The X-33 program began in 1994 as part of NASA’s Reusable Launch Vehicle (RLV) program. It awarded a contract to Lockheed Martin to build and fly an uncrewed technology demonstrator. Much of the vehicle was entirely new, including the linear aerospace rocket motor and the composite cryogenic propellant tanks. The leap exceeded what existing and developing technology could support. NASA announced March 1, 2001, that it would not add Space Launch Initiative funds to the X-33 program. As a result, the X-33 program came to completion when the cooperative agreement between NASA and Lockheed Martin expired March 31, 2001. NASA invested nearly $1 billion in the project, and Lockheed Martin put in nearly $400 million.
For more information: http://www1.msfc.nasa.gov/NEWSROOM/news/releases/2001/01-062.html.