Design and Testing of a Propellant Management System for Bimodal Chemical-Electrospray Propulsion

The most prominent modes of propulsion for spacecraft are chemical and electric propulsion, and missions that require maneuvering need to select one mode over the other, or in some cases need to carry two separate propulsion subsystems, including management of incompatible propellants. This is particularly limiting for small spacecraft, which lack the volume, mass and power resources to accommodate high-performance propulsion components. Most systems are then confined to either the chemical mode (typically high thrust, low specific impulse) or the electric mode (typically low thrust, high specific impulse). However, recent development of green monopropellants as replacements to hydrazine in chemical thrusters has created a family of propellants that are also compatible with electric thrusters. Notably, the monopropellant AF-M315E/ASCENT and other hydroxylammonium nitrate (HAN) based ionic liquids can be used in electrospray thrusters. This work proposes a bimodal propulsion system that integrates a chemical mono-propellant thruster with electrospray thrusters into a unified system with common propellant. The design, fabrication, and validation of a propellant feed line connecting the central tank to electrospray thrusters is presented here. Key elements of this design include solenoid valves for flow control and capillary tubes for pressure conditioning. A subsequent section of nonconductive tubing allows for water (present in the composition of many HAN-based propellants) to evaporate at its vapor pressure and form bubbles in the propellant line. These bubbles physically segment the conductive ionic liquid and provide electrical isolation between the electrospray thruster and the central tank; this is required to apply high voltage to operate the electrospray thrusters without current leaks or shorts. A prototype propellant line was fabricated and its flow characteristics and electrical impedance were measured in a series of tests to validate the design. These tests successfully demonstrated the feasibility of this design for a bimodal spacecraft propulsion system.