I was bored, trying to come up with something to improve my Starship Generations website, when I thought of doing a Google search for unpiloted supersonic drones: Such a drone would be needed for the Skyhook Launch System that I've envisioned (topic #9, Starship Generations). It would provide a powered supersonic descent for the 4000-km long tether through the atmosphere to prevent tether buckling, and subsonic maneuverability for an airborne rendezvous at 10-km altitude, with a Boeing 747-400 Freighter Aircraft carrying the 50-tonne payload on its back.

What I found was pretty cool...

Introducing the declassified Lockheed D21 supersonic spy drone. It is capable of sustained flight at Mach 4 at an altitude of 27 km. This is highly desirable in a Skyhook rendezvous, where nose-first airspeeds for the Grappling Pod drone and payload can reach Mach 1 at around 15-km altitude, Mach 2 at around 27-km altitude, Mach 3 at around 50-km altitude, and Mach 4 at around 85-km altitude. It would continue accelerating but the air becomes so thin above 100-km altitude that it probably wouldn't make any difference how much faster it went.