20 km. It is therefore not surprising that after 1975 the development of a second generation supersonic airliner became a challenge to the aeronautical community. Since then, a huge amount of money has been spent on R&D programs aimed at developing advanced technology for a new generation of HSCT aircraft. Arguably it is stunning that, despite 27 years of Concorde's satisfactory passenger service and so many technological advancements applied in all sectors of civil aviation, none of these programs have resulted in a viable development project for the near future aimed at producing an advanced supersonic commercial aircraft.
1.6.1 Why Fly Supersonically?
Although wide‐body seating during long‐distance flights of a long‐range subsonic airliner offers high spatial comfort, the high‐priced tickets of first class and business class seating do not compensate in the form of significantly reduced boarding and traveling times. The essential economic issue is the air traveler's value of time. Some SST economic studies base the value of time on the actual earning rate for business travel and on one half the earning rate for personal travel. Concorde's concept of flying at Mach 2.0 across the Atlantic was a technical success and high‐speed flying has remained attractive, especially to hasty officials.
Concordes were flagship aircraft flying at premium fares giving prestige to their passengers and operators. However, its substantial operating costs made high fares necessary: in the year 2000 the return ticket price London–New York was roughly 10,000 US dollars compared to 8,000 dollars for first class and 5,000 dollars for business class tickets of subsonic airliners. Nevertheless, Concorde's relatively high load factors and the fact that the ticket prices at the turn of the century were increasing by approximately 15% per year showed that a niche market existed for much faster passenger transport than any subsonic airliner can offer. It seems fair to assume that today a significant percentage of airline passengers is prepared to pay a premium fare, making this type of executive traveling commercially attractive to airlines. The unique achievements of the Concorde program justified sustained supersonic cruising from the technical viewpoint during its lifetime. Although technology has progressed steadily since Concorde was conceived, it was decades ahead of its time and nowadays we cannot do significantly better. Nevertheless, new technical innovations and organizational approaches will be mandatory to develop and operate a second generation SCT in the economic and regulatory environment of the 21st century.
Having surveyed the abundance of research achievements and project proposals generated during the half century after Concorde's first flight, one could anticipate that significantly improved concepts have become available in most aeronautical disciplines and production capabilities that could lead to a realistic program for development, production, and operation of an environmentally acceptable and economically viable second generation supersonic airliner. A crucial condition for such a program is that a new HSCT will be developed and produced by a consortium of R&D institutes and companies in America, Europe, and East‐Asia. Since all engineers involved in the first generation supersonic airliners are no longer available to apply their knowledge to such a development, considerable effort will be required to bring together and educate sufficiently experienced staff. The availability of relevant progress reports of previous projects will be indispensable to make such an international project team manageable and effective.
1.6.2 Requirements and Operations
Arguments in favor of developing and producing a modernized version of the Concorde would not immediately get acclaim from airlines. In the present commercial aviation market its 110 passenger cabin would be too small, its transatlantic design range too short, its fuel economy too low, and its engines too noisy when taking off. Although Concorde's technical complexity made it a very costly aircraft to purchase, its high operating costs were associated primarily with its poor fuel efficiency, high maintenance, and upgrading costs.
A new high‐speed transport aircraft would fly over the Atlantic, the Pacific, and uninhabited areas, covering about 80% of the most attractive long‐range routes where supersonic flight is legally permitted. The size of the market, estimated as being between 500 and 1,000 aircraft, suggests that there will only be room for a single development program and only international cooperation would make such a program feasible. Enabling a potential trip time reduction of 50% or more when compared to current subsonic flights, supersonic air travel is the one technology that offers a large step forward in functional capability and a large increase in service. This increased productivity potential could result in SCT that is economically viable and environmentally acceptable and thereby could capture a significant portion of the long‐range travel market.
Since an SCT will have to comply with the same international regulations as the contemporary subsonic fleets, take‐off performance and engine design must be improved considerably relative to Concorde's capabilities. Cruise speed is a major factor affecting the operating costs and it is the primary performance characteristic that has to be considered in drawing up the top level specifications, and its choice has far‐reaching consequences for the design and development as well as the operation of the aircraft.
The Boeing 2707‐200 was designed to achieve a range of 6,600 km, similar to the trans‐Atlantic routes served by Concorde. Such a maximum range would be of limited interest for the market of a future SCT since the most important part of its market will be the long distances over water, in particular the trans‐Pacific routes with ranges of more than 10,000 km.
The SCT must be able to take‐off from and land on existing airfields and comply with the associated noise criteria applicable to present‐day jetliners and the plane's dimensions must be compatible with the existing infrastructure of the relevant airports. Accordingly, the accessibility to the aircraft must allow for parallel embark and disembark, service, and fueling in order to enable rapid turn‐around.
In order to serve the many routes that have overland legs, subsonic/transonic flight performance must be at least as good as supersonic cruising and the plane should be able to cruise at speeds up to Mach 1.2 without producing an offensive sonic boom, thereby enabling increasing the cruise speed over land by 50% relative to present‐day jetliners.
1.6.3 Block Speed, Productivity, and Complexity
The block time for intercontinental supersonic flight rapidly improves through the low Mach number region; it levels out at speeds above Mach 3.0. Greater speeds will not be paid off with appreciable time saving to the passenger as well as increased productivity to the airliner, and the cost of cruising faster than Mach 2.0 can be large since it complicates the airframe and systems development effort. In particular, the structure of a high Mach number aircraft is subject to kinetic heating of the airframe skin. This requires a complicated air conditioning system and the usage of expensive heat‐resisting structural materials, whereas the combination of materials having different coefficients of expansion may increase structural stresses.
Complicated variable‐geometry engines are required when flying at high Mach numbers and, since the best cruise altitude increases as well, the installed power plant becomes heavier and more costly. Moreover, a heavier fuselage structure is required to cope with the higher cabin pressure differential and increased fuel tank pressurization to prevent fuel boil‐off.
A cruise speed lower than Mach 2.0 leads to less wing sweep than Concorde's leading edge sweep, which is better suited to low speed operation, higher bypass ratio engines that reduce take‐off noise, and cruise altitudes that reduce global impact of emissions. A cruise speed of Mach 1.6 to Mach 1.8 offers a practical possibility for increasing the block speed to about twice that of present‐day jetliners.
These considerations demonstrate that a considerable development effort is required to combine the need for high fuel efficiency in supersonic cruising flight with acceptable development costs and friendliness
