During ground handling everyone within was required to wear anti-static straps to prevent any discharge of static electricity that might ignite the booster. 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 using 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. The Chinese government never reported spotting the D-21B in flight. The first one failed to turn around and continued straight on, crashing in the Soviet Union. Another test flight was conducted on 20 February 1970 in a successful attempt to correct any problems. The second operational mission, however, was not until 16 December 1970. The D-21B reached Lop Nor and returned to the recovery point, but the hatch had a partial parachute failure and was lost at sea with its photographs. During the third operational mission, on 4 March 1971, the D-21B flew to Lop Nor and returned, and released the hatch, which deployed its parachute, but the midair recovery failed and the hatch fell into the water. The destroyer that tried to retrieve the hatch ran it down and it sank. The fourth, and last, operational flight of the D-21B was on 20 March 1971. It was lost over China on