Patent Description:
Gas turbine engines are typically inefficient to operate at low power settings. Operation of a gas turbine engine at idle is the typical lowest power setting available once the gas turbine engine has been started. In some instances, thrust produced at idle may be greater than the thrust needed for ground-based operations, such as taxiing and waiting in a parked position prior to takeoff or after landing. This can result in excess fuel consumption and may reduce component life with many repeated taxi, takeoff, and landing cycles. The excess thrust is typically managed by using brakes. Brake wear and deterioration can occur during taxi prior to takeoff and after landing. <CIT> discloses an aircraft engine having a low-idle mode and a high-idle mode, wherein in the low-idle mode power is directed from a lower spool to off takes, and in the high-idle mode the speed of the high spool is increased. <CIT> discloses a device for removing power from at least one rotating shaft of a turbojet and transforming the excess power into electrical energy.

According to a first aspect there is provided an engine system of an aircraft that includes an energy storage system, a gas turbine engine, and a controller. The gas turbine engine includes a low spool, a high spool, a low-spool generator operably coupled to the low spool, and a high-spool electric motor operably coupled to the high spool. The controller is configured to detect a braking condition of the aircraft, determine a storage capacity state of the energy storage system, transfer power from the low-spool generator to the energy storage system based on the storage capacity state of the energy storage system, transfer power to the high spool through the high-spool electric motor to support combustion in the gas turbine engine while a rotational speed of the low spool is reduced responsive to the low-spool generator extracting energy from the low spool, and split energy output of the low-spool generator between the energy storage system and the high-spool electric motor based on the storage capacity state and a target speed of the high spool, wherein the storage capacity state indicates a capacity to receive a power surge from the low-spool generator.

Optionally, the controller can be further configured to detect the braking condition based on a thrust command and an operating mode of the aircraft.

Optionally, power transfer can be performed based on determining that the operating mode of the aircraft is a taxi mode or a landing mode.

Optionally, the low-spool generator can be a low-spool electric machine configurable between a low-spool generator mode and a low-spool motor mode of operation, and the high-spool electric motor can be a high-spool electric machine configurable between a high-spool generator mode and a high-spool motor mode of operation.

Optionally, the controller can be configured to determine a demand associated with one or more accessories driven by the high spool and one or more target engine pressures, and control the high-spool electric motor to adjust a rotational speed of the high spool during the braking condition based on the demand associated with the one or more accessories driven by the high spool and the one or more target engine pressures.

Optionally, the controller can be configured to monitor for a flutter condition during landing of the aircraft and adjust operation of the low-spool generator and/or the high-spool electric motor to reduce the flutter condition.

Optionally, the braking condition can include operating the gas turbine engine at a minimum fuel limit, and the low-spool generator can reduce a speed and thrust output of the low spool below nominal minimum values associated with the minimum fuel limit by adding a load to the low spool.

Optionally, the gas turbine engine can include a thrust reverser, and power transfer to the high spool through the high-spool electric motor can be performed when the thrust reverser is deployed.

According to a second aspect there is provided a method that includes detecting a braking condition of an aircraft having a gas turbine engine including a low spool and a high spool and determining a storage capacity state of an energy storage system of the aircraft. Power can be transferred from a low-spool generator to the energy storage system based on the storage capacity state of the energy storage system. Power can be transferred to the high spool through a high-spool electric motor to support combustion in the gas turbine engine while a rotational speed of the low spool is reduced responsive to the low-spool generator extracting energy from the low spool. The method includes splitting the energy output of the low-spool generator between the energy storage system and the high-spool electric motor based on the storage capacity state and a target speed of the high spool; wherein the storage capacity state indicates a capacity to receive a power surge from the low-spool generator.

Optionally, the method can include detecting the braking condition based on a thrust command and an operating mode of the aircraft.

Optionally, the method can include determining a demand associated with one or more accessories driven by the high spool and one or more target engine pressures, and controlling the high-spool electric motor to adjust a rotational speed of the high spool during the braking condition based on the demand associated with the one or more accessories driven by the high spool and the one or more target engine pressures.

Optionally, the method can include monitoring for a flutter condition during landing of the aircraft, and adjusting operation of the low-spool generator and/or the high-spool electric motor to reduce the flutter condition.

<FIG> schematically illustrates an aircraft <NUM> that includes a pair of hybrid electric propulsion systems 100A, 100B (also referred to as hybrid gas turbine engines 100A, 100B or hybrid propulsion systems 100A, 100B). Each of the hybrid electric propulsion systems 100A, 100B includes a gas turbine engine <NUM> with a low spool <NUM> configured to drive rotation of a fan <NUM>. Gas turbine engine <NUM> also includes a high spool <NUM> that operates at higher speeds and pressures than the low spool <NUM>. A low-spool electric machine <NUM> can extract or add rotational power, for instance, by modifying torque and speed of the low spool <NUM> and fan <NUM>. A high-spool electric machine <NUM> can be configured to extract or add rotational power to the high spool <NUM>.

At least one power source <NUM> of the aircraft <NUM> can provide electrical power to or receive electrical power from the low-spool electric machine <NUM> and/or the high-spool electric machine <NUM> of the gas turbine engines <NUM> and/or other components of the aircraft <NUM>. The power source <NUM> can be an energy storage system that stores electrical and/or mechanical energy. For example, the power source <NUM> can include one or more of a battery, a super capacitor, an ultra-capacitor, a flywheel, and the like. Where the aircraft <NUM> includes an additional thermal engine (not depicted), such as an auxiliary power unit, the power source <NUM> can also be coupled to one or more components of the additional thermal engine. The power source <NUM> can be coupled to other energy producing or consuming systems of the aircraft <NUM>, such as an electrical power distribution system, an environmental control system, an anti-ice/deicing system, and/or other such aircraft systems (not depicted).

The aircraft <NUM> also includes a wheel and brake system <NUM> that allows for ground-based movement, steering, and braking of the aircraft <NUM>. Embodiments of the present disclosure control operation of the hybrid electric propulsion systems 100A, 100B to reduce brake wear of the wheel and brake system <NUM> as further described herein.

While the example of <FIG> illustrates a simplified example of the gas turbine engine <NUM>, it will be understood that any number of spools can be incorporated, and inclusion or omission of other elements and subsystems are contemplated. Further, systems described herein can be used in a variety of applications and need not be limited to gas turbine engines for aircraft applications.

<FIG> illustrates a hybrid electric propulsion system <NUM> (also referred to as hybrid gas turbine engine <NUM>, hybrid propulsion system <NUM>, or engine system <NUM>) as a further example of the hybrid electric propulsion system 100A, 100B of <FIG>. In the example of <FIG>, the hybrid electric propulsion system <NUM> includes gas turbine engine <NUM> operably coupled to an electrical power system <NUM> as part of a hybrid electric aircraft, such as aircraft <NUM> of <FIG>. One or more mechanical power transmissions <NUM> (e.g., 150A, 150B) can be operably coupled between the gas turbine engine <NUM> and the electrical power system <NUM>. The gas turbine engine <NUM> includes one or more spools, such as low spool <NUM> and high spool <NUM>, each with at least one compressor section and at least one turbine section operably coupled to a shaft (e.g., low pressure compressor <NUM> and low pressure turbine <NUM> coupled to inner shaft <NUM> and high pressure compressor <NUM> and high pressure turbine <NUM> coupled to outer shaft <NUM>).

The electrical power system <NUM> can include a low-spool electric machine <NUM> and a high-spool electric machine <NUM>. The low-spool electric machine <NUM> can be configurable between a low-spool motor mode of operation and a low-spool generator mode of operation. Alternatively, the low-spool electric machine <NUM> can be implemented as a low-spool electric motor 12A and a low-spool generator 12B coupled to the mechanical power transmission 150A. Similarly, the high-spool electric machine <NUM> can be configurable between a high-spool motor mode of operation and a high-spool generator mode of operation. Alternatively, the high-spool electric machine <NUM> can be implemented as a high-spool electric motor 13A and a high-spool generator 13B coupled to the mechanical power transmission 150B. The low-spool electric machine <NUM> and high-spool electric machine <NUM> can have a large capacity, such as a megawatt or more.

In the example of <FIG>, the mechanical power transmission 150A can include a gearbox operably coupled between the inner shaft <NUM> and the low-spool electric machine <NUM>. The mechanical power transmission 150B can include a gearbox operably coupled between the outer shaft <NUM> and the high-spool electric machine <NUM>. The mechanical power transmission 150A, 150B and/or electric machines <NUM>, <NUM> can include a clutch or other interfacing element(s) to selectively engage or disengage the electric machines <NUM>, <NUM>.

The electrical power system <NUM> can also include low-spool power conditioning electronics <NUM> and high-spool power conditioning electronics <NUM>. The low-spool power conditioning electronics <NUM> can include motor drive electronics 212A and rectifier electronics 212B. Similarly, the high-spool power conditioning electronics <NUM> can include motor drive electronics 213A and rectifier electronics 213B. The motor drive electronics 212A, 213A are operable to condition current to electric motors 12A, 13A (e.g., DC-to-AC converters) respectively. The rectifier electronics 212B, 213B are operable to condition current from the generators 12B, 13B (e.g., AC-to-DC converters) respectively. The low-spool power conditioning electronics <NUM> and high-spool power conditioning electronics <NUM> can interface with an energy storage management system <NUM> that further interfaces with an energy storage system <NUM>. The energy storage management system <NUM> can be a bidirectional DC-DC converter that regulates voltages between energy storage system <NUM> and electronics 212A, 212B, 213A, 213B. The energy storage system <NUM> can include one or more energy storage devices, such as a battery, a super capacitor, an ultra-capacitor, and the like. The energy storage management system <NUM> can facilitate various power transfers within the hybrid electric propulsion system <NUM>. The energy storage management system <NUM> may also transfer power to one or more electric motors on the aircraft <NUM> of <FIG>, or to external loads <NUM> and receive power from one or more external power sources <NUM> (e.g., power source <NUM> of <FIG>, aircraft power, auxiliary power unit power, cross-engine power, and the like).

A power conditioning unit <NUM> and/or other components can be powered by the energy storage system <NUM>. The power conditioning unit <NUM> can distribute electric power to support actuation and other functions of the gas turbine engine <NUM>. For example, the power conditioning unit <NUM> can power an integrated fuel control unit <NUM> to control fuel flow to the gas turbine engine <NUM>. The power conditioning unit <NUM> can also power a plurality of actuators (not depicted), such as bleed actuators, vane actuators, and the like.

One or more accessories <NUM> can also be driven by or otherwise interface with the gas turbine engine <NUM>. Examples of accessories <NUM> can include oil pumps, fuel pumps, and other such components. As one example, the accessories <NUM> include an oil pump driven through gearing, such as mechanical power transmission 150B, in response to rotation of the high spool <NUM> and/or the high-spool electric motor 13A. Alternatively, accessories <NUM> can be electrically driven through power provided by the energy storage management system <NUM> or other such sources of electrical power.

Engagement and operation of the low-spool electric motor 12A, low-spool generator 12B, high-spool electric motor 13A, and high-spool generator 13B can change depending upon an operating state of the gas turbine engine <NUM> and any commands received. Collectively, any effectors that can change a state of the gas turbine engine <NUM> and/or the electrical power system <NUM> may be referred to as hybrid electric system control effectors <NUM>. Examples of the hybrid electric system control effectors <NUM> can include the electric motors 12A, 13A, generators 12B, 13B, integrated fuel control unit <NUM>, and/or other elements (not depicted).

Transfer of power between the low spool <NUM> and high spool <NUM> can be performed by configuring the electric machines <NUM>, <NUM> in opposite modes of operation. For example, configuring the low-spool electric machine <NUM> in a low-spool generator mode of operation or engaging operation of the low-spool generator 12B can extract mechanical power and output electrical power that can drive the high-spool electric machine <NUM> in a high-spool motor mode of operation or upon engaging operation of the high-spool electric motor 13A. Further, a portion of the electric power output from the low-spool electric machine <NUM> can be stored in the energy storage system <NUM> and/or distributed elsewhere in the aircraft <NUM> of <FIG> through the energy storage management system <NUM>.

As an example, during a braking condition, lowering fuel flow to a minimum fuel limit at engine idle can support combustion in the gas turbine engine <NUM> but may result in excess thrust leading to wear on the wheel and brake system <NUM>. Engaging the low-spool generator 12B or operating the low-spool electric machine <NUM> in a low-spool generator mode can further reduce the rotational speed of the low spool <NUM> during the braking condition due to a mechanical load increase on the low spool <NUM>. Using the low-spool generator 12B during a braking condition can reduce thrust below a nominal minimum thrust that typically results from operating the gas turbine engine <NUM> at idle while maintaining the minimum fuel limit, which may also be referred to as hybrid electric idle. The thrust reduction can also reduce the wear of brakes of the wheel and brake system <NUM> during the braking condition.

To maintain combustion and provide rotational power to the accessories <NUM>, power from the low-spool generator 12B and/or from the energy storage system <NUM> can be transferred to the high-spool electric machine <NUM> in high-spool motor mode or the high-spool electric motor 13A while the low spool <NUM> is slowed. Speed and torque control of the high spool <NUM> can be monitored and adjusted by the addition of rotational power based on the demand associated with the one or more accessories <NUM> driven by the high spool <NUM> and one or more target engine pressures. When thrust reversers <NUM> are deployed on the gas turbine engine <NUM>, a greater amount of compensation may be needed from the high-spool electric motor 13A. If engine braking through engaging the low-spool generator 12B is performed while the aircraft <NUM> of <FIG> is airborne, performance conditions, such as flutter, can be monitored to determine possible stability issues. Accordingly, the low-spool generator 12B can be disengaged or operated in a reduced output mode if needed to maintain stability within the gas turbine engine <NUM>. Flutter may appear as unstable spinning of the fan <NUM>, which can be detected through speed, vibrations, or other types of sensors.

<FIG> is a schematic diagram of control signal paths <NUM> of the hybrid electric propulsion system <NUM> of <FIG> and is described with continued reference to <FIG> and <FIG>. A controller <NUM> can interface with the low-spool power conditioning electronics <NUM>, high-spool power conditioning electronics <NUM>, energy storage management system <NUM>, integrated fuel control unit <NUM>, accessories <NUM>, and/or other components (not depicted) of the hybrid electric propulsion system <NUM>. In embodiments, the controller <NUM> can control and monitor for fault conditions of the gas turbine engine <NUM> and/or the electrical power system <NUM>. For example, the controller <NUM> can be integrally formed or otherwise in communication with a full authority digital engine control (FADEC) of the gas turbine engine <NUM>. Alternatively, the controller <NUM> can be an aircraft level control or be distributed between one or more systems of the aircraft <NUM> of <FIG>. In embodiments, the controller <NUM> can include a processing system <NUM>, a memory system <NUM>, and an input/output interface <NUM>. The controller <NUM> can also include various operational controls, such as a hybrid engine control <NUM> that controls the hybrid electric system control effectors <NUM>, for instance, based on a thrust command <NUM>, an operating mode <NUM>, a braking command <NUM>, and/or a thrust reverser command <NUM>. The thrust command <NUM> can be a throttle lever angle or a command derived based on a throttle lever angle control of the aircraft <NUM> of <FIG>. The operating mode <NUM> can indicate whether the aircraft <NUM> is in a pre-takeoff taxi, takeoff, climb, cruise, descent, landing, or post-landing taxi mode, or other such operating modes, for example. The operating mode <NUM> can be determined by a system of the aircraft <NUM> based on tracking various parameters, such as weight-on-wheels, altitude, velocity, and other such aircraft parameters. The braking command <NUM> can indicate that the aircraft <NUM> is to be slowed based on a pilot command or other input. The thrust reverser command <NUM> can indicate a pilot request to deploy the thrust reversers <NUM> of <FIG>.

The processing system <NUM> can include any type or combination of central processing unit (CPU), including one or more of: a microprocessor, a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. The memory system <NUM> can store data and instructions that are executed by the processing system <NUM>. In embodiments, the memory system <NUM> may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored data and algorithms in a non-transitory form. The input/output interface <NUM> is configured to collect sensor data from the one or more system sensors and interface with various components and subsystems, such as components of the motor drive electronics 212A, 213A, rectifier electronics 212B, 213B, energy storage management system <NUM>, integrated fuel control unit <NUM>, accessories <NUM>, and/or other components (not depicted) of the hybrid electric propulsion system <NUM>. The controller <NUM> provides a means for controlling the hybrid electric system control effectors <NUM> using a hybrid engine control <NUM> that can be dynamically updated during operation of the hybrid electric propulsion system <NUM>. The means for controlling the hybrid electric system control effectors <NUM> can be otherwise subdivided, distributed, or combined with other control elements.

The controller <NUM> with hybrid engine control <NUM> can apply control laws and access/update models to determine how to control and transfer power between the low spool <NUM> and high spool <NUM>, as well as power transfers between multiple gas turbine engines <NUM>. For example, sensed and/or derived parameters related to speed, flow rate, pressure ratios, temperature, thrust, and the like can be used to establish operational schedules and transition limits to maintain efficient operation of the gas turbine engine <NUM>. For instance, the operating mode <NUM> of the gas turbine engine <NUM> can have different power settings, thrust requirements, flow requirements, and temperature effects. With respect to the aircraft <NUM> of <FIG>, each of the gas turbine engines <NUM> can have different settings and splits between electric and fuel-burn based operations in one or more of the operating modes. The hybrid engine control <NUM> can control electric current provided to the low-spool electric motor 12A and high-spool electric motor 13A and loading effects of the low-spool generator 12B and high-spool generator 13B. The hybrid engine control <NUM> can also determine a power split between delivering fuel to the combustor <NUM> and using the low-spool generator 12B and/or high-spool electric motor 13A to modify rotation speeds and thrust of the gas turbine engine <NUM>.

In embodiments, the controller <NUM> can blend the power distribution between the hybrid electric system control effectors <NUM> and fuel burn in the combustor <NUM>. From a pilot's perspective, the setting of a throttle lever angle produces thrust command <NUM> without the pilot having to distinguish between whether motor-based thrust or fuel burn based thrust is needed, although the pilot may control whether fuel is on or off. With respect to the aircraft <NUM>, the hybrid electric propulsion systems 100A, 100B can be independently controlled such that one of the hybrid electric propulsion systems 100A, 100B is operating in a fuel burning mode while the other of the hybrid electric propulsion systems 100A, 100B is operated using the low-spool electric motor 12A and/or the high-spool electric motor 13A.

<FIG> illustrate an example scenario of a response of the engine system <NUM> of <FIG> to a braking condition. <FIG> is a plot <NUM> of low-spool speed <NUM> and low-spool thrust <NUM> before and after a braking condition <NUM>. <FIG> is a plot <NUM> of fuel flow <NUM> before and after the braking condition <NUM>. <FIG> is a plot <NUM> of low-spool torque <NUM> and a high spool torque <NUM> before and after the braking condition <NUM>. <FIG> is a plot <NUM> of low-spool electrical power output <NUM> and high-spool electric power output <NUM> before and after the braking condition <NUM>. In the example scenario of <FIG>, gas turbine engine <NUM> is running on a minimum fuel limit and cannot reach a lower thrust. The braking condition <NUM> occurs at about a time of <NUM> seconds, and the controller <NUM> switches to a thrust spoiling mode where a further reduction of the low-spool speed <NUM> and low-spool thrust <NUM> is initiated while the fuel flow <NUM> remains at the minimum fuel limit. The controller <NUM> puts the low-spool electric machine <NUM> in low-spool generator mode or engages the low-spool generator 12B depending on the system configuration. The low-spool generator 12B absorbs some energy from the low spool <NUM>, increasing low-spool torque <NUM> and low-spool electrical power output <NUM>. The controller <NUM> can also put the high-spool electric machine <NUM> into high-spool motor mode or engage the high-spool electric motor 13A that reduces high spool torque <NUM> and high-spool electric power output <NUM>. Although the sequence of <FIG> illustrates one example scenario, many other braking and power transfer scenarios can be achieved by the engine system <NUM> of <FIG>.

Referring now to <FIG> with continued reference to <FIG>, <FIG> is a flow chart illustrating a method <NUM> for providing hybrid electric idle and braking for an aircraft, in accordance with an embodiment. The method <NUM> may be performed, for example, by the aircraft <NUM> through the hybrid electric propulsion systems 100A, 100B of <FIG>. For purposes of explanation, the method <NUM> is described primarily with respect to the hybrid electric propulsion system <NUM> of <FIG>; however, it will be understood that the method <NUM> can be performed on other configurations (not depicted).

Method <NUM> pertains to the controller <NUM> executing embedded code for braking and thrust control using hybrid engine control <NUM> along with other control functions, where the controller <NUM> can be an aircraft-level control or distributed between aircraft and engine system levels of control.

At block <NUM>, the controller <NUM> can detect a braking condition of the aircraft <NUM>. The braking condition can be received as an input signal based on a pilot request, such as braking command <NUM>. The braking condition can be an intermittent braking condition that occurs for a predetermined interval. For example, the intermittent braking condition can be used to reduce acceleration, such as inching forward during ground-based operation of the aircraft <NUM>. After the predetermined interval elapses, the braking condition may be removed, for instance, until another braking command <NUM> is received. There can be an interval between braking applications to support intermittent braking. For example, braking can be applied using a braking on-off duty cycle which can be configured within a range. For instance, the duty cycle can be a configured with braking active <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or another value between braking fully on and fully off during intermittent braking. Further, the braking condition may be detected based on operating conditions of the gas turbine engine <NUM> and/or the aircraft <NUM>. For example, the braking condition can be determined based on a thrust command <NUM> and an operating mode <NUM> of the aircraft <NUM>, such as an idle thrust in a taxi mode or a landing mode. The braking condition need not be associated with an explicit command to apply brakes. Other operating modes above idle where reduced power settings are desired may also or alternatively be a valid operating condition for the process described herein.

At block <NUM>, the controller <NUM> can determine a storage capacity state of the energy storage system <NUM> of the aircraft <NUM>. The storage capacity state can indicate a capacity to receive a power surge from the low-spool generator 12B of one or more of the gas turbine engines <NUM>. The determination can include considering whether both gas turbine engines <NUM> are operating in a fuel combustion state, an electrical demand of the aircraft <NUM> from the energy storage system <NUM>, and storage capacity remaining. The ability of the energy storage system <NUM> to accept an influx of current from the low-spool generator 12B of either or both of the gas turbine engines <NUM> of the aircraft <NUM> may use a customized charging schedule where the ability of the energy storage system <NUM> to be charged can have a non-linear mapping. For instance, a charging rate between a <NUM>% and <NUM>% capacity may differ from a charging rate between <NUM>% and <NUM>% capacity.

At block <NUM>, power can be transferred from a low-spool generator 12B to the energy storage system <NUM> based on the storage capacity state of the energy storage system <NUM>. The controller <NUM> can be configured to split energy output of the low-spool generator 12B between the energy storage system <NUM> and the high-spool electric motor 13A based on the storage capacity state and a target speed of the high spool <NUM>. The energy transfer can be performed in response to detecting the braking condition or during braking.

At block <NUM>, power can be transferred to the high spool <NUM> through the high-spool electric motor 13A to support combustion in the gas turbine engine <NUM> while a rotational speed of the low spool <NUM> is reduced responsive to the low-spool generator 12B extracting energy from the low spool <NUM>. For example, the controller <NUM> can determine a demand associated with one or more accessories <NUM> driven by the high spool <NUM> and one or more target engine pressures. The high-spool electric motor 13A can be controlled to adjust a rotational speed of the high spool <NUM> based on the demand associated with the one or more accessories <NUM> driven by the high spool <NUM> and the one or more target engine pressures. Power transfer to the high spool <NUM> through the high-spool electric motor 13A can be performed or modified when the thrust reverser <NUM> is deployed. The power transfer can be performed in response to detecting the braking condition or during braking.

The controller <NUM> can also be configured to monitor for a flutter condition during landing of the aircraft <NUM> and adjust operation of the low-spool generator 12B and/or the high-spool electric motor 13A to reduce the flutter condition. The braking condition can include operating the gas turbine engine <NUM> at a minimum fuel limit. The low-spool generator 12B can reduce a speed and thrust output of the low spool <NUM> below nominal minimum values associated with the minimum fuel limit by adding a load to the low spool <NUM>.

Also, it is clear to one of ordinary skill in the art that, the hybrid electric idle and braking described herein can be combined with aircraft and propulsion system control features, such as fuel flow control, power management, emergency operation, and the like.

Claim 1:
An engine system (<NUM>; 100A, 100B) of an aircraft (<NUM>), the engine system comprising:
an energy storage system (<NUM>; <NUM>);
a gas turbine engine (<NUM>) comprising a low spool (<NUM>), a high spool (<NUM>), a low-spool generator (12B) operably coupled to the low spool, and a high-spool electric motor (13A) operably coupled to the high spool; and
a controller (<NUM>) configured to:
detect a braking condition (<NUM>) of the aircraft (<NUM>);
determine a storage capacity state of the energy storage system (<NUM>; <NUM>);
transfer power from the low-spool generator (12B) to the energy storage system based on the storage capacity state of the energy storage system;
transfer power to the high spool (<NUM>) through the high-spool electric motor (13A) to support combustion in the gas turbine engine (<NUM>) while a rotational speed of the low spool (<NUM>) is reduced responsive to the low-spool generator extracting energy from the low spool; and
split energy output of the low-spool generator (12B) between the energy storage system (<NUM>; <NUM>) and the high-spool electric motor (13A) based on the storage capacity state and a target speed of the high spool (<NUM>), wherein the storage capacity state indicates a capacity to receive a power surge from the low-spool generator.