Waverider

A waverider is a type of supersonic aircraft design that improves its supersonic lift-to-drag ratio[?] by "surfing" on its own shockwave. To date no aircraft has been based on the waverider design with the possible exception of the XB-70 Valkyrie, possible because it is debatable how much lift the aircraft gained from compression lift. The waverider remains a well-studied design for high-speed aircraft in the Mach 5 and higher hypersonic regime.

History

The waverider concept was first developed by Terence Nonweiler of Queen's University, Belfast[?], and first described in print in 1951 as a re-entry vehicle. It consisted of a delta-wing planform 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 equasions 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 turbulant air on the top of the wing.

In 1960 work on the Blue Streak was cancelled as the missile was seen as being obsolete before it could enter service. Work on waverider then moved to the Royal Aircraft Establishment, where it continued until 1965 as a research program into high-speed civilian airliners (Mach 6). 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 became 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 featuring a concave lower surface that "trapped" the shock wave for lift. 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 lead to an appearance on Scottish Television.

Hawker Siddeley[?] examined the waverider in the later 1960's 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 long first stage was designed as a classical waverider, with airbreathing propulsion for return to the launch sight. The upper stage was designed as a lifting body, and would have carried an 8000 pound payload to low Earth orbit.

In 1981 Rasmussen at the University of Maryland[?] 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 manuvering over planets with an atmosphere. If used over Venus for instance, the spacecraft could aeromanuver 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 isn't particularily high, compared to a traditional wing, but more than enough to manuever given the amount of distance the vehicle covers.

The problem is to dump the heat generated in this process. In a simple way of looking at it, the energy used to place the vehicle into orbit will be returned to it when it returns to Earth, so if you consider that the vehicle likely used a very large rocket to get into space, you can gain some indication of the magnetude of the problem.

Most re-entry vehicles have been based on the blunt-nose reentry design pioneered by von Karijan[?]. He demonstrated that a shock wave is forced to "detatch" from a curved surface, forced out from in a larger configuration that requires considerable energy to form. Energy lost in forming the 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. This means that the vehicle has problems manuvering during re-entry. If the spacecraft is meant to be able to return to it's point of launch "on command", then some sort of manuvering 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 1000km to the east of the spacecraft by the time it flies over. A considerable amount of research was dedicated to combining the blunt-nose system with a wings, leading to the development of the lifting body designs in the US.

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 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 upside-down W.

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 a lot 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 comparison between various vehicles would put capsules at one end, re-entering quickly with very high heating loads, waveriders at the other with extremely long gliding profiles at high altitude, and the Space Shuttle 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.

It is controversial, but some researchers 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's theoretically soluble. The wing is said to perform even better (theoretically) if it can be constructed of tight mesh, because that reduces its drag, while lift remains constant. 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 reynolds numbers.

The temperature problem can be solved with some combination of a transpiring surface, exotic materials, and possibly heat-pipes. In a transpiring suface, small amounts of a coolant such as water are pumped through small holes int he aircraft's skin. This design works for Mach-25 spacecraft reentry shields, and therefore should work for any aircraft that can carry the weight of coolant. Exotic materials such as carbon-carbon composite do not conduct heat, but rather endure it. They tend to be brittle. Oddly, heat pipes may be an excellent, unexplored solution. They are passive (no pumps), and conduct heat better than most exotic solid materials. They would disperse the heat away from the active parts of the wing.

External links:

Accurate Automation Corporation (http://www.accurate-automation.com/)

- has built several model waveriders for low-speed study, including LoFLYTE and NASA's X-43