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As an aircraft moves through the air, the air molecules near theaircraft are disturbed and move around the aircraft.Exactly how the air re-acts to the aircraft depends upon theratio of the speed of the aircraft to thespeed of sound through the air.Because of theimportance of this speed ratio, aerodynamicists have designated itwith a special parameter called theMach numberin honor of Ernst Mach, a late 19th century physicist who studied gasdynamics.
For aircraft speeds which are much greater than the speed of sound,the aircraft is said to be hypersonic.Typical speeds for hypersonic aircraft are greater than 3000 mphand Mach number M greater than five, M > 5.We are going to define ahigh hypersonicregime at M > 10 to account for re-entry aerodynamics.The chief characteristic of hypersonic aerodynamics is that thetemperatureof the flow is so great that the chemistry of the diatomic molecules of theairmust be considered. At low hypersonic speeds, the molecular bondsvibrate, which changes the magnitude of the forces generated bythe air on the aircraft. At high hypersonic speeds, the moleculesbreak apart producing an electrically charged plasma around theaircraft. Large variations inair densityandpressureoccur because ofshock waves, andexpansions.
The only manned aircraft to fly in the low hypersonic regime were theX-15 and the Space Shuttle during re-entry. The X-15 is shown on the figure.The X-15 used arocketpropulsion system to achieve sustained Mach 6 flight.Recently, an un-manned X-43A used ascramjet,or supersonic combustionramjet,to make two hypersonic flights; one at Mach 7, the other at Mach 10.Because of the pressure losses associated with theterminal shockof theinlet,a ramjet has very limited performance beyond Mach 5.Becauseliftanddragdepend on the square of thevelocity,hypersonic aircraft do not require a largewing area.For Mach numbers greater than 5, the frictional heating ofthe airframe by the air becomes so high thatvery special nickel alloys are required for thestructure. For some proposed hypersonic aircraft,the skin is actively cooled by circulating fuel throughthe skin to absorb the heat.
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