An opposed flow burner is examined as an instrument to screen and characterize fuel before full-scale hybrid rocket testing. This device requires small amounts (similar to 10 g) of solid fuels, and it can save time and material in early phases of fuel characterization. Although impinging jet configurations have been investigated in the past, the full range of operation of these systems in terms of hybrid rocket motor flowfield conditions has not been fully explored. The regression rate, flame structure, and flame temperature in an opposed burner configuration is investigated, and an analysis to relate the results to hybrid rocket applications is developed. Hydroxyl-terminated polybutadiene, dicyclopentadiene, and paraffin are investigated via an opposed flow burner over an oxidizer mass flux range of 4 to 25 kg/s/m(2). Results show solid-fuel regression rate sensitivity to laminar and turbulent flow regimes. Aluminized hydroxyl-terminated polybutadiene regresses similar to 34% slower than neat fuel in the opposed flow burner. Infrared spectroscopy reveals peak flame temperatures of the samples tested, ranging from 1850 to 2100 K. Although the opposed flow burner is not a perfect representation of hybrid rocket motor operation, it may prove useful in small-scale screening of fuels.

This paper describes the evaluation of paraffin fuel additives with an opposed flow burner. An opposed flow burner can provide regression rate data by consuming solid fuel samples with an impinging gaseous oxidizer jet. Baseline experiments, conducted at oxygen volumetric flow rates ranging from 8 to 50 SLPM, show that hybrid motor and opposed burner results agree for the relative regression rates of HTPB and paraffin. With baseline measurements established, the relative regression rates for polymeric, metal, and energetic additives to paraffin were measured at 25 SLPM. The polymeric additives decrease the opposed burner regression rate as expected, but at a larger magnitude than seen in hybrid rocket literature. Nano and flake aluminum did not readily combust in the opposed burner, due to short residence times and the disparity between aluminum ignition and entrained molten paraffin temperatures. In addition, flake aluminum presumably increased melt layer viscosity and decreased opposed burner regression rates. Because ammonia borane has a decomposition temperature below the vaporization point of paraffin, it likely decomposes within the melt layer. However, the added energy from ammonia borane did not increase regression rates as expected due to slow decomposition. Mechanically activated titanium carbon and pyrophoric titanium chromium manganese increase the regression rate of paraffin by 47% and 31% respectively. It is believed that both additives reacted or decomposed in or near the melt layer, and provided heat to the fuel before entrainment. Though the opposed burner is a useful tool, the results from this study indicate that its applicability may be limited to certain additive types.

In this paper we report the burning rate characteristics of hydrogen peroxide and micron-aluminum propellants. Theoretical calculations show that the sea level specific impulse of this simple binary mixture is comparable to standard composite propellant. In addition, the aluminum particle size, hydrogen peroxide concentration, and mixture ratio can be adjusted over a flat peak performance regime to attain specific thrust profiles and durations. We measured the burning rates in a windowed pressure vessel at pressures ranging from 7 to 14MPa. Results show that mixture burning rates span from 0.5 to 4.5cm/s at 7MPa with power law burning rate pressure exponents ranging from 0.33 to 1.07. In this study, we focus a statistical analysis on the determination of the most influential variables affecting the burning rate and apply a thermal analysis to determine the combustion regimes of these mixtures. The statistical analysis provided a multivariate regression model for the logarithm of the burning rate with a correlation coefficient of 0.93. The model suggests aluminum diameter is the most important factor affecting the overall burning rate, and H2O2 concentration as the most influential variable on the burning rate pressure dependence. The burning rate dependence on theoretical combustion temperature shows two distinct combustion regimes attributed to kinetic and diffusion controlled combustion. A simple thermal analysis confirms the experimental burning rate pressure dependence observed for these two regimes.

Several experimental and theoretical studies over the last few decades have addressed the aluminum-water reaction. Practical applications making use of the heat generated and the products of the aluminum water reaction can be grouped into two main categories: power generation and propulsion. Power generation is typically achieved by feeding the hydrogen produced into a fuel cell. In propulsion applications, the products of the reaction burn at high pressure and are expelled at high velocity through a converging/diverging nozzle. With a focus on propulsion applications, we presented in a previous paper the results of nanoaluminum/ice (ALICE) small-scale static experiments. We showed that ALICE mixtures are stable, as well as insensitive to electrostatic discharge, impact and shock. Since then, a sounding rocket was successfully launched, powered by the ALICE propellant; the first time a propellant of this type has been flown. Although this formulation is not a practical formulation, the flight established a stepping stone for better performing propellant mixtures. Hydrogen peroxide and micron aluminum mixtures are under development and have shown promise to improve performance. Additional characterization with several nano energetic materials and bi-modal mixtures is also reported.

Iron oxide is a commonly used burn rate modifier for composite propellants. One downside to using iron oxide is that it contributes nothing aside from its catalytic effect. In particular, it detracts from the overall performance of the propellant since its addition rapidly decreases the specific impulse of the motor. It is of interest to find a catalyst for solid propellants that contributes, not only catalytically, but also provides added energy and gas to the motor. This investigation focuses primarily on one such substance; an iron complex of the energetic ligand bistetrazolamine (Fe-BTA) that has shown promise recently in the combustion synthesis of ultra high surface area metal foams. Presented here is an investigation into the use of this material as a catalytic burning rate modifier that also contributes to the specific impulse with additional gaseous products and exothermicity. More specifically, an energetic additive is used that dynamically forms catalytic transition metal products that can modify the burning rate. The focus of the investigation is on the potential use of Fe-BTA as a suitable replacement for iron oxide. Other similar metal complexes could also be considered. Tests were performed with a pressurized strand burner with an inert argon atmosphere, ranging in pressure from atmospheric to 100 atmospheres. Equilibrium calculations are performed to show the possible increase in predicted specific impulse over iron oxide catalyzed propellants. Fe-BTA, when added to AP composite in low quantities (less than 1%) shows promise with comparable catalytic burn rate modifying capabilities to iron oxide.