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Mach speed:
Mach number (Ma or M) (generally pronounced, sometimes or mæk/) is the speed of an object moving through air, or any fluid substance, divided by the speed of sound as it is in that substance. It is commonly used to represent an object's speed (such as an aircraft or missile), when it is travelling at the speed of sound (or multiples thereof).


An F/A-18 Hornet at transonic speed and displaying the Prandtl–Glauert singularity just before reaching the speed of sound

where
is the Mach number
is the speed of the source (the object relative to the medium) and
is the speed of sound in the medium
The Mach number is named after Czech/Austrian physicist and philosopher Ernst Mach. Because the Mach number is often viewed as a dimensionless quantity rather than a unit of measure, with Mach, the number comes after the unit; the second Mach number is "Mach 2" instead of "2 Mach" (or Machs). This is somewhat reminiscent of the early modern ocean sounding unit "mark" (a synonym for fathom), which was also unit-first, and may have influenced the use of the term Mach. In the decade preceding man's flying faster than sound, aeronautical engineers referred to the speed of sound as Mach's number, never "Mach .
Hypersonic
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This article is about hypersonic speeds in aerodynamics. For other uses, see Hypersonic (disambiguation).


Boeing X-43 at Mach 7
In aerodynamics, hypersonic speeds are those that are highly supersonic. Since the 1970s, the term has generally been assumed to refer to speeds of Mach 5 (5 times the speed of sound) and above. The hypersonic regime is a subset of the supersonic regime.
The precise Mach number at which a craft can be said to be fully hypersonic is elusive, especially since physical changes in the airflow (molecular dissociation, ionization) occur at quite different speeds. Generally, a combination of effects become important "as a whole" around Mach 5. The hypersonic regime is often defined as speeds where ramjets do not produce net thrust. This is a nebulous definition in itself, as there exists a proposed change to allow them to operate in the hypersonic regime (the Scramjet).