[TangoTrotFox]

Really useful.

You've gotta consider how much things change as you get higher into the atmosphere. The obvious thing is that atmospheric oxygen begins to become insufficient to operate engines as you go higher up, so you need engines that use a different oxidizer than atmospheric oxygen which is extremely complex. And even the efficiency and characteristic of engine exhaust changes radically depending on atmospheric density and the characteristics of its exhaust. In other words an engine that works great at sea level might work awfully when you start to get to a fraction of the same pressure.

There's an immense amount of flexibility in being able to travel hypersonic. The SR-71 Blackbird is an amazing example of the potential. First flown back in 1964 (!!) it was capable of hitting around mach 3, some 2200 miles per hour. That's 6 hours to get from one side of the Earth to the other. It's really quite remarkable how much our overall transportation technology seems to have stagnated since the 60s. I'd like to imagine everything is just classified but everything from the SLS to the F-35 to even things like the Zumwalt just seems to indicate that we've simply technology regressed. Kind of disconcerting to imagine the reasons for that.