Prediction of Dual-Mode Spacecraft Propellant Performance of Binary and Ternary Ionic Liquid Mixtures

The theoretical chemical and electrospray performance of [Bmim][dca], [Bmim][NO3], or [Emim][EtSO4] mixed with HAN in aqueous solution for dual mode chemical monopropellant/bipropellant and electrical electrospray rocket propulsion have been investigated. A ternary mixture comprised of 80 percent HAN, 20 percent of each ionic liquid fuel, and no water yielded the maximum specific impulse and chamber temperature: 290 seconds and 2700 Kelvin, respectively. The specific impulse was computed for a 1000 V accelerating voltage. For a binary mixture of [Bmim][dca] and HAN the upper limit on the specific impulse performance was 4655 to 6683 seconds and the lower bound was 2567 to 3745 seconds. For [Bmim][NO3] and Han the limits were 4743 to 6683 seconds and 2595 to 3745 seconds, respectively. For [Emim][EtSO4] and HAN the bounds were 4128 to 6683 seconds and 2380 to 3745 seconds, respectively. Nomenclature Ve = exit velocity, m/s e = fundamental charge, Coulomb ∆φ = change in electric potential, Volt m = accelerated particle mass, kg Isp = specific impulse, s RA = ion fraction g0 = gravitational constant, m/s 2 T = thrust, N ηsys = system efficiency Pin = input power, W W1 = weight ratio of the first ionic liquid (IL) in the solution W2 = weight ratio of the second ionic liquid in the solution R1 = ion fraction of the first ionic liquid in solution R2 = ion fraction of the second ionic liquid in solution x = mixed fraction V1,e = exit velocity of combinations derived from the first ionic liquid in solution V2,e = exit velocity of combinations derived from the second ionic liquid in solution V12,e = exit velocity of an ion of the second IL coupled with a neutral molecule of the first IL V12,e = exit velocity of an ion of the second IL coupled with a neutral molecule of the first IL V12,e = exit velocity of an ion of the second IL coupled with a neutral molecule of the first IL V21,e = exit velocity of an ion of the first IL coupled with a neutral molecule of the second IL Introduction ne of the primary limiting factors of spacecraft mission design is the available propulsion modes. A chemical system is limited by its low specific impulse and an electrospray system is limited by its low thrust performance. A dual-mode propulsion system concept increases the flexibility of spacecraft mission design by utilizing both high-thrust chemical and high-specific O impulse electric propulsion modes 1 . Several other considerations for spacecraft mission design are the mass and volume requirements. It has been shown that a dual-mode propulsion system utilizing a single monopropellant, and therefore a single tank, for both modes is feasible and maximizes mission flexibility 1 . This paper examines the performance of various ternary mixtures in the chemical and electric modes to assess the suitability for use in a dual-mode system. Since the propulsion modes available limit the type of spacecraft maneuvers, through a dualmode propulsion system missions previously inaccessible by a spacecraft utilizing a single mode are accessible due to the availability of both modes. This results in an increase in mission flexibility and adaptability as the mission objectives change. With the availability of both modes a dual-mode propulsion system enables a spacecraft to launch without a well-defined mission plan or thrust history. Furthermore, the mass and volume savings of utilizing a single ionic liquid outweigh the performance drop of the high-thrust chemical and the high-specific impulse electric modes as compared to modern state-of-the-art chemical and electric thrusters utilizing separate propellants. In a dual-mode system the electric thruster uses a fraction of the fuel, thus, despite the increased performance of a bipropellant chemical thruster, a monopropellant system that avoids the retention of unused oxidizer mass would maximize mission flexibility 2 . An ionic liquid is essentially a molten, or liquid, salt. When heated to its melting temperature all salts attain this state; however, room temperature ionic liquids (RTIL’s) remain in this state well below room temperature. Research concerning ionic liquids has begun to increase over the last decade, with the number of papers rising from 120 to 2000, annually 3 . Therefore, properties of many synthesized ionic liquids are still being researched and are unavailable. Current research, focused on identifying energetic ionic liquids suitable for propulsion and explosive applications, has illustrated that certain ionic liquids are combustible at the decomposition temperature 4,5 . This implies that certain ionic liquids may be suitable for spacecraft propulsion as a storable propellant. An electrospray propulsion system is one that uses an applied electric field to extract charged droplets from an emitter array 6 . One of the limitations on electrospray propellants is the vapor pressure. A propellant with a high vapor pressure yields a reduced and uncontrollable emission. However, due to a combination of electrical conductivity and low vapor pressure, some ionic liquids may be suitable propellants for an electrospray system 7 . Emissions for electrospray systems utilizing an ionic liquid can achieve specific impulses of 200-3000 seconds, depending on whether the emission is a charged droplet or a purely ionic regime (PIR) 8 . Based on their physical properties several imidazole-based ionic liquids have been recommended for use in electrospray systems 9 . Objective The focus of this study is to determine the theoretical chemical and electrospray performance of selected ionic liquid combinations. Three ionic liquid combinations are investigated. In the chemical analysis ternary mixtures are considered because water may be required to ensure mixing. For the chemical performance internal chamber temperature and specific impulse will be considered. In the electrospray analysis binary mixtures were investigated to simplify the analysis. This simplification will not significantly affect the results because water has a small molecular weight and is a neutral molecule. For the electrospray performance the specific impulse and thrust will be considered. The ionic liquids investigated in this study were limited to those investigated by Berg and Rovey 1 since they may be suited for performance in both modes, specifically 1-butyl-3-methylimidazolium nitrate ([Bmim][NO3]), 1-butyl-3-methylimidazolium dicyanamide ([Bmim][dca]), and 1-ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO4]). Chemical Performance Analysis Two of the key parameters used to predict the performance of a chemical propulsion system are the internal chamber temperature and the specific impulse. The former consideration provides insight into the feasibility of the propellant combination by providing a value critical to determining the suitability of materials for the thruster. The latter provides a measure of the propellant utilization of the system, analogous to miles per gallon for automobiles. Therefore, a comparison of the anticipated performance of various ionic liquid monopropellants can be obtained through the comparison of these two parameters. NASA’s Chemical Equilibrium with Applications (CEA) serves as a useful tool in gathering information about the anticipated internal chamber temperature and specific impulse of various ionic liquid monopropellants. This program uses numerical methods in tandem with a thermodynamics database to solve systems of nonlinear equations to determine compositions at chemical equilibrium. The current version (CEA2) allows the user to specify what type of system is being considered, provide information about the geometry of the system, and provide information of the species to be simulated. The simulated conditions in CEA were a pressure of 300 psia in a supersonic nozzle with an exit to throat ratio of 50 using an infinite area combustor. Each ionic liquid combination was defined using the elemental composition and the enthalpy of formation of the chemicals. Refer to Table 1 for the chemical formula and enthalpy of formation for the chemicals investigated. Under these conditions CEA was used to simulate ternary mixtures of water, an ionic liquid fuel, and a HAN oxidizer and the resulting chamber temperature and specific impulse. In gathering this data, an initial combination of the zero percent HAN, 100 percent of the ionic liquid, and zero percent water was selected. At this combination the weight percentage of water was increased in increments of five percent and the ionic liquid and HAN ratios were decreased proportionately to their respective weight ratios, in order to maintain balanced coefficients that sum to one. This was repeated until CEA indicated that an equilibrium solution could not be found. Then the HAN weight ratio was incremented by ten percent, the ionic liquid weight ratio was decreased by ten percent, and the water was reset to zero percent. The process was then repeated until for all combinations. The resulting specific impulse and chamber temperature ternary plots can be found in Figure 1. Table 1. Chemical Formulae and Enthalpy of Formation of Chemicals Investigated. Chemical Formula Enthalpy of Formation, ΔHf o [kJ/mol] [Bmim][NO3] C8H15N3O3 -261.4 [Bmim][dca] C10H15N5 206.2 [Emim][EtSO4] C8H16N2O4S1 -579.1