Describing how he modified MPM’s X-15 to model Walker’s August 1963 ride, Jeff writes, “Walker’s record flight was made in the third and final X-15 built (66672). By this time, the detachable lower ventral fin was no longer being used except on the reconfigured X-15A-2 (66671).” The white areas underneath represent frost from the X-15’s super-cold fuel of anhydrous ammonia and liquid oxygen.