Although there has been evidence to suggest that work on various classified successors to the SR-71, or some of its roles, has been attempted, none of the tantalizing signs have materialized into anything substantial. Outside of the black world, this has always been relatively easy to explain. Though few question the compelling military imperative for high speed ISR capability, the astronomical development costs have made the notion a virtual nonstarter.
The concept of a reusable hypersonic vehicle was an outgrowth of Darpa’s Falcon program, which included development of small launch vehicles, common aero vehicles (CAV) and a hypersonic cruise vehicle (HCV). As structural and aerodynamic technologies for both the CAV and HCV needed testing, Lockheed Martin was funded to develop a series of unpowered hypersonic test vehicles (HTV).