There seems to be some claims that the SR-71 was used in Special
Relativity experiments using Atomic Clocks in the 1980s, but this
cannot be easily backed up.  Was the SR-71 actually used to prove
Einsteins SR in a repeat experiment of that conducted in 1971 with
Cesium Atomic Clocks and 4 commercial jet airliners?

Given that the original 1971 work (see J.C. Hafele and R. E. Keating,
Science 177, 166 of 1972) was not controversial at the time, I doubt
anyone would have approved the expense of an SR-71 redux of this
experiment.  Furthermore, most countries then (and now) would not be
too happy with any SR-71's overflying their soil, for any proffered
explanation whatsoever (scientific or merely claimed to be).

Keep looking up!  ;)