A McKendree cylinder is a type of hypothetical rotating space habitat originally proposed at NASA's Turning Goals into Reality conference in 2000 by NASA engineer Tom McKendree. As with other space habitat designs, the cylinder would spin to produce artificial gravity by way of centrifugal force. The design differs from the classical designs produced in the 1970s by Gerard K. O'Neill and NASA in that it would use carbon nanotubes instead of steel, allowing the habitat to be built much larger. In the original proposal, the habitat would consist of a cylinder approximately 460 km (290 mi) in radius and 4,600 km (2,900 mi) in length, containing 13 million km2 (5 million sq mi) of living space, nearly as much land area as that of Russia.
As originally proposed, the McKendree cylinder is simply a scaled-up version of the O'Neill cylinder. Like the O'Neill cylinder, McKendree proposed dedicating half of the surface of the colony to windows, allowing direct illumination of the interior. The habitat would be composed of a pair of counter-rotating cylinders which would function like momentum wheels to control the habitat's orientation.