Travis R. Sippel

Combustion and Characterization of Nanoscale Aluminum and Ice Propellants

Nanoscale aluminum and water propellants have received considerable interest as of late because of their application to underwater and space propulsion. With the increasing availability of nanoaluminum, such propellants are becoming more feasible. Although particle reactivity is enhanced by decreasing particle diameter, storage problems emerge, resulting in shortened propellant shelf life. This paper demonstrates that these storage problems can be mitigated by freezing the water, creating aluminum and ice (ALICE) solid propellants. Theoretical performance, burning rates, and safety (storage stability, impact, ESD, and detonability) results are also presented and comparisons are made with published data. This initial work shows that ALICE may be practical as a propellant that could be applied in a variety of ways including in situ propellant synthesis on the moon or mars.

Dependence of Nano-Aluminum and Water Propellant Combustion on pH and Rheology

Over the past few years, the combustion of nano-aluminum/water (nAl/H2O) propellants has been widely reported, but further progress has been slowed for the following reason: the loosely correlated trends in combustion data are insufficient in guiding further research efforts, and they cannot be used to significantly improve the Isp observed in static rocket motor tests. It was previously found that different mixing techniques (hand, planetary and resonant mixers, duration and temperature), or equivalence ratio gave rise to different burning rates, but the influence of pH and rheology on nAl/H2O propellants was not considered. We find that the effects of pH on nAl/H2O propellants are profound, and correlate well with viscosity, low pressure deflagration limits, burning rate exponents, and rocket motor performance. Our findings suggest that coagulation can influence the pressure exponent over a wide range of values (0.34–0.68). For particle diameters <1 µm, dispersion during mixing is affected more by electrostatic repulsion from charged ions than from mechanical agitation, and this is reflected through zeta potential and viscosity measurements at different pH levels. Additionally, we observe that pH has an influence on nAl/H2O reaction kinetics during ignition, as the propellant transitions from low temperature oxidation to high temperature combustion.

Detailed Characterization of Al/Ice Propellants

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.

Microwave-Supported Plasma Combustion Enhancement of Composite Solid Propellants Using Alkali Metal Dopants

This effort explores the microwave-supported plasma enhancement of a composite solid propellant flame through a novel alkali metal doping technique. The technique enables formation of a high temperature plasma within the propellant flame in order to perturb the burning rate of a composite solid propellant. The aim of this effort is to investigate the use of a microwave-supported plasma combined with an alkali earth metal doping technique to produce unsteady features within the combustion of aluminized composite solid propellant. The technique employs targeted energy deposition to the propellant flame from a subcritical microwave field through the flame-induced ionization of an alkali earth metal atom (i.e. sodium, in the form of sodium nitrate, NaNO3) and formation of high free electron sites within the flame, which serve as microwave energy deposition sites for plasma kernel formation. Equilibrium propellant combustion calculations using ionic chemistry indicate that propellant formulations containing 10-20 wt.% Al and 15-40 wt.% NaNO3 produce the strongest equilibrium flame electron concentrations. However, using a 1 kW uncharacterized multimodal cavity, this effort shows that at 1 atm pressure, low levels of doping (3.5 wt.%) can result in microwave-supported formation of plasmas within the propellant flame that are capable of enhancing propellant burning rate by as much as 67%. High speed filtered and unfiltered video indicates that plasma enhancement frequently initiates in the near-burning surface portion of the flame, where plasma kernel formation is prevalent due to both the abundance of atomic sodium (inferred regions of high free electron concentration) and high localized flame temperatures. These results also suggest the importance of burning aluminum particles as additional localized energy deposition sites via eddy current heating and as a high temperature source to enhance sodium ionization.

Theoretical Performance Analysis of Metal Hydride Fuel Additives for Rocket Propellant Applications

Metal hydrides gained interest as propellant additives in the 1960s, but were dismissed due to their sensitive nature. Recently, the main focus of metal hydride research has been on their use in hydrogen storage, but due to advancements in material synthesis, processing, stabilization and handling, their use as a propellant additive is being revisited. A comprehensive theoretical performance investigation was conducted to determine the affect of metal hydride additives on the performance of both hybrid and liquid propellants. Thermo-chemistry equilibrium calculations were performed to produce theoretical rocket performance properties for each propellant formulation. Dicyclopentadiene (DCPD) and RP-1 were chosen as the hybrid and bi-propellant fuels that would be augmented by the metal hydrides. These fuels were selected for their ability to prevent material dehydrogenation and degradation. The use of specific metal hydride fuel additives is shown to raise a propellant formulation’s overall specific impulse, by as much as 17% when beryllium hydride is considered. Other metal hydrides, such as alane and lithium aluminum hydride, increase propellant specific impulse by approximately 4%. Density specific impulse is seen to increase by 6% and 7% with zirconium hydride and titanium hydride, respectively. By examining each formulation’s performance dependence on its hydrogen content it is revealed that specific impulse tends to increase with volumetric hydrogen density and gravimetric hydrogen content. Density specific impulse on the other hand tends to increase with volumetric hydrogen density, but decrease with increased gravimetric hydrogen content.

Development of a Novel Thermoelectric Propellant Temperature Controller For Strand Burning Studies

Development of a device capable of precise temperature control and rapid thermal equilibration of propellant strands is discussed. The device is comprised of three dual-stage thermoelectrics controlled by a pulse width modulating, proportional, integral, derivative hardware controller and a set of gaseous nitrogen cooling blocks for conveyance of heat out of a pressurized combustion vessel. The device is capable of operating over temperatures ranging from -25 to 70°C and at pressures from vacuum to 11.7 MPa (1700 psi). Operability testing shows the device is capable of thermal equilibration to any defined temperature with an error of ±3.75°C in less than ten minutes. The device will not only be useful in investigating temperature dependence of solid propellant burning rate but will be particularly useful in determining the burning rate effects of phase change in propellants such as aluminum and ice (ALICE) and hydrogen peroxide, aluminum, and water (PALICE) frozen propellants.