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This work presents fuel-optimal altitude maintenance of Low-Earth-Orbit (LEO) spacecrafts experiencing non-negligible air\uddrag and J2 perturbation. A pseudospectral (direct) method is first applied to roughly estimate an optimal fuel consumption\udstrategy, which is employed as an initial guess to precisely determine itself. Based on the physical specifications of KOrea\udMulti-Purpose SATellite-2 (KOMPSAT-2), a Korean artificial satellite, numerical simulations show that a satellite ascends\udwith full thrust at the early stage of the maneuver period and then descends with null thrust. While the thrust profile is\udpresumably bang-off, it is difficult to precisely determine the switching time by using a pseudospectral method only.\udThis is expected, since the optimal switching epoch does not coincide with one of the collocation points prescribed\udby the pseudospectral method, in general. As an attempt to precisely determine the switching time and the associated\udoptimal thrust history, a shooting (indirect) method is then employed with the initial guess being obtained through the\udpseudospectral method. This hybrid process allows the determination of the optimal fuel consumption for LEO spacecrafts\udand their thrust profiles efficiently and precisely

Fuel-Optimal Altitude Maintenance of Low-Earth-Orbit Spacecrafts by Combined Direct/Indirect Optimization