nologies are likely to be guarded as proprietary and competition-sensitive and are not available to groups such as NRC committees, which work in a public forum. This was true for much of the results of the HSR Program, which is not yet public information, as well as details of ongoing work, such as the Quiet Supersonic Platform Program funded by the Defense Advanced Research Projects Agency (DARPA). Because of its limited access to detailed technical data, the committee was reluctant to accept the role of recommending which individual technical concepts and approaches, many of them in the early stages of development, should be funded. Fortunately, it was not necessary for the committee to do so. As demonstrated by the Quiet Supersonic Platform Program, once the government identifies specific areas of interest and allocates development funding, industry and other research organizations are able and willing to provide detailed, innovative research proposals within the framework of a competitive government acquisition program, where competition-sensitive information is more likely to be protected. Therefore, with the concurrence of the study sponsor, the committee carried out the intent of the statement of task using the following approach:
Identify the technical barriers to sustained commercial supersonic flight, including flight over land.
Characterize the gap between the state of the art and the technology required to overcome each barrier.
Establish the feasibility of closing each gap by considering if at least one promising approach is available.
Identify what would have to be demonstrated to show that the gap has been closed.
To provide a framework for analyzing the technology needs for commercial supersonic flight over the next 25 years, the committee defined a set of three notional supersonic vehicles:
Small. An SBJ with about 8 to 15 passengers, a range of 4,000 to 5,000 nautical miles (NM), a cruise speed of about Mach 1.6 to 1.8, and sonic boom low enough to enable supersonic flights over both land and water.
Medium. An overland supersonic commercial transport with about 100 to 200 passengers, a range of 4,000 to 5,000 NM, a cruise speed of about Mach 1.8 to 2.2, and sonic boom low enough to enable operations over both land and water.
Large. A high-speed civil transport (HSCT) with about 300 passengers, a range of 5,000 to 6,000 NM, and a cruise speed of about Mach 2.0 to 2.4.
The committee also compared the technical challenges for commercial supersonic aircraft with the likely challenges for a military supersonic strike aircraft. A strike aircraft would have to overcome many of the same challenges as a commercial aircraft—for example, a high lift-to-drag ratio and acceptable takeoff and landing characteristics; efficient and durable engines; and advanced airframe materials and structures. A strike aircraft would need to meet these challenges while also meeting military requirements for stealth and weapons integration, but without necessarily meeting all the same environmental constraints.
For each class of aircraft, the committee used a combination of engineering judgment, historical trends, and simplified equations to identify key challenges and the research areas required to overcome those challenges. While noise and emissions are certainly major barriers to the development of an HSCT, significant advances in the traditional aeronautics engineering disciplines, such as structures, propulsion, and aerodynamics, are still required to close the business case and certificate new systems. Supersonic transports with overland capability (and military strike aircraft of comparable size) will require improvements in the four major factors related to economics (lift-to-drag ratio, air vehicle empty weight fraction, specific fuel consumption, and thrust-to-weight ratio) equivalent to about 10 percent over the present state of the art in each parameter, as well as additional advances related to the environment and certification. For SBJs, most parameters are already within the state of the art. HSCTs, on the other hand, will require significant advances, equivalent to about 15 percent for each of the four major economic parameters. Affordable supersonic flight is an exercise in integration: A viable commercial supersonic aircraft cannot be achieved until solutions to the individual technology challenges are brought together in one integrated airframe-engine design.
The committee also validated the importance of cruise speed as a key factor in determining the technological difficulty associated with development of a commercial supersonic aircraft. NASA’s HSR Program, which ran from 1985 to 1999, envisioned an HSCT with 300 passengers and a cruise speed of Mach 2.4. In 1997 the NRC concluded that the focus on Mach 2.4 was too aggressive and probably not justified by the business analysis. The study concluded that an aircraft with a cruise speed of Mach 2.0 might have a net productivity similar to that of a Mach 2.4 aircraft and would have an easier time overcoming some of the most difficult economic, technological, and environmental challenges.
As cruise speed increases, the most efficient cruise altitude increases also, and the technical challenges to developing an economically viable and environmentally acceptable commercial supersonic aircraft increase significantly above approximately Mach 2. For aircraft with cruise speeds less than Mach 2, an NOx emission index of 15 appears satisfactory, and water vapor emissions are unlikely to pose difficulties at the associated altitudes. Aircraft with cruise speeds in excess of Mach 2 will normally cruise in the stratosphere, where engine emissions have a greater potential to cause climate change and depletion of atmospheric ozone. At higher speeds, the NOx emissions index may need to be as low as 5.