The fourth and last operational flight attempt took place on March 20 using D-21 #527. This mission too would end in failure, when the drone was lost three-quarters of the way through the flight over a heavily defended area. This also turned out to be the last flight for the D-21 program. Out of the 21 D-21 missions flown, only four were modestly successful.
D-21B #527 Launched from B-52H on 20 March 1971....flew 2,935 nm, 4th and last operational mission, camera not recovered. D-21 Program closed down
仔细检查了尾部残骸，找到了直接的证据。USE ON SHIP 527 ONLY