Performance and Applications of Ionic Electrospray Micro-Propulsion Prototypes

Electric propulsion systems using electrospray ion sources are a scalable and high specific impulse technology which could enable small spacecraft to perform high ΔV missions. This paper presents an overview of a recently developed source design which achieves 10’s of μN of thrust at less than 1 W of input power with a specific impulse up to ∼3000 s. Demonstration devices, fabricated conventionally without microfabrication methods yet with an active area of ∼1 cm, emit positive and negative ion beams from an ionic liquid passively supplied from a coupled porous reservoir. Directly measured thrusts (up to ∼28 μN) from two simultaneously operating thruster modules are shown to be consistent with summing the calculated total force from each module. The two particle beams are configured to be at opposite polarity, in progression towards a charge neutralized system without a dedicated neutralizer. Influences of reservoir pore size and filling state are discussed in the context of performance and lifetime. Specifically, recent results have demonstrated that increasing the reservoir pore size can induce significant droplet or heavy particle populations within an otherwise ionic beam. Large reductions in specific impulse and propulsive efficiency due to these transitions are discussed here. For example, the calculated specific impulse of a negative EMI-BF4 beam could be reduced from ∼2800 s to ∼700 s by changing the reservoir pore size alone. Meanwhile, capillary actions within the reservoir aid in containing liquid via a negative Laplace pressure, thereby preventing lifeending liquid to extractor grid bridges/shorts. Finally, the technology status is reviewed through highlighting critical developments required to arrive at a functional, and applicable, propulsion system.