Monopropellant-Electrospray Multimode Thruster Testing Results: Electrospray Mode

Multimode propulsion is the combination of two or more propulsive modes into a system that shares a single propellant, providing more flexibility than traditional propulsion systems. We report for the first time the electrospray operation of a multimode microthruster that can be switched between chemical monopropellant and capillary electrospray modes. We also demonstrated for the first time that the multimode propellant FAM-110A (59% wt. HAN, 41% wt. [Emim][EtSO4], <1% H2O), which was specifically designed for multimode monopropellant-electrospray propulsion, can be stably electrosprayed from a multi-emitter capillary thruster. Using a six-emitter subset of the thruster, stable electrospray operation is demonstrated for 10+ hours using the conventional ionic liquid propellant Emi-Im, and for 4+ hours using multimode propellant FAM-110A. Retarding potential analysis of the Emi-Im plume shows that the energy profile is nearly identical to that of similarly-sized single capillary emitters in the literature, implying that the thruster is operating in the mixed ion-droplet regime. The effects of hydraulic impedance on electrospray operating regime are briefly discussed. A simple analysis of capillary electrospray thrusters in the literature suggests that an emitter hydraulic impedance of Z > 10¹⁶ Pa-s/m³ is required to reach sufficiently low flow rates to suppress droplet emission.