engine  101  further comprises a second heat exchanger arrangement  200 . The second heat exchanger arrangement  200  is configured to provide for heat exchange between the fuel in the fuel tank  40  and the heat transfer medium. In  FIGS. 16A and 16B , the solid black arrows represent the flow of the fuel, and the black arrows with a white fill represent the flow of the heat transfer medium. 
     In the first operating mode of the heat exchanger arrangement  103  shown in  FIG. 16A , the fuel is heated. A portion of the heat transfer medium is configured to flow out from the outlet  106  of the heat exchanger arrangement  103  along a flow path  51 , to be fed into the inlet  41  of the fuel tank  40 , before then being placed in heat exchange with the fuel in the fuel tank  40 . The first heat transfer medium is then configured to exit the fuel tank  40  via the outlet  42 , and then to flow along a flow path  52  towards the inlet  105  where it re-enters the heat exchanger arrangement  103 . Also, a portion of the first heat transfer medium is configured to flow along a flow path  53  into an inlet  201  of the second heat exchanger arrangement  200 , before exiting the second heat exchanger arrangement  200  via an outlet  202  thereof, along a flow path  54 , before flowing back into the inlet  105  of the heat exchanger arrangement  103  via the flow path  52 . 
     The heat transfer medium is placed in heat exchange with the fuel in the second heat exchanger arrangement  200 . As shown in  FIG. 16A , a portion of the fuel is configured to be fed from the fuel tank  40  to the second heat exchanger arrangement  200  via a flow path  55 . Then, a portion of the fuel is configured to be fed from the second heat exchanger arrangement  200  into the one or more turbomachinery components  104  via a flow path  56 , and a portion of the fuel is configured to be fed from the second heat exchanger arrangement  200  to be fed downstream of the turbomachinery components  104  to be used for re-heating and/or a Ramjet mode of operating the engine  101 , via a flow path  57 . In this way, some of the heat from the fuel tank  40  can be added to the engine cycle by feeding it to the one or more turbomachinery components  104  and downstream of the turbomachinery components for re-heating and/or a Ramjet mode. 
     Turning now to  FIG. 16B , in a second operating mode of the heat exchanger arrangement  103  shown in  FIGS. 16A and 16B , which corresponds with a low operating flight speed of the engine  101 , the fuel is cooled. In the second operating mode of the heat exchanger arrangement  103 , the heat transfer medium is configured to flow out from the outlet  106  of the heat exchanger arrangement  103  along the flow path  51 , to be fed into the inlet  41  of the fuel tank  40 , before then being placed in heat exchange with the fuel in the fuel tank  40 . The heat transfer medium is then configured to exit the fuel tank  40  via the outlet  42 , and then to flow along the flow path  52  towards the inlet  105  where it re-enters the heat exchanger arrangement  103 . A portion of the fuel is configured to be fed from the fuel tank  40  to the second heat exchanger arrangement  200  via a flow path  55 . Then, a portion of the fuel is configured to be fed from the second heat exchanger arrangement  200  into the one or more turbomachinery components  104  via a flow path  56 , and a portion of the fuel is configured to be fed from the second heat exchanger arrangement  200  to be fed downstream of the turbomachinery components  104  to be used for re-heating and/or a Ramjet mode of operating the engine  101 , via a flow path  57 . 
     In this manner, in the example shown in  FIGS. 16A and 16B , like in the examples shown in  FIGS. 13  through to  15 B, in the first operating mode of the heat exchanger arrangement  103 , the fuel volume in the fuel tank  40  represents a heat reservoir facilitating further cooling of the air. Then, in the second operating mode of the heat exchanger arrangement  103 , the fuel is cooled down so that the first operating mode can be repeated/used again. Advantageously, in the example shown in  FIGS. 16A and 16B , by use of the second heat exchanger arrangement  200 , in the first and second operating modes of the heat exchanger arrangement  103 , heat in the fuel/fuel tank  40  can advantageously be added to the engine cycle by feeding into to be utilised in the one or more turbomachinery components  104  and also used for re-heating and/or a Ramjet mode of operating the engine  101 . 
     From the examples described above and shown in  FIGS. 13  through to  16 B, it is apparent that in operation of the engine  101 , a fuel may be used as the heat transfer medium, or that the heat transfer medium may be a separate and different fluid to the fuel, and/or that the fuel may further be placed in heat exchange in a second heat exchanger arrangement. 
     It should be understood that the disclosure also includes an aircraft, flying machine or aerospace vehicle comprising an engine  101  as described herein. 
     Various modifications may be made to the described embodiment(s) without departing from the scope of the invention as defined by the accompanying claims.