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POTENTIAL MILITARY HYPERSONIC APPLICATIONS 11 1.0 POTENTIAL MILITARY HYPERSONIC APPLICATIONS The Air Force and the NASP Joint Program Office each briefed the com- mittee on the military aspects of a hypersonic vehicle. Clearly a hypersonic vehicle has several advantages. Flight time to the target could be less than one hour at near-orbital speeds. While this is comparable to the flight time of a ballistic missile, a hypersonic vehicle could have the advantage of flexible recall and en route re-direction. Con- sidering the range to potential areas of interest and targets from the interior of the United States, the military potential is readily apparent. As shown in Figure 1-1, a hyper- sonic vehicle can reach a point on the opposite side of the earth from Omaha, Nebraska in about one hour. The dynamic pressure on the vehicle could be considerably less than 1000 psf at this flight condition. A hypersonic vehicle can also range from Omaha to Moscow, USSR to run either a strike mission or a high altitude reconnaissance mission in about 30 minutes. An ICBM can also accomplish a strike mission in about 30 minutes, with its warheads either reentering ballistically or as hypersonic glide vehicles, which can maneuver. Once launched, however, the ICBM cannot be recalled; the hypersonic aircraft can. The reconnaissance mission can also be done by other means, such as by an SR- 71 at 80,000 ft. in over three hours or by a satellite pass over Moscow at a predictable time. Again, the response time of the hypersonic vehicle is measured in minutes against many hours for the two alternate means mentioned. From the viewpoint of degree of technical difficulty, the full range of operational missions at hypersonic speed can be divided into three areas. All sustained (steady) flights at such speeds will be confined to a restricted "cor- ridor" of altitudes at each speed (Mach number). Above this corridor the aircraft cannot provide enough lift or thrust; below it, vehicle heating or structural loads are excessive. These design factors are discussed in more detail later. The approximate form of the flight corridor is shown in Figure 1- 2, where the three operational areas are defined in terms of the Mach number and altitude range of each. Area 1, including operations at up to about Mach number 8, requires the least complex hypersonic vehicles from a technical point of view. Nevertheless, it offers significant reductions in mission time. Vehicles for operation in this region appear reasonably achievable, and the stress of the flight environment appears to be tolerable for sensor operation and weapon delivery. The pattern of manned military operations in this area will be similar to those of the SR-71. Area 2 involves mission accomplish- ment from space, outside the sensible atmosphere, between Mach numbers 20 and 25. In our view, this area encom- passes the most attractive hypersonic aircraft missions because of launch flexibility, short flight time, and the ability to take advantage of the relatively benign environment of space for mission accomplishment. Maneuver- ing is another attractive feature. The major technical challenges for Area 2 are presented by the accelerating climb through the atmosphere and the subsequent re-entry. Although many of the technical advances of the space
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12 program are directly applicable, the requirement to fly with a completely recoverable air-breathing vehicle adds formidable new problems. These are addressed later in this report. Area 3, between the two areas dis- cussed above, involves sustained cruising flight in the atmosphere roughly between Mach numbers ~ and 20. This is a very stressful flight environment with high skin temperatures, control and maneu-- vering difficulties, ionized boundaries through which sensors must operate, and high infrared signatures that would make the vehicle vulnerable to detection. For these reasons, we have great reserva- tions about the military utility of sus- tained hypersonic flight in the atmo- sphere above Mach number S. In examining potential applications in any of these three areas, one must take account of some basic limitations and restrictions on the operations of hypersonic aircraft. Their minimum turning radius is measured in hundreds or even thousands of miles - propor- tional to the square of the speed; they can maneuver in flight with modest energy expenditures, in contrast to the ballistic missile, but flight path curv- atures must be small compared to those of current aircraft. One important consequence is that global or near-global range is necessary, and the plane will often have to circle the planet to return to base after the mission. Another restriction is inherent in the base support requirements associated with cryogenic fuels. They will require a complete departure from conventional airport storage and distribution facilities. For economic reasons alone, we are un- able to envision a network of airfields giving the flexibility that today's air- craft enjoy. However, some mitigating HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION factors should be considered in address- ing these logistic issues. For the last 15 years or more, hydrogen-fueled aircraft have been the subject of serious study by NASA and U.S. commercial aircraft companies, primarily to enable fast, economical, long-range flight such as supersonic trans-Pacific flights. The airport facilities required for liquid hydrogen handling have received quite detailed study, and the problems appear tractable. The military hypersonic aircraft, in common with these commercial concepts, will fly farther and higher than today's aircraft, which suggests that a much smaller number of cryogenically-equipped airports will be needed for satisfactory operation. Finally, all classes of hyper- sonic aircraft will require the same type of base facilities, and it may be of interest to examine the concept of a new type of Air Force base, capable of supporting all classes of mission, and fully-equipped for cryogenic fuels. It follows from these arguments that any forecast of missions for hypersonic aircraft must include a careful examin- ation of the unusual support require- ments. We suggest that as hypersonic technology advances, periodic studies of the logistical support requirements should be made to give confidence in the vehicle's military utility. In sum, we believe that there are clear potential advantages to the Air Force in hypersonic air-breathing capa- bility. These potential advantages are sufficient to justify an intensive tech- nology development program, including flight vehicle research sufficient to determine the military utility of hyper- sonic flight at orbital or near-orbital speeds.
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