Abstract:
An apparatus for providing propulsive power that utilizes a novel swirl generator for rapidly and efficiently atomizing, vaporizing, as necessary, and mixing a fuel into an oxidant. The swirl generator converts an oxidant flow into a turbulent, three-dimensional flowfield into which the fuel is introduced. The swirl generator effects a toroidal outer recirculation zone and a central recirculation zone, which is positioned within the outer recirculation zone. These recirculation zones are configured in a backward-flowing manner that carries heat and combustion byproducts upstream where they are employed to continuously ignite a combustible fuel/oxidizer mixture in adjacent shear layers. The swirl generator is compatible with the throttle range of conventional gas turbine engines, provides smooth combustion with no instabilities and minimum total pressure losses, enables significant reductions the in L/D ratio of the combustor and is readily packaged into various applications. Other benefits include simplicity, reliability, wide flammability limits and high combustion efficiency/thrust performance. This concept is applicable to various propulsion systems such as such as ramjet missiles, combine cycle engines, gas turbine afterburners and energy conversion devices.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to improvements in combustors and more particularly to an improved fuel/oxidizer mixing and combusting apparatus that is readily integrated into a compact, lightweight, high-performance lift thrust producing and energy conversion devices.  
         BACKGROUND OF THE INVENTION  
         [0002]    During the development of the Boeing Joint Strike Fighter (JSF) Short Take-Off Vertical Landing (STOVL) aircraft propulsion system, we found it desirable to incorporate a means for providing auxiliary thrust capability to augment the core engine lift thrust. We evaluated numerous propulsion options that were based on conventional combustor technology, but drawbacks associated with each of these options precluded their employment in the STOVL aircraft propulsion system.  
           [0003]    These drawbacks were primarily related to the overall length of the combustor, rendering the incorporation of various known propulsion options into the STOVL aircraft propulsion system unpractical due to concerns for weight and packaging. In this regard, it is relatively common for combustors without high total pressure loss instream flameholders to be designed with a length-to-diameter (L/D) ratio of 5 or more, wherein the length of the combustor is defined as being the distance between the combustor dump plane and the nozzle throat. Reductions in the L/D ratio of suitable known combustors was not possible, as a shortening of the combustor&#39;s length would not provide sufficient time for the fuel and air (oxidizer) to mix and combust prior to their discharge through the nozzle.  
         SUMMARY OF THE INVENTION  
         [0004]    In one preferred form, the present invention provides a fuel/oxidizer mixing and combusting apparatus having a combustor and a swirl generator. The swirl generator includes an inlet housing, a swirl vane pack, a centerbody assembly and a plurality of fuel injectors. The inlet housing is coupled to the combustor inlet and defines a hollow interior volume that intersects the combustor inlet at a dump step. An oxidizer flow having a velocity that is substantially defined by an axial velocity component is conducted through the hollow interior volume. The swirl vane pack is disposed within the hollow interior volume and includes a plurality of vanes that cooperate to change the velocity of the oxidizer flow so that it includes a substantial tangential velocity component. The centerbody assembly is disposed in the hollow interior volume and coupled to the swirl vane pack such that it extends rearwardly from the swirl vane pack. The fuel injectors are coupled to at least one of inlet housing, the swirl vane pack and the centerbody and dispense fuel therefrom. The swirl generator converts the oxidizer flow received therein into a swirling, three-dimensional flowfield, a first portion of which flows over the dump step to form an outer recirculation zone and a second portion of the flowfield forms a central recirculation zone that is anchored by an aft end of the centerbody assembly. A first portion of the fuel mixes with the first portion of the flowfield to fuel the outer recirculation zone and a second portion of the fuel mixes with the second portion of the flowfield to fuel the central recirculation zone.  
           [0005]    In another preferred form, the present invention provides an apparatus comprising a combustor and a swirl generator, the swirl generator is coupled to the inlet of the combustor and is operable for converting an oxidizer flow with a velocity that is substantially completely defined by an axial velocity component into a three-dimensional flowfield that includes a substantial tangential velocity component. The swirl generator includes a flow defining means and a fueling means. The flow defining means is operable for controlling both a toroidal outer recirculation zone and a central recirculation zone that is disposed inwardly of the outer recirculation zone in the combustor. The fueling means is operable for fueling the outer and central recirculation zones. Heat and combustion by-products produced during combustion are carried upstream by the outer and central recirculation zones where the heat and combustion by-products are employed to continuously ignite a combustible fuel/oxidizer mixture in a shear layer adjacent each of the outer and central recirculation zones.  
           [0006]    The present invention overcomes the aforementioned drawbacks by providing a fuel/oxidizer mixing and combusting apparatus that permits extremely rapid fuel atomization, vaporization and mixing with a combustion efficiency that is ranges from about 90% to about 99%. Consequently, we were able to reduce the length of the combustor to attain L/D ratios substantially less than that which was attained by the known combustors.  
           [0007]    In yet another preferred form, the present invention provides an apparatus for providing propulsive power having an inlet structure, a swirl generator and a combustor. The inlet structure is configured to conduct an oxidizer flow therethrough and includes a plurality of flow guiding vanes that cooperate to affect the oxidizer flow such that it is substantially uniform and axial. The swirl generator is coupled to an aft end of the inlet structure and includes a flow affecting means and a fueling means. The flow affecting means includes a plurality of vanes, a dump step and a centerbody with an aft end that terminates rearwardly of a root of the vanes. The flow affecting means is operable for converting the axialsymmetric oxidizer flow into a three dimensional flowfield that includes a substantial tangential velocity component. The flow affecting means is also operable for forming and controlling both a toroidal outer recirculation zone, which is anchored by the dump step, and a central recirculation zone, which is disposed inwardly of the outer recirculation zone and anchored by the aft end of the centerbody. The fueling means is operable for fueling the outer and central recirculation zones and a core flow. The combustor coupled to an aft end of the swirl generator and includes a nozzle. Heat and combustion by-products produced during combustion are carried upstream by the outer and central recirculation zones where they are employed to continuously ignite a combustible fuel/oxidizer mixture in a shear layer adjacent each of the outer and central recirculation zones. Combustion by-products from the core flow are expelled through the nozzle to produce thrust.  
           [0008]    The present invention overcomes the aforementioned drawbacks by providing an apparatus for providing propulsive power that utilizes swirl combustion in a manner that significantly reduces the overall weight and size of the apparatus relative to other propulsion devices that employ conventional combustor technology. The apparatus of the present invention is compatible with the throttle range of a conventional gas turbine engine, provides smooth combustion with no instabilities with minimum total pressure losses, enables significant reductions in the L/D ratio of the combustor relative to the known combustors and is readily packaged into various applications. Other benefits over the known art include simplicity, reliability, wide flammability limits and high combustion efficiency and high thrust performance. Furthermore, the apparatus of the present invention provides substantial flexibility to retune existing combustor designs to meet specific combustor requirements.  
           [0009]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein:  
         [0011]    [0011]FIG. 1 is a perspective view of a jet aircraft having a pair of lift thrust augmentors constructed in accordance with the teachings of the present invention;  
         [0012]    [0012]FIG. 2 is a schematic illustration of a portion of the jet aircraft of FIG. 1;  
         [0013]    [0013]FIG. 3 is a cut-away perspective view of a portion of the lift thrust augmentor illustrated in FIG. 1;  
         [0014]    [0014]FIG. 4 is a longitudinal section view of a portion of the lift thrust augmentor;  
         [0015]    [0015]FIG. 5 is an exploded perspective view of a portion of the lift thrust augmentor illustrating the elbow in greater detail;  
         [0016]    [0016]FIG. 6 is a longitudinal section view of the elbow;  
         [0017]    [0017]FIG. 7 is an exploded perspective view of a portion of the lift thrust augmentor;  
         [0018]    [0018]FIG. 8 is an exploded perspective view of a portion of the lift thrust augmentor illustrating the centerbody hub assembly in greater detail;  
         [0019]    [0019]FIG. 9 is a longitudinal section view of the centerbody hub assembly;  
         [0020]    [0020]FIG. 9A is a schematic view of the swirl generator of the present invention illustrating several various centerbody assembly and wall injector fueling schemes;  
         [0021]    [0021]FIG. 10 is a perspective view of a portion of the swirl generator illustrating the swirl vane pack in greater detail;  
         [0022]    [0022]FIG. 10A is a partial top plan view of an alternately constructed swirl vane pack illustrating a profiled vane configuration;  
         [0023]    [0023]FIG. 11 is an exploded perspective view of the swirl vane pack of FIG. 10;  
         [0024]    [0024]FIG. 12 is an exploded sectional view of a portion of the swirl vane pack;  
         [0025]    [0025]FIG. 13 is a longitudinal section view similar to that of FIG. 4 but illustrating a combustion event;  
         [0026]    [0026]FIG. 14 is a partially broken away perspective view of a portion of a swirl generator constructed in accordance with the teachings of an alternate embodiment of the present invention;  
         [0027]    [0027]FIG. 15 is a sectional view of a portion of a swirl generator constructed in accordance with the teachings of a second alternate embodiment of the present invention which illustrates an alternatively constructed centerbody hub assembly in detail;  
         [0028]    [0028]FIG. 16 is an exploded perspective view of a portion of a swirl generator constructed in accordance with the teachings of a third alternate embodiment of the present invention which illustrates another alternatively constructed centerbody hub assembly;  
         [0029]    [0029]FIG. 17 is a longitudinal section view of the centerbody hub assembly of FIG. 16;  
         [0030]    [0030]FIG. 18 is a partially broken away side elevation view of a swirl generator constructed in accordance with the teachings of a fourth alternate embodiment of the present invention;  
         [0031]    [0031]FIG. 19 is a perspective view of a portion of a swirl generator constructed in accordance with the teachings of a fifth alternative embodiment of the present invention which illustrates a vane for an alternative swirl vane pack with a plurality of fuel injection sites;  
         [0032]    [0032]FIG. 20 is a partial sectional view of the vane of FIG. 19;  
         [0033]    [0033]FIG. 21 is a perspective view of a portion of a swirl generator constructed in accordance with the teachings of a sixth alternative embodiment of the present invention which illustrates a vane for an alternative swirl vane pack with turbulator ramps and a plurality of fuel injection sites;  
         [0034]    [0034]FIG. 22 is a partial sectional view taken through the vane of FIG. 21;  
         [0035]    [0035]FIG. 23 is a front elevation view of a swirl generator constructed in accordance with the teachings of a seventh alternative embodiment of the present invention which illustrates a vane for a third alternative swirl vane pack having scallops;  
         [0036]    [0036]FIG. 24 is a partial sectional view of a swirl generator constructed in accordance with the teachings of a eighth alternative embodiment of the present invention which illustrates an alternative inlet housing wherein a plurality of channels are formed into the inlet ramp;  
         [0037]    [0037]FIG. 24A is a sectional view taken along the line  24 A- 24 A of FIG. 24;  
         [0038]    [0038]FIG. 25 is a partial sectional view of a swirl generator constructed in accordance with the teachings of a ninth alternative embodiment of the present invention which illustrates another alternative inlet housing illustrating the incorporation of fuel injection sites into the inlet ramp;  
         [0039]    [0039]FIG. 26 is a partial sectional view of a swirl generator constructed in accordance with the teachings of a tenth alternative embodiment of the present invention which illustrates an alternative combustor illustrating the use of a quarl extension;  
         [0040]    [0040]FIG. 27 is a partial sectional view of a swirl generator constructed in accordance with the teachings of a eleventh alternative embodiment of the present invention which illustrates another alternately configured centerbody hub assembly;  
         [0041]    [0041]FIG. 28 is a partial sectional view of a swirl generator constructed in accordance with the teachings of a twelfth alternative embodiment of the present invention which illustrates another alternately configured centerbody hub assembly;  
         [0042]    [0042]FIG. 29 is a front elevation view of a ramjet missile that incorporates a swirl generator constructed in accordance with the teachings of the present invention;  
         [0043]    [0043]FIG. 30 is a longitudinal section view taken along the line  30 - 30  of FIG. 29;  
         [0044]    [0044]FIG. 31 is a longitudinal section view of a ramshell that incorporates a swirl generator constructed in accordance with the teachings of the present invention;  
         [0045]    [0045]FIG. 32 is a partial longitudinal section view of a combined cycle engine having a plurality of ramjet engines that incorporate a swirl generator constructed in accordance with the teachings of the present invention;  
         [0046]    [0046]FIG. 33 is a section view taken along the line  33 - 33  of FIG. 32;  
         [0047]    [0047]FIG. 34 is a partial longitudinal section view of a second combined cycle engine having a plurality of ramjet engines that incorporate a swirl generator constructed in accordance with the teachings of the present invention;  
         [0048]    [0048]FIG. 35 is a section view taken along the line  35 - 35  of FIG. 34;  
         [0049]    [0049]FIG. 36 is a longitudinal section view of a rocket-based combined cycle engine constructed in accordance with the teachings of the present invention;  
         [0050]    [0050]FIG. 37 is a rear elevation view of the rocket-based combined cycle engine of FIG. 37;  
         [0051]    [0051]FIG. 38 is a perspective view of a portion of the rocket-based combined cycle engine illustrating the variable area throat in an open condition;  
         [0052]    [0052]FIG. 39 is a perspective view similar to that of FIG. 38, but illustrating the variable area throat in a closed condition;  
         [0053]    [0053]FIG. 40 is a longitudinal section view of an aircraft engine having a conventional afterburner; and  
         [0054]    [0054]FIG. 41 is a longitudinal section view similar to FIG. 40 but illustrating an afterburner that incorporates a swirl generator constructed in accordance with the teachings of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0055]    With reference to FIG. 1 of the drawings, an exemplary jet aircraft  8  is illustrated to include a pair of lift thrust augmentors  10  that are constructed in accordance with the teachings of the present invention. A conventional gas turbine engine  12  serves as the primary source of propulsive power for the jet aircraft  8 , while the lift thrust augmentors  10  are selectively operable to produce thrust when the demand for thrust exceeds a predetermined threshold.  
         [0056]    With additional reference to FIGS. 2 through 4, each lift thrust augmentor  10  includes an air delivery portion  20 , a swirl generator  30  and a combustor/nozzle portion  40 . When combined, the swirl generator  30  and combustor/nozzle portion  40  are also known as a compact swirl-augmented thruster (COSAT). The air delivery portion  20  includes a butterfly valve  50  and in the particular example provided, an elbow  60 . It will be readily apparent to those skilled in the art that the air delivery portion  20  may be configured in any manner to package the lift thrust augmentor  10  into a particular application and as such, the elbow  60  may be omitted or the bend angle changed to suit the specific needs of a given application.  
         [0057]    The butterfly valve  50  and the elbow  60  are coupled in fluid connection to the gas turbine engine  12 . More specifically, hot high-pressure air is bled from the bypass fan  12   a  of the gas turbine engine  12  and diverted through a lateral jetscreen feed system (not specifically shown) to an attitude control system  70 . The butterfly valve  50 , which is normally maintained in a closed condition and coupled in fluid connection with the attitude control system  70 , is opened to divert a predetermined amount (i.e., mass flow rate) of the hot high-pressure bleed air to the lift thrust augmentors  10 . In the particular embodiment provided, about 30% of the airflow that is directed into the attitude control system  70  is redirected to the lift thrust augmentors  10  when the lift thrust augmentors  10  are operated to provide maximum thrust.  
         [0058]    The air that passes by the butterfly valve  50  is routed through the elbow  60  to the swirl generator  30 . As will be described in greater detail below, the swirl generator  30  injects fuel in to the air and promotes the efficient mixing of the air and fuel. A torch igniter  80 , such as that described in copending U.S. patent application Ser. No. 10/217,972 entitled “Torch Igniter”, the disclosure of which is hereby incorporated by reference as if fully set forth herein, is employed to initiate a combustion event wherein the fuel/air mixture is burned in the combustor  40   a  of the combustor/nozzle portion  40 . Those skilled in the art will appreciate that igniters, including electric spark igniters, plasma jet igniters, lasers and microwaves, may be employed in the alternative. As the construction of the combustor  40   a  is well known in the art, a detailed discussion of the combustor will not be provided herein. For example, those skilled in the art will appreciate that the combustor  40   a  may be wholly formed of a suitable high temperature material, or may utilize a perforated liner that facilitates air-cooling of the combustor  40   a,  or may utilize a wall that is partially or wholly comprised of fluid conduits that facilitate a flow of fluid through or about the combustor wall that operates to cool the combustor  40   a  during its operation.  
         [0059]    Thereafter, hot combustion by-products are expelled through a nozzle  40   b,  such as a convergent nozzle (FIG. 4) or a convergent/divergent nozzle (FIG. 2), to produce thrust. It will be understood that the particular combustible mixture (i.e., a liquid fuel and air) that is utilized in this example is not intended to limit the scope of the disclosure in any way. In this regard, those skilled in the art will understand that any type of fuel (e.g., liquid, slurry, gas) and any type of oxidant (e.g., air, hydrogen peroxide, oxygen) may be utilized in the swirl generator and lift thrust augmentor of the present invention. Accordingly, when the term “mixing” is used in the context of a fuel/oxidant mixture, it will be understood to include atomization and vaporization if the fuel is not injected in gaseous state.  
         [0060]    With reference to FIGS. 5 and 6, the elbow  60  is illustrated to turn the airflow  90  about 90° and is relatively constant in its cross-sectional area. The elbow  60  includes an elbow housing  94 , a plurality of flow guide vanes  96 , a pair of side covers  98  and an aerodynamic fairing  100 . As will be readily apparent to one skilled in the art, the components of the elbow  60  may be fabricated out of any appropriate material, the selection of which is largely dependant on the temperature of the air entering the air delivery portion  20  (FIG. 2). In the particular example provided, the inlet temperature may be relatively high and as such, materials including Cress stainless steel series (e.g., 304, 321), Haynes 240, ceramic matrix composites (e.g., C/SiC, SiC/SiC, Si 3 N 4 , Al 2 O 3 /Al 2 O 3 ) would be appropriate for the construction of the components of the elbow  60 . As those skilled in the art will appreciate, various thermally protective coatings (e.g., ceramics) and/or heat transfer techniques that rely on a cooling effect provided by a working fluid (e.g., fuel, air) may additionally or alternatively be employed to render the components of the elbow  60  suitable for a given set of inlet conditions.  
         [0061]    The elbow housing  94  is shown to include an inlet portion  102  and an outlet portion  104  that are interconnected by an arcuate turning portion  106 . The elbow housing  94  may be unitarily formed via conventional processes such as casting or machining (from bar stock), or may be a multi-component fabrication that is fixedly coupled together as by welding. The lateral sidewalls  110  of the turning portion  106  include a plurality of concentric slots  112  that are formed therethrough, while the endwall  114  in the turning portion  106  includes a fairing aperture  116 . The fairing aperture  116  is sized to receive a portion of the aerodynamic fairing  100 , as well as a conduit  120  (FIG. 4) that couples the swirl generator  30  to an ignition controller  122  (FIG. 4) that actuates the lift thrust augmentor  10 , as well as the fuel source  126  (FIG. 4) of the jet aircraft  8 .  
         [0062]    The flow guide vanes  96  are curved about a single axis and slide into the turning portion  106  through the concentric slots  112 . Accordingly, the flow guide vanes  96  extend across the inner dimension of the turning portion  106  in a direction that is generally perpendicular to the direction of the air flow and concentric with the radius of the turn in the turning portion  106 . The flow guide vanes  96  are also configured such that they extend between the inlet portion  102  and the outlet portion  104  of the elbow housing  94 . Accordingly, the radially inward flow guide vanes, such as flow guide vane  96   a,  are relatively shorter than the radially outer flow guide vanes, such as flow guide vane  96 d.  
         [0063]    The leading and trailing edges  130  and  132 , respectively, of the flow guide vanes  96  preferably engage the opposite ends of the concentric slots  112 , while the opposite lateral sides  136  of the flow guide vanes  96  are abutted against the side covers  98 , which are fixedly secured to the elbow housing  94  via a plurality of screws (not specifically shown). Additionally or alternatively, the flow guide vanes  96  may be welded in place to secure them in place within the concentric slots  112 .  
         [0064]    The flow guide vanes  96  are employed to mitigate air flow distortions and flow separation induced by the associated upstream butterfly valve  50  (FIG. 2) as well as to prevent further flow separation, the creation of secondary flows and large scale profile distortions due to centrifugal forces as the inlet flow undergoes the 90° turn through the turning portion  106 . If unabated, secondary flows, separations and distortions tend to complicate the design and operation of the downstream fuel injection, reduce the mixing effectiveness and provide stimuli for combustion instabilities and inefficiencies. This would, in turn, tend to reduce the life cycle of the lift thrust augmentor  10  and create unacceptable thrust pulsations. Consequently, the configurations of the air delivery portion  20  (generally) and the elbow  60  (specifically) provide a high degree of uniformity in the flow of air (i.e., a uniform axial airflow) to the swirl generator  30 .  
         [0065]    As best seen in FIG. 6, the flow guide vanes  96  cooperate with the elbow housing  94  to define a plurality of inlet flow channels  140 . As the path that is defined by inlet flow channel  140   a  is relatively shorter between the inlet portion  102  and the outlet portion  104  as compared with the inlet flow channel  140   e  and as transient flow differences between the inlet flow channels  140  are highly undesirable, the flow guide vanes  96  of the particular embodiment illustrated are positioned in a radial direction in a manner that provides inlet flow channels  140  with dissimilar cross-sectional areas such that the inlet flow channels  140  produce a series of inlet flows that are relatively uniform in flow velocity. Accordingly, the inlet flow  150  to the swirl generator  30  has a velocity that is substantially completely defined by an axial velocity component.  
         [0066]    In FIGS. 4 through 6, the conduit  120  that couples the swirl generator  30  to the ignition controller  122  (FIG. 2) and the fuel source  126  (FIG. 2) is illustrated to extend through the endwall  114  of the turning portion  106 , through a conduit aperture  154  in the flow guide vane  96   d  and out the outlet portion  104  of the elbow housing  94  where it is coupled to the swirl generator  30 . The presence of the conduit  120  in the interior of the turning portion  106  would ordinarily initiate a flow separation, which as mentioned above, reduces the effectiveness and efficiency of the lift thrust augmentor  10 . The aerodynamic fairing  100 , however, is employed to reduce or eliminate altogether, the flow separations that would be induced by the presence of the conduit  120 .  
         [0067]    In the example provided, the aerodynamic fairing  100  includes a hollow hub  156  and an airfoil portion  158  that is coupled to the hub  156 . The conduit  120  extends through the hollow interior of the hub  156  and may be coupled thereto by any appropriate retaining means, including an interference fit (e.g., shrink fit or press fit), brazing and welding. The airfoil portion  158 , which surrounds the conduit  120 , extends in the in the down-wind direction in a tapered manner and terminates at its trailing edge  164  with a relatively small amount of trailing edge bluntness. Alternatively, the airfoil portion  158  may be configured to abut the windward side of the conduit  120 . The airfoil portion  158  is positioned within the flow channel  140   e  so as to reduce or eliminate altogether the flow separations that would be induced by the presence of the conduit  120 . The hub  156  and/or airfoil portion  158  may be fixedly coupled to the elbow housing  94  by any appropriate retaining means, including an interference fit (e.g., shrink fit or press fit), brazing and welding.  
         [0068]    With reference to FIGS. 4 and 7, the swirl generator  30  is illustrated to include an inlet housing  200 , a centerbody hub assembly  204 , a swirl vane pack  206  and a wall injector assembly  208 . In the particular example provided, the inlet housing  200  is illustrated to include an upper inlet housing segment  200   a,  which is coupled to and integrally formed with the outlet portion  104  of the elbow  60 , an optional wall injection housing  228  (which will be described in detail, below) and an optional lower inlet housing segment  200   b,  which is coupled to the combustor/nozzle portion  40  and which includes an inlet extension  210  and an inlet ramp  212 . Alternatively, the inlet housing  200  may be unitarily formed. Also alternatively, the upper inlet housing segment  200   a  may be separate from the outlet portion  104  of the elbow  60  and/or the lower inlet housing segment  200   b,  if included, may be integrally formed with the combustor/nozzle portion  40 . Accordingly, those skilled in the art will appreciate that one or more of the upper and lower inlet housing segments  200   a  and  200   b  and the wall injection housing  208  may not exist as a discrete component. In the presently preferred configuration of the lift thrust augmentor  10 , the upper inlet housing segment  200   a  is integrally formed with the elbow  60  so as to minimize the overall length of the lift thrust augmentor  10 .  
         [0069]    The inlet housing  200  defines a hollow interior volume  220  into which the centerbody hub assembly  204  extends. The injection of fuel into the lift thrust augmentor  10  is illustrated to include fuel injection through the wall  224  of the inlet housing  200  at a location forwardly of the inlet ramp  212 . The sizing and purpose of the inlet ramp  212  will be discussed in greater detail, below.  
         [0070]    As all fuel injection occurs downstream of the swirl vane pack  206 , the presence of the inlet extension  210  effectively shifts the various points of fuel injection in an upstream direction relative to the inlet ramp  212  so as to provide additional time for the fuel to mix (i.e., for the liquid fuel of this example to atomize, mix and vaporize) prior to entering the combustor  40   a.  Those skilled in the art will appreciate that the amount of additional mixing time that is afforded by the inlet extension  210  is a function of its length. Those skilled in the art will also appreciate that the need for an inlet extension is based on the state of the air flow, such as the velocity, temperature, pressure and the characteristics of the fuel that is being used. Therefore, in some of the applications that we have conceived for the swirl generator  30 , such as ramjets, the air flow is at a sufficiently high temperature such that even liquid fuels are rapidly mixed (i.e., atomized, mixed and vaporized), which permits the aft end of the centerbody hub assembly  204  to be positioned so as to extend downstream of the inlet ramp  212  into the combustor  40   a.    
         [0071]    In the particular example provided, wall injection is accomplished through the wall injector assembly  208 . The wall injector assembly  208  includes an annular wall injection housing  228  (which is considered to be part of the injection housing  200 ) and a plurality of fuel injectors  230  that are circumferentially spaced about the wall injection housing  228 . The inside diameter of the wall injection housing  228  is equal in diameter to the inner diameter of the lower inlet housing segment  200   b  so as to be flush and not to induce flow separations, etc. that would tend to impede the efficiency of the swirl generator  30 . The inside diameter wall injection housing  228 , however, is somewhat smaller in diameter than the inside diameter of the upper inlet housing segment  200   a  for reasons that will be described in detail, below.  
         [0072]    The wall injection housing  228  is disposed between the aft end of the upper inlet housing segment  200   a  and the forward end of the lower inlet housing segment  200   b.  Conventional Viton O-rings  238  or other sealing devices that are well known in the art are employed to seal the interface between the opposite faces of the wall injection housing  228  and the upper and lower inlet housing segments  200   a  and  200   b.  Threaded fasteners (not shown) are employed to fixedly couple the elbow  60 , the upper inlet housing segment  200   a,  the wall injection housing  228 , the lower inlet housing segment  200   b  and the combustor/nozzle portion  40  together.  
         [0073]    In the example shown, the fuel injectors  230  comprise eight flush-mount simplex fuel injectors such as Woodward FST Simplex Injectors, which are commercially available from Woodward FST, Inc. of Zeeland, Mich. Those skilled in the art will appreciate, however, that other amounts and/or types of fuel injectors, including stand-off or wall flush simplex fuel injectors, orifice injectors or variable area poppet fuel injectors with variations in drop sizes and cone angles (i.e., solid or hollow cone) may also be used.  
         [0074]    In FIGS. 7 through 9, the centerbody hub assembly  204  is illustrated to include the conduit  120  and a centerbody assembly  236 , which includes a forward centerbody  240 , a conduit retainer  242 , and an aft centerbody  244  and an igniter  246 . In the particular embodiment illustrated, the igniter  246  is a conventional spark igniter which is commercially available from sources such as those that are manufactured by Champion Spark Plug Company of Toledo, Ohio. Accordingly, a detailed description of the construction of the igniter  246  will not be provided herein. Those skilled in the art will appreciate that other types of igniters, such as a plasma jet igniter (also well known in the art and commercially available from sources such as Unison Industries Inc. of Jacksonville Fla.), microwave ignition devices, and laser ignition devices may be employed as an alternative to a spark igniter.  
         [0075]    As noted above, the conduit  120  extends through the elbow  60  (FIG. 4) to couple the swirl generator  30  (FIG. 4) to the ignition controller  122  (FIG. 4) and the fuel source  126  (FIG. 4). In the particular embodiment provided, the conduit  120  is a hollow tube into which an electrical cable  250  and a plurality of fuel conduits  252  are housed. The electrical cable  250  electrically couples the ignition controller  122  to the igniter  246  such that electrical energy is transmitted to the igniter  246  when the ignition controller  122  is actuated to permit the igniter  246  to produce a discharge arc (not shown). The fuel conduits  252  couple in fluid connection of the fuel source  126  to a plurality of fuel injectors in the aft centerbody  244 . Those skilled in the art will appreciate, however, that a single fuel conduit (not shown) may be alternatively employed, wherein the single fuel conduit supplies fuel to a fuel manifold within or coupled to the aft centerbody  244  to which are coupled the fuel injectors. Also alternatively, the swirl generator  30  may be constructed without a discrete fuel conduit  252  wherein such function would additionally be provided by the conduit  120  that houses the electrical cable  250 . Those skilled in the art will appreciate that these fuel splits could alternatively be accomplished through a set of orifices that effectively limit the mass flow rate of fuel therethrough.  
         [0076]    The forward centerbody  240  is a generally hollow structure having a central aperture  270  into which the conduit  120  is fixedly coupled, as through brazing for example. The exterior surface of the forward centerbody  240  includes an aerodynamically contoured leading surface  272  as well as a mounting flange  274  to which the swirl vane pack  206  is mounted. The aerodynamically contoured leading surface  272 , which is illustrated to be generally spherically shaped in the particular example provided, serves to guide the inlet flow  150  (FIG. 4) exiting the elbow  60  radially outwardly around the forward centerbody  240  and into the swirl vane pack  206 . The mounting flange  274  has a diameter that is generally smaller than the trailing edge of the aerodynamically contoured leading surface  272  to thereby create an abutting flange  278  at the intersection of the mounting flange  274  and the leading surface  272 . The aft end of the interior of the forward centerbody  240  includes a counterbored portion  280  that is configured to receive the conduit retainer  242 .  
         [0077]    The conduit retainer  242  is an annular structure with an open center that is configured to receive therethrough portions of the igniter  246  and the electrical cable  250 . A plurality of conduit mounting apertures  284  are formed through the conduit retainer  242  and are sized to matingly receive an associated one of the fuel conduits  252 . The fuel conduits  252  are preferably fixedly coupled to the conduit retainer  242  in their associated conduit mounting apertures  284  through an appropriate joining process, such as brazing. Conventional threaded fasteners, such as socket head cap screws  290 , are preferably employed to fixedly but releasably couple the conduit retainer  242  to the forward centerbody  240 .  
         [0078]    The aft centerbody  244  may be configured in several different manners to optimize the efficiency of the lift thrust augmentor  10  and tailor its thrust output to a desired thrust output level. In the particular example provided the aft centerbody  244  includes first and second injector rings  300  and  302 , respectfully, and an aft bluff boat-tail  304 .  
         [0079]    A conventional Viton O-ring  310  or other well known sealing device is employed to create a seal between the first injector ring  300  and the aft face of the forward centerbody  240 . The first injector ring  300  has a generally hollow center through which the igniter  246  is received, and an outside diameter that is relatively larger in diameter than that of the mounting flange  274 . The first injector ring  300  is illustrated to include a plurality of circumferentially spaced apart fuel injectors  320 , such as simplex injectors having a flow number (FN) equal to about 8.5, which are commercially available from Woodward FST, Inc. of Zeeland, Mich. Those skilled in the art, however, will appreciate that other types of fuel injectors, including a plurality of orifices, could be additionally or alternatively employed. Although the injectors  320  are illustrated as being configured to inject fuel in a radially outward direction, those skilled in the art will appreciate that the fuel injectors  320  may be alternatively configured to inject fuel in an upstream direction, a downstream direction or any combination of the radially outward, downstream and upstream directions.  
         [0080]    The fuel injectors  320  are coupled in fluid connection to a manifold  324  that is formed into the first injector ring  300 . In the example provided, the manifold  324  is coupled in fluid connection to an associated one of the fuel conduits  252  to receive fuel therefrom. Those skilled in the art will appreciate, however, that the manifold  324  could also be coupled in fluid connection to each of the fuel conduits  252 . A Viton O-ring  326  or other well known sealing device is employed to seal the interface between the conduit retainer  242  and the front face of the first injector ring  300 .  
         [0081]    A conventional Viton O-ring  330  or other well known sealing device is employed to create a seal between the second injector ring  302  and the aft face of the first injector ring  300 . The second injector ring  302  has a generally hollow center that is at least partially threaded so as to threadably engage a threaded portion  336  of the igniter  246  in a conventional manner. The second injector ring  302  may include no fuel injectors (i.e., constitute a “blank” injector ring as shown in FIG. 9A), or may include a plurality of circumferentially spaced fuel injectors  340 , such as simplex or orifice injectors, depending on the desired output of the lift thrust augmentor  10 . Although the injectors  340  are illustrated as being configured to inject fuel in a radially outward direction (FIG. 8), those skilled in the art will appreciate that the fuel injectors  340  may be alternatively configured to inject fuel in an upstream direction, a downstream direction or any combination of the radially outward, downstream and upstream directions.  
         [0082]    The second injector ring  302  may be keyed or otherwise aligned to the first injector ring  300  in any conventional manner to maintain the first and second injector rings  300  and  302  in a condition wherein they are aligned about a common axis. In this condition, the outer surface of the second injector ring  302  substantially coincides with the outer surface of the first injector ring  300  to thereby prevent the generation of any flow separations or discontinuities.  
         [0083]    In the embodiments wherein the second injector ring  302  includes fuel injectors  340 , a manifold  344 , which is formed into the second injector ring  302 , is employed to couple in fluid connection the fuel injectors  340  to an associated fuel conduit  252 . The manifold  344  may be coupled in fluid connection to the manifold  324  of the first injector ring  300 , or to an aperture  345  that is formed through the first injector ring  300  as is shown in FIG. 8. The interface between the first and second injector rings  300  and  302  is sealed by a Viton O-ring  346  in an area proximate the aperture  345 .  
         [0084]    The aft bluff boat-tail  304  includes a flange portion  360  and a flow-effecting portion  362 . The flange portion  360  abuts the aft face of the second injector ring  302  and includes a pair of apertures  364  through which conventional socket head cap screws  366  are received. The cap screws  366  extend through similar apertures formed in the first and second injector rings  300  and  302  and the conduit retainer  242  and threadably engage apertures (not shown) in the forward centerbody  240  to fixedly couple these components to one another. The flange portion  360  also includes a hollow center  370  into which a tip  246   a  of the igniter  246  extends. The hollow center  370  is chamfered on its aft end so as to provide additional space about the tip  246   a  for a flame kernel.  
         [0085]    The flow-effecting portion  362  is coupled to the outer perimeter of the flange portion  360 . In the particular embodiment provided, the flow-effecting portion  362  is frusto-conically shaped and includes a plurality of circumferentially spaced apart fuel injectors  380  that are configured to inject fuel in a predetermined direction. Like the second injector ring  302 , the flow-effecting portion  362  may be alternately configured to include no fuel injectors (i.e., constituting a “blank” bluff body). Although the injectors  380  are illustrated as being configured to inject fuel in an upstream direction, those skilled in the art will appreciate that the fuel injectors  380  may be alternatively configured to inject fuel in a radially outward direction, a downstream direction or any combination of the radially outward, downstream and upstream directions.  
         [0086]    In the example illustrated, the flow-effecting portion has an initial outer diameter that matches the outer diameter of the first and second injector rings  300  and  302 . The flow-effecting portion  362  terminates at its aft end at a sharp edge  386  that operates to initiate flow separation and to anchor and radially extend the central recirculation zone  610  (FIG. 13) to increase its size and flameholding capabilities.  
         [0087]    Although the centerbody hub assembly  204  has been illustrated thus far as being formed from a plurality of discrete components, those skilled in the art will appreciate that various known manufacturing techniques, including direct metal fabrication, may be employed so as to reduce the actual number of components that are utilized. For example, the forward centerbody  240 , the swirl vane pack  206  and the first injector ring  300  may all be unitarily formed, which would thereby eliminate the need for the conduit retainer  242 .  
         [0088]    Referring to FIG. 9A, the swirl generator  30  of the present invention is schematically illustrated to show several of the various fueling options that may be employed. The fuel injectors  230  may comprise, for example, orifices A, flush-mount simplex injectors B or stand-off simplex injectors C. The fuel injectors of the aft centerbody assembly  244  (i.e., fuel injectors  320 ,  340 ,  380  and  840 ) may include simplex injectors D or orifices E, or may be omitted in part (blank) as designated by reference letter F.  
         [0089]    In FIGS.  4 , and  10  through  12 , the swirl vane pack  206  is illustrated to include a mounting hub  400 , a plurality of vanes  402  and a shroud  404 . In the particular example provided, the swirl vane pack  206  is an assembly wherein the components comprising the swirl vane pack  206  are fabricated, assembled and fixedly secured to one another. Those skilled in the art will appreciate, however, that alternative fabrication techniques may be employed to reduce the number of components that comprise the swirl vane pack  206 . For example, technologies such as hot isostatic pressing, casting and direct metal fabrication may be employed to form the swirl vane pack  206 , either wholly or partially, or in combination with the centerbody assembly  236  or portions thereof as described above.  
         [0090]    The mounting hub  400  is an annular structure that is received over the mounting flange  274  (FIG. 9) of the forward centerbody  240  (FIG. 9) in juxtaposed relation with the abutting flange  278  (FIG. 9) of the forward centerbody  240  and the front face of the first injector ring  300  (FIG. 9). The cap screws  366  (FIG. 8) exert a clamping force that fixedly but removably secures the aft centerbody  244  to the forward centerbody  240 . As the mounting hub  400  abuts the mounting flange  274  and the front face of the first injector ring  300 , the clamping force is also transmitted between the abutting flange  278  and the front face of the first injector ring  300 , which operates to fixedly secure (both axially and radially) the mounting hub  400  therebetween.  
         [0091]    The vanes  402  of the swirl vane pack  206  are configured with a swirl number that ranges from about 0.4 to about 1.2 so as to permit the combustor/nozzle portion  40  to achieve a combustor length-to-diameter (L/D) ratio (as measured from a plane at which the dump step  636  in FIG. 13 to the throat of the nozzle portion  40   b ) that is less than about 2.0, and preferably less than about 1.6 and more preferably about 1.0 or less. In the particular example provided, we utilized twelve vanes  402  having a straight configuration that is skewed to the centerline of the mounting hub  400  so as to provide a swirl number of 0.54. Those skilled in the art will appreciate that various other vane configurations may alternatively be employed, including vanes with different skew angles and/or an arcuate or helical profile (FIG. 10A). As the swirl vane pack  206  is comprised of a plurality of discrete components, the vanes  402  are configured with tabs  410   a  and  410   b  on their opposite ends. The radially inward tabs  410   a  are configured to engage apertures  412  that are formed on the opposite faces of the mounting hub  400 . The tabs  410   a  and the apertures  412  cooperate to align the vanes  402  to the mounting hub  400  so that the vanes  402  may be coupled to the mounting hub  400  in a conventional manner, such as brazing or welding.  
         [0092]    The shroud  404  includes a pair of end caps  420  and a pair of circumferentially extending portions  422 . The end caps  420  include a plurality of apertures  424  that are configured to receive the radially outward tabs  410   b  on the vanes  402 . In a manner similar to that of the apertures  412  of the mounting flange  274 , the apertures  424  cooperate to align the vanes  402  to the end caps  420 . The circumferentially extending portions  422  are disposed around the perimeter of the vanes  402  between the tabs  410   b  and the end caps  420 , circumferentially extending portions  422  and vanes  402  are fixedly secured together, as through welding.  
         [0093]    With reference to FIG. 4, the swirl vane pack  206  is illustrated to be fixedly coupled to the centerbody hub assembly  204  in the manner described above and disposed between the wall injector assembly  208  and a recessed step  500  formed in the upper inlet housing segment  200   a.  In this location, the trailing edges of the vanes  402  of the swirl vane pack  206  are located upstream of all fuel injection sites, which eliminates any potential for flashback which would damage the vanes  402 . The shroud  404  of the swirl vane pack  206  is preferably sized to engage in a press-fit manner the recessed step  500  in the lower inlet housing segment  200   b  to thereby structurally couple the swirl vane pack  206  and the centerbody hub assembly  204  to the inlet housing  200 .  
         [0094]    The swirl generator  30  is configured such that the vanes  402  impart tangential velocities to the axial inlet flow  150  to convert the inlet flow  150  into a spiraling, three dimensional swirling flow structure or flowfield  510  (FIG. 4). The flowfield  510  has a dramatic effect on the rate of fuel mixing, atomization, droplet vaporization, flame propagation, combustion efficiency, combustion stability, combustion intensity and widens flammability limits. Those whom are skilled in the art will appreciate that radial velocities will be affected by the swirling effect, but that the major impact of swirling effect concerns the aforementioned tangential velocity component.  
         [0095]    With reference to FIG. 13, the high tangential velocities produced by the vanes  402 , whether straight or profiled in their configuration, creates a very intense shear layer  600  and enhances the large scale vortex or central recirculation zone  610  that is generated and anchored by the bluff end  614  of the aft centerbody  244 , even at relatively low levels of swirl (i.e., a level of swirl that is greater than or equal to about 0.4). In the case of vanes  402  having a flat configuration, especially high shear stresses are created that promote very efficient mixing of the fuel that is introduced into the inlet housing  200  via the wall injector assembly  208  and the injectors  320 ,  340  and  380  in the aft centerbody  244  due to high intensity turbulence that is generated by the trailing edge vortices that are induced by the flow separation on the lee side of the vanes  402 . Combustion in the central recirculation zone  610  is initiated by a flame kernel  630  that is produced by the igniter  246  that is housed in the centerbody hub assembly  204 . Additionally or alternatively, igniters  80   a  (similar or identical to igniter  246  or igniter  80 ) may also be employed near the dump plane  636   a.    
         [0096]    The inlet ramp  212 , which is optional, aids in increasing the size of the dump step  636  that occurs at the dump plane  636   a.  In the particular example provided, the inlet ramp  212  helps to create a relatively large 90° dump step  636  at the transition between the inlet housing  200  and the inlet of the combustor  40   a  that serves to considerably improve flame propagation rates and the combustor&#39;s operability limits. More specifically, the dump step  636  creates a toroidal outer recirculation zone  640  along the combustor wall that is initially ignited by a flame kernel that is produced by the torch igniter  80  or the igniter(s)  246  and/or  80   a  (FIG. 4). The length of the outer recirculation zone  640  is a function of the height of the dump step  636  and the strength of the swirl number of the swirl vane pack  206 . Generally speaking, for a given constant swirl number, the length, size and robustness of the outer recirculation zone  640  are directly related to the height of the dump step. The inlet ramp  212  and its shape not only provide a means to easily tune the flow height of the dump step  636  and the flow direction, but also increases the local flow velocities to thereby intensify the separated shear layer turbulence and increases the rate of mass entrainment of fuel into the shear layer. The ramp shape and reduced flow gap height also accelerates early merging of the shear layers  650  and  600  of the outer recirculation zone  640  and the central recirculation zone  610 , respectively, which is essential for combustors having a relatively short length. Those skilled in the art will appreciate that the shape of the ramp can be altered to change the maximum height and, therefore, the volume of the outer recirculation zone and the gaps between the outer and central recirculation zones.  
         [0097]    Portions of the fuel that are dispensed by the fuel injectors  320 ,  340  and  380  are employed to substantially fuel the central recirculation zone  610 , while portions of the fuel that are dispensed by the fuel injectors  230  of the wall injector assembly  208  are employed to fuel both the outer recirculation zone  640  and fuel the central recirculation zone  610 . Any portions of the fuel that is dispensed by the injectors  230 ,  320 ,  340  and  380  that is not employed to fuel the central or outer recirculation zones  610  and  640  is employed to generally fuel the main core flow  700 , which as those skilled in the art will appreciate, consists of the entire flow of combusting fuel and air except the central recirculation zone  610  and the outer recirculation zone  640 . The central recirculation zone  610  and the outer recirculation zone  640 , once formed, contain a fixed trapped mass. An exchange of mass occurs between each of the central and outer recirculation zones  610  and  640  and the core flow  700 , but there is no net change in mass for steady flow conditions in either of the central and outer recirculation zones  610  and  640 .  
         [0098]    As each of the injectors  320 ,  340  and  380  are coupled in a discrete manner to the fuel source  126 , the amount of fuel that is dispensed by the injectors  320 ,  340  and  380  may be tailored in a desired manner to fine tune flame stabilization and combustion performance during throttling. Accordingly, the injectors  320 ,  340  and  380  may be independently controlled so as to provide a relatively wide range of flexibility to control combustor characteristics, depending on a particular application.  
         [0099]    The shear layers  650  and  600  of the outer recirculation zone  640  and the central recirculation zone  610  provide reduced velocity regions to hold the flame, and maintain and propagate the combustion process. More specifically, the outer recirculation zone  640  and the central recirculation zone  610  provide flame stabilization and act as a robust ignition source for the core flow  700  by supplying heat and recirculated chemically reacting by-products, such as OH, H and O radicals, to the main fuel/air mixture of the combustor  40   a.  In this regard, each recirculation zone carries the heat and chemically active species from the flame in the respective shear layer and recirculating flow volume upstream where they act to ignite the fresh combustible fuel/air mixture entering the shear layer to thereby provide a continuous pilot for the core flow  700 .  
         [0100]    As noted above, the aft centerbody  244  of the swirl generator  30  may be configured in various different arrangements to achieve desired design parameters. For example, the aft centerbody  244  may be configured with a channeled aft bluff boat-tail  304   a  as illustrated in FIG. 14. The aft bluff boat-tail  304   a  is generally similar to the aft bluff boat-tail  304  of FIG. 8, except that it includes a plurality of channels  800  that are formed about the perimeter of the flow-effecting portion  362   a.  The channels  800  are formed at an angle relative to the centerline of the aft bluff boat-tail  304   a  that maintains the effective flow direction provided by the swirl vane pack  206 . Those skilled in the art will appreciate that other channels may be selected to control turbulent transport and mixing including ones that are opposite of the tangential direction provided by the swirl vane pack. The ramp-like geometry of the channels produces a spectrum of turbulence scales that enhances mixing to promote flame intensity and propagation from the central recirculation zone  610  into the core flow  700  (Figure  13 ). Like the aft bluff boat-tail  304 , the aft bluff boat-tail  304   a  may include one or more fuel injectors, or may have a “blank” configuration (i.e., a configuration without one or more fuel injectors).  
         [0101]    In the embodiment of FIG. 15, the aft centerbody  244   b  is illustrated to be generally similar to the aft centerbody  244  of FIG. 9, except that it does not include a discrete bluff body. In this regard, the second injector ring  302  essentially forms a bluff body as the aft centerbody  244   b  terminates abruptly at the rear face of the second injector ring  302 .  
         [0102]    The embodiment of FIGS. 16 and 17 is generally similar to that of FIG. 15 except that the igniter  246  has been replaced with a fuel injector  840 , such as a simplex atomizer having, for example, a  1000  spray angle. The fuel injector  840 , like the previously discussed fuel injectors that are housed in the aft centerbody  244 , is individually coupled to a fuel conduit  252  so as to permit the fuel injector  840  to be selectively deployed. In this embodiment, the central recirculation zone is also ignited by the flame kernel that is produced by the torch igniter  80  and/or the igniter  80   a  that are described above as initiating combustion in the outer recirculation zone.  
         [0103]    Another embodiment is illustrated in FIG. 18 wherein the fuel injectors in the wall of the inlet housing  200 f are replaced by a plurality of cross-flow strut injectors  900 . Each of the cross-flow strut injectors  900  is swaged into the aft centerbody  244 f for structural support and coupled in fluid connection to fuel conduits  906  that extend through the inlet housing  200 f. Each cross-flow strut injectors  900  has a plurality of orifices  910  that promote atomization of the fuel flowing therethrough. Additionally, this embodiment includes a channeled aft bluff boat-tail  304   f  and a center-mount fuel injector  840 .  
         [0104]    The embodiment of FIGS. 19 and 20 provides a fuel injection scheme that is very similar to that of the embodiment of FIG. 18. Instead of cross-flow strut injectors, however, this embodiment utilizes a plurality of fuel injection sites  1000   a  that are formed into a trailing edge  1002   g  of at least a portion of the vanes  402   g  of the swirl vane pack. The injection sites  1000   a  are coupled in fluid connection to an associated fuel conduit that extends through the conduit  120  (FIG. 4); a plurality of internal channels  1004  in the vanes  402   g  serve to transmit the fuel through the vanes  402   g  to the injection sites  1000   a.  As illustrated in FIG. 20, each of the injection sites  1000   a  is an orifice  1006  that has an appropriate length to diameter ratio and which is formed into the trailing edge  1002   g  at a predetermined angle relative to an axis of the vane  402   g.  Although the injection sites  1000   a  are illustrated as being generally oriented in a downstream direction, those skilled in the art will appreciate that additionally or alternatively the injection sites could also be skewed to the axis of the vane  402   g.  Additionally or alternatively, the holes  1006  may be formed on a lateral surface of the vanes  402   g  to inject fuel in a desired direction (see, e.g., injection sites  1000   b  and  1000   c  in phantom in FIG. 20, which are also formed with an appropriate orifice length to diameter ratio). Despite the complexity of the vane arrangement, this embodiment is advantageous in that the exterior surfaces of the vanes  402   g  form an effective means for transferring heat from the air flow to the fuel in the vanes  402   g  which operates to cool the swirl vane pack as well as to increase the temperature of the fuel that is injected, which tends to increase the rate by which the fuel is mixed (i.e., improve atomization, decrease the size of the droplets in the fuel spray and directly increase the rate of droplet vaporization).  
         [0105]    The embodiment of FIGS. 21 and 22 is generally similar to that of FIGS. 19 and 20, except that an array or row of turbulator ramps  1050  are formed or mounted onto at least a portion of the vanes  402   h  of the swirl vane pack  206  to further enhance mixing. The turbulator ramps  1050  and the channels  1052  that are formed between each pair of turbulator ramps  1050  are employed to generate vortices that emanate from the trailing edge  1002   h  of the vanes  402   h;  the vortices enhance turbulent transport and provide highly controlled fine scale mixing. Like the previous embodiment, a plurality of fuel injection sites  1000   h  that are formed into a trailing edge  1002   h  (i.e., into the tubulators  1050 ) which are coupled in fluid connection to an associated fuel conduit that extends through the conduit  120  (FIG. 4); a plurality of integrally formed channels  1002   h  in each of the vanes  402   h  serve to transmit the fuel through the vanes  402   h.  Those skilled in the art will also appreciate that the turbulators  1050  may also be utilized in vane configurations that do not inject fuel (i.e., in vanes without injection sites formed therein).  
         [0106]    The example of FIG. 23 is generally similar to that of FIG. 4, except that the trailing edge  1002   i  of the vanes  402   i  includes a plurality of scallops  1100  rather than turbulators. The scallops  1100  are illustrated to be formed in a uniform manner wherein the crest  1102  is relatively wider than the root  1104 . Those skilled in the art will appreciate, however, that the scope of the present invention is not limited to any particular scallop pattern. Those skilled in the art will appreciate, too, that the vanes  402   i  may also be configured with a plurality of fuel injection sites in the manner described above for the embodiments of FIGS. 19 through 22.  
         [0107]    In the embodiment of FIGS. 24 and 24A, the inlet ramp  212   j  is illustrated to be generally similar to the inlet ramp  212  of FIG. 4, except that a plurality of channels  1150  are formed about the perimeter of the inlet ramp  212   j.  The channels  1150  are formed at a predetermined angle relative to the longitudinal axis of the inlet housing  200   j  and serve to enhance turbulent transport and fine scale mixing.  
         [0108]    In the embodiment of FIG. 25, a plurality of circumferentially spaced apart fuel injection sites  1200  are formed about the circumference of the inlet ramp  212   k.  The fuel injection sites  1200  are operable for injecting fuel into the flow field which aids in flame stabilization and communication between the outer and central recirculation zones. As those skilled in the art will appreciate, the channels of the previous embodiment may additionally be incorporated into the inlet ramp  212   k.    
         [0109]    Those skilled in the art will appreciate that the wall of the combustor  40   a  in an area proximate to the outer recirculation zone tends to absorb a relatively large amount of heat during the combustion process. An optional quarl expansion  1250  may be provided as shown in FIG. 26 at the dump step  636 . The quarl expansion  1250  is an annular element having a generally triangular cross section; the quarl expansion  1250  is employed to “fill” the backwards facing step at the dump plane such that the angle of the dump step  636  is reduced from 90°. The angle β of the quarl expansion  1250  may be varied in a known manner to affect the size of the central recirculation zone and the rate of heat transfer to the wall of the combustor  40   a  and the inlet housing  200 .  
         [0110]    The embodiment of FIG. 27 illustrates that the configuration of the centerbody assembly aft of the leading surface need not have the configuration of a right cylinder. In the particular embodiment illustrated, the portion of the centerbody assembly  236   m  aft of the leading surface  272   m  has a generally frusto-conical shape that is symmetrical about a longitudinal axis of the swirl generator  30   m.  Those skilled in the art should also appreciate that the portion of the centerbody assembly that is positioned aft of the leading surface may be other than conical (e.g., ogival). Those skilled in the art will also recognize that multiple fuel injection sites, similar to fuel injectors  320 ,  340 ,  380  and/or  840  of FIGS. 8 and 16 could be incorporated into the centerbody assembly  236   m.    
         [0111]    The embodiment of FIG. 28 is generally similar to the embodiment of FIG. 14 in that it includes two injection rings  1300  and  1302  that are positioned forwardly of the aft bluff boat-tail  304   a.  This embodiment differs from the embodiment of FIG. 14 in that the injection rings  1300  and  1302  are configured in a manner that is generally similar to that of the first injection ring  300  and a third injection ring  1304  is coupled to the aft end of the aft bluff boat-tail  304   a.  The injection ring  1304  is configured generally similar to that of the second injection ring  302  discussed above (i.e., includes fuel injectors and threadably engages the threaded portion  336  of the igniter  246 ). Additionally, the injection ring  1304  includes a plurality of optional grooves or channels  1310  that may be continuous with the grooves or channels of the bluff boat-tail body  800  which produces a spectrum of turbulent scales that enhance fuel and air transport and fine scale mixing to promote flame intensity and propagation from the central recirculation zone  610  into the core flow  700 . As those skilled in the art will appreciate, the injection rings  1300 ,  1302  and/or  1304  may be “blank” and/or may include grooves or channels  1310  for producing fine scale turbulence. Furthermore, the injection ring  1304  may be configured with an aft facing fuel injector such as that which is shown in FIGS. 16 and 17.  
         [0112]    While the swirl generator of the present invention has been illustrated and described thus far as being a component of a lift thrust augmentor, those skilled in the art will appreciate that the invention, in its broader aspects, may be utilized in diverse other applications. In FIGS. 29 and 30, for example, the swirl generator  30  is illustrated in conjunction with a ramjet missile  2000 .  
         [0113]    In this example, the ramjet missile  2000  includes a forebody  2002 , fins  2004 , a booster engine  2006  and a ramjet engine  2010  having an air inlet  2012 , the swirl generator  30  and a ramjet combustor/nozzle  2014 . The forebody  2002  conventionally houses the payload (not shown), the fuel (not shown), batteries (not shown) and the control portion (not shown) of the ramjet missile  2000 , while the fins  2004  conventionally stabilize and guide the ramjet missile  2000 . The air inlet  2012  includes a movable or consumable cover (not shown) that is selectively operable for sealing the air inlet  2012  during the operation of the booster engine  2006 . In the example provided, the booster engine  2006  includes a solid propellant  2018  that burns during a boost phase of the missile&#39;s operation; hot combustion by-products are expelled from the nozzle  2020  of the booster engine  2006  to generate thrust.  
         [0114]    Subsequent to the boost phase of the missile&#39;s operation, the booster engine  2006  of the exemplary ramjet missile illustrated is ejected, the movable cover is translated or consumed so as to permit air to flow into the air inlet  2012  and the task of propulsion switches from the booster engine  2006  to the ramjet engine  2010 . The speed of the ramjet missile  2000  at this cruise stage and the configuration of the air inlet  2012  cooperate to induce an airflow  2026  through the air inlet  2012  that is directed toward the swirl generator  30 . Although not shown, those skilled in the art will appreciate from the above discussion that flow guide vanes (similar to the flow guide vanes  96  in the elbow  60 ) may be employed in the air inlet  2012  to provide an axisymmetric (i.e., uniform and axial) airflow to the swirl generator  30 . Those skilled in the art will recognize that a nose inlet may be used followed by an annular air transfer duct that acts as an isolator and carries the air aft to the swirl generator  30  and the combustor of the ramjet combustor/nozzle  2014 . The swirl generator  30  is employed to generate a turbulent flowfield and to inject fuel therein in the manner described above. As in the example of the lift thrust augmentor, the swirl generator  30  operates to form both an outer recirculation zone (proximate the dump step  2030  at the transition between the inlet housing  2032  and the inlet of the combustor/nozzle  2014 ), as well as a central recirculation zone (which is anchored by the aft end of the aft centerbody assembly  244 ).  
         [0115]    Integration of the swirl generator  30  into the ramjet missile  2000  permits significant reductions to the L/D ratio of the combustor  2014   a,  which in turn affords substantial reductions in the overall weight of the ramjet missile  2000 . Furthermore, the swirl generator  30  provides relatively high combustor efficiency so that a booster engine  2006  may be separately packaged and thus be ejectable as the propulsion switches from the booster engine  2006  to the ramjet engine  2010 .  
         [0116]    Another application is illustrated in FIG. 31 wherein the swirl generator  30 n is illustrated in conjunction with a ramshell  2300 . The ramshell  2300  is a gun-launched projectile that employs the above-described ramjet technology to accelerate the ramshell  2300  to a velocity of about Mach 4 to about Mach 6 to minimize the time to target and maximize the penetration capability of the projectile. The ramshell  2300  includes a housing  2302 , a projectile  2304 , a plurality of inlet struts  2306 , the swirl generator  30   n  and a combustor/nozzle portion  2308 .  
         [0117]    The housing  2302  is a hollow shell that is inwardly tapered at it&#39;s front end to define an air inlet  2310 . The inlet struts  2306  are fixedly coupled to the interior of the housing  2302  and to the projectile  2304  to centrally mount the projectile  2304  in a forward portion of the housing  2302 . As the ramshell  2300  is spin-stabilized, the inlet struts  2306  have a spiral shape to maintain alignment with the incoming air flow. The air flow is compressed through the inlet section  2310   a  and shocked to subsonic velocities near the aft end of the inlet struts  2306 . While the projectile  2304  is illustrated as being a solid metallic rod, those skilled in the art will appreciate that any form of payload, including an explosive charge, may be employed in the alternative.  
         [0118]    The swirl generator  30   n  is mounted on the aft end of the projectile  2304  and the swirl vane pack  206  serves to support the housing  2302  aft of the inlet struts  2306 . Fuel injection is somewhat different from that of the swirl generator  30  of FIG. 4 in that the primary purposes of the swirl generator  30   n  are to augment the central recirculation zone (similar to the central recirculation zone  610  of FIG. 13) and to control the rate of mixing in the shear layer above the central recirculation zone and to control the rate of flame propagation into the core flow. Accordingly, wall injection is not employed in this embodiment, and the fuel injectors  2320  in the centerbody assembly  236   n  are fueled by a reservoir  2330  that is internal to the aft centerbody assembly  244   n.  The reservoir  2330  includes a pressurized bladder  2332  that surrounds the fuel  2334  to maintain the pressure of the fuel  2334  at sufficient levels during the operation of the ramshell  2300 . The centrifugal force of the rotating fuel  2334  in the reservoir  2330  keeps the cooler, high density liquid fuel against the outer perimeter of the reservoir  2330  to assist in thermal protection of the combustor/nozzle portion  2308 .  
         [0119]    Fuel injection in the ramshell  2300  is preferably designed to maintain a somewhat fuel-rich condition in the central recirculation zone. As the main propulsive combustion initiates in the shear layer above the central recirculation zone, the level of mixing and heat release is controlled through the design of the swirl generator  30   n  so that only the air flow in the vicinity of the shear layer is burned. Operation in this manner leaves the outer region near the interior side of the housing  2302  relatively cooler and protects the housing  2302  from the high heat flux near the throat of the combustor/nozzle portion  2308 , before mixing of the hot combusting gases with the outermost air is complete.  
         [0120]    [0120]FIGS. 32 and 33 illustrate yet another application of the swirl generator of the present invention. In this example, a swirl augmented combined cycle engine  2400  is illustrated to include a core turbojet engine  2402 , a plurality of ramjet engines  2404  that surround the core turbojet engine  2402  and a flow controller  2406 . The core turbojet engine  2402  conventionally includes a low pressure compressor  2410 , a high pressure compressor  2412 , combustors  2414 , an air bypass  2416  and a high pressure turbine  2418 . As those skilled in the art will appreciate, the core turbojet engine  2402  may optionally include an afterburner and a variable area nozzle.  
         [0121]    The ramjet engines  2404  are configured in a manner that is similar to the ramjet missile of FIG. 29 and  30 . Briefly, each of the ramjet engines  2404  includes an air inlet  2420 , a swirl generator  30  and a ramjet combustor/nozzle  2430 .  
         [0122]    The flow controller  2406  is coupled to the core turbojet engine  2402  and includes a forward movable element or diverter  2440 , which is employed to selectively control the intake of air into the core turbojet engine  2402  and the ramjet engines  2404 , and an aft movable element or diverter  2442 , which is employed to selectively close off the outlet of the core turbojet engine  2402  and the ramjet engines  2404 . In the particular example provided, the forward and aft movable elements  2440  and  2442  are hingedly mounted to the housing  2450  of the core turbojet engine  2402  and pivotable between a first condition (illustrated in broken line), which closes off the air inlet  2420  and combustor/nozzle  2430 , respectively, of each ramjet engine  2404 , and a second condition (illustrated in solid line), which closes off the intake side of the low pressure compressor  2410  and the outlet of the high pressure turbine  2418 , respectively. The forward and aft movable elements  2440  and  2442  may be moved through any of the various conventionally known means, including hydraulic actuators (not shown).  
         [0123]    The core turbojet engine  2402  produces all of the propulsive power that is output by the swirl augmented combined cycle engine  2400  from zero velocity through a predetermined transition-in velocity of, for example, at about Mach 2. Therefore, the forward and aft movable elements  2440  and  2442  are maintained in the first condition at speeds below the predetermined transition-in velocity.  
         [0124]    At the predetermined transition-in velocity, the ramjet engines  2404  are activated to provide additional thrust. Flow entering the ramjet engines  2404  is subjected to additional ramjet compression  2460  and is further compressed to subsonic speeds in a transfer duct/shock isolator  2462 . The air flow enters the swirl generator  30  and is converted into a highly turbulent flowfield into which fuel is injected in the manner described above. As the core turbojet engine  2402  is also producing thrust, the forward and aft movable elements  2440  and  2442  are maintained in a position between the first and second conditions.  
         [0125]    When air speed reaches a predetermined transition-out velocity of, for example, above about Mach 3 to about Mach 4, the forward and aft movable elements  2440  and  2442  are positioned in the second condition and thrust production shifts entirely to the ramjet engines  2404 . For flight speeds up to the predetermined transition-out velocity, thrust augmentation may be provided by tapping-off air from the high pressure compressor  12  and burning this air with added fuel (up to stoichiometric conditions) in the ramjet engines  2404 .  
         [0126]    As those skilled in the art will appreciate, the position of the forward and aft movable elements  2440  and  2442  may be controlled in response to flow sensors (not shown) in a selective manner to thereby affect the amount of air that is directed to the core turbojet engine  2402  and the ramjet engines  2404 . Alternatively, the forward and aft movable elements  2440  and  2442  may be controlled such that they move continuously from the first condition to the second condition at a predetermined rate upon the sensing of an air speed equivalent to the predetermined transition-in velocity or other event which would prompt the transition from one propulsion mode to another.  
         [0127]    Another swirl augmented combined cycle engine  2400   a  is illustrated in FIGS. 34 and 35. The swirl augmented combined cycle engine  2400   a  is similar to the swirl augmented combined cycle engine  2400  of FIGS. 32 and 33 in that it employs a turbojet engines  2402   a,  a ramjet engines  2404   a  and a flow controller  2406   a,  which selectively controls the input of air to the turbojet engine  2402   a  and the ramjet engine  2404   a.  The swirl augmented combined cycle engine  2400   a,  however, is segregated into a plurality of engine cells  2500 , with each engine cell  2500  including a turbojet engine  2402   a,  a ramjet engine  2404   a  and a flow controller  2406   a.  Operation of each engine cell  2500  is identical to the operation of the swirl augmented combined cycle engine  2400  of FIGS. 32 and 33 and as such, need not be described in detail. The plurality of engine cells  2500  are operated in a manner such that propulsion is regulated between the turbojet engines  2402   a  and the ramjet engines  2404   a  in a uniform manner across the engine cells  2500  (i.e., transition from turbojet propulsion to ramjet propulsion is substantially simultaneous across all of the engine cells  2500 ).  
         [0128]    [0128]FIGS. 36 and 37 illustrate yet another application of the swirl generator of the present invention. In this embodiment, a rocket-based combined cycle engine  4000  is illustrated to include a housing  4002 , one or more rocket engines  4004 , a ramjet engine  4006  and a nozzle  4008 . The housing  4002  houses the rocket engines  4004  and the ramjet engine  4006  and defines an air inlet  4020 . The rocket engines  4004 , which may be coupled to the housing  4002  such that they are located in the dump step  4022  (or quarl surface) and/or inside the aft end of the aft centerbody assembly  244   p,  may employ a liquid, slurry or solid fuel, depending upon the application and considerations for the altitude, range and speed that are mandated by the mission. The ramjet engine  4006  includes a swirl generator  30   p  which is generally similar to the swirl generator  30 , except for the aforementioned rocket engine  4004  that is mounted to aft centerbody assembly  244   p.    
         [0129]    The rocket engines  4004  provide low speed thrust and additionally serve to pump air into the air inlet  4020 . Air flowing through the air inlet  4020  is converted into a highly turbulent flow field into which fuel is injected and mixed (via the swirl generator  30   p ) in the manner described in detail above. In this regard, the air flowing through the air inlet is employed in an afterburning operation by the ramjet engine  4006  to augment the thrust that is generated by the rocket engines  4004  at all speeds. The pumping action is a result of the momentum transfer from the high velocity rocket exhaust and the entrained ambient air. The momentum transfer is the result of the turbulent exchange through the shear layers separating the exhaust of each rocket engine  4004  and the entrained air. Alternatively, the rocket engines  4004  may be used to rapidly accelerate the rocket-based combined cycle engine  4000  above a predetermined speed threshold after which propulsion is transitioned to the ramjet engine  4006 .  
         [0130]    Those skilled in the art will appreciate that a forward flap or diverter (not shown) may be employed to close-off the air inlet  4020  during low speed operation of the rocket-based combined cycle engine  4000  to effect pure rocket thrust generation up to a predetermined speed threshold after which propulsion is transitioned to the ramjet engine  4006 . This mode of operation produces higher ramjet combustor pressures and associated thrust without the occurrence of backflow through the air inlet  4020  at lower speeds.  
         [0131]    For optimum thrust in any mode of operation, the nozzle  4008  preferably includes a variable area throat  4030 . The variable area throat  4030  is selectably configured to match the flow rate and backpressure requirements of the rocket-based combined cycle engine  4000  for maximum and efficient thrust generation. Those skilled in the art will appreciate, however, that other nozzle throat concepts may be employed in the alternative, including consumable throat inserts and ejectable throat inserts.  
         [0132]    With additional reference to FIGS. 38 and 39, an exemplary variable area throat  4030  is illustrated to include a plurality of throat closure elements  4032  that are rotatable in the housing  4002  through an angle of about 90° between an open position, which is illustrated in FIGS. 37 and 38, and a closed position, which is illustrated in FIG. 39. Although only six elements are shown, those skilled in the art will appreciate that that the number of throat closure elements  4032  may be varied to coordinate with the particular upstream geometry of the a rocket-based combined cycle engine  4000 . In the particular example illustrated, gaps  4034  are aligned to the six upstream rocket engines  4004  in the low speed mode to minimize erosion of the throat closure elements  4032  during the operation of the rocket engines  4004 . When the throat closure elements  4032  are positioned in the closed position during the operation of the ramjet engine  4006 , the area of the throat is substantially reduced.  
         [0133]    In the embodiment illustrated, the throat closure elements  4032  rotate about an axis on a plane or facet on the inner surface of the housing  4002 . The operational mechanism for rotating the throat closure elements  4032  may be housed in the housing  4002  or mounted on an adjacent structure (e.g., fins) to which the rocket-based combined cycle engine  4000  is mounted. Preferably, the throat closure elements  4032  are operated in opposed pairs so as to minimize rotational torques on the rocket-based combined cycle engine  4000  when the throat closure elements  4032  are moved between the open and closed positions.  
         [0134]    When the throat closure elements  4032  are positioned in the closed position, the throat closure elements  4032  cooperate to provide the throat with an approximately circular shape; the extent to which the throat is circular is dependent upon the number of throat closure elements  4032  that are employed and whether or not the edges of the throat closure elements  4032  are contoured.  
         [0135]    During the operation of the rocket-based combined cycle engine  4000 , compressed air enters the air inlet  4020  and is directed to the swirl generator  30   p.  The air inlet  4020  also functions as an isolator at supersonic flight speeds where the air is further compressed and brought to subsonic speeds prior to being directed into the swirl generator  30   p.  The length of the air inlet  4020  is dictated by packaging requirements including fuel, propellants, warhead type (for missile applications), plumbing, controller(s), actuators and batteries. The minimum length of the air inlet  4020  is dictated by the isolation requirements that are necessitated during supersonic flight speeds.  
         [0136]    During the combined operation of the rocket engines  4004  and the ramjet engine  4006 , the swirl generator  30   p  functions to augment core flow mixing where flame stabilization is achieved in the recirculation zones in the backward facing areas between the exhaust nozzles  4004   a  of the rocket engines  4004  that are mounted in the outer step and/or an annular lip region at the end of the aft bluff boat-tail  304   p.  Special igniters are not required since the hot rocket exhaust will serve to ignite the fuel/air mixture for afterburning and ramjet operation. In addition, fuel and/or rocket propellant may be continuously bled through the otherwise idle rocket engines to help cool them and to prevent back flow of the ramjet&#39;s hot combustion by-products. The total engine flow passes through the variable area throat  4030  and is expelled to the atmosphere. Although the primary application anticipated for this technology is missile propulsion, those skilled in the art will appreciate that the rocket-based combined cycle engine  4000  may also be employed for aircraft propulsion where reduced weight and complexity would be desired.  
         [0137]    Another application is of the swirl generator of the present invention is shown in FIG. 41, which illustrates the retrofitting of a conventional afterburner  5000  of the military gas turbine engine  5002  that is shown in FIG. 40. Briefly, the military gas turbine engine  5002 , which may be a turbojet engine or a turbofan engine, includes a coaxially-mounted afterburner  5000  having a diffuser tailcone  5004  with one or more fuel spray rings  5006 , one or more concentric V-gutter flameholder rings  5008  and a variable area nozzle  5010 . The variable area nozzle  5010  is fully opened for afterburning operation and is reduced for non-afterburning operation.  
         [0138]    Returning to FIG. 41, retrofit of the military gas turbine engine  5002  entails the substitution of afterburner  5020  for the conventional afterburner  5000  (FIG. 40). The afterburner  5020  includes swirl generator  30 r having a variable-angle swirl vane pack  206   r,  a centerbody assembly  236   r,  which has a collapsible centerbody cone  5022 , an expanding burner  5024 , which has a quarl step  5026 , an array of fuel injectors  5028  that are embedded into the base of the vanes  402   r  for main afterburning, an array of circumferentially spaced apart fuel injectors  5030  (illustrated as injection orifices in the particular example illustrated) located on the aft centerbody assembly  244   r  for ignition and piloting the afterburning, and one or more igniters (not shown) that are located in the aft end or base of the aft centerbody assembly  244   r  and/or in the recess of the quarl step  5026 .  
         [0139]    In order to maximize the benefits of the swirl augmentation provided by the swirl generator  30   r,  attachment of the swirl generator  30   r  should be as close as possible to the turbine exit plane. Accordingly, the length of the conical diffuser (i.e., the collapsible centerbody cone  5022 ) can be shortened relative to the embodiment of FIG. 40.  
         [0140]    The hot gases, which consist mostly of air exiting the turbine and relatively cold air from the bypass fan of the main engine, enter the swirl generator  30   r  where they are swirled and the streams are mixed to form a highly turbulent, three-dimensional flowfield. The fuel that is injected into this high shear stress-laden swirling flow is rapidly atomized and mixed. Atomization and mixing are controlled by a novel design of the swirl generator  30   r.    
         [0141]    The swirling mixture of the afterburner fuel, hot turbine gases and colder bypass fan air are slowed down across the quarl step  5026  as the flow enters the combustor  5024 , and creates a central recirculation zone and an outer recirculation zone similar to the central recirculation zone  610  and outer recirculation zone  640  of FIG. 13. The central recirculation zone is governed by the combined effects of the swirl strength (a characteristic of the variable-angle swirl vane pack  206   r ) and the blunt aft end of the aft centerbody assembly  244   r.  The outer recirculation zone is created by separation of the fuel/air mixture as it flows over the quarl step  5026 . Combustion in the afterburner  5020  is very robust, stable and highly efficient such that the energetic, high temperature by-products of the combustion event are expanded through the variable area nozzle  5010  to provide high levels of thrust.  
         [0142]    During non-afterburning operation, the swirl generator  30   r  serves as a channel between the main engine and the variable area nozzle  5010 . To avoid significant pressure losses due to the presence of the vanes  402   r,  the variable-angle swirl vane pack  206   r  is controlled such that the angle of the vanes  402   r  is changed to 0° so as to remove the swirl from the flow and thereby maintain the axial character of exhaust flow. Vanes  402   r  having a flat profile are presently preferred. As an alternative to the variable angle swirl vane pack  206   r,  a two-position swirl vane pack (not shown) may also be employed. Also during non-afterburning operation, the collapsible centerbody cone  5022  that is attached to the aft centerbody  244   r  is extended to create a flowfield with relatively greater aerodynamic efficiency and relatively lower pressure losses.  
         [0143]    While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments failing within the foregoing description and the appended claims.