Development of a Restartable, Green-Propellant Thruster for Small Spacecraft

Hydrazine thrusters currently dominate the spacecraft propulsion market. During recent years, health concerns over the use of hydrazine and the possibility of higher performance propellants have led to increased research in ionic liquid propellants as alternatives. Two major “oxidizer” elements have dominated the field, Ammonium DiNitramide (ADN) and Hydroxyl Ammonium Nitrate (HAN). Matching these “oxidizers” with varying fuel elements, stabilizers, and water contents allows propellant developers to tailor propellants to match requirements for a specific research program. There are two major drawbacks for ionic liquid propellants. The first is that the thrusters require a high temperature pre-heat prior to firing. This imposes a time and energy requirement on the propellant system that may not work for all missions. The second drawback is that the higher performance ionic liquid propellants push the combustion temperature to or beyond the material capabilities of existing catalyst materials. The research funded by this fellowship attempted to address these drawbacks by eliminating the catalyst bed entirely. This research does not attempt to eliminate or replace existing ionic liquid thrusters, but to provide an additional use case for ionic liquid propellants. The proposed solution was to adapt a GOX/ABS hybrid rocket to serve as an igniter and combust the ionic liquid in an open chamber. A key technology involved in this proposal was the use of a moderate voltage spark across the fuel grain surface in order to ignite the hybrid rocket. This ignition method provides low energy, reliable relight capability for hybrid rockets. Since the ignition method relies on the fuel surface, the relight capability lasts as long as the fuel. Early work during the fellowship focused on advancing this ignition technology and adapting it for use as an igniter for ionic liquids. Figure 1 shows an early test of the igniter technology. Once the igniter was working reliably, work proceeded on attempting to ignite an ionic liquid. I worked with aqueous HAN at varying concentrations as an analogue for an actual propellant blend. The major reasons for this were that HAN is readily available, the research pointed to HAN decomposition as the first step in most HAN based propellant blends, and the early reactions in HAN decomposition are highly endothermic. This meant that if HAN would reliably decompose, it should be possible to adapt it to ignite ionic liquid propellant blends. The earliest tests used water (0% HAN concentration) in order to work out procedures and test conditions. One primary objective of the water tests was to determine a flow ratio between the igniter flow and the main propellant flow that would quickly bring the test chamber temperature above the decomposition temperature of HAN. Since HAN has a lower specific heat than water, if the igniter could heat water to decomposition temperature, then the HAN should easily decompose. Even with performing the water tests prior to starting HAN testing, the initial HAN tests resulted in uncontrolled HAN decomposition. Figure 2 shows the gas cloud created when the chamber blew out. This test showed that the system could initiate thermal decomposition of HAN, but also showed that the pressure and flow ratios were not set properly. Continued testing eventually led to successful controlled decomposition of HAN. In order to properly control the flow rates in this system and keep the proper top pressure ratios, a significant flow restriction was inserted in the HAN line. This dropped the pressure drop across the injector and thus increased droplet size. Due to this and other issues with the reliability of the test chamber, a new test chamber was needed. A next generation test apparatus was designed to limit the issues with the previous system. The new system eliminated the windows and significantly limited the sealing surfaces. The system also utilized the newest designs for the igniter. This system also features replaceable injection elements to allow for increased control of droplet size and distribution. This system has been fabricated and gone through some preliminary tests; however, no HAN testing has been done on the new system yet.