SST Reincarnated: Birth of the High-Speed Civil Transport

For much of the next decade, the most active sonic boom research took place as part of the Air Force’s Noise and Sonic Boom Impact Technology (NSBIT) program. This was a comprehensive effort started in 1981 to study the noises resulting from military training and operations, espe­cially those involving environmental impact statements and similar assess­ments. Although NASA was not intimately involved with NSBIT, Domenic Maglieri (just before his retirement from the Langley Center) and the recently retired Harvey Hubbard compiled a comprehensive annotated bib­liography of sonic boom research, organized into 10 major areas, to help inform NSBIT participants of the most relevant sources of information.[460]

One of the noteworthy achievements of the NSBIT program was to continue building a detailed sonic boom database (known as Boomfile) on all U. S. supersonic aircraft by flying them over a large array of newly developed sensors at Edwards AFB in the summer of 1987. Called the Boom Event Analyzer Recorder (BEAR), these unmanned devices

recorded the full sonic boom waveform in digital format.[461] Other con­tributions of NSBIT were long-term sonic boom monitoring of combat training areas, continued assessment of structures exposed to sonic booms, studies of the effects of sonic booms on livestock and wildlife, and inten­sified research on focused booms (long an issue with maneuvering fighter aircraft). The latter included a specialized computer program (derived from that originated by NASAs Thomas) called PCBoom to predict these events.[462] In a separate project, fighter pilots were successfully trained to lay down super booms at specified locations (an idea first broached in the early 1950s).[463]

By the mid-1980s, the growing economic importance of nations in Asia was drawing attention to the long flight times required to cross the Pacific Ocean or the ability to reach most of Asia from Europe. The White House Office of Science and Technology (OST), reversing the administration’s initial opposition to civilian aeronautical research, took various steps to gain support for such activities. In March 1985, the OST released a report, "National Aeronautical R&D Goals: Technology for America’s Future,” which included a long-range supersonic transport.[464] Then, in his State of the Union Address in January 1986, President Reagan ignited interest in the possibility of a hypersonic transport—the National Aero-Space Plane (NASP)—dubbed the "Orient Express.” The Battelle Memorial Institute, which established the Center for High-Speed Commercial Flight in April 1986, became a focal point and influential advocate for these proposals.[465]

NASA had been working with the Defense Advanced Research Projects Agency (DARPA) on hypersonic technology for what became the NASP since the early 1980s. In February 1987, the OST issued an updated National Aeronautical R&D Goals, subtitled "Agenda for Achievement.”

It called for both aggressively pursuing the NASP and developing the "fundamental technology, design, and business foundation for a long – range supersonic transport.”[466] In response, NASA accelerated its hyper­sonic research and began a new quest to develop commercially viable supersonic technology. This started with contracts to Boeing and Douglas aircraft companies in October 1986 for market and feasibility studies on what was now named the High-Speed Civil Transport (HSCT), accom­panied by several internal NASA assessments. These studies soon ruled out hypersonic speeds (above Mach 5) as being impractical for pas­senger service. Eventually, NASA and its industry partners settled on a cruise speed of Mach 2.4.[467] Although only marginally faster than the Concorde, the HSCT was expected to double its range and carry three times as many passengers. Meanwhile, the NASP survived as a NASA – DOD program (the X-30) until 1994, with its sonic boom potential stud­ied by current and former NASA specialists.[468]

The contractual studies on the HSCT emphasized the need to resolve environmental issues, including the restrictions on cruising over land because of sonic booms, before it could meet the goal of efficient long­distance supersonic flight. On January 19-20, 1988, the Langley Center hosted a workshop on the status of sonic boom methodology and under­standing. Sixty representatives from Government, academia, and industry attended—including many of those involved in the SST and SCR efforts and several from the Air Force’s NSBIT program. Working groups on sonic boom theory, minimization, atmospheric effects, and human response deter­mined that the following areas most needed more research: boom carpets, focused booms, high-Mach predictions, atmospheric effects, acceptability metrics, signature prediction, and low-boom airframe designs.

The report from this workshop served as a baseline on the latest knowledge about sonic booms and some of the challenges that lay ahead. One of these was the disconnect between aerodynamic efficiency and lowering shock strength that had long plagued efforts at boom min­imization. Simply stated, near-field shockwaves from a streamlined airframe coalesce more readily into strong front and tail shocks, while the near-field shock waves from a higher-drag airframe are less likely to join together, thus allowing a more relaxed N-wave signature. This paradox (illustrated by Figure 6) would have to be solved before a low – boom supersonic transport would be both permissible and practical.[469]