Aerodynamics

Other factors determined by research in the supersonic range of speeds of artillery projectiles include the shape of the projectiles and the rate of gas flow. The so-called tear-drop shape, which is the ideal streamlined shape for subsonic speeds, is extremely uneconomical in the supersonic range, because of the large frontal surface that must compress the atmosphere and give rise to energy-destroying shock waves of great amplitude.

If gaseous flow occurs through a constricted tube, for example the nozzle of a rocket, at subsonic speeds, the speed of the flow increases and the pressure decreases in the throat of the constriction as predicted by Bernoulli’s principle. The opposite phenomena take place at supersonic speeds, and speed of flow increases in a divergent tube. The exhaust gas of a rocket, therefore, increasing to sonic speed in the throat of a venturi tube or rocket nozzle, further increases its speed and consequent thrust in the diverging flare of the nozzle, thereby multiplying the efficiency of the rocket system. Another factor, long known to rocket designers, is the direct influence of ambient atmospheric pressures on the efficiency of the flight of planes in supersonic speed ranges. That is, the closer the surrounding medium is to a perfect vacuum, the more efficient is the power plant of the plane. The range of the supersonic plane can also be increased by reducing the area, or cross section, displacing atmosphere. Increasing the weight by increasing the length, but at the same time making the plane more slender and equipping it with a needle nose, are necessary features of design for planes operating in the supersonic range in the atmosphere. In the years following World War II, the U.S. Air Force and the U.S. Navy established research institutions that included among their facilities wind tunnels capable of testing plane models and airplane parts in currents of air traveling at supersonic speeds. See Wind Tunnel.

A major development in aeronautics resulting from wind-tunnel research was the discovery by the American physicist Richard Travis Whitcomb of the area rule, a new principle for the design of supersonic aircraft. According to this principle, the sharp rise in drag that occurs at transonic speeds results from the distribution of the total cross-sectional area at each point along the airplane. By pinching in the fuselage where the wings are attached, the reduction in the combined cross-sectional area of the fuselage and the wing produces a decrease in the drag characteristics of the aircraft. Whitcomb’s so-called wasp-waist design made possible an increase of 25 percent in the supersonic-speed range without requiring any additional engine power.

The term supersonics was formerly used more broadly to include the branch of physics now known as ultrasonics, which deals with high-frequency sound waves, usually in the range above 20,000 hertz (Hz).

See also Jet Propulsion.