I've read declassified CIA documents about ISINGLASS and RHEINBERRY, proposed very high speed recon platforms.
One "advantage" — apparently not to be emphasized to the pilot — was that there was no way for a pilot to eject and be captured in enemy territory, a la Francis Gary Powers. At the velocities the a/c would reach over the USSR, if it had a mishap the environment would be so severe the Soviets wouldn't even be able to determine from the wreckage if it was manned at all.
One "advantage" — apparently not to be emphasized to the pilot — was that there was no way for a pilot to eject and be captured in enemy territory, a la Francis Gary Powers. At the velocities the a/c would reach over the USSR, if it had a mishap the environment would be so severe the Soviets wouldn't even be able to determine from the wreckage if it was manned at all.