Boeing 2707 supersonic jet vs the Concorde: The evolution (and collapse) of supersonic flight

While Concorde is the name that springs to mind in the context of supersonic air travel, there were three contenders locked in the battle to build a commercially successful supersonic transport (SST).

Such was the excitement for supersonic flight at the time, dozens of airlines were signing up for the new planes. In fact, Qantas had ordered 10, including four Concordes and six of Boeing's planned supersonic jet.

The story has its gestation in the 1960s, a period when the US believed it owned the skies. Boeing and Lockheed were the dominant commercial aircraft builders. The US was locked in the "space race" with the Soviet Union, both sides competing to send their astronauts into ever more adventurous missions in orbit around the Earth. On May 25, 1961, President Kennedy unveiled plans to put a man on the moon before the end of the decade, delivering a knockout blow and trumping the Soviets once and for all.

While the US was keen to demonstrate and maintain its technological superiority in the skies, an interloper suddenly appeared on the scene, and the US was in danger of being left behind. In 1962 the British and French governments signed an agreement to jointly build a new SST airliner that would travel at twice the speed of sound. They called it the Concorde.

Najeeb Halaby, the Kennedy administration's newly appointed director of the Federal Aviation Authority, argued it would be a stunning setback if the US failed to develop its own SST aircraft. Halaby pinned the loss at 50,000 jobs and $US4 billion if US carriers turned to foreign suppliers for an SST aircraft. When Pan Am flagged its intention to order the Concorde, Kennedy experienced a rush of blood and announced that the US was going to build its own SST, bigger and faster than the European Concorde. So fixated was the government that it agreed to kick in 75 per cent of the development costs.

Lockheed and Boeing submitted plans that were approved for further development. While Concorde was designed to fly at Mach 2 with 100 passengers, Lockheed's L-2000 was basically a Concorde on steroids, flying at Mach 3 with 278 passengers and a range of 6400 kilometres. Boeing's 2707 was only slightly more modest – Mach 2.7, 270 passengers, but a range of 6700 kilometres. In the head-to-head design competition the Boeing carried the day.

The most obvious difference with the Boeing 2707 was the radical swing-wing design. For take-off and landing the wing would be extended, offering greater lift, a lower take-off and landing speed and a shorter runway requirement. As flight speed increased the wing would fold back to a delta shape to offer less drag, enabling it to reach its cruise speed.

More speed and a bigger payload gave the 2707 a competitive advantage over Concorde. More than two dozen airlines placed orders for more than 100 aircraft. Qantas asked for six 2707s.

It was ambitious from the start, and the requirement to demonstrate US aircraft building superiority and out-Concorde the Concorde proved a major hurdle. Even flying at Mach 2, Concorde experienced heat of more than 100C and extreme expansion and contraction of its fuselage that caused metal fatigue, halving the lifespan of its airframe from a traditional jet-engine commercial aircraft.

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Increasing that speed and flying a bigger, heavier aircraft, as the Boeing 2707 was designed to be, created a whole new set of challenges for its builders. Instead of aluminium, the aircraft would be built from titanium, a far more expensive material. The swing-wing design was perfectly workable, as proven by General Dynamics F-111, for many years the main tactical strike aircraft of the RAAF, but translating that technology from a relatively small aircraft to one capable of carrying 270 passengers was a huge technological leap. Engineering the swing-wing required massive strengthening which dramatically increased weight, causing the 2707 designers to revert to the fixed delta wing.

By this time Concorde was in testing phase but that had revealed some unexpected problems – from the public response on the ground. So ear-shattering was Concorde's sonic boom it quickly became obvious that supersonic flights over land were unviable. The only routes where SST aircraft would be able to operate were over water, yet the SST would only make sense for US carriers if they were able to operate on trans-continental routes. Faced with the prospect of operating their home-built SST on trans-Atlantic and trans-Pacific routes alone, their enthusiasm for the 2707 sank like lead balloon.

The Boeing 2707 project was terminated when the US government pulled the plug in March 1971. Although the two prototype 2707s never flew, many of the designs that Boeing developed in pursuit of the supersonic dream have found their way into modern aircraft, such as the flat-top supercritical wing, which reduces drag.

Meanwhile, back in the 1960s the Soviets had jumped on the bandwagon and come up with their own design for an SST aircraft, the TU-144, basically a Concorde knock-off. Relying on time-proven methodology, the Soviets stole thousands of technical drawings of the Concorde and built what was basically a doppelganger, albeit larger. The Soviets rushed their SST program to completion and the TU-144 made its maiden flight on December 31, 1968, two months before Concorde's.

The TU-144 carried more passengers than Concorde and flew faster but it was technologically inferior. While the Concorde could maintain supersonic speed with its fuel-gobbling afterburners off, a mode known as supercruise, the TU-144 had to keep its afterburners activated, reducing its range. Delta wing aircraft have a higher landing speed, and longer runway requirements, than conventional wing aircraft. This was particularly so in the case of the TU-144, with a landing speed of about 329km/h. Concorde had a more refined wing profile that provided greater lift at low speeds, giving the aircraft a touchdown speed of 300km/h. This is still relatively high and so the Concorde's designers gave it ceramic brakes, the first use of this technology in a commercial aircraft, while the TU-144 deployed twin drogue chutes to slow it down on landing.

While noise levels in the Concorde cabin were about standard for aircraft of its time, the engines on the TU-144 were so loud that conversation was impossible. Flights were frequently cancelled due to technical issues, and Soviet authorities were terrified lest its shortcomings be revealed. Those fears were borne out when a TU-144 exploded spectacularly in front of the world's aviation industry at the 1973 Paris Airshow

The TU-144 made its first commercial flight in 1977, two years after Concorde. Although seven TU-144s were certified for flight it operated as a passenger aircraft for less than a year, with a total of just 55 flights, once a week, between Moscow and Almaty in Kazakhstan.

The days of supersonic flight were numbered after an Air France Concorde crashed shortly after takeoff in 2000. The aircraft made its last commercial flight in 2003, but supersonic flight remains a tantalising prospect. An aircraft that could fly at Mach 2.4 – close to 3,000km/h – could shunt you from Melbourne to London in under eight hours if it was capable of doing it with just one refuelling stop, and reducing its sonic footprint to allow it to fly at supersonic speeds over land. Many challenges remain. SSTs are hungry monsters. Concorde burned two tonnes of fuel taxiing from its stand to the runway. They travel at high altitudes and have an impact on the ozone layer, with consequences for the climate, and the world is acutely conscious of such threats.

Supersonic aircraft might be on the back burner of the major aircraft manufacturers, but at least one designer is chasing the dream with all the passion of a true believer. Colorado-based Boom Technology plans to test fly its XB-1 Supersonic Demonstrator, "Baby Boom" in 2018. This is a 1/3 scale model of a Mach 2.2, 55-seat SST that Boom is hoping to get into commercial production in 2023. Exciting times ahead.

See also: The supersonic jets aiming to take-off by 2023

See also: 1249 km/h: 787 Dreamliner passenger jet breaks speed record

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