One of the least known members of a family of aircraft that includes the SR-71 is the D-21 reconnaissance drone.  The drone was designed to be launched from the back of a modified A-12, the first version of the Blackbird.  A ramjet engine that requires very high speeds to operate powers the D-21 and the Blackbird was the only aircraft capable of the speed required.  Initial testing began in 1963, however the drone was not released from the carrier aircraft in flight until March 1966.  Three successful test flights were made, but on the fourth flight the drone crashed into the launch plane and one of the two crewmen was killed.  The drone was redesigned and fitted with a booster rocket to generate the speed needed for engine operation.  In this configuration, it could be launched from a pylon mounted on the wing of a B-52.  Testing of this version began in 1967 and eventually four unsuccessful operational missions were flown over China.  The program was canceled in 1971.

Manufacturer
Lockheed

Markings
4200^th Support Squadron, Beale AFB, California, 1971

Serial Number
533

Designation
D-21B