<h2>AAS 99-132</h2>
<h2>Fuel Optimal Planar Interplanetary  Trajectories Using Low-thrust Exhaust-Modulated Propulsion                                                   </h2>
<h4>S. Vadali, R. Nah, E. Braden, I. Johnson, Jr.                                                                                                                                       </h4>
Texas A&M U., College Station, TX                                                                                                                                         
<h2> Abstract </h2>
This paper deals with the determination of fuel optimal trajectories using a plasma thruster currently being developed for manned/cargo missions to Mars. This device can generate variable thrust/Isp at constant power, within the range 1,000-35,000 sec. We use rotating polar coordinates and divide the trajectory into two phases, patched together at an intermediate point between two planets. Since the geocentric and plant-centric coordinate systems are rotating, and the transformations between the respective variables in the different phases are nonlinear (excluding the mass), the costates are discontinuous at the patch points. Results for a cargo mission to Mars are presented.