To deal with airframe heat at Mach 3 and to reduce weight, the SR-71’s skin is 85 percent…

“To deal with airframe heat at Mach 3 and to reduce weight, the SR-71’s skin is 85 percent titanium and 15 percent carbon composites. Ironically—because the spy plane was a creature of the cold war—its titanium was purchased from the Soviet Union, although the use to which it would be put remained a closely guarded secret.” - The Ultimate Spy Plane | Arts Culture | Smithsonian Magazine