Improving the specific impulse of colloidal electrospray thrusters
Abstract
Electrospray thrusters are unique among electric propulsion due to their inherent scalability and their potential to cover a broad range of specific impulses and thrust. Capillary emitters, in particular, offer advantages such as enhanced controllability and reliability over externally wetted and porous emitters. However, prototypes of capillary emitter electrospray thrusters have so far achieved modest specific impulses compared to other types of emitters. The maximum specific impulse of electrosprays is closely tied to the minimum flow rate at which cone-jets can emit stably. This study investigates how altering the geometry of capillary emitters can increase the specific impulse while maintaining the advantages that capillary emitters offer over externally wetted and porous emitters. Time-of-flight and direct mass flow rate measurements were conducted to characterize electrosprays of various ionic liquids in a vacuum. It was found that electrosprays with emitters with smaller diameters resulted in a lower stable minimum flow rate by more than an order of magnitude compared to those with larger diameters, leading to a two- to threefold increase in the maximum specific impulse.