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Photo of a hypersonic experimental plane
 with some of its characteristics

As an aircraft moves through the air, the air molecules near the aircraft are disturbed and move around the aircraft. Exactly how the air re-acts to the aircraft depends upon the ratio of the speed of the aircraft to the speed of sound through the air. Because of the importance of this speed ratio, aerodynamicists have designated it with a special parameter called the Mach number in honor of Ernst Mach, a late 19th century physicist who studied gas dynamics.

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 mph and Mach number M greater than five, M > 5. We are going to define a high hypersonic regime at M > 10 to account for re-entry aerodynamics. The chief characteristic of hypersonic aerodynamics is that the temperature of the flow is so great that the chemistry of the diatomic molecules of the air must be considered. At low hypersonic speeds, the molecular bonds vibrate, which changes the magnitude of the forces generated by the air on the aircraft. At high hypersonic speeds, the molecules break apart producing an electrically charged plasma around the aircraft. Large variations in air density and pressure occur because of shock waves, and expansions.

The only manned aircraft to fly in the low hypersonic regime were the X-15 and the Space Shuttle during re-entry. The X-15 is shown on the figure. The X-15 used a rocket propulsion system to achieve sustained Mach 6 flight. Recently, an un-manned X-43A used a scramjet, or supersonic combustion ramjet, to make two hypersonic flights; one at Mach 7, the other at Mach 10. Because of the pressure losses associated with the terminal shock of the inlet, a ramjet has very limited performance beyond Mach 5. Because lift and drag depend on the square of the velocity, hypersonic aircraft do not require a large wing area. For Mach numbers greater than 5, the frictional heating of the airframe by the air becomes so high that very special nickel alloys are required for the structure. For some proposed hypersonic aircraft, the skin is actively cooled by circulating fuel through the skin to absorb the heat.

While a ramjet is well suited for the cruise portion of the flight, a ramjet can not generate static thrust. So another propulsion system is needed to accelerate the aircraft until the ramjet can generate sufficient thrust. Turbine based combined cycle (TBCC) engines are being studied as one approach to the low speed ramjet problem.


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Jun 12 2014

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