Persistent vortex generating high regression rate solid fuel grain for a hybrid rocket engine

Abstract

A cylindrically-shaped hybrid rocket engine solid fuel grain defines an axial combustion port. A fuel grain material comprises a compounded blend of thermoplastic fuel and aluminum. The fuel grain comprises fused stack layers, each layer comprising a plurality of fused abutting concentric beaded structures arrayed to define the combustion port; the port exhibits a rifling pattern or rifling inducing geometry along the port wall. When an oxidizer is introduced into the combustion port combustion occurs along the exposed port wall. Each beaded structure defines a geometry that increases the combustion surface area while inducing a vortex flow of oxidizer and fuel gas. As each layer ablates, an abutting layer exhibiting a similar geometry, is revealed, undergoes a gas phase change, and ablates. This process repeats and persists until oxidizer flow is terminated or the fuel grain material is exhausted. The fuel grain may be manufactured by an additive manufacturing process.

Claims

1. A fuel grain for a hybrid rocket, the fuel grain comprising: a plurality of layers of fuel grain material, each layer comprising a plurality of concentric ring-shaped beaded structures of different radii fused together to form a disc defining a central opening therein; the plurality of layers stacked and bonded to form a cylindrical fuel grain with the central opening of each one of the plurality of layers aligned to form a combustion port extending axially through the fuel grain and bounded by a boundary wall; wherein the fuel grain material comprises a combustible substance; and an inner circumferential surface of each ring-shaped beaded structure comprising an irregular surface, such that as a ring-shaped bead forming the boundary wall ablates due to combustion in the combustion port, an inner circumferential wall of an adjacent ring-shaped bead comprising an irregular surface presents to form the boundary wall.

2. The fuel grain of claim 1 wherein the irregular surface of the boundary wall provides a larger surface area and an increased regression rate of the fuel grain relative to a fuel grain lacking an irregular surface while simultaneously inducing oxidizer/fuel gas flowing through the combustion port to continually trip, thereby creating a consistent eddy current and enabling improved combustion and higher Isp.

3. The fuel grain of claim 1 wherein the plurality of layers of fuel grain material comprises an inner layer forming the boundary wall prior to combustion in the combustion port, an outer layer forming an outer layer of the fuel grain, and a plurality of intermediate layers disposed therebetween, wherein progressing from the inner layer to the outer layer an irregularity in the irregular surface is less pronounced.

4. The fuel grain of claim 1 wherein the irregular surface comprises projections configured to form a progressive axial twist through the combustion port, the axial twist for inducing a swirling gaseous flow within the combustion port.

5. The fuel grain of claim 4 wherein the progressive axial twist comprises a helical grooved rifling pattern of projections.

6. The fuel grain of claim 1 wherein the progressive axial twist comprises a polygonal rifling geometry.

7. The fuel grain of claim 1 produced by an additive manufacturing process.

8. The fuel grain of claim 1 wherein the grain material comprises Acrylonitrile Butadiene Styrene (ABS) thermoplastic and a plurality of micron scale or nanoscale elemental aluminum particles or a plurality of nanoscale elemental aluminum core particles capped with an oligomer polymer.

9. The fuel grain of claim 1 wherein the grain material comprises Acrylonitrile Butadiene Styrene (ABS) thermoplastic by mass ranging from 80% to 95% and aluminum powder by mass correspondingly ranging from 20% to 5%, the particle size of which can vary from 15 nanometers to 44 microns.

10. The fuel grain of claim 1 further comprising a thermally insulating material encasing the fuel grain.

11. The fuel grain of claim 1 the irregular surface comprising one or more of a plurality of ribs, a plurality of undulations, a plurality of protrusions and recesses, a plurality of depressions.

12. The fuel grain of claim 1 the irregular surface comprising one or more of a corrugation pattern, a truncated pyramidal pattern, a rifled truncated pyramidal pattern, or a rifled polygonal pattern.

13. The fuel grain of claim 1 wherein a shape of the combustion port comprises a circular shape, an oval shape, an elliptical shape, a polygonal shape, a quatrefoil shape, a star shape, or an irregular shape.

14. The fuel grain of claim 1 wherein the fuel grain defines an outer diameter of 19.0 inches and the combustion port has an initial diameter of 4 inches prior to consumption of fuel grain material during combustion.

15. A plurality of fuel grain segments each according to claim 1 further comprising ABS material between a surface of a first fuel grain segment and an abutting surface of a second fuel grain segment thereby creating a fusion bond between the first and second fuel grain segments.

16. The fuel grain of claim 1 wherein a composition of the fuel grain material of each one of the concentric ring-shaped beaded structures is substantially uniform.

17. The fuel grain of claim 1 wherein the combustible substance comprises a formulation of thermoplastic and passivated nanoscale metallic material.

18. The fuel grain of claim 1 wherein the irregular surface of each ring-shaped beaded structure comprises a sustaining internal topological pattern as each ring-shaped beaded structure ablates and another ring-shaped beaded structure is revealed due to combustion and ablation to the combustion port wall.

19. The fuel grain of claim 1, wherein the irregular surface of each ring-shaped beaded structures forms a sustaining rifling pattern or geometry for both increasing the surface area of the combustion port wall and for generating a vortex flow of oxidizer and fuel gas flowing through the combustion port.

20. The fuel grain of claim 1 wherein a material of each ring-shaped beaded structure comprises a solidified material, further comprising a polymer or a solidified polymer-metal blend formulation suitable for combusting in a hybrid rocket engine.

21. The fuel grain of claim 1 wherein a material of each ring-shaped beaded structure comprises a blend of Acrylonitrile Butadiene Styrene (ABS) and aluminum powder.

22. The fuel grain of claim 1 wherein the combustion port defines a polygonal shape in a cross section with an orientation of each layer adjusted to create a progressive helical twist axially through the combustion port, forming a rifling pattern to induce a swirling oxidizer/fuel gaseous flow within the center combustion port.

23. A fuel grain for a hybrid rocket, the fuel grain comprising: a first fuel grain section comprising: a first plurality of concentric ring-shaped beads of different radii fused together to form a first disc, the first disc defining a first combustion port; an inner circumferential surface of each of the first plurality of circular ring-shaped beads comprising an irregular surface, such that as a ring-shaped bead forming a first combustion port wall ablates due to combustion in a combustion port, an inner circumferential surface of an adjacent ring-shaped bead comprising an irregular surface presents to form the first combustion port wall; a material of the first fuel grain section comprising a combustible substance; a second fuel grain section comprising: a second plurality of concentric ring-shaped beads of different radii fused together to form a second disc, the second disc defining a second combustion port; an inner circumferential surface of each of the second plurality of circular ring-shaped beads comprising an irregular surface, such that as a ring-shaped bead forming a second combustion port ablates due to combustion in a combustion port, an inner circumferential wall of an adjacent ring-shaped bead comprising an irregular surface presents to form the second combustion port wall; a material of the second fuel grain section comprising a combustible substance; and the first and second fuel grain sections bonded together and the first and second combustion ports aligned to form the fuel grain.

24. The fuel grain of claim 23 further comprising a first connecting member in a lower surface of the first fuel grain section for mating with a second connecting member in an upper surface of the second fuel grain section.

25. A hybrid rocket engine comprising: a fuel grain further comprising: a plurality of layers of fuel grain material each layer comprising a plurality of concentric ring-shaped beaded structures of different radii fused together to form a disc, the disc defining a central opening; the plurality of layers stacked and bonded to form a cylindrical fuel grain such that the central opening of each one of the plurality of layers is aligned to form a combustion port extending axially through the fuel grain and bounded by a boundary wall; wherein the fuel grain material includes at least one combustible substance; an inner circumferential surface of each ring-shaped beaded structure comprising an irregular surface, such that as a ring-shaped bead forming the boundary wall ablates due to combustion in the combustion port, an inner circumferential wall of an adjacent ring-shaped bead comprising an irregular surface presents to form the boundary wall; an oxidizer source, the oxidizer for flowing through the combustion port during engine operation; a valve for controlling flow of oxidizer through the combustion port; a nozzle in fluid communication with the combustion port; and a shell for housing the fuel grain, the oxidizer source, and the valve, the nozzle extending from the shell.

26. The hybrid rocket engine of claim 25 further comprising an insulating film surrounding the fuel grain.

.Iadd.27. A fuel grain for a hybrid rocket, the fuel grain comprising: multiple beads of fuel grain material, in which the beads are fused together to form a generally cylindrical fuel grain defining a combustion port extending axially through the generally cylindrical fuel grain, in which the combustion port is bounded by a boundary wall, in which the fuel grain material comprises a combustible substance, and in which the boundary wall is formed of a subset of the beads of combustible fuel grain material, and in which the boundary wall is configured to induce an eddy current in a fluid flowing through the combustion port. .Iaddend.

.Iadd.28. The fuel grain of claim 27, in which the fuel grain is configured such that when the boundary wall ablates due to combustion in the combustion port, a new surface of fuel grain material is exposed to the combustion port. .Iaddend.

.Iadd.29. The fuel grain of claim 27, in which the boundary wall presents a larger surface area to the combustion port relative to a boundary wall that is not formed of beads of fuel grain material. .Iaddend.

.Iadd.30. The fuel grain of claim 27, in which the boundary wall is textured with one or more of ribs, dimples, undulations, protrusions, or depressions. .Iaddend.

.Iadd.31. The fuel grain of claim 27, in which when viewed along a longitudinal axis of the combustion port, the boundary wall defines alternating protrusions and depressions. .Iaddend.

.Iadd.32. The fuel grain of claim 27, in which the fuel grain is fabricated in an additive manufacturing process. .Iaddend.

.Iadd.33. The fuel grain of claim 27, in which the fuel grain material comprises an Acrylonitrile Butadiene Styrene (ABS) thermoplastic. .Iaddend.

.Iadd.34. The fuel grain of claim 27, in which the fuel grain material comprises a mixture of a hybrid rocket fuel material and a nanoscale metallic material. .Iaddend.

.Iadd.35. The fuel grain of claim 34, in which the fuel grain material comprises between 80% and 95% by mass of the hybrid rocket fuel material and between 5% and 20% by mass of the nanoscale metallic material. .Iaddend.

.Iadd.36. The fuel grain of claim 27, comprising a thermally insulating material encasing the fuel grain. .Iaddend.

.Iadd.37. A fuel grain assembly comprising: multiple of the fuel grains of claim 28, in which an end of fuel grain is bonded to an end of an adjacent fuel grain to form an elongated, generally cylindrical fuel grain assembly, and in which the combustion ports of the multiple fuel grains are aligned to define an elongated combustion port of the fuel grain assembly. .Iaddend.

.Iadd.38. A hybrid rocket engine comprising: the fuel grain of claim 28; an oxidizer source configured to provide a flow of an oxidizer through the combustion port during operation of the hybrid rocket engine; a valve configured to control the flow of the oxidizer through the combustion port; a nozzle in fluid communication with the combustion port; and a casing, in which the fuel grain, the oxidizer source, and the valve are housed within the casing, and in which the nozzle extends beyond an end of the casing. .Iaddend.

.Iadd.39. A fuel grain for a hybrid rocket, the fuel grain comprising: a generally cylindrical body formed of a fuel grain material comprising a combustible substance, in which a combustion port extends axially through the body, in which the combustion port is bounded by a boundary wall, in which the boundary wall is formed of the fuel grain material, and in which the boundary wall is configured to induce an eddy current in the combustion port in a fluid flowing through the combustion port, and in which the fuel grain is configured such that when the boundary wall ablates due to combustion in the combustion port, a new surface of fuel grain material is exposed to the combustion port. .Iaddend.

.Iadd.40. The fuel grain of claim 39, in which the body comprises beads of the fuel grain material, in which the beads are fused together to form the body. .Iaddend.

.Iadd.41. The fuel grain of claim 39, in which the boundary wall is textured with one or more of ribs, dimples, undulations, protrusions, or depressions. .Iaddend.

.Iadd.42. The fuel grain of claim 41, in which the boundary wall presents a larger surface area to the combustion port relative to a boundary wall that is not textured. .Iaddend.

.Iadd.43. The fuel grain of claim 39, in which when viewed along a longitudinal axis of the combustion port, the boundary wall defines alternating protrusions and depressions. .Iaddend.

.Iadd.44. The fuel grain of claim 39, in which the fuel grain is fabricated in an additive manufacturing process. .Iaddend.

.Iadd.45. The fuel grain of claim 39, in which the fuel grain material comprises an Acrylonitrile Butadiene Styrene (ABS) thermoplastic. .Iaddend.

.Iadd.46. The fuel grain of claim 39, in which the fuel grain material comprises a mixture of a hybrid rocket fuel material and a nanoscale metallic material. .Iaddend.

.Iadd.47. The fuel grain of claim 46, in which the fuel grain material comprises between 80% and 95% by mass of the hybrid rocket fuel material and between 5% and 20% by mass of the nanoscale metallic material. .Iaddend.

.Iadd.48. The fuel grain of claim 39, comprising a thermally insulating material encasing the fuel grain. .Iaddend.

.Iadd.49. A fuel grain assembly comprising: multiple of the fuel grains of claim 39, in which an end of fuel grain is bonded to an end of an adjacent fuel grain to form an elongated, generally cylindrical fuel grain assembly, and in which the combustion ports of the multiple fuel grains are aligned to define an elongated combustion port of the fuel grain assembly. .Iaddend.

.Iadd.50. A hybrid rocket engine comprising: the fuel grain of claim 39; an oxidizer source configured to provide a flow of an oxidizer through the combustion port during operation of the hybrid rocket engine; a valve configured to control the flow of the oxidizer through the combustion port; a nozzle in fluid communication with the combustion port; and a casing, in which the fuel grain, the oxidizer source, and the valve are housed within the casing, and in which the nozzle extends beyond an end of the casing. .Iaddend.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of a solid fuel grain section made in accordance with the present invention;

(2) FIG. 2 is a top view of the solid fuel grain section of FIG. 1;

(3) FIG. 3 is a sectional view of the grain section of FIG. 1, taken along line 3-3 of FIG. 2;

(4) FIG. 4A is a side view of a solid fuel grain comprised of a plurality of solid fuel grain sections;

(5) FIG. 4B is an exploded side view of the solid fuel grain of FIG. 5A;

(6) FIG. 5 is a perspective view of the plurality of solid fuel grain sections of FIG. 45A wrapped with insulating film;

(7) FIG. 6 is a sectional view of an exemplary rocket incorporating the solid fuel grain of FIGS. 4A, 4B, and 5;

(8) FIG. 7 is an enlarged sectional view of the motor case of the rocket of FIG. 6, showing a flame configuration; and

(9) FIGS. 8A, 8B, and 8C are top views of the fuel grain section of FIG. 1 as successively consumed by a flame.

(10) FIG. 9 depicts the coordinate system and orientation of the fuel grain for use with FIGS. 10-13.

(11) FIG. 10 depicts a quarter sectional view of the fuel grain section of FIG. 1 featuring a concentric corrugation topology grain pattern.

(12) FIGS. 11 and 12 depict quarter sectional views of a fuel grain section featuring a concentric rifled truncated pyramidal topology grain pattern.

(13) FIGS. 13A and 13B depict a top view and a perspective view of the fuel grain section of FIG. 1 featuring a concentric rifled polygonal topology grain pattern.

DETAILED DESCRIPTION OF THE INVENTION

(14) The present invention is a solid fuel grain for a hybrid rocket engine. The solid fuel grain can be manufactured 65 using a fused deposition type additive manufacturing apparatus as described and claimed in the commonly-owned application Ser. No. 15/818,381 and the issued U.S. Pat. Nos. 9,453,479 and 9,822,045, as referenced above.

(15) FIGS. 1-3 illustrate various views of an exemplary solid fuel grain section 10 suitable for use in a hybrid rocket engine. Additional details of fuel grains, including their fabrication, are described and claimed in one or more of the following commonly-owned and related applications, the contents of which are hereby incorporated by reference:

(16) U.S. Pat. No. 9,453,479 issued on Sep. 27, 2016

(17) U.S. Pat. No. 9,822,045 issued on Nov. 21, 2017

(18) patent application Ser. No. 15/818,381 filed on Nov. 20, 2017

(19) patent application Ser. No. 15/262,661 filed on Sep. 12, 2016

(20) With reference to FIGS. 1-3, the fuel grain section 10 has a generally cylindrical shape and defines a center port 16. In this exemplary embodiment, the center port 16 has a substantially circular cross-section, but the center port 16 could have other geometries, such as a star, clover leaf, or polygon without departing from the spirit or scope of the present invention.

(21) The solid fuel grain section 10 is formed as a fusion (bonded) stack of layers with each such layer formed as a series of abutting fused concentric ring-shaped beads of solidified material 12 arrayed around the center port 16. In one embodiment, a heat gun with an ABS stick is used to bond the individual layers. Viscous ABS is applied to the sectional end caps before aligning and joining the grain sections. As is known by those skilled in the art, other adhesives can be used to join the grain sections.

(22) As is further described below, when incorporated into a hybrid rocket engine, an oxidizer is introduced into the solid fuel grain section 10 along a pathway defined by the center port 16, with combustion occurring along the exposed surfaces (also referred to as the boundary wall or combustion port wall) of the solid fuel grain section 10 port wall. Accordingly, each concentric ring-shaped structure possesses a geometric pattern 14 that serves to increase the surface area for combustion, compared to a smooth concentric circular structure or smooth walls as consistent with cast-molded constructions. As each such concentric ring-shaped bead ablates or undergoes phase change from either solid to gas or solid to entrained liquid droplet, the abutting concentric bead is exposed to the flame sheet. This process continues and persists during the hybrid rocket engine's operation until either oxidizer flow is terminated or the solid fuel is exhausted. Unlike prior art constructions that improve regression rate by increasing the surface area exposed to the flame sheet using a multi-port architecture at the sacrifice of fuel loading, the solid fuel grain of the present invention presents increased surface area as a means to improve regression rate, but without the disadvantages associated with multi-port configurations.

(23) Although the fuel grain section 10 may be manufactured in various sizes or dimensions, in an exemplary embodiment, the fuel grain section 10 has an outer diameter, d2, of 19.0 inches. Although a wide range of diameters and fuel grain lengths (or sectional lengths) are possible, the center port 16 has an initial diameter, d1, (i.e., before combustion) of 4.0 inches in this exemplary embodiment (although a larger diameter is shown in FIG. 3 to enable a better view of the interior of the fuel grain section 10). Although a fuel grain with any grain diameter can be fabricated, traditionally a ratio of about 5:1 (outer diameter to inner diameter) is used for a hybrid rocket fuel grain.

(24) Each of the stacked fused layers in this exemplary embodiment would have an approximate thickness ranging from 0.005 inches to 0.015 inches depending upon the fabrication technique employed.

(25) In one fabrication technique, each of the stacked layers 12 is formed by the deposition of viscous polymer which is extruded following a roughly circular tool path forming a plurality of solidified abutting ring-shaped beads of material. Viewed in cross section as depicted in FIG. 11, each ring-shaped bead of solidified material 90 is oval or elliptical in cross sectional shape, which flattens on its bottom under its own weight as the material cools and flattens on the top as the weight of the next extruded layer of abutting ring-shaped beads of material is deposed above it.

(26) As for the concentric ring-shaped beaded structures, the objective is to increase the surface area presented to the flame zone for combustion within the center port 16 in a manner that is persistent throughout the hybrid rocket engine operation. In this exemplary embodiment, and as illustrated in FIGS. 1-3, the surface pattern presented to the flame zone is characterized by a series of projections and depressions (according to other embodiments the surface pattern comprises a plurality of ribs, a plurality of undulations, a plurality of protrusions and recesses) extending radially into the center port and in this case forming elongated undulations that extend axially through the center port. These undulations are present in each concentric circular ring-shaped beaded structure such that as one ring-shaped beaded structure is ablated, the next-presented ring-shaped structure is revealed, presenting the same geometric pattern, but with an increased radius.

(27) In FIGS. 1-3 as well as in FIGS. 10-13B, the individual undulations are identifiable and have a substantially cylindrical shape. However, in practice, depending upon the scale and layer thickness, such internal topology can take the form of a dimple pattern 14 as shown in FIGS. 1-3, a corrugation pattern 92 as shown in FIG. 10, a truncated pyramidal pattern 110 as shown in FIG. 11, a truncated pyramidal pattern 120 as shown in FIG. 12, and an irregular pattern 131 as shown in FIGS. 13A and 13B, all of which may or may not be perceptible to a viewer's unaided eye. Alternatively, the geometric pattern 14, 92, 110, 120, 131 of each ring-shaped concentric beaded structure may take other forms (e.g., an irregular surface) in order to achieve the objective of increasing the surface area available for combustion and to ensure that increased surface area persists throughout operation of the hybrid rocket engine. Any surface features that create the irregular surface and increase the surface area of the combustion port (also referred to as the port wall or the flame wall) are considered within the scope of the present invention.

(28) In one exemplary embodiment, each fused stacked layer is formed from a series of fused concentric ring-shaped beads of solidified material featuring a pattern designed to increase surface area of the boundary wall or flame wall, as compared to a smooth construction, and to present grooved, protrusion, or contoured patterns. In one embodiment the center port wall (also referred to as the boundary wall or flame wall) features a rifling pattern designed to induce oxidizer vortex flow persisting throughout the hybrid rocket engine's operation as the fuel is consumed.

(29) In addition to the fused deposition techniques of additive fabrication, as referred to in the cited commonly-owned patent references, there are a number of other additive manufacturing methods that can be employed to produce hybrid rocket fuel grains according to the present invention and using a formulation of polymer and nanocomposite aluminum additive, without departing from the spirit and scope of the present invention, including: Stereolithography, Selective Laser Sintering, Powder Bed Printing, and Inkjet Head Printing.

(30) For the examples shown in the various Figures described herein, a composition of the fuel grain is about 95% by mass Acrylonitrile Butadiene Styrene (ABS), a thermoplastic possessing combustion characteristics desirable for hybrid rocket engine fuel, and about 5% nanocomposite aluminum. Fuel having this structure is available from several sources, as known by those skilled in the art.

(31) With a Young's Modulus of 2.0-2.6 GPa, ABS is 460 times less elastic than HTPB and 38 times less elastic than paraffin wax, making it an ideal material for fabricating a hybrid rocket fuel grain and its combustion chamber center port.

(32) Ultra-high energetic nano-particle sized aluminum, especially aluminum powder produced without an aluminum oxide shell and passivated (by encapsulating or capping the particle in a polymer shell) for safe handling and use, increases the fuel grain burning rate by as much as 50% using only a 5% concentration, compared to a fuel grain fabricated in ABS with a 25% concentration of standard military grade 44-micron particle size aluminum.

(33) Referring now to FIGS. 4A-4B, the individual fuel grain sections 10a, 10b, 10c, and 10d can be assembled and joined together from multiple separately fabricated sections to form a complete solid fuel grain 40. In this exemplary embodiment, each solid fuel grain section 10 has a height, h1, of 23 inches, such that the overall height, h2, of the complete solid fuel grain 40 is 92 inches. Furthermore, in this exemplary embodiment, to ensure proper alignment, the topmost solid fuel grain 10a has at least one connecting member 100a extending from its lower surface and at least one cavity 102a defined in its lower surface for receiving a mating connecting member 104b. Similarly, the intermediate solid fuel grain sections 10b, 10c, each have at least one connecting member 100b and 100c, extending from their respective lower surfaces and one connecting member 104b, 104c, extending from their respective upper surfaces, and further each have at least one cavity 102b, 102c defined in their respective lower surfaces and at least one cavity 106b, 106c defined in their respective upper surfaces. Finally, the lowermost solid fuel grain section 10d has at least one connecting member 104b extending from its upper surface and at least one cavity 106d defined in its upper surface for receiving a mating connecting member 100c in the fuel grain section 10c.

(34) Accordingly, when heated above its glass transition temperature but below the aluminum powder's ignition temperature, viscous ABS can be spread or sprayed on the upper and lower surfaces to create a strong fusion bond between the sections 10a, 10b, 10c, 10d during assembly. In this way, solid fuel grain sections 10a, 10b, 10c, 10d can be readily stacked, aligned, and mated to one another to form the complete solid fuel grain 40.

(35) Referring now to FIG. 5, after the solid fuel grain sections 10a, 10b, 10c, 10d are assembled, the solid fuel grain sections 10a, 10b, 10c, 10d collectively define a center port 46 through the solid fuel grain 40. The solid fuel grain 40 is preferably wrapped in a film 50 made of phenol or other suitable thermally resistant material. Placed between the inner wall of a fuel motor case (not shown in FIG. 6) and the outer surface of the solid fuel grain, the film 50 acts as an insulation layer to reflect heat and prevent damage to fuel motor cases made from either metal or non-metallic materials such as carbon fiber reinforced polymer composite. Once wrapped in the film 50, the solid fuel grain 40 can be placed into a motor case of a rocket.

(36) FIG. 6 is a sectional view of an exemplary hybrid rocket engine 70 housed within an aeroshell 72 to form a complete hybrid rocket powered vehicle 70 incorporating the solid fuel grain 40 as described above with respect to FIGS. 4A, 4B, and 5. The exemplary hybrid rocket powered vehicle 70 generally comprises an aeroshell body 72, a nozzle 82 at one distal end of said aeroshell body 72, and a payload section 74 at an opposite distal end of said aeroshell body 72. Enclosed within the aero shell body 72 of the hybrid rocket powered vehicle 70 is a hybrid rocket engine including an oxidizer tank 76, a valve 78, a motor case 60, and an oxidizer injector 80 housed typically within a forward cap (not shown) that also houses the ignition system (not shown). The motor case 60 houses a pre-combustion chamber (not shown), a post-combustion chamber 64, and the solid fuel grain 40, which as described above is wrapped in insulating film 50.

(37) The solid fuel grain 40 wrapped in a thermal insulating film 50 can be cartridge loaded into the motor case 60 of the hybrid rocket engine. Alternatively, the exemplary solid fuel grain 40 wrapped in thermal insulating film 50 could be wound with a fiber-reinforced polymer composite to form the motor case without departing from the spirit and scope of the present invention. In another exemplary embodiment, the solid fuel grain 40 can be inserted into a thermal protection cylinder fabricated from insulating material such as phenolic or cork without departing from the spirit and scope of the present invention. In yet another exemplary embodiment, the fuel grain 40 can be formed to embody either or both the pre-combustion chamber and the post-combustion chamber 64 without departing from the spirit and scope of the present invention.

(38) FIG. 7 is an enlarged sectional view of the motor case 60 of the hybrid rocket powered vehicle 70 of FIG. 6, showing the flame zone within the fuel grain center port 46. As shown, an oxidizer 94 (either a liquid or a gas) is injected into the motor case 60 along a pathway defined by the center port 46 of the solid fuel grain 40 and flows within the center port 46, forming a boundary layer 65 bordered by the center port 46 wall. The boundary layer 65 is usually turbulent throughout a large portion of the length of the center port 46. Within the boundary layer 65 is a turbulent diffusion flame zone 66 that extends throughout the entire length of the center port 46 and depending upon the characteristics of the solid fuel selected, either causing a phase change to a gas or entrained liquid droplets of fuel to form. Evaporation from the oxidizer/fuel gas/entrained liquid droplet interface produces a continuous flow of fuel gas that mixes with oxidizer gas at the flame zone 66 to maintain combustion along the exposed surface area of the center port 46 wall. At steady state, the regression rate of the melt surface and the gas-gas or gas-entrained liquid droplet interface is the same, and the thickness of the gaseous or entrained liquid layer is constant.

(39) Because the port wall surface pattern 14, 91, 110, 120, 131 exposed to the flame zone 66 possesses increased surface area compared to cast-molded constructions, the exemplary solid fuel grain 40 causes increased regression rate and corresponding increased thrust impulse without the decreased fuel volumes associated with multi-port designs. Additionally, the undulating wall surface pattern that runs the length of the fuel grain port also causes the mixture of fuel gas (or entrained fuel droplets) and atomized or gaseous oxidizer to continually trip, creating a consistent circular eddy current flow which contributes to more thorough combustion and a higher Isp.

(40) The continual trip referred to above is a mechanism of motion of oxidizer and fuel gas through the port. Due to the rough, semi-circular ribbed pattern along the port wall (as described elsewhere herein), the oxidizer/fuel gas mixture, as it flows along the boundary with the port wall, will trip over the ribs, and create an eddy current. This tripping mechanism causes the port wall to regress more rapidly, requiring a longer time for the fuel gas mixture to clear the port into the nozzle; thus contributing to improved combustion and less propellant waste. This mechanism, together with the much higher surface area that is created by a ribbed pattern, results in a significantly higher than typical regression rate as well as higher specific impulse that obtained with prior art designs.

(41) Also, unlike the prior art constructions that increase the surface area using a multi-port architecture (which sacrifices fuel loading), the solid fuel grain 40 of the present invention allows a smooth burning process whereby, as each concentric ring-shaped beaded structure forming each layer of the fusion stacked layer center port 46 wall is ablated, a new concentric ring-shaped beaded structure, the plurality of which forms the expanded center port 46 wall, is presented to the flame zone 66, as shown in FIGS. 8A-8C, illustrating ablation of the center port wall at three different stages. This burning process continues until either oxidizer flow is terminated or the solid fuel grain 40 material is exhausted.

(42) Generally, energetic materials suitable for use in the present invention are a class of material with high amount of stored chemical energy that can be released. Highly energetic materials include ultrafine aluminum powder, the particle size of which can vary from micron to nanoscale, including particles that are a composite of aluminum and polymer in nanoscale. As known by those skilled in the art, generally a nanocomposite is a material comprising two or more constituent solids, the size of which measures 100 nanometers (nm) or less. Even though the nano-scale aluminum particle cores are completely encapsulated in a polymer based oligomer coating and thus passivated, there remains the possibility that this highly energetic pyrophoric material can still be reactive with oxygen or water vapor. As a safety precaution, the nanocomposite aluminum, the ABS thermoplastic, and the compounded ABS-nanocomposite materials are stored in containers designed to store flammable material, preferably infilled with a non-reactive noble gas at all times prior to their use as feedstock in an additive manufacturing process.

(43) In one application, the compounded material is stored within a climate controlled environment.

(44) As a further safety measure, after fabrication each fuel grain or fuel grain section is shrink-wrapped to encase the fuel grain or fuel grain section in a thin plastic film to prevent atmospheric exposure prior to its use in a hybrid rocket engine. In another embodiment the fuel grain is spray coated with a polymeric material or paint that serves to prevent atmospheric exposure. According to another embodiment the fuel grain or grain segment is inserted into an air-tight packaging cylinder and a vacuum drawn to remove all air. The packaging cylinder is sealed before it is removed from the print bed chamber.

(45) FIG. 9 depicts the coordinate system and orientation of the fuel grain for use with FIGS. 10-13B

(46) FIG. 10 is a quarter sectional view of the fuel grain section of FIG. 1 featuring a concentric ring-shaped corrugation build pattern or fuel grain 92, a port wall surface pattern 91, and several layers of fused concentric beads in cross section 90.

(47) FIG. 11 is a quarter sectional view of the fuel grain section of FIG. 1 featuring a concentric ring-shaped truncated pyramidal build pattern or fuel grain 113, a port wall surface pattern 110, and several layers of fused concentric beads in cross section 111.

(48) FIG. 12 is a quarter sectional view of the fuel grain section of FIG. 1 featuring a concentric ring-shaped rifled truncated pyramidal build pattern or fuel grain 123, a port wall surface pattern 120 with the build and surface patterns staggered layer by layer to form in its plurality a persistent rifling pattern.

(49) FIG. 13A depicts a top view and FIG. 13B a perspective view showing the port wall surface pattern 131 of the fuel grain section of FIG. 1. FIGS. 13A and 13B feature a concentric ring-shaped rifled polygonal pattern for fuel grain 132 with each such polygonal build pattern staggered and twisted (i.e., rifled) layer-by-layer to form in its plurality a persistent rifling pattern.

(50) The embodiments of FIGS. 12 and 13A/13B present a persistent rifling pattern to the oxidizer flowing through the center port 46 to induce axial flow.

(51) The embodiments of FIGS. 11-13B depict exemplary constructions of a hybrid rocket fuel grain engineered to both increase the amount of surface area available for combustion as a means to improve regression rate, to improve specific impulse, to generate an oxidizer vortex flow, and to reduce fuel waste by inducing oxidizer axial flow within the center port 46 (see FIG. 7) to allow more time for oxidizer and fuel gases (or oxidizer and entrained liquid droplets) to mix and combust more thoroughly. Any surface area pattern or topology that furthers one or more of these objectives, and is sustainable throughout the fuel grain cross-section (i.e., as one fuel grain layer ablates the next fuel gran layer presents a desirable surface area pattern) is considered within the scope of the present invention.

(52) One of ordinary skill in the art will recognize that additional embodiments are also possible without departing from the teachings of the present invention or the scope of the claims which follow. This detailed description, and particularly the specific details of the exemplary embodiments disclosed herein, is given primarily for clarity of understanding, and no unnecessary limitations are to be understood therefrom, for modifications will become obvious to those skilled in the art upon reading this disclosure and may be made without departing from the spirit or scope of the claimed invention.