Purely by co-incidence I have just this evening finished reading Rich Grahams 1996 book. We had Rich over for our first lecture of the season at the Cambs RAeS in September.
At the end of our lecture the obvious question was asked about the existence of the follow on projects. Now for someone as passionate about the SR71 programme he kept a straight face in dispelling the myth.
Lets hope the aerodynamicists and technologists have solved the unstart issue.
According to the book. the UAVs Global Hawk and Darkstar were the successors to the SR and U2 recognising the deficiencies of the satellites available at the time.