Grandson of X-15:

The technological parts that would make two-stage-to-orbit launch a reality are all probably “in hand” already, and it is a matter of system engineering to bring them all together, Lt. Gen. Michael Hamel, head of the Space and Missile Systems Center said in Los Angeles Friday. Speaking with reporters at AFA’s National Symposium on Space, Hamel anticipates a next-generation “hybrid” program marrying a reusable first stage and expendable second stage will get underway within six years. The first stage might achieve speeds up to Mach 6 or 7, Hamel said, noting, however, that this was achieved with the X-15 program in the 1960s.