It's always worth digging into the history of these things. Not only does it give you the how and why these things were engineered within the greater context of history, but also what interesting things may have preceded or been derived from them.
As cool as the D-21/M-21 combination is, it was unfortunately doomed to fail. One of the M-21s was lost when its drone was launched at supersonic speeds - it bounced off the shock cone and crashed back into its mothership, causing the thing to violently break up. Afterwards they would only launch a few more D-21s from subsonic aircraft like the B-52.
www.youtube.com/watch?v=GMyC2u…image.tuku.china.com/tuku.mili…atomictoasters.com/wp-content/…