Motivation and background
Xenon has long dominated the Electric Propulsion (EP) market as the de-facto standard propellant for Hall Effect Thrusters (HETs). Its ongoing use however is constrained fundamentally by its high cost, scarcity, and volatile pricing. Research to date on alternative propellants for HETs has focused on elements such as krypton, on the halogen iodine, and on a variety of metals like magnesium, zinc, and bismuth. Within this context, water has emerged as a particularly promising candidate.
Water is ubiquitous on Earth, is indubitably lower cost than xenon, and comes with a variety of potential system level benefits. Water can be stored as a liquid at densities higher than krypton or argon. Additionally, as it can be stored at low pressures propellant tank mass can be saved compared to xenon. Water is also easy to transport and is non-hazardous. This removes the need for specialized propellant loading equipment and personnel, reducing launch site activity complexity and associated costs. Another major advantage of water is its potential for in-situ resource utilization (ISRU).
Encouraged by these promising properties, the IPPL are carrying out research and development for a dual-mode chemical-electric propulsion architecture that uses water as a unified propellant (see Figure 1). When higher thrust manoeuvres are required, the dual-mode system relies on a MEMS based bi-propellant chemical thrust, the Iridium Catalyzed Electrolysis (ICE) thruster. When high mass utilization efficiency is important the dual-mode system has a HET-based EP branch which can be implemented in one of two ways. The HET can either operate using the products of water electrolysis, i.e. oxygen to the anode and hydrogen to the cathode, or using water vapor for the anode and an inert gas for the cathode.
Satellites will typically contain separate chemical and electric propulsion systems with distinct propellant storage and management subsystems. One of the chemical-electric propulsion architecture’s unique features is that it offers a single unified propellant management subsystem, enabling very significant mass savings in addition to those already afforded by the use of lightweight, low pressure water storage tanks. In combining within a single architecture high thrust and high mass utilization efficiency propulsion solutions, the architecture enables access to a new spectrum of mission scenarios not currently possible without the mass penalty of flying two separate propulsion systems. Water-based HETs are a key piece required to unlock the system level benefits of this novel propulsion architecture and have thus motivated further research into strengthening our experimental understanding of their physics.
Approach
The concept of a Hall Effect thruster designed to operate on the products of water electrolysis, i.e. a HET which operates with oxygen flow to the anode and hydrogen flow to the cathode, was first proposed by Schwertheim and Knoll at the 16th Reinventing Space (RiSpace) Conference in 2018. Early design work for the oxygen-based thruster began with initial sizing of the discharge channel length, width, and mean diameter. This was carried out using a simplified quasi-0D version of the IPPL’s fully kinetic particle-in-cell (PIC) code, PlasmaSim. Optimizing for 1-2 kW level oxygen operation, a thruster with channel lengths of 35-60 mm, a channel width of 5 mm, and a channel mean diameter of 20 mm was proposed. The laboratory model thruster subsequently built, referred to internally as the Water ElecTrolysis Hall Effect Thruster or WET-HET, can be seen pictured in Figure 2.
Extensive thrust performance characterization of the lab model WET-HET was subsequently carried out in the Boltzmann vacuum chamber using the IPPL’s hanging pendulum style thrust balance. Oxygen mass flow rates between 0.96 and 1.85 mg/s, discharge powers in the range of 400 to 2880 W, and peak radial magnetic flux density values roughly between 200 and 800 G were explored. Additionally, IPPL researchers evaluated the impact of changing channel wall materials and channel lengths, testing with alumina and boron nitride at lengths of 35, 45 and 60 mm. For these early tests, operators used a plasma bridge neutralizer running with 15 sccm of krypton as a cathode suitable for operation with hydrogen had yet to be designed.
Simulated performance trends as predicted by the 0D version of PlasmaSim initially used for the WET-HET channel sizing matched experimental trends collected as part of this initial characterization test campaign. Performance levels were however significantly lower than had been predicted by the code. This was attributed mainly to the omission of non-ionizing dissociation reactions as part of the simplifying assumption used, a shortcoming that was successfully addressed in a subsequent quasi-2D implementation of the code.
Encouraged by the promising preliminary results obtained with the WET-HET, design and build of a second-generation breadboard model HET began in the scope of an Innovate UK/Entreprise Singapore funded collaboration with UK SME URA Thrusters Ltd. and Singaporean start-up Aliena Pte Ltd. As part of this project, URA Thrusters Ltd. and the IPPL worked on breadboard model thruster (anode) design, the AQUAHET, while Aliena Pte. Ltd. worked on the development of a LaB6 based hollow cathode compatible with hydrogen and krypton, the Hydrocat. AQUAHET design work began, as it had with the WET-HET, with discharge channel sizing using the improved pseudo-2D version of PlasmaSim. Optimizing for 1-2 kW level water vapor and oxygen operation, a breadboard model thruster with channel lengths of 7-10-13 mm, channel widths of 4-5 mm, and a channel mean diameter of 40 mm was proposed.
Once again, extensive thrust performance characterization of the breadboard model AQUAHET and accompanying Hydrocat was subsequently carried out in Boltzmann using the hanging pendulum thrust balance. Oxygen anode mass flow rates between 1.0 and 2.5 mg/s, hydrogen cathode mass flow rates between 0.05 and 0.2 mg/s, discharge powers between 120 and 3200 W, and magnet currents between 0.5 and 5.5 A were explored. Additionally, water vapor anode mass flow rates between 1.50 and 1.75 mg/s, discharge powers up to 1 kW, and magnet currents between 0.5 and 2.5 A were tested at.
Obtained and/or anticipated results
Best performance of the AQUAHET and Hydrocat operating with oxygen to the anode and krypton to the cathode was achieved at 3200 W. At that operating point, thrust of 51 mN, anode specific impulse of 3118 s, and anode thrust efficiency of 24.4 % was measured. With oxygen to the anode and hydrogen to the cathode, best performance was achieved at 1168 W. In this case, thrust of 20 mN, anode specific impulse of 1216 s, and anode thrust efficiency of 10.2 % was measured. Finally, when probing at power levels up to 1 kW using water vapor feed to the anode and krypton for the cathode, at 1063 W we measure 14.5 mN of thrust, 845 s of anode specific impulse, and 5.6 % of anode thrust efficiency.
Future work will focus on plasma diagnostics in the near and far field plume for the AQUAHET and Hydrocat operating with oxygen or water vapor to anode and krypton to the cathode.
Relevant publications
- Rosati Azevedo E., Berhe M., Jones-Tett K., Sadler J., Potrivitu G.-C., Laterza M., Lim J.W.M., Tejeda J.M., Moloney R., Knoll A (2024) Joint Development of a Water Electrolysis Propelled Hall Effect Thruster and LaB6 Hollow Cathode. The 38th International Electric Propulsion Conference, Toulouse, France. http://hdl.handle.net/10044/1/115897
- Tejeda J.M., Potrivitu G.-C., Rosati Azevedo E., Moloney R., Knoll A. (2024) Experimental demonstration of a water electrolysis Hall Effect Thruster (WET-HET) operating with a hydrogen cathode. Acta Astronautica, 542-554. https://doi.org/10.1016/j.actaastro.2024.03.043
- Rosati Azevedo E., Fil P., Knoll A., Sadler J. (2024) Early performance characterization of a 2 kW Water Propelled Hall Effect Thruster. The 38th International Electric Propulsion Conference, Toulouse, France.
http://hdl.handle.net/10044/1/115884
