The Boeing 2707 Supersonic Transport
The 2707, Boeing’s contender for a supersonic transport, may seem like ancient history now, but it was advanced then in both concept and technology. Perhaps it was too much so.
Because the jet race had already been won by the UK with the de Havilland DH.106 Comet and the then-designated USSR with the Tupolev Tu-104, the US was left without choice if it wished to turn the tides in the supersonic sector, especially since the same two countries were preparing to launch such designs of their own, respectively in the form of the Aerospatiale-British Aerospace Concorde and the Tupolev Tu-144 in the early 1960s.
Consensus in this early pure-jet period was that supersonic airline travel would be the next logical evolution of the subsonic one.
Submissions to fill this segment were made by several aircraft manufacturers in the United States. Boeing, for example, considered a Mach 1.8 aircraft, accommodating 227 passengers. Lockheed’s concept was more ambitious and radical. It produced a design proposal incorporating an airfoil-shaped fuselage and a doubly compounded delta wing projected to achieve Mach 3 speeds. Capacity, however, was not unlike that of Boeing’s aircraft at 218. Designated NAC-60, North American’s concept closely resembled the military B-70 Valkyrie, itself a supersonic design with canards, a compoundly-swept delta wing, and four aft-mounted engines grouped in pairs. It was also slated for the Mach 3 speed realm.
Boeing’s 2707-100, numerically considered the first of the second supersonic generation of airliners after its 707, was ultimately selected on December 31, 1966. Unlike the UK and USSR aircraft, it was intended, from the outset, to eclipse the boundaries of traditional configuration, structure, and speed, offering an extended service life.
Featuring titanium construction to withstand the 500-degree Fahrenheit structural temperatures generated by the friction of its intended, 1,800-mph/Mach 3 cruise speed, it sported a variable geometry delta wing, which pivoted on screw jacks and titanium bearings to cater to the extreme velocity variations, ranging from low subsonic approach speeds in the extended position to high supersonic cruise ones in the retracted one. Trailing edge flaps were fitted for the former portion of flight.
The tailplane, with separate vertical and horizontal surfaces, was otherwise conventional.
Power was to have been provided by four General Electric, wing-underside attached engines.
A full-scale wooden mockup of the supersonic airliner, intended to carry 300 passengers, was built.
Although the 113 optioned orders placed by 26 worldwide airlines seemed promising in June of 1967, the ambitious design had exceeded the technological expertise to transform it into reality. Aside from the inherent instability it demonstrated during wind tunnel tests, the weight of the swing-wing aircraft was prohibitively excessive, carrying a 40,000-pound penalty, thus leaving less available for the fuel needed to provide the range that carriers sought.
The immediate passenger capacity/payload reduction and/or fuel/gross weight increase solutions proved inadequate.
Supersonic flight, other than in the limited, high-altitude military form, was little understood at the time, especially for routine, scheduled commercial operations, and obstacles existed far beyond the drawing board of the design teams. Public reactions, sometimes bordering on hysteria, for instance, included protests concerning the sonic boom, its resultant property damage on the ground, the exclusion of overland flights (which reduced the potential airline market of the aircraft), the rise in world temperatures, the melting of the polar caps, the destruction of land- and ocean-dependent flora and fauna, and the reduction in radiation protection from the ozone layer.
Bureaucratically, the program was continually delayed by airframe and powerplant reassessments and the granting of the necessary governmental funding of the design.
Because the type, as envisioned n its initial version, failed to offer acceptable payload and range capabilities, a second, the 2707-200, was proposed. Although it featured an elongated fuselage and Tu-144-resembling canards above and behind the cockpit, it weighed in at 750,000 pounds, which was 25 percent higher than envisioned and greater than that of a 500-passenger 747-100, and therefore failed to meet the FAA’s finalized design submission deadline.
Even its 23,000-pound weight reduction program placed its gross weight 52,000 pounds above the target.
While its radical, swing-wing feature was technologically feasible, it failed to offer the needed parameters, because Boeing was unable to integrate the pivots, engines, and undercarriage in an efficient engineering package.
Forced to abandon this variable-geometry airfoil concept, it produced a third version, the 2707-300. Featuring a 268.8-foot overall length, it incorporated the fixed, supersonic standard delta wing planform utilized by Concorde and the Tu-144, with a 141.8-foot span and an 8,497-square-foot area. The horizontal and vertical tailplane, with a 50.1-foot height, remained conventional.
Powered by four 60,000 thrust-pound General Electric GE4/J5P turbojets, it offered double the capacity and one-third more speed than its UK and USSR competitors, however, and was intended to transport 234 passengers 5,000 miles at 1,890-mph speeds at 60,000-foot service ceilings. The prototype’s 640,000-pound gross weight was expected to increase to 710,000 pounds on production aircraft.
Simplicity, coupled with a size reduction and the elimination of the variable geometry airfoil’s weight and aerodynamic obstacles, resulted in lower production and unit costs, which, in turn, Boeing believed would have attracted greater sales. That figure was then envisioned as being as high as 500.
In October of 1968, or five months before Concorde first flew, the definitive 2707-300 was chosen as the US’s supersonic transport design and construction of its prototype commenced in September of the following year, provisioning it as the third airliner in its class to enter the market. But it never would.
Continually subjected to a design and development program that was, at times, even more turbulent than Concorde’s, it fought for survival.
The obstacles, as befitting of early 1960s commercial supersonic technology, were numerous and insurmountable, including escalating research and production costs, increasing gross weights, decreasing ranges and payloads, rising seat-mile costs, excessive fuel burns and engine noise, the need for higher than subsonic fares, and the fear that first class passengers would switch to the higher-speed transport, leaving the conventional, subsonic ones without the yield on which they depended for profitability.
Limited in route application, the type could only be economically viable with high load factors on very long range routes.
Innovative technology, it had become increasingly apparent, could not support the supersonic concept on a commercial level. Yet, blinded at times by the need to recapture the title lost during the subsonic race and that “pride-goeth-before-a-fall” dynamic necessitated by the desire to regain national prestige, the program remained aloft with the continued, albeit obstacle-ridden, granting of federal funds.
Part of this buoyancy, needless to say, was airline interest in the product, but, as occurred with Concorde, this began to wane, since they were already financially strapped with orders for widebody 747s.
Public, government, and aircraft manufacturer doubts concerning the 2707-300’s ability to ever economically achieve its fore-claimed noise, payload, and range design goals caused declining confidence to coincide with diminishing enthusiasm for the concept.
Presidential support for a supersonic transport program fluctuated widely. Escalating development costs spawned by increasing technological hurdlers and requiring additional governmental funding only resulted in increasing opposition to it. Because Tupolev seemed unable to solve its own Tu-144 problems and Concorde’s fuel-burn resulted in initial sales of only ten aircraft to Air France and British Airways (a number too small to pose any competitive threat), continued 2707-300 funding could no longer be justified.
On March 18, 1971, therefore, the House voted against it, echoed several days later by the Senate. Although supporters attempted to restart the program by rechanneling the $85.3 million for its termination into further development, and although the House itself voted in favor of this action on May 12, the Senate rejected it five days later.
Fifteen percent of the first 2707-300 airframe had been cut at the time and a 296-foot stretched version, to accommodate 321 passengers, was then envisioned.
All three US, UK, and USSR programs had been plagued by unprecedented opposition to new technology that many believed would have been detrimental to the atmosphere, the earth’s environment, and humanity. Because of its tremendous technological leap, exploding development costs, and irresolvable engineering difficulties, it never became the hoped-for reality in the US and, after a few route proving flights, the Tupolev Tu-144 itself was withdrawn from service in the USSR.
That only one such supersonic aircraft, Concorde, ever entered the scheduled sector, that it only accounted for a handful of sales to the carriers whose governments they represented, and that the exorbitant fuel costs required to sustain its speed all indicate that, while a commercial design was then technologically feasible, that It was not economically possible.