The aerodynamics of the X-15 were a dead end in the face of the heat barrier. The correct way would have been the Dyna Soar, but Kennedy's lunar occurrence killed it.Sometimes I wonder what might have been if the X-15 had been further developed, all the way up to serving as a true spaceplane.
HereWasn't there an advanced X-15 variant proposed that had built in ramjets?