This paper presents a new computationally efficient and robust algorithm that provides energy-optimal impulsive control input sequences that drive a linear timevariant system to a desired state at a specified time. This algorithm is applicable to a broad class of problems where the cost is expressed as a time-varying norm-like function of the control input. This degree of generality enables inclusion of complex, time-varying operational constraints in the control planning problem. First, optimality conditions for this problem are derived using reachable set theory, which provides a simple geometric interpretation of the meaning of the dual variables. Next, an optimal impulsive control algorithm is developed that iteratively refines an estimate of the dual variable until the optimality criteria are satisfied to within a user-specified tolerance. The iteration procedure simultaneously ensures global convergence and minimizes computation cost by discarding inactive constraints. The algorithm is validated through implementation in challenging example problems based on the recently proposed Miniaturized Distributed Occulter/Telescope small satellite mission, which requires periodic formation reconfigurations in a perturbed orbit with time-varying attitude constraints. Additionally, it is demonstrated that the algorithm can be implemented with run times more than an order of magnitude faster than other approaches.