Topic S3.02 seeks to mature and demonstrate in-space electric propulsion technologies, specifically calling for long-lifetime hollow cathodes for use with iodine propellant and high specific impulse thrusters. Hollow cathodes are used in Hall Effect Thrusters (HETs) and ion engines, two types of high specific impulse electrostatic spacecraft propulsion that can be used for unmanned planetary exploration and support secondary payload options. The compact cathode directly supports several Hall effect thrusters currently being developed at Busek and NASA, including the xenon fueled Busek BHT-600, an iodine compatible version of the BHT-600 and JPL's experimental MaSMi thruster. These thrusters may propel NASA Discovery, New Frontiers and Explorers Class science missions. Traditionally, these mission classes would use xenon propellant; however, iodine propellant would expand the mission envelope without increasing the mission cost. Iodine yields performance comparable to xenon but provides many additional benefits at the system level. In addition, a fully fueled, non-active system may be stored on the ground or on orbit for long periods of time, which facilitates spacecraft spares and minimizes down-times in the event of a failure. This program will ultimately lead to flight hardware to support iodine-fueled missions.
Hollow cathodes are used in Hall effect thrusters, a type of electric propulsion that is attractive for military and commercial missions due to high performance, small size, low mass, and relatively low cost. Low power all thrusters can accomplish all required in-space propulsion functions including orbit raising, orbit circularization, inclination changes, station-keeping, repositioning, and end-of-life de-orbiting. The design will specifically support small spacecraft with power levels up to 1 kW. The next stage for commercial users is the all-electric satellite, where apogee insertion and on-orbit functions are both handled by electric thrusters. The low plume divergence enabled by an axial cathode of the type being developed minimizes spacecraft interactions, which is critical for geosynchronous spacecraft with operational lifetimes of 10-15 years. The thruster and cathode would be also very well sized for an ESPA-class (180 kg) spacecraft flown as a secondary payload on an Air Force Evolved Expendable Launch Vehicle (EELV).