Reactive Nano Scale Emulsions for High Performance Hybrid Propulsion

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. An alternative approach to enhancing the combustion performance of hybrid propellants is to disperse liquid fuel additives within a solid fuel binder. Reactive and/or volatile liquid fuels do not possess the same difficulties as many solid additives in igniting and burning completely but can still possess high energy densities and thus are promising candidates for rocket applications aimed at increasing fuel regression rates. Liquid fuels can be homogenously dispersed into a fuel binder via emulsification, stabilized using surfactants and cured to form a biphasic dispersion fuel (BDF) in which volatile liquid fuel droplets comprise a significant portion of the total propellant mass. In addition to the benefits of a liquid additive’s heat of combustion and reduced heat of vaporization relative to the fuel binder, BDF formulations could show an improved combustion performance due to secondary atomization effects, or microexplosions, which result from significant differences in the boiling points of the fuel binder and liquid fuel additive. As emulsified droplets are entrained from the fuel surface, heat is transferred to the interior containing the liquid fuel additive, causing the internal pressure to increase to the point of explosion and subsequent secondary droplet breakup. This secondary droplet breakup could help to increase mixing and reduce the amount of unburned fuel droplets leaving the nozzle. Additionally, if sufficient heat is transferred to the fuel grain during combustion, the trapped liquid near the surface could boil and eventually burst, causing an increase in the fuel burning surface area. Surface explosions and an increased surface area translate to a larger mass transfer rate of fuel to both the turbulent melt layer of the propellant and the flame zone, in turn producing more thrust. Thus, a hybrid fuel composed of a solid fuel binder and a dispersed reactive and/or volatile liquid fuel could potentially exhibit increased combustion efficiency, regression rate, and thrust relative to the current state of the art, without requiring a complex fuel port design. The research presented in this grant focused on investigating the effects of incorporating micron scale liquid fuel droplets into hybrid rocket propellants and evaluating the combustion performance of these synthesized biphasic dispersion fuel (BDF) formulations. The major goal of the research was thus to demonstrate improvements in the regression rate and combustion efficiency of hybrid fuels, relative to the current state of the art, via dispersed liquid fuel additives. The most apparent challenges associated with these research goals were identifying which liquid fuel additives contribute the most significant combustion performance gains, identifying ideal liquid fuel additive/binder/surfactant combinations, and determining which emulsification techniques provide the most scalable, repeatable, and effective method for producing hybrid rocket biphasic dispersion fuel grains. Potential liquid fuel additives were screened for their theoretical effectiveness in enhancing combustion, and their ability to form kinetically stable emulsions when mixed with molten paraffin wax and an appropriate surfactant molecule, as determined via experimentation. Despite the theoretical promise of volatile, hydrogen rich hypergols such as 2,3-dihydrofuran and ethylenediamine, safety and cost concerns limited the scope of the research to include water, ethanol, and formamide as potential additives. These three additives were chosen to provide a baseline combustion performance comparison between neat paraffin fuels and paraffin based BDFs. Water provided a useful non reactive comparison to ethanol and formamide, helping to isolate the effect of microexplosions on the combustion performance of hybrid rocket fuels, without the added benefit of a liquid fuel additive’s heat of combustion. Ethanol was chosen due to its volatility, low cost, and low toxicity. Formamide is inexpensive, has been shown to produce stable non aqueous emulsions in various n-alkanes, and also possesses the highest theoretical density impulse of any liquid fuel additive considered for this research. Thus water, ethanol and formamide were chosen to gain a knowledge base surrounding biphasic dispersion synthesis, casting, and testing prior to moving forward with more exotic additives. BDF formulations were prepared by the dropwise addition of liquid fuel additive into molten paraffin wax while mixing with a high-shear homogenizer in an inert, pressurized, and temperature controlled mixing vessel. Once mixed, emulsified fuels were injected into casting molds and quickly cooled to form cylindrical, biphasic dispersion fuels grains. The regression rates and combustion efficiencies of each biphasic dispersion fuel grain was obtained using an optically accessible hybrid rocket combustor. The combustor features an acrylic chamber and a sacrificial quartz liner which enables instantaneous measurement of the fuel grain diameter throughout each test, with the use of a high-speed camera. High-speed videos were recorded at frame rates between 10000 and 15000 frames per second and processed using a MATLAB code to determine the time and spatially averaged fuel grain diameter at prescribed time intervals. Biphasic dispersions formed between formamide and paraffin were not stable long enough to cast into usable fuel grains, leaving water and ethanol as liquid additives for the formation of biphasic dispersion fuels. Three different biphasic dispersion fuel formulations, each containing 20% liquid additive by mass, were tested in the OCC and compared to the performance of neat paraffin wax, as summarized below in Table 1.1. Note that BDF 1 and BDF 3, as labeled in Table 1.1, each contained 20% water by mass. BDF 2 contained 20% aqueous ethanol (95% ethanol/5% water) by mass. Figure 1 shows the measured mean regression rate as a function of calculated mean oxidizer mass flux for each test. BDF 1 showed up to a 30% increase in regression rate and a 1% increase in combustion efficiency on average, relative to neat paraffin fuels tested at similar conditions. BDF 2 showed up to an 86% increase in regression rate and a 4% decrease in combustion efficiency on average, relative to neat paraffin fuels tested at similar conditions. BDF 3 showed up to a 543% increase in regression rate and a 4% increase in combustion efficiency on average, relative to neat paraffin fuels tested at similar conditions. As seen in high speed videos, the improvements in regression rate for BDF 1 and BDF 2 are attributed to microexplosions that occurred on the fuel surface and in the turbulent boundary layer during combustion. The combustion performance improvements measured for BDF 3 are largely attributed to an increase in grain porosity due to the leaking of emulsified water prior to testing, as determined via x-ray computed tomographic analysis. While the dispersion of liquid fuel droplets within solid fuel binders has shown greater potential to create formative changes to the current state of hybrid rocket fuels, there are several topics that require further investigation in order to mature this technology. The synthesis of a stable biphasic dispersion fuel grain hinges on finding a suitable surfactant or surfactant combination to prevent the separation of emulsified droplets prior to casting and cooling fuel grains. Further testing with different surfactant molecules and liquid fuel additives could produce more effective combinations and provide further combustion performance enhancements. Additionally, polymeric fuel binders, such as HTPB, may be more effective at preventing the migration of dispersed liquid droplets through the fuel binder during storage, and also do not possess the same thermal and structural limitations as paraffin wax. For further details regarding this research, please refer to the following journal publication. Mathews, J. D., Gabl, J. R., and Pourpoint, T. L., “Biphasic Dispersion Fuels for High Regression Rate Hybrid Rockets,” Journal of Propulsion and Power, Vol. 35, No. 5, 2019. doi:10.2514/1.B37431