The determination of fuel-optimal, planar, Earth-Mars trajectories of spacecraft using low-thrust, variable speci c impulse I sp propulsion is discussed. The characteristics of a plasma thruster currently being developed for crewed/cargo missions to Mars are used. This device can generate variable I sp within the range of 1000-35,000 s, at constant power. The state equations are written in rotating, polar coordinates, and the trajectory is divided into two phases, patched together at an intermediate point between the Earth and Mars. The gravitationaleffects of the sun, Earth, and Mars are included in the two phases. The formulation of the problem treats the spacecraft mass as a state variable, thus, coupling the spacecraft design to the trajectory design. The optimal control problem is solved using an indirect, multiple shooting method. Results for a 144-daycrewed mission to Mars are presented. The variation of the I sp during spacecraft's escape from the Earth's gravitational eld shows an interesting periodic behavior with respect to time. The results obtained are also compared with those obtained by assuming a three-phase trajectory, with the Earth, sun, and Mars, in uencing the spacecraft, one per phase, in sequence.
The International Symposium on Geotechnical Applications of Remote Sensing and Remote Data Transmission was held January 31, 1986, at Cocoa Beach, Fla. The symposium was organized by the American Society of Testing and Materials (ASTM) Committee D18 on Soil and Rock and the International Committee on Remote Sensing and Data Transmission (ICRSDT) of the International Association of Hydrological Sciences. Cochairs for the symposium were Ivan Johnson of A. Ivan Johnson, Inc., Arvada, Colo., and Bernt Pettersson of Brown and Root, Inc., Houston, Tex. Johnson is President of ICRSDT and Pettersson is Chairman of ASTM D18 Subcommittee on Surface and Subsurface Reconnaissance.
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