Propulsive Performance of Mechanically Activated Aluminum–Water Gelled Composite Propellant

Abstract: Aluminum-water combustion method has been advocated to produce hydrogen and synthesize ceramic materials with high purity. In the present study, aluminum-water combustion was employed to synthesize alumina, along with possible co-generation of power. The exhaust gases from aluminum-water combustion comprises of 70% (by mole basis) of hydrogen. This hydrogen gas was burnt with excess air, around an A/F ratio of 16:1 to produce large mass flow rate of gases at temperatures around 1873 K. These gases could be further used to run a turbine for generating power. The possible power generation for this method was estimated to be 6.37 MW per kg of aluminum. The purity and the fraction of alumina in the residue of aluminum-water combustion was mainly influenced by oxidizer to fuel (O/F) ratio. The stoichiometric O/F ratio of 1 resulted in 94% pure α-alumina at all pressures, which was confirmed by X-Ray diffraction (XRD) analysis.

Abstract: This paper presents a model for the purpose of estimating the fraction of aluminum powder that will form agglomerates at the surface of deflagrating composite propellants. The basic idea is that the fraction agglomerated depends upon the amount of aluminum that melts within effective binder pocket volumes framed by oxidizer particles. The effective pocket depends upon the ability of ammonium perchlorate modals to encapsulate the aluminum and provide a local temperature sufficient to ignite the aluminum. Model results are discussed in the light of data showing effects of propellant formulation variables and pressure.

Abstract: The combination of aluminum and water was theoretically analyzed to assess its performance potential for space propulsion, in particular for microrocket applications and whenever a compact package is desirable. Heat of reaction, impulse density, and handling safety are features making this combination interesting for chemical thrusters, especially because thrust is higher than typical of satellite electric thrusters. Ideal specific impulse I s p , thrust coefficient, adiabatic flame temperature, and combustion products were calculated for chamber pressures 1-10 atm, nozzle area ratios 25-100, and mixture ratios O/F 0.4-8.0. I s p reaches up to 3500 m/s. Also, the effect of hydrogen peroxide addition to aluminum and water on performance was explored. This combination improves performance slightly at the expense of simplicity, making it less attractive for microrocket engines. Ignition delay times were conservatively estimated assuming that aluminum was coated with its oxide and ignition occurred after the melting of the aluminum oxide. For this purpose heating and kinetics times were evaluated, the first by a one-dimensional physical model, the second by a reduced scheme. Results indicate that the heating time of a 0.1-μm-diameter aluminum particle may be of order 0.4 μs, whereas overall kinetics takes 10 μs: thus, the Al/water combination looks practical in principle for microrocket chambers characterized by short residence times.

Abstract: Micrometer-sized aluminum is widely used in energetics; however, performance of propellants, explosives, and pyrotechnics could be significantly improved if its ignition barriers could be disrupted. We report morphological, thermal, and chemical characterization of fuel rich aluminum-polytetrafluoroethylene (70–30 wt-%) reactive particles formed by high and low energy milling. Average particle sizes range from 15–78 μm; however, specific surface areas range from approx. 2–7 m2 g−1 due to milling induced voids and cleaved surfaces. Scanning electron microscopy and energy dispersive spectroscopy reveal uniform distribution of PTFE, providing nanoscale mixing within particles. The combustion enthalpy was found to be 20.2 kJ g−1, though a slight decrease (0.8 kJ g−1) results from extended high energy milling due to α-AlF3 formation. For high energy mechanically activated particles, differential scanning calorimetry in argon shows a strong, exothermic pre-ignition reaction that onsets near 440 °C and a second, more dominant exotherm that onsets around 510 °C. Scans in O2-Ar indicate that, unlike physical mixtures, more complete reaction occurs at higher heating rates and the reaction onset is drastically reduced (approx. 440 °C). Simple flame tests reveal that these altered Al-polytetrafluoroethylene particles light readily unlike micrometer-sized aluminum. Safety testing also shows these particles have high electrostatic discharge (89.9–108 mJ), impact (>213 cm), and friction (>360 N) ignition thresholds. These particles may be useful for reactive liners, thermobaric explosives, and pyrolants. In particular, the altered reactivity, large particle size and relatively low specific surface area of these fuel rich particles make them an interesting replacement for aluminum in solid propellants.

Abstract: The efficacy of using aluminum-water and magnesium-water as propellants for underwater thruster applications has been investigated by the authors. The theoretical specific impulse for both reactant systems is high, and the products of reaction (alumina, magnesia, and hydrogen) are environmentally benign. The attractiveness of these systems as “green” propellants has been commented on previously, however, no practical experimentation with these systems has been made. The present work describes the testing of a linear combustor with magnesium-water and aluminum-water under conditions of pressure and oxidizer-fuel ratios and with a metal powder feed system that could be employed in actual rocket engines. Measurements of off-design specific impulse are compared with theoretical predictions that take into account two-phase losses. Measurements of heat fluxes available to vaporize regeneratively the liquid water oxidizer are presented as well. Perhaps of most importance, observations of the degree of product oxide accumulation in the combustor are presented. These measurements and observations are used to determine the effectiveness of these two metal fuel systems as practical green propellants.

Abstract: The combustion of mixtures of an ultrafine electroexplosive aluminum powder with water thickened by a 3% polyacrylamide additive is investigated. The reaction in a combustion regime is accompanied by the formation of a superheated foamy layer in gel-like water. The incompleteness of aluminum burnout in a stoichiometric mixture, which is explained by boiling-out of water from the reaction zone, is shown. The maximum combustion temperatures are determined in various conditions by means of thermocouple measurements and combustion-product composition calculations. The possibility of producing ultrafine or monolithic corundum as a reaction product is shown.