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.
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