Patent Grant
8932417
The following disclosure relates generally to the manufacture of propellants and, more particularly, to methods and systems for mixing solid propellant formulations.
Solid propellant is used in various types of rocket and missile motors, and is typically composed of a polymer, such as polybutadine, cellulose acetate, or polyvinylchloride, and a finely ground oxidizer, such as a perchlorate or nitrate salt. Various types of modifiers, such as plasticizers, burn-rate catalysts, or stabilizing agents, can be added to the propellant mixture to modify the structural or performance characteristics. In the manufacture of solid propellants, the polymer, which acts as the fuel, is typically mixed with the oxidizer(s) and modifier(s) in an uncured, fluid state. A curing or cross-linking agent is then added to the mixture before it is cast in the desired shape and cured.
Conventional planetary mixers and paint shaker-type equipment are often used to mix solid propellants. Planetary and double planetary mixers, such as those provided by Baker Perkins Inc., of 3223 Kraft Ave. S.E., Grand Rapids, Mich. 49512, and Charles Ross & Son Company, of 710 Old Willets Path, Hauppauge, N.Y. 11788, typically include two curved blades that extend into the propellant mix vessel and rotate about their own axes, while orbiting the mix vessel on a common axis. The blades continually advance around the periphery of the mix vessel during the mixing process, while the mix vessel remains stationary. Paint shaker-type mixers, such as those provided by Red Devil Equipment Co., of 14900 21st Ave. N., Plymouth, Minn. 55447, shake the mix vessel back and forth at a high frequency to fully mix the propellant ingredients together.
The ratio of polymer to oxidizer in solid propellant formulations is generally such that the resultant combustion products are low molecular weight gaseous products, such as carbon monoxide, nitrogen, hydrogen, water, and hydrogen chloride. To modify the linear burn rate of propellant, the oxidizer is typically ground to a specific particle size, such as about 100 to 300 microns. Finer particle distributions generally produce higher burn rates and, consequently, greater thrust. Using an oxidizer that is ground too fine, however, can result in propellant formulations that are very viscous and difficult to mix using the current state-of-the-art processes. As a result, conventional propellant formulations are generally constrained to linear burn rates ranging from about 0.1 inch per second to about 1.5 inches per second at chamber pressures of about 1,000 psi. Another shortcoming of conventional mixing processes is the amount of time it typically takes to achieve a relatively homogenous propellant mixture. Conventional planetary mixers, for example, typically require about 6 to 8 hours to provide a relatively homogenous mixture, while conventional paint shaker-type equipment can require from 1 to 2 hours.
Accordingly, it would be advantageous to have a method of producing relatively homogenous propellant mixtures in a relatively short period of time. In addition, it would also be advantageous to have a method of producing relatively homogenous propellant mixtures having oxidizer particles that are small or fine enough to provide linear burn rates higher than 1.5 inches per second.
The following disclosure describes various embodiments of methods and systems for mixing and manufacturing solid composite propellant formulations. In one embodiment, for example, a method of manufacturing propellant includes placing a first component (e.g., a polymer) and a second component (e.g., an oxidizer) in a mix vessel. In this embodiment, the oxidizer can have an average particle size that is substantially less than that typically found in conventional solid propellant formulations to achieve a higher burn rate. The method can further include mixing the first component with the second component by rotating the mix vessel about a first axis (e.g., a spin axis passing through the center of the mix vessel) while revolving the mix vessel about a second axis (e.g., a planetary axis) that is offset from the first axis. As described in greater detail below, mixing the first and second components together in this manner can cause high shear forces in the mixture that result in a relatively homogenous product that is substantially free of air bubbles.
Certain details are set forth in the following description and in
Many of the details, dimensions, and other features shown in the Figures are merely illustrative of particular embodiments of the invention. Accordingly, other embodiments can have other details, dimensions, and features without departing from the spirit or scope of the present invention. In addition, further embodiments of the invention can be practiced without several of the details described below.
In the Figures, identical reference numbers identify identical, or at least generally similar, elements. To facilitate the discussion of any particular element, the most significant digit or digits of any reference number refer to the Figure in which that element is first introduced. For example, element 210 is first introduced and discussed with reference to
Various aspects and embodiments of the invention described below can be implemented in accordance with computer-readable instructions, such as routines executed by a computer or data processor configured or constructed to perform one or more of the method steps described below. Various aspects of the invention can be stored or distributed on computer-readable media, including magnetic and optically readable computer disks, stored as firmware in chips (e.g., EEPROM chips), as well as distributed electronically over a computer network (including wireless networks). Those skilled in the relevant art will recognize that portions of the invention may reside on a special purpose computer, such as a propellant mixer computer, while corresponding portions may reside on a remote server computer. The computers and other processing devices used to implement various embodiments of the invention described below can include one or more central processing units or other logic-processing circuitry, memory, input devices (e.g., touch-pads, keyboards, etc.), output devices (e.g., display devices and printers), and storage devices (e.g., magnetic, fixed, and floppy disk drives, and optical disk drives), known to those of ordinary skill in the art. Such computers may be general-purpose devices that can be programmed to run various types of applications, or single-purpose devices optimized or limited to a particular function (e.g., a propellant mixing function) or class of functions.
Propellant formulations can also include a blend of oxidizer sizes. For example, in one embodiment, a propellant formulation manufactured in accordance with the method 100 can include a first portion of oxidizer having a first range of particle size (e.g., an “ultra fine” size), a second portion of oxidizer having a second range of particle size (e.g., a “medium” size), and a third portion of oxidizer having a third range of particle size (e.g., a “coarse” size). In one embodiment, the ultra fine particle size can range from about 0.3 microns to about 5 microns, the medium particle size can range from about 5 microns to about 30 microns, and the coarse particle size can range from about 150 microns to about 300 microns. In another embodiment, the total amount of oxidizer in a propellant mixture can account for about 85% of the mixture by weight, with the first portion of ultra fine oxidizer accounting for about 50-55% of the total, the second portion of medium oxidizer accounting for about 20-35% of the total, and the third portion of coarse oxidizer accounting for about 2-10% of the total. In other embodiments, other propellant formulations manufactured in accordance with the method 100 can include other proportions of oxidizer having other particle sizes. After reducing the size of at least a portion of the oxidizer particles, the method proceeds to block 106.
In block 106, the method places the desired portion of oxidizer in a mixing vessel. In block 108, the method places a second portion of a fuel (e.g., a polymer), or a second portion of fuel and a third portion of a modifier (e.g., a plasticizer, burn rate catalyst, stabilizer, etc.) in the mixing vessel with the oxidizer. In block 110, the method rotates the mixing vessel in a first direction about a first axis, while (at least partially simultaneously) revolving the mixing vessel in a second direction about a second axis. In this embodiment, the first axis can be a central axis, such as a spin axis, which is aligned with a centerline of the vessel, and the second axis can be a planetary axis that is offset from, and spaced apart from, the central axis. Various embodiments of the first and second axes and the associated vessel motions are described in greater detail below with reference to
In block 112, a curing agent can be added to the propellant mixture prior to casting. In block 114, the propellant mixture can be poured or otherwise cast in rocket motor casing, missile casing, or other suitable mold, and cured to harden the propellant in a desired shape. After block 114, the method ends.
In another aspect of this embodiment, a propellant mix vessel 210 is removably carried in a receptacle 220 on the rotor 230. The mix vessel 210 can include a body 213 and a removable lid 212. The capacity of the mix vessel 210 can range from about one quart to about 50 gallons, depending on the desired quantity of propellant, the capability of the mixing system 200, and other factors. A second driver 206b (or, optionally, the first driver 206a), can be operably coupled to the receptacle 220 and configured to rotate or spin the mix vessel 210 in a second direction 216 about a second or central axis 214 while the rotor 230 is revolving in the first direction 236 about the planetary axis 232. In addition to the foregoing components, the mixing system 200 can also include an adjustable counterweight system 234 positioned approximately opposite to the mix vessel 210. Prior to operation, the counterweight system 234 can be adjusted to reflect the corresponding weight of the mix vessel 210 and reduce vibration of the mix system 200 during operation.
In the illustrated embodiment, the rotation of the rotor 230 about the planetary axis 232, and/or the rotation of the mix vessel 210 about the central axis 232, can be controlled in accordance with time, speed, and/or other operational parameters received from a user (not shown) via a control panel 208. In other embodiments, all or a portion of the various mixing parameters can be preprogrammed on suitable computer-readable media and executed automatically, or at least partially automatically, by a computer or other processing device which is operably connected to the mixing system 200.
To mix propellant in accordance with the present invention, the user places a desired formulation (composed of, for example, a first portion of fuel and a second portion of oxidizer of suitable particle size to provide the desired burn rate) in the mix vessel 210 and secures the lid 212. The mix vessel 210 is then placed in the receptacle 220 and the top cover 204 is closed. The user can then start the mixing system 200 by pressing the appropriate button or buttons on the control panel 208.
In operation, the rotor 230 revolves about the first axis 236 in the first direction 236 (e.g., the clockwise direction). For at least a portion of the time the rotor 230 is revolving, the mix vessel 210 rotates about the central axis 214 in the second direction 216 (e.g., the counter-clockwise direction). In the illustrated embodiment, the second direction of rotation 216 is opposite to the first direction of rotation 236. In other embodiments, however, the first and second directions of rotation can be the same (e.g., both clockwise or both counter-clockwise). Furthermore, in the illustrated embodiment, the spin or central axis 214 of the mix vessel 210 intersects the planetary axis 232 at a point in space 250 that is spaced apart from the mix vessel 210. In other embodiments, however, the central axis 214 can be at least generally parallel to the planetary axis 232, as described in greater detail below with reference to
The relative rotational speeds of the mix vessel 210 and/or the rotor 230 can be varied to achieve desired mixing results depending on the particular composition of the propellant formulation and/or other factors. For example, when the oxidizer comprises about 70 to 90% of the mixture (e.g., about 85% of the mixture), the rotor 230 can revolve at about 500 to 1000 revolutions per minute (RPM) about the planetary axis 232, and the mix vessel 210 can spin at about 600 to 800 RPM in the opposite direction about the central axis 214. In other embodiments, other rotational speeds of the rotor 230 and/or the mix vessel 210 can be used to accommodate other propellant formulations, reduce mix time, or for other reasons. In addition, the rotational speeds can also be varied according to a preset program. For example, the rotational speed of one or both of the rotor 230 and/or the mix vessel 210 can be increased or decreased in a stepwise fashion; and rotation of one or both of the rotor 230 and/or the mix vessel 210 can be stopped or slowed down while rotation of the other of the rotor 230 or the mix vessel 210 is sped up.
There are commercially available paint mixing systems that are at least generally similar in structure and function to the propellant mixing system 200 described above with reference to
The arrangement of multiple mix vessels illustrated in
Modifiers, such as plasticizers and butadiene (e.g., hydroxy-terminated polybutadiene (HTPB)), from fuel storage 408 are weighed out in blocks 426 and 428, respectively, before being mixed together in block 430. The wet mixture is then combined with the dry oxidizer mixture in block 432. Various embodiments of the systems and methods described above with reference to
There are a number of advantages associated with the propellant mixing methods described above with reference to
The methods disclosed herein can also reduce propellant manufacturing cycle times by approximately 50-90% due to higher efficiency mixing. For example, conventional blade-type planetary mixing systems can require about 6-8 hours to achieve a homogenous mixture, and conventional paint shaker-type mixers can require about 1-2 hours to achieve this result. In contrast, the mixing methods described herein can produce homogenous propellant mixtures in cycle times as low as 10 minutes or less, such as about 6-8 minutes. The mixing methods described herein can also reduce or eliminate trapped gases or air in propellant mixtures which can lead to poor performance during propellant burn.
From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the various embodiments of the invention. Further, while various advantages associated with certain embodiments of the invention have been described above in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the invention. Accordingly, the invention is not limited, except as by the appended claims.
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