Abstract:
A pulsejet system and method requires no pulsejet internal moving parts. Each pulsejet includes a combustion chamber having an upstream inlet port joined to an inlet diffuser, boundary layer air ports enveloping the combustion chamber, and a downstream exit port joined to a discharge nozzle. Each pulsejet discharges into an ejector to increase net thrust. Each ejector includes an augmentor cell having side walls and perforated end plates. The perforated end plate between each pair of pulsejets is shared to permit the discharge thrust to equalize across the pulsejet group. Air and fuel mix in the combustion chamber and are detonated by a reflected back-pressure wave. Detonation/deflagration reverse pressure waves compressing boundary layer air flow act as a pneumatic throat to temporarily choke off inlet fresh air at the upstream inlet port. The pneumatic throat replaces the conventional mechanical valve used for this purpose.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to pulsejets and more specifically to a pneumatically controlled pulsejet coupled with an ejector providing increased thrust.  
       BACKGROUND OF THE INVENTION  
       [0002]     Pulsejets provide an inexpensive means to propel an aircraft or other propulsion device. Pulsejets as known in the art are extraordinarily simple devices, generally having only one moving part in the engine (a “mechanical” type air inlet valve). The main disadvantages of known pulsejet designs are a low propulsion efficiency and a limited mechanical durability due to the life expectancy of the air valve used in the engine. Pulsejet engine designs are therefore not commonly used as the main engines (i.e., the engine normally used for axial propulsion) of an aircraft.  
         [0003]     In order to improve the thrust capability of a pulsejet engine, thrust augmentors, well known in the art, are often employed. By adding a thrust augmentor to the discharge side of a pulsejet engine, the thrust from the pulsejet engine can be increased by a factor ranging from approximately 1.5 to approximately 4.0. The drawback of known thrust augmentors is that the thrust augmentor itself is a separate structure added onto the pulsejet which increases the overall weight and air drag of the engine/augmentor combination. Because of the air inlet mechanical valve design, however, an augmented pulsejet engine is still not a good choice for the main propulsion engine of an aircraft due to its limited endurance, and propulsion inefficiency in applications where limited engine quantity is a design condition.  
         [0004]     The air valve typically used on known pulsejet designs is a “mechanical” valve. The mechanical valve is typically located in the air inlet of a pulsejet engine and operates by deflecting to a minimum aperture size to allow air into the pulsejet engine. When a detonation of a fuel/air mixture occurs in the pulsejet engine, a backpressure wave from the detonation closes the mechanical valve, temporarily shutting off the air inlet to the pulsejet engine. Many mechanical valve designs known in the art suffer from a frequent mechanical failure rate and often fail from fatigue of the metal components used. The mechanical valve therefore becomes a limiting factor in the design life of a pulsejet engine, and therefore has reduced the application of pulsejet engines.  
         [0005]     The prior art also includes pulsejet engines with acoustic valves or valve-less designs. These designs, however, do not restrict engine combustion products from traversing back through the inlet. This backflow creates a loss in thrust unless (as in the case of a Hiller-Lockwood type pulsejet reactor) the inlet is pointed aft to provide thrust. The acoustic inlets suffer from severe performance losses. The reactors suffer from poor volumetric and integration considerations.  
         [0006]     A need therefore exists for a pulsejet engine which reduces the maintenance and failure rate of existing designs due to the mechanical valve, and for an improved overall pulsejet/augmentor design to increase the potential uses of this otherwise simple engine type.  
       SUMMARY OF THE INVENTION  
       [0007]     In a preferred embodiment of the present invention a pulsejet engine with no mechanical valve, and therefore no engine moving parts, is provided. Each pulsejet engine includes a combustion chamber having an upstream inlet port joined to an inlet diffuser. A plurality of boundary air plenums having boundary layer air ports envelope the combustion chamber and are positioned to inject boundary layer air into the combustion chamber. A downstream combustion chamber exit port is joined to a discharge nozzle. An ejector is structurally combined with each pulsejet which by directing pulsejet effluent entrains ambient air and increases the net thrust of the pulsejet engine. Two or more pulsejets and ejectors of the present invention when arranged in one or more groups of pulsejets provide a reliable propulsion source.  
         [0008]     In operation of the invention, fresh air enters the inlet diffuser of the pulsejet at an inlet throat. Boundary layer air is directed into the boundary layer air ports enveloping the combustion chamber. Fuel is injected forward of the combustion chamber in the inlet diffuser, or within the combustion chamber via a fuel injection system, and combines with the air into a fuel/air mixture in the combustion chamber. The fuel/air mixture is detonated by reflected pressure waves from a previous cycle of operation of the pulsejet. The detonation produces reverse pressure waves which force boundary layer air to localize about the inlet throat. This temporarily chokes off the fresh air and the fuel supply at the upstream inlet port of the combustion chamber forcing all of the pressurized effluent to exit the rear of the pulsejet as thrust. The reverse pressure waves used to temporarily choke off the inlet air and fuel supply eliminate the need for the mechanical valves now used for this purpose. By eliminating the mechanical valve, a pulsejet with no moving engine parts results.  
         [0009]     In one preferred embodiment, each ejector includes an augmentor cell. The augmentor cell includes side walls, potentially perforated end plates and inlet and exhaust apertures. The pulsejets are grouped into pairs of pulsejets and the perforated end plates of the augmentor cell between pairs of pulsejets is shared which permits the discharge thrust from the pairs of pulsejets to equalize across the perforated end plates. Equalizing the discharge thrust permits the overall group of pulsejets to operate at an evenly distributed thrust level.  
         [0010]     In another preferred embodiment, at least one moveable cowl is provided on at least one of the inlet and exhaust apertures. The movable cowl has several uses. Movable cowls are used to reduce the exhaust aperture size of one or more augmentor cells which provides improved control of the thrust from each pulsejet or pulsejet group. Movable cowls also isolate the inlet and exhaust apertures of the augmentor cells to protect the pulsejet engines within the augmentor cells when the pulsejet engines are not in use.  
         [0011]     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  
       [0012]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0013]      FIG. 1  is a side elevation view of a preferred embodiment of a pulsejet engine of the present invention;  
         [0014]      FIG. 2  is a perspective view of a group of pulsejets of  FIG. 1  having ejectors wherein the ejectors are formed as augmentor cells of the present invention;  
         [0015]      FIG. 3  is an elevation view of a single augmentor cell of the present invention having the pulsejet structurally mounted to the augmentor cell structure;  
         [0016]      FIG. 4  is an alternate preferred embodiment of the present invention having the pulsejet inlet diffuser and combustion chamber arranged horizontally and the discharge nozzle arranged vertically to discharge downward into an ejector of the present invention;  
         [0017]      FIG. 5  is a sectioned elevation view of a pulsejet known in the art;  
         [0018]      FIG. 6  is the sectioned elevation view of  FIG. 5  further showing pressure waves closing a mechanical valve and providing discharge thrust;  
         [0019]      FIG. 7  is the sectioned elevation view of  FIG. 6  further showing the partial opening of the mechanical valve and inflow of a fuel/air mixture into the combustion chamber;  
         [0020]      FIG. 8  is the sectioned elevation view of  FIG. 7  further showing a fully opened mechanical valve and a fuel/air mixture compression cycle;  
         [0021]      FIG. 9  is the sectioned elevation view of  FIG. 8  further showing a fuel/air mixture detonation following a collision between the fuel/air mixture with reflected backpressure waves from a previous detonation;  
         [0022]      FIG. 10  is a sectioned elevation view of a preferred embodiment of a pulsejet engine of the present invention during a detonation cycle;  
         [0023]      FIG. 11  is the sectioned elevation view of  FIG. 10  further showing the deflagration step following detonation wherein the air and fuel flow are choked off by a pressure wave generated during the detonation phase;  
         [0024]      FIG. 12  is the sectioned elevation view of  FIG. 11  further showing the expansion of a fuel/air fuel mixture into the combustion chamber of a pulsejet engine of the present invention following thrust exhaust of the previously detonated air fuel mixture;  
         [0025]      FIG. 13  is the sectioned elevation view of  FIG. 12  further showing the fuel/air mixture in the combustion chamber colliding with reflected pressure waves from the discharge nozzle of the pulsejet of the present invention prior to detonation of the air fuel mixture; and  
         [0026]      FIG. 14  is the sectioned elevation view of  FIG. 3  further showing a rotatable cowl at an inlet aperture of the pulsejet of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0028]     Referring to  FIG. 1 , a pulsejet engine  10  in accordance with a preferred embodiment of the present invention is shown. The pulsejet engine  10  includes a body  12  having an inlet end  14  and an exhaust end  16 . Propulsion thrust from the pulsejet engine  10  discharges from an exhaust end  16  in a propulsion exhaust direction A. Air, normally at atmospheric pressure, enters the inlet end  14 . The air mixes with a fuel (discussed in reference to  FIG. 11 ) which is detonated to produce thrust to propel a platform (not shown) in a platform travel direction B. In the exemplary preferred embodiment shown, both the air flow and the burned fuel/air mixture travel in the propulsion exhaust direction A approximately parallel with a pulsejet engine longitudinal centerline C.  
         [0029]     Referring to  FIG. 2 , an exemplary pulsejet bank  18  of the present invention is shown. Each pulsejet bank  18  includes a plurality of pulsejets  20 . Each pulsejet  20  together with a pair of sidewalls  22  form a structural member. Additional structural members including intercostals and webs interconnecting each pulsejet to the sidewall(s)  22  are not shown for clarity. An outward facing sidewall  22  has been removed from the view of  FIG. 2  for clarity. Each sidewall  22  is curved to entrain and direct air together with the pulsejet exhaust to maximize thrust from each of the pulsejets  20  in the thrust direction D shown. A plurality of perforated end plates  24  are connectably attached to the sidewall  22  adjacent to each of the pulsejets  20 . Each of the perforated end plates  24  has a plurality of apertures  26  there through. The apertures  26  permit equalization of flow between each of the pulsejet  20  exhaust flows such that a pulsejet  20  within the pulsejet bank  18  which is operating above or below a nominal operating condition is equalized with the remaining pulsejets  20  of the pulsejet bank  18 . Air enters each of the pulsejets  20  through a pulsejet inlet  28 . The exhaust gas producing thrust from each of the pulsejets  20  is discharged from a pulsejet exhaust  30  in the thrust direction D. Each adjacent pair of perforated end plates  24  connectably joined to opposed sidewalls  22  form each of a plurality of augmentor cells  32 . Only one sidewall  22  is shown in  FIG. 2  for clarity. Fuel is supplied to each of the pulsejets  20  through a fuel injection system (shown and discussed with reference to  FIG. 10 ).  
         [0030]     Referring now to  FIG. 3 , an exemplary pulsejet bay  34  is detailed. Each pulsejet bay  34  includes one augmentor cell  32  and one pulsejet  20 . Each pulsejet  20  includes an inlet diffuser  36 , a combustion chamber  38 , and a discharge nozzle  40 . Structural members join each pulsejet  20  with one or both of the sidewalls  22  to form a unitary load bearing structure. In one preferred embodiment a plurality of fins  42  surround the discharge nozzle  40 . The fins  42  are used to structurally interconnect the pulsejet  20  with one or both of the sidewalls  22  of the pulsejet bay  34 . In another preferred embodiment, a plurality of webs or intercostals (not shown) join each pulsejet  20  with one or both of the sidewalls  22 . Discharge from each of the pulsejets  20  is in the thrust direction D as shown. A portion of the discharge from the pulsejet  20  exits through each of the plurality of apertures  26  in the ejector cross flow direction E. Air enters the inlet diffuser  36  in the air inlet flow direction F. The inlet diffuser  36  is connectably joined to the combustion chamber  38 , and the combustion chamber  38  is connectably joined to the discharge nozzle  40 . In another preferred embodiment, each of the inlet diffuser  36 , the combustion chamber  38 , the discharge nozzle  40 , and the sidewall(s)  22  can also be provided as an integral unit cast or formed from a single piece of material.  
         [0031]     Referring to  FIG. 4 , another preferred embodiment of the present invention is shown. In this embodiment, the inlet diffuser  36  and the combustion chamber  38  are co-aligned on a horizontal axis G. A bend  44  connectably joins the combustion chamber  38  to the discharge nozzle  40 . The discharge nozzle  40  is aligned along a vertical axis H. The discharge nozzle  40  discharges in the thrust direction D into the augmentor cell  32 . This embodiment of the present invention permits the inlet for the pulsejet to be aligned horizontally while the discharge is aligned vertically providing additional flexibility in the arrangement of the pulsejets. A perpendicular alignment between the inlet and the discharge of the pulsejet are shown, however, any angle can be used to suit arrangement constraints while considering engine operability.  
         [0032]      FIGS. 5 through 9  depict a complete operating cycle for a pulsejet engine known in the art. Referring to  FIG. 5 , a pulsejet  50  known in the art is detailed. The pulsejet  50  includes an inlet diffuser  52  which receives air in the inlet flow direction J. An exhaust nozzle  54  discharges flow from the pulsejet  50  in the exhaust flow direction K. A mechanical valve  56  is included in the inlet diffuser  52  to prevent a backflow of detonated gas from back flowing into the inlet diffuser  52 . In  FIG. 5 , a detonation stage of a fuel/air mixture  58  in a combustion chamber  60  is shown.  
         [0033]     Referring to  FIG. 6 , after the fuel/air mixture  58  of  FIG. 5  detonates in the combustion chamber  60 , a plurality of reverse pressure waves  62  are generated in a deflagration stage. The reverse pressure waves  62  and the resultant combustion gas travel toward the inlet diffuser  52  and cause the mechanical valve  56  to close preventing flow of the gas through the inlet diffuser  52 . A plurality of forward pressure waves  64  is also generated during the deflagration stage. The forward pressure waves  64  and combustion gas travel in the direction of the exhaust nozzle  54  generating thrust from the pulsejet.  
         [0034]     Referring to  FIG. 7 , after a majority of the combustion gas exhausts through the exhaust nozzle  54 , the pressure in the combustion chamber  60  reduces due to reflected expansion waves. The reflected expansion waves create a differential pressure gradient across inlet diffuser  52  to the combustion chamber  60  which force the mechanical valve  56  to begin to open. As the mechanical valve  56  opens a new supply of air and fuel enters the combustion chamber  60 . A plurality of expanding air pressure waves  66  create low pressure regions that lead a fuel/air mixture  68  into the combustion chamber  60 .  
         [0035]     Referring to  FIG. 8 , in a compression stage compression of the fuel/air mixture  68  begins to occur in the combustion chamber  60 . The mechanical valve  56  is fully open allowing air flow through the inlet diffuser  52  in the inlet flow direction J. A plurality of high temperature discharge nozzle backpressure waves  70  reflect from the exhaust nozzle  54 . The discharge nozzle backpressure wave  70  temperature is approximately 1,500 degrees Fahrenheit (815° C.). The discharge nozzle backpressure waves  70  travel in the nozzle backpressure direction L. When the discharge nozzle backpressure waves  70  contact the fuel/air mixture  68  the fuel/air mixture  68  initially compresses in the combustion chamber  60 .  
         [0036]     Referring to  FIG. 9 , when the high temperature discharge nozzle backpressure waves  70  contact the fuel/air mixture  68  (shown in  FIG. 8 ) and the fuel/air mixture  68  temperature rises to its ignition temperature a detonation of the fuel/air mixture  68  occurs in the combustion chamber  60 . The fuel/air mixture  68  detonates at a fuel/air detonation point  72  and a new cycle for the pulsejet  50  begins. The detonation, exhaust, compression and new detonation cycle occurs rapidly in the pulsejet engine, i.e., approximately 60 to 100 cycles per second as is known in the art. Fuel is either continuously pressurized and fed by a fuel injection system (shown and discussed in reference to  FIG. 10 ) or is pulse pressurized to enter at the optimum time of each engine operating cycle. Detonation is normally initiated and can also be controlled using a detonation device (not shown) such as a spark plug in the combustion chamber.  
         [0037]      FIGS. 10 through 13  show a single cycle of operation of a pulsejet of the present invention. Referring to  FIG. 10 , a pulsejet  100  of the present invention is shown. The pulsejet  100  includes an inlet diffuser  102  connected to an upstream inlet port  104  of a combustion chamber  106 . The combustion chamber  106  is enveloped by a boundary layer air plenum  108 . The boundary layer air plenum  108  provides a plurality of boundary layer air ports  110  (designated as exemplary boundary layer air ports  110 ′,  110 ″, and  110 ′″) for introduction of a boundary layer air supply (not shown) through supply lines  112 . Boundary layer air is provided by an external source (not shown) which can include compressed air, oxygen generating candles, or bleed air. Boundary layer air enters the combustion chamber  106  through a plurality of apertures  114  in a body section  116  (shown in an exemplary conical shape) of the combustion chamber  106 . The apertures  114  in the body section  116  can have the same aperture size, or can increase or decrease in size, as viewed in  FIG. 10  from right to left as the apertures  114  are positioned along the body section  116 . The body section  116  and the combustion chamber  106  can also be provided in other geometric shapes. One or more boundary layer air ports  110  can be used.  
         [0038]     The combustion chamber  106  tapers down and connects to a discharge nozzle  118  at a downstream exit port  120 . A fuel supply (not shown) is fed or injected into the inlet diffuser  102  upstream of the upstream inlet port  104  through one or more fuel supply lines  122 . Fuel supply lines  122  can also enter the combustion chamber  106 , or divide between both the upstream inlet port  104  and the combustion chamber  106 . A detonation stage is depicted in  FIG. 10 . A fuel and air mixture detonates in the combustion chamber  106  at a fuel/air detonation point  124 .  
         [0039]     Referring to  FIG. 11 , following the detonation stage shown in  FIG. 10 , a deflagration stage of the pulsejet  100  is shown. A fuel/air deflagration mixture  126  is shown. As the fuel/air mixture continues to burn and expand beyond the fuel/air deflagration mixture  126 , a plurality of reverse pressure waves  128  form. The reverse pressure waves  128  travel in the reverse pressure wave direction M toward the inlet diffuser  102  (shown in  FIG. 10 ). A plurality of forward pressure waves  130  also form. The forward pressure waves  130  travel in the thrust direction N into the discharge nozzle  118 . The reverse pressure waves  128  contact an entering boundary layer air volume  132  and compress the boundary layer air volume  132  in the direction of the inlet diffuser  102 .  
         [0040]     A fresh air stream  134  combines with fuel supplied through the fuel supply line  122  (shown in  FIG. 10 ) to form a fuel/air mixture  136 . The boundary layer air volume  132  contacts the fuel/air mixture  136  and a choke point  138  is formed. At the choke point  138 , the pressure of the now compressed boundary layer air volume  132  equals or exceeds the pressure of the fresh air stream  134  and further flow of the fresh air stream  134  into the combustion chamber  106  is temporarily blocked. The pressure of the boundary layer air volume  132  driven by the reverse pressure waves  128  also exceeds the pressure of the fuel injection system (not shown) at the fuel supply line  122 , or a sensor of the fuel injection system signals a fuel cut-off therefore preventing input of fuel during the deflagration stage.  
         [0041]     High pressure within the combustion chamber  106  still exists at the stage where the choke point  138  is created. The pressure in the combustion chamber  106  is relieved as thrust in the thrust direction N as the forward pressure waves  130  travel toward the discharge nozzle  118 . The high pressure of the reverse pressure waves  128  force more and more of the boundary layer airflow injected through the boundary layer air plenum  108  (shown in  FIG. 10 ) away from the discharge nozzle facing end of the boundary layer air plenum  108  towards the inlet diffuser facing end. Boundary layer air flow is constricted to flow through an increasingly smaller injection area which causes the velocity and subsequent penetration of the boundary layer air flow into the fresh air stream  134  to increase. In effect, this creates a pneumatic throat or venturi which not only chokes the fresh air stream  134  from entering the pulsejet  100 , but also prevents combustion byproducts from exiting the engine via the inlet diffuser  102  (shown in  FIG. 10 ). The choke point  138  location is determined in part by the shape of the body section  116  of the combustion chamber  106 , and by the pressure of the reverse pressure waves  128 .  
         [0042]     Referring now to  FIG. 12 , during an expansion stage the deflagration pressure and its effect on the boundary layer air volume  132  is reduced by thrust discharge through the discharge nozzle  118  and back reflection of the reverse pressure waves  128  from the choke point  138  (identified in  FIG. 11 ). The reverse pressure waves  128 , traveling in the direction P encounter the choked flow, reflect and travel in the direction of expansion direction arrows  0 . This reflection, together with the forward pressure waves  130  exiting the combustion chamber  106 , create a diffusion process which subsequently decreases the pressure in the combustion chamber  106 . The pressure differential between the fuel/air mixture  136  and the pressure in the combustion chamber  106  causes the fuel/air mixture  136  to flow again into the combustion chamber  106  in the direction of expansion arrows O.  
         [0043]     The reduced pressure in the combustion chamber  106  also allows the boundary layer air volume  132  to redistribute itself throughout the boundary layer air plenum  108  (described in reference to  FIG. 10 ) and the combustion chamber  106  from the boundary layer air ports  110 . As the boundary layer air flow is redistributed, it is allowed to pass through an ever increasing passage porosity (i.e., the injection area increases). With constant injection pressure and airflow, an increased area necessitates lower velocity injection due to fundamental gas laws. A lowered combustion chamber pressure and increased fresh air charge also help guide the boundary layer air flow to the outer combustor walls of the body section  116 . This serves to partially cool and isolate the hot combustor section from the inlet and also stabilizes subsequent combustion processes by focusing the combustion processes to the fuel/air detonation point  124  (shown in  FIG. 10 ). In this expansion stage, the forward pressure waves  130  have reached the discharge nozzle  118 . A plurality of discharge nozzle back-pressure waves  140  in the form of rarefaction waves begin to form in this stage. The discharge nozzle back-pressure waves  140  create a sub-ambient expansion which partially induces ejector airflow and combustion byproducts from the last cycle into the discharge nozzle  118 . The discharge nozzle back-pressure waves  140  also travel in the direction P.  
         [0044]     Referring now to  FIG. 13 , in a compression stage the fuel/air mixture  136  traveling in an air/fuel flow direction  0  begins to contact the discharge nozzle back-pressure waves  140 . The fuel/air mixture  136  begins to compress in the combustion chamber  106 . A stabilizing volume of the previously expanded boundary layer air volume  132  is shown as it compresses along the perimeter of the combustion chamber  106 . The compression stage shown in  FIG. 13  shows the plurality of discharge nozzle back-pressure waves  140  immediately before detonation of the fuel/air mixture  136  similar to the detonation shown in  FIG. 10 . Detonation begins a new cycle for the pulsejet.  
         [0045]     Referring back to  FIG. 12 , the combustion chamber  106  includes a taper section  142 . The taper section  142  ends at a taper distal end  144  which is the connecting point for the discharge nozzle  118 . The geometry of the taper section  142  helps provide the constriction of the out flowing gases and the generation of the discharge nozzle back-pressure waves  140 .  
         [0046]     Referring to  FIG. 14 , an exemplary pulsejet bay  146  in accordance with a preferred embodiment of the present invention is shown. An upper aperture  148  of the pulsejet bay  146  can be partially or completely closed by a cowl  150 . In the exemplary embodiment shown, the cowl  150  is mounted to the pulsejet bay  146  by a hinge  152  or similar mechanical element. The cowl  150  rotates along an arc R about a hinge center-line S to the closed, phantom position shown. The cowl  150  is controlled by a control system (not shown). Air inlet flow to the pulsejet  100  in the pulsejet bay  146  can be controlled by the single cowl  150  shown or by two or more cowls (not shown) similar to the cowl  150 . Similar devices provided at the bottom aperture  148  of each of the pulsejet bays  146  can be used to control the thrust produced in each pulsejet bay  146 . In another preferred embodiment, the cowl  150  is provided as a flexible member which rolls out from a reel (not shown) which replaces the hinge  152 , to the closed, phantom position shown.  
         [0047]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. For example, the exhaust end  16  of the pulsejet engine  10  is shown having a cruciform shape. The exhaust end  16  can also be round, tapered/conical, or shaped to easily be structurally integrated with surrounding structure. Such variations are not to be regarded as a departure from the spirit and scope of the invention.