Patent Publication Number: US-10759540-B2

Title: Hybrid propulsion systems

Description:
TECHNICAL FIELD 
     This disclosure relates to hybrid propulsion systems. 
     BACKGROUND 
     A gas turbine engine is a type of internal combustion engine that may be used to power an aircraft, another moving vehicle, or an electric generator. The turbine in a gas turbine engine may be coupled to a rotating compressor that increases a pressure of fluid flowing into the turbine. A combustor may add fuel to the compressed fluid and combust the fuel/fluid combination. The combusted fluid may enter the turbine, where it expands, causing a shaft to rotate. The rotating shaft may drive the compressor, a propulsor, other devices, and loads including an electric generator. The propulsor may use the energy from the rotating shaft to provide propulsion for the system. 
     SUMMARY 
     In general, this disclosure describes hybrid propulsion systems that enable vehicles to be propelled using combinations of electrical motors and combustion motors (e.g., thermodynamic engines such as gas turbine engines). As one example, in a series hybrid propulsion system, the combustion motors may provide mechanical energy to operate one or more electrical generators, and the electrical motors may utilize power generated by the electrical generators to operate one or more propulsors. As another example, in a parallel hybrid propulsion system, the combustion motors may provide mechanical energy to operate one or more electrical generators and one or more propulsors, and the electrical motors may utilize power generated by the electrical generators to operate the propulsors that are also operated by the combustion motors. As another example, in a series-parallel hybrid propulsion system, the combustion motors may provide mechanical energy to operate one or more electrical generators and one or more propulsors, a first set of the electrical motors may utilize power generated by the electrical generators to operate the propulsors that are also operated by the combustion motors, and a second set of the electrical motors may utilize power generated by the electrical generators to operate one or more propulsors that are different than the propulsors operated by the combustion motors. 
     In one example, a system includes a plurality of power units configured to output electrical energy onto one or more electrical busses; one or more propulsors; and one or more electrical machines, each respective electrical machine configured to drive a respective propulsor of the one or more propulsors using electrical energy received from at least one of the one or more electrical busses. 
     In another example, a method of propelling an aircraft includes: outputting, by a plurality of power units, electrical energy onto one or more electrical busses; and driving, by one or more electrical machines and using electrical energy received from at least one of the electrical busses, one or more propulsors. 
     In another example, an airframe includes a plurality of power units configured to output electrical energy onto one or more electrical busses; one or more propulsors; and one or more electrical machines, each respective electrical machine configured to drive a respective propulsor of the one or more propulsors using electrical energy received from at least one of the one or more electrical busses. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual block diagram illustrating a system that includes a hybrid propulsion system, in accordance with one or more techniques of this disclosure. 
         FIG. 2  is a conceptual block diagram illustrating a system that includes a hybrid propulsion system in a series configuration, in accordance with one or more techniques of this disclosure. 
         FIG. 3  is a conceptual block diagram illustrating a system that includes a hybrid propulsion system in a series configuration with propulsive energy storage, in accordance with one or more techniques of this disclosure. 
         FIG. 4  is a conceptual block diagram illustrating a system that includes a hybrid propulsion system in a parallel configuration, in accordance with one or more techniques of this disclosure. 
         FIG. 5  is a conceptual block diagram illustrating a system that includes a hybrid propulsion system in a series-parallel configuration, in accordance with one or more techniques of this disclosure. 
         FIG. 6  is a conceptual block diagram illustrating a system that includes a hybrid propulsion system in a series-parallel configuration with propulsive energy storage, in accordance with one or more techniques of this disclosure. 
         FIG. 7  is a conceptual diagram illustrating an example electrical layout for a hybrid propulsion system, in accordance with one or more techniques of this disclosure. 
         FIG. 8  is a schematic diagram of an aircraft that includes a hybrid propulsion system, in accordance with one or more techniques of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Vehicles may include combustion motors that convert chemical potential energy (e.g., fuel) to propulsion and/or to electrical power. In addition to combustion motors, vehicles may include electrical machines to create propulsion. A vehicle that includes both combustion motors and electrical machines may be referred to as a hybrid vehicle. The motors in hybrid vehicles may be configured as series, parallel, or series-parallel. 
     In a series configuration, the combustion motor(s) may not directly provide power to propulsors, but instead may provide power in the form of rotational mechanical energy to one or more electric generators. The generator(s) may provide electrical power to the electrical machine(s), which in turn provide power (i.e., rotational mechanical energy) to one or more propulsors. In some examples, a vehicle with motors in a series configuration may include an energy storage system (ESS) capable of storing electrical energy for subsequent use by the electrical machines. The ESS may be charged with electrical energy generated by the generator(s) using mechanical energy from the combustion motor(s), electrical energy received from a source external to the vehicle (e.g., ground power in the case of an aircraft), and/or electrical energy generated by one or more other components of the vehicle. Some other components of the vehicle that may generate electrical energy include, but are not limited to, the electrical machines (e.g., in a descent phase of flight in the case of an aircraft), solar panels, and the like. 
     In a parallel configuration, the combustion motor(s) and the electrical machine(s) each may directly provide power to common propulsors. For instance, a combustion motor and an electrical machine may be configured to provide power (i.e., rotational mechanical energy) to a common propulsor. The electrical machine may provide the power to the propulsor using electrical power generated via the combustion motor (e.g., at a time when the electrical machine is not providing power to the propulsor), electrical power received from an ESS, or electrical power generated by another combustion motor. In this way, the electric machine may provide a “boost” of available power (e.g., for peak thrust operations). Similar to the ESS in the series configuration, the ESS in the parallel configuration may be charged with electrical energy generated by the generator(s) using mechanical energy from the combustion motor(s), electrical energy received from a source external to the vehicle (e.g., ground power in the case of an aircraft), and/or electrical energy generated by one or more other components of the vehicle. 
     In a series-parallel configuration, the combustion motor(s) and the electrical machine(s) may directly provide power to propulsors. However, as opposed to the parallel configuration in which each propulsor is mechanically powered by at least a combustion motor, the series-parallel configuration includes at least one propulsor that is powered exclusively by one or more electrical machines. That is, the series-parallel configuration includes a first set of electrical machines configured to provide power to a first set of propulsors that are also directly powered by combustion motors and a second set of electrical machines configured to provide power to a second set of propulsors that are not directly powered by combustion motors. Similar to the ESS in the series and parallel configurations, the ESS in the series-parallel configuration may be charged with electrical energy generated by the generator(s) using mechanical energy from the combustion motor(s), electrical energy received from a source external to the vehicle (e.g., ground power in the case of an aircraft), and/or electrical energy generated by one or more other components of the vehicle. 
       FIG. 1  is a conceptual block diagram illustrating a system  2  that includes a hybrid propulsion system, in accordance with one or more techniques of this disclosure. As shown in  FIG. 1 , system  2  includes an electrical bus  4 , one or more power units  6 A- 6 N (collectively, “power units  6 ”), one or more series propulsion modules  12 A- 12 N (collectively, “series propulsion modules  12 ”), one or more non-hybrid propulsion modules  18 A- 18 N (collectively, “non-hybrid propulsion modules  18 ”), one or more parallel propulsion modules  24 A- 24 N (collectively, “parallel propulsion modules  24 ”), an energy storage system (ESS)  34 , and a controller  36 . System  2  may be included in, and provide propulsion to, any vehicle, such as an aircraft, a locomotive, or a watercraft. System  2  may include additional components not shown in  FIG. 1  or may not include some components shown in  FIG. 1 . 
     Electrical bus  4  provides electrical power interconnection between various components of system  2 . Electrical bus  4  may include any combination of one or more direct current (DC) bus, one or more alternating current (AC) electrical bus, or combinations thereof. As one example, electrical bus  4  may include a DC bus configured to transport electrical power between power units  6  and series propulsion modules  12 . As another example, electrical bus  4  may include plurality of redundant DC buses configured to transport electrical power between power units  6  and series propulsion modules  12 . 
     Power units  6  provide electrical power for use by various components of system  2 . As shown in  FIG. 1 , each of power units  6  includes one or more combustion motors and one or more associated electrical machines. For instance, power unit  6 A includes combustion motor  8 A and electrical machine  10 A, and power unit  6 N includes combustion motor  8 N and electrical machine  10 N. In operation, combustion motor  8 A utilizes consumes fuel to produce rotational mechanical energy, which may be provided to electric machine  10 A via drive shaft  7 A. Electric machine  10 A converts the rotational mechanical energy into electrical energy and outputs the electrical energy to electrical bus  4 . Each of the combustion motors included in power units  6  may be any type of combustion motor. Examples of combustion motors include, but are not limited to, reciprocating, rotary, and gas-turbines. 
     Each of power units  6  may have the same or different power generation capacities. As one example, when operating at peak power, power unit  6 A may be capable of generating a greater amount of electrical power than power unit  6 N. In this way, one or more of power units  6 A- 6 N may be enabled, e.g., depending on a power demands of series propulsion modules  12 , other components of system  2 , or both. As another example, when operating at peak power, power unit  6 A and power unit  6 N may be capable of generating the same amount of electrical power. 
     Series propulsion modules  12  convert electrical energy to propulsion. As shown in  FIG. 1 , each of series propulsion modules  12  may include one or more electrical machines and one or more propulsors. For instance, series propulsion module  12 A includes electrical machine  14 A and propulsor  16 A, and series propulsion module  12 N includes electrical machine  14 N and propulsor  16 N. In operation, series propulsion modules  12  may operate in a plurality of modes including, but not limited to, an electric-only mode, a regeneration mode, and a neutral mode. 
     When series propulsion module  12 A operates in the electric-only mode, electrical machine  14 A may consume electrical energy received via electrical bus  4  and convert the electrical energy to rotational mechanical energy to power propulsor  16 A. When series propulsion module  12 A operates in the regeneration mode, electrical machine  14 A converts rotational mechanical energy received from propulsor  16 A into electrical energy, and provides the electrical energy to electrical bus  4 . Electrical bus  4  may distribute the electrical energy to another one of series propulsion modules  12 , one of parallel propulsion modules  24 , ESS  34 , or combinations thereof. When series propulsion module  12 A operates in the neutral mode, propulsor  16 A may “windmill” and/or reduce its fluid resistance (e.g., feather and/or blend with contours of an airframe). 
     Each of series propulsion modules  12  may have the same or different propulsion capacities. As one example, when operating at peak power, series propulsion module  12 A may be capable of generating more propulsive power than series propulsion module  12 A. As another example, when operating at peak power, series propulsion module  12 A may be capable of generating the same amount of propulsive power as series propulsion module  12 A. As another example, series propulsion module  12 A may positioned at an outboard portion of a wing to provide greater yaw control while series propulsion module  12 N may be positioned at an inboard portion of the wing in order to provide primary propulsion. 
     Non-hybrid propulsion modules  18  provide propulsion using fuel. Non-hybrid propulsion module  18  may be considered “non-hybrid” in that non-hybrid propulsion modules  18  neither generate electrical power for use to generate propulsive force, nor consume electrical power to provide propulsive force. As shown in  FIG. 1 , each of non-hybrid propulsion modules  18  may include one or more combustion motors and one or more propulsors. For instance, non-hybrid propulsion module  18 A includes combustion motor  20 A and propulsor  22 A, and non-hybrid propulsion module  18 N includes combustion motor  20 N and propulsor  22 N. Non-hybrid propulsion modules  18  may operate in plurality of modes including, but not limited to, a combustion-only mode and a neutral mode. When non-hybrid propulsion module  18 A operates in the combustion-only mode, combustion motor  20 A may consume fuel (e.g., from a fuel tank) to provide rotational mechanical energy to propulsor  22 A. When non-hybrid propulsion module  18 A operates in the neutral mode, propulsor  22 A may “windmill” and/or reduce its resistance (e.g., feather and/or blend with contours of the airframe). 
     Each of non-hybrid propulsion modules  18  may have the same or different propulsion capacities. As one example, when operating at peak power, non-hybrid propulsion module  18 A may be capable of generating a propulsive power than non-hybrid propulsion module  18 N. As another example, when operating at peak power, non-hybrid propulsion module  18 A may be capable of generating the same amount of propulsive power as non-hybrid propulsion module  18 N. As another example, non-hybrid propulsion module  18 A may positioned at an outboard portion of a wing in order to provide higher yaw control while non-hybrid propulsion module  18 N may be positioned at an inboard portion of the wing in order to provide primary propulsion. 
     Parallel propulsion modules  24  provide propulsion using fuel and electrical energy. As shown in  FIG. 1 , each of parallel propulsion modules  24  may include one or more electric machines, one or more combustion motors, and one or more propulsors. For instance, parallel propulsion module  24 A includes electric machine  26 A, combustion motor  30 A, and propulsor  32 A; and parallel propulsion module  24 N includes electric machine  26 N, combustion motor  30 N, and propulsor  32 N. Parallel propulsion modules  24  may operate in one or more of a plurality of modes including, but not limited to, a combustion-only mode, a combustion-generating mode, a dual-source mode, an electric-only mode, a generating mode, a regenerating mode, and a neutral mode. 
     When parallel propulsion module  24 A operates in the combustion-only mode, combustion motor machine  30 A may consume fuel (e.g., from a fuel tank) to provide rotational mechanical energy to propulsor  32 A while electric machine  26 A may neither generate electrical power nor consume electrical power. When parallel propulsion module  24 A operates in the combustion-generating mode, combustion motor machine  30 A may consume fuel (e.g., from a fuel tank) to provide rotational mechanical energy to propulsor  32 A and electric machine  26 A, and electric machine  26 A may convert a portion of the rotational mechanical energy to electrical power that is output to electrical bus  4 . When parallel propulsion module  24 A operates in the electric-only mode, combustion motor machine  30 A may be deactivated (e.g., not consume fuel) and electric machine  26 A may convert electrical power received from electrical bus  4  into rotational mechanical energy to power propulsor  32 A. When parallel propulsion module  24 A operates in the dual-source mode, combustion motor machine  30 A may consume fuel (e.g., from a fuel tank) to provide rotational mechanical energy to propulsor  32 A while electric machine  26 A may provide additional rotational mechanical energy to propulsor  32 A using electrical energy sourced via electrical bus  4 . When parallel propulsion module  24 A operates in the generating mode, combustion motor machine  30 A may consume fuel (e.g., from a fuel tank) to provide rotational mechanical energy to electric machine  26 A, and electric machine  26 A may convert to rotational mechanical energy to electrical power that is output to electrical bus  4 . As compared to the combustion-generating mode, when parallel propulsion module  24 A operates in the generating mode, propulsors  32  may be feathered or otherwise reduce or eliminate the amount of power taken from combustion motors  30  (e.g., de-clutch from a drive shaft) such that a majority of the power is used by electrical machines  26  to generate electrical power. When parallel propulsion module  24 A operates in the regenerating mode, electric machine  26 A may convert to rotational mechanical energy received from propulsor  32 A to electrical power that is output to electrical bus  4 . When parallel propulsion module  24 A operates in the neutral mode, propulsor  22 A may “windmill” and/or reduce its fluid resistance (e.g., feather and/or blend with contours of the airframe). 
     Each of parallel propulsion modules  24  may have the same or different propulsion capacities. As one example, when operating at peak power, parallel propulsion module  24 A may be capable of generating a propulsive power than parallel propulsion module  24 N. As another example, when operating at peak power, parallel propulsion module  24 A may be capable of generating the same amount of propulsive power as parallel propulsion module  24 N. As another example, parallel propulsion module  24 A may positioned at an outboard portion of a wing to provide higher yaw control while parallel propulsion module  24 N may be positioned at an inboard portion of the wing in order to provide primary propulsion. 
     For modules that include electric machines and combustion motors (i.e., power units  6  and parallel propulsion modules  24 ), the electric machines may be discrete components included in their own housing, or may be integral to (i.e., included/embedded in) a same housing as the combustion motors. As one example, electric machine  26 A may be included in same housing and/or directly mounted to combustion motor  30 A. As another example, electric machine  26 A may be attached to combustion motor  30 A via a drive shaft. 
     Additionally, for modules that include electric machines and combustion motors, the modules may include an additional starter, be started by their respective electric machine(s), or be started through some other means. As one example, combustion motor  8 A may include a starter that is different than electric machine  10 A. As another example, electric machine  10 A may operate as a starter for combustion motor  8 A. 
     Energy storage system (ESS)  34  may provide energy storage capacity for system  2 . ESS  34  may include any devices or systems capable of storing energy (e.g., electrical energy). Examples of devices that may be included ESS  34  include, but are not limited to, batteries, capacitors, supercapacitors, flywheels, pneumatic storage, and any other device capable of storing electrical energy or energy that may be converted to electrical energy (without combustion). ESS  34  may be coupled to electrical bus  4  and may be capable of providing electrical energy to electrical bus  4  and receiving electrical energy (e.g., for charging) from electrical bus  4 . 
     In some examples, ESS  34  may include multiple energy storage systems. For instance, ESS  34  may include a first energy storage system configured to store and provide electrical energy for propulsion and a second energy storage system configured to store and provide electrical energy for other systems, such as avionics and/or hotel loads. In some examples, ESS  34  may include a single energy storage system. For instance, ESS  34  may include a single energy storage system configured to store and provide electrical energy for propulsion and other systems. 
     In some examples, one or more components of ESS  34  may be swappable. For example, one or more batteries of ESS  34  may be swappable while an aircraft including system  2  is on the ground. As such, the aircraft may be quickly able to return to a fully charged state without the need to charge the batteries on the ground. 
     Controller  36  may control the operation of one or more components of system  2 . For instance, controller  36  may control the operation of electrical bus  4 , power units  6 , series propulsion modules  12 , non-hybrid propulsion modules  18 , parallel propulsion modules  24 , and ESS  34 . In some examples, controller  36  may include a single controller that controls all of the components. In other examples, controller  36  may include multiple controllers that each control one or more components. Where controller  36  includes multiple controllers, the controllers may be arranged in any configuration. As one example, controller  36  may include a separate controller for each module type. For instance, controller  36  may include a first controller that controllers power units  6 , a second controller that controls series propulsion modules  12 , a third controller that controls non-hybrid propulsion modules  18 , and a fourth controller that controls parallel propulsion modules  24 . As another example, controller  36  may include a separate controller for each module, or sub-module, within the module types. For instance, controller  36  may include a separate controller for each of power units  6 , a separate controller for each of series propulsion modules  12 , a separate controller for each of non-hybrid propulsion modules  18 , and a separate controller for each of parallel propulsion modules  24 . 
     Controller  36  may comprise any suitable arrangement of hardware, software, firmware, or any combination thereof, to perform the techniques attributed to controller  36  herein. Examples of controller  36  include any one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. When controller  36  includes software or firmware, controller  36  further includes any necessary hardware for storing and executing the software or firmware, such as one or more processors or processing units. 
     In general, a processing unit may include one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. Although not shown in  FIG. 1 , controller  36  may include a memory configured to store data. The memory may include any volatile or non-volatile media, such as a random access memory (RAM), read only memory (ROM), non-volatile RAM (NVRAM), electrically erasable programmable ROM (EEPROM), flash memory, and the like. In some examples, the memory may be external to controller  36  (e.g., may be external to a package in which controller  36  is housed). 
     In operation, system  2  may include and be propelled by any combination of series propulsion modules  12 , non-hybrid propulsion modules  18 , and parallel propulsion modules  24 . As one example, in what may be referred to as a “series configuration,” system  2  may include one or more power units  6  and one or more series propulsion modules  12 . Further details of the series configuration are discussed below with reference to  FIG. 2 . As another example, in what may be referred to as a “series configuration with propulsive energy storage,” system  2  may include one or more power units  6 , one or more series propulsion modules  12 , and ESS  34 . Further details of the series configuration with propulsive energy storage are discussed below with reference to  FIG. 3 . As another example, in what may be referred to as a “parallel configuration,” system  2  may include one or more parallel propulsion modules  24 . Further details of the parallel configuration are discussed below with reference to  FIG. 4 . As another example, in what may be referred to as a “series-parallel configuration,” system  2  may include one or more series propulsion modules  12  and one or more parallel propulsion modules  24 . Further details of the series-parallel configuration are discussed below with reference to  FIG. 5 . As another example, in what may be referred to as a “series-parallel configuration with propulsive energy storage,” system  2  may include one or more series propulsion modules  12 , one or more parallel propulsion modules  24 , and ESS  34 . Further details of the series-parallel configuration with propulsive energy storage are discussed below with reference to  FIG. 6 . 
     Where multiple propulsion modules are present (e.g., multiple instances of a specific type of propulsion module, multiple different types of propulsion modules, or combinations thereof), the multiple propulsion modules may be controlled independently, collectively in groups, or completely collectively. As one example, in an example where system  2  includes multiple series propulsion modules  12 , each of series propulsion modules  12  may be independently controlled. As another example, in an example where system  2  includes multiple series propulsion modules  12 , all of series propulsion modules  12  may be collectively controlled. As another example, in an example where system  2  includes multiple series propulsion modules  12 , a first set of series propulsion modules  12  may be collectively controlled and a second set of series propulsion modules  12  may be collectively controlled independently from the first set of series propulsion modules  12 . As another example, in an example where system  2  includes multiple series propulsion modules  12  and multiple parallel propulsion modules  24 , the series propulsion modules  12  may be collectively controlled and the parallel propulsion modules  24  may be collectively controlled independently from the series propulsion modules  12 . 
     Any or all of the combustion motors described above (i.e., combustion motors  8 , combustion motors  20 , and/or combustion motors  30 ) may, in some examples, be recuperated. That is, system  2  may include one or more recuperators configured to improve the cycle efficiency of the combustion motor(s). For instance, the recuperator may place an exhaust air flow that is downstream from a combustor in a combustion motor in a heat exchange relationship with a compressed airflow that is upstream from the combustor such that the recuperator transfers thermal energy from the exhaust airflow to the compressed airflow. 
       FIG. 2  is a conceptual block diagram illustrating a system  2 A that includes a hybrid propulsion system in a series configuration, in accordance with one or more techniques of this disclosure. System  2 A may represent one example of system  2  of  FIG. 1  that includes power unit  6 A, series propulsion module  12 A, and series propulsion module  12 B. As shown in  FIG. 2 , system  2 A also includes propulsion electrical bus  4 A, critical electrical bus  4 B, non-critical electrical bus  4 C, controller  35 , controllers  37 A- 37 D (collectively, “controllers  37 ”), AC/DC converters  42 A and  42 B (collectively, “AC/DC converters  42 ”), and DC/AC converters  44 A and  44 B (collectively, “DC/AC converters  44 ”). 
     Controller  35  and controllers  37  may collectively perform the functions of controller  36  of  FIG. 1 . Controller  35  may operate as a system master controller and controllers  37  may operate as sub-controllers. For instance, controller  37 A may operate as an engine controller to control operation of combustion motor  8 A, controller  37 B may operate as a generator controller to control operation of electric machine  10 A and AC/DC converters  42 , controller  37 C may operate as a propulsor controller to control operation of series propulsion modules  12  and DC/AC converters  44 , and controller  37 D may operate to control operation of ESS  34 A. Any of controller  35  and controllers  37  may be combined into one controller or further subdivided into additional controllers. As one example, controller  37 A and controller  37 B may be combined into a single controller that controls operation of combustion motor  8 A, electric machine  10 A and AC/DC converters  42 . As another example, controller  37 C may be subdivided into a first controller that controls DC/AC converter  44 A and series propulsion module  12 A, and a second controller that controls DC/AC converter  44 B and series propulsion module  12 B. 
     Controller  35  and controller  37  may be any type of controller capable of controlling operation of the corresponding devices/modules. For instance, controller  37 A may be an engine control unit (ECU) that may be partial authority or full authority (i.e., controller  37 A may be a full authority digital engine controller (FADEC)). Controller  35  and controller  37  may be implemented in any combination of hardware and software. 
     As shown in the example of  FIG. 2 , electrical busses  4  of  FIG. 1  may be divided into propulsion electrical bus  4 A, critical electrical bus  4 B, and non-critical electrical bus  4 C. Propulsion electrical bus  4 A may operate to transport electrical power used for propulsion components of system  2 A. For instance, propulsion electrical bus  4 A may facilitate the transfer of electrical power between power unit  6 A and series propulsion modules  12 . In the example  FIG. 2 , propulsion electrical bus  4 A is implemented as a DC bus or busses (e.g., a 700 volt DC bus). However, in other examples, propulsion electrical bus  4 A may be implemented as an AC bus, AC busses, or a combination of one or more DC bus(es) and one or more AC bus(es). 
     Critical electrical bus  4 B may operate to transport electrical power used by critical devices/systems of system  2 A. Examples of critical devices/systems include, but are not limited to, engine controllers, avionics, flight control systems, and the like. Critical electrical bus  4 B may be implemented as any combination of one or more DC bus(es) and/or one or more AC bus(es). For instance, critical electrical bus  4 B may be implemented as a 28 volt DC electrical bus. 
     Non-critical electrical bus  4 C may operate to transport electrical power used by non-critical devices/systems of system  2 A. Examples of non-critical devices/systems include, but are not limited to, hotel loads  50 , engine starters (e.g., a starter of combustion motor  8 A), and the like. Non-critical electrical bus  4 C may be implemented as any combination of one or more DC bus(es) and/or one or more AC bus(es). For instance, non-critical electrical bus  4 C may be implemented as a 28 volt DC electrical bus. 
     Hotel loads  50  include devices and systems that consume electrical power for non-critical purposes (e.g., purposes other than propulsion and flight control). Examples of hotel loads  50  include, but are not limited to, cabin lighting, cabin climate control, cooking, and the like. 
     AC/DC converters  42  may operate as rectifiers to convert AC electrical power generated by one or more components of system  2 A into DC electrical power. For instance, AC/DC converters  42  may convert AC electrical power generated by electric machine  10 A into DC electrical power that is output via propulsion electrical bus  4 A. 
     DC/AC converters  44  may operate as inverters to convert DC electrical power received from one or more components of system  2 A into AC electrical power. For instance, DC/AC converters  44  may convert DC electrical power received via propulsion electrical bus  4 A into AC electrical power that is used by series propulsion modules  12  to provide propulsion to system  2 A (e.g., used by electric machines  14 A and  14 B to respectively drive propulsors  16 A and  16 B). 
     As discussed above with reference to  FIG. 1 , in some examples, ESS  34  may be capable of storing and providing electrical energy for propulsion and other systems/devices. In general, the size and/or weight of ESS  34  may be dependent on the electrical storage capacity of ESS  34 . The greater the electrical storage capacity, the greater the size and/or weight of ESS  34 . The amount of electrical energy used for propulsion may be significantly greater than the amount of electrical energy used for other systems/devices of system  2 A. As such, the size and/or weight of ESS  34  in examples where ESS  34  is used to store electrical energy for propulsion may be greater than the size and/or weight of ESS  34  in examples where ESS  34  is not used to store electrical energy for propulsion. 
     In accordance with one or more techniques of this disclosure, system  2 A may not include an energy storage system configured to store or provide electrical energy for propulsion. For instance, as shown in  FIG. 2 , ESS  34 A may not be configured to output electrical energy to series propulsion modules  12  for driving propulsors  16 . Similarly, in some examples, ESS  34 A may not be configured to receive electrical power generated by power unit  6 A, which is configured to output electrical energy to series propulsion modules  12  for driving propulsors  16 . As such, by not using ESS  34 A to store or provide electrical energy for propulsion, the size and/or weight of ESS  34 A may be reduced relative to energy storage systems that are configured to store or provide electrical energy for propulsion. 
     As discussed above, in some examples, ESS  34 A may not be configured to receive electrical power generated by power unit  6 A. In some of such examples, ESS  34 A may be charged while system  2 A is on the ground and be sized to have enough energy storage capacity to power systems/devices attached to critical bus  4 B and non-critical bus  4 C for a projected flight time. Additionally or alternatively, ESS  34 A may be configured to receive electrical energy generated by power unit  6 A or any other electrical power source of system  2 A (e.g., a different combustion operated generator, solar panels, a ram-air turbine, or the like). 
     In operation, system  2 A may function in a plurality of modes including, but not limited to, an electric-only mode and a neutral mode. In the electric-only mode, controller  37 A may cause combustion-motor  8 A to burn fuel to generate rotational mechanical energy, which is used to drive electric machine  10 A via drive shaft  7 A. Controller  37 B may operate electric machine  10 A to convert the rotational mechanical energy into AC electrical power, and operate AC/DC converters  42  to rectify the AC electrical power into DC electrical power for output to propulsion electrical bus  4 A. Controller  37 C may operate DC/AC converters  44  to convert DC electrical power received from propulsion electrical bus  4 A into AC electrical power for output to electrical machines  12 . Controller  37 C may operate electrical machines  12  to convert the AC electrical power into rotational mechanical energy to drive a respective propulsor of propulsors  16 . 
     In the neutral mode, controller  37 A may shutdown combustion-motor  8 A such that combustion-motor  8 A ceases to burn fuel. Additionally, in some examples, controller  37 C may modify a shape/position/orientation of one or more aspects of series propulsion modules  12  to reduce wind resistance. As one example, where propulsors  16  include variable pitch propellers, controller  37 C may “feather” the blades of the propellers (i.e., rotate the blades to be substantially parallel with the airflow). As another example, controller  37 C may fold-up all or portions of propulsors  16 . 
     The hybrid system  2 A may present one or more advantages. As one example, as discussed above, system  2 A may reduce a weight of the energy storage system. As another example, system  2 A may enable power to be imparted on the DC bus from engine driven generators. As another example, system  2 A may allow propulsor motors to receive independent varying level of power to enable the thrust differential between propulsors. 
       FIG. 3  is a conceptual block diagram illustrating a system  2 B that includes a hybrid propulsion system in a series configuration with propulsive energy storage, in accordance with one or more techniques of this disclosure. System  2 B may include components similar to system  2 A of  FIG. 2 . However, as shown in  FIG. 3 , system  2 B includes an energy storage system that is configured to store and provide electrical energy for propulsion. For instance, system  2 B includes ESS  34 B, which is coupled to propulsion electrical bus  4 A and configured to provide propulsive electrical energy to series propulsion modules  12  via propulsion electrical bus  4 A. Additionally, ESS  34 B may be configured to receive electrical energy via propulsion electrical bus  4 A. 
     In addition to the electric-only and neutral modes described above, system  2 B may operate in a dual-source electric-only mode, a regenerating mode and a generating mode. In the dual-source electric-only mode, controllers  37 A and  37 B may operate power unit  6 A and AC/DC converters  42  in a manner similar to the electric-only mode discussed above. Additionally, controller  37 D may cause ESS  34 B to output DC electrical power onto propulsion electrical bus  4 A. Controller  37 C may operate DC/AC converters  44  and series propulsion modules  12  in a manner similar to the electric-only mode discusses above, with a difference being that electrical energy used by series propulsion modules  12  for propulsion is contemporaneously sourced from power unit  6 A and ESS  34 B. 
     In the regenerating mode, controller  37 C may cause series propulsion modules  12  to operate as generators (e.g., operate as ram air turbines) by converting rotational mechanical energy of propulsors  16  into AC electrical power. Controllers  37 C may operate DC/AC converters  44  to convert the AC electrical power into DC electrical power for output to propulsion electrical bus  4 A. Controller  37 D may operate ESS  34 B to charge from propulsion electrical bus  4 A using the DC electrical power output by DC/AC converters  44 . 
     In the generating mode, controllers  37 A and  37 B may operate power unit  6 A and AC/DC converters  42  in a manner similar to the electric-only mode discussed above. However, as opposed to controller  37 C operating DC/AC converters  44  and series propulsion modules  12  to utilize the generated power for propulsion, controller  37 D may cause ESS  34 B to store the generated power (i.e., to charge). 
     The hybrid system  2 B may present one or more advantages. As one example, where the regenerating mode is used while an aircraft including system  2 B is descending, ESS  34 B may obtain enough charge on decent to enable system  2 B to operate in the dual-source electric-only mode on take-off and/or ascent without the need to charge ESS  34 B on the ground. Additionally or alternatively, the generating mode may be used while the aircraft is on the ground such that ESS  34 B may obtain enough charge to enable system  2 B to operate in the dual-source electric-only mode on take-off and/or ascent without the need to charge ESS  34 B on the ground from an external charging source. As such, system  2 B may enable hybrid aircraft to utilize airports that lack ground charging facilities. 
     As another example, the dual-source electric mode may enable system  2 B to provide a similar amount of thrust with a relatively smaller sized combustion motor. As such, system  2 B enables a weight reduction in hybrid aircraft. For similar reasons, system  2 B may enable a reduction in emissions from aircraft. 
     As another example, system  2 B may allow the transfer of excess power on the DC bus to the ESS. For instance, ESS  34 B may be “trickle” charged using excess power generated by power unit  6 A (e.g., during cruise). As another example, system  2 B may allow power to be imparted on the DC bus from one or both of engine driven generators and ESS. The level of power demand placed on the DC bus is shared between ESS and engine driven generator system at varying percentage of power share depending on the operational needs of the platform, available stored electrical energy and fuel. As another example, application of power from both the ESS and engine driven generator system in system  2 B may allow the power available on the bus to be higher than that from a standalone turbo-generator offering a “boost” to the available power for peak power demand operations. As another example, application of power from both the ESS and engine driven generator system in system  2 B may allow fluctuating power demands on the bus to be met while maintaining a constant power demand on the engine. As another example, system  2 B may allow propulsor motors to receive independent varying level of power to enable the thrust differential between propulsors. As another example, system  2 B may allow the aircraft to self-start without the need to an external starter or APU. As another example, system  2 B may deliver power for all hotel loads and avionics. As another example, system  2 B may deliver all power to all critical functions/systems. 
     While illustrated in  FIGS. 2 and 3  as including a single power unit and multiple series propulsion modules, systems  2 A and  2 B are not so limited. For instance, one or both of systems  2 A and  2 B may include multiple power units and/or a single series propulsion module. 
     Including multiple power units may present one or more advantages. As one example, a series hybrid system with multiple power units may be more fault tolerant than a series hybrid system with a single power unit. For instance, in a series hybrid system that includes two power units, flight power would still be available in the event that one of the power units failed or otherwise was shutdown. 
       FIG. 4  is a conceptual block diagram illustrating a system  2 C that includes a hybrid propulsion system in a parallel configuration, in accordance with one or more techniques of this disclosure. System  2 C may represent one example of system  2  of  FIG. 1  that includes parallel propulsion module  24 A, and ESS  34 B. As shown in  FIG. 4 , system  2 C also includes propulsion electrical bus  4 A, critical electrical bus  4 B, and non-critical electrical bus  4 C, controller  35 , controllers  37 , and AC/DC converters  42 . 
     Controllers  35  and  37  may perform operation similar to those discussed above. For instance, controller  35  may operate as a system master controller, controller  37 A may control operation of combustion motor  30 , controller  37 B may control operation of electric machine  26 A and AC/DC converters  42 , and controller  37 D may control operation of ESS  34 B. 
     However, as opposed to systems  2 A and  2 B that included series propulsion modules (e.g., propulsion modules without a mechanical linkage between combustion motor and propulsor), system  2 C includes parallel propulsion module  24 A. While illustrated as including a single parallel propulsion module, system  2 C is not so limited and may include a plurality of parallel propulsion modules. 
     In operation, system  2 C may function in a plurality of modes including, but not limited to, a combustion-only mode, a dual-source mode, a combustion-generating mode, an electric-only mode, a generating mode, and a regenerating mode. In the combustion-only mode, controller  37 A may cause combustion-motor  30 A to burn fuel to generate rotational mechanical energy, which drives propulsor  32 A. In the combustion-only mode, electrical machine  26 A may not supply or remove rotational energy (other than minimal frictional losses and the like) drive shaft  28 A. For instance, electric machine  26 A may be clutched or otherwise mechanically decoupled from drive shaft  28 A. 
     In the dual-source mode, controller  37 A may cause combustion-motor  30 A to burn fuel to generate rotational mechanical energy, which drives propulsor  32 A. Additionally, controller  37 B may cause electric machine  26 A to add rotational energy to drive propulsor  32 A using electrical energy supplied from ESS  34 B. In some examples, as opposed to causing drive shaft  28 A to rotate faster than the speed caused by combustion motor  30 A, electric machine  26 A may provide additional torque to drive shaft  28 A. As such, in examples where propulsor  32 A is a variable pitch propeller, controller  37 A may adjust the pitch such that a higher level of thrust is obtained without increasing the rotational speed to propulsor  32 A. 
     In the combustion-generating mode, controller  37 A may cause combustion-motor  30 A to burn fuel to generate rotational mechanical energy, which drives propulsor  32 A. Additionally, controller  37 B may cause electric machine  26 A to convert rotational energy generated by combustion motor  30 A into AC electrical power. Controller  37 B may cause AC/DC converters to convert the AC electrical power into DC electrical power for output onto propulsion electrical bus  4 A. Controller  37 D may cause ESS  34 B to store the generated electrical power. 
     In the electric-only mode, controller  37 A may cause combustion motor  30 A to shutdown and cease consuming fuel. Controller  37 B may cause electric machine  26 A to convert electrical power sourced from ESS  34 B into rotational mechanical energy to drive propulsor  32 A. 
     In the generating mode, the components of system  2 C may perform functions similar to the combustion-generating mode with a difference being that controller  37 A may cause propulsor  32 A to decouple from combustion motor  30 A or, if variable pitch, cause propulsor  32 A to feather. As such, controller  37 A may cause all, or at least a vast majority, of the rotational mechanical energy generated by combustion motor  30 A to be available for conversion into electrical energy by electric machine  26 A. 
     In the regenerating mode, the components of system  2 C may perform functions similar to the electric-only mode with a difference being that the flow of electrical power is reversed. For instance, parallel propulsion module  24 A may convert rotational mechanical energy received via propulsor  32 A into electrical energy that is stored by ESS  34 B. 
     The parallel hybrid system  2 C may present one or more advantages. As one example, the dual-source mode may enable system  2 C to provide a similar amount of thrust with a relatively smaller sized combustion motor. As such, system  2 C enables a weight reduction in hybrid aircraft. For similar reasons, system  2 B may enable a reduction in emissions from aircraft. 
     As another example, as the combustion-generating mode enable system  2 C to store propulsion energy for future use (i.e., in ESS  34 B), the combustion-generating mode may enable controller  37 A to operate combustion motor  30 A at an optimal level (e.g., a most fuel efficient level) without wasting energy, even if the energy resulting at the optimal level is not immediately required. In other words, system  2 C may allow the transfer of excess power on the DC bus to the ESS for future use. As another example, application of mechanical power from electric machine motoring may allow the power available on the propulsor shaft to be higher than that from a standalone engine, thereby offering a “boost” to the available power for peak power thrust operations. As another example, application of power from the ESS powered motor may allow fluctuating power demands on the shaft to be met while maintaining a constant power demand on the engine. As another example, system  2 C may allow an aircraft to self-start without the need to an external starter or APU. As another example, system  2 C may deliver power for all hotel loads and avionics. As another example, system  2 C may deliver all power to all critical functions/systems. 
       FIG. 5  is a conceptual block diagram illustrating a system  2 D that includes a hybrid propulsion system in a series-parallel configuration, in accordance with one or more techniques of this disclosure. System  2 D may include components similar to system  2 A of  FIG. 2  and system  2 C of  FIG. 4 . However, as shown in  FIG. 5 , the ESS  34  included in system  2 D is not configured to provide electrical energy for propulsion. 
       FIG. 6  is a conceptual block diagram illustrating a system  2 E that includes a hybrid propulsion system in a series-parallel configuration with propulsive energy storage, in accordance with one or more techniques of this disclosure. System  2 E may include components similar to system  2 D of  FIG. 5 . However, as shown in  FIG. 6 , system  2 E includes an energy storage system that is configured to store and provide electrical energy for propulsion. For instance, system  2 E includes ESS  34 B, which is coupled to propulsion electrical bus  4 A and configured to provide propulsive electrical energy to series propulsion modules  12  and/or parallel propulsion module  24 A via propulsion electrical bus  4 A. Additionally, ESS  34 B may be configured to receive electrical energy via propulsion electrical bus  4 A. 
     The series-parallel system  2 E may be configured to operate in any of the modes described above with reference to the series and parallel configurations. Additionally, the series-parallel system  2 E may operate in one or more additional modes. As one example, the series-parallel system  2 E may operate in a dual source mode in which electrical energy used by series propulsion modules  12  is sourced from an energy storage system (e.g., ESS  34 ) and one or more power units (e.g., power units  6 ). In this dual source mode, the parallel propulsion modules (e.g., parallel propulsion module  24 A) may burn fuel to drive propulsors or may be inactive. As another example, the series-parallel system  2 E may operate in a triple source mode in which the parallel propulsion modules (e.g., parallel propulsion module  24 A) may burn fuel to drive propulsors (e.g., propulsor  32 A) and their electrical machines output electrical energy via propulsion electrical bus  4 A, electrical energy used by series propulsion modules  12  is simultaneously sourced from all three of an energy storage system (e.g., ESS  34 ), one or more power units (e.g., power units  6 ), and the parallel propulsion modules. 
     The series-parallel hybrid system  2 E may present one or more advantages. As one example, system  2 E may allow power to be imparted on the DC bus from one or both of engine driven generators and an ESS. As another example, power demand level placed on the DC bus may be shared between ESS and engine driven generator system at varying percentage of power share depending on the operational needs of the platform, available stored electrical energy and/or fuel. As another example, application of mechanical power from electric machine motoring may allow the power available on the propulsor shaft to be higher than that from a standalone engine, thereby offering a “boost” to the available power for peak power thrust operations. As another example, application of power from the ESS powered motor may allow fluctuating power demands on the shaft to be met while maintaining a constant power demand on the engine. As another example, application of power from both the ESS and engine driven generator system may allow fluctuating power demands on the bus to be met while maintaining a constant power demand on the engine. As another example, system  2 E may allow propulsor motors to receive independent varying level of power to enable the thrust differential between propulsors. As another example, system  2 E may allow an aircraft to self-start without the need to an external starter or APU. As another example, system  2 E may deliver power for all hotel loads and avionics. As another example, system  2 E may allow deliver all power to all critical functions/systems. 
       FIG. 7  is a conceptual diagram illustrating an example electrical layout for a hybrid propulsion system, in accordance with one or more techniques of this disclosure. As shown in  FIG. 7 , system  3  includes propulsion electrical bus  4 A, critical electrical bus  4 B, non-critical electrical bus  4 C, series propulsion module  6 A, parallel propulsion module  24 A, AC/DC converters  42 , DC/AC converters  44 , controller  37 A, critical power panel  62 , propulsion power panel  60 , ESS  34 , ESS rack  64 , ESS battery  66 , engine starter  68 , and external power interface  70 . Propulsion electrical bus  4 A, critical electrical bus  4 B, non-critical electrical bus  4 C, series propulsion module  6 A, series propulsion module  6 A, parallel propulsion module  24 A, AC/DC converters  42 , DC/AC converters  44 , and controller  37 A may perform operations described above. 
     As shown in  FIG. 7 , ESS  34  may include ESS rack  64 , ESS battery  66 , and external power interface  70 . ESS rack  64  may operate to provide electrical power to any of propulsion electrical bus  4 A, critical electrical bus  4 B, and non-critical electrical bus  4 C. ESS rack  64  may be coupled to each of the electrical busses  4 A- 4 C, ESS battery  66 , and external power interface  70 . ESS rack  64  may facilitate the transfer of electrical power amongst various components. As one example, ESS rack  64  may utilize electrical power stored in ESS battery  66  to supply 28 volt DC power to critical panel  62  via critical electrical bus  4 B. As another example, ESS rack  64  may utilize electrical power stored in ESS battery  66  to supply 28 volt DC power to hotel loads  50  via non-critical electrical bus  4 B. As another example, ESS rack  64  may utilize electrical power stored in ESS battery  66  to supply 700 volt DC power to propulsion power panel  60  via propulsion electrical bus  4 A. As another example, ESS rack  64  may utilize electrical power stored in ESS battery  66  to supply 28 volt DC power to engine starter  68  to start a combustion engine. 
     External power interface  70  may enable system  3  to receive power from one or more external sources and/or provide power to one or more external loads. For instance, external power interface  70  may include one or more electrical receptacles (e.g., plugs) that may be connected to a terrestrial power grid at an airport to facilitate charging of ESS battery  66 . 
     As shown in  FIG. 7 , propulsion electrical bus  4 A may include propulsion power panel  60 , which may facilitate the transfer of electrical power between ESS  34 , series propulsion module  6 A, and parallel propulsion module  24 A. Propulsion power panel  60  may include one or more mechanical or solid state power switches to facilitate the power routing. In some examples, propulsion power panel  60  may be capable of routing power amongst any arbitrary combination of ESS  34 , series propulsion module  6 A, and parallel propulsion module  24 A. For instance, propulsion power panel  60  may include a full cross-point switching matrix. 
     As shown in  FIG. 7 , critical electrical bus  4 B may include critical power panel  62 , which may facilitate the transfer of electrical power to critical systems/devices. In some examples, one or more additional systems/devices may be included in critical power panel  62 . For instance, system master controller  72 , which may be an example of controller  35  or controller  36 , may be included in critical power panel  62 . 
     Each of propulsion power panel  60 , critical panel  62 , and ESS rack  64  may be discrete physical components. The physical components may be located in a common area of an airframe, or at different areas around the airframe. System  3  may include various electrical protections elements in between and/or amongst power panel  60 , critical panel  62 , and ESS rack  64 . For instance, system  3  may include components and be configured such that critical electrical bus  4 B is functionally and physically independent from other power systems (e.g., electrical busses  4 A and  4 C). 
       FIG. 8  is a schematic diagram of an aircraft that includes a hybrid propulsion system, in accordance with one or more techniques of this disclosure. As shown in  FIG. 8 , aircraft  1000  include system  2 F, which includes series propulsion modules  12 A and  12 B, ESS  34 B, and power unit  6 A. In the example of  FIG. 8 , system  2 F further includes recuperator  70  and thermal management system (TMS)  80 . As discussed above, recuperator  70  may place an exhaust air flow that is downstream from a combustor (i.e., a combustor of combustion motor  8 A) in a combustion motor in a heat exchange relationship with a compressed airflow that is upstream from the combustor such that recuperator  70  transfers thermal energy from the exhaust airflow to the compressed airflow. 
     TMS  80  may be configured to manage thermal aspects of system  2 F. For instance, TMS  80  may manage a temperature of a battery of ESS  34 B. In some examples TMS  80  may include one or more fans. In some examples, TMS  80  may be fanless. As shown in  FIG. 8 , TMS  80  may include a heat ejector  82  and a heat exchanger  84 . 
     The following examples may illustrate one or more aspects of the disclosure: 
     Example 1 
     A system comprising: one or more power units configured to output electrical energy onto one or more electrical busses; a plurality of propulsors; and a plurality of electrical machines, each respective electrical machine configured to drive a respective propulsor of the plurality of propulsors using electrical energy received from at least one of the one or more electrical busses. 
     Example 2 
     The system of example 1, further comprising one or more electrical energy storage devices operably coupled to at least one of the one or more electrical busses, wherein the electrical energy storage devices are configured to both: charge using electrical energy sourced via the at least one of the one or more electrical busses; and discharge to provide electrical energy to the at least one of the one or more electrical busses. 
     Example 3 
     The system of example 2, wherein pairs of the propulsors and the electrical machines each comprise respective series propulsion units, the system further comprising: one or more parallel propulsion units. 
     Example 4 
     The system of example 1, wherein the system does not include an energy storage system configured to provide electrical energy to the plurality of electrical machines for driving the propulsors. 
     Example 5 
     The system of example 4, further comprising one or more electrical energy storage devices configured to provide electrical energy to one or more devices other than the plurality of electrical machines. 
     Example 6 
     The system of any combination of examples 1-5, wherein at least one of the plurality of electrical machines is configured to: generate electrical energy using mechanical energy derived from a corresponding propulsor; and output the generated electrical energy onto the one or more electrical busses. 
     Example 7 
     The system of any combination of examples 1-6, wherein the one or more electrical busses comprise direct current (DC) electrical busses. 
     Example 8 
     The system of any combination of examples 1-7, wherein the one or more power units comprise a plurality of power units. 
     Example 9 
     The system of example 8, wherein an amount of electrical energy generated by each of the plurality of power units is independently controllable. 
     Example 10 
     A method of propelling an aircraft, the method comprising: outputting, by one or more power units, electrical energy onto one or more electrical busses; and driving, by each respective electrical machine of a plurality of electrical machines and using electrical energy received from at least one of the electrical busses, a respective propulsor of a plurality of propulsors. 
     Example 11 
     The method of example 10, further comprising: charging, by one or more electrical energy storage systems, using electrical energy sourced via the at least one or more electrical busses; and discharging, by the one or more electrical energy storage systems, to provide electrical energy to the at least one of the one or more electrical busses. 
     Example 12 
     The method of example 11, wherein the discharging to provide electrical energy to the one or more electrical busses, the outputting electrical energy to the one or more electrical busses, and the driving the propulsors using electrical energy received from the electrical busses are performed simultaneously in a dual source mode. 
     Example 13 
     The method of any combination of examples 10-12, wherein the one or more power units comprise at least a first power unit and a second power unit, and wherein outputting the electrical energy comprises: outputting, at a first time and by the first power unit, a first amount of electrical energy via the one or more electrical busses; and outputting, at the first time and by the second power unit, the first amount of electrical energy via the one or more electrical busses. 
     Example 14 
     The method of example 13, wherein outputting the electrical energy comprises: outputting, at a second time and by the first power unit, a second amount of electrical energy via the one or more electrical busses; and outputting, at the second time and by the second power unit, a third amount of electrical energy via the one or more electrical busses, wherein the third amount of electrical energy is different than the second amount of electrical energy. 
     Example 15 
     The method of any combination of examples 10-14, wherein pairs of the propulsors and the electrical machines each comprise respective series propulsion units, the method further comprising: driving, by one or more combustion motors that are not included in the series propulsion units, one or more propulsors that are not included in the series propulsion units. 
     Example 16 
     An airframe comprising: one or more power units configured to output electrical energy onto one or more electrical busses; a plurality of propulsors; and a plurality of electrical machines, each respective electrical machine configured to drive a respective propulsor of the plurality of propulsors using electrical energy received from at least one of the one or more electrical busses. 
     Example 17 
     The airframe of example 16, further comprising one or more electrical energy storage devices operably coupled to at least one of the one or more electrical busses, wherein the electrical energy storage devices are configured to both: charge using electrical energy sourced via the at least one of the one or more electrical busses; and discharge to provide electrical energy to the at least one of the one or more electrical busses. 
     Example 18 
     The airframe of example 17, wherein pairs of the propulsors and the electrical machines each comprise respective series propulsion units, the airframe further comprising: one or more parallel propulsion units. 
     Example 19 
     The airframe of example 16, wherein the airframe does not include an electrical energy storage system configured to provide electrical energy to the plurality of electrical machines for driving the propulsors. 
     Example 20 
     The airframe of any combination of examples 16-19, wherein the one or more power units comprise a plurality of power units. 
     Example 21 
     A system comprising: a plurality of power units configured to output electrical energy onto one or more electrical busses; one or more propulsors; and one or more electrical machines, each respective electrical machine configured to drive a respective propulsor of the one or more propulsors using electrical energy received from at least one of the one or more electrical busses. 
     Example 22 
     The system of example 21, further comprising one or more electrical energy storage devices operably coupled to at least one of the one or more electrical busses, wherein the electrical energy storage devices are configured to both: charge using electrical energy sourced via the at least one of the one or more electrical busses; and discharge to provide electrical energy to the at least one of the one or more electrical busses. 
     Example 23 
     The system of example 22, wherein pairs of the propulsors and the electrical machines each comprise respective series propulsion units, the system further comprising: one or more parallel propulsion units. 
     Example 24 
     The system of example 21, wherein the system does not include an energy storage system configured to provide electrical energy to the plurality of electrical machines for driving the propulsors. 
     Example 25 
     The system of example 24, further comprising one or more electrical energy storage devices configured to provide electrical energy to one or more devices other than the plurality of electrical machines. 
     Example 26 
     The system of any combination of examples 21-25, wherein at least one of the plurality of electrical machines is configured to: generate electrical energy using mechanical energy derived from a corresponding propulsor; and output the generated electrical energy onto the one or more electrical busses. 
     Example 27 
     The system of any combination of examples 21-26, wherein the one or more electrical busses comprise direct current (DC) electrical busses. 
     Example 28 
     The system of any combination of examples 21-27, wherein the one or more electrical machines comprise a plurality of electrical machines, and wherein the one or more propulsors comprise a plurality of propulsors. 
     Example 29 
     The system of any combination of examples 21-28, wherein an amount of electrical energy generated by each of the plurality of power units is independently controllable. 
     Example 30 
     A method of propelling an aircraft, the method comprising: outputting, by a plurality of power units, electrical energy onto one or more electrical busses; and driving, by one or more electrical machines and using electrical energy received from at least one of the electrical busses, one or more propulsors. 
     Example 31 
     The method of example 30, further comprising: charging, by one or more electrical energy storage systems, using electrical energy sourced via the at least one or more electrical busses; and discharging, by the one or more electrical energy storage systems, to provide electrical energy to the at least one of the one or more electrical busses. 
     Example 32 
     The method of example 31, wherein the discharging to provide electrical energy to the one or more electrical busses, the outputting electrical energy to the one or more electrical busses, and the driving the one or more propulsors using electrical energy received from the electrical busses are performed simultaneously in a dual source mode. 
     Example 33 
     The method of any combination of examples 30-32, wherein the plurality of power units comprises at least a first power unit and a second power unit, and wherein outputting the electrical energy comprises: outputting, at a first time and by the first power unit, a first amount of electrical energy via the one or more electrical busses; and outputting, at the first time and by the second power unit, the first amount of electrical energy via the one or more electrical busses. 
     Example 34 
     The method of any combination of examples 30-33, wherein outputting the electrical energy comprises: outputting, at a second time that is different than the first time and by the first power unit, a second amount of electrical energy via the one or more electrical busses; and outputting, at the second time and by the second power unit, a third amount of electrical energy via the one or more electrical busses, wherein the third amount of electrical energy is different than the second amount of electrical energy. 
     Example 35 
     The method of any combination of examples 30-34, wherein pairs of the propulsors and the electrical machines each comprise respective series propulsion units, the method further comprising: driving, by one or more combustion motors that are not included in the series propulsion units, one or more propulsors that are not included in the series propulsion units. 
     Example 36 
     An airframe comprising: a plurality of power units configured to output electrical energy onto one or more electrical busses; one or more propulsors; and one or more electrical machines, each respective electrical machine configured to drive a respective propulsor of the one or more propulsors using electrical energy received from at least one of the one or more electrical busses. 
     Example 37 
     The airframe of example 36, further comprising one or more electrical energy storage devices operably coupled to at least one of the one or more electrical busses, wherein the electrical energy storage devices are configured to both: charge using electrical energy sourced via the at least one of the one or more electrical busses; and discharge to provide electrical energy to the at least one of the one or more electrical busses. 
     Example 38 
     The airframe of example 37, wherein pairs of the propulsors and the electrical machines each comprise respective series propulsion units, the airframe further comprising: one or more parallel propulsion units. 
     Example 39 
     The airframe of example 36, wherein the airframe does not include an electrical energy storage system configured to provide electrical energy to the plurality of electrical machines for driving the propulsors. 
     Example 40 
     The airframe of any combination of examples 36-39, wherein the one or more power units comprise a plurality of power units. 
     Example 41 
     An aircraft propulsion system comprising: one or more parallel propulsion units, each of the parallel propulsion units comprising: a propulsor of a first set of propulsors; a gas turbine engine configured to drive the propulsor; and an electrical machine selectively configurable to: generate, for output via one or more electrical busses, electrical energy using mechanical energy derived from the propulsor or the gas turbine engine; and drive the propulsor of the first set of propulsors using electrical energy received via the one or more electrical busses; and one or more series propulsion units, each of the series propulsion units comprising: a propulsor of a second set of propulsors; and an electrical machine selectively configurable to: generate, for output via the one or more electrical busses, electrical energy using mechanical energy derived from the propulsor or the gas turbine engine; and drive the propulsor of the second set of propulsors using electrical energy received from one or more electrical busses. 
     Example 42 
     The system of example 41, further comprising one or more electrical energy storage devices operably coupled to at least one of the one or more electrical busses, wherein the electrical energy storage devices are configured to both: charge using electrical energy sourced via the at least one of the one or more electrical busses; and discharge to provide electrical energy to the at least one of the one or more electrical busses. 
     Example 43 
     The system of any combination of examples 40-42, further comprising: one or more power units configured to generate and output electrical energy via at least one of the one or more electrical busses. 
     Example 44 
     The system of any combination of examples 40-43, wherein each of the power units, the parallel propulsion units, and the series propulsion units are independently controllable. 
     Example 45 
     The system of any combination of examples 40-44, wherein the one or more parallel propulsion units includes only a single parallel propulsion unit and the one or more series propulsion units include a plurality of series propulsion units. 
     Example 46 
     The system of example 45, wherein the single parallel propulsion unit is positioned on a centerline of the aircraft, and wherein the plurality of series propulsion units are positionally mirrored across the centerline of the aircraft. 
     Example 47 
     A method of propelling an aircraft, the method comprising: driving, by one or more parallel propulsion units of the aircraft, one or more propulsors of a first set of propulsors; outputting, by the one or more parallel propulsion units of the aircraft, electrical energy onto one or more electrical busses; and driving, by one or more series propulsion units of the aircraft and using electrical energy received via the one or more electrical busses, one or more propulsors of a second set of propulsors that is different than the first set of propulsors. 
     Example 48 
     The method of example 47, further comprising: outputting, by one or more power units, electrical energy onto the one or more electrical busses. 
     Example 49 
     The method of example 48, further comprising: operating the aircraft in a dual source mode by at least simultaneously driving the one or more propulsors of the first set of propulsors, outputting electrical energy by the one or more power units, and driving the one or more propulsors of the second set of propulsors. 
     Example 50 
     The method of example 49, further comprising: charging, by an electrical storage system of the aircraft, using electrical energy sourced via the at least one of the one or more electrical busses; and discharging, by the electrical storage system, to provide electrical energy to the at least one of the one or more electrical busses. 
     Example 51 
     The method of example 50, further comprising: operating the aircraft in a triple source mode by at least simultaneously driving the one or more propulsors of the first set of propulsors, outputting electrical energy by the one or more power units, discharging the electrical storage system, and driving the one or more propulsors of the second set of propulsors. 
     Example 52 
     The method of any combination of examples 47-51, further comprising: operating the aircraft in an electric-only mode by at least simultaneously driving the one or more propulsors of the first set of propulsors, driving the one or more propulsors of the second set of propulsors, and causing the parallel propulsion units to refrain from burning fuel. 
     Example 53 
     An aircraft propulsion system comprising: one or more parallel propulsion units, each of the parallel propulsion units comprising: a propulsor of a first set of propulsors; a gas turbine engine configured to drive the propulsor; and an electrical machine selectively configurable to: generate, for output via one or more electrical busses, electrical energy using mechanical energy derived from the propulsor or the gas turbine engine; and drive the propulsor of the first set of propulsors using electrical energy received via the one or more electrical busses; and one or more series propulsion units, each of the series propulsion units comprising: a propulsor of a second set of propulsors; and an electrical machine selectively configurable to: generate, for output via the one or more electrical busses, electrical energy using mechanical energy derived from the propulsor or the gas turbine engine; and drive the propulsor of the second set of propulsors using electrical energy received from one or more electrical busses. 
     Example 54 
     The system of example 53, further comprising one or more electrical energy storage devices operably coupled to at least one of the one or more electrical busses, wherein the electrical energy storage devices are configured to both: charge using electrical energy sourced via the at least one of the one or more electrical busses; and discharge to provide electrical energy to the at least one of the one or more electrical busses. 
     Example 55 
     The system of any combination of examples 53-54, further comprising: one or more power units configured to generate and output electrical energy via at least one of the one or more electrical busses. 
     Example 56 
     The system of any combination of examples 53-55, further comprising: one or more controllers configured to operate the aircraft in a dual source mode by at least simultaneously causing the parallel propulsion units to drive the first set of propulsors using fuel, causing the power units to output electrical energy via the one or more electrical busses, causing the electrical storage system to discharge to output electrical energy via the one or more electrical busses, and causing the series propulsion units to drive the second set of propulsors using electrical energy received via the one or more electrical busses. 
     Example 57 
     The system of any combination of examples 53-56, further comprising: one or more controllers configured to operate the aircraft in a dual source electric-only mode by at least simultaneously causing the parallel propulsion units to drive the first set of propulsors using electrical energy received via the one or more electrical busses without the gas turbine engines using fuel, causing the power units to output electrical energy via the one or more electrical busses, causing the electrical storage system to discharge to output electrical energy via the one or more electrical busses, and causing the series propulsion units to drive the second set of propulsors using electrical energy received via the one or more electrical busses. 
     Example 58 
     The system of any combination of examples 53-57, wherein each of the power units, the parallel propulsion units, and the series propulsion units are independently controllable. 
     Example 59 
     The system of any combination of examples 53-58, wherein the one or more parallel propulsion units includes only a single parallel propulsion unit and the one or more series propulsion units include a plurality of series propulsion units. 
     Example 60 
     The system of example 59, wherein the single parallel propulsion unit is positioned on a centerline of the aircraft, and wherein the plurality of series propulsion units are positionally mirrored across the centerline of the aircraft. 
     Example 61 
     An aircraft propulsion system comprising: a plurality of electrical busses comprising a propulsion bus, a critical bus, and a non-critical bus; an electrical energy storage system coupled to each of the plurality of electrical busses; one or more power units configured to generate and output electrical energy via the propulsion bus; one or more electrical machines configured to drive respective propulsors using electrical energy received via the propulsion bus; one or more hotel loads configured to receive energy via the non-critical bus; and one or more critical loads configured to receive energy via the critical bus. 
     Example 62 
     The system of example 61, wherein at least one of the electrical machines is included in a parallel propulsion module. 
     Example 63 
     The system of any combination of examples 61-62, wherein at least one of the electrical machines is included in a series propulsion module. 
     Example 64 
     The system of any combination of examples 61-63, wherein the electrical energy storage system includes an interface for receiving electrical energy from an electrical energy source external to the aircraft. 
     Example 65 
     The system of example 64, wherein the interface is further configured to provide electrical energy to a load external to the aircraft. 
     Example 66 
     The system of any combination of examples 61-65, wherein the propulsion bus comprises a relatively high voltage direct current (DC) bus, and the critical and non-critical busses comprise relatively low voltage DC busses. 
     Example 67 
     The system of any combination of examples 61-66, wherein the propulsion bus includes a propulsion power panel. 
     Example 68 
     A method comprising: outputting, by one or more power units, electrical energy via a propulsion electrical bus; driving, by one or more electrical machines, respective propulsors using electrical energy received via the propulsion electrical bus; outputting, by an electrical energy storage system, electrical energy via a non-critical electrical bus and a critical electrical bus; receiving, by one or more hotel loads, electrical energy via the non-critical electrical bus; and receiving, by one or more critical loads, electrical energy via the critical electrical bus. 
     Example 69 
     The method of example 68, further comprising: outputting, by the electrical energy storage system, electrical energy via the propulsion electrical bus. 
     Example 70 
     The method of example 69, further comprising: charging, by the electrical energy storage system, using electrical energy received via the propulsion electrical bus. 
     Example 71 
     The method of any combination of examples 68-70, further comprising: outputting, by one or more parallel propulsion units, electrical energy via the propulsion electrical bus. 
     Example 72 
     The method of example 71, wherein driving the propulsors comprises: driving, by electrical machines included in the parallel propulsion units, propulsors of the parallel propulsion units using electrical energy received via the propulsion electrical bus. 
     Example 73 
     An airframe comprising: a plurality of electrical busses comprising a propulsion bus, a critical bus, and a non-critical bus; an electrical energy storage system coupled to each of the plurality of electrical busses; one or more power units configured to generate and output electrical energy via the propulsion bus; one or more electrical machines configured to drive respective propulsors using electrical energy received via the propulsion bus; one or more hotel loads configured to receive energy via the non-critical bus; and one or more critical loads configured to receive energy via the critical bus. 
     Example 74 
     The airframe of example 73, wherein at least one of the electrical machines is included in a parallel propulsion module. 
     Example 75 
     The airframe of any combination of examples 73-74, wherein at least one of the electrical machines is included in a series propulsion module. 
     Example 76 
     The airframe of any combination of examples 73-75, wherein the electrical energy storage system includes an interface for receiving electrical energy from an electrical energy source external to the airframe. 
     Example 77 
     The airframe of any combination of examples 73-76, wherein the interface is further configured to provide electrical energy to a load external to the airframe. 
     Example 78 
     The airframe of any combination of examples 73-77, wherein the propulsion bus comprises a relatively high voltage direct current (DC) bus, and the critical and non-critical busses comprise relatively low voltage DC busses. 
     Example 79 
     The airframe of any combination of examples 73-78, wherein the propulsion bus comprises a plurality of redundant propulsion busses. 
     Example 80 
     The airframe of any combination of examples 73-79, further comprising: a recuperator for at least one of the power units. 
     Example 81 
     A system or airframe comprising any combination of examples 1-9, 16-20, 21-29, 36-40, 41-46, 53-60, 61-67, and 73-80. 
     Example 82 
     A method comprising any combination of examples 10-15, 30-35, 47-52, and 68-72. 
     Example 83 
     A computer-readable storage medium storing instructions that, when executed, cause one or more controllers to perform the method of any combination of examples 10-15, 30-35, 47-52, and 68-72. 
     Various examples have been described. These and other examples are within the scope of the following claims.