NASA Marshall has developed a system for generating iodine vapor from solid iodine for use as a propellant in a Hall or ion thruster propulsion system. Xenon has generally been the preferred propellant for these spacecraft ion propulsion systems but more recently, iodine-based systems have gained significant attention due to comparable performance to xenon, and the system-level advantages of low storage pressure and higher storage density with more propellant per unit volume.
The solid iodine, in comparison to gaseous xenon, must be sublimated into a vapor for ionization, and a heat source must be used to increase the sublimation rate of the solid iodine to a level that is useful for propulsive purposes.
NASA's iodine vapor feed system is based on a mechanism that holds and maintains the solid iodine in contact with a heated surface; in this case, the walls of the propellant tank. The mechanism provides a robust and reliable steady-state delivery of sublimated iodine vapor to the ion propulsion system by ensuring good thermal contact between the solid iodine and the tank walls.
To date, the technology development effort includes extensive thermal, mechanical, and flow modelling together with testing of components and subsystems required to feed iodine propellant to a 200-W Hall thruster. The feed system has been designed to use materials that are resistant to the highly reactive nature of iodine propellant. Dynamic modeling indicates that the feed system tubing can be built in such a way as to reduce vibrationally induced stresses that occur during launch.
Thermal modeling has been performed to demonstrate that the feed system heater power levels are sufficient to heat the tank and propellant lines to operating temperatures, and sublime the iodine in the storage tank to supply propellant for reliable and long-term operation.