Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
  • F02K—JET-PROPULSION PLANTS
  • F02K9/00—Rocket-engine plants, i.e. plants carrying both fuel and oxidant therefor; Control thereof
  • F02K9/08—Rocket-engine plants, i.e. plants carrying both fuel and oxidant therefor; Control thereof using solid propellants
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
  • F02K—JET-PROPULSION PLANTS
  • F02K9/00—Rocket-engine plants, i.e. plants carrying both fuel and oxidant therefor; Control thereof
  • F02K9/08—Rocket-engine plants, i.e. plants carrying both fuel and oxidant therefor; Control thereof using solid propellants
  • F02K9/10—Shape or structure of solid propellant charges
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
  • F02K—JET-PROPULSION PLANTS
  • F02K9/00—Rocket-engine plants, i.e. plants carrying both fuel and oxidant therefor; Control thereof
  • F02K9/08—Rocket-engine plants, i.e. plants carrying both fuel and oxidant therefor; Control thereof using solid propellants
  • F02K9/24—Charging rocket engines with solid propellants; Methods or apparatus specially adapted for working solid propellant charges
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
  • F02K—JET-PROPULSION PLANTS
  • F02K9/00—Rocket-engine plants, i.e. plants carrying both fuel and oxidant therefor; Control thereof
  • F02K9/08—Rocket-engine plants, i.e. plants carrying both fuel and oxidant therefor; Control thereof using solid propellants
  • F02K9/32—Constructional parts; Details not otherwise provided for
  • F02K9/34—Casings; Combustion chambers; Liners thereof
  • F02K9/346—Liners, e.g. inhibitors

Definitions

• This disclosure relates to solid fuel rocket motors, and to fast response motors suitable for use as attitude control thrusters.
• Solid fuel rocket motors are commonly used in various configurations to propel rockets and missiles. Small solid fuel rocket motors may also be used to control the attitude and steering of a missile, rocket, or other projectile. Small solid fuel rocket motors used to control attitude are commonly called attitude thrusters or divert thrusters. Solid fuel rocket motors may also be used to turn a vertically-launched missile or rocket into near-horizontal flight. Such rocket motors are commonly called pitch-over thrusters.
• a p propellant surface area
• R b propellant burn rate
• the propellant surface area A p , the propellant burn rate R b and the propellant density P p are important factors that may be used to determine the thrust produced by a solid fuel rocket.
• the force produced by a rocket motor results in a linear or angular acceleration of the missile or other body propelled by the rocket motor.
• the net change in the linear or angular velocity of the missile or other body is proportional to the force produced by the motor integrated over time.
• the time integral of the force produced by a rocket motor is commonly called the "impulse" of the motor.
• FIG. 1 is a cross-sectional side view of a conventional solid fuel rocket motor.
• FIG. 2A is a cross-sectional end view of a propellant charge at ignition.
• FIG. 2B is a cross-sectional end view of a propellant charge at a time t after ignition.
• FIG. 2C is a cross-sectional end view of a propellant pellet at ignition.
• FIG. 2D is a cross-sectional end view of a propellant pellet at a time t after ignition.
• FIG. 3 is a cross-sectional side view of an exemplary hybrid solid fuel rocket motor.
• FIG. 4A is a chart of simulation results showing the thrust of an exemplary hybrid solid fuel rocket motor.
• FIG. 4B is a chart of simulation results comparing internal pressures for propellant, pellet propellant, and hybrid solid fuel rocket motors.
• FIG. 5 is a cross-sectional end view of a propellant solid fuel rocket motor.
• FIG. 6 is a chart of simulation results showing the thrust of an exemplary hybrid solid fuel rocket motor.
• FIG. 7A is a cross-sectional end view of a propellant charge having a star-shaped center perforation.
• FIG. 7B is a cross-sectional end view of a propellant charge having a finocyl center perforation.
• FIG. 8A is a cross-sectional side view of an exemplary hybrid solid fuel rocket motor.
• FIG. 8B is a cross-sectional side view of an exemplary hybrid solid fuel rocket motor.
• a conventional solid fuel rocket motor 100 may include a case 110, a propellant charge 120 having a longitudinal opening 130, a nozzle 140, and an igniter 150.
• the propellant charge 120 may commonly be called the "grain". Note that the term “grain” is used to describe the propellant charge 120 as a whole, but does not refer to the weight of the propellant charge, the particle size of the material composing the propellant charge, or the surface texture of the propellant charge. Note that the propellant charge 120 may not be a single continuous mass of propellant material. For example, for ease of fabrication, the propellant charge 120 may be fabricated as two or pieces that are bonded or otherwise assembled within the rocket motor 100.
• the propellant charge 120 may be any material including at least one fuel ingredient and at least one oxidizer ingredient.
• the propellant charge may be a so-called “single-base” material, such as nitrocellulose, where the same ingredient serves as both the fuel and oxidizer components.
• the propellant charge may be a so-called “double base” material, such as a mixture of nitrocellulose and nitroglycerine, having two or more ingredients that serve as both the fuel and oxidizer components.
• the propellant charge may be a so-called “composite” material wherein the fuel ingredient and the oxidizer ingredient may be in the form of fine particles held in a matrix of a synthetic rubber or plastic binder.
• a composite propellant charge may include other ingredients such as a plasticizer.
• the binder and plasticizer may include energetic materials such as nitrocellulose, nitroglycerine, or other energetic materials.
• the propellant charge 120 may be formed by casting the selected material directly within the case 110.
• the propellant charge may be formed by casting or machining external to the case and may then be bonded or otherwise assembled into the case.
• a variety of propellant compositions are known. Exemplary propellant compositions are described in Chapter 12 of Rocket Propulsion Elements, by George Sutton and Oscar Biblarz (John Wiley & Sons, 2001).
• a longitudinal cavity 130 may be formed in the propellant charge 120.
• the longitudinal cavity 130 may commonly be centered on the longitudinal axis of the case 110.
• the burning area may rapidly spread to include the entire surface of the propellant charge 120 facing the longitudinal cavity 130 and, in some cases, the end of the propellant charge 120 proximate to the nozzle 140.
• a propellant charge with a longitudinal cavity may be termed a "center-perforated grain" or an "inside burning grain”.
• the igniter 150 may be a small charge of flammable material that, when burned, releases a predetermined amount of hot combustion gases.
• the combustion of the igniter may be initiated, for example, by an electric current flowing through a heater wire adjacent to, or embedded in, the flammable igniter material.
• the temperature and pressure of the gases produced by the igniter 150 may need to exceed predetermined values.
• the cavity 130 may be sealed by an environmental seal 145.
• the environmental seal 145 may also serve to protect the rocket motor 100 from environmental effects, such as humidity and precipitation.
• the environmental seal 145 may be designed to rupture or blow free from the motor after the pressure within the cavity 130 exceeds a predetermined pressure level, which may be, for example, between 100 and 500 pounds per square inch (PSI).
• a predetermined pressure level which may be, for example, between 100 and 500 pounds per square inch (PSI).
• PSI pounds per square inch
• the environmental seal may be retained in the nozzle by means of shear pins that fracture when the pressure exceeds the predetermined level.
• the environmental seal may be a burst disc having intentional structural weakness that allows the burst disc to rupture in a controlled manner when the pressure exceeds the predetermined level.
• the environmental seal 145 may be disposed at or near the portion of the nozzle 140 having the smallest cross-sectional area, commonly termed the throat 142.
• the environmental seal 145 may be disposed at other locations within the nozzle 140.
• the longitudinal cavity may be tapered slightly, as shown in FIG. 1, to maintain a relatively constant core velocity and minimize erosive burning.
• the longitudinal cavity 130 may be shaped as a cylinder or slightly tapered cylinder.
• the thrust provided by a solid fuel rocket motor having a cylindrical longitudinal cavity may increase continuously as the grain is burned.
• FIG. 2A shows a cross sectional end view of an exemplary solid fuel rocket motor including a propellant charge 220 with a cylindrical longitudinal cavity.
• the burning surface of the propellant charge 220, immediately after ignition, is shown by the heavy line 222 A.
• the area of the burning surface 222 A of the propellant charge 220 is given by ⁇ (L c )(D c o), where L c is the length of the propellant charge 220 normal to the plane of FIG. 2, and D c o is the initial diameter of the longitudinal cavity.
• FIG. 2B shows a cross sectional end view of the exemplary solid fuel rocket motor of FIG. 2 A at a time t after ignition.
• the burning surface of the propellant charge 220 is indicated by the heavy line 222B.
• the diameter of the burning surface 222B is equal to D c o+2R bc t, where R bc is the burn rate of the propellant charge.
• R bc is the burn rate of the propellant charge.
• FIG. 2B shows that the thrust of a solid fuel rocket motor having a propellant charge with a cylindrical longitudinal cavity will increase continuously as the propellant charge is burned.
• a propellant structure where the burning surface area increases as the result of combustion is said to have a "progressive burn”.
• FIG. 2C shows a cross sectional end view of an exemplary propellant pellet 225, which, for example, may be shaped as cylindrical disc having an initial diameter D p o. Note that FIG. 2C and FIG. 2D are not drawn to the same scale as FIG. 2A and FIG. 2B.
• the size of the pellet 225 is exaggerated with respect to the size of the propellant charge 220.
• the circumferential burning surface of the propellant pellet, immediately after ignition, is shown by the heavy line 222C.
• the total area of the pellet burning surface consists of surface 222C and the two circular faces of the propellant pellet 225.
• the total burning surface area of a disc shaped pellet is given by ⁇ (L p )(D p o) + 0.5( ⁇ )(D p o) , where L p is the length of the propellant pellet 225 normal to the plane of FIG. 2.
• FIG. 2D shows a cross sectional view of the exemplary propellant pellet 225 at a time t after ignition.
• the circumferential burning surface of the propellant pellet 225 is shown by the heavy line 222D.
• the diameter of the burning disc is equal to D p o-2R bp t, where R bp is the burn rate of the propellant pellets.
• R bp is the burn rate of the propellant pellets.
• a solid fuel rocket motor 300 may include a case 310, a propellant charge 320, a plurality of propellant pellets 325, a nozzle 340, and an igniter 350.
• a rocket motor having both a bulk propellant charge and propellant pellets will be referred to as a "hybrid solid fuel rocket motor”.
• hybrid rocket motor commonly used to describe a rocket motor where either one of the fuel or oxidizer ingredients is a solid and the other is a liquid.
• bulk propellant may be employed to distinguish the propellant charge 320 from the propellant pellets 325.
• the hybrid solid fuel rocket motor 300 may include a large plurality (hundreds or thousands) of propellant pellets 325.
• the propellant pellets 325 may be disposed within a central cavity in the propellant charge, as shown, or elsewhere within the case 310.
• the solid fuel pellets 325 may be, for example, gas generator pellets that are produced in large quantities for use in automobile air bags.
• Each of the plurality of propellant pellet 325 may be formed in a shape similar to that of a medicine tablet such as an aspirin.
• Each propellant pellet 325 may be formed in a short, generally cylindrical, shape with convex faces similar to an aspirin tablet.
• the convex faces may minimize the contact area between adjacent propellant pellets and thus prevent the propellant pellets from stacking or agglomerating.
• the convex faces may thus ensure that the plurality of propellant pellets 325 will have a very large total burnable surface area.
• Each propellant pellet may be formed in other shapes including flat discs, spheres, elongated cylinders, elongated cylinders with rounded ends, and other shapes.
• Each propellant pellet 325 may be composed of at least some of an energetic fuel material and an oxidizer material.
• Each propellant pellet may contain additional binder and/or plasticizer material.
• the binder material and the plasticizer material may be reactive and may serve as a fuel material and/or an oxidizer material.
• Suitable compositions for propellant pellets are well known. Suitable compositions used for propellant pellets include, for example, compositions that are predominantly guanidine (or guanidinium) nitrate and basic copper nitrate, cobalt nitrate, and combinations thereof, as described in U.S. Patent 5,608,183. At least 60% of the total mass of the propellant pellets 325 may be composed of guanidine nitrate and basic copper nitrate.
• the propellant pellets 325 may have a combustion temperature, for example between 1500°C and 1800°C, less than the combustion temperature of the propellant charge 320.
• the average temperature of the combustion gasses may be an average of the combustion temperatures of the composite and pellet propellants.
• the relative size of the propellant charge and the quantity of propellant pellets may be selected such that the average temperature of the combustion gasses is low enough (less than about 2500°C) to allow components of the rocket motor that are exposed to the combustion products to be fabricated from Molybdenum or TZM (Titanium-Zirconium-Molybdenum) alloy.
• the propellant pellets 325 may be randomly disposed within the rocket motor as shown in FIG. 3.
• the propellant pellets 325 may be stacked in an ordered manner. Many stacking arrangements may be possible depending on the size of rocket motor and the size of the propellant pellets.
• the rocket motor may include rods, guides, or other structure (not shown in FIG. 3) to position and retain the stacked pellets.
• the propellant pellets 325 may be retained within the rocket motor by means of a perforated pellet retainer 335.
• the term "perforated” is intended to encompass any type or shape of openings in the structure of the pellet retainer, and does not imply any particular method of forming or creating the openings.
• the pellet retainer may prevent the unburned pellets from being ejected through the nozzle 340 while allowing passage of the exhaust gases produced when the propellant pellets 325 and the propellant charge 320 are ignited.
• the pellet retainer 335 may be formed of a thin metal material with machined or chemically formed perforations.
• the pellet retainer 335 may be formed of a woven or etched screen or mesh.
• the pellet retainer 335 may be formed as a single physical element, or may include a plurality of physical pieces such as, for example, a plurality of closely spaced metal discs, wires, or rods. However the pellet retainer is formed, the pellet retainer 335 will provide perforations or passages for the exhaust gases to flow to the nozzle 340.
• the perforations may be circular, square, rectangular, or elongated slits, or any other shape that allows passage for the combustion products while retaining the fuel pellets.
• the perforations may be adapted to retain burning fuel pellets within the rocket motor until combustion has reduced the size of the pellets to less than a predetermined size.
• at least one dimension of the perforations or openings in the pellet retainer 335 may be significantly smaller than any dimension of the unburned fuel pellets 325, such that a fuel pellet may not pass through the pellet retainer until combustion of the fuel pellet is nearly complete.
• combustion of the fuel pellets 325 is nearly complete, at least some of the fuel pellets may be swept through the perforations in the pellet retainer 335 into the nozzle 340 by the flow of the combustion gases.
• the pellet retainer 335 may be positioned to form a cavity 332 between the pellet retainer 335 and the throat 342 of the nozzle 340.
• the cavity 332 may be initially devoid of propellant pellets.
• the length of the cavity 332 may be such that at least a portion of the partially burned fuel pellets passing through the pellet retainer are completely burned before the fuel pellets pass through the throat 342.
• Fuel pellets that are completely burned before passing through the throat 342 may fully contribute to the thrust of the rocket motor. Each fuel pellet that is still burning as it is ejected through the throat 342 may result in an incremental reduction in the total thrust provided by the rocket motor.
• pellet retainer 335 may, to some extent, restrict the flow of combustion gases from the burning fuel pellets 325 and the propellant charge 325 to the nozzle 340. To ensure that the pressure within the rocket motor is governed by the cross-sectional area of the throat 342, the total cross-sectional area of the perforations in the pellet retainer 335 may be larger than the cross- section area of the throat 342.
• An igniter 350 may be disposed within the case to ignite the propellant charge 320 and the plurality of propellant pellets 325. Note that the igniter may directly ignite only a portion of the propellant charge 320 and/or the plurality of propellant pellets 325. The hot combustion gases from the ignited propellant may then rapidly ignite the remaining propellant.
• FIG. 4A shows the thrust, as a function of time, produced by three rocket motors using different propellants.
• the graph 400A plots results from simulations of the three different rocket motors.
• the solid line 471 is a plot of the thrust produced by a propellant charge, which is presumed to have a cylindrical longitudinal cavity.
• the surface of the propellant charge may be fully ignited at approximately 0.002 seconds.
• the thrust produced by the propellant charge may increase linearly from 0.002 seconds to 0.016 seconds due to a continuous increase in the burnable surface area.
• the propellant charge with a cylindrical longitudinal cavity exhibits a progressive burn. At 0.016 seconds, the propellant may be consumed, and the thrust may rapidly decrease.
• the solid line 471 represents the result of a simulation of the burn of an idealized propellant charge. Minor localized variations in the burning rate of the propellant charge and end effects may cause the thrust produced by a real rocket motor to deviate from the ideal.
• the dashed line 472 is a plot of the thrust produced by a plurality of propellant pellets.
• the surface of the propellant pellets may be fully ignited at about 0.004 seconds.
• the thrust produced by the propellant pellets may then decrease, roughly linearly, from 0.004 seconds to about 0.016 seconds due to a continuous decrease in the burnable surface area.
• the dashed line 472 demonstrates that propellant pellets provide a regressive burn. At about 0.016 seconds, the propellant pellets may be consumed.
• the simulation assumed the propellant pellets were shaped as short cylindrical tablets, where the thickness of the tablet is smaller than the radius of the cylinder.
• the decreased in thrust between 0.004 seconds and 0.016 seconds is due to the reduction in the diameter and thickness of the cylinder as the pellets burn.
• the burn out at 0.016 seconds occurs when the combustion burns through the thickness of the pellets.
• the rate of decrease of the thrust produced by other types of propellant pellets may be determined by the shape of the pellets, and may not be linear in time.
• the broken line 473 is a plot of the total thrust produced by an exemplary hybrid solid fuel rocket motor.
• the total thrust of the hybrid solid fuel rocket motor is essentially constant from about 0.006 seconds to 0.016 seconds.
• the interval of essentially constant total thrust occurs because the regressive burn (the reduction in thrust over time) of the pellet propellant effectively compensates for the progressive burn (the increase in thrusts over time) of the propellant charge during this time interval.
• a rocket motor where the thrust is essentially constant with time after the initial ignition period is described as having a "neutral burn".
• FIG. 4B shows the internal pressure of the three rocket motors.
• the graph 400B plots additional results from simulations that produced the data previously shown in FIG. 4A.
• the solid line 474 is a plot of the internal pressure of the motor using a propellant charge.
• the maximum internal pressure, which occurs near the end of the progressive burn, is over 10,000 PSIA (pounds per square inch absolute).
• the dashed line 475 is a plot of the internal pressure of the rocket motor using a plurality of propellant pellets.
• the maximum internal pressure, which occurs early in the burn when the pellets are fully ignited, is over 11,000 PSIA.
• the broken line 476 is a plot of the internal pressure of the exemplary hybrid solid fuel rocket motor.
• the maximum internal pressure is about 8000 PSIA.
• FIG. 4B illustrates the principle that, for equal total impulse, a rocket motor with a neutral propellant burn will have a maximum internal pressure that is lower than a rocket motor having either a progressive or regressive propellant burn.
• the external case of a rocket motor must be able to contain the maximum internal pressure.
• a rocket motor having either a progressive or regressive burn may require a stronger, and potentially heavier and more costly, case than a rocket motor having a neutral propellant burn.
• a hybrid solid fuel rocket motor may include a cylindrical propellant charge with a cylindrical central cavity and a plurality of pellets shaped as discs, where the thickness of each disc is substantially less that its diameter.
• the burn time of the propellant charge may be approximately (D C ⁇ max -D c o)l ⁇ 2Rbc), where D C ⁇ tnax is the outside diameter of the propellant charge.
• the burn time of the propellant pellets may be approximately dp/(2Rbp), where d p is the thickness for a disc-shaped a propellant pellet.
• F c U e *R bc * P pc * A c (t) (4)
• F c force produced by the propellant charge
• R hc propellant burn rate
• P pc propellant density
• a c (t) Propellant burning surface area as a function of time.
• N the number of propellant pellets
• R hpc propellant pellet burn rate
• P pp propellant pellet density
• a p (t) Propellant pellet burning surface area as a function of time.
• a hybrid solid fuel rocket motor may have a neutral burn if the following relationship is satisfied:
• FIG. 5 shows an exemplary rocket motor which combines a propellant charge 520 and a plurality of propellant pellets 525 to provide an initial period of high thrust followed by a period of sustained lower thrust.
• This type of rocket motor commonly called a "dual-burn" motor may be useful to quickly accelerate a missile to a cruising speed that may then be sustained.
• the propellant pellets 525 are shown as being smaller than the propellant pellets shown in FIG. 3 to imply that the propellant pellets 525 burn for a shorter period of time. However, as previously discussed, the burn time will be determined by both the shape and the burn rate of the propellant pellets 525.
• the propellant pellets in a dual-burn hybrid solid fuel rocket motor may be smaller than, larger than, or the same size as the propellant pellets in a neutral burn hybrid solid fuel rocket motor.
• FIG. 6 shows a graph 600 which plots the thrust, as a function of time, produced by a hypothetical dual-burn hybrid solid fuel rocket motor.
• the solid line 671 is a plot of the thrust produced by a propellant charge, which is adapted to provide a neutral burn.
• the propellant charge may have a star- shaped center perforation, as shown in FIG. 7 A, or a finocyl perforation, as shown in FIG. 7B.
• the propellant charge is assumed to be fully ignited by time tl and to provide constant thrust until a later time t3.
• the solid line 671 represents a hypothetical result for an idealized propellant charge. Minor localized variations in the burning rate of the propellant charge and end effects may cause the thrust produced by a real rocket motor to deviate from the ideal.
• the dashed line 672 is a plot of the thrust produced by an plurality of propellant pellets.
• the surface of the propellant pellets is assumed to be fully ignited at time tl.
• the propellant pellets then provide high thrust until they are consumed at time t2.
• the burn time, from time tl to time t2, of the propellant pellets may be substantially shorter than the burn time, from time tl to time t3, of the propellant charge.
• the broken line 673 is a plot of the total thrust produced by the hypothetical hybrid solid fuel rocket motor.
• the hypothetical hybrid solid fuel rocket motor provides high thrust from an initial period from tl to t2, followed by sustained lower thrust from time t2 to time t3.
• FIG. 8A and FIG. 8B show two additional exemplary hybrid solid fuel rocket motors.
• Each rocket motor 800A,B may include a propellant charge 820 A,B and a plurality of propellant pellets 825 A,B.
• Each plurality of propellant pellets 825A,B may be retained within the respective rocket motor by an appropriately formed pellet retainer 835 A,B.
• the rocket motors of FIGS. 3, 5, 8 A, and 8B are examples and that a propellant charge and propellant pellets may be integrated within a hybrid solid fuel rocket motor in many other configurations.
• a "set" of items may include one or more of such items.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Testing Of Engines (AREA)
• Solid-Fuel Combustion (AREA)

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• 2008
  • 2008-03-21 US US12/053,475 patent/US7685940B1/en active Active
• 2009
  • 2009-02-10 EP EP09739312A patent/EP2281114B1/en active Active
  • 2009-02-10 WO PCT/US2009/033694 patent/WO2009134510A2/en not_active Ceased
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