INVESTIGATION OF A MULTIMODAL ELECTROSPRAY PROPULSION SYSTEM FOR SMALL SPACECRAFT

Abstract

Spacecraft have scaled down at a faster rate than the propulsion systems required for orbital maneuvers. As a result, the majority of small satellites launched into orbit do not have propulsion capabilities. The addition of a micro-propulsion system would allow for greater mission capability for earth-orbiting satellites. Electrospray thrusters are a promising micropropulsion technology which could be used to meet the propulsion needs of small satellites, or for fine attitude control of larger spacecraft. Electrospray thrusters operate by applying strong electric fields to electrically conductive propellants which are ejected at high speeds to produce thrust. These thrusters have many benefits over other forms of propulsion such as highly efficient propellant usage, the lack of any compressed gases required for operation, and no requirement for a neutralization process. Combining multiple forms of spacecraft propulsion using a single common propellant is referred to as multimodal propulsion. A multimodal system would potentially provide a thruster with the capacity of selecting either high specific impulse mode or a high thrust mode. Interestingly, electrospray thrusters have the ability to operate in different modes based on the type of particle being emitted. Thus, this thesis investigates a micro-propulsion system which combines a droplet and ion mode electrospray emitter into a unified system using EMI-BF4 as the common ionic liquid propellant. Two porous wedge emitters each were manufactured using Laser milling: a high relative thrust droplet mode emitter fabricated from a commercially available P3 borosilicate glass; and a high efficiency ion mode emitter fabricated from a carbon xerogel substrate (fabricated in-house). To characterize the multimodal thruster, a full beam and time-of-flight (ToF) experimental setup were developed at the RMC Advanced Propulsion and Plasma Exploration Laboratory (RAPPEL). The full beam experimental setup placed the thruster in close proximity of a beam collector aligned with a suppression grid designed to eliminate secondary electrons. A custom Einzel lens and Bradbury-Nielsen gate were used to focus and deflect the emitted beam, allowing for the flights times of emitted particles to be measured at the collector. These times were then used to indirectly estimate the thrust and specific impulse of thrusters operated within the low-pressure Bell-Jar chamber. The electrospray thruster was operated using a custom high voltage bi-polarity switch which alternated between positive and negative voltage of 0 to 3kV every three seconds. Both full beam and ToF measurements were successfully made for the multimodal thruster. The ion mode emitter had an onset voltage around 1400 V with an estimated thrust performance of 0.14 µN and specific impulse of 4040 s. Droplet mode had an onset voltage around 1375 V with an estimated performance of thrust at 14.5 µN and specific impulse of 140 s. The prototype thruster described in this research has demonstrated how various electrospray emitters could be combined into a multimodal system which could provide effective thrust for small spacecraft.