Following the record breaking flight of the X-15A2, despite the application of an ablative coating ( called MA-25S ) there was considerable heat damage to the airframe. Rather than relegate the remains to a museum, the USAF decided to rebuild it with a new delta wing.  The aim of this was to compare the performance of the existing airframe & ablative combination to that of a new build X-15C using construction/heatshield methods originally developed for the cancelled X-20 Dyna Soar project. Following initial test flights, there was sufficient confidence in the coating to permit an orbital flight to be considered, using strap-on solid boosters to attain the required speed after a Mach 3 airlaunch from a B-70 Valkyrie.

( AMT Valkyrie, Monogram X-15 with scratchbuilt wings, boosters from a 1/288 shuttle )