An underwater missile launch system includes one or more missile loading modules for supporting a plurality of missiles in a stackable, in-line configuration within a pressure vessel. The missiles are arranged inside the modules, which may be stacked in groups inside a single pressure vessel, or payload bay. Each module is preferably substantially identical including a common size, shape, and payload of missiles in common with the module above and below it. A one-way positioning latch is provided that prevents the upper missiles from dropping down on top of the lower missiles, while allowing the lower missiles to later pass up through the same launch tube as the upper missiles, after the upper missiles have been ejected.