The smaller shuttlepods were introduced during the second season of Star Trek: The Next Generation to alleviate the costs associated with building exteriors and mock-ups of the larger, more elaborate types of warp-capable passenger shuttles. These ships were designed to carry fewer people, and had slower speeds. Originally intended by designers to be impulse-only, dramatic requirements often had the shuttles traveling long distances that would require warp speeds, and the Type 18 has been explicitly stated to have warp drive. The Type 15 were often specified as impulse only, so we must assume that when they were shown in use for long range travel they were fitted with a warp drive that was not previously part of its outfitting (we could likewise assume that impulse-only Type 18s exist). Many shuttle designations and specifications were never spoken onscreen, but are derived from background graphics and artwork, and are reiterated by the Star Trek: The Next Generation Technical Manual and the Star Trek: Deep Space Nine Technical Manual.