Debuting in the early 1960s with Mach 3.2 speed and an 85,000-foot ceiling, the SR-71 Blackbird spy plane was universally regarded as the greatest aircraft in history.
By the 1980s, there was sentiment to relegate the Blackbird to history, because its imagery and radar data couldn't be used in real time. And as then-Defense Secretary Dick Cheney lamented, operating the Blackbird cost $85,000 per hour. Which put the kibosh on Lockheed's planned Mach-5 SR-71 replacement, shown below.
But at least, soon, we'll have the Mach-6 SR-72: