| Propulsion

Green Liquid Monopropellant Thruster for In-Space Propulsion

Hydrazine has been the standard baseline liquid monopropellant for space propulsion over many decades since replacing hydrogen peroxide. Hydrazine was preferred over peroxide due to its easier storability and higher performance. Like peroxide, hydrazine can be readily decomposed by passing it over a catalyst. For these advantages, hydrazine has a significant drawback: it is toxic (carcinogen and mutagen), and its handling and refurbishment requires special procedures. Accidental spill is a concern, and strict and expensive mitigation procedures are required. In recent years, a need has arisen to find green or reduced toxicity hydrazine substitutes with equal or greater propulsive performance than hydrazine.

Certain ionic liquids offer the potential for better performance with greatly reduced toxicity. Ionic liquids are readily storable and have high density. They also have very low freezing point and a very wide liquidus range. Several formulations of ionic liquid monopropellants have been developed, including the blend called AF-M315E developed by the Air Force Research Laboratory/Edwards AFB. For all the advantages that ionic liquid monopropellants offer, they have one significant disadvantage: extremely low vapor pressure. This makes them storable and provides extremely low toxicity, but the lack of vaporizability makes ionic liquids very difficult to ignite. The current state of the art is to initiate ignition by injecting the monopropellant over a highly preheated specific catalyst. For example, the AF-M315 can be decomposed over an iridium-based catalyst preheated to >400 °C. Thus, a large amount of electrical power is required from the spacecraft.

The innovation developed in this work involves chemical ignition of AF-M315E using a liquid hypergolic compound without the use of a catalyst, avoiding the need for preheat power. The wide liquidus range of ionic liquids such as the AF-M315E means that runaway reactions needed for ignition to occur must be initiated in liquid phase. Therefore, the ignition system must effectively mix the mono-propellant and hypergol liquids together within a small volume and over a high surface area, while providing ample residence time for the two liquids to react. This innovation accomplishes this via a design that integrates injectors for the monopropellant and hypergol liquids, high surface area/small volume/long residence time mixer for the two liquids, and a high-pressure cavity within a single coaxial element.

The applications of the thruster include in-space propulsion for space exploration as well as earthbound missions. The element is geometrically scalable, and multiple such elements can be integrated into a thruster depending on its design requirements. The thruster can be used to propel a variety of spacecraft including CubeSats.

This work was done by Prakash B. Joshi, Jeffrey L. Wegener, and John C. Magill of Physical Sciences Inc. for Glenn Research Center. NASA is seeking partners to further develop this technology through joint cooperative research and development. For more information about this technology and to explore opportunities, please contact here. LEW-19328-1