Patent Publication Number: US-2023139068-A1

Title: Electrical machines for integration into a propulsion engine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to, and is a continuation application of, U.S. patent application Ser. No. 17/170,229 filed Feb. 8, 2021, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Field 
     The present specification generally relates to electrical machines for incorporation into gas turbine engines. 
     Technical Background 
     Incorporating an electrical machine (e.g., an electrical generator) into a propulsion engine to generate electrical power from mechanical energy generated by the propulsion engine may enhance the capabilities of aircraft by eliminating the need for heavy and bulky energy storage devices on the aircraft. For example, the electrical power generated by the electrical machine may be used to operate an accessory propulsor (e.g., an electric fan, motor, or the like) to supplement thrust provided via the turbine engine. Introduction of such an electrical machine, however, may introduce challenges relating to size, weight, accessibility, and aerodynamic performance. 
     SUMMARY 
     An electrical machine includes a stator assembly coupled to an engine stator component of a propulsion engine. The stator assembly includes a stator support assembly fixedly attached to the engine stator component and a stator disposed on a supporting surface of the stator support structure. The electrical machine also includes a rotor assembly including a rotor support structure connected to a shaft of the propulsion engine and a rotor attached to the rotor support structure such that the rotor is disposed radially inward of the stator. The rotor exchanges rotational energy with the shaft to operate as either an electrical motor or an electrical generator. 
     In another embodiment, an electrical machine includes a stator assembly coupled to an engine stator component of a propulsion engine. The stator assembly includes a stator support assembly fixedly attached to the engine stator component and a stator disposed on a supporting surface of the stator support structure. The electrical machine also includes a rotor assembly comprising a rotor support structure directly connected to a shaft of the propulsion engine and a rotor attached to the rotor support structure. The rotor is disposed radially inward of the stator such that the stator assembly circumferentially surrounds the rotor. In embodiments, the rotor rotates in conjunction with the shaft to generate a power signal. In embodiments, the electrical machine receives power from an external source to provide rotational energy to the shaft. 
     In another embodiment, an electrical machine includes a stator assembly coupled to an engine stator component of a propulsion engine. The stator assembly includes a stator support assembly fixedly attached to the engine stator component. A stator disposed on a supporting surface of the stator support structure. The electrical machine also includes an electrical machine shaft coupled to an end of a shaft of the propulsion engine via an intermediate shaft member extending axially between the end of the shaft and the electrical machine shaft. The electrical machine also includes a bearing support frame extending from the propulsion engine, the bearing support frame including an axial portion extending in an axial direction. The electrical machine also includes electrical machine bearings radially extending from the axial portion of the bearing support frame to rotatably contact the electrical machine shaft. The electrical machine also includes a sealing member disposed axially aft of the electrical machine bearings, the sealing member extending from the axial portion of the bearing support frame to the electrical machine shaft. The electrical machine also includes a rotor assembly including a rotor support structure connected to the electrical machine shaft and a rotor attached to the rotor support structure such that the rotor is disposed radially inward of the stator. In embodiments, the rotor rotates in conjunction with the shaft of the propulsion engine via the intermediate shaft member to generate a power signal. In embodiments, the electrical machine receives power from an external source to provide rotational energy to the shaft. 
     In another embodiment, an electrical machine includes a stator assembly coupled to an engine stator component of a propulsion engine. The stator assembly includes a stator support assembly fixedly attached to the engine stator component and a stator disposed on a supporting surface of the stator support structure. The electrical machine also includes an electrical machine shaft coupled to an end of a shaft of the propulsion engine via an intermediate shaft member extending axially between the end of the shaft and the electrical machine shaft. The electrical machine also includes a bearing support frame extending from the propulsion engine, the bearing support frame defining a bearing cavity in conjunction with the electrical machine shaft. The electrical machine also includes first and second electrical machine bearings radially extending from the bearing support frame to rotatably contact the electrical machine shaft. The electrical machine also includes a sealing member disposed axially aft of the electrical machine bearings, the sealing member extending from the bearing support frame to the electrical machine shaft. The electrical machine also includes a rotor support structure connected to the electrical machine shaft and a rotor attached to the rotor support structure. The rotor rotates in conjunction with the electrical machine shaft to exchange energy with the shaft of the propulsion engine. 
     In another embodiment, a propulsion engine includes a core portion generating exhaust that travels in an axial direction and a turbine section coupled to a shaft. The turbine section receives the exhaust and generates mechanical energy to rotate the shaft. The propulsion engine also includes a turbine frame attached to the turbine section, and the turbine frame includes an outer casing coupled to the turbine section and an inner hub supporting the shaft via a bearing assembly comprising an engine bearing supporting the shaft. The propulsion engine also includes an electrical machine including a stator assembly comprising a stator support assembly attached to the inner hub and a stator attached to the stator support structure; an electrical machine shaft coupled to an end of the shaft via an intermediate shaft member extending axially between the end of the shaft and the electrical machine shaft; a bearing support frame attached to the inner hub and extending radially inward therefrom to define a bearing cavity extending between the bearing support frame and electrical machine shaft; electrical machine bearings radially extending from the bearing support frame to rotatably contact the electrical machine shaft; and a rotor assembly. The rotor assembly includes a rotor support structure connected to the electrical machine shaft and a rotor attached to the rotor support structure and extending radially inward of the stator. The rotor rotates in conjunction with the shaft via the intermediate shaft member to exchange energy with the shaft. 
     Additional features, advantages, and embodiments of the processes and systems described herein will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that such features, advantages, and embodiments are contemplated and considered within the scope of the disclosure, based on the teachings disclosed hereupon. 
     It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the subject matter claimed and described herein. The accompanying drawings are provided to facilitate a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the subject matter claimed and described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments set forth in the drawings are illustrative and exemplary in nature and not intended to limit the subject matter defined by the claims. The following detailed description of the illustrative embodiments can be understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
         FIG.  1    depicts a cross-sectional view of the propulsion engine depicted in  FIG.  1    above a central axis A-A thereof, according to one or more embodiments described herein; 
         FIG.  2    depicts an enlarged view of an electrical machine of the propulsion engine depicted in  FIG.  1    according to one or more embodiments described herein; 
         FIG.  3    schematically depicts a sectional view of an electrical machine that may be incorporated into the propulsion engine depicted in  FIG.  1   , according to one or more embodiments described herein; 
         FIG.  4    schematically depicts a sectional view of an electrical machine that may be incorporated into the propulsion engine depicted in  FIG.  1    according to one or more embodiments described herein; and 
         FIG.  5    schematically depicts a sectional view of an electrical machine that may be incorporated into the propulsion engine depicted in  FIG.  1   , according to one or more embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to electrical machines for integration into a propulsion engine such as a turbine engine. The electrical machines described herein may be disposed at various axial locations within the propulsion engine (e.g., at an aft end of the propulsion engine, between ends of a shaft a propulsion engine) to facilitate an exchange of rotational energy between the shaft and the electrical machine. For example, the electrical machines described herein may include a stator assembly coupled to an engine stator component and a rotor assembly that is connected with the shaft of the propulsion engine such that a rotor of the rotor assembly rotates in conjunction with the shaft to facilitate an exchange of rotational energy between the rotor assembly and the shaft. The electrical machines described herein may be operated in a generator mode, in which rotational energy from the shaft generates a power signal in a stator that is provided by an electrical connector to other components of the propulsion engine or aircraft; and a motor mode, in which electrical power is provided to the electrical machine from an external source (e.g., a power storage device disposed on the aircraft) such that the rotor alters the rotational speed of the shaft. 
     In embodiments, the rotor assembly may be directly or indirectly attached to the shaft of the propulsion engine using different attachment structures that may have differing effects on the vibration of the shaft. For example, in embodiments, the rotor assembly includes a rotor support structure directly connected to the shaft of the propulsion engine such that the rotor assembly is supported by a bearing assembly already incorporated into the propulsion engine. In such embodiments, the rotor support assembly may be designed to have a desired effect on the natural vibration frequencies of the shaft of the propulsion engine. For example, in embodiments, the structure of the rotor support structure is designed to alter a vibration frequency of the shaft. In embodiments, the electrical load of the electrical machine (e.g., via switchable coil connections in the rotor and stator) may be modulated to controllably dampen vibrations of the shaft of the propulsion engine to improve the long-term operability thereof. 
     In embodiments, the rotor assembly is indirectly attached to the shaft of the propulsion engine via an electrical machine shaft rotatably coupled to the shaft of the propulsion engine with an intermediate shaft member allowing axial and radial displacements of the electrical machine shaft. The electrical machine in such embodiments may be supported by its own generator bearings to protect the electrical machine from vibrations of the shaft of the propulsion engine. Such separate shaft and bearing embodiments may also be beneficial in that the intermediate shaft member in that the intermediate shaft member may enable decoupling of the electrical machine shaft from the engine shaft to protect the engine from the electrical machine in the event of a malfunction. 
     In embodiments, the electrical machine may be positioned in an aft portion of the propulsion engine (e.g., proximate to a turbine rear frame). Such positioning beneficially renders the electrical machine accessible for maintenance and repair while the propulsion engine and electrical machine are installed an aircraft (e.g., on a wing, on a fuselage, or the like). Additionally, in embodiments, the electrical machine may be positioned within a tail cone of the propulsion engine and be directly accessible for repairs after performance of a non-invasive procedure (e.g., opening of core cowl, removal of aft skin, and removal tail cone) on the remainder of the propulsion engine. This way, the electrical machine may be efficiently maintained while not disrupting operation of the other components of the propulsion engine. Additionally, various components of the electrical machine (e.g., the rotor assembly and the stator assembly) may be designed to allow independent removal thereof from the propulsion engine. Such non-invasive access to the electrical machine beneficially facilitates maintenance and repair of the electrical machine while the propulsion engine is installed on an aircraft (e.g., on a wing, a fuselage, or the like of the aircraft). 
     Referring to  FIG.  1   , a propulsion engine  100  is schematically depicted. The propulsion engine  100  may take various forms depending on the implementation. In the embodiments described herein, the propulsion engine  100  is a high-bypass turbofan engine. However, other types of turbine engines are contemplated and are within the scope of the present disclosure. As depicted in  FIG.  1   , the propulsion engine  100  includes an electrical machine  300  disposed in an aft portion  104  of the propulsion engine  100 . The aft portion  104  is disposed axially downstream (e.g., in a direction parallel to a central axis A-A of the propulsion engine  100 ) of a core portion  220  of the propulsion engine  100 . In embodiments, the electrical machine  300  converts mechanical energy (e.g., generated from exhaust gases generated in the core portion  220 ) produced by the propulsion engine  100  into electrical energy that may be used to power electrical devices of the propulsion engine  100  or components disposed elsewhere on an aircraft (including components that incorporate the propulsion engine  100 ). As described herein, positioning the electrical machine  300  in the aft portion  104  of the propulsion engine  100  beneficially renders the electrical machine  300  accessible for maintenance, repair, and replacement while the propulsion engine  100  is disposed on an aircraft (e.g., on a wing or fuselage of the aircraft). The electrical machine  300  is designed to be integrated into the propulsion engine  100  via a set of connections that may be removed without invasively disassembling the entirety of the propulsion engine  100  (e.g., removing without detaching the propulsion engine  100  from the aircraft). 
     In embodiments, the electrical machine  300  may be attached to an inner hub  226  of a turbine rear frame  222  of the propulsion engine  100 . Example embodiments of the structure of the electrical machine  300  are described in greater detail herein. Still referring to  FIG.  1   , it should be understood that the depicted arrangement of the propulsion engine  100  is only exemplary is not intended to be limiting. For example, in alternative embodiments, the electrical machine  300  may be disposed axially forward of the core portion  220 . 
     Positioning the electrical machine  300  in the aft portion  104  provides accessibility, but creates additional design considerations for the propulsion engine  100 . Exhaust gases generated via the core portion  220  are at relatively high temperatures (e.g., in excess of about 700° C. or more in various embodiments), which renders cooling the electrical machine  300  beneficial. Additionally, the aft portion  104  of the propulsion engine  100  may not be directly connected to an aircraft incorporating the propulsion engine  100 . Given this, electrical signals routed to and from the electrical machine  300  are routed through the propulsion engine  100 . In embodiments, for example, the propulsion engine  100  includes an electrical system (not depicted in  FIG.  1   ) including a plurality of electrical lines that connect the electrical machine  300  to an electrical machine control unit (not depicted in  FIG.  1   ). In embodiments, the electrical lines are disposed within cooling ducts that provide coolant (e.g., from a bypass section disposed radially outward from the core portion  220 ) to the aft portion  104 . In embodiments, the electrical lines are disposed externally to the cooling ducts. 
     In embodiments, electrical machine control unit converts the power signal generated by the electrical machine  300  (between an alternating current signal and direct current signal, or vice versa) for provision to additional components of the propulsion engine  100  or incorporating aircraft. In embodiments, as described in greater detail herein, the electrical machine control unit may also provide control signals to the electrical machine  300  to change the mode of operation thereof (e.g., between an electrical generator mode and an electrical motor mode) and/or the load thereof to change the rotational energy exchange between the electrical machine  300  and additional components of the propulsion engine  100 . In embodiments, the electrical machine control unit may be disposed in a location within the propulsion engine  100  that is displaced from the electrical machine  300  (e.g., axially forward of the core portion  220 ), or elsewhere on the aircraft (e.g., in a pylon). 
     Referring still to  FIG.  1   , the propulsion engine  100  includes a fan  201 , a low pressure compressor  202 , a high pressure compressor  204 , and a combustor  206 , which mixes air compressed via the high pressure compressor  204  with fuel for generating combustion gases that flow downstream through a high pressure turbine  208  and a low pressure turbine  210  to generate pressurized exhaust. A first shaft  212  joins the high pressure compressor  204  to the high pressure turbine  208 . A second shaft  216  joins the low pressure turbine  210  to the fan  201  and the low pressure compressor  202 . In embodiments, the high pressure compressor  204 , the combustor  206 , and the high pressure turbine  208  may collectively form the core portion  220 . The core portion  220  may generate combustion gases that are channeled to the low pressure turbine  210 , which in turn powers the fan  201  via the second shaft  216 . The low pressure turbine  210  may include a plurality of rows of blades that rotate in response to the combustion gases from the core portion  220  and thereby cause the second shaft  216  to rotate, thereby powering the fan  201 , low pressure compressor  202 , and the electrical machine  300 . 
     The turbine rear frame  222  is disposed aft of the low pressure turbine  210  (e.g., offset from the low pressure turbine  210  in an aft direction (e.g., an axial direction  272 ) extending parallel to the central axis A-A). The turbine rear frame  222  includes a plurality of struts  224  extending between an inner hub  226  and an outer casing  228 . The turbine rear frame  222  provides an exhaust flow path for exhaust flowing from the low pressure turbine  210 . The inner hub  226  and the outer casing  228  may circumferentially surround the second shaft  216  and the plurality of struts  224  may be distributed around the second shaft  216 . In embodiments, the plurality of struts  224  function as outlet guide vanes to straighten the exhaust airflow, which may flow over a tail cone  230  to improve performance of the propulsion engine  100 . It should be understood that the turbine rear frame  222  may include any number of struts  224  in any arrangement consistent with the present disclosure. 
     Referring still to  FIG.  1   , the propulsion engine  100  includes a core cowl  250  and an aft skin  252 . The core cowl  250  delineates a flow path for air compressed by the fan  201 . In embodiments, the aft skin  252  is connected to the outer casing of the turbine rear frame  222  via a bolted connection (not depicted). In embodiments, exhaust exits the propulsion engine  100  via an outlet  254  defined by the turbine rear frame  222 , the aft skin  252 , and the tail cone  230 . 
     Referring now to  FIG.  2   , a detailed view of a sectional portion of the electrical machine  300  depicted in the dashed boundary of  FIG.  1    is shown. In the depicted embodiment, the electrical machine  300  includes a stator assembly  302  and a rotor assembly  304 . The stator assembly  302  is directly connected to the propulsion engine  100  via a first engine stator component  260  and a second engine stator component  270 . The first and second engine stator components  260  and  270  may vary depending on the particular location at which the electrical machine  300  is disposed within the propulsion engine  100 . For example, as described herein with respect to  FIG.  1   , the electrical machine  300  is disposed in the aft portion  104  of the propulsion engine  100 . In such embodiments, the first engine stator component  260  may be a flow-defining structure such as the turbine rear frame  222  depicted in  FIG.  1    (e.g., the stator assembly  302  may be attached to the inner hub  226 ) and the second engine stator component  270  may be another component extending radially between the second shaft  216  and the inner hub  226  (e.g., a turbine component such as a bearing support structure or the like). Alternative locations of the electrical machine  300  are contemplated and within the scope of the present disclosure. For example, in embodiments, the electrical machine  300  is disposed between the ends of the second shaft  216  (e.g., axially forward of an aft end  217  of the second shaft  216  and the turbine rear frame  222 ). In such embodiments, the first engine stator component  260  may be another flow path-defining structure (e.g., mid turbine frame or the like). Various points of connection between the electrical machine  300  and the propulsion engine  100  are contemplated and within the scope of the present disclosure. 
     The stator assembly  302  includes a stator  314  and the rotor assembly  304  includes a rotor  346 . In the depicted embodiment, the rotor assembly  304  is disposed radially inward of the stator assembly  302  (e.g., the entire rotor assembly  304  is disposed more proximate to the second shaft  216  than the stator assembly  302 ). In embodiments, the stator assembly  302  circumferentially surrounds the rotor assembly  304 , such that the rotor assembly  304  is disposed radially between the stator assembly  302  and the second shaft  216  of the propulsion engine  100 . In embodiments, the electrical machine  300  is operated as an electrical generator converting rotational energy of the second shaft  216  (e.g., during flight operation of the propulsion engine  100 ) into electrical energy that may be conveyed to other components of an aircraft. In embodiments, the electrical machine  300  is operated as a motor to provide torque to the second shaft  216  (e.g., to increase operational efficiency of the propulsion engine  100 ). 
     The inner-rotor construction of the electrical machine  300  facilitates compactness (e.g., aerodynamic performance) and operability thereof. For example, the inner-rotor construction of the electrical machine  300  may facilitate long-term operability thereof over embodiments where the stator assembly  302  is disposed radially inward of the rotor assembly  304  by reducing rotational loads imparted on the structural components supporting the rotor  346  during operation of the electrical machine  300 . Additionally, the inner-rotor construction of the electrical machine  300  depicted in  FIG.  2    may facilitate the stator assembly  302  acting as a shield to prevent damaged rotor components from traveling in a radially outward direction and impacting additional components of the propulsion engine  100 . For example, if the electrical machine  300  malfunctions, a piece of the rotor assembly  304  may break and become disconnected from the second shaft  216 . Such a broken component may disrupt operation of the propulsion engine  100  if left unimpeded. Due to the inner-rotor construction of the electrical machine  300 , however, such broken rotor components are contained within a rotor cavity  370  delineated by the stator assembly  302 . Containment of debris by the stator assembly  302  reduces the likelihood of electrical machine malfunction affecting operation of other components of the propulsion engine  100  or high energy components being released from the propulsion engine  100 . Embodiments with an outer-rotor construction are contemplated and within the scope of the present disclosure, but may include a debris shield disposed radially outward of the rotor assembly  304 . 
     In the embodiment depicted in  FIG.  2   , the electrical machine  300  is directly connected to the second shaft  216  via a rotor support structure  338  of the rotor assembly  304 . In embodiments, the rotor support structure  338  includes an attachment element (e.g., a groove, protrusion, or the like, which is not depicted) that slidably engages with a corresponding attachment element on the second shaft  216 . For example, in embodiments, the rotor support structure  338  may slidably engage with the second shaft  216  at an aft end  217  of the second shaft  216 . A locking nut (not depicted) may secure the rotor support structure  338  to the second shaft  216  such that the rotor assembly  304  rotates in conjunction with the second shaft  216  to facilitate generation of electrical power via the rotation of the second shaft  216 . 
     Such embodiments where the rotor assembly  304  is attached directly to the second shaft  216  may be referred to herein as “embedded generator embodiments.” In embedded generator embodiments, the rotor assembly  304  may be supported at least in part by a bearing assembly supporting the second shaft  216  (e.g., an engine bearing assembly—not depicted—supporting the aft portion  104  of the propulsion engine  100 , disposed axially proximate to the turbine rear frame  222 ). Embedded generator embodiments may be beneficial in that the electrical machine  300  may function as a shaft natural frequency damper. In embodiments, components of the rotor assembly  304  (e.g., the rotor support structure  338 ) may be structurally designed to modify a natural frequency of the second shaft  216  (e.g., to alter the natural vibrational frequency of the second shaft  216  as compared to embodiments not incorporating the electrical machine  300 ). For example, in embodiments, the stiffness and volume of the rotor support structure  338  may be selected to remove the natural vibration frequency of the second shaft  216  from a range where vibration of the second shaft  216  is likely to excite vibrational modes of other components of the propulsion engine  100 , thereby avoiding structural integrity issues associated with high amplitude oscillations. 
     In embodiments, the electrical machine  300  may be designed to variably impact the natural vibration frequencies of the second shaft  216  via external control thereof. In the depicted embodiment, for example, the electrical machine  300  is communicably coupled (e.g., via electrical lines  336  and  342  and an electrical connection device  328 , described herein) to an electrical machine control unit  380 . The electrical machine control unit  380  may control the electrical machine  300  based on power demand by altering the electrical load. In embodiments, the electrical machine control unit  380  is disposed in an axially different location of the propulsion engine  100  than the electrical machine  300 . In embodiments, the electrical machine control unit  380  is disposed at the same axial location as the electrical machine  100  within the propulsion engine. 
     In embodiments, for example, the electrical machine control unit  380  receives instructions from another component (e.g., an engine power control unit) associated with the aircraft to modulate the electrical machine load to alter the rotational energy extracted from the second shaft  216  via rotation of the rotor  346 , thereby controllably dampening vibrations of the second shaft  216 . For example, the electrical machine control unit  380  may alter the electrical load of the electrical machine  300  (e.g., by switching electrical connections between coils thereof off and on) in response to vibrations of the second shaft  216  being detected in order to dampen the detected vibrations. Such electrical load control by the electrical machine control unit  380  may occur in both a generator working mode and a motor working mode of the electrical machine  300 . 
     Referring still to  FIG.  2   , the rotor assembly  304  further includes a rotor attachment arm  340  extending axially forward from an end of the rotor support structure  338 . The rotor attachment arm  340  maintains the rotor  346  in spaced relation to the stator  314  and second shaft  216 . In embodiments, the rotor  346  comprises a plurality of permanent magnets circumferentially distributed about the stator  314  such that rotation of the rotor  346  about the stator  314  generates an AC power signal. It should be understood that alternative configurations for the rotor  346  are envisioned depending on the implementation of the electrical machine  300 . For example, in embodiments, the rotor  346  may include a plurality of electromagnets and active circuitry. Various implementations are envisioned wherein the electrical machine  300  is configured as an induction type generator, a switched reluctance generator, an asynchronous AC electrical machine, or any suitable type of electric generator. 
     In embodiments, the stator assembly  302  circumferentially surrounds the rotor assembly  304 . In embodiments, the stator assembly  302  (e.g., the stator support assembly  308 , described herein) includes a plurality of circumferential segments that can each be individually detached from the propulsion engine  100  to facilitate removal thereof. Such embodiments incorporating a plurality of circumferential segments may be particularly beneficial in embodiments where the electrical machine is centrally disposed within the propulsion engine  100  (e.g., away from the ends of the second shaft  216 ), as accessing the electrical machine  300  for maintenance or replacement may be more time consuming in such embodiments. 
     In the depicted embodiment, the stator assembly  302  is attached to the first and second engine stator components  260  and  270  via first and second connection bolts  327  and  329 , respectively. The stator assembly  302  includes a stator support assembly  308  holding the stator  314  in a desired position relative to the rotor  346 . The stator support assembly  308  includes a stator support arm  312 . The stator support arm  312  extends in the axial direction  272  (e.g., parallel to the second shaft  216 ) and defines a stator support surface  313  where the stator  314  is attached to the stator support assembly  308 . 
     In embodiments, the stator support arm  312  extends over the entirety of the rotor  346  in the axial direction  272  to define the rotor cavity  370  extending between the stator support arm  312  and the second shaft  216 . In embodiments, the stator support arm  312  comprises a length in the axial direction  272  that is greater than that of the rotor  346 . In addition to providing structural support to the stator  314 , the stator support arm  312  contributes to containment of any debris associated with the rotor assembly  304  (e.g., in conjunction with a debris shield disposed radially outward of the stator support arm  302 ), thereby preventing release of high energy components outside of the propulsion engine  100 . That is, the stator support arm  312  may act as a shield to prevent malfunctioning of the rotor assembly  304  from disrupting the operation of other components of the propulsion engine  100  or to prevent the rotor assembly  304  from emitting high energy debris outside of the propulsion engine  100 . In embodiments, the stator support arm  312  is the sole containment mechanism of the propulsion engine  100  for containing such debris from the rotor assembly  304 . That is, the inner-rotor construction of the electrical machine  300  may eliminate the need for debris shields surrounding the electrical machine  300 . 
     In embodiments, the stator support arm  312  comprises a substantially cylindrical structure surrounding the second shaft  216 . In embodiments, the substantially cylindrical structure is an integrated, continuous body. In embodiments, the stator support arm  312  comprises a plurality of circumferential segments, with each of the plurality of circumferential segments being connected to one another in order to facilitate individual removal of each circumferential segment radially away from the second shaft  216 . In embodiments, the plurality of circumferential segments are non-continuous circumferentially. That is, in such embodiments, the stator support arm  312  may comprise gaps around a circumference thereof. 
     In embodiments, electrical and fluid connections of the electrical machine  300  are facilitated through the structure of the stator support assembly  308 . In the embodiment depicted in  FIG.  2   , for example, the electrical machine  300  includes a connector support  316  extending radially between the first engine stator component  260  and the stator support arm  312 . In embodiments, the connector support  316  includes at least one opening  318  for supporting an electrical connection device  328 . It should be understood that embodiments are also envisioned where the electrical machine  300  does not include the connector support  316  or where the connector support  316  is disposed in a different location than that depicted in  FIG.  2   . The electrical connection device  328  may conductively connect an electrical line  336  from the stator  314  to an external electrical line  342 . In embodiments, the connector support  316  functions as a holder of the electrical connection device  328  to facilitate electrically connecting the electrical machine  300  to other components of the propulsion engine  100 . In embodiments, the connector support  316  comprises a plurality of openings  318  that are distributed around the circumference of the stator support assembly  308 . An electrical connector may extend through each one of the plurality of openings to facilitate provision of electrical signals generated by the electrical machine  300  to external components. It should be understood that alternative locations are envisioned for the electrical connection device  328 . That is, the electrical connection device  328  may be disposed along the electrical lines  336  and  342  at alternative locations than that depicted in  FIG.  2    (e.g., axially forward of the electrical machine  300 ). In such embodiments with alternative positioning of the electrical connection device  328 , the electrical machine  300  may not include the connector support  316 . 
     In embodiments, the electrical machine  300  includes a cooling system  350  distributing coolant to various portions of the electrical machine  300 . In the depicted embodiment, the cooling system  350  includes an inlet manifold  352  and a stator manifold  354 . The inlet manifold  352  receives coolant air from portions of the propulsion engine  100  that are external to the electrical machine  300 . In embodiments, the inlet manifold  352  is a portion of or connected to another cooling duct of the propulsion engine  100 . In embodiments, the inlet manifold  352  may be routed through the first engine stator component  260  (e.g., through one of the struts  224  of the turbine rear frame  222  depicted in  FIG.  1   ). The stator manifold  354  provides coolant to the stator  314  to maintain a temperature thereof within a suitable operating range. In embodiments, one or more of the electrical lines and the electrical connection device is disposed within the cooling system  350 . For example, in the depicted embodiment, the electrical lines  336  and  342  extend through the stator manifold  354  and the inlet manifold  356 , respectively, and are conductively connected within the stator manifold  354  via the electrical connection device  328 . Embodiments are also envisioned where electrical lines are disposed outside of the cooling system  350 . In embodiments, the stator support arm  312  comprises holes or openings to lead to cooling manifolds other than those depicted in  FIG.  2   . 
     In the embodiment depicted in  FIG.  2   , electrical and fluid connections via the cooling system  350  are disposed at an aft end of the stator  314  (e.g., via the connecting portion  358  of the stator manifold  354 ). It should be appreciated that alternative embodiments are contemplated and within the scope of the present disclosure. For example, in embodiments, the electrical line  336  may extend from an axially forward end of the stator  314 , and the stator manifold  354  may extend through the stator support arm  312  to facilitate attachment and connection to the axially forward end of the stator (e.g., the stator manifold  354  may not include the connecting portion  358  including a bend, as in the depicted embodiment). In such embodiments, the electrical connection device  328  may also be disposed axially forward of the stator  314 . For example, the electrical connection device  328  may be supported within or externally to a portion of the stator manifold  354  extending through the stator support arm  312  to facilitate electrical connection with the external electrical line  342 . Such embodiments may not include the connector support  316  (the depicted embodiments may also not include the connector support  316 ). Various combinations of electrical connection and fluid coupling structures are contemplated and within the scope of the present disclosure. 
     In embodiments, the electrical machine  300  comprises a thermal shield  348 . In embodiments, the thermal shield  348  does not directly attach to the second shaft  216  but rather circumferentially surrounds the aft end  217  of the second shaft  216 , the stator assembly  302 , and the rotor assembly  304 . In embodiments, the thermal shield  348  is attached to the first engine stator component  260 . For example, in embodiments, the thermal shield  348  is attached to the connection flange  310  of the first engine stator component  260  via the first connection bolt  327 . In embodiments, the thermal shield  348  comprises at least two components. In embodiments, for example, the thermal shield  348  comprises a stator portion circumferentially surrounding the stator assembly  302  and a rotor portion extending axially aft of the rotor  346 . In embodiments, the separate portions of the thermal shield  348  may be separately removed from the propulsion engine  100  to facilitate access to the rotor assembly  304  without disruption the connections of the stator assembly  302 . 
     The view depicted in  FIG.  2    corresponds to a single circumferential section of the electrical machine  300 . As such, it should be understood that the electrical machine  300  may include any number of the components depicted in  FIG.  2    distributed around the circumference of the second shaft  216 . The components depicted in  FIG.  2    may also be split axially. That is, each component depicted in  FIG.  2    is a continuous section (e.g., the stator support arm  312 ) may be split into a plurality of segments extending in the axial direction  272  that extend from one another. In embodiments, the electrical machine  300  includes a plurality of cooling systems that are similar to the cooling system  350  depicted in  FIG.  2    (e.g., having a plurality of manifolds, electrical connectors, and electrical lines extending therethrough) distributed around the circumference thereof. Moreover, the components of the electrical machine  300  (e.g., the stator support assembly  308 , thermal shield  348 , etc.) may be connected to the propulsion engine  100  at any number of points along the circumference thereof. That is, the electrical machine  300  may include a plurality of first and second connection bolts  327  and  329  distributed around its circumference. 
     Having described various components of the electrical machine  300  and the propulsion engine  100 , various advantages of the structures described with respect to  FIGS.  2  and  3    can now be appreciated. For example, referring to  FIG.  1   , to render the electrical machine  300  entirely accessible, the tail cone  230  may be removed. After removal of the tail cone  230 , at least a portion of the electrical machine  300  may be removed from the propulsion engine  100 . Depending on the type of operation being performed, all or a portion of the electrical machine  300  may be removed, depending on the process followed. For example, in embodiments, the plurality of first and second connection bolts  327  and  329  attaching the stator assembly  302  to the first and second engine stator components  260  and  270  may be removed to facilitate removal of the thermal shield  348 . A connection between the rotor support structure  338  and the second shaft  216  may then be loosened to facilitate removal of the rotor assembly  304  for replacement and/or maintenance. The stator assembly  302  may also be removed from the first and second engine stator components  260  and  270 . 
     In embodiments, rather than removal of the stator assembly  302 , the rotor assembly  304  may be removed from the second shaft  216  without removal of the connections at the plurality of first and second connection bolts  327  and  329 . The non-radially overlapping structure of the stator assembly  302  and rotor assembly  304  facilitates access and removal of the rotor assembly  304  from the propulsion engine  100  without disruption of the stator assembly  302 , facilitating prompt and effective maintenance operations. 
     The manner with which the electrical machine  300  is positioned and connected within the propulsion engine  100  thus facilitates access and removal of the electrical machine  300  without removing any components of the propulsion engine  100  that are disposed forward (e.g., the opposite of the axial direction  272  depicted in  FIG.  2   ) or radially-inward of the turbine rear frame  222 . Accessing the electrical machine  300  in such a non-invasive manner facilitates maintenance or replacement of various components of the electrical machine  300  while the propulsion engine  100  is disposed on a wing or fuselage of an aircraft, which minimizes time that the aircraft may be out of commission if the electrical machine  300  needs repairs. Furthermore, the manner with which the electrical machine  300  is connected to various components of the propulsion engine  100  provides for a streamlined process for removal of the electrical machine  300  from the propulsion engine  100 . 
     Referring now to  FIG.  3   , a cross-sectional view of an electrical machine  400  that may be integrated into a propulsion engine (such as the propulsion engine  100  described herein with respect to  FIG.  1   ) is schematically depicted. The electrical machine  400  may include components of the electrical machine  300  described herein with respect to  FIG.  2   . Accordingly, like reference numerals are utilized in  FIG.  3    to indicate the incorporation of such like components. The electrical machine  400  is also an embedded generator embodiment, including a rotor  346  fixedly attached to the second shaft  216 . The electrical machine  400  includes the stator assembly  302  described with respect to the electrical machine  300  depicted in  FIG.  2   . The electrical machine  400  further includes a rotor assembly  402  that differs in structure from the rotor assembly  304  described with respect to  FIG.  3    in that the rotor assembly  402  includes a rotor support structure  404  extending axially forward from its point of attachment to the second shaft  216 . In embodiments, the rotor support structure  404  is connected to the second shaft  216  at an aft end  217  of the second shaft  216  (e.g., by engaging features on the second shaft  216 ). As depicted in  FIG.  3   , the rotor support structure  404  extends in an axially forward direction, and the rotor attachment arm  340  extends axially rearward from an end of the rotor support structure  404 . Such axially forward extension of the rotor support structure  404  provides additional space rearward of the electrical machine  400  for disposal of additional components (e.g., coolant manifolds, oil supply lines etc.) that may be incorporated into the electrical machine  400  and propulsion engine  100 . In embodiments, the rotor support structure  404  extends only in the radial direction  274 . 
     Referring now to  FIG.  4   , a cross-sectional view of an electrical machine  500  that may be integrated into a propulsion engine (such as the propulsion engine  100  described herein with respect to  FIG.  1   ) is schematically depicted. The electrical machine  500  may include components of the electrical machine  300  described herein with respect to  FIG.  2   . Accordingly, like reference numerals are utilized in  FIG.  4    to indicate the incorporation of such like components. The electrical machine  500  differs from the electrical machine  300  in that the electrical machine  500  is not directly connected to the second shaft  216 , but rather indirectly thereto via an electrical machine shaft  502 . The electrical machine shaft  502  is attached to the aft end  217  of the second shaft  216  by an intermediate shaft member  504 . In embodiments, the intermediate shaft member  504  is attached to the second shaft  216  such that axial and radial vibrations of the second shaft  216  are not transferred to the electrical machine shaft  502 . For example, in embodiments, the intermediate shaft member  504  comprises a quill shaft comprising a first spline (not depicted) at a forward end  505  thereof. The first spline may be inserted into an opening at the aft end  217  of the second shaft  216  to rotationally couple the second shaft  216  and the electrical machine shaft  502 . A second spline (not depicted) at an aft end  507  of the intermediate shaft member  504  may be inserted into a connection end  503  of the electrical machine shaft  502 . Such spline couplings between the electrical machine shaft  502  and the second shaft  216  may permit axial and radial movement of the electrical machine shaft  502  relative to the second shaft  216  such that the electrical machine  500  does not alter the natural vibration frequencies of the second shaft  216 . In embodiments, rather than a quill shaft, the intermediate shaft member  504  may include a bellows spring member permitting relative axial and radial movement of the electrical machine shaft  502  relative to the second shaft  216 . In embodiments, the intermediate shaft member  504  includes a shear section that is structured to decouple (e.g., rupture) when placed under a predetermined shear load. In embodiments, the intermediate shaft member  504  and the electrical machine shaft  502  may be integrated into a single component. 
     The electrical machine shaft  502  is radially supported via a generator bearing assembly  516  attached to the second engine stator component  270  via a bolted connection  552 . The generator bearing assembly  516  includes a bearing support frame  514  extending radially between the electrical machine shaft  502  and the second engine stator component  270 . As depicted, the bearing support frame  514  includes an axial portion  518  defining a bearing cavity  520  in conjunction with the electrical machine shaft  502 . First and second bearing support arms  519  and  521  extend from the bearing support frame  514  in the axial direction  272 . A first generator bearing  522  extends between the first bearing support arm  519  and a first portion of the electrical machine shaft  502  and a second generator bearing  524  extends between the second bearing support arm  521  and a second portion of the electrical machine shaft  502  to rotatably contact the electrical machine shaft  502  (e.g., inner races connected to the electrical machine shaft  502  may house the first and second generator bearings  522  and  524  to provide such rotatable contact). The first and second generator bearings  522  and  524  may include various types of bearings (e.g., ball bearings, roller bearings, or the like) depending on the implementation. The first and second generator bearings  522  and  524  protect the electrical machine  500  from radial and axial movements of the second shaft  216 . 
       FIG.  4    also depicts an engine bearing assembly  538  associated with the propulsion engine  100 . For example, in embodiments, the engine bearing assembly  538  may support the second shaft  216  via the second engine stator component  270  (e.g., the second engine stator component  270  may include a support structure extending radially inward of the inner hub  226  of the turbine rear frame  222 ). The engine bearing assembly  538  includes an engine bearing support arm  540  extending from the second engine stator component  270 . The engine bearing support arm  540  defines an engine bearing cavity  541  in conjunction with the second shaft  216 . An engine bearing  542  is disposed within the engine bearing cavity  541  and extends between the engine bearing support arm  540  and the second shaft  216 . The engine bearing  542  rotatably contacts the second shaft  216  such that the second engine stator component  270  supports the second shaft  216 . 
     The electrical machine  500  is thus supported by dedicated bearings (e.g., the first and second generator bearings  522  and  524 ) on the electrical machine shaft  502  to protect the electrical machine  500  from vibrations of the second shaft  216 . In embodiments, the engine bearing cavity  541  and the bearing cavity  520  are fluidly isolated one another to mitigate the risk contamination of the engine bearing assembly  538  during maintenance of the electrical machine  500 . For example, in the embodiment depicted in  FIG.  4   , the generator bearing assembly  516  includes a first sealing member  526  extending between the first bearing support arm  519  and the electrical machine shaft  502  and a second sealing member  528  extending between the second bearing support arm  521  and the electrical machine shaft  502 . The first and second sealing members  526  and  528  may be constructed of a suitable compliant material (e.g., labyrinth air seals) to generate seals at the interfaces between the first and second sealing members  526  and  528  and the electrical machine shaft  502 . 
     The engine bearing assembly  538  further comprises a sealing member  544  disposed between the engine bearing support arm  540  and the second shaft  216 . The sealing member  544  generates a seal at the interface between it and the second shaft  216 . The sealing member  544  is disposed axially between the engine bearing  542  and the generator bearing assembly  516  such that the engine bearing assembly  538  is fluidly isolated from the generator bearing assembly  516 . Such isolation of the bearing assemblies facilitates provision of lubricant from separate sources to reduce the risks of contamination during maintenance. For example, in the depicted embodiment, the propulsion engine  100  comprises an engine bearing lubrication system  546  and a generator bearing lubrication system  530 . The generator bearing lubrication system  530  includes an oil supply line  531  supported by the bearing support frame  514 . In embodiments, the oil supply line  531  extends through an opening in the axial portion  518  into the bearing cavity  520 . The generator bearing lubrication system  530  further includes an oil ejection nozzle  532  including outlets disposed proximate to the first and second generator bearings  522  and  524  such that oil from a lubricant source (not depicted) travels through the oil supply line  531  and is ejected onto surfaces of the first and second generator bearings  522  and  524  to provide lubrication and cooling during operation thereof. The generator bearing lubrication system  530  also includes an oil scavenge  534  facilitating circulation of oil out of the bearing cavity  520 . 
     The engine bearing lubrication system  546  includes an oil supply line  548  and an oil ejection nozzle  550  including an outlet disposed proximate to the engine bearing  542  to provide lubrication during operation thereof. By utilizing separate bearing lubrication systems (e.g., separate oil supply lines  531  and  548 ) to provide lubricant to the generator bearing assembly  516  and the engine bearing assembly  538 , the embodiment depicted in  FIG.  4    mitigates the risks associated with performing maintenance on the electrical machine  500 . 
     While the depicted embodiment incorporates a generator bearing lubrication system  530  and an engine bearing lubrication system  546  that are oil-based, it should be understood that alternative lubrication systems using different types of lubricants are contemplated and within the scope of the present disclosure. Various types of fluid-based lubricants (e.g., synthetic polymer-based lubricants), gas-based lubricants, or solid lubricants may also be used in accordance with the present disclosure. Isolating the separate lubrication systems associated with the generator and engine bearings generally avoids complications in the engine bearing assembly resulting from maintenance of the electrical machine  500 , thereby avoiding disrupting operation of the remainder of the propulsion engine  100 . 
     Referring still to  FIG.  4   , the electrical machine  500  further differs from the electrical machine  300  described herein with respect to  FIG.  2    in that the electrical machine comprises a rotor assembly  506  that differs in structure from the rotor assembly  304  described herein. The rotor assembly  506  comprises a rotor support structure  508  that is attached to the electrical machine shaft  502  via a mounting flange  510  of the electrical machine shaft  502 . The rotor support structure  508  is attached to the mounting flange  510  by a connecting bolt  512  extending through the mounting flange  510  and the rotor support structure  508 . The rotor support structure  508  extends from the electrical machine shaft  502  and the rotor attachment arm  340  extends axially therefrom to support the rotor  346  in a desired position. In embodiments, the rotor support structure  508  extends diagonally from the electrical machine shaft  502  (e.g., similar to the rotor support structures  338  and  404  described herein with respect to  FIGS.  2  and  3   ). 
     In the depicted embodiment, the rotor  346  is disposed radially inward of the stator assembly  302 . As discussed herein, such inner rotor design facilitates independent removal of the rotor assembly  506 . Because the mounting flange  510  of the electrical machine shaft  502  is disposed axially aft of the bearing support frame  514 , the connecting bolt  512  may be accessed without removal of the bearing support frame  514 , which allows removal of the rotor assembly  506  independently from the stator assembly  302 . Additionally, due to the inner-rotor construction of the electrical machine  500 , the stator support arm  312  may act as a debris shield for containing any broken components of the rotor assembly  506 . It should be appreciated that embodiments incorporating various aspects of the electrical machine  500  (e.g., the electrical machine shaft  502 , the intermediate shaft member  504 , the generator bearing assembly  516 ) are also envisioned where the rotor  346  is disposed radially outward of the stator assembly  302 . 
     The separate shaft coupling of the electrical machine  500  via the electrical machine shaft  502  further facilitates separation of the electrical machine  500  from the propulsion engine  100  by decoupling the intermediate shaft member  504 . As depicted in  FIG.  4   , the propulsion engine  100  includes a decoupling device  536  extending from the second engine stator component  270 . The decoupling device  536  axially overlaps the intermediate shaft member  504  such that, upon activation of the decoupling device  536 , the decoupling device  536  performs an action on intermediate shaft member  504  decouple the electrical engine shaft  502  from the second shaft  216  to protect the second shaft  216  from malfunctioning of the electrical machine  500  by mechanical disconnect of the electrical machine  500  from the second shaft  216 . 
     To facilitate disassembly of the electrical machine  500  from the propulsion engine  100  via the splines of the intermediate shaft member  504 , the manner with which the stator assembly  302  is connected to the propulsion engine  100  may be modified as compared with the generator assembly  300  described herein with respect to  FIG.  3   . As depicted in  FIG.  4   , the stator assembly  302  is not directly connected to the second engine stator component  270 , but rather to the bearing support frame  514 . The bearing support frame  514  is connected to the second engine stator component  270  via a first connection bolt  552 , and the stator support arm  312  is connected to the bearing support frame  514  via a second connection bolt  554 . An axial portion  556  of the bearing support frame  514  extends between the first and second connection bolts  552  and  554  to axially separate the stator support arm  312  from the second engine stator component  270 . In embodiments, the first connection bolt  552  may be detached to facilitate removal of the electrical machine shaft  502  from the intermediate shaft member  502  via the splined connection. Thus, the spline coupling provided by the intermediate shaft member  504  facilitates removal of the entire electrical machine  504  (e.g., the electrical machine shaft  502 , the bearing support frame  514 , the generator bearing assembly  516 , the rotor assembly  506 , and the stator assembly  302 ) as a single module to reduce the risk of contamination. After removal of the electrical machine  500 , the intermediate shaft member  502  may be removed as well. 
     In embodiments, the second connection bolt  554  may be loosened to facilitate removal of the stator assembly  302  without using the decoupling device  536  (e.g., removal of the connection bolts  327 ,  554 , and  512  may facilitate removal of the rotor assembly  506  and stator assembly  302  from the propulsion engine  100  independently of the bearing support frame  514 ). As such, the depicted design facilitates flexibility in the operations that may be performed to remove the electrical machine  500  (or portions thereof) from the propulsion engine  100 . In embodiments, the bearing support frame  514  and stator support arm  312  may be structured from different materials for desired function and durability. In embodiments, the stator support arm  312  and the bearing support frame  514  are integrated into a single part. 
     Referring now to  FIG.  5   , a cross-sectional view of an electrical machine  600  that may be integrated into a propulsion engine (such as the propulsion engine  100  described herein with respect to  FIG.  1   ) is schematically depicted. The electrical machine  600  may include components of the electrical machine  500  described herein with respect to  FIG.  5   . Accordingly, like reference numerals are utilized in  FIG.  5    to indicate the incorporation of such like components. The electrical machine  600  differs from the electrical machine  500  described herein with respect to  FIG.  4    in that the electrical machine  600  includes a generator bearing assembly  602  and an engine bearing assembly  604  that are disposed in a common sump  640  defined at least in part by the bearing support frame  514  and the second engine stator component  270 . As depicted in  FIG.  5   , the generator bearing assembly  602  includes the first and second generator bearings  522  and  524  described with respect to  FIG.  4   , but only includes a single generator bearing sealing member  622  disposed axially aft of the second generator bearing  524 . The engine bearing assembly  604  includes the engine bearing  542  described with respect to  FIG.  4   , but includes a single engine bearing sealing member  606  disposed axially forward of the engine bearing  542 . The sealing members  622  and  606  seal off the common sump  640  to contain lubricant supplied to the bearings. 
     In the electrical machine  600 , the common sump  640  is not sealed axially between the first and second generator bearings  522  and  524  and the engine bearing  542 . That is, the bearing cavity  520  and the bearing cavity  541  described with respect to  FIG.  4    are not fluidly isolated from one another. Such lack of sealing between the bearings allows a common lubrication source to be used to lubricate the first and second generator bearings  522  and  524  and the engine bearing  542 . For example, in the depicted embodiment, a bearing lubrication system  608  is used to supply lubricant to the engine bearing  542  and the first and second generator bearings  522  and  524 . The bearing lubrication system  608  includes an oil supply line  610  extending into the coolant cavity  562  from a lubricant source (not depicted). The oil supply line  610  branches into a generator portion  614  extending axially aft into the bearing cavity  520  and an engine portion  612  extending axially forward into the bearing cavity  541 . An oil nozzle  618  at an end of the generator portion  614  includes outlets providing oil to the first and second generator bearings  522  and  524 . An oil nozzle  620  disposed at an end of the engine portion  612  includes an outlet for supplying oil to the engine bearing  542 . The bearing lubrication system  608  further includes one or more drain passages  626  disposed proximate to each of the bearings  522 ,  524 , and  542  for receiving oil after the oil has been applied to the bearings  522 ,  524 , and  542 . Embodiments may incorporate drain passages in the portion of the second engine stator component  270  disposed proximate to the decoupling device  536 . The drain passages  626 ,  628 ,  630  may drain the oil near the bearings  522 ,  524 , and  542  into the common sump  640 . A scavenge  624  may route oil to a scavenge line for filtration and reuse. 
     The common sump  640  of the electrical machine  600  thus facilitates utilization of one bearing lubrication system  608  (e.g., including a single oil supply line  610  from a lubrication source), and includes a simpler structure than the multiple lubrication systems (e.g., the generator bearing lubrication system  530  and the engine bearing lubrication system  546 ) associated with the electrical machine  500  described with respect to  FIG.  4   . The reduction in oil lines, seals, and connections in the electrical machine  600  may reduce the weight and complexity of the electrical machine  600  over the electrical machine  500  described with respect to  FIG.  4   . 
     In view of the foregoing description, it should be appreciated that an electrical machine may be integrated into a propulsion engine of an aircraft. A stator assembly of the electrical machine may be coupled to one or more engine stator components of the propulsion engine, while the rotor assembly may be coupled directly or indirectly to a shaft of the propulsion engine to facilitate an exchange of rotational energy between the shaft and the electrical machine. The rotor assembly may either be directly connected to the shaft by a rotor support structure attached to the shaft or indirectly connected to the shaft via an intermediate shaft member and an electrical machine shaft. In such embodiments, the electrical machine may be supported on its own bearings to protect the electrical machine from vibrations of the shaft and protect the shaft from vibrations of the electrical machine. The electrical machine may also be removed in its entirety by a splined connection with the intermediate shaft member to avoid risks of magnetic contamination of the electrical machine during maintenance. The electrical machine may be constructed and positioned to facilitate relatively easy access and removal thereof to facilitate maintenance while the propulsion engine is installed on an aircraft. 
     As used herein, the term “about” means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. When the term “about” (or “substantially” or “approximately”) is used in describing a value or an end-point of a range, the specific value or end-point referred to is comprised. Whether or not a numerical value or end-point of a range in the specification recites “about,” two embodiments are described: one modified by “about,” and one not modified by “about.” It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. For example, the approximating language may refer to being within a 1, 2, 4, 10, 15, or 20 percent margin in either individual values, range(s) of values and/or endpoints defining range(s) of values. 
     Directional terms as used herein—for example up, down, right, left, front, back, top, bottom—are made only with reference to the figures as drawn and are not intended to imply absolute orientation. 
     Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order, nor that with any apparatus specific orientations be required. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or that any apparatus claim does not actually recite an order or orientation to individual components, or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, or that a specific order or orientation to components of an apparatus is not recited, it is in no way intended that an order or orientation be inferred, in any respect. This holds for any possible non-express basis for interpretation, comprising: matters of logic with respect to arrangement of steps, operational flow, order of components, or orientation of components; plain meaning derived from grammatical organization or punctuation, and; the number or type of embodiments described in the specification. 
     As used herein, the singular forms “a,” “an” and “the” comprise plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” component comprises aspects having two or more such components, unless the context clearly indicates otherwise. 
     Further aspects of the invention are provided by the subject matter in the following clauses: 
     1. An electrical machine comprising: a stator assembly coupled to an engine stator component of a propulsion engine, the stator assembly comprising: a stator support assembly fixedly attached to the engine stator component; and a stator disposed on a supporting surface of the stator support structure; and a rotor assembly comprising a rotor support structure connected to a shaft of the propulsion engine and a rotor attached to the rotor support structure such that the rotor is disposed radially inward of the stator, wherein the rotor exchanges rotational energy with the shaft to operate as either an electrical motor or an electrical generator. 
     2. The electrical machine of any preceding clause, wherein the stator assembly circumferentially surrounds the rotor assembly such that the rotor assembly is disposed radially between the shaft and the stator assembly. 
     3. The electrical machine of any preceding clause, wherein the stator support assembly comprises a stator support arm extending in an axial direction parallel to the shaft, the stator support arm defining the supporting surface, wherein the stator support arm is at least as long as the rotor assembly in the axial direction such that an entirety of the rotor assembly is disposed in a rotor cavity delineated by the stator support arm and the stator support arm shields the propulsion engine from the rotor assembly. 
     4. The electrical machine of any preceding clause, further comprising: an electrical connection device coupled to the stator support assembly; and an electrical line extending from the stator to the electrical connection device. 
     5. The electrical machine of any preceding clause, further comprising an electrical machine control unit, the electrical machine control unit being electrically connected to the stator via the, wherein the electrical machine control unit is configured to switch operation of the electrical machine between a generator mode to generate electrical power from rotation of the shaft and a motor mode in which the stator adds rotational energy to the shaft. 
     6. The electrical machine of any preceding clause, wherein the rotor assembly is directly connected to an end of the shaft. 
     7. The electrical machine of any preceding clause, wherein no portion of the stator assembly extends axially aft of the rotor assembly. 
     8. The electrical machine of any preceding clause, further comprising: an electrical machine shaft coupled to an end of the shaft of the propulsion engine via an intermediate shaft member extending axially between the shaft and the electrical machine shaft; and a generator bearing assembly comprising: a bearing support frame extending between the electrical machine shaft and the engine stator component; and generator bearings supporting the electrical machine shaft, wherein the electrical machine shaft rotates within generator bearing in conjunction with the shaft of the propulsion shaft to rotate the rotor. 
     9. The electrical machine of any preceding clause, further comprising a cooling system comprising one or more cooling manifolds directing coolant from a coolant source to regions proximate to the stator and the rotor. 
     10. An electrical machine comprising: a stator assembly coupled to an engine stator component of a propulsion engine, the stator assembly comprising: a stator support assembly fixedly attached to the engine stator component; and a stator disposed on a supporting surface of the stator support structure; and a rotor assembly comprising a rotor support structure directly connected to a shaft of the propulsion engine and a rotor attached to the rotor support structure, wherein: the rotor is disposed radially inward of the stator such that the stator assembly circumferentially surrounds the rotor, and at least one of: the rotor rotates in conjunction with the shaft to generate a power signal, and the electrical machine receives power from an external source to provide rotational energy to the shaft. 
     11. The electrical machine of any preceding clause, wherein the rotor support structure is directly connected to an end of the shaft of the propulsion engine by a removable connection such that the rotor assembly is independently removable from the propulsion engine. 
     12. The electrical machine of any preceding clause, wherein the rotor support structure is directly connected to the shaft between ends of the shaft of the propulsion engine. 
     13. The electrical machine of any preceding clause, wherein the rotor support structure extends in a radial and an axial direction from the end of the shaft to mechanically alter a natural vibration mode of the shaft. 
     14. The electrical machine of any preceding clause, further comprising an electrical machine control unit, the electrical machine control unit being electrically connected to the stator, wherein the electrical machine control unit is configured to actively modulate an electrical machine load to influence electrical machine rotation to dampen vibrations of the shaft. 
     15. The electrical machine of any preceding clause, further comprising a cooling system comprising one or more cooling manifolds directing coolant from a coolant source to regions proximate to the stator and the rotor. 
     16. The electrical machine of any preceding clause, wherein an electrical line extending from the stator is at least partially routed through the one or more cooling manifolds. 
     17. An electrical machine comprising: a stator assembly coupled to an engine stator component of a propulsion engine, the stator assembly comprising: a stator support assembly fixedly attached to the engine stator component; and a stator disposed on a supporting surface of the stator support structure; and an electrical machine shaft coupled to an end of a shaft of the propulsion engine via an intermediate shaft member extending axially between the end of the shaft and the electrical machine shaft; a bearing support frame extending from the propulsion engine, the bearing support frame including an axial portion extending in an axial direction; electrical machine bearings radially extending from the axial portion of the bearing support frame to rotatably contact the electrical machine shaft; a sealing member disposed axially aft of the electrical machine bearings, the sealing member extending from the axial portion of the bearing support frame to the electrical machine shaft; and a rotor assembly comprising: a rotor support structure connected to the electrical machine shaft; and a rotor attached to the rotor support structure such that the rotor is disposed radially inward of the stator, wherein at least one of: the rotor rotates in conjunction with the shaft of the propulsion engine via the intermediate shaft member to generate a power signal, and the electrical machine receives power from an external source to provide rotational energy to the shaft. 
     18. The electrical machine of any preceding clause, wherein the rotor support structure is connected to the electrical machine shaft axially rearward of the generator bearing to facilitate removal of the rotor from the electrical machine shaft. 
     19. The electrical machine of any preceding clause, further comprising a decoupling device attached to the propulsion engine, the decoupling device positioned to axially overlap the intermediate shaft member to decouple the intermediate shaft member. 
     20. The electrical machine of any preceding clause, further comprising a seal extending between the bearing support frame and the electrical machine shaft, the seal fluidly isolating the electrical machine bearings from bearings of the propulsion engine. 
     21. An electrical machine comprising: a stator assembly coupled to an engine stator component of a propulsion engine, the stator assembly comprising: a stator support assembly fixedly attached to the engine stator component; and a stator disposed on a supporting surface of the stator support structure; and an electrical machine shaft coupled to an end of a shaft of the propulsion engine via an intermediate shaft member extending axially between the end of the shaft and the electrical machine shaft; a bearing support frame extending from the propulsion engine, the bearing support frame defining a bearing cavity in conjunction with the electrical machine shaft; first and second electrical machine bearings radially extending from the bearing support frame to rotatably contact the electrical machine shaft; a sealing member disposed axially aft of the electrical machine bearings, the sealing member extending from the bearing support frame to the electrical machine shaft; a rotor support structure connected to the electrical machine shaft; and a rotor attached to the rotor support structure, wherein the rotor rotates in conjunction with the electrical machine shaft to exchange energy with the shaft of the propulsion engine. 
     22. The electrical machine of any preceding clause, wherein: the bearing cavity is defined by the bearing support frame in conjunction with the electrical machine shaft, and an engine bearing of the propulsion engine is disposed within the bearing cavity. 
     23. The electrical machine of any preceding clause, further comprising: an oil supply line extending through the bearing support frame into a common sump defined at least in part between the bearing support frame and the electrical machine shaft; a first oil nozzle extending from the oil supply line and disposed proximate to the electrical machine bearings; and a second oil nozzle extending from the oil supply line and disposed proximate to the engine bearing. 
     24. The electrical machine of any preceding clause, wherein: the stator assembly comprises a stator support arm connected to the engine stator component via an axial portion of the bearing support frame; and the oil supply line extends through the axial portion of the bearing support frame into the common sump. 
     25. The electrical machine of any preceding clause, further comprising an engine bearing support arm connected to the second engine stator component, the engine bearing support arm defining an engine bearing cavity in conjunction with the shaft of the propulsion engine. 
     26. The electrical machine of any preceding clause, further comprising a first sealing member and a second sealing member extending between the bearing support frame and the electrical machine shaft to seal off the bearing cavity. 
     27. The electrical machine of any preceding clause, further comprising: a first oil supply line extending through the bearing support frame into the bearing cavity; and a second oil supply line extending into the engine bearing cavity. 
     28. The electrical machine of any preceding clause, further comprising a decoupling device attached to the propulsion engine, the decoupling device axially overlapping the intermediate shaft member such that the decoupling device decouples the intermediate shaft member upon activation. 
     29. The electrical machine of any preceding clause, wherein the intermediate shaft member permits movement of the electrical machine shaft relative to the shaft of the propulsion engine in an axial direction and a radial direction. 
     30. The electrical machine of any preceding clause, wherein the intermediate shaft comprises a quill shaft comprising a first spline disposed at a first end thereof and a second spline disposed at a second end thereof, wherein the first and second splines are inserted into openings at ends of the shaft of the propulsion engine and the electrical machine shaft to permit the movement of the electrical machine shaft in the axial and radial directions. 
     31 The electrical machine of any preceding clause, wherein entireties of the rotor and the rotor support structure are disposed radially inward of the stator assembly. 
     32. The electrical machine of any preceding clause, wherein the rotor support structure is connected to an end of the electrical machine shaft by a removable connection such that the rotor support structure and rotor are independently removable from the propulsion engine. 
     33. The electrical machine of any preceding clause, wherein the intermediate shaft comprises a shear section structured to decouple when placed under a predetermined shear load. 
     34. A propulsion engine comprising: a core portion generating exhaust that travels in an axial direction; a turbine section coupled to a shaft, wherein the turbine section receives the exhaust and generates mechanical energy to rotate the shaft; a turbine frame attached to the turbine section, the turbine frame comprising: an outer casing coupled to the turbine section; and an inner hub supporting the shaft via a bearing assembly comprising an engine bearing supporting the shaft; and an electrical machine comprising: a stator assembly comprising a stator support assembly attached to the inner hub and a stator attached to the stator support structure; an electrical machine shaft coupled to an end of the shaft via an intermediate shaft member extending axially between the end of the shaft and the electrical machine shaft; a bearing support frame attached to the inner hub and extending radially inward therefrom to define a bearing cavity extending between the bearing support frame and electrical machine shaft; electrical machine bearings radially extending from the bearing support frame to rotatably contact the electrical machine shaft; and a rotor assembly comprising: a rotor support structure connected to the electrical machine shaft; and a rotor attached to the rotor support structure and extending radially inward of the stator, wherein the rotor rotates in conjunction with the shaft via the intermediate shaft member to exchange energy with the shaft. 
     35. The propulsion engine of any preceding clause, wherein: the turbine frame further comprises a plurality of struts extending between the outer casing and the inner hub, and at least one of the plurality of struts defines an internal cavity having a cooling duct disposed therein. 
     36. The propulsion engine of any preceding clause, wherein the electrical machine further comprises a cooling system comprising a stator manifold to provide coolant from a coolant source to the stator assembly and the rotor assembly. 
     37. The propulsion engine of any preceding clause, wherein the intermediate shaft member and the electrical machine shaft are an integrated component. 
     38. The propulsion engine of any preceding clause, further comprising a decoupling device axially overlapping the intermediate shaft member such that the decoupling device decouples the intermediate shaft member upon activation. 
     39. The propulsion engine of any preceding clause, wherein the intermediate shaft member permits movement of the electrical machine shaft relative to the shaft of the propulsion engine in an axial direction and a radial direction. 
     40. The propulsion engine of any preceding clause, further comprising a sealing member extending between the bearing support frame and the electrical machine shaft to seal the bearing cavity. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus, it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.