Lockheed D-21 Aircraft, Project Tagboard

The fate of the D-21 that had disappeared on the first operational flight was finally revealed in February 1986 when an official from the CIA returned a panel to Ben Rich that he had been given by a Soviet KGB agent. The drone had self-destructed over Siberia and the Soviets had recovered the wreckage. The Tupolev design bureau reverse-engineered the wreck and produced plans for a Soviet copy, named the Voron (Raven), but it was never built.
In the late 1990s NASA considered using a D-21 to test a hybrid "rocket-based combined cycle" engine, which operates as a ramjet or rocket, depending on its flight regime. Ultimately NASA used a derivative of the agency's X-43A hypersonic test vehicle for the experiments.