Wednesday, September 21, 2011
Tuesday, September 20, 2011
Characterization of Hybrid Rocket Propellant Regression and Performance
Research over the last week focused mainly on the methods employed by others to characterize the regression rate and performance of hybrid rocket fuels. The theory behind hybrid rocket fuel regression was also studied.
For rubberized fuels, regression proceeds through the desorption of high molecular mass particles from the fuel grain surface into a detached boundary layer in the fuel port where combustion occurs in the presence of oxidizer flow. Due to the exothermic nature of the combustion reaction, heat is released largely in this combustion zone. The desorption of fuel particles is governed largely by convective heat transfer to the fuel surface. Radiative heat transfer also plays a role. However, without convection, fuel particles are not carried into the combustion zone. Since heat is generated in the combustion zone, the regression of fuels is determined through the rate of heat transfer and convection that occurs between this boundary layer and the fuel surface within the combustion port. The effects of boundary layer growth and mass flux within the combustion port both play competing roles in the rate of convective heat transfer to the fuel. Their influence is a function of axial position within the fuel port. Near the point of oxidizer injection, the boundary layer is very thin, possessing high temperature and velocity gradients, resulting in high shear stresses and convective heat transfer to the fuel surface. As the boundary layer grows with axial distance from the injector, these effects are less pronounced and reduce the rate of regression. However, the total mass flux in the fuel port increases with axial position due to mass addition in the form of upstream fuel injection. This effect tends to increase the convective heating of the fuel surface with increasing mass flux and axial position. Regression is more sensitive to mass flux than boundary layer growth and is therefor high near the injector, decreasing to a minimum and finally increasing agin until the end of the fuel port. At high oxidizer mass fluxes and lower pressures, the effects of convection dominate radiation. However, at high pressures and low oxidizer mass fluxes, the effects of radiation become significant, increasing regression over that predicted with purely convective models. Such a scenario occurs near the end of motor burn, where the port area has widened, decreasing oxidizer flux values and pressure is high because of increased exposed fuel surface area.
When non-rubber, liquefying, fuels like paraffin are used, additional effects are present that can significantly increase rates of fuel regression. These fuels produce a liquid film of low-viscosity fuel at their surface in a combustion port. Regions of high shear stress in the boundary layer cause this film to destabilize and form ripples, from which fuel droplets are ejected and convected into the combustion zone. These effects tend to amplify the factors that effect the regression of rubberized fuels, giving liquefying fuels much higher burn rates in comparison. Additionally, Hydrogen peroxide can be decomposed in a catalyst bed into oxygen gas and water vapor; these gases, now at temperatures of roughly 800 degrees Celsius, can be used as a hypergolic oxidizer in paraffin based fuel grains. The high injection temperature of the oxidizer serves to further increase the burn rate of the fuel. This concept draws on the ideas of liquid monopropellants and is a good subject for investigation. Other oxidizers like HAN have been proposed as monopropellants and could be investigated.
Common test setups for characterizing the burn rate and performance of hybrid rocket fuels consist of a lab-scale motor with instrumentation and a test stand remotely controlled via USB standoff. A firing program is executed by a computer, which controls the timing of ignition, oxidizer flow initiation, and inert gas purge. Electrically actuated solenoid valves are used to control these events from a distance. Several pressure transducers and a load cell are used to measure the motor's performance.
Oxidizers are pressure fed using an inert gas like nitrogen or helium, regulated down from roughly 2000 psi to operating pressures. Oxidizer flow rates are measured and controlled using various types of venturies. Two that look promising are the tapped venturi with a differential transducer that measures pressure differences between an inlet and nozzle throat to find the rate of flow. Another type is the cavitating venturi. Here, the liquid oxidizer is passed through a converging passage, increasing its velocity and lowing its pressure according to continuity. The velocity is high enough after the converging section to lower the fluid's static pressure bellow its vapor pressure, limiting the flow due to the presence of a gaseous bubble in the divergent section. Thus, the flow is only dependent of a recorded upstream pressure. The cavitating venturi seems to be simpler in construction. Hybrid tests stands use the inert pressurization gas as a post-firing purge. This removes any hazardous oxidizer from the fuel port and injection system.
For a given fuel and chamber pressure, tests are run using thermodynamics software to determine the oxidizer to fuel mass ratio that produces the highest specific impulse. For tests, fuel grain length are varied to change oxidizer to fuel ratios and pressure. Nozzle throat area is varied to change chamber pressure. Lastly, oxidizer flow rate is varied to change average oxidizer mass fluxes in the fuel port. Thrust, firing time, oxidizer flow rate, chamber pressure, fuel mass differences before and after firing, and fuel port diameter changes are measured. These data allow for the calculation of specific impulse along with burn rate and its relationship with oxidizer mass flux and pressure. These experiments will serve as a model for my testing and experimental design.
For rubberized fuels, regression proceeds through the desorption of high molecular mass particles from the fuel grain surface into a detached boundary layer in the fuel port where combustion occurs in the presence of oxidizer flow. Due to the exothermic nature of the combustion reaction, heat is released largely in this combustion zone. The desorption of fuel particles is governed largely by convective heat transfer to the fuel surface. Radiative heat transfer also plays a role. However, without convection, fuel particles are not carried into the combustion zone. Since heat is generated in the combustion zone, the regression of fuels is determined through the rate of heat transfer and convection that occurs between this boundary layer and the fuel surface within the combustion port. The effects of boundary layer growth and mass flux within the combustion port both play competing roles in the rate of convective heat transfer to the fuel. Their influence is a function of axial position within the fuel port. Near the point of oxidizer injection, the boundary layer is very thin, possessing high temperature and velocity gradients, resulting in high shear stresses and convective heat transfer to the fuel surface. As the boundary layer grows with axial distance from the injector, these effects are less pronounced and reduce the rate of regression. However, the total mass flux in the fuel port increases with axial position due to mass addition in the form of upstream fuel injection. This effect tends to increase the convective heating of the fuel surface with increasing mass flux and axial position. Regression is more sensitive to mass flux than boundary layer growth and is therefor high near the injector, decreasing to a minimum and finally increasing agin until the end of the fuel port. At high oxidizer mass fluxes and lower pressures, the effects of convection dominate radiation. However, at high pressures and low oxidizer mass fluxes, the effects of radiation become significant, increasing regression over that predicted with purely convective models. Such a scenario occurs near the end of motor burn, where the port area has widened, decreasing oxidizer flux values and pressure is high because of increased exposed fuel surface area.
When non-rubber, liquefying, fuels like paraffin are used, additional effects are present that can significantly increase rates of fuel regression. These fuels produce a liquid film of low-viscosity fuel at their surface in a combustion port. Regions of high shear stress in the boundary layer cause this film to destabilize and form ripples, from which fuel droplets are ejected and convected into the combustion zone. These effects tend to amplify the factors that effect the regression of rubberized fuels, giving liquefying fuels much higher burn rates in comparison. Additionally, Hydrogen peroxide can be decomposed in a catalyst bed into oxygen gas and water vapor; these gases, now at temperatures of roughly 800 degrees Celsius, can be used as a hypergolic oxidizer in paraffin based fuel grains. The high injection temperature of the oxidizer serves to further increase the burn rate of the fuel. This concept draws on the ideas of liquid monopropellants and is a good subject for investigation. Other oxidizers like HAN have been proposed as monopropellants and could be investigated.
Common test setups for characterizing the burn rate and performance of hybrid rocket fuels consist of a lab-scale motor with instrumentation and a test stand remotely controlled via USB standoff. A firing program is executed by a computer, which controls the timing of ignition, oxidizer flow initiation, and inert gas purge. Electrically actuated solenoid valves are used to control these events from a distance. Several pressure transducers and a load cell are used to measure the motor's performance.
Oxidizers are pressure fed using an inert gas like nitrogen or helium, regulated down from roughly 2000 psi to operating pressures. Oxidizer flow rates are measured and controlled using various types of venturies. Two that look promising are the tapped venturi with a differential transducer that measures pressure differences between an inlet and nozzle throat to find the rate of flow. Another type is the cavitating venturi. Here, the liquid oxidizer is passed through a converging passage, increasing its velocity and lowing its pressure according to continuity. The velocity is high enough after the converging section to lower the fluid's static pressure bellow its vapor pressure, limiting the flow due to the presence of a gaseous bubble in the divergent section. Thus, the flow is only dependent of a recorded upstream pressure. The cavitating venturi seems to be simpler in construction. Hybrid tests stands use the inert pressurization gas as a post-firing purge. This removes any hazardous oxidizer from the fuel port and injection system.
For a given fuel and chamber pressure, tests are run using thermodynamics software to determine the oxidizer to fuel mass ratio that produces the highest specific impulse. For tests, fuel grain length are varied to change oxidizer to fuel ratios and pressure. Nozzle throat area is varied to change chamber pressure. Lastly, oxidizer flow rate is varied to change average oxidizer mass fluxes in the fuel port. Thrust, firing time, oxidizer flow rate, chamber pressure, fuel mass differences before and after firing, and fuel port diameter changes are measured. These data allow for the calculation of specific impulse along with burn rate and its relationship with oxidizer mass flux and pressure. These experiments will serve as a model for my testing and experimental design.
Sunday, September 11, 2011
Project Refinement #1
At present, serious research has been done into energetic materials for possible incorporation into a hybrid rocket's propellant grain. Of these, lithium aluminum hydride, aluminum hydride, ammonia borane, and lithium borohydride have the potential for vast hydrogen storage and energy content. Such additives will contribute greatly to the adiabatic flame temperature during combustion, improving specific impulse. It is also reported in the research that increased heat at the fuel surface and in the chamber can cause more rapid heat transfer to the fuel grain, thereby increasing burn rate. Increased temperatures are also reported to melt or boil the metal oxides that coat liberated metals at the fuel surface and in the gas flow. When these light metals are allowed to more fully combust in the presence of oxidizer flow, more energy is liberated and delivered to gaseous products and the fuel surface. Energetic materials like Mono Methyl Hydrazinium Nitroformate (MMHNF), guanidinium azo-tetrazolate, and other tetrazoles with low sensitivity and negative oxygen balance are considered. They all have the potential to greatly increase the burn rate of the fuel. Compounds containing tetrazole provide large volumes of nitrogen gas upon reaction. The production of nitrogen during propellant combustion lowers the average molecular mass of the gaseous products and proves a high temperature expandable gas, utilized in a nozzle for the evolution of jet kinetic energy. More research is needed to select specific nitrogen-rich compounds for testing. These selections will be made on the grounds of ease of synthesis, stability, cost, and safety.
A polymer chemist at the University of Pittsburgh has been contacted and has agreed to help select and synthesize a polymeric binder for the fuel constituents. While HTPB is commonly used, it has a low density and heat of formation compared to glycidyl azide polymer (GAP), poly bis-azido methyl oxetane (Poly-BAMO), azidomethyl methyl oxetane (AMMO), polyglycidyl nitrate(PGN), and nitrato methyl-methyl oxetane (NMMO). Polymeric binders will be evaluated based primarily on their compatibility with the above energetic fuels. This is important since hydrides and borohydrides of light metals along with ammonia borane are strong reducing agents and may react with binders, plasticizers, curative agents, and possibly other fuel additives. Secondary means of evaluation for the binders include density, heat of formation, and potential reaction products.
Another area of interest is the oxidizer used by the hybrid rocket motor. Nitrous oxide is commonly used and is likely a good starting point. Unless pressure fed, nitrous oxide cannot be injected at pressures above its vapor pressure of 750psi. This seriously limits the achievable chamber pressure and performance of the rocket motor. Hydrogen peroxide is also widely used, however can only be obtained in 30% concentration. This puts lower limits on the reactivity of the fuel constituents in the form of their affinity for oxygen, since the principle oxidizing agent will be water. Luckily, the metal hydrides and borohydrides selected are reactive in water, producing hydrogen gas. This exothermic process will be enhanced by hydrogen peroxide. Another interesting alternative comes from the concept of monopropellant rocket fuels. Here, energetic oxidizing salts can be dissolved at high concentrations in water or the hydrogen peroxide solution. Some possibilities are ammonium nitrate (AN), hydrazinium nitroformate (HNF), and ammonium dinitramide (ADN). Hydroxylammonium nitrate (HAN) is also a possibility if no other strong oxidizers are present to react with its reducing hydroxylammonium cation. Unless easier means of synthesis are found, ADN, will be ruled out for its difficult preparation, including a nitration procedure that must take place at -35 to -45 deg C. Both the synthesis of HNF and its alkyl derivative MMHNF can be readily synthesized provided the availability of nitroform (trinitromethane). If nitroform must be synthesized, the lab time needed to produce HNF and MMHNF increases vastly. Procedures remain simple even if synthesis of nitroform is required. The synthesis of HAN is a straight forward neutralization between nitric acid and hydroxylamine. A high concentration solution of HAN seems promising. Of course, any liquid oxidizer with as low a vapor pressure as water will certainly need a pressure feed system. Prior work has given me quite some experience with high pressure, nitrogen fed, pneumatic systems. Pressure regulated nitrogen systems are optimal for high pressure feed. However a lower cost option would entail the use of bottled CO2. The liquid would be passed through an expansion chamber and pressure regulated to feed the oxidizing fluid at pressures around 800psi.
A polymer chemist at the University of Pittsburgh has been contacted and has agreed to help select and synthesize a polymeric binder for the fuel constituents. While HTPB is commonly used, it has a low density and heat of formation compared to glycidyl azide polymer (GAP), poly bis-azido methyl oxetane (Poly-BAMO), azidomethyl methyl oxetane (AMMO), polyglycidyl nitrate(PGN), and nitrato methyl-methyl oxetane (NMMO). Polymeric binders will be evaluated based primarily on their compatibility with the above energetic fuels. This is important since hydrides and borohydrides of light metals along with ammonia borane are strong reducing agents and may react with binders, plasticizers, curative agents, and possibly other fuel additives. Secondary means of evaluation for the binders include density, heat of formation, and potential reaction products.
Another area of interest is the oxidizer used by the hybrid rocket motor. Nitrous oxide is commonly used and is likely a good starting point. Unless pressure fed, nitrous oxide cannot be injected at pressures above its vapor pressure of 750psi. This seriously limits the achievable chamber pressure and performance of the rocket motor. Hydrogen peroxide is also widely used, however can only be obtained in 30% concentration. This puts lower limits on the reactivity of the fuel constituents in the form of their affinity for oxygen, since the principle oxidizing agent will be water. Luckily, the metal hydrides and borohydrides selected are reactive in water, producing hydrogen gas. This exothermic process will be enhanced by hydrogen peroxide. Another interesting alternative comes from the concept of monopropellant rocket fuels. Here, energetic oxidizing salts can be dissolved at high concentrations in water or the hydrogen peroxide solution. Some possibilities are ammonium nitrate (AN), hydrazinium nitroformate (HNF), and ammonium dinitramide (ADN). Hydroxylammonium nitrate (HAN) is also a possibility if no other strong oxidizers are present to react with its reducing hydroxylammonium cation. Unless easier means of synthesis are found, ADN, will be ruled out for its difficult preparation, including a nitration procedure that must take place at -35 to -45 deg C. Both the synthesis of HNF and its alkyl derivative MMHNF can be readily synthesized provided the availability of nitroform (trinitromethane). If nitroform must be synthesized, the lab time needed to produce HNF and MMHNF increases vastly. Procedures remain simple even if synthesis of nitroform is required. The synthesis of HAN is a straight forward neutralization between nitric acid and hydroxylamine. A high concentration solution of HAN seems promising. Of course, any liquid oxidizer with as low a vapor pressure as water will certainly need a pressure feed system. Prior work has given me quite some experience with high pressure, nitrogen fed, pneumatic systems. Pressure regulated nitrogen systems are optimal for high pressure feed. However a lower cost option would entail the use of bottled CO2. The liquid would be passed through an expansion chamber and pressure regulated to feed the oxidizing fluid at pressures around 800psi.
Saturday, September 3, 2011
Initial direction based off of preliminary research
Hybrid rocket motors offer several theoretical advantages over solid and liquid propulsive systems. When compared to liquid bipropellant systems, hybrids offer increased simplicity and as a result have a lower cost to develop. Also, hybrids with metallized fuel grains are theoretically superior for high speed endo-atmospheric flight since their propellants are more volumetrically energy-dense than liquids. When compared to solids, hybrids offer increased specific impulse and safety due to the ability to throttle a hybrid rocket motor, and even shut it down completely if need be. These characteristics make hybrids the ideal propulsive system for civilian space flight and reliable, cheap, possibly single-stage, transportation. Hybrids have not received as much attention as theory would dictate because of the low regression rate of hybrid propellants and the complex, dynamic nature of their combustion process, which is not fully understood.
These problems lend themselves easily to the two part, engineering project that I currently envision, spanning the classes of Advanced Computer Science and Innovations (ACSI) and Research Science. Both parts can be explored simultaneously. Part one is the application of a Finite Pointset Method (FPM) for Computational Fluid Dynamics (CFD), coupled with iterative equations of gas phase kinetics and equilibrium in order solve the Navier Stokes and Energy Equations for the 3D turbulent flows and dynamic combustion and regression in hybrid rocket motors. My model will be completely free and open to the public for improvement and implementation. Part two is the synthesis and application of green oxidizers (not containing halogens) to be dissolved in low concentration hydrogen peroxide solution. This safe, storable, liquid oxidizer is to be combusted with solid fuel grains containing high energy-density aluminum particles, hydrogen-rich compounds like borohydrides, and nitrogen rich additives like tetrazoles to increase the burn rate. Such additives will be suspended in a polymer matrix of Hydroxyl Terminated PolyButadiene (HTPB).
At this stage, work has already been done on the successful preparation of HTPB composite fuels. Research shows that sodium borohydride is easily acquired and other boranes and borohydrides may be synthesized in the lab. Also, some possible preparations of green oxidizers like ammonium dinitramide (ADN) and HydroxylAmine Nitrate (HAN), and Hydrazinium Nitroformate (HNF) are documented, yet some may be more challenging than others. Further research is needed on the synthesis of these substances so that some may be eliminated due to complexity. Procedures for the synthesis of tetrazoles are documented, as well as their physical and chemical properties. Of course, this research will start small, with the synthesis and characterization of each additive in HTPB and then in combinations. The oxidizer used in initial tests will be standard and well documented like nitrous oxide, hydrogen peroxide, or gaseous oxygen.
These problems lend themselves easily to the two part, engineering project that I currently envision, spanning the classes of Advanced Computer Science and Innovations (ACSI) and Research Science. Both parts can be explored simultaneously. Part one is the application of a Finite Pointset Method (FPM) for Computational Fluid Dynamics (CFD), coupled with iterative equations of gas phase kinetics and equilibrium in order solve the Navier Stokes and Energy Equations for the 3D turbulent flows and dynamic combustion and regression in hybrid rocket motors. My model will be completely free and open to the public for improvement and implementation. Part two is the synthesis and application of green oxidizers (not containing halogens) to be dissolved in low concentration hydrogen peroxide solution. This safe, storable, liquid oxidizer is to be combusted with solid fuel grains containing high energy-density aluminum particles, hydrogen-rich compounds like borohydrides, and nitrogen rich additives like tetrazoles to increase the burn rate. Such additives will be suspended in a polymer matrix of Hydroxyl Terminated PolyButadiene (HTPB).
At this stage, work has already been done on the successful preparation of HTPB composite fuels. Research shows that sodium borohydride is easily acquired and other boranes and borohydrides may be synthesized in the lab. Also, some possible preparations of green oxidizers like ammonium dinitramide (ADN) and HydroxylAmine Nitrate (HAN), and Hydrazinium Nitroformate (HNF) are documented, yet some may be more challenging than others. Further research is needed on the synthesis of these substances so that some may be eliminated due to complexity. Procedures for the synthesis of tetrazoles are documented, as well as their physical and chemical properties. Of course, this research will start small, with the synthesis and characterization of each additive in HTPB and then in combinations. The oxidizer used in initial tests will be standard and well documented like nitrous oxide, hydrogen peroxide, or gaseous oxygen.
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