Abstract-Medical procedures such as lung biopsy and brachytherapy require maneuvering through tubular structures such as the trachea and bronchi to reach clinical targets. We introduce a new method to plan configurations for active cannulas, medical devices composed of thin, pre-curved, telescoping lumens that are capable of following controlled, curved paths through open or liquid-filled cavities. Planning optimal configurations for these devices is challenging due to their complex kinematics, which involve both beam mechanics and space curves. In this paper, we propose an optimization-based planning algorithm that computes active cannula configurations through tubular structures that reach specified targets. Given the target location, the start position and orientation, and a geometric representation of the physical environment extracted from pre-procedure medical images, the planner optimizes insertion length and orientation angle of each lumen of the active cannula. The planner models active cannula kinematics using a physically-based simulation that incorporates beam mechanics and minimizes energy. The algorithm typically computes plans in less than 2 minutes on a standard PC. We apply the method in simulation to anatomy extracted from a human CT scan and demonstrate configurations for a 5-lumen active cannula that maneuver it through the bronchi to targets in the lung.