 A series of experimental hypersonic projects, including an advanced version of the Martin Marietta X-24 lifting body, were turned down.Eventually issues with the Rockwell HiMAT advanced UAV led to a manned X-plane with forward sweep, the Grumman X-29, which flew in 1984.Some of the X-planes have been well publicized, while others, such as the X-16, have been developed in secrecy.