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Commercial Supersonic Technology: The Way Ahead
Supersonic Commercial Transport
The desired payload for a high-speed civil transport (HSCT) would be about 300 passengers, and the desired range would be at least 4,500 NM, with 5,000 to 6,000 NM preferred. Sonic boom reduction would not be required to provide trans-oceanic service. The minimum expected cruise speed is approximately Mach 2.0, based in part on earlier studies of aircraft utilization on long international routes. A cruise Mach number close to 2 might be acceptable to users (i.e., airlines) and would also mitigate some of the technological challenges and environmental concerns associated with Mach numbers of 2.4 and greater.
A large transport, such as an HSCT, that can fly at Mach 2 or greater with little or no sonic boom, a capability necessary for overland operations, is not viewed as technologically feasible. One alternative for providing supersonic airline service over land would be to develop a large commercial supersonic aircraft with a cruise speed close enough to Mach 1 to avoid creating a sonic boom that would propagate to the ground. Boeing is currently conducting design studies for such an aircraft, which could probably be developed without government research into the breakthrough technologies that are the subject of this report.
A second alternative for improving the economics of a large supersonic transport would be to design it for flight at a high cruise speed over water and a lower (but still supersonic) cruise speed over land. This scenario, however, raises questions about performance efficiency during long flight segments at off-design speeds. Early in the HSR Program, for example, Boeing explored the option of aerodynamically shaping a Mach 2.4 aircraft to produce a low-level boom for overland flight at Mach 1.6 to Mach 1.8. This effort was dropped when it became clear that the necessary design modifications would significantly degrade performance at Mach 2.4 and reduce the overall economic viability of the aircraft. It might be worthwhile, however, to reexamine this area using advanced technologies and lower cruise speeds (for example, Mach 2.0 over water/Mach 1.2 over land instead of Mach 2.4 over water/Mach 1.6 over land).
A third alternative would be to build a commercial supersonic transport with a payload capability similar to that of a military strike aircraft (equivalent to 100 passengers), a range capability similar to that of an SBJ, and a cruise speed of approximately Mach 2. Compared to an HSCT, the reduced size and potentially lower speed would make it much more feasible to develop the technologies necessary to reduce the sonic boom enough to permit overland operations. This transport would be capable of transcontinental service as well as transoceanic service directly to and from noncoastal cities.
A 1997 study by the NRC reviewed demand studies that predicted a market size on the order of 1,000 HSCTs, assuming that targeted levels of cost, performance, and environmental impacts can be achieved (NRC, 1997). The demand studies also assumed that sonic booms would prevent these aircraft from flying at supersonic speeds over land and that they could be operated profitably with a ticket surcharge of about 10 percent (relative to the price of travel on subsonic aircraft) for coach class travel and a surcharge of 30 percent for business and first class travel. However, generalizations in the assumptions on which some of the demand studies were based may have caused the studies to overstate the projected market size. The 1997 NRC study concluded that turn-around times seemed to be unrealistically low and that an aircraft with a cruise speed of Mach 2.0 might have a productivity similar to that of a Mach 2.4 aircraft (NRC, 1997). A cruise speed of Mach 2.2 or less would also be a more tractable goal than Mach 2.4 for the 25-year period of interest to this study.
VEHICLE CHARACTERISTICS AND ECONOMIC GOALS
The above-mentioned views of customer requirements form the basis for defining three notional commercial supersonic aircraft: an SBJ, an overland supersonic commercial transport, and an HSCT. These generic aircraft were selected as being representative of the complete spectrum of supersonic aircraft likely to be developed in the foreseeable future. For example, the committee determined that the speed, range, and payload characteristics of an overland supersonic transport would be similar to those of a nominal supersonic strike aircraft. The purpose of these notional vehicles is to help assess the need for advances in the technological state of the art; they are not intended to endorse any particular product or replace the need for detailed design and market studies to validate the vehicle performance specifications prior to advanced product development.
Implicit in aircraft design specifications are costs—development, production, operations, and maintenance costs— that are considered affordable. Each of these costs depends on many factors. The 1997 NRC report identified 22 factors that impact vehicle affordability. For the purpose of this study, takeoff gross weight (TOGW) was used as one readily measurable indicator of aircraft cost for rapid assessment of configuration trade studies. Given this simplification, affordability is tied to the ratio of payload weight to TOGW that is considered to be economically viable (or militarily cost-effective) in the applications of interest identified above.
For each of the three notional commercial supersonic aircraft, the committee used a combination of engineering judgment, historical trends, and simplified equations to identify vehicle characteristics and the technology goals that must be achieved to satisfy requirements for an environmentally acceptable and economically viable aircraft (see Table 2-1 and Figure 2-1).
Overland transport aircraft (and comparably sized military strike aircraft) will require improvements equivalent to about 10 percent over the present state of the art in the four most important factors related to economics (L/D, air vehicle empty weight fraction, specific fuel consumption, and