Propulsion system for an aircraft

Abstract

There is disclosed herein propulsion system (100) for an aircraft, comprising a first propulsor (102), a second propulsor (112), and an air diversion means (122). The first propulsor (102) has a first air intake (104) feeding a first air flow path (106), a first compressor (108) arranged in the first air flow path 5 (106), and a first exhaust (110). The second propulsor (112) has a second air intake (114), separate to the first air flow path (106), feeding a second air flow path (116), a second compressor (118) arranged in the second air flow path (116), and a second exhaust (120), wherein the second propulsor (112) is configured for a lower exhaust velocity than the first propulsor (102). The air 10 diversion means (122) is configured to controllably divert air (M) away from the first air intake (M1, 104) to the second air intake (M2, 114) based on the airspeed of the aircraft, such that a lower airspeed of the aircraft corresponds to more air (M) diverted by the air diversion means (122). There is further disclosed herein an aircraft comprising such a propulsion system.

Description

PROPULSION SYSTEM FOR AN AIRCRAFTTechnical Field

[0001] The present disclosure relates to a propulsion system for an aircraft, and an aircraft comprising such a propulsion system.Background

[0002] Aircraft propulsion systems may comprise gas turbine engines (which may be referred to generally as ‘jet engines’) wherein intake air is compressed, heated, and exhausted to thereby generate thrust.

[0003] The air intakes for engines may be sized according to a maximum desired aircraft speed, desired engine thrust and airflow (e.g., supersonic speed). However, when the aircraft has a lower or intermediate velocity and the engine operates at lower thrust (e.g., subsonic speed), the engine and intake may operate away from optimum conditions.

[0004] Simply reducing engine speed at lower aircraft speeds can be inefficient due to the nature of jet engines and the matching requirements with the air intake. Jet engines work by accelerating air, and the energy required for this acceleration is proportional to the square of the speed. Hence, at low flight speeds there is less thrust and more fuel consumption for the same momentum.

[0005] A trade-off thus emerges between maintaining enough engine speed for effective air acceleration and matching the air mass flow with the aircraft speed for optimal fuel efficiency.Summary

[0006] Aspects of the present disclosure are directed at addressing at least one or more of the above-discussed shortcomings associated with aircraft engines. More particularly, there is disclosed herein a propulsion system that provides an advantageously improved efficiency of operation, especially at reduced aircraft speeds.

[0007] Specifically, according to an aspect of the present disclosure, there is provided a propulsion system for an aircraft, comprising a first propulsor, asecond propulsor, and an air diversion means.

[0008] The first propulsor (which may be a gas turbine or ‘jet’ engine or the like) has a first air intake feeding a first air flow path, a first compressor arranged in the first air flow path, and a first exhaust. The first propulsor may comprise further components such as one or more turbines, other engine / power source / prime mover / fuel cell drive line etc., depending on the particular implementation of the first propulsor.

[0009] As used herein, an ‘air intake’ may comprise the start of the air flow path through a propulsor, such that the first air intake may correspond to engine interface region downstream from a main intake duct arranged in front of the engine forward face. That is, the propulsion system may further comprise a main air intake, which may be formed as a duct, and the first air intake and the second air intake may be arranged downstream from the main air intake. That is, the first air intake may be defined as the engine-intake interface plane.

[0010] The second propulsor has a second air intake, separate to the first air flow path, feeding a second air flow path, a second compressor arranged in the second air flow path, and a second exhaust. As with the first propulsor, the second propulsor may comprise further components such as one or more turbines, etc., depending on the particular implementation of the second propulsor.

[0011] The second propulsor is configured for a lower exhaust velocity than the first propulsor. That is, the airflow from the second exhaust has a lower exhaust velocity / pressure than the airflow from the first exhaust. This may be implemented, for example, by configuring the second flow path differently to the first flow path (e.g., different volumetric constraints), by configuring the second compressor to provide a lower pressure ratio than the first compressor, or via some other configuration. It will be appreciated that, as a result of the second exhaust having a lower exhaust velocity than the first exhaust, a more efficient acceleration of the air mass flow - and hence a more efficient thrust - can be provided for lower speeds of the aircraft. For higher / maximal speeds of the aircraft, a more or the most efficient thrust maybe achieved by directing all air mass flow through the first propulsor with the higher exhaust velocity.

[0012] The propulsion system thus comprises an air diversion means configured to controllably divert air away from the first air intake to the second air intake based on the airspeed of the aircraft, such that a lower airspeed of the aircraft corresponds to more air diverted by the air diversion means. Put another way, at lower speed of the aircraft, more air can be processed compared to a system with only a primary propulsor.

[0013] The air diversion means is preferably configured to adjust an air mass flow into the first air intake such that the air mass flow corresponds to the aircraft velocity. Preferably, the air mass flow better matches with the optimal mass flow for a current aircraft velocity, which may be sensed with any appropriate sensor as understood by those skilled in the art.

[0014] In some examples, the air diversion means is further configured to controllably divert air away from the first air intake to the second air intake based on a thrust request for the aircraft, such that a higher thrust request of the aircraft corresponds to less air diverted by the air diversion means. In this way, more air mass flow may be directed through the primary / first propulsor if the aircraft speed is low but there is a request for sudden acceleration. Put another way, the share of the air mass flow through and first and second propulsors, controlled by controlling the air diversion means, may be based on the aircraft acceleration request as an alternative or addition to the aircraft velocity.

[0015] The second air flow path is away from the first air flow path, which may be a main engine flow path, such that an air mass flow exhausted from the first air flow path may not be exhausted from the second air flow path and vice versa. In some example implementations, at least part of the air diversion means is arranged upstream of the first air intake. To achieve this, for example, the air diversion means may be formed as a bifurcation in the main air intake to feed air to the first and second propulsors. The second air flow may then be exhausted outside the first propulsor (i.e. , outside the main engine).

[0016] The second air intake may be adjacent the first air intake, such as surrounding it (i.e. , circumferentially and / or coaxially). As a result, an advantageous secondary air current can be generated nearby the first intake, which may assist in forming greater laminar flow into the first intake, thus improving the overall aerodynamics of the propulsion system.

[0017] The second compressor may be driven by at least the first propulsor. For example, the second compressor may be mounted on a drive shaft, which may be coupled, e.g., via a gear arrangement, to a drive shaft of the first propulsor. As another example, the rotational motion of the first propulsor may drive an electrical generator to generate electrical power, which may power (directly or via some energy storage means) an electric motor that drives the second compressor. An electrically driven second compressor may have greater controllability and advantageously simplified control.

[0018] In yet a further example, an air flow (e.g., from a high-pressure portion of the first propulsor such as after the first compressor or after an expander turbine in the first propulsor) may be diverted to spin a turbine, which may either directly drive the second compressor or may charge a generator for an electric motor drive for the second compressor. Such air diversion to drive the second compressor may be partially or entirely separate to that of the air diversion means controlling the share of air mass flow into the first and second propulsors.

[0019] Additionally or alternatively, the second compressor may be driven by an external power drive. The external power drive may comprise, for example, a battery-powered electric motor, a fuel cell power source, or the like. Preferably, the external power drive can power the second propulsor when the first propulsor is not operating. In this way, the second propulsor may advantageously provide more efficient thrust during taxi of the aircraft (i.e., a low aircraft speed regime).

[0020] In preferred examples, the propulsion system further comprises a heat exchanger arranged in the second air flow path, configured to heat air in the second air flow path with waste heat from the aircraft. The heat exchanger may be arranged upstream or downstream from the second compressor,preferably downstream for increased heat transfer efficiency.

[0021] By providing the heat exchanger in the second air flow path, heat transferred into the air mass flow through the second air flow path may advantageously expand the air and thereby increase the thrust provided by the second propulsor. Viewed from one perspective, the second air flow, downstream from the second compressor, provides an advantageous source of cool air. Hence, the waste heat in the aircraft can be more effectively cooled, thereby reducing the infrared signature of the aircraft. The configuration of the heat exchanger may be based on liquid coolant circulation or other configurations understood by those skilled in the art.

[0022] Conventional aircraft may typically comprise heat exchangers in cooling arrangements. However, according to the above-described example implementations of the present disclosure, this heat - which may have otherwise been without utility and merely waste - can be converted into additional thrust from the compressed air in the second air flow path. Accordingly, the efficiency of the thrust provided by the propulsion system (i.e. , from the second propulsor) can be further enhanced.

[0023] In such examples, the airflow through the second propulsor may be (additionally or alternatively) controlled according to a heat dissipation need.

[0024] According to a further aspect of the present disclosure, there is provided an aircraft comprising a propulsion system substantially as described above, which may provide substantially all or some of the thrust for the aircraft.

[0025] In such an aircraft, the vector of the second exhaust may preferably be controllable / directable. Hence, the thrust vector from the second propulsor may advantageously be controlled (in direction and / or magnitude) separately to the thrust vector of the first propulsor. Put another way, a thrust vector of the second exhaust may be controllably directable. Thus, the overall thrust vector for the aircraft may be advantageously more easily modified via modification of the contribution from the second propulsor.

[0026] For example, the second exhaust of the propulsion system may be configured to direct air towards the wake of the aircraft. ‘Wake blowing’ in thisway, i. e. , disrupting wing tip or wake vortices, may assist in reducing the noise of the aircraft, for example.

[0027] In some example implementations, the second exhaust may be configured to direct air towards a surface of the aircraft to thereby modify air flow over said surface of the aircraft. In this way, the air drag on said surface may be advantageously modified, where the drag may be reduced to improve fuel efficiency or increased to improve the dynamic motion of the aircraft (e.g., during complex aerial maneuvering). Air from the second exhaust may also be directed at a surface with a view to cooling said surface, as an addition or alternative to the above, thereby advantageously reducing the infrared profile of the aircraft.

[0028] Further advantages than those described above may be appreciated by those skilled in the art, and the above-described advantages or further advantages may be made (further) apparent through the following description of one or more embodiments of the present disclosure.Brief Description of the Drawings

[0029] One or more embodiments will be described, by way of example only, and with reference to the following figures, in which:

[0030] Figure 1 schematically shows a cross-sectional view of a propulsion system for an aircraft, according to aspects of the present disclosure;

[0031] Figure 2 schematically shows an example propulsion system for an aircraft;

[0032] Figure 3 schematically shows an aircraft having a pair of example propulsion systems;

[0033] Figures 4 to 6 schematically show cross-sectional views of various example implementations of a propulsion system for an aircraft.Detailed Description

[0034] The present disclosure is described in the following by way of a number of illustrative examples. It will be appreciated that these examples are provided for illustration and explanation only and are not intended to be limiting on thescope of the disclosure.

[0035] Furthermore, although the examples may be presented in the form of individual embodiments, it will be recognized that the present disclosure also covers combinations of the embodiments described herein.

[0036] Figure 1 shows a highly schematic propulsion system 100 for an aircraft (not shown). The propulsion system 100 comprises a first propulsor 102 and a second propulsor 112. The first propulsor 102 comprises a first inlet 104 feeding a first airflow 106, a first compressor 108 arranged in the first airflow 106 (the direction of which is indicated by an arrow), and a first exhaust 110, which in this example is formed as a nozzle. The second propulsor 112 comprises a second inlet 114 feeding a second airflow 116, a second compressor 118 arranged in the second airflow 116 (the direction of which is indicated by an arrow), and a second exhaust 120, which in this example is also formed as a nozzle.

[0037] It will be appreciated that the labels ‘first’ and ‘second’ are purely for the purposes of distinguishing components with similar names from each other. These labels may be dispensed with in the following discussed, for example when the reference numeral is suitable for clear identification of the component being referred to.

[0038] The second propulsor 112 is configured to create a lower velocity exhaust airflow, compared to that from the first propulsor 102 (i.e. , when provided with a comparable air mass flow). Those skilled in the art will be aware of a number of configurations that could achieve such a lower exhaust velocity. For example, the exhaust nozzle 120 of the second propulsor may have a different cross-sectional area, the blades of the compressor 118 may be angled differently, etc. The second propulsor 112 may thus have a lower pressure ratio than the first propulsor 102, e.g., when considering the ratio of air pressures either side of the exhausts 110 and 120.

[0039] An aircraft comprising the propulsion system 100 may travel at higher speeds during high-speed operations (such as high-speed cruising or pursuit operations) and at lower speeds during low-speed operations (such as idling or on-ground taxiing).

[0040] The first propulsor 102 may be particularly configured for efficient highspeed operations. That is, the intake 104 may be sized to receive an air mass flow corresponding to a maximum aircraft velocity (e.g., supersonic speeds), and the compressor 108 may be configured to apply a high pressure ratio (e.g., 3.0, 4.0, or more or less, depending on the type of aircraft), thereby enabling an efficient high thrust.

[0041] However, if the aircraft is performing low speed operations, the air mass flow into the intake 104 and the compression ratio of the compressor 108 may be poorly matched to the circumstance, leading to a highly inefficient operation if the first propulsor 102 were the only propulsor used to provide thrust during such low-speed operations. Put another way, during low-speed operations, the engine (i.e., the primary propulsor 102) may be operating far from its preferred design point if the engine is operated at a lower speed to provide less thrust.

[0042] Hence, the propulsion system 100 further comprises an air diversion means 122 configured to divert air away from the first propulsor 102. Reducing the air mass flow through the primary propulsor 102 may allow the primary propulsor 102 to operate at closer to its preferred design point even at lower aircraft speeds, as a lesser mass of air is accelerated.

[0043] It is realized as a part of the present disclosure that this air diverted from the primary propulsor 102 by the air diversion means 122 can provide fuel-efficient additional thrust if directed through a secondary propulsor 112 that is better configured for lower speed operation. That is, it is realized as a part of the present disclosure that such a secondary propulsor 112 can be configured in a manner such that the diverted air can be compressed and utilized for thrust such that the overall efficiency of the propulsion system 100 is enhanced. In this way, more air can be processed compared to a system with only the primary propulsor 102. The second propulsor 112 may thus be configured for a lower exhaust velocity than the first propulsor 102.

[0044] The propulsion system 100, or more particularly the air diversion means 122, may be controlled based on the aircraft speed such that a lower airspeed of the aircraft corresponds to more air M diverted by the air diversionmeans 122. Put another way, as shown in figure 1 , the air diversion means 122 may be configured to share an incoming air mass flow M between the first propulsor 102 and the second propulsor 112, these being apportioned shares M-1 and M-2 of the air mass flow M, respectively.

[0045] At a maximum speed, M-1 may be substantially equal to M, such that little or no air M is diverted to the second propulsor. However, as the speed of the aircraft decreases, the share M-2 of air M diverted to the second propulsor 112 may increase to enhance the efficiency of the propulsion system 100. Preferably, the air diversion means 122 is controlled so that the portion M-2 of air mass flow into the second propulsor 112 better matches the optimum mass flow for the current aircraft velocity, which may be based on the expected efficiency gains from diverting air mass flow M from the first propulsor 102, the relative efficiency of the second propulsor 112, and / or the thrust gain expected from the second propulsor 112.

[0046] The air diversion means 122 may take any form suitable for drawing air away from the intake 104 of the first propulsor 102 and providing said air to the intake 114 of the second propulsor 112. The air diversion means 122 is shown schematically upstream of the intake 104 of the first propulsor (which may be an engine interface region within a main intake duct). It will be appreciated that the air diversion means 122 is preferably arranged at least upstream of the compressor 108 so that a lower air mass flow is processed by the first propulsor 102, thereby better achieving the above-mentioned efficiency gains.

[0047] Figure 2 schematically shows an example implementation of a propulsion system 200, where similar reference numerals (incremented by 100) may correspond to similar components, at least in respect of the intended function of said component.

[0048] As shown in this figure, the propulsion system 200 comprises a main air intake duct 224, which in this example has a converging and diverging cylindrical profile substantially aligned with the axis of the duct / airflow of the first propulsor 202. In other examples, the main air intake 224 may have a different shape or position, depending on the aircraft design.

[0049] The main air intake 224 forms a front-most (in terms of aircraft travel direction) air interface for the propulsion system 200. The main air intake 224 is sized for an air mass flow corresponding to a maximum speed for the aircraft. The main air intake 224 further comprises an air diversion means 222 configured to divert air on its way towards the first propulsor 202 to instead be directed into the second propulsor 212. The air diversion means 222 may be formed with a diversion duct and a valve arrangement 230 configured to selectively or variably block the entrance to the diversion duct.

[0050] The valve arrangement 230 may be controlled by a controller (not shown) so as to selectively divert air based on the speed of the aircraft. In this way, the air mass flow into the first propulsor 202 may be reduced at low aircraft speeds, thereby improving the efficiency of the operation of the first propulsor 202, and this diverted air flow can provide efficient additional thrust via the second propulsor 212, this second propulsor 212 being configured for lower exhaust velocity, thus being suited for efficient operation at lower aircraft velocity.

[0051] In some implementations, the air diversion means 222 may be configured to divert the airflow to the second propulsor 212 from the upstream region of the main air intake 224. Preferable, air is drawn from direct proximity to the main intake 224 to advantageously control the (boundary layer) flow entering the main intake 224. This may be implemented by a duct 225 (shown in dotted lines in figure 2) and may be provided in addition to the previously mentioned diversion duct, or may be provided instead of conditioning the flow after the (throat section of) the intake 224. Such an alternative duct 225 may have its flow controlled via a valve arrangement or the like, similar to the valve arrangement 230 discussed above.

[0052] As indicated by the dotted arrow, the second compressor is powered / driven (at least in part) by the first propulsor 202. Such an arrangement may be configured in a number of ways, with examples provided in figures 3 to 6 and their associated discussion.

[0053] The propulsion system 200 further comprises a heat exchanger 226 arranged in the second airflow 216. In this example, the heat exchanger 226is arranged in the second propulsor 212 downstream from the second compressor 218.

[0054] The heat exchanger 226 is configured to receive waste heat 228 from sources of heat in the aircraft, which may include the engine, the cockpit, electrical systems, etc. The heat exchanger 226 may be configured in any manner understood by those skilled in the art and the particular form of the heat exchanger 226 is beyond the scope of the present disclosure. Downstream from the compressor 218, the compressed air, having a comparatively lower temperature with a controllable flow rate (through, e.g., control of the second compressor), can advantageously provide an effective cooling for the heated components of the aircraft. Accordingly, the infrared profile of the aircraft can be further reduced. The heat exchanger 226 may also serve as a cooler for engine bleed air (i.e. , air bled from the first propulsor 202).

[0055] It is further realized as a part of the present disclosure that the arrangement of the heat exchanger 226 in the airflow 216 second propulsor 212 advantageously provides an expansive effect on the air passing thereover towards the exhaust 220, which further enhances the thrust that can be provided by the second propulsor 212. Accordingly, the overall efficiency of the thrust provided by the propulsion system 200 can be further improved.

[0056] Figure 3 schematically shows an example arrangement of propulsion systems 300-a and 300-b on an aircraft 301 . In this example, there are two propulsion systems 300-a and 300-b, and both propulsors 302 and 312 of each propulsion system 300-a and 300-b are arranged on respective wings of the aircraft 301 . In other examples, there may only be one propulsion system.

[0057] It will be appreciated that, in other examples, the propulsion systems 300-a and 300-b may be arranged elsewhere and / or further propulsion systems 300 may be provided on the aircraft 301 . Additionally or alternatively, each propulsor 302, 312 of one or more propulsion systems 300 may be arranged in different locations. However, it may be preferable that the first propulsor 302 and the second propulsor 312 of each propulsion system 300are adjacent one another.

[0058] The propulsion system 300-a has an air mass flow M-a directed into its intake 324. The magnitude of this air mass flow M-a will depend primarily on the aircraft (air)speed and can be measured using any monitoring / sensor arrangement understood by those skilled in the art. The same applies to the air mass flow M-b directed into the intake 324 of the other propulsion system 300-b.

[0059] The propulsion system 300-a and / or 300-b may correspond to the propulsion system 200 described in respect of figure 2, or the propulsion systems 400, 500, 600 described in relation to figures 4, 5, or 6, for example. Accordingly, the air mass flows M-a and M-b are selective diverted from entering the first propulsor 302 to enter the second propulsor 312, using some controllable air diversion means (not shown). The air diversion means are controlled at least according to the speed of the aircraft 301 , so as to optimally match the portion of the air mass flows 300-a and 300-b through the primary propulsors 302 to the aircraft speed, and providing air diverted from the primary propulsors 302 to the 312 for efficient additional thrust.

[0060] In preferred examples, the air diversion means is controlled further based on a desired thrust of the aircraft 301 , which may be characterized by a control signal from the pi lot / control system of the aircraft 301 . Hence, if the speed of the aircraft 301 is low, but there is a large demand for thrust, less air may be diverted to the second propulsors 312 compared to if the speed of the aircraft 301 is low and there is a lower demand for thrust.

[0061] The thrust from the first propulsors 302 is indicated as F1-a and F1-b. The thrust from the second propulsors 312 is indicated as F2-a and F2-b. It can be seen that the thrust vectors for each of the first propulsors 302 and second propulsors 312 is separate. Preferably, at least the thrust vectors F2-a and F2-b are directable. Hence, these thrust vectors F2-a and F2-b may be used for wake blowing, active boundary layer suction, or the like. Such a control may advantageously reduce the noise profile of the aircraft 301 and / or make the aircraft 301 more maneuverable.

[0062] Figure 4 schematically shows an example implementation of apropulsion system 400 for an aircraft, where components with similar reference numerals to those used in previous figures (incremented by factors of 100) may correspond at least in their (intended) function to illustrated components in said previous components.

[0063] As shown in this figure, the first propulsor 402 comprises a compressor 408 and a turbine 434 arranged on a driveshaft 432. The particular design / construction of the compressor 408 and the turbine 434 is outside the scope of the present disclosure but may be well understood by those skilled in the art of gas turbine engines or similar aircraft propulsion systems. Further arranged on the driveshaft 432 is an electrical generator 436 which is configured to convert rotational motion of the compressor 408 and the turbine 434 (powered by fuel combustion, not shown) into electrical energy, in a manner understood by those skilled in the art.

[0064] The generator 436 is configured to provide the generated electrical energy to an electric motor 440 (as indicated by the dotted arrow). This energy may be provided via an intermediate energy storage such as a battery. In preferred examples, the battery or some other external power drive may be further configured to drive the electric motor 440, such that the second propulsor 412 can be powered even when the primary propulsor 402 is not operating (and hence not energizing the generator 436).

[0065] The electric motor 440 is connected to a driveshaft 438 which drives the compressor 418 in the second propulsor 412. In some examples, the motor 440 may be controllable in its speed, either via electrical control or via some gearing system.

[0066] Downstream from the compressor 418 is a heat exchanger 426 provided with waste heat 428 from the aircraft (which may include waste heat from the first propulsor 402 or elsewhere in the propulsion system 400), similar to the arrangement discussed in relation to figure 2. Figure 5 shows an alternative arrangement where a heat exchanger 526 is arranged upstream of the compressor 518. The other illustrated components in figure 5 may substantially correspond to those of figure 4 (with reference numerals incremented by 100).

[0067] The placement of the heat exchanger 526 upstream of the compressor 518, or downstream as shown in figures 2 and 4, may depend on the preferred thermodynamic behavior, expected ram air temperature, etc.

[0068] Figure 6 shows an example alternative arrangement for powering the second compressor 618 with the first propulsor 602. As before, components with similar reference numerals to those used in previous figures (incremented by factors of 100) may correspond at least in their (intended) function to illustrated components in said previous components.

[0069] In this example, a hose 644 (or pipe, line, etc.) is configured to bleed air from downstream of the first compressor 608. This air may pass over the heat exchanger 626 and be expanded through a second turbine 642, thereby spinning the turbine 642, which in turn drives the compressor 618. In other examples, the air may not be passed over the heat exchanger 626.

[0070] In yet further examples, not illustrated, a driveshaft of the first propulsor may drive a second driveshaft of the second propulsor, e.g., via a gearing arrangement or the like. This arrangement, the example arrangements disclosed in previous figures, and other arrangements that can be readily contemplated in view of the foregoing disclosure, may be combined in any manner depending on the particular constraints and preferences for operation of the propulsion system.

[0071] While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments are shown and described above by way of example in relation to the drawings, with a view to clearly explaining the various advantageous aspects of the present disclosure. It should be understood, however, that the detailed description herein and the drawings attached hereto are not intended to limit the disclosure to the particular form disclosed. Rather, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the following claims.

Claims

C L A I M S1 . A propulsion system (100) for an aircraft, comprising: a first propulsor (102) having a first air intake (104) feeding a first air flow path (106), a first compressor (108) arranged in the first air flow path (106), and a first exhaust (110); a second propulsor (112) having a second air intake (114), separate to the first air flow path (106), feeding a second air flow path (116), a second compressor (118) arranged in the second air flow path (116), and a second exhaust (120), wherein the second propulsor (112) is configured for a lower exhaust velocity than the first propulsor (102); and an air diversion means (122) configured to controllably divert air (M) away from the first air intake (M1 , 104) to the second air intake (M2, 114) based on the airspeed of the aircraft, such that a lower airspeed of the aircraft corresponds to more air (M) diverted by the air diversion means (122).

2. The propulsion system according to claim 1 , further comprising a main air intake (224), wherein the first air intake and the second air intake are arranged downstream from the main air intake (224).

3. The propulsion system according to claim 1 or claim 2, wherein the first propulsor (102) is a gas turbine engine.

4. The propulsion system according to any preceding claim, wherein at least part of the air diversion means is arranged upstream of the first compressor.

5. The propulsion system according to any preceding claim, wherein the air diversion means is configured to adjust an air mass flow into the first air intake such that the air mass flow corresponds to the aircraft velocity.

6. The propulsion system according to any preceding claim, wherein the second compressor is driven by at least the first propulsor.

7. The propulsion system according to any preceding claim, wherein the second compressor is driven by an external power drive.

8. The propulsion system according to any preceding claim, further comprising a heat exchanger (226) arranged in the second air flow path, configured to heat air in the second air flow path with waste heat from the aircraft.

9. The propulsion system according to any preceding claim, wherein the second air intake is adjacent the first air intake.

10. The propulsion system according to claim 9, wherein the second air intake surrounds the first air intake.11 . The propulsion system according to any preceding claim, wherein the air diversion means is further configured to controllably divert air away from the first air intake to the second air intake based on a thrust request for the aircraft, such that a higher thrust request of the aircraft corresponds to less air diverted by the air diversion means.

12. The propulsion system according to any preceding claim, wherein a thrust vector of the second exhaust is controllably directable.

13. An aircraft comprising the propulsion system according to any preceding claim.

14. The aircraft according to claim 13, wherein the second exhaust is configured to direct air towards the wake of the aircraft.

15. The aircraft according to claim 13 or claim 14, wherein the second exhaust is configured to direct air towards a surface of the aircraft to thereby modify air flow over said surface of the aircraft.

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