The Scaled Composites Model 348 White Knight Two (WK2) is a jet-powered cargo aircraft which is used to lift the SpaceShipTwospacecraft to release altitude. It was developed by Scaled Composites from 2007 to 2010 as the first stage of Tier 1b, a two-stage to suborbital-space manned launch system. WK2 is based on the successful mothership to SpaceShipOne, White Knight, which itself is based on Proteus.