Effect of Ionic Liquid Composition on Colloid Thruster Emission and Thrust Performance

Experiments to explore the effect of the molecular structure of ionic liquid propellants on the emission mode from a colloid thruster are reported. The colloid thruster is an electrospray device that has application in small thrust space missions. These thrusters can be operated at or between two limiting modes, that of droplets or ions. The droplet mode generally produces higher thrust, whereas the ion mode higher specific impulse. A design challenge is to select a propellant that operates best in a desired mode or that has the capability of operating throughout the range from droplets to ions thus offering large turndown ratios. Eight ionic liquids were selected for electrospray experiments: 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-(2-hydroxyethyl)-3-methylimidazolium dicyanamide, 1-butyl-1-methylpyrrolidinium dicyanamide, 1-butyl-3-methylimidazolium hexafluorophosphate, 1,2-dimethyl-3-propylimidazolium tricyanomethide, 1-ethyl-2-methylpyrazolium tetrafluoroborate, allylmethylimidazolium dicyanamide, and 1-butyl-3-methylimidazolium dicyanamide. The electrospray results reveal propellants that operate best in which mode, whether droplet, mixed, or ionic emission regimes. The performance results also provided data to validate a theoretical approach that uses molecular structures to derive quantitative structural property relationships for predicting the mode of emission. Details are presented in a separate paper.