Roddenberry further specified that the "Enterprise" would operate mainly in space, have a crew of 100–150, and be incredibly fast. Jefferies and Roddenberry did not want the "Enterprise" to look like any of the rocket ships already used by the aerospace industry or in popular culture; many of Jefferies' designs were rejected for being "too conventional". To meet Roddenberry's requirement that the ship look believable, Jefferies tried "to visualize what the fourth, fifth or tenth generation of present-day equipment would be like". Jefferies' experience with aviation let him him imbue his designs with what he called "aircraft logic". He imagined the ship's engines would be too powerful to be near the crew, requiring them to be set apart from the hull. While Jefferies initially rejected a disk-shaped component, worried about the similarities to flying saucers, a spherical module eventually flattened into a disk. During one visit to Jefferies, Roddenberry and NBC staff were drawn to a sketch of the "Enterprise" resembling its final configuration. Jefferies had created a small model of this design that, when held from a string, hung upside-down – an appearance he had to "unsell". Jefferies kept the hull smooth, with a sense that the ship's components were serviced from inside. When designing the Klingon starship in the third season, Jefferies rearranged and changed the shape of the "Enterprise"s basic modules: a main body, two engine pods, and a neck with a head on it. Some of Jefferies' rejected design concepts – such as spherical hull sections and warp engines that encircle a ship – inspired future "Star Trek" vessel designs.