Solar electric propulsion cargo spacecraft for Mars missions

One of the topics available to the 1990-91 Aerospace Engineering senior class was the development of a preliminary design of an unmanned cargo ferry that would support the Mars mission by bringing equipment and supplies from a low Earth orbit (LEO) to a low Mars orbit (LMO). Several previous studies initiated by NASA have indicated that low-thrust transportation systems seem to offer the best performance for Mars missions. Such systems are characterized by long spiral times during escape and capture maneuvers, high payload mass fractions, and, typically, low propellant mass fractions. Of two main low-thrust candidates, nuclear electric propulsion (NEP) and solar electric propulsion (SEP), only the first one received extensive consideration because it seemed to represent the most promising concept for a manned mission to Mars. However, any sustained Mars initiative will have to include an unmanned cargo transportation system, for which an SEP concept deserves very careful consideration. The key assumptions and requirements established in cooperation with the Space Exploration Initiative office at the NASA Langley Research Center were (1) vehicle is assembled at the Space Station Freedom (SSF); (2) Earth-to-orbit delivery of the vehicle components, propellant, and payload is via shuttle-C; (3) vehicle's cargo mass is 61,000 kg; (4) vehicle delivers cargo to LMO at an altitude of 500 km and inclination of 70 deg; (5) vehicle returns (without cargo) to SSF; (6) vehicle should be reusable for at least three missions; and (7) vehicle is powered by ion argon thrusters. Two configurations were developed by two student teams, working mostly independently.