According to different publications including the standard dictionary, it supposedly begins at around Mach 5 and goes on up to orbital speed. That's a lot of mach.
But that other thread about scramjets got me thinking. Officially, there is no barrier to the first break of Mach (1.0) just that historically is had been really hard to get past it unless you either had enough thrust or enough acceleration (same thing).
Compressibility begins as soon as you start moving, just it is extremely negligible until you reach speed near the average critical Mach of most jet airliners, roughly M0.8; you are within the region known as transonic by then until you pass that barrier and become supersonic.
This is where I get confused somewhat. It had been said that the hypersonic Mach number is just an average, being that some effects can begin at speeds of Mach 3 or as late as Mach 7. Some engineers won't consider hypersonic unless those speeds in question are well above Mach 10.
I remember (could be wrong) that one of the original reasons that NASA choose the speed of M2.4 for the ex-HCST was that if it went past that there would be a sudden increase in friction heating that was not generally linear.
I believe that should be the initial barrier to hypersonic flight as its general characteristic is excessive heating and M2.4 is where it begins. Granted that decision may and probably is due to their researched shape and is not the same for all shapes.
What about an uppermax? I do not know how it makes sense that meteorites can travel at hypersonic velocities of Mach 100+ as they burn up in our atmosphere. Meaning that word should not be used for the noun in question. Though the word hypersonic in not always used for ICBM's or orbital space flights, I think as long as we are talking about flights within the atmosphere, it should be restricted to such.
For example, if say the flight was accelerated by an air breather, the uppermax would occur at a point where the engine cannot further accelerate the body. Right now it is suggested that a scramjet can accelerate a space plane of sorts into orbit. I figure the point at which there is maximum velocity does not occur at thrust = drag. Rather it is where the engine does not get enough oxygen and thus the thrust drops, and then thrust equals drag.
How is this possible? Two words: Oxygen disassociation. Fact: if the heat around a hypervelocity vehicle gets hot enough, the diatomic molecules separate into ions.
As you accelerate to just under Mach 3, the atmosphere so far is a calorically perfect gas. From that point to about Mach 7, you get vibration excitation within the atmosphere. From there to about 5 kilometers per second, there is oxygen disassociation. Which means the engine is not going to be able to burn diatomic oxygen as it continues to accelerate. It is at this point I think the word hypersonic has it's upper maximum, which depending on a bunch of ambient circumstances, can be anywhere from M15 to M20.
If at this speed you continued to orbital speed within the atmosphere, you would be experiencing nitrogen disassociation. And if you for whatever reason decided to accelerate to escape speed within the atmosphere, around 16 kilometers per second, you would be experiencing complete atmospheric ionization. Basically you're flying in a superheated acid-particle soup.
Perhaps this region past hypersonic should/could be named "ultrasonic" or "astrosonic" cuz since scramjets can't work out here only rockets can continue your acceleration. On the other hand, sound and space have nothing to do with each other so using the word sonic at all is dumb.
I usually refer to this region as some fraction of orbital speed. Like if orbital speed is 18000 mph at some altitude and you're going 16200 mph, you would be going 0.90 Orbital.