Today you get a bunch of photographs, of a single subject, taken at a variety of venues — and it’s all about the D-21 drone.
This first shot, taken at the Pima Air & Space Museum near Tucson, Arizona, is probably the clearest one I’ve got:
The D-21 drone was developed in the 1960’s as a Mach 3 reconnaissance drone, largely in response to the shoot-down of Gary Powers’ U-2 over the USSR in 1960. The idea was that the SR-71 could be used for reconnaissance over some sites, but for the really hazardous spots you’d rather use an unmanned reconnaissance aircraft. So since the SR-71 was under development (originally as the CIA’s A-12), the idea was to launch a reconnaissance drone (at first called a Q-12) off the back of an A-12 — the drone would then fly a pre-programmed path, ending over an ocean somewhere. The film it carried would be ejected on a parachute, and the drone would automatically self-destruct.
Here’s another D-21 shot, from the National Museum of the U.S. Air Force in Dayton, Ohio:
In 1963, for no particularly clear reason (that I can find, anyway), the elements of the program got renamed — the reworked A-12 motherships were dubbed M-21s, and the drones renamed from Q-12s to D-21s (M for “mother,” and D for “daughter”). Here’s a M-21 / D-21 pair at the Museum of Flight in Seattle:
All was well with the program until an in-flight accident occurred in 1967 — a D-21 hit the tail of its mothership immediately after deployment, destroying both craft and ending in the drowning death of one of the M-21’s crew. Deciding that the A-12s / M-21s / SR-71s were a bit too precious to be risked as carrier aircraft (and deploying something off the back of one at Mach 3 a bit too tricky to reliably pull off), the powers that be switched to a scheme in which the drones (now called M-21Bs in their updated version) would be dropped from the wing of a B-52 bomber and brought up to speed (since they were powered by ramjets) by a solid rocket booster.
Ultimately this proved unreliable as well, and since satellite reconnaissance was now both more productive and less controversial than aircraft overflights, the D-21 program was cancelled in 1971. FWIW, the top two of my D-21 images are of D-21B drones; the lower one is of a surviving D-21 (although it’s mislabeled as a D-21B on the museum website).