The central ship of the player's fleet is the mothership, a large base which can construct other ships; unlike other spacecraft, in the single-player campaign the mothership is unable to move. Present in each level are stationary rocks or dust clouds, which can be mined by specialized harvesting ships (resource collectors) which then empty their loads at the mothership in the form of "resources", the game's only currency. Resources can be spent by the player on building new ships, which are constructed by the mothership. Buildable ships come in a variety of types, which are discovered over the course of the game. They include resource collectors, fighters and corvettes, frigates, destroyers, and heavy cruisers, as well as specialized non-combat ships such as research vessels and repair corvettes. Fighter ships need to dock with support ships or return to the mothership periodically to refuel, while salvage corvettes can capture enemy ships and tow them to the mothership to become part of the player's fleet. In some levels, new ship types can be unlocked by capturing an enemy ship of that type, through research performed at the research vessel, or through plot elements. At the beginning of the campaign, the player may select between controlling the "Kushan" or "Taiidan" fleet; this affects the designs of the ships and changes some of the specialized ship options, but has no effect on the plot or gameplay.