Hypersonic is an aerodynamics term referring to speeds that are highly supersonic. In the 1970s the term generally came to refer to speeds of Mach 5 and above.
Supersonic airflow is decidedly different than subsonic flow. Practically everything about the way an aircraft flies changes dramatically as an aircraft accelerates to supersonic speeds. Even with this strong demarcation, there is still some debate as to the definition of “supersonic”. One definition is that the aircraft, as a whole, is travelling at Mach 1 or greater. More technical definitions state that you are only supersonic if the airflow over the entire aircraft is supersonic, which occurs around Mach 1.2 on typical designs. From Mach 0.8-1.2 is therefore considered transonic.
Considering the problems with this simple definition, it should be no surprise that a definition of hypersonic would be even more difficult, considering that there is no physical change in airflow that makes it “hyper.” Some phenomena, such as increasing temperature and the formation of a shock layer, begin to affect vehicle design at about Mach 3. Others, such as chemically reacting flow, don’t begin to have a significant effect until about Mach 7 or more. Generally these effects become important “as a whole” around Mach 5. The hypersonic regime is often defined as speeds where ramjets do not produce net thrust, a nebulous definition in itself. A proposed change to allow it to operate in the hypersonic regime is the Scramjet.
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