A waverider is a hypersonic aircraft design that improves its supersonic lift-to-drag ratio by using the shock waves being generated by its own flight as a lifting surface, a phenomenon known as compression lift. The waverider remains a well-studied design for high-speed aircraft in the Mach 5 and higher hypersonic regime, although no such design has yet entered production. The Boeing X-51 scramjet demonstration aircraft was tested from 2010 to 2013. In its final test flight, it reached a speed of Mach 5.1 (5,400 km/h; 3,400 mph).
History
Early work
The waverider design concept was first developed by Terence Nonweiler of the Queen's University of Belfast, and first described in print in 1951 as a re-entry vehicle. It consisted of a delta-wing platform with a low wing loading to provide considerable surface area to dump the heat of re-entry. At the time, Nonweiler was forced to use a greatly simplified 2D model of airflow around the aircraft, which he realized would not be accurate due to spanwise flow across the wing. However, he also noticed that the spanwise flow would be stopped by the shockwave being generated by the aircraft, and that if the wing was positioned to deliberately approach the shock, the spanwise flow would be trapped under wing, increasing pressure, and thus increasing lift. In the 1950s, the British started a space program based around the Blue Streak missile, which was, at some point, to include a crewed vehicle. Armstrong-Whitworth were contracted to develop the re-entry vehicle, and unlike the U.S. space program, they decided to stick with a winged vehicle instead of a ballistic capsule. Between 1957 and 1959, they contracted Nonweiler to develop his concepts further. This work produced a pyramid-shaped design with a flat underside and short wings. Heat was conducted through the wings to the upper cool surfaces, where it was dumped into the turbulent air on the top of the wing. In 1960, work on the Blue Streak was canceled as the missile was seen as being obsolete before it could have entered service. Work then moved to the Royal Aircraft Establishment (RAE), where it continued as a research program into high-speed (Mach 4 to 7) civilian airliners. This work was discovered by engineers at North American Aviation during the early design studies of what would lead to the XB-70 bomber. They re-designed the original "classic" delta wing to incorporate drooping wing tips in order to trap the shock waves mechanically, rather than using a shock cone generated from the front of the aircraft. This mechanism also had two other beneficial effects; it reduced the amount of horizontal lifting surface at the rear of the aircraft, which helped offset a nose-down trim that occurs at high speeds, and it added more vertical surface which helped improve the directional stability, which decreased at high speed.
Caret wing
Nonweiler's original design used the shock wave generated by the aircraft as a way to control spanwise flow, and thereby increase the amount of air trapped under the wing in the same way as a wing fence. While working on these concepts, he noticed that it was possible to shape the wing in such a way that the shock wave generated off its leading edge would form a horizontal sheet under the craft. In this case, the airflow would not only be trapped horizontally, spanwise, but vertically as well. The only area the air above the shock wave could escape would be out the back of the sheet where the fuselage ended. Since the air was trapped between this sheet and the fuselage, a large volume of air would be trapped, much more than the more basic approach he first developed. Furthermore, since the shock surface was held at a distance from the craft, shock heating was limited to the leading edges of the wings, lowering the thermal loads on the fuselage. In 1962 Nonweiler moved to Glasgow University to become Professor of Aerodynamics and Fluid Mechanics. That year his "Delta Wings of Shapes Amenable to Exact Shock-Wave Theory" was published by the Journal of the Royal Aeronautical Society, and earned him that society's Gold Medal. A craft generated using this model looks like a delta wing that has been broken down the center and the two sides folded downward. From the rear it looks like an upside-down V, or alternately, the "caret", ^, and such designs are known as "caret wings". Two to three years later the concept briefly came into the public eye, due to the airliner work at the RAE that led to the prospect of reaching Australia in 90 minutes. Newspaper articles led to an appearance on Scottish Television. Hawker Siddeley examined the caret wing waverider in the later 1960s as a part of a three-stage lunar rocket design. The first stage was built on an expanded Blue Steel, the second a waverider, and the third a nuclear-powered crewed stage. This work was generalized in 1971 to produce a two-staged reusable spacecraft. The 121-foot (37 m) long first stage was designed as a classical waverider, with air-breathing propulsion for return to the launch site. The upper stage was designed as a lifting body, and would have carried an 8000-pound (3.6 t) payload to low Earth orbit.
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