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Lockheed D-21 22 October 2014

The D-21 was a supersonic reconnaissance drone used briefly in the 1960s and 1970s.

Kelly Johnson at Lockheed had developed the A-12 reconnaissance aircraft for the CIA, but after Gary Powers' U-2 was shot down in 1960 the company started to look at alternatives to manned reconnaissance aircraft, even very high and fast ones.

What was initially known as the Q-12 was an idea for an expendable reconnaissance drone, intended to fly at around Mach 3.5 and 90,000 feet; it would eject its camera and exposed film at the end of the trip, then self-destruct. In shape it looked much like a shrunken A-12 (or SR-71), built round a single ramjet engine modified from the Marquardt ramjet that powered the Bomarc long-range SAM. The camera and film unit would come down on a parachute to be snagged in mid-air by a C-130 Hercules (already being used to retrieve film from reconnaissance satellites), or if that failed would float on water to be picked up by a ship.

In order to get the drone up to speed and altitude for the ramjet to work, it would be launched off the back of a modified A-12 (it was small enough to fit between the tail fins), flying at Mach 3 and around 80,000 feet. This was where the problems started. Under the code name TAGBOARD, the system was re-designated as D-21 (for the drone) and M-21 (for the mothership aircraft), and two A-12s were set aside for conversion. This mostly consisted of a larger cockpit for a second crewmember, and a launch pylon.

Four launches were conducted from M-21s. The first time out, the drone released, but stayed close to the M-21's back for several seconds before pulling up and away. The second drone to be launched reached its operating speed and altitude, but some 1,200 nautical miles down-range a hydraulic pump failure caused it to be lost. The third flight was generally successful, but the camera hatch was not released.

The fourth flight was the disastrous one: rather than fly in an outside loop like the previous launches, this one had the M-21 flying straight and level. The D-21 engine failed to ignite cleanly, and the drone struck the tail of the M-21, leading to the loss of the aircraft; the pilot survived, but the back-seater drowned after landing.

This didn't seem workable, so Kelly Johnson proposed that the drones should instead be launched from a lower and slower carrier aircraft, such as a B-52. Now known as the D-21B, the drone would need a rocket booster to get it up to height and speed for the ramjet; the solid-fuel rocket turned out to be bigger and heavier (at 44 feet and six tons) than the drone itself (42 feet and five tons).

Out of twelve test launches off B-52s, four were completely successful. The rest failed for various reasons, ranging from the rocket engine failing to ignite to the whole assembly falling off the carrier before launch thanks to a stripped nut. But this was good enough to put the system into service.

The D-21B made a total of four operational flights, between November 1969 and March 1971, with the aim of photographing the Lop Nor nuclear test site in China. The first one failed to reverse course and carried on flying, self-destructing somewhere over Siberia. (Many years later, it was revealed that the Soviets had found enough wreckage for the Tupolev design bureau to reverse-engineer it and plan a copy, to be known as the Voron (Raven), but they never attempted to build it.)

Another test firing was conducted to try to fix the guidance problem; the second operational flight was more successful, but the camera unit suffered a partial parachute failure and was lost at sea. On the third flight, the parachute worked, but the C-130 failed to snag the package; the destroyer attempting the retrieval got too close and ran over the floating unit, and it sank. On the fourth flight, the drone was lost for unknown reasons on the return leg of its flight; the wreckage was found by the Chinese, and is now in the China Aviation Museum near Beijing.

The poor success rate, better reconnaissance satellites, and Nixon's rapprochement with China all combined to make the programme look like a bad investment, and it was cancelled.

The A-12, of course, was the immediate inspiration for the even more successful SR-71.

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