Design of a Bimodal Chemical-Electrospray Propulsion System using Ionic Liquid Monopropellants

Abstract

Two primary propulsion modes currently exist for spacecraft: chemical (e.g. monopropellant, cold gas, solid propellant) and electric (e.g. Hall thruster, ion engine, electrospray). Chemical propulsion typically offers high thrust and low specific impulse, while electric propulsion provides the inverse of low thrust and high specific impulse. As such, having access to both of these modes on the same spacecraft is extremely useful for a wide range of applications. The conventional propellants used by chemical and electric thrusters are highly incompatible, making this particularly difficult for small spacecraft, which lack the mass, power, and volume to accommodate two separate propulsion systems. However, recent advancements in green monopropellants - developed as less-toxic alternatives to hydrazine in chemical monopropellant thrusters - have created a new family of propellants that are also compatible with electric thrusters. In particular, hydroxylammonium nitrate (HAN) based green monopropellants are also ionic liquids, which is the standard propellant for electrospray thrusters. This thesis outlines a design that takes advantage of this to create a bimodal propulsion system with access to both chemical monopropellant and electrospray propulsion. The proposed system builds upon existing technology, commercially available green monopropellant thrusters and the MIT iEPS electrospray thrusters, connected to a single, shared monopropellant tank. The design primarily focuses on propellant conditioning for the electrospray thrusters. Key technical objectives of this design include 1) addressing the need for pressure conditioning of the propellant and 2) ensuring electrical isolation between the thrusters and propellant line during firing. A prototype propellant line was fabricated to test the system and proved that the design sufficiently addresses the technical objectives. This successfully validates the design and proves its feasibility for a bimodal spacecraft propulsion system.