Boeing 2707 SST Design

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.

A bow shock wave exists for free-stream Mach numbers above 1.0. In three dimensions, the bow shock is in reality a cone in shape (a Mach cone) as it extends back from the nose of the airplane. The Mach cone becomes increasingly swept back with increasing Mach numbers. As long as the wing is swept back behind the Mach cone, there is subsonic flow over most of the wing and relatively low drag. A delta wing has the advantage of a large sweep angle but also greater wing area than a simple swept wing to compensate for the loss of lift usually experienced in sweepback. But, at still higher supersonic Mach numbers, the Mach cone may approach the leading edge of even a highly swept delta wing. Because of the high sweep, the landing speeds of airplanes with delta wings are very fast.

Sweepback has been used primarily in the interest of minimizing transonic and supersonic wave drag. At subsonic Mach numbers, however, the disadvantages are dominant. They include high induced drag (due to small wing span or low aspect ratio), high angles of attack for maximum lift, and reduced effectiveness of trailing-edge flaps. The straight-wing airplane does not have these disadvantages. For an airplane which is designed to be multimission, for example, subsonic cruise and supersonic cruise, it would be advantageous to combine a straight wing and swept wing design. This is the logic for the variable sweep or swing-wing. Although not necessarily equal to the optimum configurations in their respective speed regimes, it is evident that an airplane with a swing-wing capability can in a multimissioned role, over the total speed regime, be better than the other airplanes individually.

The swing-wing design attempts to exploit the high lift characteristics of a primarily straight wing with the ability of the sweepback wing to enable high speeds. During landing and takeoff, the wing swings into an almost straight position. During cruise, the wing swings into a sweepback position. There is a price to pay with this design, however, and that is weight. The hinges that enable the wings to swing are very heavy. One major drawback of the swing-wing airplane is the added weight and complexity of the sweep mechanisms.

Swept wings decreased controllability and combat load at takeoff, unless the wings could be pivoted forward during takeoff and landing and swept back during flight. Test articles from wartime German experiments had pointed the way, and the Bell X-5 provided additional data during the early 1950s. The British also had a variable-sweep concept plane called the Swallow, which underwent extensive testing at Langley. The NASA contribution in this development included variable in-flight sweeping of the wings and the decision to locate the pivot points outboard on the wings rather than pivot the wings on the centerline, solving a serious instability problem. All of this eventually led to the TFX program, which became the F-111. It was a long and controversial program but the eventual success of the variable geometry wing on the F-111 and the Navy's Grumman F-14 Tomcat owed much to NASA experimental work.

Boeing began small-scale studies of SST designs in 1952, and in 1958 established a permanent research committee which grew to a $1 million effort by 1960. The committed evaluated a number of alternative designs, all under the name Model 733. Most of the configurations were large delta wing planforms. In 1959 one swing-wing design was evaluated based on Boeing's efforts in the TFX project. In 1960 an evaluation of a baseline 150-seat aircraft for trans Atlantic routes incidated that the swing-wing version promised to be considerably better than the delta wing designs.

NASA did considerable work, starting in 1959, on basic configurations for the SST. In 1959, a delegation from NASA Langley briefed E. R. Quesada, head of the FAA, on the technical feasibility of a supersonic transport (SST). The NASA group advocated a variable geometry wing and an advanced, fan-jet propulsion system. The briefing, later published as NASA Technical Note D-423, "The Supersonic Transport: --A Technical Summary," analyzed structures, noise, runways and braking, traffic control, and other issues related to SST operations on a regular basis. An SST, the report concluded, was entirely feasible. The FAA concurred, and within a year, a joint program with NASA had allocated contracts for engineering component development. Eventually, the availability of advanced Air Force aircraft provided the opportunity to conduct flight experiments as well. There evolved four basic types of layout which were studied further by private industry. Lockheed chose to go with a fixed-wing delta design; whereas, Boeing initially chose a swing-wing design.

Many of the airplane-configuration studies undertaken by NASA during the 1959-62 period were concerned with the design of the North American B-70 Mach 3 bombing airplane and with preliminary designs for a commercial supersonic transport. The SST had become of interest as a national development project and it was felt to be the responsibility of NASA to take the lead in determining the most promising general configurations from which a successful SST might be developed.

Toward this end, the NASA Langley Research Center sponsored what was known as the SCAT (supersonic commercial air transport) program to investigate four basic SST configurations designated SCAT 4, 15, 16, and 17. SCAT designs 4 and 15 were quickly disposed of, leaving SCAT 16, having a wing with controllable sweep, and SCAT 17, having a canard configuration with a fixed delta wing. These two design arrangements were studied intensively both by NASA, in its wind tunnels, and by certain major aircraft companies. Ames, because of its long-standing interest in delta-wing and canard configurations, gave most of its attention to SCAT 17; whereas Langley, for similar reasons, devoted most of its efforts to SCAT 16. The Boeing and the Lockheed aircraft companies were later awarded Government contracts for SST designs based essentially on the SCAT 16 and the SCAT 17 configurations.

Boeing's design, submitted to the FAA on 15 January 1964, was nearly identical to the swing-wing Model 733 studied in 1960. Known officially as the Model 733-197, it was also referred as either the 1966 Model and the Model 2707. This latter name became the best known to the public, while internally Boeing continued to use 733 designation. The design was surprisingly similar to the B-1 Lancer strategic bomber, though the engines were mounted in individual nacelles rather than the twin pods on the Lancer.

The size of the SST design grew to meet airline payload requirements. The new SST was intended to carry 250 passengers, more than twice as many as the Concorde, and to fly at Mach 2.7, versus the Mach 2.2 (1,400 mph) of the Concorde. It was required to have a trans-Atlantic range of 4,000 miles. The higher speed required the aircraft be built of either stainless steel or titanium. At speeds above Mach 2.2 atmospheric friction would cause the aluminium used in the Concorde or Tu-144 to loose structural integrity.

A downselect resulted in North American and Curtiss-Wright being dropped from the program, with both Boeing and Lockheed asked to offer designs using either of the remaining engine designs and able to meet more demanding FAA requirements. In November 1964 a second design review was held. Boeing had enlarged the original design into a 250-seat model, the Model 733-290.

Both Boeing and Lockheed prepared more detailed designs for final source selection in 1966. Boeing's configuration was the 300-seat Model 733-390. Problems with this wing-tail configuration were exhaust scrubbing and acoustic noise/fatigue on the passenger cabin and aft fuselage; and pitch-up in both swept and unswept wing positions. Shortly before the end of the final competition, concerns about the Boeing design led to a drastic redesign. In the original configuration, the engines were to have been under the fixed portion of the swing-wing, but in response to design concerns -- including fears about what the jet exhaust might do to the tail (some suggested it might burn off!) -- the wings, tail and engine arrangement were drastically reworked for the competition deadline.