Modern Flying - The Return of Supersonic Aircraft

It was the epitome of modern commercial aviation: the Concorde took its passengers from Paris to New York in just three and a half hours. In October 2003, the supersonic jet's flight operations were canceled. Is technology now returning?

On 24 October 2003, a British Airways Concorde arrived at London Heathrow for the last time. Three years earlier, the airline, which once operated up to seven of the supersonic aircraft, had already partially grounded its Concorde fleet. The reason were hairline cracks on the wings, which had initially not been considered relevant to safety, but expanded over time. By the time of this closure in July 2000, a total of 13 of the Supersonic jets had been in commercial service worldwide for a quarter of a century without injuring a single person.

The disaster occurred on 25 July 2000, at Air France, the second major Concorde operator. A banal puncture caused by an engine part lost by a DC-10 on runway 26 at Paris Charles de Gaulle airport, which the pilot of the Concorde caught with the front tire of his left main landing gear at his start to New York, caused the supersonic jet on the left side to go up in flames. A more than one-meter long piece of the tread had come loose from the tire and hit a leak in the flat tank. The escaping fuel ignited in the engine jet, the supersonic aircraft came with a long firetail under the left surface still up from the runway, but then crashed only a few kilometers further before an emergency landing attempt.


Myth burns up

It was the beginning of the end of the civil supersonic flight. After clarifying the cause, there were still a few changes that should prevent such an accident in the future. But in October 2003 the myth of supersonic flight came to an end. This was not only caused by the serious accident that had occurred three years earlier in Paris. Rather, it was the economic aspects that heralded the end of the Concorde. The high maintenance costs and the exorbitant fuel consumption of the aircraft - per fuel unit it only managed about half to one-third of the passenger-kilometers of long-haul subsonic aircraft - were offset by maximum passenger capacity of 128 passengers, who had to pay correspondingly high ticket prices. As a result, the capacity of the aircraft used for regularly scheduled flights between Europe and the USA could often not be fully utilized. The burnt-out wreck of the Air France 4590 was only another impulse to finally question the technology of civil supersonic flying.

Besides, Concorde was only able to make limited use of the enormous time saved due to its flight services, because, for noise protection reasons, civil flights in this speed range over landmasses are not eligible for approval. Only over the Atlantic and at a corresponding altitude could the Concorde accelerate to its travel performance of Mach 2, twice the speed of sound.

Incarnation through technical progress

Today's much more efficient engine technologies compared to 16 years ago, as well as bionics knowledge, have made it possible not only to reduce noise emissions from aircraft parts but also the engines themselves. A few years ago, this revived the dream of ultra-fast passenger transport that Concorde flights had dreamed of at that time. There are currently around 12 projects around the world dealing with the development of new super- and hypersonic jets - including those involving renowned government aerospace agencies and aircraft manufacturers.

In April last year, NASA announced a contract with Lockheed Martin for the development of a particularly quiet Supersonic experimental aircraft. The "Quiet Supersonic Transport" Lockheed Martin X-59 is supposed to turn the formerly deafening overshoot bang into a significantly quieter, muffled impact sound at a speed of Mach 1.4.


 
 

Various variables play a decisive role in reducing the supersonic bang.©Lockheed Martin

Boeing and Airbus - formerly French Aerospatiale and already involved in the development of Concorde - are or have been active in new supersonic projects. At the time of the last Concorde flights, Aerion was still involved in a project for which Airbus later contributed components and know-how and in which a joint venture with Lufthansa Technik was also involved. Aerion AS2, a tri-jet business aircraft with up to 12 seats, is scheduled to take off for the first time as early as 2021. A first AS2 transatlantic flight is announced for 2023. However, this will no longer take place with the participation of the Franco-German aircraft manufacturer, whose American rival Boeing has now joined the project.


 
 

The Aerion AS2 heralds a new era in supersonic flying. In the future, the metropolises London and New York should be reachable in only 2 hours.©Aerion Corporation

Enlarged and Faster

The Spike S-512 Supersonic Jet is supposed to carry up to 18 passengers at a speed of Mach 1.6 over a distance of more than 6,000 nautical miles - non-stop flights from London to Hong Kong would thus be possible in half the flight time required today. But despite its larger capacity, the S-512 is still a business jet.

The "Boom Supersonic" project already makes people think bigger. The Boom XB-1, which is currently being developed as a test vehicle for non-noise supersonic flights, will later be followed by a commercial aircraft for 55 passengers. At present, wind tunnel and engine tests are still being carried out in the laboratory. For the wind tunnel, a scale model of the tri-jet, two-seater XB-1 is available. According to the manufacturer, the first aircraft is currently about to be manufactured.

The larger Boom would be the return to supersonic commercial aviation with 55 seats. Only Charles Bombardier thinks even bigger at the moment. Already four years ago, the aircraft designer presented his project "Skreemr" - a machine that is supposed to accelerate up to 75 passengers to Mach 10 using a ramjet engine, as it is known from rockets. At this speed, New York would be just 30 minutes away from mainland Europe by air.

The speed targeted would not be reached for the first time. Already 15 years ago NASA flew a B-42, a Hypersonic X-43A named unmanned test aircraft, to a height of 12 kilometers, before the engines of the test drone ignited and catapulted the X-43A to Mach 9.8 - the highest speed ever reached in the history of aviation. In 2013, the American military once again managed to reach a speed of Mach 5.5 with a scramjet drive on the X-51A, which was also unmanned. A project of the German Aerospace Center in Bremen, the "SpaceLiner", also relies on rocket propulsion. It is intended to enable even higher speeds, with which up to 50 passengers can travel from Europe to Australia in just 90 minutes. According to DLR Project Manager Dr. Martin Sippel, 30 to 40 years are likely to elapse before that happens.


Engine Technology and Aerodynamics

Whether such rocket engines will be able to assert themselves in commercial aviation, however, is questionable, due to the high load on passengers for acceleration at such high speeds. However, the idea of flying supersonic fast with modern engines at significantly lower fuel consumption compared to the Concorde is not that far-fetched. None of the new concepts also uses an afterburner - a technology the Concorde still had to resort to and which does not have a positive effect on noise development. Avoiding the supersonic bang, on the other hand, is something one wants to get under control with today's significantly better understanding of sound development.

If a stretched aircraft body breaks through the sound barrier, shock waves are generated in front of and behind it, which form a double bang at this speed over the entire flight distance. With the computer simulations that are available today, it is already possible to design the aircraft nose and tail in such a way that the strength of these shock waves is reduced. NASA also intends to achieve further improvements by modifying the aircraft fuselage. The aim is to separate the shock waves in front of and behind the fuselage more clearly and to influence them in such a way that they no longer reach the ground as a sharp bang, but only as a dull boom. If the XB-1 succeeds in this in practice in the next few years, it would be a breakthrough - the goal is a noise limit of 75 DB(A), which is no louder than slamming a car door. If this succeeds, small commercial aircraft could also be developed as supersonic jets based on the knowledge gained. Experience has shown, however, that they will probably no longer reach the passenger capacity of the Concorde.


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