for purposes of promoting an understanding of the principles of the invention , reference will now be made to the embodiments illustrated in the drawings , and specific language will be used to describe the same . it will nonetheless be understood that no limitation of the scope of the invention is intended by the illustration and description of certain embodiments of the invention . in addition , any alterations and / or modifications of the illustrated and / or described embodiment ( s ) are contemplated as being within the scope of the present invention . further , any other applications of the principles of the invention , as illustrated and / or described herein , as would normally occur to one skilled in the art to which the invention pertains , are contemplated as being within the scope of the present invention . referring to fig1 , there are illustrated some aspects of a non - limiting example of an aircraft 10 in accordance with an embodiment of the present invention . aircraft 10 includes a fuselage 12 , wings 14 , an empennage 16 and two propulsion systems 18 . in one form , aircraft 10 is a twin engine military turbofan aircraft . in other embodiments , aircraft 10 may be any fixed - wing aircraft , including turbofan aircraft , turbojet aircraft and turboprop aircraft . in still other embodiments , aircraft 10 may be a rotary - wing aircraft or a combination rotary - wing / fixed - wing aircraft . in various embodiments , aircraft 10 may have a single propulsion engine or a plurality of propulsion engines . in addition , in various embodiments , aircraft 10 may employ any number of wings 14 . empennage 16 may employ a single or multiple flight control surfaces . referring to fig2 , there are illustrated some aspects of a non - limiting example of a propulsion system 18 in accordance with an embodiment of the present invention . propulsion system 18 includes a gas turbine engine 20 as a main engine , i . e ., a main propulsion engine , which includes an auxiliary turbine system 22 . engine 20 is a primary propulsion engine that provides thrust for flight operations of aircraft 10 . in one form , engine 20 is a two - spool engine having a high pressure ( hp ) spool 24 and a low pressure ( lp ) spool 26 . in other embodiments , engine 20 may include three or more spools , for example , and may include an intermediate pressure ( ip ) spool and / or other spools . in one form , engine 20 is a turbofan engine , wherein lp spool 26 is operative to drive a propulsor 28 in the form of a turbofan ( fan ) system , which may be referred to as a turbofan , a fan or a fan system . in other embodiments , engine 20 may be a turboprop engine , wherein lp spool 26 powers a propulsor 28 in the form of a propeller system ( not shown ), e . g ., via a reduction gearbox ( not shown ). in still other embodiments , propulsor 28 may take other forms , such as a helicopter rotor or tilt - wing aircraft rotor or a propfan . in one form , two propulsion systems 18 are coupled to fuselage 12 of aircraft 10 . in other embodiments , one or more propulsion systems 18 may be coupled to other portions of aircraft 10 . for example , one or more propulsion systems 18 may be coupled to each wing 14 and / or empennage 16 in addition to or in place of fuselage - mounted propulsion systems 18 . in one form , engine 20 includes , in addition to auxiliary turbine system 22 and fan system 28 , an accessory gearbox 23 , a bypass duct 30 , a compressor system 32 as part of hp spool 24 , a diffuser 34 , a combustion system 36 , a high pressure ( hp ) turbine 38 as part of hp spool 24 , a low pressure ( lp ) turbine 40 as part of lp spool 26 , a nozzle 42 a , and a nozzle 42 b . accessory gearbox 23 is coupled to hp spool 24 and compressor 32 via conventional means , e . g ., a bevel gear set and shafting 25 . in other embodiments , accessory gearbox 23 may be coupled to hp spool 24 and / or lp spool 26 via other mechanical arrangements . in one form , compressor 32 is a variable compressor . in other embodiments , compressor 32 may not be a variable compressor . in one form , compressor 32 is a variable geometry compressor . in other embodiments , compressor 32 may be other types of variable compressors that may or may not employ variable geometry , e . g ., including geared compressors that are configured to operate at more than one speed relative to a given shaft input speed . bypass duct 30 and compressor 32 are in fluid communication with fan system 28 . nozzle 42 b is in fluid communication with bypass duct 30 . diffuser 34 is in fluid communication with compressor 32 . combustion system 36 is fluidly disposed between compressor 32 and hp turbine 38 . lp turbine 40 is fluidly disposed between hp turbine 38 and nozzle 42 b . in one form , combustion system 36 includes a combustion liner ( not shown ) that contains a continuous combustion process . in other embodiments , combustion system 36 may take other forms , and may be , for example , a wave rotor combustion system , a rotary valve combustion system , a pulse detonation combustion system and / or a slinger combustion system , and may employ deflagration and / or detonation combustion processes . fan system 28 includes a fan rotor system 48 driven by lp spool 26 . in various embodiments , fan rotor system 48 includes one or more rotors ( not shown ) that are powered by lp turbine 40 , which may operate at the same or different rotational speeds . fan system 28 may include one or more stages of vanes ( not shown ). bypass duct 30 is operative to transmit a bypass flow generated by fan system 28 around the core of engine 20 . compressor 32 includes a compressor rotor system 50 . in various embodiments , compressor rotor system 50 includes one or more rotors ( not shown ) that are powered by hp turbine 38 . hp turbine 38 includes a turbine rotor system 52 . in various embodiments , turbine rotor system 52 includes one or more rotors ( not shown ) operative to drive compressor rotor system 50 . turbine rotor system 52 is drivingly coupled to compressor rotor system 50 via a shafting system 54 . lp turbine 40 includes a turbine rotor system 56 . in various embodiments , turbine rotor system 56 includes one or more rotors ( not shown ) operative to drive fan rotor system 48 . turbine rotor system 56 is drivingly coupled to fan rotor system 48 via a shafting system 58 . in various embodiments , shafting systems 54 and 58 include a plurality of shafts that may rotate at the same or different speeds and directions . in some embodiments , only a single shaft may be employed in one or both of shafting systems 54 and 58 . lp turbine 40 is operative to discharge the engine 20 core gas flow to nozzle 42 a . during the operation of gas turbine engine 20 , not including the use of auxiliary turbine system 22 , air is drawn into the inlet of fan system 28 and pressurized by fan rotor system 48 . some of the air pressurized by fan rotor system 48 is directed into compressor 32 as core gas flow , and some of the pressurized air is directed into bypass duct 30 as bypass flow . compressor 32 further pressurizes the core gas flow received therein from fan system 28 , which is then discharged into diffuser 34 . diffuser 34 reduces the velocity of the pressurized air , and directs the diffused core gas flow into combustion system 36 . fuel is mixed with the pressurized air in combustion system 36 , which is then combusted . the core gas flow , in the form of hot gases exiting combustion system 36 , are directed into hp and lp turbines 38 and 40 , e . g ., sequentially , which extract energy in the form of mechanical shaft power to drive compressor 32 and fan 28 via respective shafting systems 54 and 58 . in the depicted embodiment , the engine 20 core flow is discharged through nozzle 42 a , and the bypass flow is discharged through nozzle 42 b . in other embodiments , other nozzle arrangements may be employed , e . g ., a common nozzle for core and bypass flow ; a nozzle for core flow , but no nozzle for bypass flow ; or another nozzle arrangement . it is desirable that engine 20 product peak thrust output during aircraft 10 takeoff , and during some aircraft 10 maneuvering operations . in addition , it is desirable that engine 20 operate at high efficiency during cruise conditions , including supercruise conditions , i . e ., supersonic cruise without the use of thrust augmentation ( e . g ., afterburners ). conventionally , a fixed geometry gas turbine engine sized for takeoff thrust conditions yields a greater than ideal specific fuel consumption during cruise conditions because the engine is running at an “ off - design ” point during cruise conditions . on the other hand , a fixed geometry gas turbine sized for peak efficiency during cruise conditions may have insufficient thrust for desirable takeoff and maneuver performance . in order to maximize thrust at high power , e . g ., takeoff and maneuver conditions , engine 20 is configured as a variable cycle gas turbine engine . in particular , engine 20 employs auxiliary turbine system 22 for selectively expanding and contracting the turbine flow capacity of engine 20 . in some embodiments , compressor 32 may be variable , e . g ., a variable geometry compressor , which in conjunction with auxiliary turbine system 22 further enhances the cycle variability of engine 20 . referring to fig3 , some aspects of a non - limiting example of auxiliary turbine system 22 in accordance with an embodiment of the present invention are schematically depicted . in one form , auxiliary turbine system 22 includes an auxiliary turbine 60 , a valve 62 , a controller 64 , inlet ducting 66 and 68 , and exhaust ducting 70 . in one form , auxiliary turbine 60 is mechanically coupled to accessory gearbox 23 via a shaft 72 , and is coupled to compressor 32 via accessory gearbox 23 . in other embodiments , auxiliary turbine 60 may be coupled to compressor 32 or one or more other hp spool 24 components , e . g ., shafting system 54 , via other mechanical arrangements . in one form , inlet ducting 66 is coupled to a plenum 74 at one end , and is coupled to valve 62 at the other end . plenum 74 is disposed between the outlet of hp turbine 38 and the inlet of lp turbine 40 , and is operative to receive a portion of the core gas flow exiting hp turbine 38 for use by auxiliary turbine 60 . in other embodiments , the portion of core gas flow for use by auxiliary turbine 60 may be obtained from one or more other turbine stages , in addition to or in place of the hp turbine 38 outlet . in addition , in other embodiments , the portion of core gas flow for use by auxiliary turbine 60 may be obtained via other arrangements , which may or may not employ the use of a plenum for the extraction of the portion of the core gas flow for auxiliary turbine 60 , depending upon the needs of the particular application . inlet ducting 68 is coupled to valve 62 at one end , and to the inlet of auxiliary turbine 60 at the other end . in one form , portions of inlet ducting 68 that pass through bypass duct 30 are disposed within an aerodynamic strut 76 in order to minimize losses . in other embodiments , other arrangements may be employed . exhaust ducting 70 is coupled to the outlet of auxiliary turbine 60 at one end , and is configured to direct the exhaust from auxiliary turbine 60 into bypass duct 30 at the other end for conversion to thrust , e . g ., via nozzle 42 b . in other embodiments , auxiliary turbine system 22 may be configured to discharge the auxiliary turbine 60 exhaust flow to other locations , for example and without limitation , into nozzle 42 a or overboard engine 20 . although the depicted embodiment envisions the use of inlet ducting 66 , inlet ducting 68 and exhaust ducting 70 as set forth herein , other embodiments may employ other arrangements to channel flow to and from valve 62 and auxiliary turbine 60 . in addition , although depicted embodiment envisions auxiliary turbine 60 being disposed outside of bypass duct 30 , in other embodiments , auxiliary turbine 60 may be disposed in other locations , including radially inward of bypass duct 30 , inside bypass duct 30 , or upstream or downstream of bypass duct 30 . valve 62 is configured to regulate the portion of the core gas flow that is received by auxiliary turbine 60 . in one form , valve 62 is configured to modulate the portion of the core gas flow received by auxiliary turbine 60 between a minimum flow amount and a maximum flow amount in accordance with the needs of the particular application . valve 62 is also configured to close to prevent flow to auxiliary turbine 60 . valve 62 is controlled by controller 64 to selectively allow or disallow flow through valve 62 . valve 62 may take any suitable form , and may be , for example and without limitation , a butterfly valve , a gate valve , a poppet valve or any other suitable valve type . valve 62 is actuated by an actuation mechanism ( not shown ) under the direction of controller 64 . controller 64 is communicatively coupled to valve 62 via a communications link 78 . communications link 78 may take any suitable form , and may be , for example , a wired and / or wireless and / or optical link capable of transmitting control signals to valve 62 . in some embodiments , valve 62 may provide feedback information to controller 64 indicative of valve position , in which case communications link 78 is also configured to transmit feedback signals to controller 64 from valve 62 . in some embodiments , communications link 78 may also be configured to provide electrical power for actuating valve 62 . controller 64 is configured to execute program instructions to control valve 62 to selectively prevent or allow flow to auxiliary turbine 60 , and to regulate the flow rate to a desired level during engine 20 operations where such flow is desired . the flow regulation , including starting and stopping flow to auxiliary turbine 60 may be based on , for example and without limitation , one or more lookup tables and / or rate schedules , and / or may be based on , for example and without limitation , sensed and / or calculated engine 20 parameters , engine 20 inlet conditions , aircraft 10 speed and / or power lever angle . in one form , controller 64 is microprocessor based and the program instructions are in the form of software stored in a memory ( not shown ). however , it is alternatively contemplated that controller 64 and the program instructions may be in the form of any combination of software , firmware and hardware , including state machines , and may reflect the output of discreet devices and / or integrated circuits , which may be co - located at a particular location or distributed across more than one location , including any digital and / or analog devices configured to achieve the same or similar results as a processor - based controller executing software or firmware based instructions . in other embodiments , controller 64 may not be configured with the level of functionality associated with a processor - based controller , but rather may be a simple controller configuration . in one form , controller 64 is a gas turbine engine controller , such as a full authority digital electronic control ( fadec ) unit . in other embodiments , controller 64 may take any suitable form , and in some embodiments may be a dedicated controller for operating valve 62 . during aircraft 10 takeoff , the power ( thrust ) output of engine 20 is enhanced by employing auxiliary turbine system 22 to expand turbine flow capacity by opening valve 62 to allow a portion of core gas flow to flow through auxiliary turbine 60 . auxiliary turbine 60 extracts power from the gas flow and transmits the power via accessory gearbox 23 to compressor 32 , thereby increasing the output of compressor 32 , and hence engine 20 . the exhaust gas from auxiliary turbine 60 is directed into bypass duct 30 , from where it will contribute to the thrust output of engine 20 . during some operating conditions , such as aircraft 10 takeoff , controller 64 may command valve 62 to open fully , thereby providing a maximum flow to auxiliary turbine 20 , yielding a higher takeoff power output by engine 20 than a similar engine not equipped with auxiliary turbine system 22 . the amount by which valve 62 opens may vary with conditions , for example and without limitation , ambient / inlet conditions . during other operating conditions of aircraft 10 that require high thrust levels , controller 64 may command valve 62 to open partially or fully , e . g ., depending operating conditions and / or pilot input . during cruise conditions , including supercruise flight , controller 64 may command valve 62 to close fully , thereby contracting the turbine flow capacity of engine 20 , which may result in increased fuel efficiency , as engine 20 is effectively operating closer to design point at the cruise power condition . embodiments of the present invention include a variable cycle gas turbine engine , comprising : a compressor configured to compress a core gas flow ; a combustor in fluid communication with the compressor and configured to combust the core gas flow ; a primary turbine drivingly coupled to the compressor and configured to receive the core gas flow , wherein the primary turbine is configured to drive the compressor ; an auxiliary turbine drivingly coupled to the compressor ; and a valve configured to selectively direct a portion of the core gas flow to the auxiliary turbine , wherein the auxiliary turbine is configured to extract power from the portion of the core gas flow and supply the power to the compressor when the valve is open . in a refinement , the valve is in fluid communication with the primary turbine and operative to receive the portion of the core gas flow from the primary turbine . in another refinement , the valve is configured to modulate the portion of the core gas flow between a minimum flow amount and a maximum flow amount . in yet another refinement , the valve is configured to close to prevent flow to the auxiliary turbine . in still another refinement , the valve is operative to open during a takeoff power condition of the engine . in yet still another refinement , the valve is operative to close during a cruise power condition of the engine . in a yet further refinement , the variable cycle gas turbine engine further comprises a fan and a fan bypass duct in fluid communication with the fan , wherein variable cycle gas turbine engine is configured to direct the exhaust of the auxiliary turbine into the fan bypass duct . in a still further refinement , the variable cycle gas turbine engine further comprises an accessory gearbox coupled to the compressor , wherein the auxiliary turbine is drivingly coupled to the compressor via the accessory gearbox . embodiments of the present invention include a variable cycle gas turbine engine , comprising : a compressor configured to compress a core gas flow ; a combustor in fluid communication with the compressor and configured to combust the core gas flow ; a primary turbine drivingly coupled to the compressor and configured to receive the core gas flow , wherein the primary turbine is configured to drive the compressor ; and an auxiliary turbine system having an auxiliary turbine drivingly coupled to the compressor , wherein the auxiliary turbine system is configured to selectively receive a portion of the core gas flow ; generate shaft power using the portion of the core gas flow ; and supply the shaft power to the compressor . in a refinement , the auxiliary turbine system includes a valve configured to selectively direct a portion of the core gas flow to the auxiliary turbine . in another refinement , the primary turbine is a high pressure turbine ; and wherein the valve is in fluid communication with the discharge of the high pressure turbine , and is operative to receive the portion of the core gas flow from the discharge of the high pressure turbine . in yet another refinement , the valve is configured to modulate the portion of the core gas flow between a minimum flow amount and a maximum flow amount . in still another refinement , the variable cycle gas turbine engine further comprises a low pressure turbine ; a fan driven by the low pressure turbine ; and a fan bypass duct in fluid communication with the fan , wherein variable cycle gas turbine engine is configured to direct the exhaust of the auxiliary turbine into the fan bypass duct . in yet still another refinement , the variable cycle gas turbine engine is configured wherein the portion of the core gas flow is received by the auxiliary turbine from upstream of the low pressure turbine . in a further refinement , the variable cycle gas turbine engine further comprises an accessory gearbox coupled to the compressor , wherein the auxiliary turbine is drivingly coupled to the compressor via the accessory gearbox . in a yet further refinement , the compressor is a variable geometry compressor . embodiments of the present invention include a variable cycle gas turbine engine , comprising : a compressor configured to compress a core gas flow ; a combustor in fluid communication with the compressor and configured to combust the core gas flow ; a turbine drivingly coupled to the compressor and configured to receive the core gas flow , wherein the turbine is configured to drive the compressor ; and means for selectively expanding and contracting a turbine flow capacity . in a refinement , the means for selectively expanding and contracting the turbine flow capacity includes an auxiliary turbine system having an auxiliary turbine drivingly coupled to the compressor , wherein the auxiliary turbine system is configured to selectively receive a portion of the core gas flow ; generate shaft power using the portion of the core gas flow ; and supply the shaft power to the compressor . in another refinement , the means for selectively expanding and contracting the turbine flow capacity includes a valve configured to selectively direct a portion of the core gas flow to the auxiliary turbine . while the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment , it is to be understood that the invention is not to be limited to the disclosed embodiment ( s ), but on the contrary , is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims , which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as permitted under the law . furthermore it should be understood that while the use of the word preferable , preferably , or preferred in the description above indicates that feature so described may be more desirable , it nonetheless may not be necessary and any embodiment lacking the same may be contemplated as within the scope of the invention , that scope being defined by the claims that follow . in reading the claims it is intended that when words such as “ a ,” “ an ,” “ at least one ” and “ at least a portion ” are used , there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim . further , when the language “ at least a portion ” and / or “ a portion ” is used the item may include a portion and / or the entire item unless specifically stated to the contrary .