<h2>AAS 95-451</h2><h2>Electric Propulsion for Orbit Transfer in a Resistive Medium</h2><h4>M. Guelman, Aerospace Eng., Technion, Haifa, Israel </h4><h2> Abstract </h2>This paper presents a power limited low thrust optimal guidance law for spacecraft transfer in a central gravitational field with drag effects considered.  Drag forces acte along and opposite the vehicle inertial velocity vector.  For optimal thrust control synthesis a stationary atmosphere with constant density is assumed.  An optimal guidance law is obtained analytically as a particular solution to the case of transfer with free final time.  The extremal guidance law is shown to be colinear with the spacecraft velocity vector multiplied by a time varying gain that depends on a single constant of integration.  By adequately choosing the integration constant and the free final time, transfer to a final orbit with predesigned semimajor axis and excentricity is performed.  Based on the extremal thrust acceleration guidance, a suboptimal guidance for spacecraft orbit transfer in an atmosphere with variable density is developed.  Numerical results for a representative case are presented.<br><br>