Abstract:
An apparatus and method are provided for decomposition of a propellant. The propellant includes an ionic salt and an additional fuel. Means are provided for decomposing a major portion of the ionic salt. Means are provided for combusting the additional fuel and decomposition products of the ionic salt.

Description:
U.S. GOVERNMENT RIGHTS  
       [0001] The invention was made with U.S. Government support under contract NAS3-01008 awarded by the National Aeronautics and Space Administration (NASA). The U.S. Government has certain rights in the invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    (1) Field of the Invention  
           [0003]    The invention relates to propellant combustion. More particularly, the invention relates to combustion of monopropellants.  
           [0004]    (2) Description of the Related Art  
           [0005]    Use of monopropellants is known in rocket propulsion and auxiliary/emergency power generation. U.S. Pat. No. 5,485,722 identifies the catalytic decomposition/combustion of hydroxylammonium nitrate (HAN)-based monopropellants. In general most HAN-based propellants are mixtures of three primary components: HAN, water, and a fuel. For miscibility the fuel is typically ionic, partly ionic, or polar. The fuel may be an ionic salt like HAN. Triethylammonium nitrate (TEAN) and 2-hydroxyethyl-hydrazine nitrate (HEHN) are two of the most common organic types. Partly ionic fuels may include amino acids (e.g., glycine). Polar fuels may include alcohols (e.g., ethanol or methanol). Most of these propellant mixtures are roughly 60%-80% HAN. HAN concentrations may be reduced by addition of more water. This has been used to lower combustion temperatures to levels survivable by materials involved in traditional monopropellant thrusters. Some of the more exotic mixtures use small amounts of ammonium nitrate as a further oxidant and/or mix multiple fuels into the blend.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, one aspect of the invention involves an apparatus having a source of HAN-based propellant. A reactor dissociates a major portion of the HAN in the propellant. A combustor combusts products of the dissociation with an additional fuel in the propellant. In various implementations, the propellant may comprise a mixture of the HAN, the additional fuel including alcohol, and water. The source may include a tank containing the propellant and an automatically-controlled valve governing flow between the tank and the reactor. The reactor may include a circuitous heated passageway. The reactor may include a catalyst bed through which the propellant passes. There may be a porous barrier between the reactor and the combustor. There may be means for feeding back heat from the combustor to the reactor. A turbine may be driven by products of the combustion. A generator may be driven by the turbine. A hydraulic pump may be driven by the turbine. The apparatus may include a hull for supporting the apparatus within a body of water. The apparatus may include a propeller driven by the turbine to propel the apparatus through the body of water. The apparatus may include an explosive warhead.  
           [0007]    Another aspect of the invention involves a method for operating a combustion system using a HAN-based propellant. The propellant is introduced to a reactor. The propellant is decomposed in the reactor to dissociate at least a major portion of the HAN. An output of the reactor is directed to a combustor. The output is combusted in the combustor so as to combust dissociation products of the dissociation HAN with unreacted fuel in the propellant. In various implementations, the combusting may release at least 60% of an energy of the propellant. The decomposing may decompose a majority of the HAN in the propellant. The decomposing may include passing propellant through a porous catalyst. The directing may include passing reaction products through a porous barrier. The directing may include counterflow passing of reaction products relative to combustion products. The method may include feeding back heat from the combustor to the reactor in an amount effective to initiate the decomposing.  
           [0008]    Another aspect of the invention involves an apparatus including a source of propellant. The propellant includes at least 50%, by weight, of an ionic salt and an additional fuel. The apparatus includes means for decomposing a major portion of the ionic salt. The apparatus includes means for combusting the additional fuel and the decomposition products. In various implementations, the propellant may include a mixture of HAN, alcohol, and at least 5% water. The means for decomposing may include a porous catalyst bed. The additional fuel may include one or more organic ionic salts.  
           [0009]    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.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a partially schematic view of a thruster system.  
         [0011]    [0011]FIG. 2 is a partially schematic view of a first emergency power unit system.  
         [0012]    [0012]FIG. 3 is a partially schematic view of a second emergency power unit system.  
         [0013]    [0013]FIG. 4 is a partially schematic view of a torpedo.  
         [0014]    [0014]FIG. 5 is a partially schematic view of a propulsion system of the torpedo of FIG. 4.  
         [0015]    [0015]FIG. 6 is a partially schematic view of an alternate thruster system. 
     
    
       [0016]    Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0017]    [0017]FIG. 1 shows an exemplary system  20  configured for use as a rocket thruster (e.g., for a spacecraft). The system includes a pressure vessel or tank  22  containing a body  24  of monopropellant. A headspace  26  of the tank is pressurized with an inert gas (e.g., helium) such as via a pressurization tube  28  penetrating an upper end of the tank. A lower end of the tank is penetrated by an outlet conduit  30  having therein a valve  32  controlled by a spacecraft control system (not shown). An exemplary valve  32  is a solenoid valve having a body  34  with an outlet port sealable by a downstream head of a piston  36  whose position is controlled via a coil  38 . Downstream of the valve  32 , the conduit  30  extends to a headspace  40  of a decomposition chamber or fume reactor  42 . The exemplary decomposition chamber  42  is formed in an upstream portion of a vessel  44  separated from a downstream portion by a porous thermal barrier  46  (e.g., aluminum oxide or zirconium oxide). The downstream portion serves as a combustion chamber  48  with a convergent/divergent nozzle  50  having a throat  52  and an outlet  54 . Downstream of the headspace  40  the decomposition chamber  42  contains means for facilitating decomposition of the monopropellant. Exemplary means may include a catalyst (e.g., rhenium) bed  60  between the headspace  40  and barrier  46  and/or a heater  62  (e.g., electric heater or fluidic conductive heater). In an exemplary operation, the heater (if present) is engaged to preheat the decomposition chamber whereupon the valve  32  is open to admit monopropellant to the decomposition chamber. In a discrete charge mode of operation, the valve may then be closed. In a continuous mode of operation, the valve may be left open. The heat and/or catalytic reaction causes the HAN to decompose into warm gaseous products. These products along with fuel vapor or particles and miscellaneous components (e.g., water vapor) flow through the barrier  46  into the combustion chamber  48 . In alternative embodiments, the decomposition and combustion chambers may be remote of each other and the communication of the decomposition chamber output products may be via appropriate conduits and controlled via appropriate valves.  
         [0018]    Means may be provided for triggering combustion within the combustion chamber. Exemplary means include a high voltage coil  80  having respective terminals coupled to a ground conductor  82  and a high voltage conductor  84 . The high voltage conductor is, in turn, coupled to a spark electrode (e.g., anode)  86  having an operative distal end  88  in the combustion chamber. The coil may be coupled to the control system for operation in either discrete or continuous modes. The exemplary anode  86  is concentrically surrounded by an insulator  90  separating the anode from a cathode sleeve  92  which may be coupled to ground. The thermal conductive properties of the vessel  44  and barrier  46  may be selected to limit feedback of heat from the combustion chamber to the decomposition chamber. However, these properties may advantageously be selected to permit sufficient heat feedback to encourage the fumeoff reaction within the decomposition chamber while not permitting any (or at least substantial) combustion within the decomposition chamber. Alternative combustion triggering mechanisms to the spark igniter involve hot wire igniters and/or glow plugs.  
         [0019]    An exemplary monopropellant comprises HAN, an additional fuel (e.g., an alcohol such as methanol) and water. Various examples are identified above. A group of these monopropellants may generally be characterized as having a major portion (e.g., at least about 75% (percentages by weight unless oterwise indicated)) of one or more ionic salts. Water contents are in the vicinity of at least 4%, more narrowly, 5-20%. Other fuel, if present, may be in the vicinity of up to about 20%. Particularly interesting monopropellants are HAN-based (i.e., at least 50% HAN by weight or, more broadly, at least 50% of the non-water mass). The decomposition chamber&#39;s fumeoff reaction entails evaporation of the water and dissociation of the HAN into gaseous products (oxidizers and water vapor). The additional fuel will evaporate if it is appropriately volatile (e.g., if alcohol or a light amino acid) or disperse with the gaseous fume products as minute molten particles. The dissociation is via a partial exothermic reaction essentially of the HAN such as:  
         7 HAN→4N 2 O+N 2 +4HNO 3 +12H 2 O  
         [0020]    with minor amounts (e.g., ˜3% by weight) of NO &amp; NO 2 .  
         [0021]    The presence of alcohol and water serve to reduce the effective reaction temperature. The theoretical temperature of this reaction is ˜560° C. (above the auto-ignition temperature of the gaseous mixture) but, due to the heat absorption by water and fuel evaporation and the heat loss to the system, will be less than theoretical. An exemplary actual temperature will be in the vicinity of 500K (e.g., less than 800K and more narrowly, between 400K and 700K. The heat generated by the reaction largely vaporizes the alcohol and water without major reaction of these compounds. Initially, the decomposition chamber should be hot enough to initiate the reaction (e.g., about 120-130° C. in a vacuum but potentially less with a catalyst bed). Exemplary fumeoff pressures are in the vicinity of 100 to 200 psia, more narrowly, 150-200 psia. The decomposition chamber pressure is advantageously slightly higher than combustion chamber pressure which is application dependent. Successful decomposition operation will have complete fuming (no pooling) with small amounts of combustion (e.g., 5-15%) being acceptable. By way of example, a weight percent mixture of 75.4% HAN, 16.0% methanol, and 8.6% water may react at one atmosphere at an approximate temperature somewhat over 400K. Pure HAN reacted at that pressure would have an associated temperature over 800K.  
         [0022]    The combustion occurs with substantially higher peak temperatures and pressures. By way of example, combustion may produce temperatures well over 2000K and peak pressures well over 1000 psia. The combustion of the gaseous products releases the majority of the energy of the mixture (e.g., about 80% for the exemplary mixture, more broadly, 60-95%, and 70-85%). For efficiency of near complete combustion, advantageously there is no to minimal non-fuming, pooling, and/or exploding in the decomposition chamber. Such performance will be dependent upon mixing and flow characteristics that may be unique to each application. Advantageously, overall combustion is at least 95%. The combustion temperature will depend on the thermal management system of each thruster application and the propellant mixture. The combustion is via an exothermic reaction of the HAN decomposition products with the unreacted fuel, for which the two key reactions are between the HAN&#39;s nitrous oxide and nitric acid on on hand and the fuel on the other hand. For example, with methanol fuel:  
         3N 2 O+CH 3 OH→2H 2 O+CO 2 +3N 2    
         6HNO 3 +5CH 3 OH→13H 2 O+5CO 2 +3N 2    
         [0023]    In situations wherein the monopropellant includes both HAN and another salt such as TEAN or HEHN, this other salt advantageously serves the role of the alcohol by substantially reacting only in the combustor. Such fuel components would be expected to be carried as minute molten particles from the decomposition chamber to the combustion chamber in the flow of HAN decomposition products.  
         [0024]    The physical separation of HAN decomposition and fuel combustion may be used for one or more purposes. These may include providing high numbers of cycles and high reliability of ignition. Direct monopropellant ignition may suffer from reliability problems. The use of a catalyst may improve reliability. However, if the catalyst or other decomposition means is exposed to the extreme heat of combustion, it may be expended over a short number of cycles. Decomposition remote from combustion may provide the combustion chamber with input that may be reliably ignited while protecting the decomposition means and permitting their reuse over a large number of cycles. The number of cycles required will vary based upon the given application. For many applications, it would be advantageous to configure the thermal isolation of the decomposition chamber (in view of its physical parameters and the parameters of a particular required combustion (including cycle time)) so as to preserve the decomposition means for at least one hundred cycles. In some applications, the desired number of cycles may exceed ten thousand.  
         [0025]    [0025]FIG. 2 shows a system  110  configured for use as a hydraulic emergency power unit (EPU) for an aircraft. A decomposition/combustion subsystem  120  may be generally similar to the system  20  of FIG. 1 and is not discussed separately. The outlet of the combustion chamber of the subsystem  120  is coupled to the inlet  122  of a case  124  of a turbine subsystem  125 . The exhaust products discharged by the subsystem  120  are used to drive a turbine  126  having a shaft  128  mounted within the case for rotation about a shaft axis. The exhaust products pass through the case and are discharged from an outlet  130 . The shaft couples the turbine to an impeller  140  of a centrifugal pump  141 . The pump has a case  142  with an inlet  144  and outlet  146  for pumping hydraulic fluid for the EPU. Alternate EPUs may utilize an electric generator  150  (FIG. 3) in lieu of or addition to the pump.  
         [0026]    [0026]FIG. 4 shows a torpedo  200  having a hull  202  extending from a bow or nose  204  to a stem or tail  206 . At the stern, the torpedo has a propeller  208  and a number of guidance fins  210  providing control surfaces. A control system  212  controls the guidance fins and is coupled to an explosive warhead  214  within the hull. The control system is also coupled to a decomposition/combustion subsystem  220  (FIG. 5) which may be generally similar to the system  20 . The illustrated subsystem  220  does not similarly pressurize its tank  222 , instead relying on a pump  224  between the tank and the decomposition chamber  226 . As with the EPU embodiments, the exhaust from the combustion chamber  228  may be directed to a turbine system  230 . The shaft  232  of the turbine is, in turn, coupled to the propeller  208  to drive the propeller about the shaft axis to propel the torpedo through the water. The exhaust may be discharged into the water via a turbine outlet  234 .  
         [0027]    [0027]FIG. 6 shows an alternate thruster system to that of FIG. 1. The system  310  has a monopropellant inlet conduit  312  directing the monopropellant from a source (e.g., a tank as heretofore described) to a solenoid valve  314 . A length  316  of this conduit downstream of the valve extends to a combustor/thruster body  320 . In the exemplary embodiment, the thruster body has an upstream portion  322  surrounding a combustion chamber  324  and a high thermal mass downstream portion  326  surrounding a portion of the nozzle  327 . In the exemplary embodiment, it surrounds and defines a major portion of the converging volume  328  of the nozzle upstream of the throat  330 . The diverging volume  332  downstream of the throat may be defined by a separate element. The body downstream portion  326  bounds a circuitous aft-to-fore counterflow path  340  for the monopropellant. The exemplary path is shown bounded by a helical passageway having an inlet  342  from the conduit  316  and an outlet  344  at the downstream end of the combustion chamber. In the exemplary embodiment, the outlet  344  is at an outboard portion of the combustion chamber separated from a downstream inboard portion by an annular wall  350 . In the exemplary embodiment, monopropellant flowing along the circuitous passageway is decomposed at least in part by heat of exhaust products expelled through the volume  328  in a counterflow heat exchange. Additional catalyst may be provided within the passageway or upstream thereof. The decomposed output exiting the passageway outlet  344  passes forward through the annular space  352  between a combustion chamber outer wall  354  of the body upstream portion and inner wall  350 . Reaching a forward/upstream end of the combustion chamber, the output is ignited via an igniter  360  and discharged downstream through the interior  362  of the annular wall  350  and therefrom through the volume throat  330  and volume  332 . During this passage, as described above, thermal conduction through the surface bounding the volume  328  assists in the decomposition of further monopropellant. Advantageously, the thermal conductive properties of the body  326  are selected to provide an advantageous level of decomposition.  
         [0028]    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. For example, details of any particular use will influence details of appropriate implementations. Accordingly, other embodiments are within the scope of the following claims.