The Blackbird with the most radical “after-market” alteration, #959 (“Big Tail”) started her career in 1966 like any other SR-71. In 1975 she was selected as the test platform for a new OBC (optical bar camera), surveillance and ECM (electronic counter-measures) suite located in a nine-foot “boom” attached to the aft end of the fuselage.
The alterations were tested in November 1975, and proved conclusively that the airframe suffered no appreciable performance loss from the modifications. However, other tests also showed that the new ECMs did not offer any increased protection from missile attack; and in the face of budget cuts, the program to retrofit other blackbirds was cancelled.
Air Force Armament Museum, Eglin AFB, Eglin, Florida