Abstract:
Monopropellant and pre-mixed bipropellant storage and supply systems for rocket engines and other work producing systems are subject to damage when detonation progresses upstream from a combustion chamber to and through supply lines. Interposing one or more micro porous or micro fluidic elements into the supply conduit can limit the flame front that accompanies such unintended detonation, but inevitably restrict the flow of the propellant to the combustion chamber. A tiered micro fluidic element where a bulk of the element has relatively large pores but forms a structurally robust supports a second, relatively thin region having appropriately small mean pore diameter provides an effective flashback barrier that can resist catastrophic failure during such detonations. Such elements can be used in isolation, or they can be incorporated into detonation wave arrestors or pressure wave-triggered cut-off valves or the like to decrease the incidence of unintended detonations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims benefit of priority to U.S. Provisional Patent Application No. 61/223,611, entitled “Propulsion Systems and Components Thereof” and filed on Jul. 7, 2009, which is specifically incorporated by reference herein for all that it discloses or teaches. Further, the present application is related to U.S. patent application Ser. No. 11/950,174, entitled “Spark Integrated Propellant Injector Head With Flashback Barrier”, filed on Dec. 4, 2007 and to U.S. patent application Ser. No. 12/633,770 entitled “Regeneratively Cooled Porous Media Jacket”, filed on Dec. 8, 2009. Further, the present application is related to: 
         [0000]    U.S. patent application Ser. No. 12/831,703, entitled “Detonation Wave Arrestor” (Attorney Docket No. 488-011-USP2),
 
U.S. patent application Ser. No. 12/831,767, entitled “Flashback Shut-off” (Attorney Docket No. 488-011-USP3), and
 
U.S. patent application Ser. No. 12/831,824, entitled “Aluminum Porous Media” (Attorney Docket No. 488-011-USP4),
 
all three of which are filed on Jul. 7, 2010, which are also specifically incorporated by reference herein for all they disclose or teach.
 
     
    
     BACKGROUND 
       [0002]    Disclosed are materials of variable density or tiered porosity micro-fluidic porous media structures of sintered metal or other materials, and methods of making same. While micro-fluidic materials may be used in filters, heat exchangers, catalyst beds, and lightweight structural materials, the disclosed tiered porosity materials and the corresponding processes for making these disclosed materials find particular use in components of rocket propulsion systems, such as injector heads, flashback arrestors and shut-off valves, and in similar components in other work producing systems where a detonation-susceptible fluid propellant or such energetic materials must be safely fed from a storage container to a chemical reaction chamber, a combustion chamber or the like where work is extracted from the resulting of heat of reaction. 
         [0003]    Generally speaking, work extracting cycles that can implement the flashback arrestor element may include without limitation gas turbine cycles (e.g., Brayton similar cycles) Otto cycles, diesel cycles, and constant pressure expansions of combusted products (e.g., similar to pneumatic machines). Accordingly, it should be understood that materials, devices, and methods described herein may have other applications in addition to rocket propulsion. 
         [0004]    A monopropellant is a single liquid that serves as both fuel and oxidizer. A monopropellant decomposes into a hot gas in the presence of an appropriate catalyst, upon introduction of a high-energy spark, or upon introduction of similar point source ignition mechanism. Monopropellants, for example, can be used in a liquid-propellant rocket engine. A common example of a monopropellant is hydrazine, often used in spacecraft-attitude control jets. Another example is HAN (hydroxylammonium nitrate). 
         [0005]    Another form of propellant is called a bipropellant, which consists of two substances usually stored separately: the fuel and the oxidizer. Anytime a combustion process is employed, pre-mixing of combustion components may be desirable. Examples of fuels which can benefit from pre-mixing prior to combustion include, without limitation, natural gas, gasoline, diesel, kerosene, ethane, ethylene, ethanol, methanol, methane, acetylene, and nitro methane. Examples of oxidizers that can be pre-mixed with said fuels include, without limitation, air, oxygen/inert gas mixes, oxygen, nitrous oxide, and hydrogen peroxide. Fuel components can be mixed with oxidizing components in many different ratios to make a pre-mixed bipropellant and thus obtain a desired combustion reaction. The flashback arrestor element described herein is specifically relevant to any situation where the combustion components are mixed prior to entering a combustion chamber. 
         [0006]    Chemically reacting monopropellants and pre-mixed bipropellants contain constituents that liberate chemical energy through thermal decomposition and/or combustion. The chemical energy release is initiated by a mechanism designed within the chemical reaction chamber (where the majority of chemical energy release occurs). Commonly, this initiation mechanism is incorporated in the vicinity of a chemical reaction chamber&#39;s injector head. 
         [0007]    Deflagration is a common form of combustion where the flame speed travels at velocities less than the speed of sound. Deflagration combustion is commonly associated with low pressures. However, contained or pressurized combustion may result in the more powerful detonation phenomenon. 
         [0008]    A detonation is a phenomenon characterized by supersonic flame front propagation. Usually associated with detonation waves are pressure/temperature spikes and shock waves. The physics and corresponding reaction phenomenon are sufficiently different from a deflagration to warrant separate designations and analysis. The aforementioned conditions can result in a transient phenomenon containing immense power that can be used for destructive or carefully controlled constructive purposes. 
         [0009]    An ignition source is any energy mechanism that causes a chemical combustion process to initiate. In combustion reactions, the reactants are at a higher energy state than the products following combustion. However, to release the energy stored within the chemical bonds of the reactants, a certain quantity of energy (activation energy) must first be provided. The sources of the initiation energy in a combustion process are referred to as ignition sources. Many ignition sources exist including, without limitation, electrical sparks, catalysts (substances which lower the activation energy by providing a surface which increases a reaction&#39;s chemical kinetics), heat sources, impact loads, compression, or any combination thereof. 
         [0010]    If an ignition source exists downstream of a detonable mixture/detonable single component, in particular monopropellants and premixed bipropellants, flames can propagate (also known as “flashback”) through a feed line and into a storage container causing catastrophic system failure An ignition source downstream of a detonable mixture can cause a detonation to propagate upstream. 
         [0011]    Rocket engines commonly operate with monopropellants that can have very high gas and/or liquid densities as compared to more conventional air/fuel mixtures or low-pressure fuel and oxidizer mixtures. Flashback at these much higher monopropellant energy densities is not readily controlled. As a result, high energy density monopropellants that have small quenching distances (e.g., fluid gap, pore, and/or effective fluid passageway diameters small enough such that flames cannot propagate through the passageway) have been traditionally avoided because of the flashback failure mechanism that is very difficult to control. 
       SUMMARY 
       [0012]    A tiered porosity flashback-suppressing element intended to advance safety in the use of highly combustible gases and liquids, particularly at high propellant densities (high gas pressure or liquid phase), in a tubing flow path or propellant conduit is described herein. Such a flashback arrestor may be used, for example, in spacecraft propulsion, energy generation, work producing cycles, and general combustion reactions employing monopropellants and pre-mixed bipropellants. Accordingly, disclosed herein are materials, methods and devices relating to various components of such propulsion and work producing systems including, without limitation, micro-fluidic porous media elements, injector heads, flashback arrestors and shut-off valves in the field of rocket propulsion or other applications wherein combustible materials may be subject to flashback. The materials are variable density micro-fluidic porous media elements of sintered metal or other materials, and methods of making same. The flashback arrestors comprise such porous media elements and other elements to provide a flashback arrestor or shut-off valve for use with high temperature and pressure propellants in feed lines. 
         [0013]    Accordingly, provided is a tiered porosity flashback suppressing element capable of permitting flow therethrough of propellant from a propellant supply while capable of resisting catastrophic failure incident to a detonation of propellant in a propellant conduit between the element and a combustion chamber or the like. Such an element has an overall shape and at least two overlapping regions, each region having a characteristic mean pore diameter, and the regions differing from one another in its characteristic pore diameter. The first of these overlapping regions has a characteristic pore diameter likely too large to suppress the passage of a flame front therethrough incident to the detonation of propellant, but robust enough to resist catastrophic failure during detonation. The second of these overlapping regions has a characteristic pore diameter small enough to suppress the passage of a flame front therethrough incident to the detonation of propellant, but not robust enough without the first region to resist catastrophic failure during detonation. 
         [0014]    This first region and the second region could each be formed of sintered particles, or of overlapping thin layers or foils of photoeched, electron discharge machined, or laser ablated, materials. 
         [0015]    Also provided is a process for making such a tiered porosity flashback suppressing element by forming a stable shape having pores of a first mean pore diameter, then using various techniques of treating at least one portion of said shape to form an overlapping region having a second mean pore diameter or size that differs from the mean pore diameter of the rest of the stable shape. Such treatments include controlled detonation of propellant charges, applying one or more layers of a second material having this second mean pore size or diameter. Such layer or layers can be applied using photoetched or laser ablated thin foils, or thin foils of a sintered material formed by electron discharge machining this material after bonding to the shape or before bonding to the shape. 
         [0016]    Other implementations are also described and recited herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a perspective schematic view of an orbital craft or spacecraft with several attitude or apogee thrusters using the presently disclosed flashback-arresting devices. 
           [0018]      FIG. 1   a  is an enlarged schematic cross section of an example monopropellant propulsion system in the orbital vehicle using flashback-arresting devices according to the presently disclosed technology. 
           [0019]      FIG. 2  illustrates an example flowchart for monopropellant or bipropellant systems using detonation-arresting devices for propulsion systems, working fluid production systems, and/or electricity generation systems. 
           [0020]      FIG. 3  illustrates an example embodiment of a detonation wave arrestor with a disk-shaped tiered porosity flashback-suppressing element. 
           [0021]      FIG. 4  illustrates an alternate implementation of a flashback arrestor with a conical-shaped tiered porosity flashback-suppressing element. 
           [0022]      FIG. 5  illustrates yet another alternate implementation of a flashback arrestor with a hemispherical-shaped tiered porosity flashback-suppressing element. 
           [0023]      FIG. 6  illustrates the disc-shaped tiered porosity microfluidic element in perspective shown in  FIG. 3 . 
           [0024]      FIG. 7  illustrates the cone-shaped tiered porosity microfluidic element in cross-section shown in  FIG. 4 . 
           [0025]      FIG. 8  illustrates the tiered porosity hemisphere-shaped microfluidic element in cross-section shown in  FIG. 4 . 
           [0026]      FIG. 9  demonstrates an exemplary combustible mixture quenching curve generated for a nitrous oxide blended fuel mixture. 
           [0027]      FIG. 10  demonstrates exemplary micro-fluidic porous media pressure drop vs. mass flow data for a 10-micron porous metal media element. 
           [0028]      FIG. 11  illustrates an example of a fabrication process for a tiered porosity micro-fluidic porous medium wherein the metal powders are mixed with a binding agent. 
           [0029]      FIG. 12  illustrates an exemplary process for laying up a very thin micro-fluidic porous media membrane that can subsequently be bonded onto other structures. 
           [0030]      FIG. 13  illustrates another exemplary process for manufacturing a thin micro-fluidic porous media element or membrane that involves EDM machining thin slices of micro-fluidic porous media from a larger block 
           [0031]      FIG. 14  illustrates another exemplary process for manufacturing a tiered porosity micro-fluidic porous medium that involves EDM removal (either wire or plunge EDM) of material from a micro-fluidic porous media prebonded onto a lower pressure drop porous media substrate. 
           [0032]      FIG. 15  illustrates another exemplary process for manufacturing a micro-fluidic porous media element or membrane that involves stacking and rotating foils with predrilled micro-fluidic passageways. 
           [0033]      FIG. 16  illustrates an exemplary process for increasing a micro-fluidic porous media element&#39;s rating on detonation wave strength by exposing the element to weak but progressively increasing strength detonation waves in the fabrication 
           [0034]      FIG. 17  illustrates a tiered porosity micro-fluidic porous medium that incorporates multiple small porosity micro-fluidic porous thin elements embedded in larger porous media structure to provide redundancy to flashback embedded in a single structure. 
           [0035]      FIG. 18  illustrates a laser process for ablating extremely uniform diameter pores through a sheet or foil. 
           [0036]      FIG. 19  illustrates multiple layers of laser ablated sheets or foils assembled to form a tiered porosity element. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some of these specific details. For example, while various features are ascribed to particular embodiments, it should be appreciated that the features described with respect to one embodiment may be incorporated with other embodiments as well. Similarly, however, no single feature or features of any described embodiment should be considered essential to the invention, as other embodiments of the invention may omit such features. 
         [0038]      FIG. 1  illustrates a cross-sectional view of an example monopropellant propulsion system  110  in an orbital vehicle  100  that may be using flashback-arresting devices  102 ,  104 ,  106  according to the presently disclosed technology, which includes a monopropellant tank  112 . An ignition interface  106  is located between the vehicle body  110  and a combustion chamber  114 , which feeds into an expansion nozzle  116 . In the illustration, the vehicle would be propelled from left to right. 
         [0039]    Propellant from the monopropellant tank  112  is fed to the combustion chamber  114  via monopropellant lines or tubing  118 . Flashback-arresting shut-off valve  102  may shut off the fuel in the event of the flashback. A flashback arrestor  104  diverts the energy caused by a flashback away from the lines  118  and tank  112 . Flashback-arresting ignition interface  106  may contain a micro-fluidic porous media structure of sintered metal or other heat resistance materials. Further, the shut-off valve  102  and/or the flashback arrestor  104  may also contain a micro-fluidic porous media structure. Note that while the flashback arresting devices  102 ,  104 ,  106  are disclosed in  FIG. 1  with respect to an orbital vehicle thruster such as an apogee “kicker” or orbital adjustment thruster, such devices may also be used in other propellant and/or power generation systems. 
         [0040]    Thus each of these devices include one or more micro-fluidic porous media structures, in particular one or more tiered porosity flashback suppressing elements made of materials and using methods as will be detailed below. It should be understood that while it is preferred to incorporate such tiered porosity elements into valve structures, blast deflector structures and the like, in some situations and with some monopropellants or pre-mixed bipropellants, it may be possible or even desirable to interpose such elements alone in the propellant flow path. 
         [0041]      FIG. 2  illustrates an example flowchart  200  for monopropellant or bipropellant systems using flashback-arresting devices for flashback protection in propulsion systems (e.g., thruster  220 ), working fluid production systems (e.g., gas generator  222 ), and/or electricity generation systems (e.g., power plant  224 ). In a first depicted implementation, monopropellant tank  226  is the fuel/oxidizer source for a power generation system  220 ,  222 , or  224 . Flashback valve  210 , flashback arrestor or diverter  236 , and/or regulator  232  contain flashback-arresting technology as presently disclosed. The flashback arresting technology prevents or stops detonation waves from propagating upstream and causing catastrophic system failure in monopropellant feed lines and/or monopropellant tank  226 . Further, the presently disclosed flashback arresting technology (e.g., flashback arrestor  236 ) may also divert energy of the detonation waves away from the feed lines and/or monopropellant tank  226 . 
         [0042]    In a second depicted implementation, bipropellant tanks (i.e., fuel tank  228  and oxidizer tank  230 ) are premixed before injection into the power generation system  220 ,  222 , or  224 . Example fuels for such systems include, without limitation, natural gas, gasoline, diesel, kerosene, ethane, ethylene, ethanol, methanol, methane, acetylene, and nitro methane. Example oxidizers for such systems include, without limitation, air, oxygen/inert gas mixtures, oxygen, nitrous oxide, and hydrogen peroxide. Fuel components can be mixed with oxidizing components in many different ratios to obtain a desired combustion reaction. 
         [0043]    Flashback valve  234 , flashback arrestor  236 , and/or the regulator  232  may contain flashback-arresting or suppressing technology as presently disclosed. The flashback arresting technology prevents or stops detonation waves from propagating upstream towards the tanks  226 ,  228 ,  230  and causing catastrophic system failure in feed lines downstream of where fuel is premixed with oxidizer. Further, the presently disclosed flashback arresting technology (e.g., flashback arrestor  236 ) may also include detonation wave arrestor/diverter to divert energy of the detonation waves away from the feed lines and/or fuel tank  228  and oxidizer tank  230 . 
         [0044]    Further,  FIG. 2  illustrates three alternative power generation systems (i.e., thruster  220 , gas generator  222 , or power plant  224 ), each with a corresponding injector head  238 . Other power generation systems are also contemplated herein. For example, various work-extracting cycles may implement the flashback arresting technology (e.g., gas turbine (Brayton) cycles, Otto cycles, diesel cycles, and constant pressure cycles). The injectors  238  may also be equipped with the aforementioned flashback arresting technology that prevents or stops detonation waves from propagating upstream of the injectors  238  and causing catastrophic system failure. The implementations shown in  FIG. 2  demonstrates the flashback arrestor  236  implemented to protect single ignition source, that is one flashback valve  210  and one flashback diverter  230  for protecting tank  226 , and one flashback valve  234  and one flashback diverter  236  to protect the tanks  228 ,  230 . However, in an alternate embodiment, a single flashback arrestor may be implemented to protect multiple sources of combustible mixture. Moreover, the flashback valves and diverters may be implemented at any point between an ignition source and a container of combustible mixture. 
         [0045]      FIG. 3  illustrates an example geometry and composition of an assembly of components of an embodiment of a flashback arrestor assembly  300 . The flashback arrestor assembly  300  may include a detonation wave deflector  302 , a cap  304 , a flame arrestor structure  306 , a burst membrane  308 , a bottom compression fitting  310 , and a top compression fitting  312 . A number of screws or other mechanisms may hold together the flashback arrestor assembly  300 . For example, in the illustrated embodiment, the cap  304  and the flame arrestor structure  306  have threads  320  for screws that hold together the detonation wave deflector  302  and the burst membrane  308  between the cap  304  and the flame arrestor structure  306 . The cap  304  has an opening  322  along its central axis and the flame arrestor structure  306  has an opening  324  along its central axis. In an alternate embodiment, each of the openings  322  and  324  may be located in a direction perpendicular to, or any other direction, the central axis of the cap  304  and the flame arrestor structure  306 . In the example implementation of the flashback arrestor  300 , the bottom compression fitting  310  connects with the flame arrestor structure  306  via the opening  324  and the top compression fitting  312  connects with the cap  304  via the opening  322 . 
         [0046]    Each of the bottom compression fitting  310  and the top compression fitting  312  provides a path for propellant fluids (gases, liquids, or a combination thereof) through cavities in their bodies. The bottom compression fitting  310  may be designed so that it may be connected to tubes, pipes or other mechanism designed for transporting such fluids towards the bottom compression fitting  310  from the tanks  226 ,  228 ,  230 . Similarly, the top compression fitting  312  may be designed so that it may be connected to tubes, pipes or other mechanism designed for transporting a fluid away from the bottom compression fitting  312  towards the injectors  238 . The flame arrestor structure  306  may be designed to incorporate a receptor  326  on one of its surface to hold a tiered porosity element  330 . Note that while in the embodiment illustrated in  FIG. 3 , the receptor  326  is shown to have a flat overall shape with a ribbed or waffled surface structure, which presents channels or pores for carrying propellant to the upstream side of the tiered porosity element. The receptor  326  also provides mechanical support for element  330  as will be detailed below. While the receptor and its elements are shown as a relatively flat disk in  FIG. 3 , as will be discussed below, receptor  326  may have various alternate geometrical structures. In such alternate embodiments, the porous tiered porosity element  330  may also have a geometrical structure that is not flat. The detailed designs of the various components of the flashback arrestor assembly  300  are illustrated in further detail below. 
         [0047]    The flashback arrestor assembly  300  is configured to be positioned in the path of fluid from a fluid reservoir such as the tanks  226 ,  228 ,  230  to the injectors  238 . Thus, the fluid from a tank may travel through a connecting pipe, tube, or other mechanism towards the bottom compression fitting  310 . The bottom compression fitting  310  is connected to the flame arrestor structure  306  in a manner so that the fluid from the bottom compression fitting  310  travels towards the receptor  326  containing the porous media element  330 . As discussed above, the porous media element  330  allows the fluid to pass through it, but is structured to resist a flame front from progressing through it, as will be detailed below. Moreover, the fluid may also travel in the direction of the surface of the receptor  326  and thus, perpendicular to the flow of the fluid through the porous media. In  FIG. 3 , a directional arrow  332  denotes the path of the fluid along the surface of the receptor  326 , whereas a directional arrow  334  denotes the path of fluid through the porous media  330 . 
         [0048]    The bottom surface of the detonation wave deflector  302  is designed so that it deflects the fluid travelling thorough the porous media element  330  towards the periphery of the detonation wave deflector  302 . Moreover, the side surface of the detonation wave deflector  302  is designed in a manner so that when the burst membrane  308  is fitted around the detonation wave deflector  302 , a number of flow paths are formed along the side surface of the detonation wave deflector  302 . The fluid coming from the porous media element  330  and the fluid traveling along the surface of the receptor  326  may travel through such flow paths formed between the detonation wave deflector  302  and the burst membrane  308  towards the cap  304 . Directional arrows  336  denote such path of fluid flow between the detonation wave deflector  302  and the burst membrane  308 . Preferably, the face or surface of the element  330  facing downstream of the normal propellant flow comprises a region having a characteristic pore diameter small enough to suppress the passage of a flame front through it to the rest of the porous element. This tiered porosity is obtained or created as will be detailed below. 
         [0049]    The outer surface of the detonation wave deflector  302  that is designed to be adjacent to the cap  304  may also be designed in a manner so as to form a number of flow paths  338  between the detonation wave deflector  302  and the cap  304 . The fluid traveling between the detonation wave deflector  302  and the burst membrane  308  along paths  336  may flow though the path  338  towards the central opening in the body of the cap  304 . Subsequently, the fluid may flow through the opening in the cap  304  towards the top compression fitting  312  and from there towards a pipe, tube, or other mechanism connecting the top compression fitting to the injector  238 . 
         [0050]    In an alternate embodiment, the tiered porosity flashback-suppressing element can be incorporated, either alone or in an arrestor assembly as described above, into a shut-off valve. For example, a shut-off valve may be placed adjacent to the receptor  326  and attached to the burst membrane  308  so that in the case of a flashback, the shutoff valve closes off the flow of fluid from the tank  226 ,  228 ,  230  to the injector  238 . As discussed below, such a shut-off valve may be attached to the burst membrane  308  in a manner to trigger a shut-off in case of a bursting of the burst membrane  308 . 
         [0051]    Such a shut off valve is preferably part of a propellant shut-off assembly for isolating a propellant source in the event of a flashback. The propellant shut-off assembly may include a burst membrane configured to fail in the presence of the flashback and the shut-off valve closing bias is attached to the burst membrane. The shut-off valve is held open by the burst membrane while the burst member is intact. 
         [0052]    In case of an incident causing flashback, the porous media element  330  operates as a thermal sponge that absorbs combustion energy at rates higher than the rate at which a detonation wave can release combustion energy. As a result, the porous media element  330  provides a detonation quenching. However, because in the normal operation, the porous media element  330  is also providing a path for combustible fluid, the porous media element  330 &#39;s effective microchannel diameter sizing and surface area are strategically chosen for each particular application based on combustible fluid mass flow rate requirements and allowable pressure drop. While the quenching distance of the porous media element  330  may be sufficient to arrest a primary detonation wave, the energy release from a line flashback can cause secondary ignitions through mechanical failures and/or heat transport through solid material. This conductive heat transport can produce hot spots in direct contact with un-combusted combustible fluid sufficient to ignite a propellant upstream of the flashback arrestor assembly  300 . 
         [0053]    However, the detonation wave deflector  302  together with the burst membrane  308  provides additional protection to the sources of combustible fluids from the potential harm caused by such additional detonation wave. Specifically, the detonation wave deflector  302 , together with the burst membrane  308 , allows the detonation products travelling from the opening in the top compression fitting  312  to be vented before they reach the porous media element  330  or at least in the immediate vicinity of the porous media element  330 . Moreover, the detonation wave deflector  302 , when hit by a combustion wave, disperses the shock wave away from the porous media element  330 . Specifically, the detonation wave deflector  302  directs the shock wave energy towards the burst membrane  308 . thus, the ability of the porous media element to resist catastrophic failure from such a propellant can be enhanced. 
         [0054]      FIG. 4  illustrates an alternate implementation of a flashback arrestor assembly  1000  and various components thereof. Specifically, the flashback arrestor assembly  1000  is shown to have a detonation wave deflector  1002  and a flame arrestor structure  1004 . The bottom surface of the detonation wave deflector  1002  is designed to have a cone shape. The cone shaped bottom surface  1008  may include a number of circular steps  1010  as well as a number of grooves  1012  expanding outwards from the center of the detonation wave deflector  1002 . Similarly, the flame arrestor structure  1004  may have a cone shaped protruding surface  1014  and a number of circular steps or channels on the cone shaped protruding surface  1014  around its central axis. A porous media element  1006  that is shaped in the form of a cone may be positioned between the cone shaped bottom surface of the detonation wave deflector  1002  and against the cone shaped protruding surface  1014  of the flame arrestor structure  1004 . This conical shape, while more difficult to form than the disk shaped element described above, can provide a more robust structure, all other things being equal. Also, its relatively greater surface area for a given diameter can help reduce propellant flow pressure drop through the element. 
         [0055]      FIG. 5  illustrates an alternate implementation of a flashback arrestor assembly  1100  and various components thereof. Specifically, the flashback arrestor assembly  1100  is shown to have a detonation wave deflector  1102  and a flame arrestor structure  1104 . The bottom surface of the detonation wave deflector  1102  is designed to have a hemispherical shape. The hemisphere shaped bottom surface  1108  may include a number of circular steps  1110  as well as a number of grooves  1112  expanding outwards from the center of the detonation wave deflector  1102 . Similarly, the flame arrestor structure  1104  may have a hemisphere shaped protruding surface  1114  and a number of circular steps or channels on the hemisphere shaped protruding surface  1114  around its central axis. A tiered porosity flashback suppressing element  1106  that is shaped in the form of a hemisphere may be positioned between the hemisphere shaped bottom surface of the detonation wave deflector  1102  and against the hemisphere shaped protruding surface  1114  of the flame arrestor structure  1104 . As in the case of the conical shaped element, the outer or convex surface region is or contains the smaller mean pore diameter media. 
         [0056]    These three representative shaped tiered porosity elements are shown in  FIGS. 6 ,  7  and  8  with their “downstream” regions in an upward facing orientation.  FIG. 6  shows element  330  in perspective. Each have perimeter edges ( 610 ,  710 ,  810 ) that extend across and bound the thickness dimension of the element, a downstream facing surface or region ( 620 ,  720 ,  820 ) of small diameter pore media intimately bonded or integrally formed to another, usually main region ( 630 ,  730 ,  830 ) of porous media having a characteristic porosity likely inadequate to prevent a flame front penetration. Preferably, the perimeter edges are intimately bonded, welded or otherwise sealed to the tubing, propellant conduit, or in when the elements are integrated into a flashback shutoff valve or blast deflector, these perimeter edges are firmly sealed or welded to the body portion of such devices so that only the region having the pore size small enough to dependably suppress or stop the flame front is presented to the downstream propellant. 
         [0057]      FIG. 9  demonstrates an exemplary combustible mixture quenching curve generated for a nitrous oxide blended fuel mixture. The detonation wave generated from a volume filled with a combustible fluid density can effectively be quenched by a micro-fluidic porous media element. The ability of a micro-fluidic porous media element to quench said wave is dependent on the lowest flow-resistant path through the structure. In general the smaller the effective flow passageway diameter (corresponds to pore diameter in a sintered particle element) and/or the more tortuous path, the higher the probability that a detonation wave will be quenched. Design of good micro-fluidic porous media elements will require very high product reliabilities for mitigating a detonation wave that could potentially be generated for a given propellant and propellant density. Micro-fluidic porous media should therefore be tested and rated for a given application. Because a detonation wave&#39;s interaction(s) with a complex micro-fluidic structure is an inherently complex process, this phenomenon can be more accurately evaluated experimentally rather than analytically. To explore this phenomenon experimentally, one can fill a test fixture with a combustible mixture and intentionally ignite the mixture within a contained volume. If the element is sealed such that the only path for fluid flow is through the porous element, the flame propagation phenomenon through said element can be explored. In this implementation, it is critical that volumes are entirely “sealed” from one another by a porous element. Live data monitoring during the ignition, or post inspection of the porous element can indicate if the flame has propagated through the porous element. When this process is repeated over a range of porous media elements and fluid densities of the same combustible mixture, curves can be fit to the data. These curves are useful in specification of a porous element for specific uses. As shown in graph  800  of  FIG. 9 , as the propellant density increases, the quenching distance and therefore corresponding pore size necessary to prevent flashback decreases (the “pass” points  802  are left of and below the curve). 
         [0058]    Equation 1 below covers gases and liquids, it uses the mass flux moving through the structure rather than the fluid velocity as there is no ambiguity in terms of what velocity you are speaking of when using “fluid velocity”. All combustion reactions (from which detonations could be derived) are most commonly based on mass or molar flow rates of constituents rather than fluid velocities. 
         [0000]    
       
         
           
             
               
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             = 
             
               
                 
                   - 
                   
                     C 
                     ρ 
                   
                 
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                     m 
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                   ″2 
                 
               
               - 
               
                 
                   μ 
                   
                     ρ 
                      
                     
                         
                     
                      
                     K 
                   
                 
                  
                 
                   
                     m 
                     . 
                   
                   ″ 
                 
               
             
           
         
       
     
         [0000]    Wherein dP/ds—Pressure change along a fluid streamline moving through the element, K—micro-fluid porous media permeability coefficient, C—micro-fluidic porous media Form coefficient, μ—Fluid dynamic viscosity, ρ—fluid density, {dot over (m)}″—mass flux of fluid moving through micro-fluidic porous media element. 
         [0059]    The special case of further derivation of Equation 2 below, for ideal gases flowing through a structure of thickness, L (this equation is consistent with our  FIG. 2  data): 
         [0000]    
       
         
           
             
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         [0060]    P 1 —Pressure immediately upstream of micro-fluidic porous media element, P 2 —Pressure immediately downstream of micro-fluidic porous media element, L—micro-fluidic porous media element thickness, K—micro-fluid porous media permeability coefficient, C—micro-fluidic porous media Form coefficient, R—gas constant, T—gas temperature in micro-fluidic porous media element, μ—Fluid dynamic viscosity, ρ—fluid density, {dot over (m)}″—mass flux of fluid moving through micro-fluidic porous media element. 
         [0061]      FIG. 10  shows graph  900  of exemplary micro-fluidic porous media (gas) flow data for a 10-micron porous metal media element. In addition to the quenching characteristics of a porous element, the flow characteristics must meet the requirements for the intended application. This data can be extracted from a conventional micro-fluidic porous media element experimentally. The data can then be post processed to accurately size the surface area of the micro-fluidic porous media element for the intended application&#39;s mass flow rates. In general, the smaller the pore size in a porous media, the larger the pressure drop through the medium by changes in the porous media&#39;s flow coefficients (typically larger C and smaller K values shown in Eq. 1 and Eq. 2). 
         [0062]    For ideal detonation wave quenching, the micro-fluidic porous media must consist of sufficiently small fluid channel diameters and/or tortuous paths to quench effectively back propagation of a flame front. At the same time, the micro-fluidic porous media must be made sufficiently thin to avoid excessive pressure drop during normal operation (i.e., it permits the flow of propellant into the combustion chamber). To quench a flame, typical flame propagation into a medium is on the order of 1&#39;s to 100&#39;s of quenching diameters into the medium. For example, for a combustible fluid that requires 10 micron pores to quench a flame, the thickness of the membrane necessary to quench the flame may be as small as ˜100 microns. However, the combustion process may generate combustion pressures that drive the preferred membrane thickness to be significantly greater in order to provide mechanical strength during a combustion event. If the micro-fluidic porous media is nominally designed for both small quenching distances and very large thicknesses to accommodate the combustion pressures, very large fluid pressure drops may ensue when flowing a combustible fluid through the micro-fluidic porous media structure, i.e., a thick membrane of small pore size will interfere with the normal flow of propellant into the combustion chamber. 
         [0063]    Accordingly, the disclosed process for obtaining and design for a flashback arrestor is one in which the pore diameters of the micro-fluidic porous media varies in the thickness direction of the micro-fluidic porous media. Near the front or downstream facing surface of the micro-fluidic porous media where the combustion event may be initiated (e.g., on the combustion chamber side of the flashback arrestor), the effective pore diameters in this region should be much smaller than in the region of the much thicker porous structure which lies below (e.g., on the upstream or propellant tank side of the flashback arrestor). This micro-fluidic porous media structure transfers mechanical loads from near the surface where the combustion event has occurred and ensures that the overall structure does not mechanically fail. Therefore, the process for creating variable density micro-fluidic porous media should meet the requirements for preventing flashback with much lower pressure drops than micro-fluidic porous media structures that have uniform pore structures throughout. 
         [0064]    The goal is to provide a porous media element that prevents flashback with minimum propellant flow pressure drop through the micro-fluidic porous media. This element may be composed of metal or other materials. In one embodiment, the membrane is composed of metal or other materials that are ductile, highly thermally conductive to dissipate heat, and can take many thermal cycles without cracking. The pores are approximately within a range of 10 nanometers to 100 microns in diameter. 
         [0065]    One method for providing such characteristics with a very thin micro-fluidic porous media membrane to minimize fluid pressure drop through the membrane utilizes the fabrication process disclosed below. 
         [0066]      FIG. 11  illustrates an example of a fabrication process for a variable density or tiered porosity micro-fluidic element. 
         [0067]    In one embodiment, in order to create very thin sintered metal membranes with reproducible thicknesses, a process is used in which a binding agent or other fluid medium is mixed with metal powders in a batch process. The binder (for example, a mixture of paraffin based waxes) has physical properties such that at slightly elevated temperatures, it will melt and become fluid, thus allowing conventional mixing with selected metal powders to create a homogenous blended composite ( FIG. 11 ). This blending process, schematically shown at  1000  exemplifies one embodiment in which the mixing of the binder (element  1002 ) with the powder (element  1004 ) was done by mechanical mixing (element  1006 ). At ambient (room) temperature, the binder/metal mixture will remain in a plastic (e.g., clay-like) state. At this stage, the plasticity of the mix provides amenability to placement on a molding surface or substrate to form the composite material to a prescribed thicknesses and shape. Very thin (˜0.020 inches thick) structures are reproducibly made using this method. The part is subsequently heated at a temperature lower than that of the metal-sintering temperature, initially to melt out binding agent leaving a porous part of the prescribed thickness and shape. The part may then be subjected to higher temperatures to become sinter-bonded. A pressing mechanism may also be deployed at the time of sintering that will apply a force to the part for consolidation and strengthening. 
         [0068]    Another or additional process step may involve “oxidation reduction.” Micro-fluidic porous media structures made of metals such as aluminum may be treated by an additional chemical process in which the aluminum oxide patina is at least partially reduced back to aluminum metal. An oxide patina reduces the particle to particle bond strength, which will compromise the strength of the micro-fluidic porous media structure. Reducing agents may be used to reduce the aluminum oxide patina during the mixing and/or sintering processes. Liquid reductants (i.e. ammonia and ammonia based compounds, oxalic acid, formic acid, dilute nitric acid, sodium mercury amalgams, dilute hydrochloric acid containing amalgams), metal reductants (i.e. zinc, tin, magnesium), hydride reductants (i.e. LiAlH, NaBH, BiH 3 ) powders or suspensions of powdered reductants may be mixed in prior to sintering. A releasing agent such as alkyl stearates or stearic acid may be used in order to release the micro-fluidic porous media structure from the mold. 
         [0069]    Another variation involves the use of dissolvable pore space occupiers. Silicon or silicon dioxide beads may be mixed into the homogenous batch process shown in  FIG. 11  and subsequently dissolved with an etchant such a KOH, NaOH, HF, or Buffered Oxide Etch (BOE). Once the silicon or silicon dioxide beads have been dissolved, the micro-fluidic porous media structure and the bead cavities remain. Merging. 
         [0070]    To fabricate variable density porous injector head components, PSMP or (Plastic State Mould Process, may be used to create very thin elements that may be pre-sintered as thin membranes and subsequently merged with other pre-sintered elements in a process termed merging. One embodiment is shown in steps A-D of  FIG. 12 . Merging is a process whereby a pre-molded porous element made of materials with one pore size, is pressed onto another pre-molded element, made material of second (different, and often smaller, pore size). Merging may also include stacking more than two pre-molded elements into multiple layers of elements with varying densities. The stack of elements is subsequently heated and pressed to create variable porous layering within a single part, as shown as step D in  FIG. 12 . The sintering/pressing process may also include a method for evacuating or displacing the oxygen from the process at sintering temperatures to avoid oxidation decomposition of the part. The equivalent of pressing may also be done by heating the part under fixed constraints that don&#39;t expand as much as the part such that a large internal pressure is applied throughout the part. 
         [0071]    More particularly,  FIG. 12  illustrates one embodiment for the procedure for fabricating tiered porosity flashback suppressing elements. Step A shows a cross sectional view illustrating the operation of “striking off” a first layer of a PSMP produced composite within a mold  1104 . Step B shows a second mold  1108  being placed atop the first. Step C shows the striking off of the second layer of material onto a surface of the first layer. Step D shows how sintering heat and force are used to consolidate the layers into a single variable density element  1100  having tiered pore sizes. While  FIG. 12  shows a flat or disk shaped element, it should be understood that other shapes having tiered porosity may be made using this process. 
         [0072]      FIG. 13  illustrates one embodiment for the procedure for fabricating tiered porosity flashback suppressing elements using Electrical Discharge Machining In this process, an EDM wire  1200  is used to slice a preformed micro-fluidic porous media plug  1202  to produce a very thin slice  1204  (&lt;0.030 in) micro-fluidic porous media element. Similar to the merging process, this small pored structure can go through an oxidation reduction process as required (see paragraph [0062]) and a Merging process (see paragraph [0064]) to combine the thin sliced micro-fluidic porous media element to another mechanical structure that provides sufficient mechanical backing and fluid wetting on the upstream (propellant side) of the resulting thin element region. 
         [0073]      FIG. 14  illustrates one embodiment for the procedure for fabricating tiered porosity flashback suppressing elements using Electrical Discharge Machining on prebonded layers or regions. In this process, a micro-fluidic porous media element is diffusion bonded using, for example, an oxidation reduction process (as required) and Merging process. An EDM wire or plunge EDM  1310  is used to remove excess micro-fluidic porous media material until only a very thin region overlies and is prebonded to the low-pressure drop, larger diameter pore, porous media  1302 . 
         [0074]      FIG. 15  shows another embodiment method of fabricating a tortuous path micro-fluidic porous media comprises using integrated circuit processing methods to form silicon micro-fluidic porous media, silicon dioxide, silicon nitride or metal micro-fluidic porous media. This uses the standard masking process. A thin (e.g., ˜200 micron thick silicon, silicon dioxide (or in the case of silicon nitride or metal, a film, for example) is coated with a photoresist, covered by a mask, exposed to develop the mask, etched by either anisotropic or isotropic processes such as BOE, HF, KOH, RIE or similar process. The mask is removed, and the wafer or film is released as a thin layer. Several micro-fluidic porous media elements may be fabricated on a single wafer or film. For instance, 30-40½-inch micro-fluidic porous media elements may be made from a single wafer of 100 millimeters (4 inches). The micro-fluidic porous media elements may be masked again, and etched away from the wafer structure, leaving the separate micro-fluidic porous media elements. The individual slices are then bonded using a method such as anodic bonding, annealing or fusing, or similar method. During bonding, each slice is registered, a process by which one slice is placed atop another, aligned with fiducials placed during fabrication. The process of registering each slice is shown in  FIG. 5 . Drawing 500 shows how the use of rotation of one element slice with respect to another may define small, definable-size paths. The size of the paths, given appropriate fabrication, will vary continuously from fully open to submicron sizes depending on the degree of rotation of each micro-fluidic porous media slice. This alignment allows for different pore sizes without necessarily requiring a new mask and processing for each pore size desired. This process may also be used to accomplish a continuous, smoothly curved or a jagged path, depending on the desired tortuousity. 
         [0075]      FIG. 15  is a top down view of the embodiment  1400  of the method using integrated circuit processing methods to produce a micro-fluidic porous media comprising three stacked micro-fluidic porous media elements. The diagrams show the overlap of each layer with the registration of A, relative to B, relative to C. 
         [0076]    Mechanical alteration to an existing porous element can minimize flow alteration while increasing the element&#39;s flashback resistance. A number of post manufacturing processes may effectively achieve the same result of reduction in mean pore diameter in a layer or region of the element. These post manufacturing processes may include, without limitation, cold pressing, water hammering, ball peening, or detonation “burn in” of a first porosity medium to form a layer of having a second, preferably smaller mean pore size. In one embodiment, the method of detonation burn-in has produced desirable results. In this process, a detonable fluid is loaded within a fixture to a density below the predicted flashback failure point of the porous element and the fluid is intentionally detonated. If this process is repeated with progressively higher combustible fluid density, mechanical alteration particularly near the surface structure on the combustion side of a porous element can be achieved to effectively decrease the pore size of the membrane near this surface. In addition to mechanical alteration of the structure, this process can be used to validate a flashback arresting device&#39;s characteristics prior to use as a flashback arresting device. 
         [0077]      FIG. 16  demonstrates test data of a burn in alteration of a porous element. Both the unaltered element&#39;s flashback arresting points as well as the flashback failure points are shown. The exemplary burnt-in micro-fluidic porous media repeatably failed at densities over ˜30% higher than the unaltered porous elements, as shown by the distance between the two dotted vertical lines in graph  1600 . This test data indicates that burn-in alteration of a porous element can increase the flashback resistance of said element. 
         [0078]    Not only must the micro-fluidic porous media element be able to quench the detonation wave by dissipating heat in the micro-fluidic structure at a higher rate than it is being chemically released, but the structure must also be designed to tolerate the high transient combustion wave (e.g. detonation) pressures that ultimately will be incident on the micro-fluidic porous media element. This structural requirement can be met through a number of design means including, without limitation, working with geometries that minimize exposure of the micro-fluidic porous media elements to maximum strength combustion waves, and providing sufficient material of a given type to dissipate the energy of the combustion wave without causing material failure or alteration of the micro-fluidic structure. 
         [0079]    For example, it is possible simply to increase the thickness of a micro-fluidic porous media element in order to dissipate effectively the detonation wave shock energy without mechanically failing. However, this method would also increase the pressure drop through the micro porous element. As discussed above, any design must balance the needs of desired flow of propellants to the combustion chamber with the ability to provide flashback protection characteristics. However, if the issues of pressure drop and the scale of the element can be overcome, there are valid methods by which to increase the mechanical strength of the micro porous element. 
         [0080]    To achieve this end, mechanical backing reinforcement may be placed in strategic locations to increase the micro-fluidic porous media element&#39;s (and backing structure&#39;s) tolerance to shock energy without mechanically failing. In this configuration, a very thin membrane-like structure that quenches the combustion wave is bonded onto a stronger mechanical substrate that is also permeable and/or contains fluid passageways to the thinner micro-fluidic porous media element. This backing structure could be, for example, a higher permeability porous media element bonded to the much thinner micro-fluidic porous media element. Alternatively, the micro-fluidic porous media element&#39;s permeability may be designed to vary continuously. In another configuration, the very thin membrane and/or a variable density micro-fluidic porous media element can be placed on a solid backing structure that allows high load transfer without micro-fluidic porous media element failure and simultaneously contains fluid passageways to distribute fluid across the micro-fluidic porous media element. To achieve this end, mechanical reinforcement should be placed in strategic locations to minimize the internal stresses, yet maximize the porous element&#39;s fluid throughput. Such backing structures are shown as integrally formed in flame arrestor structure  306 , receptor  326  and the dome and cone shaped structures shown in  FIGS. 4 and 5  as described above. 
         [0081]    Another method by which to mitigate the mechanical failure is to utilize structural design of the micro-fluidic porous media elements that can handle much higher compressive pressure loads. Such structures may consist without limitation geometries such as a cone or hemisphere. The bulk of these shapes may have a porosity with relatively high propellant fluid flow, but likely inadequate to suppress flashback in that fluid, while the first or outer surface of such shapes have been made or treated to have the requisite mean porosity to dependably suppress such a flashback. The first or outer surface would be facing “downstream” i.e., away from the propellant storage vessel or vessels and towards the ignition source i.e., combustion chamber of a rocket engine, gas generator or power plant. 
         [0082]    Thus, these tiered porosity flashback suppressing elements can be thought of crudely as a thermal sponge that absorbs the combustion energy at rates higher than the detonation wave can release. The rate of energy absorption of a micro-fluidic porous media element increases with smaller flow passage effective diameter and to some extent the tortuosity and geometry of the fluid path. It should be noted that supersonic detonation wave quenching distances can typically be significantly smaller than the subsonic deflagration wave quenching distances given the dramatically different rates of thermal release associated with the speed of the wave. Many high energy density propellants have submicron to 100-micron detonation wave quenching distances. The disclosed elements, created by sintering pre-sorted metal media, can effectively create flow paths as small as 0.1 micron and can conceivably eventually be manufactured down into nanometer scales. The described flashback arrestor creates sufficiently small flow paths to quench high-pressure closed line detonations preventing ignition past said flashback arrestor. 
         [0083]    Preferably, then, the porous elements of whatever shape can be made of a precursor particles or sheets and should be of a material that is physically robust, has a high thermal conductivity and thermal diffusivity, and can be bonded to form a porous body having a controllable mean pore diameter. Such materials should also be chemically inert with regard to the propellant flowing therethrough. Alternatively, some reactive or catalytic but otherwise desirable precursor materials can be made inert by isolating the surfaces of the elements with an inert coating. Without limitation, inert coatings for a particular propellant (e.g. MgO, Al 2 O 3 , Yttria) may be applied to allow use of materials that may be catalytic with the propellant. 
         [0084]      FIG. 17  illustrates a tiered-porosity element  1700  incorporating multiple small porosity thin elements  1702 ,  1704  into larger porous media regions  1706 ,  1708  to provide redundancy to flashback suppression. 
         [0085]      FIG. 18  illustrates a method  1800  that leverages some advances in laser etching, but used here to make precisely ablated porous sheets or foils  1802 . These precisely processed foils  1902  are bonded together to form the region of small mean pore diameter of a tiered porosity flashback suppressing member  1902  shown in  FIG. 19 . Referring to  FIG. 18 , a laser source  1804  of appropriate power and wavelength produces a highly collimated beam  1806  that passes through a microlens array  1808 . This array is of known type in the microelectronics industry and produces a precise array of closely packed, precisely focused beams. The combination of laser source and lens array ablates a layer of material, preferably the metal foil  1802  to form a corresponding array of micron-sized pores by ablating the foil  1802 . These pores are of precise, repeatable size and shape, preferably having an hourglass or double cone shape, by taking advantage of the shape of the focused beams emitting from the microlens array. The laser ablation process is repeated many times to create ablated foils. As shown in  FIG. 19 , these foils are subsequently bonded together is a precise, repeatable manner to form a microporous medium  1900  which, when bonded to a robust porous layer, can provide a tiered porous member  1902  having the desired flow and flame arresting characteristics discussed above. 
         [0086]    The above specification, examples, and data provide a complete description of the structure and use of exemplary embodiments of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended. Furthermore, structural features of the different embodiments may be combined in yet another embodiment without departing from the recited claims.