Lockheed D-21 Aircraft, Project Tagboard

The first attempted launch of a D-21B was on 28 September 1967, but the drone fell off the B-52's launch pylon due to a stripped nut on the pylon before the aircraft reached its intended launch point. Johnson admitted that the incident was "very embarrassing". Three more launches were performed from November 1967 through January 1968. None were completely successful, so Johnson ordered his team to conduct a thorough review before renewing launch attempts. The next launch was on 10 April 1968. It also failed as the engine did not ignite. On 16 June the D-21B finally made a completely successful flight; it flew at the specified altitude and course for its full range, and the hatch was recovered. The next two launches were failures, followed by another successful flight in December. A test in February 1969 to check the inertial navigation system with an actual mission profile was a failure. The next two flights in May and July succeeded.
Operational history
Four operational missions with the D-21B took place under the codename of Senior Bowl. These were conducted over the People's Republic of China from 9 November 1969 to 20 March 1971 to spy on the Lop Nor nuclear test site. The USAF's 4200th Support Squadron, based at Beale Air Force Base, California, flew the missions, usually from Andersen Air Force Base in Guam.