Biphasic Dispersion Fuels for High Performance Hybrid Propulsion

Joshua David Mathews, Purdue University

Abstract

Hybrid rocket propulsion is a crosscutting technology with the potential for significant cost reductions and improvements in safety and simplicity over liquid and solid rocket propellants. Significant strides have been made in the past decade with both paraffin-based hybrid fuels and traditional polymeric hybrid fuels by including solid additives to increase regression rates and thrust levels. Yet low combustion efficiencies and high-mass exhaust products have limited performance gains. This dissertation describes a novel approach to synthesize high performance multiphase hybrid rocket propellants via the formation of biphasic dispersions. The techniques employed involve the emulsification of liquid fuels within solid fuel binders at elevated temperature and pressure to create an internal structure composed of homogenously dispersed micron-sized liquid fuel droplets. The primary goal of the research is to demonstrate augmented regression rates, without reducing combustion efficiency, of the biphasic dispersion fuels relative to traditional hybrid rocket fuels due to the proposed mechanism of secondary atomization. The biphasic dispersion fuels are prepared under an inert, pressurized environment via high-shear homogenization and cast into cylindrical fuel grains. The combustion performance of each biphasic dispersion fuel is analyzed using an optically accessible small-scale hybrid rocket motor that allows for the measurement of fuel regression rates, thrust, and combustion efficiency. Additionally, the average dispersed droplet diameter and internal droplet structures are investigated for each synthesized propellant formulation via optical microscopy and x-ray computed tomography techniques. Three different biphasic dispersion fuels were prepared. The first fuel formulation (BDF-1) contained 75 wt.% paraffin wax, 20 wt.% deionized water, and 5 wt.% surfactant. The second fuel formulation (BDF-2) contained 75 wt.% paraffin wax, 20 wt.% aqueous ethanol solution (95:5 ethanol to water by mass), and 5 wt.% water. The third fuel formulation (BDF-3) contained 79.5 wt.% paraffin wax, 20 wt.% deionized water, and 0.5 wt.% surfactant. The biphasic dispersion fuels formed between water and paraffin wax served as non-reactive cases to validate the effects of secondary atomization and trouble shoot the procedures required to synthesize and cast propellants. Combustion performance tests of BDF-1 showed moderate increases in both regression rate (up to a 30% increase) and mean combustion efficiency (1% increase), relative to neat paraffin wax fuels tested at similar conditions. The biphasic dispersion fuels formed between ethanol and paraffin wax (BDF-2) showed greater increases in regression rate (up to an 86% increase) but moderate decreases in mean combustion efficiency (4% decrease), relative to neat paraffin wax fuels tested at similar conditions. However, emulsion experiments indicated that no surfactant molecule was able to adequately stabilize ethanol and prevent emulsion separation. BDF-2 formulations were prepared by continuously stirring while cooling the mixture until the wax solidified, preventing further droplet separation. Similarly, emulsion experiments indicated that no surfactant molecule was able to adequately stabilize formamide and prevent emulsion separation After modification of the synthesis and casting procedures, a third biphasic dispersion fuel formulation was formed between water and paraffin wax. BDF-3 exhibited substantial increases in regression rate (up to a 543% increase) and moderate improvements in mean combustion efficiency (4% increase), relative to neat paraffin wax fuels tested at similar conditions. Droplet settling velocity analysis performed using backlit digital photography indicated an average droplet diameter on the order of 80 μm but optical microscopy analysis of diluted samples indicated average droplet diameters on the order of 5 μm. Density measurements and x-ray CT analysis of BDF-3 fuel grains indicated that a significant portion of the dispersed water droplets leaked prior to testing, leaving a porous, internal void structure.

Degree

Ph.D.

Advisors

Pourpoint, Purdue University.

Subject Area

Chemical engineering|Aerospace engineering

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