Patent Publication Number: US-2022234726-A1

Title: Pyrotechnic wheel acceleration system

Description:
BACKGROUND 
     Most aircraft are equipped with landing gear that enables travel on the ground during takeoff, landing, and taxiing phases. These landing gear comprise a plurality of wheels, which may be arranged according to configurations varying from one aircraft to another. For takeoff, aircraft traditionally rely on main engine thrust in order to reach flying speed. 
     Aircraft with low engine thrust and relatively high overall mass have lengthy takeoff distances, as it takes a long time for the low thrust to accelerate the aircraft to flying speed. Although this combination is often acceptable, for instance with long-endurance aircraft and self-launching gliders, in many cases the long takeoff distance is not desirable. Indeed, there are times when operation from reduced runway lengths is necessary—such as the operation of aircraft from aircraft ship decks or when operating from improvised airfields of limited length. While catapults and other such mechanisms effectively reduce take distance, such solutions are not always available. Thus, a means of reducing the takeoff length is desired. 
     Wheel drive systems have been proposed to assist with taxiing, and in some cases to assist aircraft with takeoff. One known wheel drive system is set forth in US Patent Publication No. 2015/0314859, and assigned to Safran Landing Systems. Such a system proposes an undercarriage leg supporting an electric drive actuator, which drives rotation of a landing gear wheel via a reduction gearset. Another known electrical drive system is set forth in US Patent Publication No. 2016/0096619, also assigned to Safran Landing Systems. While these systems are well-suited to taxiing applications, they are not suited to the task of quickly accelerating an aircraft to takeoff speeds, except potentially for very lightweight aircraft. For weight and space reasons, such known systems cannot scale in order to deliver the torque and power output necessary to accelerate even modestly sized aircraft from a ship deck or improvised runway, even with the assistance of main engine thrust. 
     Accordingly, there is a continuing need in the industry for improved techniques of reducing the takeoff length through an aircraft mounted system. 
     SUMMARY 
     The present disclosure provides examples of innovative aircraft-mounted wheel acceleration systems that utilize pyrotechnic cartridge-powered rotary propulsion units, which in turn deliver high torque and power output to one or more aircraft wheels, in order to accelerate the aircraft to flying speed in connection with main engine thrust. 
     In accordance with an aspect of the present disclosure, a wheel acceleration system is provided. The wheel acceleration system is configured to apply a motive force to at least one wheel mounted to a landing gear of an aircraft. The wheel acceleration system includes a pyrotechnic unit configured to generate expanding gases by combusting a propellant, and a rotary propulsion unit pneumatically coupled to the pyrotechnic unit. The rotary propulsion unit includes an impeller configured to be driven by the expanding gases and to deliver torque to the wheel (e.g., to an output shaft coupled to the wheel). 
     In any of the embodiments described herein, the rotary propulsion unit and/or the pyrotechnic unit is mounted to the landing gear. 
     In any of the embodiments described herein, the impeller of the rotary propulsion unit is part of a turbine or an expansion vane motor. 
     In any of the embodiments described herein, the impeller is coupled to the wheel (e.g., coupled to an output shaft of the wheel) via a reduction gearbox. 
     In any of the embodiments described herein, the rotary propulsion unit is contained within an outer housing configured to be mounted to the landing gear. 
     In any of the embodiments described herein, the wheel acceleration system can include a clutch coupling the impeller to the wheel (e.g., coupling the impeller to an output shaft to the wheel, or coupling an output shaft to the wheel). 
     In any of the embodiments described herein, the wheel acceleration system can include a flywheel coupled with rotary propulsion unit (e.g., to an output shaft of the rotary propulsion unit), the flywheel being configured to selectively engage the aircraft wheel. 
     In any of the embodiments described herein, the pyrotechnic unit and the rotary propulsion unit are contained in a common outer housing. 
     In any of the embodiments described herein, the pyrotechnic unit comprises a breech configured to receive a pyrotechnic cartridge containing the propellant. 
     In any of the embodiments described herein, the pyrotechnic cartridge is a pressure vessel and the pyrotechnic unit is configured to jettison the pyrotechnic cartridge by releasing a gas connector that couples the pyrotechnic cartridge to the pyrotechnic unit. 
     In any of the embodiments described herein, the wheel acceleration system can include a second rotary propulsion unit comprising a second impeller configured to be driven by the expanding gases of the pyrotechnic unit, the second impeller configured to be driven by the expanding gases and to deliver torque to a second wheel of the aircraft (e.g., to a second output shaft of the second wheel). 
     In any of the embodiments described herein, the pyrotechnic unit is mounted to a fuselage of the aircraft, the rotary propulsion unit is mounted to the landing gear, and the second rotary propulsion unit is mounted to a second landing gear of the aircraft. 
     In any of the embodiments described herein, the pyrotechnic unit is configured to deliver the expanding gases to the rotary propulsion unit via a first fluid circuit and to the second rotary propulsion unit via a second fluid circuit. 
     In any of the embodiments described herein, the pyrotechnic unit and the rotary propulsion unit are mounted to a fuselage of the aircraft or to the aircraft landing gear. 
     In any of the embodiments described herein, the rotary propulsion unit is configured to rotatably drive the wheel and a second wheel of the aircraft with a plurality of drive shaft assemblies. 
     In any of the embodiments described herein, the rotary propulsion unit is configured to deliver a torque output of at least 1500 Nm to the wheel (e.g., to an output shaft of the wheel). 
     In any of the embodiments described herein, the rotary propulsion unit is configured to deliver a power output of at least 800 Kilowatts to the wheel (e.g., to an output shaft of the wheel). 
     In accordance with another aspect of the present disclosure, a landing gear is provided, equipped with at least one wheel acceleration system as described herein. 
     In accordance with still another aspect of the present disclosure, an aircraft is provided, equipped with at least one wheel acceleration system of the present disclosure. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of the claimed subject matter will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1A  is a schematic diagram of a representative wheel acceleration system in accordance with an embodiment of the present disclosure; 
         FIG. 1B  is a schematic diagram of another representative wheel acceleration system in accordance with another embodiment of the present disclosure; 
         FIG. 2A  is a schematic front view of an aircraft equipped with a representative wheel acceleration system in accordance with an embodiment of the present disclosure; 
         FIG. 2B  is a schematic breakout view of an aspect of the wheel acceleration system of  FIG. 2A ; 
         FIG. 2C  is a schematic breakout view of another aspect of the wheel acceleration system of  FIG. 2A ; 
         FIG. 3A  is a schematic front view of another aircraft equipped with a representative wheel acceleration system in accordance with another embodiment of the present disclosure; and 
         FIG. 3B  is a schematic front view of the aircraft of  FIG. 3A  equipped with a representative wheel acceleration system in accordance with still another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings, where like numerals reference like elements, is intended as a description of various embodiments of the disclosed subject matter and is not intended to represent the only embodiments. Each embodiment described in this disclosure is provided as a representative example or illustration and should not be construed as preferred or advantageous over other embodiments. The representative examples provided herein are not intended to be exhaustive or to limit the claimed subject matter to the precise forms disclosed. 
     Terms such as, but not limited to, “upper,” “lower,” “inboard,” “outboard,” “top,” “bottom,” “side,” “vertical,” “horizontal,” and “lateral” in the present disclosure is meant to provide orientation for the reader with reference to the drawings and is not intended to be the required orientation of the components or to impart orientation limitations into the claims. 
     The present disclosure relates to aircraft-mounted wheel acceleration systems, in addition to aircraft landing gear systems and aircraft equipped with such wheel acceleration systems. The wheel acceleration systems of the present disclosure are aircraft-mounted systems configured to assist with the acceleration of the aircraft to flying speeds, for example with the assistance of main engine thrust. Restated, the power delivered by the wheel acceleration systems and main engine thrust together accelerate the aircraft to takeoff speeds. Such systems have wide applicability to commercial and military aircraft. Advantageously, such wheel acceleration systems effectively reduce takeoff distance, which can enable aircraft to operate from aircraft ship decks and/or improvised runways. 
     As will be described below, the wheel acceleration systems of the present disclosure utilize pyrotechnic charges to generate expanding gases, which in turn drive an impeller that delivers high torque and power to at least one wheel of the aircraft, thereby accelerating the aircraft during the takeoff phase. Following takeoff, the wheel acceleration systems may be stowed on the aircraft or at least partially jettisoned in order to reduce “dead weight.” 
     Advantageously, the wheel acceleration systems of the present disclosure provide sufficient power density to accelerate military aircraft to takeoff speeds, without overly penalizing flying weight. 
       FIG. 1A  is a schematic diagram of an aircraft  102  equipped with a landing gear  104  and a wheel  106 . The landing gear  104  may include one or more struts, legs, or the like. The aircraft  102  may be any type of aircraft  102 , for example military “fighter” type aircraft having a takeoff weight of 4,000 kg or greater, which are tasked with operating from improvised runways or ship decks. Accordingly, the landing gear  104  and wheel  106  include the landing gear structure and wheels of such aircraft  102 , including one or more struts, legs, or the like. In the representative embodiment shown, the landing gear  104  includes a landing gear leg and at least one axle shaft  108 . The wheel  106  is mounted upon the axle shaft  108  or may be coupled to the axle shaft  108  by a coupler  110  such that it is configured for rotational movement. As used herein, “aircraft wheel” refers to both the wheel itself and any tire mounted upon the wheel. In one application, the wheels  106  include the rear wheels of such aircraft (e.g., wheels mounted to the wings or a rearward portion of the fuselage), which generally experience greater friction with the runway surface during takeoff, and thus are well-suited to accelerating the aircraft due to greater traction. However, it is contemplated that the wheel acceleration systems of the present disclosure are suitable for use with the front wheels (i.e., wheels attached to a front portion of the fuselage), rather than the rear wheels. It is further contemplated that wheel acceleration systems of the present disclosure may be utilized on the front and/or rear wheels of a tandem-style landing gear, with wheels on a tail-mounted landing gear, with wheels on landing gear having a plurality of bogies, and with any other type of landing gear. 
     The aircraft  102  is equipped with a representative wheel acceleration system  120  in accordance with an embodiment of the present disclosure, which is configured to apply a motive force to the wheel  106 . It is contemplated that in some embodiments, the aircraft  102  is equipped with a plurality of wheel acceleration systems  120 . For brevity and clarity, a single wheel acceleration system  120  is described below in connection with  FIG. 1A . 
     The aircraft-mounted wheel acceleration system  120  may be mounted in a number of configurations relative to the aircraft  102 . In some embodiments, the wheel acceleration system  120  is mounted at least partially upon the landing gear  104 , while in some embodiments, the wheel acceleration system  120  is mounted at least partially upon a fuselage, wing, or other non-landing gear portion of the aircraft  102 . Representative mounting configurations are described below with respect to  FIGS. 2A-3B . 
     Referring to  FIG. 1A , the wheel acceleration system  120  includes two primary subsystems: a pyrotechnic unit  130  and a rotary propulsion unit  160 . As will be described in detail below, the pyrotechnic unit  130  is configured to generate expanding gases by combusting a propellant, whereas the rotary propulsion unit  160  is configured to utilize the expanding gases from the pyrotechnic unit  130  to create rotary motion, and thus to rotatably drive at least the wheel  106 . Although  FIG. 1A  illustrates certain components as forming part of the pyrotechnic unit  130  or the rotary propulsion unit  160 , such demarcations are representative, not limiting. In some embodiments, the rotary propulsion unit  160  is configured to drive more than one wheel. 
     In some embodiments, the pyrotechnic unit  130  is configured to utilize a pyrotechnic cartridge  132 , which is separate from the pyrotechnic unit  130  in the illustrated embodiment, but in some embodiments forms part of the pyrotechnic unit  130 . The pyrotechnic cartridge  132  includes an outer casing or shell which contains within it a propellant  134 , e.g., a solid or liquid fuel. Representative solid propellants include ammonium nitrate-based propellants suspended in a combustible binder. Representative liquid propellants include hyrdrazine, red nitric acid and jet fuel, and the like. Representative pyrotechnic cartridges  132  include MXU-4A and MXU-4A/A starting cartridges, formerly manufactured by Talley Industries, Inc. of Mesa, Ariz. Whether the propellant  134  is a solid or liquid fuel, its characteristics (e.g., grain composition, size, concentration, chemistry, and/or distribution) can be adjusted to achieve a desired burn rate, gas volume, gas temperature, and other pyrotechnic performance parameters. In some embodiments, the pyrotechnic cartridge  132  includes an integrated ignitor, e.g., an electro-explosive device (not shown). 
     A breech  136  may be provided for forming a pressure vessel configured to receive the pyrotechnic cartridge  132 , and is selectively sealed by a breech cap  138  (which itself can form part of the breech  136 ). In the illustrated embodiment, the breech cap  138  can be selectively opened in order to remove and replace the pyrotechnic cartridge  132 , e.g., after depletion of the propellant and between flights. 
     In the illustrated embodiment, the breech  136  is operatively coupled to an electrical connector  140 , which is configured to receive an ignition signal, e.g., from a pilot of the aircraft  102 , a controller on board the aircraft  102 , or remotely. The ignition signal initiates an ignitor  142  (e.g., an electro-explosive device, an electrical ignition device, a firing pin, or the like), which in turn ignites the propellant  134  within the pyrotechnic cartridge  132 . This ignition of the propellant  134  causes an exothermic reaction, which generates hot expanding gases  144  within the pressure vessel formed by the breech  136  and breech cap  138 . Some embodiments include a mechanical ignitor, either as an alternative or backup to the ignitor  142 . Such embodiments include a firing pin or similar mechanical ignition device disposed on or proximal to the breech  136 . 
     Fluid circuit  146  channels the expanding gases from the breech  136  to the rotary propulsion unit  160 . Depending on the mounting location of the pyrotechnic unit  130 , the fluid circuit  146  can have different forms. For example, in embodiments in which the pyrotechnic unit  130  and rotary propulsion unit  160  have a fixed relative location (e.g., both are mounted on the aircraft landing gear as in  FIG. 2A  or both located centrally on the fuselage of the aircraft as in  FIG. 3B ), the fluid circuit  146  can be a fixed/rigid fluid circuit. In other embodiments in which the relative positions of the pyrotechnic unit  130  and the rotary propulsion unit  160  vary during takeoff or flight, the fluid circuit  146  is a flexible fluid circuit. Such flexible fluid circuit advantageously enables relative movement of the rotary propulsion unit  160  and the pyrotechnic unit  130 , e.g., in embodiments in which the pyrotechnic unit  130  is located centrally on the aircraft fuselage and the rotary propulsion unit  160  is located on the landing gear  104  (as in  FIG. 3A ). Representative fluid circuit  146  includes high temperature-resistant flexible exhaust ducting, rigid exhaust fluid circuit, articulating exhaust joints, and the like. Although the fluid circuit  146  is shown as part of the pyrotechnic unit  130  in  FIG. 1A , it may at least partially form part of the rotary propulsion unit  160  in some embodiments. 
     A relief valve  148  is fitted to the fluid circuit  146  and configured to vent any excess gases produced by the pyrotechnic cartridge  132  which cannot be consumed by the rotary propulsion unit  160  (as described below). In the illustrated embodiment, the relief valve  148  is positioned upstream of an optional gas connector  150  (e.g., a flanged exhaust fitting) which pneumatically connects the fluid circuit  146  to the rotary propulsion unit  160 . However, in some embodiments, the relief valve  148  and gas connector  150  have different relative positions. For example, in some embodiments, the relief valve  148  is located downstream of the gas connector  150 ; in such embodiments, the relief valve  148  and at least a portion of the fluid circuit  146  form part of the rotary propulsion unit  160 . As another example, the gas connector  150  is disposed at an upstream end of the fluid circuit  146  (e.g., where the fluid circuit  146  meets the breech  136  or breech cap  138 ; accordingly, the relief valve  148  is also disposed downstream of the fluid circuit  146  in such embodiments. 
     The rotary propulsion unit  160  connects with the pyrotechnic unit  130  via the fluid circuit  146 , e.g. at the gas connector  150 . As noted above, the rotary propulsion unit  160  is configured to utilize the expanding gases  144  from the pyrotechnic unit  130  to create rotary motion, and thus to rotatably drive the wheel  106 . That is, the rotary propulsion unit  160  utilizes, for example, an expansion vane motor, a turbine, or similar rotary motor to create torque to drive the wheel  106 . 
     Still referring to  FIG. 1A , the rotary propulsion unit  160  is housed within an outer housing  162  configured for mounting upon the landing gear  104  or the fuselage, wing, or other part of the aircraft  102 . In one representative embodiment, the outer housing  162  includes one or more brackets, fasteners, and the like configured to secure the rotary propulsion unit  160  to a leg, strut, fuselage, wing, or other part of the aircraft  102 . 
     The outer housing  162  has a gas inlet  164  in fluid communication with the fluid circuit  146  and configured to receive the expanding gases  144 . The outer housing  162  rotatably supports at least one impeller  166  therein, the vanes of which are acted upon by the expanding gases  144 , which causes the impeller  166  to rotate its impeller shaft  168 , ultimately delivering torque to the wheel  106  and accelerating the aircraft  102 . The spent gases exit the outer housing  162  through one or more gas outlets  170 . 
     In an embodiment, the impeller  166  is part of an expansion vane motor, i.e., a pneumatic motor wherein the impeller  166  comprises a rotor and a plurality of expansion vanes housed within an eccentric stator. The expansion vanes expand and contract radially in order to follow the internal eccentricity of the stator. Consequently, the expanding vane surface offsets declining gas pressure within the stator, causing substantially uniform force delivery along an intake arc about the impeller shaft  168 . One advantageous feature of vane motors is relatively high torque delivery at low speeds. These features make the expansion vane motor well-suited to delivering relatively high starting torque to the impeller shaft  168 . 
     One representative expansion vane motor that can be practiced with embodiments of the present disclosure is described in SAE Technical Paper No. 861714, which is hereby incorporated by reference. See Dusenberry, G. and Carlson, D., “Development of a Hot Gas Vane Motor for Aircraft Starting Systems,” SAE Technical Paper 861714, 1986, https://doi.org/10.4271/861714. Other representative pneumatic expansion vane motors that may be employed include the LZL Vane Air Motors manufactured by Atlas Copco Tools and Assembly Systems LLC of Auburn Hills, Mich. Expansion vane motors may be utilized in all embodiments of the wheel acceleration system  120 , including those having an optional reduction gearbox, flywheel and/or clutch, as described below. In another embodiment, the impeller  166  is a turbine having fixed vane dimensions. Such turbines are well-suited to high-speed/low-torque applications, and may be utilized in all embodiments of the wheel acceleration system  120 , including those having an optional reduction gearbox, flywheel and/or clutch, as described below. 
     The rotary propulsion unit  160  of  FIG. 1A  is shown with a number of optional features which may be disposed within the outer housing  162  and utilized alone or in any combination, in any of the embodiments contemplated herein, in order to meet the torque output requirements of the particular application. An optional reduction gearbox  172  is disposed in the outer housing  162  and configured to be driven by the impeller shaft  168  in order to increase the torque delivered to the wheel  106  (e.g., via at least one output shaft  174 ). Some embodiments of the reduction gearbox  172  include a plurality of output shafts. An optional flywheel  176  is disposed on the output shaft  174 , and an optional clutch  178  is disposed between the output shaft  174  and the axle shaft  108  of the wheel  106 . The flywheel  176  and clutch  178  combination is advantageous because it enables decoupling of the output shaft  174  from the axle shaft  108 , and further enables the expanding gases  144  to accelerate the impeller  166  and the flywheel  176  during the initial burn of the propellant  134 . Once the flywheel  176  is spinning and the propellant burn is stable, the clutch  178  can be engaged in order to deliver the combined torque from the impeller  166  and the flywheel  176  to the axle shaft  108 . 
     In some embodiments, the clutch  178  is an overdriving clutch that enables the wheel  106  to spin-up upon landing without spinning the impeller  166 , reduction gearbox  172 , flywheel  176 , or other elements of the rotary propulsion unit  160 . In some embodiments, the clutch  178  is a selectable clutch configured to engage the wheel  106  or the axle shaft  108  upon receipt of an engagement signal (e.g., from the pilot) and/or automatically (e.g., when the output shaft  174  reaches a predetermined speed). 
     To clarify, some embodiments of the rotary propulsion unit  160  include the clutch  178 , but not the flywheel  176 . Other embodiments include neither the flywheel  176  nor the clutch  178 , and in such embodiments the output shaft  174  is directly coupled to the axle shaft  108  (e.g., a “live axle”), or to a shaft located in the interior of the axle shaft  108 , which in turn is connected to a hub of the wheel  106 . Indeed, in any of the embodiments contemplated herein, the output shaft  174  or flywheel  176  may deliver torque to the wheel  106  by acting directly on the wheel  106 , on the axle shaft  108 , on a shaft located in the interior of the axle shaft  108 , or by similar connection schemes. 
       FIG. 1B  is a schematic diagram of another wheel acceleration system  120   b  in accordance with another representative embodiment of the present disclosure. The wheel acceleration system  120  of  FIG. 1B  is similar to that of  FIG. 1A  except where described below. Accordingly, alike reference numerals and names have alike meanings except where described below. For brevity, certain reference numerals introduced with respect to  FIG. 1A  are not reintroduced with respect to  FIG. 1B . 
     The wheel acceleration system  120  of  FIG. 1B  is configured to minimize flying weight of the aircraft  102  following takeoff and after depletion of the propellant. This configuration described below avoids the undesirable need for the aircraft  102  to carry the pyrotechnic cartridge  132  after depletion of the propellant  134 , when it is dead weight. 
     Whereas the pyrotechnic cartridge  132  of  FIG. 1A  is configured to be manually removed from the breech  136  between flights, e.g., by a ground crewmember, the pyrotechnic cartridge  132  of  FIG. 1B  is configured to be jettisoned following depletion of the propellant  134 . In  FIG. 1B , the pyrotechnic unit  130  does not have a breech or a breech cap; rather, the pyrotechnic cartridge  132  forms its own pressure vessel which contains the propellant  134 . Accordingly, the pyrotechnic cartridge  132  connects directly to the fluid circuit  146  at a gas connector  150 , which can be a pyrotechnic fastener, an electromechanical connector, or the like. 
     Referring still to  FIG. 1B , in use, the pyrotechnic cartridge  132  is ignited prior to or during takeoff, which causes the rotary propulsion unit  160  to deliver torque to the wheel  106 . This process depletes the propellant  134 , after which time the pyrotechnic cartridge  132  becomes dead weight. Therefore, the gas connector  150  releases (jettisons) the pyrotechnic cartridge  132  following takeoff, e.g., upon retraction of the landing gear  104  or upon receipt of a jettison signal from the pilot. As noted above, the gas connector  150  can release the pyrotechnic cartridge  132  by executing a pyrotechnic sequence (in the case of a pyrotechnic fastener), by releasing a latch (in the case of an electromechanical connector), or by a similar release process. Not only does this eliminate dead weight from the aircraft  102 , but it also reduces the volume occupied by the pyrotechnic unit  130 , which can enable retraction of the landing gear  104  into a landing gear bay. 
     The pyrotechnic wheel acceleration systems described above are unlike and superior to electric taxi (“e-taxi”) systems. First, the pyrotechnic wheel acceleration systems are configured to provide much greater absolute torque and power outputs to the wheel  106 , which is necessary to accelerate relatively heavy aircraft (e.g., 4,000 kg or greater) to takeoff speeds. For example, any of the wheel acceleration systems  120  described herein can be configured to provide a torque output of at least 1500 Nm to wheel  106  through the takeoff roll (either directly to the wheel  106 , to the axle shaft  108 , or through a similar connection scheme), resulting in a power output of at least 800 Kilowatts (e.g., 800-1,000 kW) at the end of the runway length or cartridge burn, which lasts from 5-10 seconds. Configurations providing greater torque and power outputs are contemplated. By comparison, known e-taxi systems are incapable of providing such high torque and power outputs. 
     A second key distinction is that the pyrotechnic wheel acceleration systems are configured to provide much greater power densities than e-taxi systems. The superior power densities of the pyrotechnic wheel acceleration systems described herein stems from reduced power source weight, reduced motor weight, absence or minimization of power controls systems, and/or absence or minimization of cooling systems. For example, pyrotechnic cartridges suitable for the wheel acceleration systems described herein (such as those based on the MXU-4A) could weigh approximately 15 kg, as compared to an approximately 140 kg supercapacitor that would be necessary to deliver a comparable power output. Further, the rotary motors described herein, such as pneumatic expansion vane motors, can weigh approximately 25% of electric motors having comparable power output. Further still, the pyrotechnic wheel acceleration systems of the present disclosure do not require cumbersome electronic control systems that would be necessary for e-taxi systems having comparable power output. Further still, given the low duty cycle and short burn times of the pyrotechnic wheel acceleration systems described herein, liquid cooling systems are not necessary. 
     As a result of the foregoing advantages, and the unobvious utilization of a pyrotechnic-cartridge in an aircraft system, the pyrotechnic wheel acceleration systems of embodiments of the present disclosure (including the pyrotechnic unit and the rotary propulsion unit) have a power density of at least about 8.0 kW/kg. For example, in some embodiments, the power density of the pyrotechnic wheel acceleration system can be selected from one of the following power densities: at least 9.0 kW/kg; at least 10.0 kW/kg; at least 11.0 kW/kg, at least 12.0 kW/kg; at least 13.0 kW/kg, at least 14.0 kW/kg, at least 15.0 kW/kg, or about any one of these power densities. In some embodiments, the power densities of the pyrotechnic wheel acceleration system is in a range selected from one of the following ranges of power densities: between 8.0-20.0 kW/kg; between about 9.0-18.0 kW/kg; between about 10.0-16.0 kW/kg; between 8.0-12.0 kW/kg; between 10.0-12.0 kW/kg; or between about any one of these ranges. The foregoing power densities are much higher than known assisted taxi systems. 
     Relatedly, embodiments of the rotary propulsion units of the present disclosure have a power density of, for example, at least 18.0 kW/kg. In some embodiments, the power density of the rotary propulsion unit is in a range between 20.0-30.0 kW/kg. In a representative embodiment, the rotary propulsion unit includes a direct drive vane motor configured to fit inside an 20 cm-diameter aircraft wheel and to generate about 426 kW (570 hp). This rotary propulsion unit weighs between about 19.0 and about 22.0 kg, and weighs about 20.6 kg in some embodiments. Additional components of the pyrotechnic wheel acceleration system (including the cartridge, connections, valves, and the like) weigh in the range of between about 18.0 kg and about 22.0 kg, and about 20.0 kg in some embodiments. 
     In a certain embodiment, the rotary propulsion unit has a power density of 20.6 kW/kg and the overall pyrotechnic wheel acceleration system has a power density of 10.5 kW/kg. 
     By comparison, in order for any e-taxi system to provide torque and power outputs as high as 1500 Nm and/or 800 kW, the immense supercapacitors, electric motors, power electronics, and cooling hardware of such an electric system would contribute to a very low power density, rendering it unsuitable for accelerating aircraft weighing at least 4,000 kg to flying or takeoff speeds. 
     The wheel acceleration systems described herein can be mounted in a number of different configurations to suit particular applications. For example, aircraft having wing-mounted landing gear are well-suited to the representative embodiments shown in  FIGS. 2A-2C , whereas aircraft having centralized (e.g., fuselage-mounted) landing gear are well-suited to the representative embodiment shown in  FIGS. 3A-3B . 
     In  FIGS. 2A-2B , the wheel acceleration systems are mounted to inboard sides of rear wheels of the aircraft. However, this is not limiting. In variations of any of the embodiments contemplated herein, the wheel acceleration systems can alternatively be mounted to an outboard side of one wheel (such as shown in  FIG. 2C ). In still other embodiments, the aircraft includes a wheel acceleration system mounted to a front wheel, in addition to or alternatively to the wheel acceleration systems mounted to the rear wheels. 
       FIG. 2A  shows a front elevation view of a representative military “fighter” type aircraft  202  having wing-mounted landing gear  204  and rear wheels  206 . The aircraft  202  is equipped with two wheel acceleration systems  220 , each being mounted to an inboard side of one rear wheel  206  and together forming part of a common wheel acceleration system. 
       FIG. 2B  is as breakout view showing details of the starboard wheel acceleration system  220 , according to one representative mounting configuration which is applicable to all embodiments contemplated herein. As shown, each wheel acceleration system  220  includes a pyrotechnic unit (“PU”)  230  and a rotary propulsion unit (“RPU”)  260 . In some embodiments, each wheel acceleration system  220  has all of the components of the wheel acceleration system  120  of  FIG. 1A . In other embodiments, each wheel acceleration system  220  has all of the components of the wheel acceleration system  120  of  FIG. 1B . In still other embodiments, each wheel acceleration system  220  has less than all of the aforementioned components, e.g., an optional flywheel and/or optional clutch are omitted. In still other embodiments, the two wheel acceleration systems  220  are constructed identically to each other, although this is not necessary. 
     The PU  230  is disposed on an inboard side of the RPU  260 , which is mounted over the axle shaft  208  of the wheel  206  in order to facilitate torque delivery. In some embodiments, the PU  230  and RPU  260  share a common outer housing. In some embodiments in which the PU  230  has a jettisonable pyrotechnic cartridge, the entire PU  230  is configured to be jettisoned following takeoff. 
       FIG. 2C  is as breakout view showing details of the port-side wheel acceleration system  220 , according to another representative mounting configuration, which is applicable to all embodiments contemplated herein. Whereas the wheel acceleration system  220  of  FIG. 2B  is mounted over the axle shaft  208  of the wheel  206 , the wheel acceleration system  220  of  FIG. 2C  has an output shaft  274  (of the RPU  260 ) coupled directly to a hub or coupler  210  of the wheel  206 . 
     The mounting configurations of  FIGS. 2B-2C  are representative, not limiting. For example, any of the embodiments disclosed herein may utilize one or both of the mounting schemes shown in  FIGS. 2B-2C  on the inboard or outboard sides of one or more wheels, including the rear wheels and/or front wheels. 
       FIG. 3A — 3 B show a front elevation view of another representative military “fighter” type aircraft  302  having centralized, fuselage-mounted landing gear  304  and rear wheels  306 , which differs from the wing-mounted landing gear  204  and wheels  206  of the aircraft  202  of  FIG. 2A . Such centralized and narrow landing gear systems are well-suited to wheel acceleration systems having one or more centralized components (i.e., components that serve more than one wheel  306 ). The centralized components can be scaled up in order to provide power and torque to a plurality of wheels  306 , such as both rear wheels  306  in the illustrated embodiment. Such systems also provide weight savings as compared to embodiments with a plurality of pyrotechnic units, in which no components serve more than one wheel. 
     Referring to  FIG. 3A , the aircraft  302  is equipped with a wheel acceleration system  320 . The embodiment of  FIG. 3A  differs in that the wheel acceleration system  320  includes a pyrotechnic unit (“PU”)  330  centrally mounted to a fuselage  312  of the aircraft  302 , and two rotary propulsion units (“RPUs”)  360 —one mounted to the inboard side of each rear wheel  306 . Each of the PU  330  and RPUs  360  comprise the components of wheel acceleration system  120  of  FIG. 1A or 1B  (some embodiments do not include the optional flywheel and/or clutch). Accordingly, the PU  330  has at least one gas outlet from the breech or pyrotechnic cartridge (depending on the embodiment), in order to deliver expanding gases via flexible fluid circuit  346  to each of the RPUs  360 , which each deliver torque to the respective wheels  306 . Although the PU  330  is shown as external to the aircraft fuselage in  FIG. 3A , it is contemplated that in some embodiments, the PU  330  is stowed internally to the fuselage, for improved aerodynamics. 
     The flexible fluid circuit  346  enables relative movement of the PU  330  and RPUs  360 , for example during articulation, retraction, and extension of the landing gear  304 . In some embodiments, the flexible fluid circuit  346  comprises a continuous length of high temperature-resistant flexible exhaust ducting extending from the PU  330  to each RPU  360 . In other embodiments, the flexible fluid circuit  346  comprises one or more sections of rigid exhaust piping, which are coupled together by flexible fluid circuit couplers  352  (e.g., articulating exhaust joints and the like). In some embodiments, the flexible fluid circuit  346  is routed along the landing gear struts to improve aerodynamics and to facilitate stowage of the landing gear  304  during flight. Accordingly, it is contemplated that the fluid circuit  346  may be at least partially routed internally through the fuselage and/or wing of the aircraft  302 , rather than fully external to the aircraft  302 . 
     Referring now to  FIG. 3B , the aircraft  302  is equipped with a wheel acceleration system  320  having a different centralized configuration from that of  FIG. 3A . While the wheel acceleration system  320  of  FIG. 3B  still comprises the components of wheel acceleration system  120  of  FIG. 1A or 1B  (some embodiments do not include the optional flywheel and/or clutch), it differs in that it includes a centrally-mounted pyrotechnic unit (“PU”)  330  and a centrally mounted RPU  360 —both of which are mounted to the fuselage  312  of the aircraft  302 . Although the PU  330  and RPU  360  are shown as mounted external to the aircraft fuselage in  FIG. 3B , it is contemplated that in some embodiments, the PU  330  and/or RPU  360  are stowed internally to the fuselage  312 , for improved aerodynamics. 
     Because PU  330  and RPU  360  are both configured to service both wheels  306 , power is transferred from the RPU  360  to the wheels  306  via a plurality of drive shaft assemblies  380 . Each drive shaft assembly  380  includes a plurality of drive shafts  382  connected by at least one drive shaft coupler  384  (e.g., universal joints, continuously variable joints, reduction gearboxes, angled gearboxes, or the like). At an upper end, each drive shaft assembly  380  is operably connected to an output shaft of the RPU  360 , either directly or via a reduction gearbox. At a lower end, each drive shaft assembly  380  is operably connected to one of the wheels  306 , either directly (e.g., to a hub of the wheel), via an axle shaft, via a shaft internal to the axle shaft, via one or more drive shaft couplers  384 , or via similar connection scheme (as described above). In use, ignition of a pyrotechnic cartridge in the PU  330  causes the RPU  360  to deliver torque to both wheels  306  via the drive shaft assemblies  380 , thus accelerating the aircraft  302 . 
     Thus, the present disclosure provides a number of innovative wheel acceleration systems having an unobvious combination of features that are together configured to accelerate an aircraft to flying speed in connection with main engine thrust, utilizing pyrotechnic cartridge-powered rotary propulsion units, which in turn deliver high torque and power output to one or more aircraft wheels. 
     The present application may reference quantities and numbers. Unless specifically stated, such quantities and numbers are not to be considered restrictive, but exemplary of the possible quantities or numbers associated with the present application. Also in this regard, the present application may use the term “plurality” to reference a quantity or number. In this regard, the term “plurality” is meant to be any number that is more than one, for example, two, three, four, five, etc. The terms “about,” “approximately,” “near,” etc., mean plus or minus 5% of the stated value. For the purposes of the present disclosure, the phrase “at least one of A and B” is equivalent to “A and/or B” or vice versa, namely “A” alone, “B” alone or “A and B.” Similarly, the phrase “at least one of A, B, and C,” for example, means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B, and C), including all further possible permutations when greater than three elements are listed. 
     The principles, representative embodiments, and modes of operation of the present disclosure have been described in the foregoing description. However, aspects of the present disclosure which are intended to be protected are not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. It will be appreciated that variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present disclosure. Accordingly, it is expressly intended that all such variations, changes, and equivalents fall within the spirit and scope of the present disclosure, as claimed.