Universe Today
Ion drives are renowned for their efficiency. They're extremely efficient compared to chemical rockets, so they're preferred for deep space missions where propellant supplies are critical. New research shows how they could run on simple water, making them even more efficient.
Water is widespread throughout the Solar System, at least in ice form. Electrolysis can split water into hydrogen and oxygen, generating hydrogen that can be used as rocket fuel. However, that's a complicated endeavour. If the same water could be used as fuel in an ion drive, that's an intriguing possibility.
New research in the Journal of Electric Propulsion shows how it might work and what benefits it could provide. It's titled "Computational modelling of water-fuelled Hall Effect Thrusters," and the lead author is Jesús Manuel Muñoz Tejeda, who is from the Imperial Plasma Propulsion Laboratory at Imperial College London.
This research focuses on Hall-effect thrusters (HET), which aren't exactly the same as ion drives but very similar. Both systems use accelerated ions to propel spacecraft through space. Neither one can launch a spacecraft from the surface of a planet, but both are viable systems once a spacecraft has escaped a gravity well.
Ion drives are desirable for longer missions because they're more propellant efficient. NASA used them on their Deep Space 1, Dawn, and Psyche missions, while the ESA has used them on their Bepi-Colombo and LISA missions.
The authors write that Hall Thrusters are "the preferred option for a variety of in-space propulsion operations, such as station-keeping or orbital disposal." According to their paper, 72% of geo-stationary orbit satellites use Hall Effect Thrusters.
Hall Thrusters are simpler and have fewer components, while ion drives are a little more complex. Hall Thrusters generate higher thrust and lower specific impulse, while ion drives are the opposite. Both systems are in the broad category of electric propulsion.
Both systems use electricity, typically generated with solar panels, to give their propellant gas a positive charge. Then they use magnetic fields to eject the gas, creating thrust.
Xenon is the most common fuel in HETs because it doesn't need much energy to ionize, has a high atomic mass, and is easy to handle and store. However, xenon has limited availability and is subject to price volatility. Krypton is also used, and iodine is a viable alternative. Researchers are also considering argon, zinc, magnesium, and even buckminsterfullerene is under consideration. Each of these options has its own problems and obstacles.
Water is being considered for some obvious reasons. It is low-cost, non-toxic, easily stored, and synergistic with other spacecraft systems like life support and thermal cooling. It's also abundant in the Solar System.
The authors point out the need to explore water as a fuel for HETS because experiments are so expensive and time-consuming. "Therefore, the need for robust and predictive simulation tools is apparent, both for a relatively inexpensive iteration within the design parametric space and for a better understanding of the physics underlying the thruster operation," they write.
The researchers simulated the use of water as a propellant for Hall Thrusters. In one case, they used water vapour; in the second, they used oxygen derived from water by electrolysis. Hall Thrusters generate ions that are propelled to create thrust, and each simulation produced a different array of ions.
The results show that water could be a suitable propellant in both cases in HETs. "The results coming from the simulations and experiments for both oxygen and water vapour demonstrate reasonable agreement," the authors write.
Xenon is still superior when power levels are the same. "When compared with xenon-fueled HET, it becomes evident that xenon achieves superior thrust and efficiency at comparable power levels," they explain.
However, at increased power levels, water is more efficient. "It is important to observe that, for both oxygen and water vapour systems, experimental observations and the presented data demonstrate that their specific impulse improves with increasing discharge power, ultimately exceeding the values achieved by xenon-fueled HETs," the authors write.
Because they generate higher specific impulse, these systems could be desirable for deep-space missions, where efficiency and long-term operations are paramount. "All in all, although the performance of water-fueled HETs is not expected to reach that of xenon-fueled devices due to dissociation losses, the lightweight mass of the propellant, and its higher first ionization potential compared to xenon, water-fueled systems present significant advantages," the authors write.
This is preliminary research, so it doesn't reach any absolute conclusions. Simulations like this are preliminary to actual tests, and researchers will learn a lot more in the future.
The clincher might be that water's availability and synergy with other systems are very desirable in deep-space missions. "In such missions, water electrolysis can produce both oxygen and hydrogen for chemical and electric propulsion, enabling multifunctionality," the researchers explain.
