Condensable Propellant Hall Thrusters
Hall-effect thrusters (named after an electromagnetic effect discovered by Edwin Hall in 1879) are plasma rockets that are used to maneuver satellites in space. Hall-effect thrusters can be used to change a satellite’s orbit, maintain a specific orbit in the presence of perturbing effects like drag, or propel a satellite from Earth to another location in the solar system. State-of-the-art Hall thrusters use gaseous xenon for their propellant. Of all the elements that are gaseous at ambient conditions, xenon possesses the best mix of chemical properties for use as a plasma fuel. It is possible, however, to realize a Hall thruster with superior performance if other elements that are not gases could be used in place of xenon. Many candidate elements are condensed (solids) at room temperature, and thus are referred to as “condensable” propellants to distinguish them from gases like xenon.
The Ion Space Propulsion laboratory has been conducting research into condensable-propellant-fueled Hall thrusters since 2004. Bismuth, zinc and magnesium thrusters have been successfully tested using the condensable propellant test facility. These metal propellants have superior performance to xenon for many missions and are also significantly cheaper than xenon. The most challenging aspect when using metallic propellants is that they need to be vaporized before being supplied to the thruster. This vaporization process can consume large amounts of electrical power – resulting in an intolerably low system efficiency. Michigan Tech has patented and developed a unique process to vaporize metallic propellants within the Hall thruster by utilizing waste heat from the plasma discharge.
An additional potential benefit of condensable propellant Hall thrusters is that planetary research has shown that zinc and magnesium can be found in high concentrations throughout the solar system. A Hall-effect thruster running off one of these propellants could potentially refuel itself during a mission by harvesting these sources from Martian soil or asteroid regolith.