Microwave ignition of electrically operated propellants

Abstract

Microwave energy is used to ignite and control the ignition of electrically operated propellant to produce high-pressure gas. The propellant includes conductive particles that act as a free source of electrons. Incoming microwave energy accumulates electric charge in an attenuation zone, which is discharged in the form of dielectric breakdowns to create local randomly oriented currents. The propellant also includes polar molecules. The polar molecules in the attenuation zone absorb microwave energy causing the molecules to rapidly vibrate thereby increasing the temperature of the propellant. The increase in temperature and the local current densities together establish an ignition condition to ignite and sustain ignition of an ignition surface of the attenuation zone as the zone regresses without igniting the remaining bulk of the propellant.

Claims

1. A gas generation system, comprising: a combustion chamber; a source configured to generate microwave energy in a defined microwave band between 0.3 GHz and 300 GHz and to provide said microwave energy as an electrical input; and an electrically operated propellant in which an ignition condition of the electrically operated propellant is defined by satisfying both a thermal ignition threshold and an electrical ignition threshold, wherein said electrically operated propellant has a self-sustaining threshold pressure at which the electrically operated propellant once ignited cannot be extinguished and below which the electrically operated propellant can be extinguished by interruption of the electrical input in an extinguishment condition, said electrically operated propellant further including, one or more conductive additives including conductive particles suspended within the electrically operated propellant that increase a conductivity of the electrically operated propellant; and one or more polar additives including polar molecules suspended within the electrically operated propellant, wherein the one or more conductive additives and the one or more polar additives reduce an amount of the microwave energy otherwise required to satisfy the ignition condition and to sustain combustion of the electrically operated propellant, and a controller configured to control the source such that in the ignition condition, the microwave energy accumulates electric charge in the electrically operated propellant in an attenuation zone, with said electric charge being discharged in the form of dielectric breakdowns between the conductive particles through the electrically operated propellant such that a local current density J surpasses the electrical ignition threshold, and wherein in the ignition condition, the microwave energy also rapidly vibrates the polar molecules to increase a temperature of the electrically operated propellant in the attenuation zone to surpass the thermal ignition threshold, whereby the controller controls the source such that the electrical ignition threshold and the thermal ignition threshold are met only at an ignition surface of the attenuation zone whereby only the ignition surface is ignited without igniting a remaining bulk of the electrically operated propellant, to thereby generate gaseous byproducts that pressurize the combustion chamber while maintaining a combustion chamber pressure of the combustion chamber at less than the self-sustaining threshold pressure, wherein the controller is further configured to control the source to vary a phase or frequency of the microwave energy such that an anti-node of the microwave energy tracks the ignition surface as the attenuation zone regresses, wherein in the extinguishment condition the source interrupts generation of the microwave energy to interrupt the electrical input and extinguish the electrically operated propellant.

2. The gas generation system of claim 1, wherein the defined microwave band is a center frequency Fc between 0.3 GHz and 300 GHz plus or minus at most 10% of the center frequency.

3. The gas generation system of claim 2, wherein the center frequency Fc is between 1 and 10 GHz.

4. The gas generation system of claim 1, wherein the conductive particles include metal particles that provide fuel for the electrically operated propellant.

5. The gas generation system of claim 1, wherein said conductive particles constitutes at least 5% by mass of the electrically operated propellant and said polar molecules constitute at least 5% by mass of the electrically operated propellant.

6. The gas generation system of claim 1, wherein said one or more conductive additives and said one or more polar additives are configured such that at least 5% of the microwave energy coupled into the electrically operated propellant is discharged in dielectric breakdowns and at least 5% is absorbed by the polar molecules to satisfy the electrical ignition threshold and the thermal ignition threshold, respectively.

7. The gas generation system of claim 1, wherein a depth of the attenuation zone varies as the attenuation zone regresses.

8. The gas generation system of claim 1, further comprising a nozzle coupled to the combustion chamber to exhaust the gaseous byproducts to produce thrust.

9. The gas generation system of claim 1, wherein said self-sustaining threshold pressure is at least 500 psi.

10. The gas generation system of claim 9, wherein the electrically operated propellant comprises: 50 to 90 percent by mass an ionic perchlorate-based oxidizer; 10 to 30 percent by mass a binder; 5 to 30 percent by mass a metal or polymer based fuel; 5 to 40 percent by mass a metal additive; and 5 to 40 percent by mass the polar molecules.

11. The gas generation system of claim 1, wherein the ignition surface burns as the attenuation zone regresses.

12. The gas generation system of claim 1, wherein the conductive particles provide a free source of electrons for the dielectric breakdowns.

13. A method of generating a gaseous byproduct to pressurize a combustion chamber, the method comprising: providing an electrically operated propellant in which an ignition condition of the electrically operated propellant is defined by satisfying both a thermal ignition threshold and an electrical ignition threshold, said electrically operated propellant having a self-sustaining threshold pressure of at least 200 psi at which the electrically operated propellant once ignited cannot be extinguished and below which the electrically operated propellant can be extinguished by interruption of the an electrical input, said electrically operated propellant including metal particles and polar molecules suspended within the electrically operated propellant to reduce an amount of energy supplied by the electrical input required to satisfy the ignition condition; supplying microwave energy as the electrical input to the electrically operated propellant to rapidly vibrate the polar molecules to increase a temperature of the electrically operated propellant above the thermal ignition threshold; supplying the microwave energy as the electrical input to the electrically operated propellant to accumulate and then discharge electric charge in the form of dielectric breakdowns into randomly oriented local currents that exhibit a local current density J above the electrical ignition threshold thereby satisfying the ignition condition thereby igniting and sustaining ignition of an ignition surface as the electrically operated propellant burns, wherein the metal particles and polar molecules reduce the an amount of the microwave energy otherwise required to satisfy the ignition condition and to sustain the burning of the electrically operated propellant; wherein combustion of the ignited electrically operated propellant generates the gaseous byproduct that pressurizes the combustion chamber; and varying a phase or frequency of the microwave energy such that an anti-node of the microwave energy tracks the ignition surface as the attenuation zone regresses; and extinguishing the electrically operated propellant by interrupting the microwave energy while the electrically operated propellant is subject to a pressure less than the self-sustaining threshold pressure.

14. The method of claim 13, wherein the self-sustaining threshold pressure is at least 500 psi, wherein the electrically operated propellant comprises a perchlorate based oxidizer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an illustration of a gas generation system using microwave ignition of an electrically operated propellant;

(2) FIGS. 2a and 2b are illustrations of an electrically operated propellant with polar molecule and conductive particle additives and microwave stimulation of the propellant to produce dielectric heating and dielectric breakdowns to ignite an ignition surface;

(3) FIGS. 3a and 3b are illustrations of the gas generation system with varying depths of the attenuation zone in the electrically operated propellant based on the concentration of conductive particles;

(4) FIGS. 4a and 4b are illustrations of the gas generation system with varying depths of the attenuation zone in the electrically operated propellant based on the operating frequency of the microwave energy;

(5) FIG. 5 is an illustration of an embodiment of a rocket motor using microwave ignition of an electrically operated propellant;

(6) FIG. 6 is a diagram illustrating the ignition, throttling and extinguishing of the electrically operated propellant by the microwave source; and

(7) FIGS. 7a and 7b are embodiments of a phase-controlled microwave source and a linear actuator for the electrically operated propellant to position the anti-node of the microwave energy at the ignition surface of the electrically operated propellant.

DETAILED DESCRIPTION OF THE INVENTION

(8) The present invention provides for microwave ignition of electrically operated propellant. Microwave ignition overcomes the issue of burn back exhibited by various electrode configurations. Microwave ignition is scalable to combust greater propellant mass to support larger gas generation systems.

(9) Referring now to FIGS. 1, 2a and 2b, a gas generation system 10 includes a source 12 (e.g. a magnetron) configured to generate microwave energy 14 in a defined microwave band between 0.3 GHz and 300 GHz, a mass of electrically operated propellant 16 and a combustion chamber 18. Typically, the walls of the chamber around the mass of propellant are impermeable to the microwave energy and the far wall opposite the mass is permeable to the microwave energy. The microwave band is generally a narrow band above a center frequency Fc. The band is typically at most +/10% of Fc and more typically +/5% of Fc. In certain embodiments, Fc lies between 1 and 10 GHz.

(10) The electrically operated propellant 16 may be any formulation of constituent elements in which an ignition condition of the propellant is defined by satisfying both a thermal ignition threshold and an electrical ignition threshold. To ignite the propellant, the temperature of the propellant must exceed the thermal ignition threshold and the current density J must exceed the electrical ignition threshold.

(11) The base electrically operated propellant 16 includes a combination of constituent elements including an oxidizer 20, a fuel 22, and a binder 24. The oxidizer provides oxygen required to burn the fuel. The binder provides a structural material to bind the fuel and oxidizer. The binder itself is a fuel. Additional fuel may or may not be required. The additional fuel may be a metal-based fuel such as aluminum magnesium or a polymer-based fuel such as Polyvinyl alcohol (PVA) and Polyvinylidene fluoride (Kynar).

(12) To facilitate microwave ignition, the base electrically operated propellant is modified to reduce the amount of microwave energy required to satisfy both the thermal and electrical ignition conditions. Without such modification to the formulation, microwave ignition while theoretically possible would be impracticable.

(13) The base electrically operated propellant may often include an additive containing polar molecules such as H.sub.2O, glycerol, dimethul sulfoxide (DMSO), n-butanol, ethanolamine or 2-ethoxyethanol to mix the constituent elements of the propellant. The polar molecules are then removed during the baking process. This process may be modified by either increasing the original concentration of the polar molecules or changing the baking process such that a concentration of polar molecules 26 remain suspended in the final cured electrically operated propellant. For example, the polar molecule additive may constitute approximately 5 to 40 percent of the mass of the cured electrically operated propellant.

(14) A polar molecule has a net polarity 28 as a result of the opposing charges (i.e. having partial positive and partial negative charges) from polar bonds arranged asymmetrically. Water (H.sub.2O) is an example of a polar molecule since it has a slight positive charge on one side and a slight negative charge on the other. The dipoles do not cancel out resulting in a net polarity. If the bond dipole moments of the molecule do not cancel, the molecule is polar. For example, the water molecule (H.sub.2O) contains two polar OH bonds in a bent (nonlinear) geometry. The bond dipole moments do not cancel, so that the molecule forms a molecular dipole with its negative pole at the oxygen and its positive pole midway between the two hydrogen atoms.

(15) The base electrically operated propellant may include conductive particles in the form of the metal-based fuel. Alternately, one or more additives including conductive particles 30 (e.g., metals such as tungsten, magnesium, aluminum . . . or non-metals such as graphene or carbon nanotubes) may be suspended within the propellant. The conductive particles increase the conductivity of the propellant by acting as a free source of electrons 32. The fuel may comprise a metal fuel such as aluminum or magnesium selected for their IR emittance or heat absorbance properties. The additive may be, for example, gold or tungsten selected for their electrical properties to provide free electrons. The non-fuel additives typically have higher conductivity and provide more free electrons. The conductive additive may constitute approximately 5 to 40 percent of the mass of the electrically operated propellant.

(16) As shown in FIG. 2b, in an ignition condition, incoming microwave energy 14 includes both a magnetic field B 33 an electric field E 34 within an attenuation zone 36 of the propellant that oscillate at the microwave frequency. The electric field E 34 penetrates an attenuation zone 36. The remaining bulk 37 of the propellant is a region of little to no absorption. The electric field E causes the free electrons 32 to accumulate at conductive particles 30 of high potential 35 and be discharged in the form of dielectric breakdowns through the other constituents of the propellant to conductive particles 30 of low potential 39. The charge follows a path of least resistance between conductive particles. These discharges form randomly oriented local currents 38 that exhibit local current densities J that exceed an electrical ignition threshold. The net bulk current across the attenuation zone will be near zero. The randomness of the local discharges largely cancels. There might be a small bias current from the front to the back of the attenuation zone on account of a natural gradient in the amount of energy coupled to the material. Because signal attenuation (absorption) is a percentage of the available microwave power, the signal gets absorbed most rapidly at the front surface.

(17) The electric field E also causes the polar molecules 26 to vibrate rapidly (the net polarity 28 tries to switch with the electric field E but cannot move fast enough), which produces dielectric heating to raise the temperature of the propellant above a thermal ignition threshold in the attenuation zone. Ignition typically occurs around 200 C for most propellants.

(18) In this ignition condition, an ignition surface 40 of the attenuation zone ignites and burns as the zone regresses without igniting the remaining bulk of the propellant to generate gaseous byproducts 42 that pressurize a combustion chamber. The dielectric heating and dielectric breakdowns occur throughout the attenuation zone but most strongly at the ignition surface 40 because of signal attenuation causing it to ignite first. The thermal and electrical ignition thresholds may be satisfied throughout the attenuation zone but need only be satisfied at the ignition surface. The burn rate and regression of the burning propellant means that only the surface will burn as the propellant regresses. Burn back should not be a problem.

(19) In different embodiments, to satisfy both the thermal ignition threshold and the electrical ignition threshold the polar molecules and conductive particles each constitute at least 5% of the mass of the electrically operated propellant. Furthermore, the additives and their relative concentrations are typically selected so that at least 5% of the microwave energy coupled to the propellant is absorbed by the polar molecules for dielectric heating and discharged in dielectric breakdowns. More typically, 70-90% of the energy is directed to dielectric heating and 10-30% to dielectric breakdowns.

(20) In an embodiment, the electrically operated propellant 16 is formulated to exhibit a self-sustaining threshold pressure at which the propellant once ignited cannot be extinguished and below which the propellant can be extinguished by interruption of an electrical input. Sawka's hydroxyl-ammonium nitrate (HAN) based propellant (U.S. Pat. No. 8,857,338) exhibits a threshold of about 150 psi. Villarreal's perchlorate-based propellant (U.S. Pat. No. 8,950,329) can be configured to exhibit a threshold greater than 200, 500, 1,500 and 2,000 psi. In an extinguishment condition, combustion of the propellant is turned off by interrupting of the microwave energy as long as the chamber pressure has not exceeded this threshold.

(21) In an embodiment, the electrically operated propellant 16 comprises an oxidizer such as an ionic perchlorate-based oxidizer of approximately 50 to 90 percent of the mass of the electrically operated propellant, a binder of approximately 10 to 30 percent of the mass of the electrically operated propellant, a metal or polymer based fuel of approximately 5 to 30 percent of the mass of the electrically operated propellant, a metal additive of approximately 5 to 40 percent of the mass of the electrically operated propellant, said metal additive acting as a free source of electrons to increase the conductivity of the propellant and a polar molecule additive of approximately 5 to 40 percent of the mass of the electrically operated propellant. The electrically operated propellant is configured to ignite and extinguish according to the respective application and interruption of microwave energy. The electrically operated propellant has a self-sustaining threshold pressure at which pressure the propellant once ignited cannot be extinguished and below which the propellant can be extinguished by interruption of the microwave energy.

(22) Referring now to FIGS. 3a-3b and 4a-4b, a penetration depth 50 of an attenuation zone 52 in a propellant 54 is by convention defined as the depth at which 1/e.sup.2 (13%), where e is base of the natural log, of a signal energy 56 at center frequency Fc from a source 58 remains assuming an anti-node (peak) 60 is positioned at an ignition surface 62. These peaks of amplitude, called anti-nodes, are the points at which energy is transferred into the propellant most rapidly. The desired penetration depth 50 may depend on such factors as the grain size of the propellant, the size of a rocket motor engine, thrust requirements. The penetration depth 50 may vary as the propellant regresses to accommodate initial ignition conditions versus ongoing burn or changing thrust requirements. A remaining bulk 64 of the propellant is a region of little to no absorption.

(23) In general, the penetration depth is a function of the nature and concentration of the polar molecules and conductive additive and the microwave frequency Fc. As shown in FIGS. 3a-3b, the more absorbing or the greater the concentration of either additive the shallower the penetration depth 50. The concentration may be varied throughout the propellant to change the depth of the attenuation zone as the propellant burns and regresses. As shown in FIGS. 4a-4b, the lower the frequency (longer the wavelength) the shallower the penetration depth 50. The frequency may be varied to change the depth of the attenuation zone as the propellant burns and regresses.

(24) Referring now to FIGS. 5 and 6, an embodiment of a rocket motor 100 includes a source 102 (e.g. a magnetron) configured to generate microwave energy 104 in a defined microwave band between 0.3 GHz and 300 GHz. A mass of electrically operated propellant 106 is contained in a microwave impermeable case 108. A microwave permeable wall 110 defines a combustion chamber 112 to which a surface 114 of the propellant is exposed. A nozzle 116 is coupled to the combustion chamber to exhaust high-pressure gas 118 to produce thrust. A controller 120 is configured to control source 102 to set and possibly vary the amplitude and frequency of microwave energy 104 to control the burn rate, hence the thrust.

(25) In an ignition condition, controller 120 configures source 102 to generate microwave energy 104 at a frequency Fc and of sufficient amplitude 122 such that the polar heating component 124 and local current density J component 126 in an attenuation zone 127 exceed their respective thermal ignition threshold 128 and electrical ignition threshold 130 at surface 114 to ignite and burn the surface. Burning of the propellant produces the high-pressure gas 118 at a chamber pressure Pc 132.

(26) In a throttle condition, controller 120 may throttle the burn rate, hence chamber pressure 132 and the resulting thrust up and down based on mission requirements. This would typically be done by varying the amplitude 122 of the of the microwave energy. Alternately, the controller may alter the frequency or phase of the microwave energy to vary the amount of energy coupled to the propellant.

(27) In an extinguishment condition, provided the chamber pressure Pc 132 has not exceeded a self-sustaining threshold pressure 134 of the electronically operated propellant, the controller 120 may extinguish combustion of the propellant by interrupting the microwave energy. The controller may re-ignite the propellant by satisfying the ignition condition.

(28) As long as the self-sustaining threshold pressure 134 is not exceeded, the controller 120 can turn the ignition of the propellant on and off at will. When combined with the high self-sustaining threshold pressures provided by Villareal's formulation, the controller has the flexibility to deliver a dynamic and controllable thrust profile such as might be used to control the motion and displacement of a vehicle or the rate and duration of the inflation of an automotive airbag.

(29) As previously discussed, the peaks of amplitude of the microwave energy, called anti-nodes, are the points at which the microwave energy is transferred into the propellant most rapidly. As such, it is generally desirable to position the anti-node at the ignition surface at ignition and maintain that position as the propellant burns. For conciseness, the reference numbers used in FIG. 5 are used here for like elements.

(30) Referring now to FIG. 7a, in an embodiment, controller 120 modulates the phase 140 (or frequency) of the microwave energy 104 is modulated such that an anti-node 142 tracks the position of the ignition surface 114 as the attenuation zone 127, hence surface 114 regresses. This can be done open-loop based on a priori knowledge of the burn rate and regression of the propellant or closed-loop using a sensor to detect the position of surface 114 or rate of regression

(31) Referring now to FIG. 7b, in an embodiment, an actuator 150 displaces the propellant 106 to hold the ignition surface 114 in place at a fixed anti-node 152 of the microwave energy. The actuator may comprise one or more springs, a linear actuator, a screw drive, hydraulic pistons or high-pressure gas and valves. The high-pressure gas in the chamber may be partially diverted behind the propellant to assist the actuator.

(32) While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.