Patent Description:
Additive-manufactured fuel grains have been produced before, albeit for rocket motors using conventional metal casings and nozzles.

Hybrid rocket motors produce pressurized gasses from combustion of a solid fuel and a liquid oxidizer. Shortcomings of at least some prior hybrid motor configurations have included a lack of volumetric efficiency, and the need to provide hoses or other plumbing between a remote oxidizer tank, and a location where combustion occurs.

<CIT> discloses propulsion systems and a method for making a propulsion system including additively manufacturing a casing body into a single-piece structure having no bonded or bolted joints. The casing body defines a combustion chamber therein and is at least partially composed of a material useful as a solid rocket fuel and capable of being consumed during combustion.

A hybrid rocket motor has an oxidizer tank wholly or partially within a combustion chamber in which the fuel-oxidizer chemical reaction takes place.

According to an aspect of the invention, the present disclosure provides a hybrid rocket motor comprising: a solid fuel adjoining and at least partially defining a combustion chamber; and an oxidizer tank fully within the combustion chamber; wherein the solid fuel and a liquid oxidizer from the oxidizer tank are burnable in the combustion chamber to produce thrust in the motor; and wherein the oxidizer tank is supported by a series of radial ribs that extend outward from the oxidizer tank, through the combustion chamber, to a radially inner surface of the solid fuel at least partially defining the combustion chamber.

According to an embodiment of the device of any prior paragraph(s), the combustion chamber is cylindrical; and the oxidizer tank is located along a central longitudinal axis of the combustion chamber.

According to an embodiment of the device of any prior paragraph(s),the casing has injection ports for releasing the oxidizer from the oxidizer tank into the combustion chamber.

According to an embodiment of the device of any prior paragraph(s), the solid fuel includes a fuel plastic material.

According to an embodiment of the device of any prior paragraph(s), the solid fuel also includes an energetic additive to the fuel plastic material.

According to an embodiment of the device of any prior paragraph(s), the oxidizer tank includes an oxidizer tank plastic material.

According to an embodiment of the device of any prior paragraph(s), the oxidizer tank plastic material is the same as a fuel plastic material that is at least part of the solid fuel.

According to an embodiment of the device of any prior paragraph(s), the oxidizer tank plastic material different from a fuel plastic material that is at least part of the solid fuel.

According to an embodiment of the device of any prior paragraph(s), the series of radial ribs connect the tank to an outer casing of the motor that surrounds the combustion chamber.

According to an embodiment of the device of any prior paragraph(s), wherein the tank and the solid fuel are parts of a single unitary piece of material.

According to an embodiment of the device of any prior paragraph(s), the single unitary piece that includes the fuel and the tank are formed by additive manufacturing.

According to an embodiment of the device of any prior paragraph(s), the rocket motor is part of a flying vehicle.

According to another aspect of the invention, the present disclosure provides a method of making a hybrid rocket motor, the method comprising: using additive manufacturing to form a fuel element and an oxidizer tank of the rocket motor, with a combustion chamber within the fuel element, and with the oxidizer tank fully within the combustion chamber; and filling the oxidizer tank with a liquid oxidizer; and wherein the oxidizer tank is supported by a series of radial ribs that extend outward from the oxidizer tank, through the combustion chamber, to a radially inner surface of the solid fuel at least partially defining the combustion chamber.

According to an embodiment of the method of any prior paragraph(s), the using additive manufacturing includes extruding fuel material to produce the fuel element, and oxidizer tank material to produce the oxidizer tank.

According to an embodiment of the method of any prior paragraph(s), the fuel material and the oxidizer tank material are extruded by respective extruders.

According to an embodiment of the method of any prior paragraph(s), the using additive manufacturing includes forming a fill line; and.

According to an embodiment of the method of any prior paragraph(s), the method further includes, subsequent to the using additive manufacturing, filling the oxidizer tank with an oxidizer.

To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

A hybrid rocket motor includes a solid fuel element, and an oxidizer tank containing an oxidizer. The solid fuel element adjoins and at least partially defines a combustion chamber in which the solid fuel and the oxidizer are burned, to produce thrust from the hybrid rocket motor. The oxidizer tank is at least partially within the combustion chamber, and the entire oxidizer tank may be within the combustion chamber. The oxidizer tank may be protected by an insulating material, which may also serve as a structural material that contains the pressure of the oxidizer. The insulating material and the fuel material may both be polymer-based materials, although they may be different materials having different characteristics, for example including different additives to the same polymer material. The fuel element and the oxidizer tank may be made by additive manufacturing processes, for example by adding different materials in different locations as part of a single process.

Referring initially to <FIG>, a hybrid rocket motor <NUM> includes a solid fuel element <NUM> and an oxidizer tank <NUM> that contains an oxidizer <NUM>. The solid fuel element <NUM> may be the main structure of the rocket motor <NUM>, and may surround and define a combustion chamber <NUM> in which combustion between the fuel and the oxidizer occurs. The oxidizer tank <NUM> may be fully within combustion chamber <NUM>, for example being located along a longitudinal axis <NUM> of the rocket motor <NUM>, with a portion of the combustion chamber <NUM> radially outside of and fully surrounding the tank <NUM>.

The solid fuel element <NUM> may be all or a part of the supporting structure that contains the combustion chamber <NUM> and holds the tank <NUM> in place. Alternatively, there may be a single material that serves as both the fuel for the combustion, and as the structural support for the tank <NUM> (and to contain the combustion in the chamber <NUM>). There may be an outer casing <NUM> of the motor <NUM>, which may be made completely or in part of the fuel material of the fuel element <NUM>, that surrounds and contains the combustion chamber <NUM>. A series of radial ribs <NUM> may extend inward from the outer casing <NUM> to support the tank <NUM>.

The tank <NUM> may have a fill port <NUM>, for filling the tank <NUM> with the oxidizer <NUM>. In addition, the tank <NUM> may have a series of injection ports <NUM> that are used to release the oxidizer <NUM> into combustion chamber <NUM> once the combustion process is initiated by an igniter <NUM>. The fill port <NUM> may be sealed with a fill port plug <NUM> after filling of the tank <NUM> with the oxidizer <NUM>. The oxidizer injection ports <NUM> may be initially blocked with plugs <NUM> that are removed are part of the process of initiating combustion in the chamber <NUM>.

The tank <NUM> and its supporting structure (the outer casing <NUM> and the ribs <NUM>) may be made in an additive manufacturing process. "Additive manufacturing" is broadly used herein to refer to processes in which features are formed by selectively adding material, as opposed to removing material from an already-existing larger structure (subtractive manufacturing). Such a process is often referred to generally as three-dimensional printing. The additive manufacturing process allows the desired geometry of the rocket motor <NUM>, to be formed without additional manufacturing steps, such as machining. The fuel element <NUM>, the tank <NUM>, and/or the supporting structure for the tank <NUM>, may be built up layer-by-layer in the longitudinal direction of the rocket motor <NUM>. Many types of additive manufacturing processes may be used to produce the parts of the motor <NUM>. One example of a suitable process is fused deposition, where material is deposited at selected locations to build up the elements or parts layer by layer, with the deposited material fusing to previous layers of material. The fused deposition may involve movement of one or more extruders with respective heated heads, to deposit extruded material in desired locations. A variety of other additive manufacturing processes, such as selective laser sintering (SLS), stereo lithography (SLA), are possible as alternatives.

The parts of the rocket motor <NUM> maybe made of any of a variety of suitable materials. The material or materials of the parts may be selected so as to achieve desired characteristics of the motor <NUM>, as well as being usable for the manufacturing process. One suitable class of materials are polymer materials, with suitable additives for obtaining desired material characteristics. For example, one or more thermoplastics may be used, with suitable additives for use as a fuel or an insulating material. Additives may be used to increase fuel density (allows more mass to be packaged per unit volume), to increase burning rate during combustion (a higher burn rate means a higher mass flow, which means increased thrust), and/or to increase flame temperature (improves combustion efficiency and specific impulse which is thrust per unit mass of propellant). Metal hydrides could be added to the printed fuel to increase energy content. Alternatively, or in addition, ammonium perchlorate could be added to the printed plastic fuel to increase oxygen content. The additives may be in the form of particles of material, or may be in the form of droplets of liquid within a continuous material, such as a thermoplastic. As another alternative, the fuel may be a thermoplastic or other continuous material without any additives in it.

Other base or continuous materials are possible. Non-limiting examples include rubber, dense plastics, and metals or metal compounds, such as magnesium or copper oxide. The tank <NUM>, the fuel <NUM>, and the structure supporting these elements may have the same base (continuous) material. Different additives may be put into the same base material to result in different properties. Examples for such additives for the material of the fuel element <NUM> are described above.

A plastic material for the tank <NUM> may be sufficient to protect the oxidizer <NUM> from heat, since plastics are often poor conductors of heat. The material for the tank <NUM> may have additives or different materials for any (or a combination of) reasons, such as to provide better insulation against heat, to enhance structural strength, or to better resist ablation or other removal of the material during combustion in the chamber <NUM>. In the case of improved insulative properties, additives or materials that might be suitable for this would be carbon or graphite, silica, fiberglass, cork, and the like. In the case of improved strength or mechanical properties additives or materials that might be suitable for this would be aluminum alloys, titanium alloys, steel alloys, as well as carbon and glass fibers. In the case of improved resistance to ablation or erosion additives or materials that might be suitable for this would be carbon or graphite, silica, fiberglass, as well as refractory metals and alloys such as tungsten, molybdenum, nickel, Inconel, thoriated-tungsten, tungsten-rhenium, tungsten-carbide, and the like.

The oxidizer <NUM> may be any of a variety of suitable liquid oxidizer materials, for example oxygen or nitrous oxide, or a blend of liquid oxygen and nitrous oxide.

The additive manufacturing of the rocket motor <NUM> may be accomplished using one or more extruders to extrude a suitable material or materials, in suitable locations. Alternatively, material for the rocket motor <NUM> may be fused to build up the motor <NUM> layer by layer. Either way, the motor <NUM> may be built up in a configuration that would be difficult to form in a practical way by other methods of manufacture, such as molding or machining.

The manufacture of the motor <NUM> may involve placing preformed structures during the manufacturing process, with other parts of the motor <NUM> formed around the preformed structures. Examples of such preformed structures include fittings and nozzles, such as a nozzle <NUM> that is part of the motor <NUM>. Alternatively, the motor <NUM> may be nozzleless, or the nozzle <NUM> may be formed using an additive manufacturing method.

Characteristics of the tank <NUM>, such as its shape, material, and/or thickness, may be selected in order to maintain structural integrity of the tank <NUM> throughout the firing of the rocket motor <NUM>. Material of the tank <NUM> may be removed and/or weakened during burning of the fuel in the chamber <NUM>. However, at the same time the oxidizer <NUM> is being consumed by the combustion, with the oxidizer <NUM> initially at a high pressure (e.g., <NUM> psi), and then reducing in pressure. This reduces the amount of the oxidizer <NUM> remaining in the tank <NUM>, and therefore the pressure in the tank <NUM>. Thus the ability of the tank <NUM> to retain its structural integrity in storing the oxidizer <NUM> is being reduced at the same time that the internal pressure load on the tank <NUM> is being reduced as well.

The rocket motor <NUM> may only be fired for a strictly limited, for example <NUM> or <NUM> seconds (or more broadly <NUM> to <NUM> seconds), such that the effect of thermal forces on the tank <NUM>, and structural weakening of the tank <NUM>, may be limited. The time over which the rocket motor <NUM> fires may be limited by the amount of the oxidizer <NUM> initially in the tank <NUM>, for example. A limited-firing-time rocket motor <NUM> may be used as a steering thruster, for example, or may be used as a main thruster for a device with a short flight time, such as an air-to-air missile.

The motor <NUM> may be formed as described above, and then the tank <NUM> may be filled with the oxidizer <NUM> through the fill port <NUM>, with the fill port <NUM> then blocked with the plug <NUM>. Thus assembled, the motor <NUM> may then be stored, such as part of a device such as a spacecraft or missile, for a considerable time (e.g., months or years) before use.

In operation, firing of the rocket motor <NUM> would begin with activating the igniter <NUM>. The igniter <NUM> may be a combustible compound that is rapidly consumed and generates sufficient heat to initiate more general combustion of the fuel <NUM>. The ignition process also dislodges the fill injection port plugs <NUM>, to allow flow of the oxidizer <NUM> into the combustion chamber <NUM>. Flow of the oxidizer <NUM> into the combustion chamber <NUM> allows for continuation of combustion in the chamber <NUM>. Some of the fuel <NUM> is also burned away in this combustion. Parts of the tank <NUM> may also be burned as fuel and/or otherwise removed by the combustion. The combustion products (pressurized gasses produced by the combustion) exit the rocket motor <NUM> to provide thrust. The combustion products may pass through a nozzle, such as the nozzle <NUM>, that may enhance the thrust from the rocket motor <NUM>, such as by transitioning the flow of the pressurized gasses from subsonic flow to supersonic flow.

The rocket motor <NUM> may be used in any of a variety of devices, for any of a variety of purposes. The rocket motor <NUM> may be a part of a spacecraft or aircraft, to provide main thrust or to provide steering or for use as a divert thruster, to give some examples. The rocket motor <NUM> may be integrated into any of a variety of structural parts of a spacecraft or aircraft, for example being part of a fuselage, wing, or fin.

As one example of a suitable use, small versions of the hybrid rocket motor <NUM>, for example having about the size of small consumer-use household batteries (e.g., AA batteries) could be produced and stored as separate structures, ready for use. Such small thrusters could be used for maneuvering, by placing them in small holes in a fuselage or other flying vehicle structure, with the small hybrid rocket motors held in place by mechanisms such as an adhesive or mechanical mechanisms, such as snap rings.

The rocket motor <NUM> offers numerous advantages. The rocket motor <NUM> provides for the possibility of a more efficient use of space than in prior hybrid rocket motors in which the oxidizer tank or containment is located away from the combustion chamber. The rocket motor <NUM> reduces the inert mass fraction of the system and allows a more compact package, with less system volume than in other hybrid rocket motors capable of providing similar amounts of thrust. For example, the inert mass faction may be reduced <NUM>-<NUM>% (or a <NUM>-<NUM>% increase in propellant mass) compared with prior hybrid rocket motors.

<FIG> illustrates one process for making parts of the rocket motor <NUM>, in which two or more extruders <NUM> are used to deposit appropriate material, layer by layer, at appropriate locations, to build up elements of the rocket motor <NUM>. The additive manufacturing extrusion process may be used to produce the fuel element <NUM>, the oxidizer tank <NUM>, and the supporting structure that supports the tank <NUM>.

Other additive manufacturing or three-dimensional printing methods may be usable as alternatives to the extrusion process described in the previous paragraph. Such additive manufacturing processes are not limited to layerwise deposition, and may include other methods, such as, but not limited to: Selective Laser Sintering (SLS), Stereolithography (SLA), micro-stereolithography, Laminated Object Manufacturing (LOM), Fused Deposition Modeling (FDM), MultiJet Modeling (MJM), aerosol jet, direct-write, inkjet fabrication, and micro-dispense. Areas of overlap can exist between many of these methods, which can be chosen as needed based on the materials, tolerances, size, quantity, accuracy, cost structure, critical dimensions, and other parameters defined by the requirements of the object or objects to be made.

<FIG> shows an alternative configuration. <FIG> shows a rocket motor <NUM> which is nozzleless, with an oxidizer tank <NUM> within a combustion chamber <NUM> of a nozzleless rocket, thruster, or gas generator. In other respects, and details, the rocket motors <NUM> and <NUM> are similar to the rocket motor <NUM> (<FIG>) described above.

Claim 1:
A hybrid rocket motor (<NUM>, <NUM>) comprising:
a solid fuel (<NUM>) adjoining and at least partially defining a combustion chamber (<NUM>, <NUM>); and
an oxidizer tank (<NUM>, <NUM>) fully within the combustion chamber;
wherein the solid fuel and a liquid oxidizer (<NUM>) from the oxidizer tank are burnable in the combustion chamber to produce thrust in the motor; and
wherein the oxidizer tank is supported by a series of radial ribs (<NUM>) that extend outward from the oxidizer tank, through the combustion chamber, to a radially inner surface of the solid fuel at least partially defining the combustion chamber.