Electrospray as a source of nanoparticles for efficient colloid thrusters

The need for electric propulsion in the thrust range of tens of micro-Newto ns has triggered the rebirth of colloid thruster technology, The ability to deliver thrust at these levels in a controllable fashion will enhance the use of small satellites (masses smaller than some 20 kg) and the execution of space missions in which very accurate positioning of spacecrafts is requ ired. We present here a novel approach to the field of colloid thrusters. L ike previous efforts it uses electrospray as the mean of producing charged colloids. However, it differs in the spraying regime employed. Electrospray s of highly conducting propellants in single cone-jet mode allow the genera tion of droplets of high specific charge at operating voltages lower than i n the earlier era of colloid thruster research. In this article we use ener gy analysis techniques to characterize electrospray beams and to measure th eir thrust and specific impulse. We have studied three novel propellants: f ormamide, tributyl phosphate, and the ionic liquid 1-ethyl-3-methylimidazol ium bis (trifluoromethylsulfonyl)imide. The thrust, specific impulse, and e fficiency associated with a single spray of the most conducting formamide s olution are typically 0.3 muN, 300 s, and 75 % for an acceleration voltage of 1300 V. The electrospray phenomenology presented in this article is dive rse. For example, by varying the electrospraying parameters we are able to study emission modes in which 1) solvated ions are field evaporated from th e jet's surface, 2) both satellite and main droplets result from the jet's breakup, and 3) only main droplets are emitted.