Terrence L. Connell

Combustion of Frozen Nanoaluminum and Water Mixtures

mixtureexhibitedalinearburningrateof4. 8c m∕satapressureof10.7MPaandapressureexponentof0.79.Three motors of internal diameters in the range of 1.91–7.62 cm were studied. Grain configuration, nozzle throat diameter, and igniter strength were varied. The propellants were successfully ignited and combusted in each laboratory-scale motor, generating thrust levels above 992 N in the 7.62-cm-diam motor with a center-perforated grain configuration (7.62 cm length) and an expansion ratio of 10. For the 7.62 cm motor, combustion efficiency was 69%, whereas the specific impulse efficiency was 64%.Increased combustionefficiency and improvedease of ignitionwere observedat higher chamber pressures (greater than 8 MPa).

Molecular Aluminum Additive for Burn Enhancement of Hydrocarbon Fuels

Additives to hydrocarbon fuels are commonly explored to change the combustion dynamics, chemical distribution, and/or product integrity. Here we employ a novel aluminum-based molecular additive, Al(I) tetrameric cluster [AlBrNEt3]4 (Et = C2H5), to a hydrocarbon fuel and evaluate the resultant single-droplet combustion properties. This Al4 cluster offers a soluble alternative to nanoscale particulate additives that have recently been explored and may mitigate the observed problems of particle aggregation. Results show the [AlBrNEt3]4 additive to increase the burn rate constant of a toluene-diethyl ether fuel mixture by ∼20% in a room temperature oxygen environment with only 39 mM of active aluminum additive (0.16 wt % limited by additive solubility). In comparison, a roughly similar addition of nano-aluminum particulate shows no discernible difference in burn properties of the hydrocarbon fuel. High speed video shows the [AlBrNEt3]4 to induce microexplosive gas release events during the last ∼30% of the droplet combustion time. We attribute this to HBr gas release based on results of temperature-programmed reaction (TPR) experiments of the [AlBrNEt3]4 dosed with O2 and D2O. A possible mechanism of burn rate enhancement is presented that is consistent with microexplosion observations and TPR results.

Aluminum-Ice (ALICE) Propellants for Hydrogen Generation and Propulsion

An experimental investigation was conducted to determine the relative propulsive performance and viability of novel solid propellants comprised of ALICE using fundamental techniques such as steady-state strand experiments and applied experimentation such as labscale static fire rocket tests. Burning rates, slag accumulation, thrust, and pressure are some of the experimental parameters obtained. System scaling has been performed to examine the effect of larger systems on slag accumulation and performance parameters. The effect of pressure on the linear burning rate was examined and correlated using a Saint Roberts’s law fit. The pressure exponent for ALICE was 0.73, which is approximately a factor of two larger than Al/water mixtures. Three sizes rocket motors ranging from internal diameters of 0.75 to 3-in. Nozzle throat diameter and igniter strength were varied. It was found that ALICE propellants successfully ignited and combusted in each lab-scale rocket motor, generating thrust levels above 223 lbf for expansion ratios of 10 and center-perforated grain configurations (3-in length). For the 3-in motor, combustion efficiency was around 70%, while the specific impulse efficiency was 64%.

Characterization of Ammonia Borane (NH 3BH 3) Enhancement to a Paraffin Fueled Hybrid Rocket System.

Ammonia borane was studied as an additive to a paraffin wax solid fuel for a hybrid rocket system using gaseous oxygen as the oxidizer. Ammonia borane is a hydrogen rich material (19.6% hydrogen by weight) that has received recent attention as a means for chemically storing hydrogen in a solid form. Equilibrium calculations show that the addition of ammonia borane raises chamber temperatures and lowers the average molecular weight for the products of combustion, both of which correspond to an increase in propellant performance. Static motor firings were conducted using a laboratory scale hybrid rocket engine. Solid fuel compositions included paraffin wax solid fuel with a 2 percent by mass addition of carbon black and similar formulations containing various amounts of ammonia borane. Average regression rate, chamber pressure, and thrust were measured to evaluate the propellant’s performance. A 10 percent addition of ammonia borane increased the regression rate of the solid fuel, but experiments with higher quantities of ammonia borane (20 and 50 percent) showed a decreased regression rate. However, characteristic velocity and specific impulse were both increased with the addition of ammonia borane. These data suggest ammonia borane may be a good performance enhancer for applications requiring high specific impulse, such as space propulsion.

Experiment and Semi-Empirical Modeling of Lab-Scale Hybrid Rocket Performance

A simplified semi-empirical predictive model was developed to aid in the determination of operating parameters and chamber specifications for a lab-scale hybrid rocket engine and test sled design. The model combines user defined initial operating and system design parameters with empirically derived regression rate correlations, NASA CEA2000 combustion equilibrium analysis results, and conservation of mass derivations. The model facilitates parametric optimization of oxidizer flow, chamber pressure and nozzle throat diameter, through a time resolved series of functions, deriving output parameters including characteristic velocity, combustion temperature, efficiency, chamber pressure, thrust, and specific inertia. Experiments were conducted using polymethyl methacrylate (PMMA), hydroxyl-terminated polybutadiene (HTPB) and gaseous oxygen. Experimental results indicates HTPB regression rate exceeds PMMA by a factor of 2 for a given oxidizer flow rate and nozzle parameters. Additionally, the results show, a simplified model of the hybrid combustion system is sufficient to adequately predict combustion parameters in a lab-scale hybrid rocket motor.

Ammonia Borane Based-Propellants

The combustion of ammonia borane (AB) is studied as a potential fuel or propellant ingredient for propulsion applications. Theoretical equilibrium analyses indicate that due to the high hydrogen content of AB and moderate exothermicity of decomposition, it has the potential to boost Isp, thereby increase performance. Various concept experiments are performed with AB as an ingredient in liquid and solid fuels as well as an ingredient in conventional and novel composite propellants. Results show that burning rates are generally not affected or increased slightly with the addition of AB. In liquid fuel droplet experiments where AB is dissolved in methanol, decomposition of AB can occur below the vaporization temperature of the methanol and potentially lead to droplet fragmentation during combustion. In paraffin-based solid fuels, AB does not affect the regression rate when burned with pure oxygen, but has the potential to improve rocket performance in terms of specific impulse due to AB’s desirable combustion products. From spectroscopic emission measurements significant amounts of BO2 are observed indicating conversion of elemental boron to boron oxide products. In mixtures of stoichiometric aluminum and water, the burning rate increased with AB addition as long as it remained dissolved in the water. In preparation of various formulations containing AB, ammonia borane was found to have material compatibility problems with several ingredients including nanometer metal oxide particles, such as copper oxide and molybdenum trioxide, and cross linking agents for hydroxylterminated polybutadiene.

Effect of Fuel Type on Hypergolic Ignition of Hydrogen Peroxide with Gelled Hydrocarbon Fuel

Experimental counterflow and impinging jet studies and modeling analysis of hypergolic hydrogen peroxide and gelled hydrocarbon fuel mixtures were conducted to characterize condensed phase reaction rates and ignition delay times. The gelled hydrocarbon mixtures consisted of nheptane and fumed silica and sodium borohydride particles. The present results were compared with previously obtained results for similar gels, but with dodecane instead of n-heptane. Scanning electron microscopy, x-ray photoelectron spectroscopy, and simultaneous thermogravimetric and differential scanning calorimetry analysis of the sodium borohydride particles were performed to characterize particle size, size distribution, geometry, surface composition, and thermal decomposition. Preliminary rheological characterization of several gelled fuels is also reported. Counterflow experiments, conducted over a range of flow rates, hydrogen peroxide concentrations, and particle loadings, show heat release from the condensed phase reaction increases with sodium borohydride particle loading, hydrogen peroxide concentration, and residence time. These experiments were used to derive a global rate constant for the condensed phase reaction between hydrogen peroxide and sodium borohydride. The derived value was within 21% of the rate constant value obtained with dodecane as the fuel versus n-heptane. Using the condensed phase global reaction and rate constant and a detailed gas phase mechanism for n-heptane oxidation, chemical kinetics calculations were performed to interpret the effect of sodium borohydride loading on ignition delay. Impinging jet experiments indicated ignition delay decreased from 78 to 36 ms as the sodium borohydride weight percent was increased from 3 to 7 weight percent. Model results indicate the same trend. Compared to n-dodecane, ignition delays for n-heptane were longer for a given sodium borohydride particle mass loading, however, when considered on a molar basis, ignition delay times for the two fuels converge. These results imply, for a gelled distillate fuel such as RP-1 (or kerosene), the presence of lighter component hydrocarbons in the liquid fuel does not necessarily result in reduced ignition delay time. Rather, these results suggest, on a mass basis, a higher loading fraction of reactive particles would be required for a lower molecular weight hydrocarbon fuel to achieve ignition delay times comparable to a higher molecular weight fuel.

Combustion of Aluminum, Aluminum Hydride, and Ice Mixtures

The combustion of nano-aluminum, alane, and ice mixtures is theoretically studied. A multi-zone theoretical framework is established by solving the energy equation in each zone and matching the temperature distribution and the heat flux at the interfacial boundaries. The effect of replacing a portion of nano-aluminum particles with micron-sized aluminum and alane particles is examined in the pressure range of 1-10 MPa and for an additive mass fraction of 25%. The addition of micron-sized alane particles results in lower flame temperatures due to the dehydrogenation reaction of alane particles prior to their ignition. The lower flame temperatures and the longer burning times of micron-sized alane particles is responsible for lower flame speeds of these mixtures. For bimodal aluminum-ice mixtures, the lower mass fraction of alumina causes an increase in the flame temperatures. However, the mixtures exhibit mildly lower flame speeds, in view of the longer burning times of micron-sized aluminum particles. The flame thickness of a bimodal mixture of aluminum particle mixtures is higher than that of mono-dispersed mixtures due to the prevalence of two distinct reaction zones. The model results are well supported by experimentally measured burning rates.