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Photo of a high supersonic spy 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 greater than the speed of sound, the aircraft is said to be supersonic. There are some very special aircraft which fly in the high supersonic regime in which aircraft skin temperature becomes high enough that special materials must be used, but the temperature is still low enough that the air molecules remain intact. Typical speeds for high supersonic aircraft are greater than 1500 mph but less than 2500 mph. The Mach number M is then greater than three, but less than five, 3 < M < 5. In addition to the high temperatures, we encounter compressibility effects and the local air density varies because of shock waves, and expansions.

The only aircraft to cruise in this regime were the XB-70 and the SR-71/YF-12. An SR-71 is shown on the figure. Both of these aircraft employed very specialized inlet systems to bring high speed air into the engine. The XB-70 employed six special afterburning turbine engines while the SR-71 used an integrated turbo-ramjet. Because lift and drag depend on the square of the velocity, these aircraft did not require large wing area in cruising flight. Like any supersonic aircraft, the wings are swept in planform to reduce drag.

As any metal is heated, it begins to lose strength. The amount of the loss depends on the type of metal. For Mach numbers greater than 2.5, the frictional heating of the airframe by the air becomes high enough that light weight aluminum can not be used for the structure. The SR-71 was made largely of titanium, which has good high temperature characteristics, but is still light enough for aircraft structures. During heating, any metal expands. To allow for the expansion, slip joints are used in many places on the SR-71. On the ground, the SR-71 fuel tanks leak, and they do not seal until the aircraft heats up during flight.


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

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