Supersonic Transports (SST)

Although supersonic flight was first accomplished in 1947 by Captain Charles E. Yeager flying the X-l airplane, serious study of the problems of long-range, supersonic cruising aircraft did not begin until the mid-1950's. Research on such aircraft had its origins in the United States in the work which was begun in 1954 or 1955 in support of the Air Force XB-70 program, and research aimed toward a supersonic commercial airliner began in 1958. Independent studies of supersonic airliners were undertaken by the British and French in about 1956.

The British and French Governments ultimately pooled their resources and after lengthy negotiations signed an agreement in November 1962 for the joint development of a supersonic transport (SST) aircraft. Technology development continued at a rapid pace in the United States; and in June 1963, President John F. Kennedy announced the United States commitment to a supersonic transport development program.Hard economic times and mounting environmental concerns, though, combined to force the program's cancellation in March 1971, after more than $500 million of federal funds had been sunk into the program. When the United States supersonic transport (SST) program was cancelled in 1971, thc available noise-control technology left substantial room far improvement.

The demise of the United States supersonic transport gave the market for this type aircraft to the Concorde and the Soviet TU-144. The Soviet SST saw only limited service. Although the Concorde ushered in the supersonic transport (SST) era, was not a commercial success for a variety of reasons. The Concorde consumed about three times as much fuel per seat-mile as equivalent technology (circa 1976) subsonic long-range airplanes. This is largely responsible for its uncompetitive economics - twice the total operating cost (TOC) as similar technology subsonic transports and much worse than that relative to contemporary technology aircraft.

Swept-back wings of such aircraft have introduced inefficiencies due to high skin friction development resulting from the turbulent boundary layer air flow associated with such highly swept wings. This skin friction drag contributes to undesirably high fuel consumption, and results in concomitant high operating expense and short range. Furthermore, the high sweep and short span of such wings results in very inefficient subsonic flight and poor takeoff and landing performance. Accordingly, the main obstacle to widespread acceptance of the supersonic transport is its relatively poor range and fuel efficiency, resulting in uncompetitive economics. The basic cause of this uncompetitive performance is the low lift to drag ratio (L/D) of proposed SSTs, at both supersonic and subsonic speeds.

Current supersonic aircraft designs provided passengers and cargo with reduced flight times, but at the cost of the noise produced by sonic booms. Due to adverse public perception of the noise associated with sonic booms, civil regulations currently prohibit overland supersonic flights in the continental United States. As a result, successful business and commercial aircraft development has generally been limited to subsonic designs. A variety of supersonic military aircraft designs are operationally employed, however, the scope of military supersonic flight operations is sometimes limited due to sonic boom noise.

The theory of sonic boom reduction has been in existence since the 1960s. However, no supersonic aircraft that incorporates sonic boom reducing design features has ever entered production or operational use. Many design studies have been performed, but few have led to promising designs. Implementing a constrained sonic boom signature imposes an exact requirement on the distribution of a quantity called "equivalent area" along the lengthwise axis of the vehicle. Equivalent area at a given location is the sum of a term that is related to the local cross sectional area at that location, plus a term that is proportional to the cumulative lift between the nose of the aircraft and the given location. Thus, the equivalent area distribution involves a combination of the cross sectional area distribution and the lift distribution.

Prior attempts to design passenger aircraft with reduced sonic boom have typically used cross-sectional area only, at least as far aft as the beginning of the passenger cabin, to provide the required equivalent area distribution. With the lift distribution beginning aft of that point, lift must then be built up fairly rapidly in order to provide the center of lift at the center of gravity. The tradeoff between lift and cross section then produces a pinched section near the middle of the vehicle. This pinching is known as "area-ruling" and it is common even on supersonic vehicles which are not designed for reduced sonic boom. However, designing to a sonic boom requirement tends to aggravate the pinching if conventional design approaches are followed.

It is well known that the airframe configuration requirements for efficient supersonic flight are not compatible with the airframe configuration requirements for efficient slow speed flight, take-off and climb, or descent and landing. For low speed flight, and conventional take-off and landing, the optimum wing planform is generally considered to be a long span, narrow chord wing having little, if any, sweep angle.

Since the total lift developed by a lifting airfoil, with other factors such as angle of attack and dynamic pressures being equal, is substantially dependent on the aspect ratio of the airfoil, defined as the square of the span of the airfoil divided by the surface area thereof, it is apparent that a long narrow wing is capable of developing substantially greater lift-to-drag ratio than is attainable using a short broad wing of the same plan area. The use of the high aspect ratio wing offers the advantages that the angle of attack required for landing and take-off is at the low end of the spectrum. The take-off and landing speeds are lower than for low aspect ratio wings, thus permitting a relatively short take-off and landing, as well as a low speed climb to altitude. Furthermore, the drag due to lift is also at the low end of the spectrum, thereby providing high aerodynamic efficiency for subsonic cruise and low power requirement during take-off and landing.

For transonic and supersonic flight however, highly swept wings are considered preferrable because aerodynamic drag may be greatly reduced thereby, and other advantages are also obtained. For example, even during high altitude subsonic cruise the highly swept wing configuration develops a comparatively low drag coefficient, while still developing the required lift coefficient. It has been experimentally shown that lift/drag ratios of 10 to 12 may be obtained with the highly swept wing at supersonic high altitude cruise thus making such flights economically feasible even in the case of commercial transport aircraft. The highly swept wing configuration is also preferred for supersonic flight at low levels, where the combination of high dynamic pressure at the high frequency end of the gust spectrum may establish the structural strength requirements of the aircraft, since the gust loads imposed on a highly swept wing are much smaller than on a more or less straight wing due to a smaller change in lift force resulting from change in the angle of attack. This result is due to the fact that a moving aircraft experiences atmospheric turbulence only as the result of sudden changes in the angle of attack which may be said to be in the direction of the resultant of the vertical component of gust velocity and horizontal component of aircraft velocity.

However, a swept wing aircraft designed solely on the basis of supersonic high performance flight will obviously not perform satisfactorily for subsonic cruise, take-off and landing. Even present day supersonic aircraft are designed with aspect ratios higher than that considered optimum for supersonic cruising flight in order to make take-off and landing feasible. These supersonic aircraft must also climb to cruise altitude at subsonic speeds to prevent heavy shock wave ground damage and they must do this at the expense of increased fuel consumption since the relatively low aspect ratio of the wing results in increased drag due to lift while in the climb. For example, it is not unusual for a supersonic swept wing transport on a transatlantic flight to expend 30% or more of its total fuel requirement during take-off and climb to cruise altitude at subsonic speed.

There have been two recent supersonic aircraft technology development programs sponsored by the US government. They are the High Speed Civil Transport (HSCT) program sponsored by the National Aeronautics and Space Administration (NASA), and the Quiet Supersonic Platform (QSP) program sponsored by the Defense Advanced Research Project Agency (DARPA). These programs included both military and civil aircraft.

It has become clear that no new supersonic transport could be designed without first breaking many fundamental and technical barriers. A new project will benefit from the modern tools devised by aeronautical science and technology during the last period, but advances will have to be made to resolve many difficult challenges. The main areas of progress are related to the environmental impact of the vehicle, to its global performance, and to operational considerations. Meeting the challenges will require fundamental progress in aerodynamic optimization for sonic boom and drag reduction, combustion management for emission reduction, engine design to comply with noise regulations, and propulsion integration to improve performance.