Electrospray Thrusters in Chemical-Electric Multimode Propulsion for Small Satellites

Abstract

Propulsion for small spacecraft is typically one of two modes, chemical or electric. These modes offer complementary propulsive performance: chemical propulsion provides high thrust and low specific impulse, while electric propulsion provides the inverse. As such, having access to both modes on the same spacecraft (i.e. multimode propulsion) is extremely useful. Unfortunately, the conventional propellants used by chemical and electric thrusters are highly incompatible, making this particularly difficult on small spacecraft that 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 ionic liquids monopropellants, making them the natural propellant for a highly compact form of electric propulsion known as electrospray thrusters. This presents a unique opportunity for a propellant to be shared between two propulsion modes, decreasing required mass and volume to be feasible for small spacecraft. This thesis examines the use of ionic liquid monopropellants in electrospray thrusters for a multimode chemical-electric propulsion system. This thesis focuses particularly on ASCENT, a high-maturity monopropellant with flight heritage in chemical thrusters.

In this work, the performance of ASCENT in the MIT ion Electrospray Propulsion System (iEPS) is extensively characterized. Experimental work includes ion plume diagnostics, indirectly and directly obtained performance estimates, temperature-dependent performance estimates, and extended duration firing behavior. Preliminary studies of similar monopropellants are also conducted to assess their use in a multimode system. To support an upcoming technology demonstration flight, a new multimode-compatible iEPS thruster tank is designed, fabricated, and validated. The integration and operation requirements for this thruster in a flight-ready system are defined. Finally, the mission benefits of an ASCENT multimode system for CubeSats are compared against current commercial options using an Earth observation mission case study.

This work finds that an iEPS thruster with ASCENT propellant has thrust of 9-15 µN, a specific impulse of 600-750 seconds, and a total efficiency of 18-22%, depending on current setpoint. We find that ASCENT is slightly volatile in high vacuum, which causes time-dependent losses in efficiency and specific impulse from gradual propellant evaporation. This volatility may also increase thruster lifetime by mitigating the risk of thruster failure by emitter flooding. This work also identified a modified version of ASCENT, created when the propellant is exposed to iron. This modified version produces a dramatically higher thrust and thrust-to-power compared to standard ASCENT. Additionally, flight-ready configurations of a multimode system are defined for 6U, 12U, and 27U CubeSats. A case study analysis found that the benefits of a chemical-electrospray multimode system are best realized at the 12U scale and above. Overall, this thesis provides critical insights on the performance, integration, and operation of electrospray thrusters with ionic liquid monopropellants. These results can then be used to enable a multimode propulsion system for small satellites.