1. By the mid-1970s interests changed; hypersonics research funding faded but did not disappear completely. It can be inferred that Soviet Union research remained relatively active during this period. The U.S. Air Force remained active at basically a maintenance level in the hypersonics field because of the obvious importance of hypersonics to many military missions. A few research laboratories in the United States were active in order to support this interest of the Air Force but, in general, the natural result of drastically reduced funding and military interest was that many of the groups and almost all of the universities that had been doing hypersonics research now found other areas of research to pursue. Further, the development of new facilities at both government and private laboratories was reduced drastically and, in most cases, ceased entirely. Many of the researchers involved found careers in other research areas, and many of the hypersonic facilities were dismantled. Fortunately, however, the achievements and the difficulties associated with the active hypersonics area of the 1950s and 1960s were reasonably well documented in generally available literature, and selected research facilities and personnel remained active.
2. In 1985, the U.S. government embarked on a program to restart hypersonic awareness within the universities by initiating a multiyear program titled "Training and Research in Hypersonics." This was followed in 1986 by the announcement of a joint NASA and Defense Departmnt program to build the National Aerospace Plane (NASP), or the X-30 flight research vehicle, which has launched a renewed interest in hypersonics in the United States Prior to the NASP announcement, the French (HERMES), British (HOTOL), and West Germans (Sanger II) were under way with their own hypersonic programs, as were the Japanese and the Soviets. It is important to note that these vehicle configurations and the flight regimes associated with current hypersonic vehicles are very different from those that were studied in the 1950-1970 era. In the 1960s and 1970s there was a high level of confidence in designing and operating a very blunt (Gemini, Mercury, and Apollo) type of entry body, slender (either blunt or sharp-nosed) weapon type of entry body and the winged, rocket-powered X-15, which operated at Mach numbers lower than those of current interest.
3. In order to address the test facility requirements associated with the newer generation of hypersonic vehicles, it is important to revisit the achievements of the past 30 years and the reasons for some of the failures, but it is essential that we recognize that some of the problems and the facility requirements of the 1980s and 1990s are very different from those that were previously faced.
4. During the past few years, work has continued on current problems even though the optimum tools, both computational and experimental, have not been available. The research tools available can be utilized to extract the maximum amount of information possible while, in parallel, developing the required new facilities, computational codes, and flight programs. It is important to recognize that there has been and will continue to be a funding limitation requiring that a priority list be established that is consistent with national goals in hypersonics. Section II provides a review of past and current hypersonic facility studies. It will briefly review one of the extensive efforts to establish facility requirements for the hypersonic problems-of the 1970 era, followed by a similar brief review of just completed studies that were intended to define facility requirements for the solution of problems associated with the next generation of hypersonic vehicles. Section III describes some of the many problems currently associated with wing-body hypersonic airplanes and several currently operational ground-based facilities or facilities in the development stage that can be used now and in the future to obtain relevant data.
5. In 1970, McDonnell Aircraft Company reported the results of a one-year hypersonic facility (HYFAC) program study that attempted to identify high-priority research required for future hypersonic cruise aircraft, to evaluate research potential and cost of candidate research facilities, and to assess the usefulness of these facilities in support of other aerospace systems. The similarities between these requirements and those of current problems facing the hypersonic community are striking. The results of the study are described in Refs. 1-8 and represent one group's anlysis of the requirements of both ground-based and flight research facilities required in order to develop an air-breathing winged-body vehicle capable of operating at hypersonic speeds. The material presented in these reports was focused on a 300-ft, a 200-ft, or a 100-ftlong vehicle and the associated mission requirements as they were understood in 1970, which are reasonably consistent with those of today. Further, the vehicle configurations sketched in these reports and the propulsion systems described were in many ways similar to current designs, but details of the test requirements were very different. The specific subjects addressed included: 1) gasdynamic facilities, 2) turbo- machinery/ramjet facility, 3) scramjet engine research facilities, 4) structural/fluid system facility, and 5) material research facility. A detailed listing of the research objectives that the recommended facilities were to satisfy is given in Ref. 2. This list contains 102 objectives ranging from the desire to improve fundamental knowledge of hypersonic boundary-layer behavior to the development and integration of engine components into complete scramjet systems. In a later phase of the study, reported in Ref. 1, the list was reduced to 10 primary objectives.