Understanding the Supersonic and Hypersonic Aircrafts

As stated in www.emeraldinsight.com , as the airflow over an aircraft reaches and exceeds Mach 1, drag begins to rise very steeply. Most transonic aircraft lack propulsion power to push very far into the drag rise in level flight, but by 1950 it had been verified that transonic aircraft could exceed flight speeds of Mach 1 in a dive. Fighter aircraft need excess power for maneuvering. Generally this is achieved by adding afterburning to the jet engine. The next step, begun early in the 1950s, was to refine the aerodynamics and controls of fighters to permit them to fly at supersonic (over Mach 1) speeds in afterburning. The most notable changes involved adoption of greater sweep angle, highly swept delta wings, or thin, low-aspect wings with sharp leading edges. By 1960, most fighters entering service were capable of exceeding Mach 2. This was possible only at high altitudes (limited by aerodynamic heating and forces) and for brief periods (limited by high afterburning fuel consumption), but the speed was tactically useful. These were transonic airplanes that were capable of supersonic sprints.

As transonic aircraft entered service in substantial numbers for military and commercial purposes in the 1950s, it was generally anticipated that they would soon be supplanted or at least widely supplemented by truly supersonic aircraft that normally flew at over Mach 1. However, by 2000, only one type of aircraft regularly spent more than half of its time aloft in supersonic flight, the Anglo-French Concorde airliner. Obstacles to wider supersonic flight included weight, cost, and environmental impact. Theory and experiment quickly led to the conclusion that the best shape for supersonic flight was slender and arrow-like and that suitable slender aircraft could cruise supersonically with efficiency generally matching that of transonic aircraft. Slender airplanes were not inherently suited to the relatively low speeds needed for landing and take-off, however. Compromises and adaptations were necessary for controllable and efficient flight over a range of speeds that varied by 10:1 or more from maximum to stalling, leading to extra weight and expense. Moreover, supersonic flight presented even greater structural challenges than transonic flight, and this also brought cost and weight penalties. These arose in part from the high dynamic pressures involved in flight at very high speeds, but even more so from aerodynamic heating, representing the sum both of friction and of air compression in the supersonic flow.

According to history.nasa.gov , for sustained flight at more than Mach 2.5, aluminum loses too much strength due to heating to be used as a structural material unless it is cooled or protected. Steel or titanium may be used instead. In aircraft, any increase in weight brings cascading problems. This is especially true for supersonic aircraft, which tend to be most attractive for longrange missions requiring large fuel loads. High weight allied with the need for special materials and structures pushed costs up for supersonic aircraft. Moreover the supersonic shock wave reaches disturbing and even destructive levels on the ground below the path of the supersonic plane even when it flies at altitudes of 20 kilometers or more. These problems combined to drastically slow acceptance of supersonic flight. Indeed, one supersonic type that did see successful service, the U.S. Lockheed SR-71 Mach 3 strategic reconnaissance aircraft, was ultimately withdrawn from operations because its functions could be performed more economically by other means.

As mentioned by history.nasa.gov , at over Mach 4, a series of changes in aerodynamic phenomena led to the application of the label ''hypersonic.'' In principle, hypersonic flight presents attractive opportunities. In the 1960s there was a belief that supersonic aircraft might be supplemented relatively rapidly by hypersonic types. In practice, the problems of weight, cost, and environmental effects proved to be even more intractable. At hypersonic speed, heating is so intense that even steel and titanium lose strength. A number of research programs relating to hypersonic flight, stimulated in part by the demands of reentry from space, led to the accumulation of considerable knowledge of many of the issues. Progress on development of air-breathing propulsion systems was halting however, and several efforts aimed at construction of a prototype hypersonic aircraft collapsed owing to cost and technology issues. Thus at the end of the twentieth century, the promise of supersonic flight seemed just out of reach and that of hypersonic flight not yet clearly in view.