I know because my great uncle died flying the X-15 on an experimental flight [2], so it's always been an interesting story for my family.
[1] https://en.wikipedia.org/wiki/North_American_X-15 [2] https://theaviationgeekclub.com/the-story-of-mike-adams-the-...
Scramjet planes are faster than other non-rocket planes I would imagine?
Scramjets do need air, although something to consider is that the faster you go, the more air you get. So at high altitudes with very thin air, you can go faster to get more air. In fact you have to go faster (true airspeed) in thin air anyways even if your brought along liquid oxygen for your engine, because as the air thins your stall speed will go up.
The whole thing is a complicated system of equations. Engine capabilities and/or critical mach number (for planes designed for subsonic flight, like the U-2) of the airframe set an upper limit on how fast you can go. Air density decreases with altitude, increasing the stall speed (according to true airspeed, not indicated airspeed.) But for a given true airspeed, with increased altitude the temperature of the air drops, consequently dropping the speed of sound, which reduces the mach number. In subsonic planes as altitude goes up the stall speed and critical mach number eventually pass each other, beyond which your plane can no longer fly. The point of that intersection is known as the 'coffin corner'. In planes designed for supersonic flight passing the critical mach number isn't a problem, but eventually you'll reach a point where the drag on the aircraft is so great that the engines simply don't have enough power to overcome it. Increasing altitude reduces drag as the air gets thinner, but that only works for so long. Air speed inside the engines needs to be considered too. The airspeed past the turbine blades needs to be such that you won't get compressor stall. When you discard the turbine requirement, that's where things start to get interesting. A ramjet has no turbines, it compresses the air using the intake geometry. At full speed the SR-71 was actually producing most of it's thrust with a ramjet effect, most of the air going into the engines was not going through the turbines. The difference between a ramjet and a scramjet is that in a ramjet, the air inside the engine becomes subsonic again for the burn, but in a scramjet the air inside the engine remains supersonic. Scramjets could go really fast.
The big challenge is basically making sure that you mix your fuel with the air, combust it fully, and extract the energy before it flies out the back of the engine. Scramjets are the engines capable of this at the highest speeds.
I think the hypersonic planes will all be drone munition delivery (or just munitions themselves) for a long time. Maybe some day they'll carry people but there's no supersonic aviation much less hypersonic and even then the piloting at speed will probably be all automated.
So when you get to mach 6 you might as well be in space....
[1]: https://en.wikipedia.org/wiki/Mercedes-Benz_W125_Rekordwagen
@DuskStar
It's certainly a glider on reentry. Now, most speed records for planes are "in level flight" which would disqualify the shuttle during reentry. But if we ignore that it's interesting that the speed record for manned aircraft is held by a glider, and the speed records for RC aircraft are also held by gliders! https://www.youtube.com/watch?v=MoaWlKC3wIM
As a Brazilian (see previous comments about the Wright Brothers), I consider that an airplane:
* Is powered, otherwise, it's a glider;
* Can maintain level flight, otherwise, it's just "Falling with style";
* Can take off on it's own, assistance (RATO, catapults) is allowed for short take-offs (like from a carrier);
The Shuttle fails in all that in some point of the flight or another, Buran on the other hand, could be fitted with air breathing jet engines that allowed it to do all that.
And on descent it was gliding, not under its own power.
Well, by that definition I think the X-15 isn't a plane - it required a B-52 mothership to launch after all. IMO it's hard to find a definition of "plane" that includes the X-15 but not the Space Shuttle stack (not the orbiter on its own - the full stack).
I like Douglas Adams' description: The ship hung in the sky in much the same way that bricks don't.
https://en.wikipedia.org/wiki/Richard_Pearse#Flights
His flight was a bit... crashy. The Wrights managed a fully controlled powered flight first, but even then their "first flight" wasn't as far as Pearce's flight had been nine months earlier. And it also ended in a crash.
Typically the Wright Flyer is credited with "the first sustained and controlled heavier-than-air powered flight" or something similar.
If you add enough qualifiers you can be first at anything.
So as far as "Official" speed records go the fastest "airplane" is a Sr-71. Its technically the fastest manned air breathing aircraft.
There seem to be some semantic ambiguities between Aircraft, Airplane, Rocket, etc.
The other popular case, the only lost ship was also similar to the bad MCAS system in the Boeing 737max, the pilot forgot to turn off the problematic flight control system, and he didn't see the 90 yaw angle because he turned it off, so he raced straight down, and the control SW kept it that way.