21 June 1967 – In June 1967, the X-15A-2 rocket-powered research aircraft received a full-scale ablative coating to protect the craft from the high temperatures associated with supersonic flight.
This pink eraser-like substance, applied to the #2 aircraft (56-6671), was then covered with a white sealant coat before flight. This coating would help the #2 aircraft reach the record speed of 4,520 mph (Mach 6.7).(1)
Source:(1) https://www.nasa.gov/centers/dryden/history/thisweek/ECN-1751.html