Waverider

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The Boeing X-51 forebody is an example of cone-derived waverider

A waverider is a hypersonic aircraft design that improves its supersonic lift-to-drag ratio by producing a lifting surface built out of the shock waves being generated by its own flight, a technique known as compression lift. To date the only aircraft to use the technique is the Mach 3 supersonic XB-70 Valkyrie, which was waverider-like with its drooping wingtips. The waverider remains a well-studied design for high-speed aircraft in the Mach 5 and higher hypersonic regime, although no production design has used the concept to date. The Boeing X-51A scramjet demonstration aircraft is in the final stages of development and expected to make its first flight in October 2009.

History

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 he 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. While attempting to develop simplified 3D equations to model the aircraft, he noticed that the shock wave would lead to high pressure under the wing, which could be used for lift. This is the basic principle of the waverider, and was more fully developed in later research.

In the 1950s, the British started a space program based around the Blue Streak missile, which was, at some point, to include a manned vehicle. Armstrong-Whitworth were contracted to develop the re-entry vehicle, and unlike the US 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 enter service. Work on waverider then moved to the Royal Aircraft Establishment (RAE), where it continued as a research program into high-speed (Mach 4 to 7) civilian airliners. The plane shock waveriders of Nonweiler were expanded to use curved shock waves from a conical projection. The expansion to a wider range of compression surface flows allowed the design of waveriders with control of volume, upper surface shape, engine integration and centre of pressure position. Performance improvements and off-design analysis continued until 1970. During this period at least one waverider was tested at the Woomera Rocket Range, mounted on the nose of an air-launched Blue Steel missile, and a number of airframes were tested in the wind tunnel at NASA's Ames Research Center.

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. In this paper the "classic" waverider is described, a modification of the original Armstrong-Whitworth design. In the new design the wings were angled down towards the tips, and the shock waves being generated from their leading edges interacted to form a single flat "plate" shock under the fuselage. The shock wave itself was a lifting surface, generating the needed lift with little physical interaction with the airframe, and dramatically lowering heating. 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 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 manned stage. This work was generalized in 1971 to produce a two-staged reusable spacecraft. The 121-foot (36.9 m) long first stage was designed as a classical waverider, with airbreathing 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.

During the 1970s most work in hypersonics disappeared, and the waverider along with it.

In 1981, Maurice Rasmussen at the University of Oklahoma started a waverider renaissance by publishing a paper on a new 3D underside shape riding the shock wave from a conical projection, as opposed to Nonweiler's simple 2D 'caret' design riding the shock from a flat nose. These shapes have superior lifting performance and less drag. Since then, whole families of cone-derived waveriders have been designed using more and more complex conic shocks, based on more complex software. This work eventually led to a conference in 1989, the First International Hypersonic Waverider Conference, held at the University of Maryland.

One last development of the waverider is the Hypersonic Sail Waverider, which uses a rogallo wing as the lifting surface. The primary purpose for this design is to create a light-weight disposable lifting surface for interplanetary spacecraft to use while maneuvering over planets with an atmosphere. If used over Venus for instance, the spacecraft could aeromaneuver with the lift provided by the waverider to a degree that no gravitational slingshot could hope to achieve.

Design

During re-entry, hypersonic vehicles generate lift only from the underside of the fuselage. The underside, which is inclined to the flow at a high angle of attack, creates lift in reaction to the vehicle wedging the airflow downwards. The amount of lift is not particularly high, compared to a traditional wing, but more than enough to maneuver given the amount of distance the vehicle covers.

Most re-entry vehicles have been based on the blunt-nose reentry design pioneered by Theodore von Kármán. He demonstrated that a shock wave is forced to "detach" from a curved surface, forced out into a larger configuration that requires considerable energy to form. Energy expended in forming this shock wave is no longer available as heat, so this shaping can dramatically reduce the heat load on the spacecraft. Such a design has been the basis for almost every re-entry vehicle since, found on the blunt noses of the early ICBM warheads, the bottoms of the various NASA capsules, and the large nose of the Space Shuttle.

The problem with the blunt-nose system is that the resulting design creates very little lift, meaning the vehicle has problems maneuvering during re-entry. If the spacecraft is meant to be able to return to its point of launch "on command", then some sort of maneuvering will be required to counteract the fact that the Earth is turning under the spacecraft as it flies. After a single low-earth orbit, the launching point will be over 1000 km to the east of the spacecraft by the time it flies over again after one full orbit. A considerable amount of research was dedicated to combining the blunt-nose system with wings, leading to the development of the lifting body designs in the U.S.

It was while working on exactly one such design that Nonweiler developed the waverider. He noticed that the detachment of the shock wave over the blunt leading edges of the wings of the Armstrong-Whitworth design would allow the air on the bottom of the craft to flow spanwise and escape to the upper part of the wing through the gap between the leading edge and the detached shock wave. This loss of airflow dramatically reduced the amount of lift being generated by the waverider (up to a quarter), which led to studies on how to avoid this problem and keep the flow trapped under the wing.

Nonweiler's resulting design is a delta-wing with some amount of negative dihedral — the wings are bent down from the fuselage towards the tips. When viewed from the front, the wing resembles a caret symbol (^) in cross section, and these designs are often referred to as carets. The more modern 3D version typically looks like a rounded letter 'M'. Theoretically, a star-shaped waverider with a frontal cross-section of a "+" or "×" could reduce drag by another 20%. The disadvantage of this design is that it has more area in contact with the shock wave and therefore has more pronounced heat dissipation problems.

Waveriders generally have sharp noses and sharp leading edges on their wings. The underside shock-surface remains attached to this. Air flowing in through the shock surface is trapped between the shock and the fuselage, and can only escape at the rear of the fuselage. With sharp edges, all the lift is retained.

Even though sharp edges get much hotter than rounded ones at the same air density, the improved lift means that waveriders can glide on re-entry at much higher altitudes where the air density is lower. A list ranking various space vehicles in order of heating applied to the airframe would have capsules at the top (re-entering quickly with very high heating loads), waveriders at the bottom (extremely long gliding profiles at high altitude), and the Space Shuttle somewhere in the middle.

Simple waveriders have substantial design problems. First, the obvious designs only work at a particular Mach number, and the amount of lift captured will change dramatically as the vehicle changes speed. Another problem is that the waverider depends on radiative cooling, possible as long as the vehicle spends most of its time at very high altitudes. However these altitudes also demand a very large wing to generate the needed lift in the thin air, and that same wing can become rather unwieldy at lower altitudes and speeds.

Because of these problems, waveriders have not found favor with practical aerodynamic designers, despite the fact that they might make long-distance hypersonic vehicles efficient enough to carry air freight.

Some researchers controversially claim that there are designs that overcome these problems. One candidate for a multi-speed waverider is a "caret wing", operated at different angles of attack. A caret wing is a delta wing with longitudinal conical or triangular slots or strakes. It strongly resembles a paper airplane or rogallo wing. The correct angle of attack would become increasingly precise at higher mach numbers, but this is a control problem that is theoretically solvable. The wing is said to perform even better if it can be constructed of tight mesh, because that reduces its drag, while maintaining lift. Such wings are said to have the unusual attribute of operating at a wide range of mach numbers in different fluids with a wide range of Reynolds numbers.

The temperature problem can be solved with some combination of a transpiring surface, exotic materials, and possibly heat-pipes. In a transpiring surface, small amounts of a coolant such as water are pumped through small holes in the aircraft's skin (see transpiration and perspiration). This design works for Mach-25 spacecraft re-entry shields, and therefore should work for any aircraft that can carry the weight of the coolant. Exotic materials such as carbon-carbon composite do not conduct heat but endure it, but they tend to be brittle. Heatpipes are not widely used at present. Like a conventional heat exchanger, they conduct heat better than most exotic solid materials, but like a thermosiphon do not require pumping. The Boeing X-51A deals with external heating through the use of a tungsten nosecone and space shuttle-style heat shield tiles on its belly. Internal (engine) heating is absorbed by using the JP-7 fuel as a coolant prior to combustion.

External links

• Aerospace Web: Hypersonic Waveriders
• Accurate Automation Corporation has built several model waveriders for low-speed study, including LoFLYTE and NASA's X-43
• ASTRA Waverider