This paper presents a trajectory optimization method for reentry vehicles. Though the problem has been treated before, here we present a new way that is geared towards integration of the trajectory optimization problem with vehicle shape and structural sizing optimization problem. The gradient-based optimization techniques are used to obtain the optimal solution and its sensitivities with respect to problem parameters. The main thrust of this work is concentrated on finding an efficient way for solving the optimization problem and the sensitivity analysis. The new trajectory module is integrated into in a new tool for the preliminary design and optimization (PDOT) of launch and re-entry vehicles. As part of the solution the overall problem is simplified by switching from the time domain into a velocity domain. This reduces the number of constraints and allows efficient sensitivity analysis. Heat transfer constraints are imposed along the trajectory by using a cumulative constraint function. The sensitivity analysis is done throughout the search of a feasible direction providing good approximations to the optimal solution when vehicle shape or structural design parameters are changed.