In this work a trajectory optimization problem for three-stage space launcher is studied. The mission of the launcher is to put a given payload on the GEO orbit with the minimal propellant consumption. The considered flight sequence performs two boosts. The first one steers the launcher to a given GTO orbit. Then, after a ballistic flight, a second boost is used to perform the orbit transfer maneuver from the GTO to the GEO. We decompose the global optimization problem into two optimal control problems and we apply in this context the HJB approach together with a parameter optimization procedure.