The D-21 drone used a ramjet, just like its Oxcart mothership, Annie Jacobsen explains (in Area 51), and needed to be launched at supersonic speed:
For public safety reasons, the plan was to launch the triplesonic drone off the coast of California in March of 1966 for the first test, and to prepare his pilots, Colonel Slater had them swim laps each day in the Area 51 pool, first in bathing suits and then with their pressure suits on. “We’d hoist the guys up over the water in a pulley and then drop them in the pool. As soon as they hit the water the first time, the pressure suit inflated, so we had to have that fixed,” Slater recalls. When it came time to practice an actual landing in a large body of water, the Agency’s highest-ranking officer on base, Werner Weiss, got the Coast Guard to seal off a large section of Lake Mead, the largest reservoir of water in the United States, located just east of Las Vegas.
[…]
Both aircraft flew west until they were a hundred and fifty miles off the coast of California. There, the M-21, piloted by Bill Park, prepared for the D-21 launch. A camera in Slater’s airplane would capture the launch on 16-millimeter film. Down below, on the dark ocean surface, a rescue boat waited. Park hit Ignite, and the drone launched up and off the M-21. But during separation, the drone pitched down instead of up and instantly split the mother aircraft in half. Miraculously, the drone hit neither Park nor Torick, who were both trapped inside.
The crippled aircraft began to tumble through the sky, falling for nearly ten thousand feet. Somehow, both men managed to eject. Alive and now outside the crashing, burning airplane, both men were safely tethered to their parachutes. Remarkably, neither of the men was hit by the burning debris falling through the air. Both men made successful water landings. But, as Slater recalls, an unforeseen tragedy occurred. “Our rescue boat located Bill Park, who was fine. But by the time the boat got to Ray Torick, he was tied up in his lanyard and had drowned.”
Kelly Johnson was devastated. “He impulsively and emotionally decided to cancel the entire program and give back the development funding to the Air Force and the Agency,” Johnson’s deputy Ben Rich recalled in his 1994 memoir about the Lockheed Skunk Works. Rich asked Johnson why. “I will not risk any more test pilots or Blackbirds. I don’t have either to spare,” Johnson said. But the Air Force did not let the Mach 3 drone program go away so quickly. They created a new program to launch the drone from underneath a B-52 bomber, which was part of Strategic Air Command. President Johnson’s deputy secretary of defense, Cyrus Vance, told Kelly Johnson, “We need this program to work because our government will never again allow a Francis Gary Powers situation develop. All our overflights over denied territory will either be with satellites or drones.”
Three years later, in 1969, the D-21 drone finally made its first reconnaissance mission, over China, launched off a B-52. The drone flew into China and over the Lop Nur nuclear facility but had then somehow strayed off course into Soviet Siberia, run out of fuel, and crashed. The suggestion was that the drone’s guidance system had failed on the way home, and it was never seen or heard from again. At least, not for more than twenty years. In the early 1990s, a CIA officer showed up in Ben Rich’s office at Skunk Works with a mysterious present for him. “Ben, do you recognize this?” the man asked Rich as he handed him a hunk of titanium. “Sure I do,” Rich said. What Ben Rich was holding in his hand was a piece of composite material loaded with the radar-absorbing coating that Lovick and his team had first developed for Lockheed four decades before. Asked where he got it, the CIA officer explained that it had been a gift to the CIA from a KGB agent in Moscow. The agent had gotten it from a shepherd in Siberia, who’d found it in the Siberian tundra while herding his sheep. According to Rich, “The Russians mistakenly believed that this generation-old panel signified our current stealth technology. It was, in a way, a very nice tribute to our work on Tagboard.”