Abstract:
An acoustic resonance igniter uses high-pressure helium to heat a resonance cavity so a hot surface of the resonance cavity forms a source of ignition to a combustion chamber. The resonance cavity may be round or may extend linearly to increase the size of the hot surface. The combustion chamber is cooled by arranging a feed of hydrogen and oxygen which is oxygen rich and which becomes more so when ignition occurs. A second combustion chamber receives the combustion chamber output and adds additional hydrogen through ports tangential to the wall of the second combustion chamber to enrich the fuel ratio and cool the second combustion chamber. The acoustic resonance igniter is used to ignite a large rocket engine or to form a rocket thruster.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims priority on U.S. Provisional Patent App. No. 61/647,696 Filed May 16, 2012, the disclosure of which is incorporated by reference herein. 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was made with government support under contract to US Air Force Research Laboratory # FA9300-10-C-2105. The government has certain rights in the invention. The government may exercise such rights over assignee&#39;s objection in accordance with 35 U.S.C. 202 and 203 if the government finds such action necessary in accord with 35 U.S.C. 203(a)(1-4). 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to igniters such as are used in rocket engines, jet engines and combustors in general; and to igniters utilizing a hot surface, heated by acoustically heated gas, as an ignition source in particular. 
     Generally the safest, most reliable and most widely used method of igniting a combustor which does not employ a pilot light is an electrical spark. This approach is generally reliable and safe, e.g., such as used in an internal combustion engine spark plug. Historically, however, igniters for rocket engines have often used a pyrotechnic igniter or hypergolic ignition to assure reliable engine ignition. Ignition is particularly a concern in liquid rocket engines where both the fuel and oxidizer are supplied as liquids to the chamber, because any momentary delay in ignition can result in the accumulation of an explosive mixture of fuel and oxidizer, resulting in a hard start which may damage or destroy the engine. Restartable rocket engines are often necessary where the engine is used to perform orbit circularization, orbital maneuvers, or orbital transfer. Multiple pyrotechnic igniters, one for each use of the engine, have been used. Reusable engines also require multiple starts, and, while replaceable pyrotechnic igniters are possible, they may leave residues which may add to the cost of reconditioning the engine for re-flight. Another approach to reliable ignition is to use propellants which are hypergolic (ignite on contact with each other) so that multiple restarts of the engine are not generally a problem. Hypergolic fuel combinations are widely used in rocket engines employed in missiles, rocket boosters, and/or maneuvering engines, in large part because they provide a simple and reliable ignition process. Non-hypergolic propellant combinations in rocket booster stages often use a limited quantity or slug of hypergolic propellant in one or both of the propellant lines or are separately injected into the combustion chamber to initiate combustion. In such a case multiple starts become complicated. Although engines utilizing hypergolic propellants readily perform multiple restarts and are widely used, using hypergolic propellant combinations limits propellant choice and can limit performance. Moreover, generally hypergolic propellants are themselves expensive and toxic, such that the cost of procurement and handling may be significantly increased as compared to non-hypergolic propellants. 
     Electric spark ignition has been used to overcome these problems particularly with the hydrogen and oxygen propellant combination such as on the Pratt &amp; Whitney RL 10 engine. Hydrogen and oxygen are clean burning, require low ignition energy, and have wide flammability limits. However, electrical ignition sources add complexity, require electrical power and a high-voltage electrical source, and are susceptible to electromagnetic damage such as caused by lightning strikes, and generally provide low ignition energy. 
     One possible ignition source which has been considered particularly for hydrogen and oxygen propellants is an acoustic igniter. An acoustic igniter employs a nozzle which directs an under-expanded sonic or supersonic gas jet into an essentially blind hole which forms an acoustic resonance tube. This arrangement, originally used as a high frequency noise source, was subsequently investigated as a simple way of obtaining a small quantity of very hot gas, or a hot surface which can be used as a source of ignition. 
     What is needed is a practical acoustic resonance igniter for H 2  and O 2 , particularly with relatively low pressure gasses. 
     SUMMARY OF THE INVENTION 
     The acoustic resonance igniter of this invention uses a high-pressure driver gas to heat a resonance cavity to a high temperature so the hot surface of the resonance cavity acts in the manner of a glow plug. An oxygen manifold supplies a primary combustion chamber of oxygen at a first regulated pressure, and hydrogen is supplied to the combustion chamber by a hydrogen manifold at a second regulated pressure which is lower than the oxygen manifold pressure. The primary combustion chamber exhausts through an exhaust orifice into a secondary combustion chamber, such that the pressure in the primary combustion chamber is governed by the pressure of the primary oxygen manifold, the size of the oxygen inlet port to the primary combustion chamber, and a manifold pressure of the primary hydrogen manifold and the size of the hydrogen inlet port to the primary combustion chamber, and finally by the size of the exhaust port orifice. 
     The primary hydrogen manifold and inlet port and the primary oxygen manifold and inlet port are arranged such that they have a high oxygen/fuel mixture ratio, for example 33, which is near the minimum energy for ignition of hydrogen and oxygen. The manifolds and the inlets are further arranged such that combustion in the primary combustion chamber drives the mixture ratio to a higher mixture ratio, for example 100, so that the combustion gases temperature reduce or eliminate the need for cooling of the primary combustion chamber. The highly oxygen rich combustion gases exit the primary combustion chamber through the exhaust orifice into the secondary combustion chamber where the temperature of the combustion gases is raised by the injection of additional hydrogen through the hydrogen manifold. The manifold forms an annulus around the secondary chamber and has inlet ports which are drilled at an angle, as shown in  FIG. 1 , so that the secondary hydrogen enters the combustion chamber through injection ports which are arranged to inject the hydrogen tangentially to the inside cylindrical wall of the secondary combustion chamber. 
     Operation in a typical sequence is by starting helium flowing through the sonic nozzle and into the resonance cavity, after which the helium is exhausted out through one or more exhaust ports. After some short interval of heating, the exterior of the resonance cavity will be in excess of 1500° K (1230° C., 2246° F.), well above the autoignition temperature for H 2  and O 2  gas. After the short heating interval, the main propellant valves are opened, first H 2  then O 2 . Under cold-flow conditions, the mixture ratio in the primary combustion chamber is approximately 33, which is near the mixture ratio (O/F) of minimum ignition energy. Once combustion occurs in the primary chamber, a pressure drop will occur across the primary chamber throat and the mixture ratio in the region adjacent to the resonance cavity will rise to approximately 100. This mixture ratio will result in a much cooler flame temperature in the primary combustion chamber, thus enhancing the hardware survivability. In the secondary combustion chamber, the secondary hydrogen will be injected to trim out the mixture ratio to an O/F of approximately 1.5. Additionally, this secondary fuel is injected in a swirling pattern to provide film cooling to the rest of the igniter and transfer tube. 
     It is a feature of the invention to provide for ignition of low-pressure hydrogen and oxygen with high-pressure helium in a glow plug type acoustic resonance igniter. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front elevational cross-sectional view of a test configuration of the ignitor of this invention. 
         FIG. 1 a    is a cross-sectional view of the apparatus of  FIG. 1  taken along line  1   a - 1   a.    
         FIG. 1 b    is a cross-sectional view of the apparatus of  FIG. 1  taken along line  1   b - 1   b.    
         FIG. 2  is a front elevational cross-sectional view of a flight weight configuration of the ignitor of this invention. 
         FIG. 2 a    is a cross-sectional view of the apparatus of  FIG. 2  taken along line  2   a - 2   a.    
         FIG. 2 b    is a cross-sectional view of the apparatus of  FIG. 2  taken along line  2   b - 2   b.    
         FIG. 3  is a cut-away isometric view of an alternative embodiment of the ignitor of this invention, where a linear nozzle and linear resonance cavity are employed. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring more particularly to  FIGS. 1-3 , wherein like numbers refer to similar parts, a glow plug type acoustic resonance igniter  20  is shown in  FIG. 1 . The igniter  20  of  FIG. 1  is a laboratory test article. A flight weight igniter  120  is shown in  FIG. 2 , and an alternative embodiment linear igniter  220  is shown in  FIG. 3 . 
     The resonance igniter  20  shown in  FIG. 1  employs a high-pressure helium source  31 , for example helium at 200-620 psia, which is accelerated through a sonic nozzle  21  into a lower pressure chamber  22  at, for example, 60 psia. A pintle  37  is mounted in the nozzle  21  to improve flow characteristics. A pressure test port  18  is shown in  FIG. 1  for measuring pressure in the lower pressure chamber  22 . The pressure in the lower pressure chamber  22  is maintained by outlets  23 . The outlets form choked flow nozzles, whose outflows depend only upon the helium temperature and pressure but not on the external pressure to which the helium outlets  23  exhaust. Opposite the sonic nozzle  21  is a resonance cavity  24  similar to that described in U.S. application Ser. No. 13/396,919, filed on Feb. 15, 2012, which is incorporated herein by reference. 
     As described therein the resonance cavity  24  is arranged so that a sonic resonance wave is set up in the cavity which results in heating of the helium gas, particularly in the lowermost cylindrical portion  27  of the cavity. The resonance cavity  24  is shown as formed in part of a structural component  26  which also defines the lower portion of the pressure chamber  22 . One possible material for forming the resonance cavity  24  is a molybdenum alloy such as TZM Molybdenum ASTM B386 type 364 (Alloy Plate, Sheet, Strip, and Foil) and B387 type 364 (Alloy Bar, Rod, and Wire) an alloy of 0.50% Titanium, 0.08% Zirconium and 0.02% Carbon with the balance Molybdenum. 
     After some short interval of heating, the exterior of the resonance cavity  24 , i.e., the hot surface  25 , will be in excess of 1500° K (1230° C., 2246° F.), well above the autoignition temperature for H 2  and O 2  gas. The inlet manifold for oxygen  28  and the inlet manifold for hydrogen  29  are arranged with manifold pressures and the inlet orifices  30 ,  32  together with the exhaust orifice, and flow channel  34  such that the primary combustion chamber  36  has a high oxidizer rich mixture ratio, for example 33:1, which is near optimal, i.e., minimum energy for ignition from the hot surface  25  of the surrounding lower cylindrical portion  27  of the resonance cavity. The arrangement of manifold pressures and orifice sizes are preferably arranged such that when the oxygen and hydrogen ignite, increasing the volume of the gases in the primary combustion chamber  36 , the mixture ratio becomes, with or without active control, even more oxygen-rich, for example 100:1, so as to minimize heating in the primary combustion chamber  36  and the exhaust orifice  34 . The exhaust orifice  34  does not generally operate with a choked flow (i.e., does not operate such that downstream conditions do affect the pressure in the primary combustion chamber). 
     To increase the flame temperature of the combustion gases from the primary combustion chamber  36 , secondary hydrogen from an inlet  35  is added to the combustion gases in a secondary combustion chamber  38  into which the exhaust orifice  34  empties. Sufficient hydrogen to substantially lower the mixture ratio, for example to produce a mixture ratio of 1.5:1, is used to increase the energy of the combustion gases to produce a suitable torch for igniting the combustion chamber of a larger engine. To raise the energy of the combustion gases without overheating the walls  40  of the secondary combustion chamber, the secondary hydrogen gas from an inlet  43  is introduced through an annular manifold  42  which surrounds the secondary combustion chamber walls  40 . The annular manifold  42  introduction is arranged so as to cool the secondary combustion chamber walls  40 . The cooling injection the secondary combustion chamber walls  40  is arranged through ports  44  which are drilled through the chamber walls  40 , best shown in  FIG. 1 b    between the annular manifold  42  and the secondary combustion chamber, so that rows of holes, i.e., injection ports  44 , enter the secondary combustion chamber at tangents to the inner cylindrical wall  40  of the combustion chamber and downwardly at a 45° angle with respect to an axis defined by the cylindrical chamber wall  40 . The secondary hydrogen enters along the wall  40  of the combustion chamber  38  so as to induce rotation of the injected hydrogen to produce a shield of hydrogen gas around the secondary combustion chamber wall  40 , and the transfer tube  41  to the larger oxygen-hydrogen engine (not shown). 
     The acoustic resonance igniters  20 ,  120  are configured to provide positive mixture ratio control during and after the ignition transient, either passively through the selection of the regulated gas pressures and orifices  30 ,  32 ,  130 ,  132  or actively by varying gas pressures and flows. 
     The “glow plug”-style resonance cavity device, i.e., the resonance cavity  24 , is located inside a primary combustion chamber  36  which is located upstream of a secondary combustion chamber  38 . All the oxidizer and a portion of the fuel is injected into the primary combustion chamber  36 . The primary propellants flow from the primary combustion chamber  36  through an unchoked orifice  34  into the secondary combustion chamber  38  where the balance of the fuel is injected. The propellant manifolds are kept at constant pressure through the use of pressure regulators in the propellant feed systems upstream of the igniter i.e., the hot surface  25 . The orifice between the primary  36  and secondary  38  combustion chambers serves to create a differential injection back pressure between the primary fuel injector and the secondary fuel injector. Prior to ignition, the mixture ratio (O/F) of the primary combustion chamber is approximately 30:1-40:1 (which is easily ignitable) and the mixture ratio of the secondary combustion chamber is approximately 1.0:1-1.2:1. After ignition, the pressure in both combustion chambers increases and the pressure drop through the inter-chamber orifice  34  increases as well. The post-ignition mixture ratio in the primary combustion chamber is approximately 100:1-120:1 (which results in a cooler flame temperature) and the mixture ratio in the secondary combustion chamber is approximately 1.4:1-1.6:1 resulting in a higher flame temperature. The igniter exhaust nozzle  41  exhausts to a near-vacuum prior to ignition. After ignition, the igniter exhausts into a rocket engine main combustion chamber that reaches pressures of up to 2000 psia. As the main combustion chamber pressure is elevated, the igniter manifold pressures are elevated accordingly, stopping the flow of gas to the primary  36  and secondary  38  combustion chambers. With ignition of the engine the flow of helium hydrogen and oxygen to the plug type acoustic resonance igniter  20  is shut down 
     The major sub-scale operating parameters of the acoustic resonance igniter illustrated and described with respect to  FIG. 1  are listed in the table below: 
     
       
         
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                   
               
               
                   
                 Pre-Ignition 
                 Post-Ignition 
                   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Oxygen Injector 
               
             
          
           
               
                 Oxygen Manifold Inlet Area 
                 0.0254 
                 0.0254 
                 in 2   
               
               
                 Oxygen Manifold Pressure 
                 40 
                 40 
                 psia 
               
               
                 Oxygen Injector Diameter 
                 0.076 
                 0.076 
                 in 
               
               
                 Oxygen Mass Flow Rate 
                 0.00460 
                 0.00320 
                 lb m /sec 
               
             
          
           
               
                 Hydrogen Injectors 
               
             
          
           
               
                 Hydrogen Manifold Pressure 
                 30 
                 30 
                 psia 
               
               
                 Primary H 2  Injector Diameter 
                 0.033 
                 0.033 
                 in 
               
               
                 Primary H 2  Injector Flow Rate 
                 0.00014 
                 0.00003 
                 lb m /sec 
               
               
                 Hydrogen Trim Injector Diameter 
                 0.041 
                 0.041 
                 in 
               
               
                 Hydrogen Trim Injector Num. 
                 16 
                 16 
                 — 
               
               
                 Hydrogen Trim Flow Rate 
                 0.00402 
                 0.00216 
                 lb m /sec 
               
             
          
           
               
                 Primary Chamber 
               
             
          
           
               
                 Primary Mixture Ratio 
                 33 
                 101 
                 — 
               
               
                 Primary Chamber Temperature 
                 300 
                 1620 
                 K 
               
               
                 Primary Chamber Pressure 
                 18.7 
                 30.0 
                 psia 
               
               
                 Primary Chamber Throat Diameter 
                 0.15 
                 0.15 
                 in 
               
               
                 Primary Chamber Throat Area 
                 0.0177 
                 0.0177 
                 in 2   
               
               
                 Primary Mass Flow Rate 
                 0.00474 
                 0.00323 
                 lb m /sec 
               
               
                 Primary Chamber Throat ΔP 
                 4.8 
                 4.7 
                 psid 
               
             
          
           
               
                 Secondary Chamber 
               
             
          
           
               
                 Global Mixture Ratio 
                 1.1 
                 1.5 
                 — 
               
               
                 Global C* 
                 N/A 
                 8213 
                 ft/sec 
               
               
                 Global T Ad   
                 300 
                 1668 
                 K 
               
               
                 Total Mass Flow Rate 
                 0.00876 
                 0.00540 
                 lb m /sec 
               
               
                 Secondary Chamber Pressure 
                 13.8 
                 26.3 
                 psia 
               
             
          
           
               
                 Nozzle 
               
             
          
           
               
                 Nozzle Throat Diameter 
                 0.258 
                 0.258 
                 in 
               
               
                 Nozzle Throat Area 
                 0.0523 
                 0.0523 
                 in 2   
               
               
                   
               
             
          
         
       
     
     A flight weight arrangement of the plug type acoustical resonance igniter  120 , is shown in  FIG. 2 . The primary differences over the igniter  20  is the use of braze joints for assembly to reduce weight, and the use of a single gas hydrogen inlet  129  which connects to the hydrogen manifold  142  which in turn is connected through a passageway  143  and an orifice  132 , which feeds the primary combustion chamber  136 . The gaseous oxygen source  128  at about 30 psi is supplied to the primary combustion chamber  136  through an inlet orifice  130 . The igniter  120  has cooling injection holes  144 , and combustion gases exhaust though nozzle  141 . 
     An additional feature is that helium exhaust outlets  123  are arranged to exit radially from a lower pressure chamber  122  through which helium exits, the outlets  123  form a plurality of choked flow exhaust outlets in the igniter  120 . 
     The flight weight arrangement of the plug type acoustical resonance igniter  120  incorporates, a helium inlet  131 , and hot surface  125  of a lower cylindrical part  127  of the resonance cavity  124 . The resonance igniter  120  also has a mounting flange  133  with three holes  126  for receiving mounting fasteners (not shown) which mount the igniter  120  to a combustion chamber (not shown). 
     Ignition of hydrogen and oxygen requires a certain amount of the heated gases to reach the autoignition temperature. In such a situation the temperature of the hot surface, the area of the hot surface, and the velocity of the gases passing by the hot surface will all affect the ability and speed at which the hot surface igniter ignites the gases. 
     Shown in  FIG. 3  is an acoustic resonance igniter  220  arranged to increase the size of the heated surface  225  by arranging a resonance cavity  224  which extends linearly a selected distance e.g., 0.080 to 1.000 inches, across a primary combustion chamber  236 . The linear extension forms resonance cavities  224  for example of 0.040″×0.500″ up to 0.080″×1.000″. The liner resonance cavity  224  is fed by a linear jet of helium. The linear jet of helium is formed by helium from the inlet  231  at a pressure of about 200-620 psia which feeds a plurality of inlets  219  on both sides of a linear pintle  237 , centered in a linear sonic nozzle  221 . The pintle is similar to the pintles  37 ,  137  but is linearly extended as shown in  FIG. 3 . The helium exhaust outlets  223  are arranged to exit radially from a lower pressure chamber  222  through which helium exits. The outlets  223  form a plurality of choked flow exhaust outlets in the igniter  220 . The Helium from the manifold  131  as it passes through the linear sonic nozzle  221  forms a linearly extending sonic jet of helium. The linearly extending sonic jet of helium sets up resonance within the linear resonance cavity  224 , which is positioned along a plane defined by the linear extension of the linear sonic nozzle  221 . The resonance within the linear resonance cavity  224  heats the linear lower portion of the resonance cavity  227  to from a linearly extending hot ignition surface  225 . 
     The primary combustion chamber  236  contains the hot surface  225 , and, because of the larger hot surface, a larger quantity of hydrogen and oxygen gas can be fed to the primary chamber from gaseous oxygen inlet  228  and gaseous hydrogen inlet  229 . This may eliminate the necessity of the secondary chamber  236  to increase the temperature or adjust the mixture ratio of the gases, which may be used directly to ignite a larger rocket engine combustion chamber. Alternatively the secondary combustion chamber  236  such as shown in  FIGS. 1 and 2  can be used. 
     Although the acoustic resonance igniter has been described for use with gaseous hydrogen and oxygen, other propellants could be used, whether liquid or gas, including bipropellants and monopropellants including those described in U.S. application Ser. No. 13/396,919. 
     It should be understood that the resonance cavity  24 ,  124 ,  224  could be formed of a separate thin-walled structure. Such a thin-walled structure is formed of a high temperature thermally conductive material resistant to hot hydrogen, oxygen and hydroxyl vapor, such as the molybdenum alloy TZM Molybdenum ASTM B386 type 364. 
     It should be understood that the pintles  37 ,  137 ,  237  in the linear sonic nozzle  21 ,  121 ,  221  of the igniters  20 ,  120 ,  220  shown in  FIGS. 1-3  could be omitted so long as a sonic jet of helium or other suitable low molecular weight gas is formed. 
     It should be understood that a larger area of the hot surface, and the resulting greater contact time of the propellant gases e.g., hydrogen and oxygen, over the hot surface can result in faster ignition or can support the ignition of larger flows of propellant gases or both. 
     It should be understood that the acoustic resonance igniter of this invention is most effective with a lightweight molecular monatomic gas such as helium as the resonance gas. A lightweight diatomic gas such as hydrogen is also very effective. Heavier monatomic gases such as neon, argon, and krypton or heavier diatomic gases such as nitrogen and oxygen could also be effective sources of ignition depending on the temperature needed. In this regard, although the invention has been described as using high temperature to achieve reliable and fast ignition, use of a catalytic surface, such as platinum on the heated surface of the lowermost portion of the cavity, could be used in combination with a lower temperature. Use of catalyzers may not be preferred because of the problem of catalyst contamination and the associated quality control issues of determining that an active catalyst surface is present when the igniter is called upon to function. 
     It should be understood that a linear pintle in the sonic nozzle is not strictly necessary, although without a linear pintle the flow rates of helium may be greater or the effectiveness of the heating in the resonance chamber may be less. 
     It should be understood that the essentially blind resonance cavity may have an opening from which hot gas escapes without preventing the operation of the resonance cavity described above if the opening is sufficiently small. However, such an opening is not necessary in the embodiments shown in the figures. 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces all such modified forms thereof as come within the scope of the following claims.