The legacy of the North American X-15
thehighfrontier.wordpress.com
thehighfrontier.wordpress.com
With the benefit of hindsight, flying into space with the X-15's successors would have happened if there was not a need to beat the Soviets quickly with rockets.
Also throwing away the Saturn rockets after around 20 launches was just silly.
The rest is just incrementalism which is more than cancelled out by serious backsliding all the way around.
During the X-Prize launches, I observed that what Burt Rutan had done was to build himself an X-15, and then build himself a B-52 to launch it.