Abstract:
A pilot for a scramjet provides a flame front whose arrival at the wall of the scramjet combustor is delayed thereby reducing combustor heat load. By combining in-stream injection of fuel with an interior pilot and a lean (fuel-poor) outer annulus, the bulk of combustion is confined to the scramjet combustor center. This concept, referred to as “core-burning,” further reduces combustor heat load. One such pilot is for a two dimensional scramjet effective to propel a vehicle. This pilot includes a plurality of spaced apart struts separated by ducts and a strut pilot contained within each strut. A second such pilot is for an axisymmetric scramjet engine has, in sequence and in fluid communication, an air intake, an open bore scramjet isolator and a scramjet combustor. This centerbody pilot pod includes a pilot isolator disposed between the air intake and a pilot diffuser, the pilot diffuser disposed between the pilot isolator and a pilot with the pilot disposed between the pilot diffuser and a pilot combustor. The pilot pod is in axis symmetry around a central axis of the scramjet isolator and supported by a plurality of struts extending from an inner wall of the open bore to an outer surface of the centerbody pilot pod.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    N.A. 
       U.S. GOVERNMENT RIGHTS 
       [0002]    N.A. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates to an engine that utilizes air moving at supersonic speeds for compression, combustion and expansion. Such an engine is known as a scramjet. More particularly, a pilot pod is centrally disposed within an isolator of a scramjet module. As a result, the hottest combustion gases are located within the core of the combustor, rather than along the walls, thereby reducing combustor heat load. 
         [0005]    2. Description of the Related Art 
         [0006]    Engines that use ram compression instead of a mechanical compressor to pressurize air for combustion and expansion are known as ramjets. When the flowpath through the engine is designed specifically for higher speeds where supersonic combustion is superior, that is typically at speeds above Mach 5 or 6, the engine is referred to as a scramjet. A simplified version of a prior art scramjet is illustrated in  FIG. 1 . The two dimensional scramjet  10  is generally symmetric about axis  12  and includes a main isolator  14  that connects the scramjet intake  16  to a main combustor  18 . The main isolator  14  permits raising the air pressure higher than the scramjet intake  16  can generate at a given flight speed and altitude. While not required for a scramjet, the main isolator  14  is vital for dual mode ramjets that are capable of operation in both subsonic and supersonic environments. A suitable fuel  19  is introduced into the airflow through fuel injectors  20 . Combustion of the fuel/air mix generates very high temperatures and rapid expansion of gaseous combustion products. Expulsion of these combustion products through a nozzle (not shown) downstream  22  of the main combustor  18  generates thrust. 
         [0007]    The high velocity and low pressure flow of air and fuel, within the main combustor  18  makes it difficult to sustain combustion. In most scramjet engines the combustion will only take place when a suitable pilot zone ignites and incoming fuel/air mixture and then propagates across the duct with a turbulent flame front. This flame travels normal to the air at a fraction of the mean air velocity so the flame front appears to be swept back at a large angle. In prior art scramjets, with the pilot  24  situated on the main combustor wall  26  of the scramjet  10  that defines the exterior wall of the main combustor  18 , the exterior wall  26  is immediately exposed to full combustion temperature while the flame slowly moves radially inward to burn the rest of the air. Dotted isotherm lines  28  illustrate a demarcation between hottest region  30  (e.g. typical total temperature in excess of 6000° R), moderate region  32  (e.g. typical total temperature between 5000° R and 6000° R), and coolest region  34  (e.g. typical temperature less than 5000° R). The hottest region  30  generates a high combustor heat load on the exterior walls  26  of the main combustor  18  which are exposed to the most severe thermal environment. As a result, the walls must be made from exotic high temperature resistant materials such as tungsten or actively cooled with scarce fuel increasing costs and complexity. 
         [0008]    U.S. Pat. No. 4,170,110 to Radin discloses a scramjet where the intake air is divided into a central stream and peripheral boundary layer streams. The peripheral boundary layer streams are very narrow, on the order of 15 microns in thickness. A typical scramjet with a non-axisymmetric, two dimensional (2-D), pilot is illustrated in U.S. Pat. No. 5,253,474 to Correa, et al. Both U.S. Pat. No. 4,170,110 and U.S. Pat. No. 5,253,474 are incorporated by reference herein in their entireties. 
         [0009]    There remains a need for a scramjet having a reduced heat load applied to the wall of the combustor as compared to the prior art. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims. 
         [0011]    In accordance with a first embodiment of the invention, there is provided a two dimensional scramjet effective to propel a vehicle. This scramjet includes a plurality of spaced apart struts separated by ducts and a strut pilot contained within each strut. 
         [0012]    In accordance with second embodiment of the invention, there is provided a centerbody pilot pod for a scramjet engine. The scramjet engine has, in sequence and in fluid communication, an air intake, an open bore scramjet isolator and a scramjet combustor. The pilot pod includes a pilot isolator disposed between the air intake and a pilot diffuser, the pilot diffuser disposed between the pilot isolator and a pilot with the pilot disposed between the pilot diffuser and a pilot combustor. The pilot pod is in axis symmetry along a central axis of said scramjet isolator and supported by a plurality of struts extending from an inner wall of the open bore to an outer surface of the pilot pod. 
         [0013]    It is an advantage of certain aspects of the invention that flame front arrival at the wall of the scramjet combustor is delayed thereby reducing combustor heat load. A further advantage of certain aspects of the invention is that by combining in-stream injection of fuel with a lean (fuel-poor) outer annulus, the bulk of combustion is confined to the scramjet combustor center. This concept, referred to as “core-burning,” further reduces combustor heat load. 
         [0014]    Among the benefits of the core-burning embodiments of the invention are the scramjet combustor length may be reduced when instream injection is utilized. Further, the fuel injectors may be spaced for zone injection. In-stream injection is recognized as effective to reduce the combustor length relative to wall only injection resulting in reduced viscous losses and heat load within the combustor. Combustor hot spots are minimized or eliminated simplifying cooling requirements and may reduce or eliminate the need for an endothermic heat sink. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  illustrates a 2-D scramjet as known from the prior art and a thermal profile for that scramjet. 
           [0016]      FIG. 2  illustrates a strut pilot installed in a scramjet module in accordance with a first embodiment of the invention. 
           [0017]      FIG. 3  is a cross-sectional view of a strut used to contain the strut pilot of  FIG. 2 . 
           [0018]      FIG. 4  is a frontal view of the scramjet module of  FIG. 2 . 
           [0019]      FIG. 5  is a rear view the scramjet module of  FIG. 2 . 
           [0020]      FIG. 6  is a rear view of the scramjet module of  FIG. 2  illustrating fuel injection. 
           [0021]      FIG. 7  illustrates a thermal profile for the scramjet module of  FIG. 2 . 
           [0022]      FIG. 8  illustrates a centerbody pilot pod installed in a scramjet module in accordance with a second embodiment of the invention. 
           [0023]      FIG. 9  illustrates a frontal view of the scramjet of  FIG. 8 . 
           [0024]      FIG. 10  illustrates a rear view of the scramjet of  FIG. 8 . 
           [0025]      FIG. 11  is a rear view of the scramjet module of  FIG. 8  illustrating fuel injection for three mission segments. 
           [0026]      FIG. 12  illustrates a thermal profile for the scramjet module of  FIG. 8 . 
       
    
    
       [0027]    Like reference numbers and designations in the various drawings indicated like elements. 
       DETAILED DESCRIPTION 
       [0028]      FIG. 2  illustrates a 2-D scramjet module  40  having a strut pilot  43  in accordance with a first embodiment of the invention. The strut pilot  43  is fully contained within strut  44  that bridges a gap within the main isolator  14  extending between a body side  46  of a vehicle, such as a missile or the like and a cowl side  48 . The strut  44  mounted strut pilot  43  includes a pilot isolator  50 , a pilot diffuser  52 , a pilot flameholder  54 , a pilot combustor  56  and a pilot nozzle  82 . It is noted that the strut pilot  43  is essentially a ramjet fully contained within strut  44 . As illustrated in cross-sectional representation in  FIG. 3 , the strut  44  has a small leading edge radius  58  and a wedge shaped windscreen  69  that forms a relatively small angle, α, relative to a center line axis  60  to minimize air drag through the main isolator of the scramjet module. Trailing edge  62  is formed by a tapered boat tail shape with a similar angle relative to the centerline axis and may have bluff base for enhanced strength. The walls of the strut define a central cavity  64 . Among the functions of the central cavities are to house the strut pilot, to provide a channel for the flow of fluids, and to reduce the weight of the strut, including by forming lightening holes  65  to remove metal not need to provide support. 
         [0029]    With reference back to  FIG. 2 , in operation, the strut pilot  43  receives incoming pilot portion air  66  which is slowed in the pilot isolator  50  by a shock train  68  and further slowed in the pilot diffuser  52 . On entering the pilot isolator  50 , the pilot portion air  66  is at a first supersonic speed (M 1 ) and first pressure (P 1 ). Within the pilot isolator  50 , the pilot portion air  66  is divided into a supersonic core  70  and subsonic boundary layer  72 . Outgoing  74  pilot portion air enters the pilot diffuser  52  at a second subsonic speed (M 2 ) and second pressure (P 2 ) where M 1 &gt;M 2  and P 1 &lt;P 2 . A ratio of the length of the pilot isolator  50  to its width is selected to maximize an increase in air pressure, typically 8:1. 
         [0030]    Inwardly directed pilot fuel injectors  76  add fuel  19  to the pilot portion air  66  which is ignited by pilot flameholder  54  and burned in pilot combustor  56 . Hot exhaust  80  exits the strut pilot  43  through a choked nozzle  82  to ignite a fuel and air mixture flowing around the strut  44  into the main combustor  18 . A suitable fuel injector for both pilot fuel injectors  76  and outwardly directed main fuel injectors  78  is a cascade injector such as that disclosed in U.S. Pat. No. 5,220,787. U.S. Pat. No. 5,220,787 is incorporated by reference in its entirety herein. 
         [0031]    A supersonic stream of air from scramjet intake  16  is divided into pilot portion air  66  stream and main air  84  stream. The main air  84  flows around the outside of the strut  44  and through the main isolator  14 . The pilot portion air  66  streams through the pilot isolator  50  as described above. Typically, the pilot portion air  66  will constitute from about 3% to 10%, by volume, of the air captured by scramjet intake  16 . Fuel injectors  76 ,  78  introduce a suitable fuel  19  such as JP-7 (a kerosene distillate, low volatility aviation turbine fuel as defined by Military Specification MIL-T-38219B (USAF)) or hydrogen into the pilot portion air  66  and main air  84 . The fuel injectors  76 ,  78  have pilot circuits disposed within the pilot duct and main injectors on the outer surface of the strut  44  as shown in more detail in  FIG. 6 . 
         [0032]      FIG. 4  is a frontal view, along sight line A-A of  FIG. 2 , of a 2-D scramjet module  40  containing three struts  44  and four ducts  86 . The struts  44  extend from body side  46  to cowl side  48 . The main air flows through the ducts  86  that form the scramjet isolator  14  which is defined by the smallest gap  88  between the struts  44 . The struts  44  also have a structural function, carrying load between the body  46  and cowl  48  reducing the effective span of the scramjet module  40 . The net flow area of the combination of main isolator  14  and pilot isolator  50  is constant or increasing slightly to insure inlet starting at a suitable low Mach number. The blockage of the struts is minimized consistent with the volume for the pilot function, cooling and structural requirements. 
         [0033]      FIG. 5  is a rearview, as viewed along sight line B-B of  FIG. 2 , illustrating multiple strut pilot exhaust nozzles  82  situated in the interior of the scramjet module. The hot gases from the pilots continuously ignite fresh fuel/air reactants passing between the struts  44 . The flame stability is greatly enhanced by first combusting the pilot air and fuel in the pilot combustor ( 56  of  FIG. 2 ) and then igniting the main air/fuel mixture in ducts  86 . The pilot utilizes only a small fraction of the total engine flow (3%-10%, by volume) in order to burn just enough air and fuel to insure the main flow is ignited while maintaining supersonic through flow for the main flow path. 
         [0034]      FIG. 6  is the same rear view as  FIG. 5  showing the main fuel injectors  78  injecting fuel  19  into the main air  84  flowpath. The plumes of fuel  19  may be injected through apertures formed in exterior walls of the struts  44 . A zoned fuel injection strategy, as described below, is employed and  FIG. 6  illustrates a lean fuel condition where approximately the outermost 20% of the main combustor is unfueled creating a lean region along main combustor walls  26 . At cruise speed, those main fuel injectors adjacent a wall of the main isolator are in a no-flow condition while a remainder of the main fuel injectors are in a provide fuel condition. Fuel flow is reduced to a lean condition using only the main fuel injectors  78  which are designed to fuel the interior region of the main air  84  flowpath. The outer region  90  is unfueled at cruise resulting in a lean and cooler wall environment. 
         [0035]      FIG. 7  illustrates an advantage of the strut pilot  43  of this first embodiment of the invention. When the scramjet is in high speed cruise mode, utilizing lean fueling, isotherm lines  28  illustrate how the hottest region  30  is isolated from the main combustor walls  26  reducing combustor thermal loading. Main fuel injectors inject fuel into the main air  84  flowpath that streams around the struts  44 . The air/fuel mix is ignited in the scramjet main combustor  18  by the hot gaseous exhaust  80  exiting the pilot. With core burning, the pilot zone and the initiation of the combustion hottest region  30  is remote from the main combustor wall  26 . As the flame front travels from the center of the main combustor  18  towards the combustor wall  26 , the mixture is rapidly raised in temperature according to the amount of fuel mixed with the air. As the flame front crosses the gap and approaches the combustor wall  26 , it encounters the lean region with little or no fuel contained therein. This lean air can not burn, so the combustor wall never sees the full heat flux of prior art scramjets. With a shorter main combustor  18  length due to instream injection from the struts  44  and a lower heat flux, the heat load is significantly lower with core burning than possible with prior art scramjets. 
         [0036]    In accordance with a second embodiment of the invention,  FIG. 8  illustrates a main isolator  14  for a scramjet having an axisymmetric cross-sectional profile. A centerbody pilot pod  42  is mounted with axial symmetry along a center line axis of the scramjet. The centerbody pilot pod  42  includes a pilot isolator  50 , pilot diffuser  52 , pilot flameholder  54  and pilot combustor  56  and is essentially a dual mode ramjet (DMRJ) symmetrically disposed within the scramjet main isolator  14 . In operation, the pilot receives pilot portion air  66  from the scramjet intake  16 . The pilot portion air  66  is slowed in pilot isolator  50  by shock train  68  and further slowed in the pilot diffuser  52 . Pilot fuel injectors  76  add fuel to the pilot portion air stream which is ignited by the pilot flameholder  54  and burned in the pilot combustor  56 . The hot exhaust  80  exits the pilot pod to ignite the fuel and air mixture flowing around the centerbody pilot pod  42 . The centerbody pilot pod  42  is supported by a plurality of struts  44  that extend from an interior bore wall of the scramjet isolator  14  to the centerbody pilot pod  42 . 
         [0037]    A supersonic stream of air captured by scramjet intake  16  is divided into pilot portion air  66  and main air  84 . The pilot portion air  66  travels through pilot isolator  50  as discussed above, while the main air  84  travels around the pilot pod  42  and struts  44  and through the duct of the main isolator  14  that is defined by outer bore of the main isolator and the centerbody pilot pod  42 . Typically, the pilot portion air  66  will constitute about 3% to 10%, by volume, of the total airflow. Pilot fuel injectors  76  introduce a suitable fuel, such as JP-7 or hydrogen, into the pilot. Main fuel injectors  78  inject fuel into the main air  84  stream. The fuel injector array has pilot circuits disposed within the pilot duct and main injectors disposed in the outer duct as shown in more detail in  FIG. 11 . 
         [0038]      FIG. 9  is a frontal view of a centerbody pilot pod  42  viewed from the scramjet intake looking back towards the entrance to the main combustor. A number of struts  44 , typically three, extend from outer bore wall  92  of the main isolator to the centerbody pilot pod  42 . The cross-sectional area of the combination of struts and centerbody pilot pod is small relative to the cross-sectional area of the scramjet isolator minimizing pilot drag and facilitating inlet starting. The primary ducts for the main air  84  occupy about 96%, by area referenced to the inlet throat, of the scramjet bore and the central portion occupies about another 10%, by area. The duct for the pilot air portion  66  has a flow area of about 4%. This results in an open area portion of at least 100% relative to the inlet throat area. The outer bore wall  92  of the main isolator duct diverges to accommodate the net blockage of the centerbody pilot pod  42  and the struts  44 . 
         [0039]      FIG. 10  is a rearview, as viewed from the main combustor  22  looking forward to the scramjet intake of the centerbody pilot pod  42 . The hot exhaust exiting from the pilot nozzle  82  continuously ignites fresh reactants passing around it. Flame stability is greatly enhanced by first combusting the pilot air and fuel within pilot combustor  56  ( FIG. 7 ) before ignition of the main flow. This is due to the more favorable combustion environment generated within the centerbody pilot pod  42  than is practical in the main air  84  flow path. This is because combustion in a high speed system is enhanced by slowing it to subsonic speeds and raising the pressure. Forcing the entire main airstream to subsonic speed would cause unacceptable losses in performance. The centerbody pilot pod  42  of the invention does this only for a small fraction of the flow (3%-10%, by volume) in order to burn just enough air and fuel to insure the main flow is ignited while maintaining supersonic through flow for the remaining 90%-97%. If the pilot were located in the main flow, it would be in a less favorable combustion environment and require a larger, higher drag, pilot structure, such as the prior art annular pilot ( 24  in  FIG. 1 ). 
         [0040]      FIG. 11  is a rear view of the main isolator  14  showing an injection of fuel  19  into the main air  84  flow. Three operating conditions effective for different segments of a mission are illustrated in  FIG. 11 . It should be recognized that in practice, a single fuel injection operating condition is applied to the entire main isolator  14  during a mission segment. The fuel plumes  19  may be injected through apertures formed in an exterior wall of the centerbody pilot pod  42 , bases of struts  44 , and cascade injectors  94  formed in outer bore wall  92  (One typical injector is illustrated for clarity). For operation across a wide range of speed, with widely ranging inflow and fuel conditions, zoned injection is preferred to optimize engine performance. 
         [0041]    Segment A illustrates fuel injection for lowest speed, during ramjet takeover (the lowest speed where the ramjet mode can accelerate the vehicle). Pilot fuel injectors  76   a  and  76   b  are functioning at all speeds to maintain the piloting of the main combustion. Main fuel injectors  78   c  inject radially outward from the centerbody pilot pod  42  and main fuel injectors  78   d  inject axially from the strut  44  bases. The main air/fuel mix is ignited by the central pilot exhaust  80 . This configuration delays the combustion so that the combustion back pressure does not exceed the inlet tolerance and unstart the inlet. 
         [0042]    As shown at Segment B, at higher speeds, low speed injectors  78   d  are turned off and main fuel injectors  78   e  and  78   f  that inject fuel radially inward from the outer bore wall  92  are turned on. This zone provides maximum acceleration prior to reaching the cruise condition. For maximum efficiency during acceleration, the engine fuel flow is typically in excess of the stoichiometric value, or 5%-10% rich. 
         [0043]    As shown at Segment C, when the desired cruise speed is reached, the engine thrust is decreased to sustain the desired speed. In order to reduce the thrust, the fuel flow is reduced to a lean condition which is accomplished by turning off injectors  78   f  which are designed to only fuel the outer region  90  of the main air  84  flow. 
         [0044]      FIG. 12  is the same view as  FIG. 8  showing a variation on the internal piloting function. In ramjet and scramjet technology, the pilot function depends on three primary conditions. These are the temperature, the pressure and the residence time in the pilot zone. In practical systems, the size of the pilot zone determines (in combination with other factors) the residence time. If a different size vehicle or mission is selected a relatively larger pilot zone may be needed. In  FIG. 8 , the pilot zone was defined by an annular cavity pilot  54  at the end of the pilot diffuser  52 . In order to illustrate a larger pilot zone, we show a conical base pilot  55  in  FIG. 12 . This variant would be used for lower ramjet takeover speeds, smaller vehicles or higher cruise altitudes. 
         [0045]    An advantage of the invention is also illustrated in  FIG. 12 . When the scramjet is in high speed cruise mode, utilizing lean fueling, isotherm lines  28  illustrate how the hottest region  30  is isolated from the main combustor walls  26  reducing combustor thermal loading. Main fuel injectors  78  inject fuel into the main air  84  flow external to the centerbody pilot pod  42 . The air/fuel mix is ignited in main combustor  18  by the heat of combustion exiting pilot combustor  56 . With core burning, the pilot zone and the initiation of the combustion hottest region  30  is remote from the main combustor wall  26 . As the flame front travels from the center of the main combustor  18  towards the main combustor wall  26 , the mixture is rapidly raised in temperature according to the amount of fuel mixed with the air. As the flame front crosses the gap and approaches the main combustor wall  26 , it encounters the lean outer annulus region  90  with little or no fuel contained therein. This lean air can not burn, so the main combustor wall  26  never sees the full heat flux as in a prior art scramjet. The heat load, which is the heat each pound of fuel must absorb, is the integral of the combustor heat flux (equivalent to heat transfer per unit of area). With a shorter combustor length due to instream injection and a lower heat flux, the heat load is significantly lower with core burning than possible with prior art scramjets. 
         [0046]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.