Green Propulsion Advancement: Challenging the Maturity of Monopropellant Hydrazine

Recent commercial and government investments have resulted in substantial advances in the technology readiness of a number of new candidate “green” monopropellants, many of which have already completed, or will complete in the next few years, successful on-orbit demonstrations. These propellants are often touted as potential (and in some cases technology-level ready) replacements for conventional hydrazine. This paper presents a comprehensive, carefully considered set of screening criteria, against which will be evaluated a number of these emergent candidates alongside flight-proven propellants. Thereafter, a thorough assessment of both the performance advantages and disadvantages, as well as current states of the art (and technology readiness levels) for a number of these emerging green technologies as compared with each other and incumbent hydrazine flight systems are provided. Additional discussion is included addressing what it notionally means for a propellant to be green, how definitions vary throughout industry, and the relative importance of “greenness” to other performance, safety, and handling considerations. Finally, results of these studies are used to provide a candid assessment of what types of missions are either enabled by, or best suited to, the demonstrated properties of the green monopropellants vs established propellants, and where cost and risk factors favorably align for near-term infusion into specific missions (e.g., deep space missions requiring low freezing points, microsatellites with limited volume, and military and defense applications).