Patent Publication Number: US-9416917-B2

Title: Small-scale metal tanks for high pressure storage of fluids

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/542,629 filed Oct. 3, 2011 to the same inventor. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with government support under contract NNA08BB37C awarded by NASA and under contract HR0011-08-C-0101 awarded by DARPA. The government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to small storage tanks for fluids, and more particularly relates to small-scale tanks with high tank factors. 
     BACKGROUND 
     Storage of high pressure gases and liquids is a critical requirement for many applications, e.g. rocket and aircraft propulsion components, automotive airbags, pneumatic and hydraulic systems, etc. The science for design and manufacture of suitable tanks for this purpose is well documented, with many examples of commercially available tanks. Typical tanks are made in the form of spheres or cylinders, and may be manufactured from metals or composite (with or without a liner). 
     Pictures of representative commercially available tanks for high pressure storage of gases and liquids are shown in  FIG. 1  and  FIG. 2  as examples of commercially available tanks. While such tanks are relatively common in large sizes with diameters in excess of six inches, they less common in the extremely small size-class (i.e. diameters of the order of a few inches). The problem is especially difficult in extremely weight sensitive applications (i.e. rocket engines), and in applications where the pressure of the stored fluid is very high (several hundred pounds per square inch). 
     The realization of small high-pressure tanks has proved challenging for several reasons including that, given a limitation of minimum gage thickness for conventional materials, the mass of the walls ends up being much higher than what is required, thereby making the tanks much heavier than they need to be and it is difficult to form conventional materials into suitable cylindrical or spherical shapes at the small scale. An exemplary conventional metal tank is welded together from pieces bent sheet metal. For example, a first sheet is rolled into a cylinder, and two hemispherical ends are then formed in a press. The hemispherical ends are then welded onto the ends of the cylinder. The smallest gage aluminum which can be worked in such a process is 30 mil, and even that is very difficult and expensive. This is the practical gage limitation that prevents conventional methods from making thinner-walled aluminum tanks. Consequently, there are currently no commercially available high tank factor storage tanks in the 1-10 cubic inch size class. 
     A key figure-of-merit commonly used in this context is the “tank factor” which is defined as: “Failure Pressure” times “Storage Volume” divided by “Tank Weight” (the lower the tank weight for a given failure pressure and volume, the better the tank, and hence, higher the tank factor).  FIG. 3  depicts the tank factors for commonly available tanks as a function of storage volume, and clearly shows that while one can achieve high tank factors (nearing 30,000 meters for storage volumes in the 100-10,000 cubic inches range), the achievable tank factor decreases with size, there being no tanks with similar performance in the small size-scale (i.e. storage volumes of 1-10 cubic inches). 
     The tanks that do exist in the small size scale (less than 10 cubic inches) are either single-use disposable cylinders, for example, those used to inflate life-jackets, or “sample cylinders” used for capturing and transporting small samples of gas for analysis. These are limited to cylindrical shapes and have tank factors of less than 2500 meters. 
     Accordingly, it is desirable to manufacture a tank with a high tank factor (approximately 8,000 meters) in the 1-10 cubic inches volume range. In addition, it is desirable to devise a method of manufacturing such tanks that is effective and economical. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY 
     An apparatus is provided for storing fluids at high pressures in small volumes. The apparatus comprises one or more pressure vessels that are made up of multiple arrays of internal chambers with a single gas inlet and outlet for each vessel as well as gas feeder and connector lines. 
     A method is described for manufacturing small-volume tanks with high tank factors by aligning and stacking a plurality of patterned layers into a 3D shape, sandwiching the stacked layer between end wall structures, and diffusion bonding the multiple layers into a single monolithic tank with automatic fluid interconnects between internal chambers. The present invention uses a micro layer metal foil etching and diffusion bonding methodology to realize high-pressure tanks in the small size-class. 
     An exemplary embodiment of the invention is shown in  FIG. 4  in cross section form. Herein, the 2″×2″ square piece consists of two separate pressure vessels on the left and right that are made up of multiple honeycomb shaped internal chambers with a single gas inlet and outlet for each vessel as well as gas feeder and connector lines. The alignment pin referred to in  FIG. 4  is used to align the different layers and ensure a good diffusion bond between the layers for structural integrity of the internal chambers in the final structure. 
     The creation of such smaller chambers within the pressure vessels reduces the structural requirements on the outermost metal walls, thereby allowing for a light weight structure. 
     A key element of the present invention is the method used to manufacture the tanks. As discussed in regard to  FIGS. 20A-20D , the process involves: slicing a CAD model of the geometry into multiple layers; generating the necessary “pattern” artwork for each layer; using the pattern to etch each metal layer and create the pre-formed shapes; aligning and stacking of each of the layers into a 3D shape, and sandwiching between end wall structures; diffusion bonding the multiple layers into a single monolithic tank with automatic fluid interconnects between internal chambers; and external machining of the structure to release the final geometry and create access ports. 
     The invention provides A small scale metal tank for high pressure storage of fluids including: a tank factor of at least three thousand meters and a tank volume of at most ten cubic inches. The tank, including: an enclosure including a plurality of outer tank walls; an array of internal chambers within the enclosure; a plurality of fluidic interconnections between each of the internal chambers of the array of internal chambers and each other internal chamber of the array of internal chambers; and a fluidic conduit between an internal chamber of the a array of internal chambers and a point external to the enclosure. The tank, where the outer tank wall of the plurality of outer tank walls includes a flat outer tank wall. The tank, where the enclosure includes a shape that is adapted to and/or conformal to a particular mechanical application. The tank, where the array of internal chambers is formed of diffusion-bonded metal layers having diffusion-bonded seams between adjacent layers. The tank, where each chamber of the array of internal chambers has: opposed first and second end walls: a plurality of side walls extending between the opposed first and second end walls; an internal junction between a side wall of the plurality of side walls and one of the opposed first and second end walls; and a filet at the internal junction, where the filet includes no fusion-bonding seams. The tank, where either the opposed first and second end walls include a portion of an outer tank wall of the plurality of outer tank walls and the portion of the outer tank wall includes an arcuate shape that is internal and/or external. The tank, where a side wall of the plurality of side walls includes a portion of an outer tank wall of the plurality of outer tank walls and the portion of the outer tank wall includes an arcuate shape that is internal and/or external. The tank, where the at least one array of chambers includes two or more arrays of chambers, each forming an independent vessel within the enclosure and each having fluidically interconnected chambers within each of the two or more arrays of chambers and each vessel having a fluidic conduit external to the enclosure. 
     A small scale metal tank for high pressure storage of fluids having: a tank factor of at least three thousand meters; and a tank volume of at most ten cubic inches; where the tank includes: an enclosure including a plurality of outer tank walls; at least one array of internal chambers within the enclosure; an internal junction between a side wall of the plurality of side walls and one of the opposed first and second end walls; and a filet at the internal junction, where the filet includes no the fusion-bonding seams. The tank, where the outer tank wall of the plurality of outer tank walls includes a flat outer tank wall. The tank, where the enclosure includes: a shape adapted to fit adaptively and/or conformally with a particular mechanical device; and a shape that is not spherical. The tank, where the array of internal chambers is formed of diffusion-bonded metal layers having diffusion-bonded seams between adjacent diffusion-bonded layers. The tank, where each chamber of the array of internal chambers has: opposed first and second end walls: a plurality of side walls extending between the first and second end walls; an internal junction between a side wall of the plurality of side walls and one of the first and second end walls; and a filet at the internal junction, where the filet includes no the diffusion-bonding seams. The tank, where one of the first and second end walls includes a portion of an outer tank wall of the plurality of outer tank walls and the portion of the outer tank wall includes an arcuate shape that is internal and/or external. The tank, where one side wall of the plurality of side walls includes a portion of an outer tank wall of the plurality of outer tank walls and the portion of the outer tank wall includes an arcuate shape that is internal and/or external. The tank, where the at least one array of chambers includes two or more arrays of chambers, each forming an independent vessel within the enclosure and each having fluidically interconnected chambers within each of the two or more arrays of chambers and each vessel having a fluidic conduit terminating external to the enclosure. 
     A small scale metal tank for high pressure storage of fluids having: a tank factor of at least three thousand meters and a tank volume of at most ten cubic inches; where the tank includes: an enclosure including a plurality of outer tank walls; at least one array of internal chambers within the enclosure; an internal junction between a side wall of the plurality of side walls and one of the opposed first and second end walls; and a filet at the internal junction, where the filet includes no the fusion-bonding seams; where the enclosure includes: a shape adapted to fit adaptively and/or conformally with a particular mechanical device; a shape that is not spherical; and a shape that does not have a hemispherical tank end; where each chamber of the array of internal chambers includes: a plurality of diffusion-bonded metal layers having diffusion-bonded seams between adjacent the diffusion-bonded layers; opposed first and second end walls each including one diffusion-bonded layer of the plurality of the diffusion-bonded layers; a plurality of side walls each comprised of a stack of the fusion bonded layers and extending between the opposed first and second end walls; an internal junction between a side wall of the plurality of side walls and one of the opposed first and second end walls; and a filet at each the internal junction, where the filet includes no diffusion-bonding seams. The tank, where either the first end wall, the second end wall, and a side wall of the plurality of side walls of the chamber includes a portion of an outer tank wall of the plurality of outer tank walls and the portion of the outer tank wall includes an arcuate surface that is internal and/or external. The tank, further including the tank attached to the particular mechanical device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a front perspective view illustrating a prior art tank in the 100-10,000 cubic inches volume range; 
         FIG. 2  is a front perspective view illustrating a plurality of prior art tanks in the 100-10,000 cubic inches volume range; 
         FIG. 3  is a chart illustrating tank factor vs. storage volume for prior art tanks; 
         FIG. 4  is a perspective view illustrating an exemplary embodiment of the foil-layer stack and internal tank structure for a small volume, high tank factor, tank, according to an embodiment of the present invention; 
         FIG. 5  is a perspective view illustrating another exemplary embodiment of the internal tank structure with an end wall being added to a stack for a small volume, high tank factor, tank, according to an embodiment of the present invention; 
         FIG. 6  is a perspective view illustrating two additional embodiments of walled tank structures trimmed via electrical discharge machining (EDM) with respective trimmed external material for a small volume, high tank factor, tank, according to an embodiment of the present invention; 
         FIG. 7  is a perspective view illustrating another exemplary embodiment of the internal tank structure with end walls for a small volume, high tank factor, tank under hydrostatic testing, according to an embodiment of the present invention; 
         FIG. 8  is a perspective view illustrating another exemplary embodiment of the internal tank structure with end walls for a small volume, high tank factor, tank under hydrostatic testing, according to an embodiment of the present invention; 
         FIG. 9  is a top plan view diagrammatic view illustrating an exemplary arrangement of chambers into two exemplary vessels, according to the exemplary embodiment of  FIG. 4 ; 
         FIG. 10  is a cut-away perspective view illustrating an exemplary annular small volume, high tank factor, tank, according to another embodiment of the present invention; 
         FIG. 11  is a cross-sectional perspective view illustrating a first exemplary application of the exemplary annular tank, according to an embodiment of the present invention; 
         FIG. 12  is a perspective view illustrating a second exemplary rocket propulsion system using exemplary semi-annular tanks, according to an embodiment of the present invention; 
         FIG. 13  is a perspective cut-away view of a first alternate exemplary embodiment of arranging chambers into chambers, according to an embodiment of the present invention; 
         FIG. 14  is a perspective cut-away view of a second alternate exemplary embodiment of arranging chambers into chambers, according to an embodiment of the present invention; 
         FIG. 15  is a perspective cut-away view of a third alternate exemplary embodiment of arranging chambers into chambers, according to an embodiment of the present invention; 
         FIG. 16  is a composite of a perspective, cut-away, and diagrammatic views illustrating exemplary inner details of a first annular tank, according to the embodiment of  FIG. 10 ; 
         FIG. 17  is a composite of a perspective, cut-away, and diagrammatic views illustrating exemplary inner details of a second annular tank, according to an embodiment of the present invention; 
         FIG. 18  is a cross-sectional diagrammatic view illustrating exemplary domed end wall portions for the end walls of a tank, according to an embodiment of the present invention; 
         FIG. 19  is a chart illustrating the comparative performance of the present invention and prior art, according to embodiments of the present invention; 
         FIG. 20A  is a diagrammatic illustration of a first exemplary step in the process of making an exemplary device using stacked etched foil layers, according to an embodiment of the present invention; 
         FIG. 20B  is a diagrammatic illustration of a second exemplary step in the process of making an exemplary device using stacked etched foil layers, according to an embodiment of the present invention; 
         FIG. 20C  is a diagrammatic illustration of a third exemplary step in the process of making an exemplary device using stacked etched foil layers, according to an embodiment of the present invention; and 
         FIG. 20D  is a diagrammatic illustration of a fourth exemplary step in the process of making an exemplary device using stacked etched foil layers, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
       FIG. 1  is a perspective view illustrating a prior art tank  100  in the 100-10,000 cubic inches volume range. Tank  100  is a spherical tank. 
       FIG. 2  is a perspective view illustrating a plurality of prior art tanks  200  in the 100-10,000 cubic inches volume range. Two spherical tanks and two cylindrical tanks with hemispherical tank ends are shown. 
       FIG. 3  is a chart  300  illustrating tank factor vs. storage volume for prior art tanks. The prior art has no tank factors above 3,000 meters for tanks in the 1-10 cubic-inch volume range. The upper volume limit is actually slightly greater than ten cubic inches, as shown. More precisely, the chart  300  shows no tank factors above zero meters in tanks under ten cubic inch volume. Tanks that do exist in the small size scale (less than 10 cubic inches) are either single-use disposable cylinders, for example, those used to inflate life-jackets, or “sample cylinders” used for capturing and transporting small samples of gas for analysis. These are limited to cylindrical shapes and have tank factors of less than 2500 meters. 
       FIG. 4  is a perspective view illustrating an exemplary embodiment of the foil-layer stack structure  400  showing the internal tank structure  410  for a small volume, high tank factor, tank, according to an embodiment of the present invention. The internal tank structure  410  includes first vessel  406  and second vessel  408  made of an interconnected (see  FIG. 9 ) array of chambers  404  (one of 128 labeled) in a frame  402 . The chambers  404  are illustrated as hexagonal in cross-section, but the invention is not so limited. In various embodiments, various cross-sectional shapes may be used, as will be discussed and illustrated in greater detail below. The internal tank structure  410  is made by bonding foil layers together in a vertical stack  400 . The fingers in the illustration are not part of the invention, but give an approximate size reference. 
     An embodiment of the invention is shown in  FIG. 4  in cross section form. Herein, the 2″×2″ square piece consists of two separate pressure vessels  408  and  406  on the left and right, respectively, that are made up of a honeycomb of hexagonal-shaped internal chambers  404  with a single gas inlet  914  and  916  (see  FIG. 9 ) for each vessel  406  and  408  as well as gas feeder and connector lines  904  and  906 , respectively. 
     Alignment pins  508 , such as the one shown in  FIG. 5 , are inserted into alignment holes  412  and  414  to align the various layers and ensure a good diffusion bond between the layers for structural integrity of the internal chambers  404  and in the final light-weight structure  400 . 
     The creation of such smaller chambers  404  within the pressure vessels  406  and  408  reduces the structural requirements on the outermost metal frame  402 , thereby allowing for a light-weight structure  400 . 
     A key element of the present invention is the method used to manufacture the tanks. As discussed in greater detail in regard to  FIGS. 20A-20D , the process involves:
         1. Slicing a CAD model of the geometry into multiple layers;   2. Generating the necessary “pattern” artwork for each layer;   3. Using the pattern to etch each metal layer and create the pre-formed shapes;   4. Aligning and stacking of each of the layers into a 3D shape, and sandwiching between end wall structures;   5. Diffusion bonding the multiple layers into a single monolithic tank with automatic fluid interconnects between internal chambers; and   6. External machining of the structure to release the final geometry and create access ports.       

       FIG. 5  is a perspective view illustrating another exemplary embodiment of an internal tank structure  506  with an end wall  504  being added to a stack  510  for a small volume, high tank factor, tank  500 , according to an embodiment of the present invention. Edge chambers  502  (one of ten labeled) have arcuate internal surfaces  516  and a flat external surface  514 , as shown. In a preferred embodiment, the flat external surface  514  will be machined away, as illustrated in  FIG. 6 . In another embodiment, at least a portion of the flat surface  514  may be retained to assist in fitting tank  500  into another mechanical device or application. Alignment pin  508  is used to align the various layers, similar to layers  1004 ,  1616 ,  1510 , and  1620  (See  FIG. 16 ), and to ensure a good diffusion bond between the layers for structural integrity of the internal chambers  512  and in the final structure  500 . The fingers in the illustration are not part of the invention, but give an approximate size reference. The present invention realizes flat end walls  504  (also  602  and  606  in  FIG. 6 ) in the frame  506  (uncommon in pressure vessels) without sacrificing tank factor and performance. 
       FIG. 6  is a perspective view illustrating two additional embodiments of walled tanks  600  and  610  with chambers  602  and  606  (one of thirty-six labeled in each), respectively, trimmed via electrical discharge machining (EDM), with respective trimmed external material  604  and  608  for a small volume, high tank factor, tank, according to an embodiment of the present invention. The EDM trimming reduces the weight of the tanks  600  and  610  without sacrifice of required strength. Fluidic couplings  612  and  614  provide both an inlet for charging and discharging the tank  600  and  610 , respectively, through a single tube. 
       FIG. 7  is a perspective view illustrating an exemplary embodiment of the internal tank structure  700  with end walls  702  for a small volume, high tank factor, tank  600  under hydrostatic testing, according to an embodiment of the present invention. Hydrostatic testing verifies the ability of the tank  600  to withstand operational pressures. Bulging  704  of the individual chamber  404  end wall  702  portions can be seen. 
       FIG. 8  is a perspective view illustrating another exemplary embodiment of the internal tank structure  800  with end walls  802  for a small volume, high tank factor, tank  600  under hydrostatic testing, according to an embodiment of the present invention. Testing to failure defines the limits of the tanks&#39;  600  design capability. As shown, the end wall  802  has delaminated between some of the internal chambers  404 , but pressure loss has not occurred. 
       FIG. 9  is a top plan view diagrammatic view illustrating an exemplary arrangement of chambers  404  into two exemplary first and second vessels  406  and  408 , according to the exemplary tank embodiment  400  of  FIG. 4 . Fluid inlet lines  906  feed fluid to the chambers  404  (one of sixty-four labeled) of first vessel  406  from an inlet conduit  916  that extends outside of the tank  400 . Fluid inlet lines  904  feed fluid to the chambers  404  (one of sixty-four labeled) of second vessel  408  from an inlet conduit  914  that extends outside of the tank  400 . In a particular preferred embodiment, fluid inlet conduits  914  and  916  may also be used as outlet conduits in an application that first pressurizes the tank  400  with fluid through the inlet conduits  914  and  916  and then releases pressurized fluid out of the tank  400  through conduits  914  and  916 . Frame  402  includes alignment pin apertures  902  and  908 , as well as first and second mounting apertures  910  and  912 . In a preferred embodiment, each vessel  406  and  408  additionally has its own fluid outlet (not shown, but similar to inlets  914  and  916 ). The design enables realization of a complete tank  400  with automatic interconnects  918  (one of ten diagonals labeled) between internal chambers  404  to allow for fluid connectivity to each of the internal chamber  404  volumes. Interconnects  918  have a lesser depth than the depth of internal chamber  404 . 
       FIG. 10  is a cut-away perspective view illustrating an exemplary annular small volume, high tank factor, tank  1000 , according to another embodiment of the present invention. Each arcuate chamber  1002  (one of many labeled) is fluidically connected to each other arcuate chamber  1002  via fluid conduits (not shown, but see  FIG. 9  for example). The outer end wall  1004  seals the top layer of arcuate chambers  1002  in a three-dimensional array  1010  of arcuate chambers  1002 . Tank  1000  has first and second vessels (not visible in this view), as with the embodiment  400  of  FIG. 4 , and has first and second fluid inlets  1006  and  1008  for first and second vessels, respectively. The tank  1000  is formed in a disk-like flat shape that may adaptively and/or conformally shaped to be easily integrated with other devices by attachment or otherwise.  FIG. 11  and  FIG. 12  show applications in small satellite and rocket propulsion systems, respectively. The opening  1012  is shaped adaptively to a particular application and so may be conformal to a mechanical device to which it will be attached or may provide access for any pipes, regulators, valves, or other structures that may pass through opening  1012  in the particular application. 
       FIG. 11  is a cross-sectional perspective view illustrating a first exemplary rocket propulsion system  1100  for a small satellite using the exemplary annular tank  1000 , according to an embodiment of the present invention. An advantage of the inventive method is the ability to produce an external shape that can be conformal and/or adaptive with an application. The rocket propulsion system  1100  includes first and second fuel tanks  1102  and  1104 . In a particular embodiment, first and second fuel tanks  1102  and  1104  may each hold a propellant, such as monopropellant hydrazine. Annular tanks  1000  may hold a pressurant gas, such as nitrogen, to provide pressure to the hydrazine to move the hydrazine through regulator  1108  to one or more thrusters  1106  (one of four labeled). The radially exterior outer wall of tank  1000  is shaped conformally to a housing  1110  for the rocket propulsion system  1100  to make efficient use space and its inner opening  1012  is shaped adaptively to the space requirements of the regulator  1008 . In another exemplary embodiment, first fuel tank  1102  may hold a bi-propellant, such as monomethylhydrazine, and second fuel tank  1104  may hold an oxidizer, such as nitrogen tetroxide, each separately pressurized using pressurant gases from annular tanks  1000 . Those of skill in the art, enlightened by the present disclosure, will appreciate the many variations of rocket engine systems that may be advantageously created using small tanks  600  and  1000  with high tank factors, including the use of small tanks  1000  to hold propellant, including cold gas propellant. 
       FIG. 12  is a perspective view illustrating a second exemplary rocket propulsion system  1200  using exemplary semi-annular tanks  1214 ,  1215 ,  1216 , and  1217 , according to an embodiment of the present invention. Four semi-annular tanks  1214 ,  1215 ,  1216 , and  1217  equatorially surround spherical monopropellant tank  1202  and are supported by frame  1204 . Pressurant valve  1206  supplies pressurant gas over line  1210  to pressurant intake valve  1208  of monopropellant tank  1202 . The pressurant gas entering monopropellant tank  1202  through pressurant intake valve  1208  forces the monopropellant into thruster and valve assembly  1212  to provide thrust for the rocket propulsion system  1200 . In various additional embodiments, the mounting of the semi-annular tanks  1214 ,  1215 ,  1216 , and  1217  may be non-equatorial. The radially outer wall of tanks  1214 ,  1215 ,  1216 , and  1217  are shaped adaptively to the frame  1204  and the curvature of the inner walls is shaped conformally to spherical monopropellant tank  1202 . Rocket propulsion system  1200  is exemplary of the broad variation in possible shapes for tanks of the present invention. 
       FIG. 13  is a perspective cut-away view of a first additional exemplary embodiment of arranging exemplary tank chambers  1306 ,  1308 ,  1310 , and  1310  into vessel  1300 , according to an embodiment of the present invention. Vessel  1300  is preferably a corner portion of a larger vessel (not shown). Considerable variation in the shapes and wall thicknesses of tank chambers  1306 ,  1308 ,  1310 , and  1310  is within the scope of the present invention. The minimum wall thickness consistent with required tank strength is preferred and is found using a CAD system or structural analysis. In the illustrated embodiment, only wall  1302  has a thickness of 0.016 inches, while other walls, such as wall  1304 , have a thickness of 0.020 inches. Chamber  1306  is a tank interior chamber, chamber  1310  is a tank corner chamber, and chambers  1313  and  1308  are tank edge chambers. Tank corner chamber  1310  has an arcuate substitute  1314  for its two outer walls, having an arcuate surface both internally and externally. Edge chambers  1308  and  1312  each have one arcuate wall. The overall strategy is to provide square interior chambers  1306  and exterior chambers  1308 ,  1310 , and  1312  with arcuate outer walls. The apparatus reflects the method&#39;s ability to realize a very wide variety of internal and external shapes and geometrical flexibility in the plane (using CAD to convert the designs into artwork for etching of the metal layers, such as  1004 ,  1616 ,  1610 , and  1620  shown in  FIG. 16 ). 
       FIG. 14  is a perspective cut-away view of a second alternate exemplary embodiment of arranging exemplary tank chambers  1406 ,  1408 ,  1410 ,  1412 ,  1414 , and  1416  into a vessel  1400 , according to an embodiment of the present invention. Vessel  1400  is preferably a corner portion of a larger vessel (not shown). Internal tank chambers  1406  and  1408 , illustrated in a cut-away view, are hexagonal in cross section, as shown. Corner tank chamber  1412  has four of its six hexagonal sides merged into an arcuate wall  1418 , as shown. A first type of tank edge chamber  1410  and  1416  have two of their outer walls merged into an arcuate outer wall  1420  and  1424 , as shown. A second type of edge tank chamber  1414  has one arcuate outer wall  1422 , as shown. The minimum wall thickness consistent with required tank strength is preferred and is found using a CAD system. In the illustrated embodiment, only wall  1402  has a thickness of 0.008 inches, while other walls range in thickness up to a thickness of 0.022 inches, such as wall  1404 . The overall strategy is to provide hexagonal interior chambers  1406  and  1408  and hexagonal exterior chambers  1410 ,  1412 ,  1414 , and  1416  with arcuate outer walls  1420 ,  1418 ,  1422 , and  1424 , respectively. An advantage of the inventive method is the ability to produce an external shape that can be conformal with an application. Another advantage of the method used to make vessel  1400  is the ability to make external shapes that are not necessarily spherical or cylindrical, thereby allowing for more efficient usage of available space and the ability to make tanks that are conformal to the devices that use the tanks. 
       FIG. 15  is a perspective cut-away view of a third alternate exemplary embodiment of arranging square tank chambers  1506 ,  1508 ,  1510 , and  1512  into a vessel, according to an embodiment of the present invention. Vessel  1500  is preferably a corner portion of a larger vessel (not shown). Internal tank chamber  1506  has a square cross section. Corner chamber  1510  has an arcuate substitute  1512  for two of its walls, providing both an arcuate interior surface and an arcuate exterior surface. Edge chambers  1508  and  1514  each have a an arcuate outer wall  1518  and  1516 , respectively, as shown. The minimum wall thickness consistent with required tank strength is preferred and is found using a CAD system. In the illustrated embodiment, only wall  1502  has a thickness of 0.0075 inches, while other walls range in thickness up to a thickness of 0.020 inches, such as wall  1504 . The overall strategy is to provide square interior chambers  1506  and also to provide exterior chambers  1508 ,  1510 , and  1514  with arcuate outer walls  1518 ,  1512  and  1516 , as shown. 
       FIG. 16  is a composite of perspective, cut-away, and diagrammatic views illustrating exemplary inner details of a first annular tank  1000 , according to the embodiment of  FIG. 10 . Annular tank  1000  is shown in a cut-away perspective view and defines radial section AA′. Arcuate chambers, such as chamber  1002  (one of many labeled), are stacked radially and axially in a two-vessel configuration (not shown). The top foil layer  1004  seals the top layer  1602  of arcuate chambers  1002 . The radial cross sectional array  1601  illustrates top edge chamber  1604 , with a floor  1610 , a side wall  1608  and a top layer  1004 . With ten foil layers  1616  (one of ten labeled) per side  1608  of chamber  1604 , plus top layer  1004 , floor  1610 , and bottom  1620  layers, a stack  1612  of one hundred foil layers that are bonded together is shown. The top layer  1004 , bottom  1620 , and floor  1610  layers have filets  1622  (one of one thousand and eight in cross section  1601  labeled) to avoid a destructive concentration of forces at the corners. Filets  1622  are formed by etching a sixteen mil foil layer down to floor  1610  thickness and a twelve mil layer down to outside wall  1004  thickness, for example. Filets  1622  are used at all corners where chamber walls  1004 ,  1608 ,  1610 , and back and front chamber walls (not shown, but same as  1608 ) meet. The seams  1624  (one labeled) between the side  1608  and the floor  1610  or top layer  1004  or bottom  1620  are outside of the filet  1622 , so any stress at the chamber corners is engaged by solid material and not by a seam  1624 . Side walls  1608  are thinner than can be achieved by other production methods, due to minimum gauge limitations. 
       FIG. 17  is a composite of perspective, cut-away, and diagrammatic views illustrating exemplary inner details of a second annular tank  1700 , according to an embodiment of the present invention. Chamber walls  1708  of chambers  1704  (one of sixty-five labeled) each have twenty foil layers  1716  (one of twenty labeled). Top layer  1706 , floor layers  1710  (one of four labeled) and bottom layer  1720 , together with the wall layers  1716  forma stack  1712  of one hundred and six foil layers. Enlarged portion  1714  more clearly illustrates the use of filets  1722  (one of two hundred and sixty in cross section BB′ shown as array  1701 ) to resist stress concentrations at the corners. Seams  1724  are preferably outside the filet  1722 . Filets  1722  are formed by etching a sixteen mil foil layer down to floor thickness, for example. Filets  1722  are used at all corners where chamber walls  1706 ,  1708 ,  1710 , and back and front internal chamber walls (not shown, but same as  1708 ) meet. Actual chambers  1702  are shorter along their arcuate length than the chambers  1604  of the embodiment of  FIG. 16 , as shown. 
       FIG. 18  is a diagram illustrating exemplary domed end wall portions  1802  for the end wall  1800  of a tank, according to an embodiment of the present invention. Domes  1802  terminate chambers  1808  while flat portions  1804  of the end wall  1800  rest on inner chamber walls  1806 . The domed portions  1802 , which may be regarded as double filets, avoid stress concentrations at the seam  1810  between the end wall  1800  and the chamber walls  1806 . 
       FIG. 19  is a chart illustrating the comparative performance of the present invention and prior art, according to all embodiments of the present invention. The present invention creates tanks in a region  1900  bounded by the storage volume range of one-to-ten cubic inches that have tank factors in the neighborhood of eight-thousand meters, depending on the particular embodiment. None of the prior art (see also  FIG. 3 ), can match this performance. Accordingly, the present invention is novel. 
       FIG. 20A  is a diagrammatic illustration of a first exemplary step  2000  in the process of making an exemplary device using stacked  2008  (see  FIG. 20B ) etched foil layers  2009  (one of six labeled), according to an embodiment of the present invention. Each metal foil sheet  2007  is etched with patterns  2004  and  2005 , for example, and cut along demarcation lines  2006  into smaller sheets  2009 . The exemplary patterns  2004  and  2005  are determined by slicing a 3D CAD model of the device into slices having the same thickness as the metal foil sheet  2007 . The device illustrated in  FIGS. 20A-20D  is a small thruster  2010 , but the technique is broadly applicable to the small tank factor tanks of the present invention as well. 
       FIG. 20B  is a diagrammatic illustration of a second exemplary step  2001  in the process of making an exemplary device  2010  using stacked  2008  etched foil layers  2009 , according to an embodiment of the present invention. The layers  2009  are stacked  2008  in an aligned configuration, with approximately four hundred layers  2009  per device  2010 . Considerable complexity in the patterns, such as patterns  2004  and  2005 , is possible with the present method. The illustrated patterns are not intended to be limiting. 
       FIG. 20C  is a diagrammatic illustration of a third exemplary step  2002  in the process of making an exemplary device  2010  using stacked  2008  etched foil layers  2009 , according to an embodiment of the present invention. In the third exemplary step  2002 , the entire stack  2014 , of which stack  2008  is a part is subjected to pressure  2016  in a mechanical press  2012 , as well as heat sufficient to bond the metal foil layers  2008  together. The device  2010  has taken form within the entire stack  2014 . 
       FIG. 20D  is a diagrammatic illustration of a fourth exemplary step in the process of making an exemplary device using stacked  2008  etched foil layers  2009 , according to an embodiment of the present invention. Internal surfaces of the device  2010  may be machined smooth using a finishing tool  2018  intruded  2020  into the entire stack  2014  against the interior surfaces of the device  2010 . The exterior of the device  2010  may be trimmed by cutting and finished with grinders and polishers. External flanges, features, and couplings, if desired, may be formed in the trimming and finishing portion of step  2003 . 
     The present invention overcomes the limitation of low tank factors in the small size-class by realizing highly-efficient and light-weight tanks for high-pressure storage of liquids and gases in small storage volumes. As shown in  FIG. 19 , use of the present invention to realize such small-scale tanks allows for tank factors nearing 8,000 meters in storage volumes as low as 1-10 cubic inches. Other key unique features of the present invention include:
     1. Presence of internal walls, exemplified as walls  1608  and  1708 , to provide structural integrity and strength while reducing overall weight and external wall thickness;   2. Realization of a complete tank  600 ,  610 ,  1000 , or  1700  with automatic interconnects  918  between internal chambers  404 ,  1002 , or  1704  to allow for fluid connectivity to each of the internal chamber  404 ,  1002 , or  1704  volumes;   3. Ability to realize wall thicknesses, such as for walls  1304 ,  1420 ,  1518 ,  1614 , and  1714 , that are much smaller than those allowable by minimum gage limitations;   4. Ability to realize a very wide variety of internal shapes ( 1300 ,  1400 , and  1500 ) and geometrical flexibility in the plane (using CAD to convert the designs into artwork for etching of the metal layers  2009 );   5. Ability to realize an external shape  600 ,  610 ,  1000  that can be conformal with an application;   6. Ability to make external shapes  600 ,  610 ,  1000 , and  1700  that are not necessarily spherical or cylindrical, thereby allowing for more efficient usage of available space;   7. Ability to realize flat end walls  504 ,  602 , and  606  (uncommon in pressure vessels) without sacrificing tank factor and performance;   8. Placement of end wall fillets  1622  and  1722  in the small-scale tanks  1000  and  1700  to remove stress concentrations and improve performance;   9. Provision for annular  1000  and  1700  and other shapes so as to allow for plumbing channels and other structure  1108  through the tank  1000  (in the middle hole or elsewhere); and   10. Use of scalloped or domed end walls  1802  to further reduce the size and thickness of the external walls for a given level of pressure.   

     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description and following claims will provide those skilled in the art with a convenient road map for implementing the exemplary and additional embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention.