Review and Evaluation of the Air Force Hypersonic Technology Program (1998)

In this chapter the committee discusses the background of the study, provides a brief technical prologue, presents the Statement of Task for the study, describes the strategy for conducting the study, and delineates the plan for the remainder of the report.

Since the historic fifth Volta Congress on High Speeds in Aviation,¹ which was held in Rome, Italy, in 1935, military and civilian engineers have developed aircraft and missile systems that can fly faster than the speed of sound in the atmosphere. The first official, manned, supersonic flight took place only 12 years later, in 1947. Yet today only one supersonic airliner, the Concorde, is in regularly scheduled operation. Although the Concorde is a marvel of technology, its development required significant investments by two nations, France and England. Larger, faster, more efficient supersonic commercial aircraft are under consideration, but they are still only a future possibility. Military aircraft and missile systems, worldwide, have routinely operated above the speed of sound for the past 40 years; some, like the SR-71 reconnaissance aircraft, can cruise at three times the speed of sound (i.e., at Mach 3). Missile and experimental aircraft systems have reached much higher speeds.

More than 30 years ago, the rocket-powered X-15 reached hypersonic speeds in the atmosphere (in this case, top speeds of six to seven times the speed of sound, or Mach 6 to Mach 7). Since that time, hypersonic projects have come and gone, including the air-breathing National Aero-Space Plane² Program. Air-breathing vehicles are more efficient and promise more flexible operations than rocket-powered vehicles. Most orbital air-breathing vehicle concepts would be gradually changed from air-breathing to rocket propulsion at speeds of Mach 10 to Mach 15, depending on the mission requirements. During the transition phase, the vehicle would rapidly gain altitude to avoid the weight penalties caused by high aerodynamic and thermal loads at lower altitudes. Very high speeds are best achieved outside the sensible atmosphere.³

For a variety of reasons, including that more was promised than the available technologies and underlying physics could provide, none of the hypersonic projects has resulted in an advanced operational capability for missiles or aircraft that can cruise at high speeds in the atmosphere. Some key reasons are discussed below.

The purpose of this section is to provide readers with some background for understanding the remainder of the report, which is focused on the content and pace of the Air Force program for the development of hypersonic propulsion technology. The objective of the program is to develop a technology base to support the future development of a hypersonic, scramjet-powered, hydrocarbon-fueled, air-launched missile that can reach speeds up to Mach 8.⁴ The speed of Mach 8 appears to be the upper limit of what may be technically feasible using hydrocarbon fuels (Curran, 1997).

The characteristics of an operational air-breathing hypersonic missile will be determined by a combination of the desired capabilities, the necessary technologies, and the resources allocated by the Air Force. The magnitude of the technical problems for hypersonic aircraft depends on the maximum Mach number. From a technical standpoint, technology for a missile with Mach 8 speed would have to overcome several difficult problems to become operational by 2015. These technical problems are more challenging than the problems associated with a missile with a maximum speed of Mach 4 or Mach 6.

Many examples could be cited of how the difficulty of the technical problems escalates rapidly as the Mach number increases. The best known example is that the stagnation temperature of the oncoming air flow increases from about 1,100°F at Mach 4 to about 2,500°F at Mach 6 and about 4,200°F at Mach 8. The temperatures after combustion inside the engine are even higher, about 4,000, 4,400, and 5,100°F for Mach 4, Mach 6, and Mach 8, respectively. A few brittle materials could survive the very high temperatures inside the engine, but they oxidize readily. No combination of base material and oxidation-resistant coating that could survive has been developed. No known or projected materials are both practical for use in a scramjet engine and able to survive the maximum temperatures without active fuel-cooling at Mach 8 flight in the atmosphere.

Step-changes in the technology will be required as the Mach number increases. Although the exact Mach number at which any step-change occurs depends on several factors, the changes that will be necessary in the Mach number range of 6 to 8 will be very challenging. These include, for example: (1) an endothermic fuel-cracking system to provide adequate cooling capacity (the cooling results when heat is absorbed to crack, or reform, the fuel into its lighter parts); (2) cooled engine structures that can function throughout the operational envelope of the missile; (3) high-temperature materials for leading edges subjected to external air flow; and (4) methods of piloting and stabilizing the combustion process. Some of this technology has already been developed and is available for specific engineering applications. Some requires further development. (See Chapter 2 for a discussion of the most pressing technical challenges.)

Figures 1-1 and 1-2 illustrate the flight environment of a typical hypersonic missile of the type considered in this study. The first figure shows the flight profiles for the notional Air Force Mach 8 scramjet missile and for a Mach 6 variant with the same metallic engine structure. Both engines are actively cooled with endothermic fuel. The Mach 6 missile has a range of approximately 1,200 nautical miles, which is about a 60 percent increase over the range of the Mach 8 missile (780 nautical miles). However, the Mach 6 missile takes about two minutes longer to travel 780 nautical

FIGURE 1-1 Altitude and range profiles for Mach 8 and Mach 6 missiles. Legend: Profile of missile altitude as a function of downrange distance (nautical miles) for nominal Mach 8 and Mach 6 air-launched, scramjet-powered missiles. Both missiles are assumed to use the same propulsion system and endothermic fuels for cooling, although endothermic fuels may not be necessary for a Mach 6 missile. Sample flight times (in minutes) and sample lift/drag ratios are also shown. For these plots, the vehicle’s angle of attack was adjusted to maintain altitude as weight decreased. To maintain munitions kinetic energy, the profiles incorporate a payload ejection segment. (See the legend of Figure 1-2 for more information regarding the munitions.) Source: Air Force Hypersonic Technology Program Office, United Technologies Corporation (Pratt & Whitney), and Boeing North American.

FIGURE 1-2 Altitude and Mach number profiles. Legend: Profile of missile altitude as a function of Mach number for the two missiles in Figure 1-1, showing flight dynamic pressures and air total temperatures (T₁ in degrees Rankine). The Mach 8 profile separates the munition at about 39,000 feet, allowing the 250 pound low-drag penetrator to impact at about 4,700 feet per second for an impact kinetic energy of 8.6×10⁷ foot-pounds. For the Mach 6 profile, engineering estimates indicate that a similar ejection would result in munition impact at about 3,500 feet per second for an impact kinetic energy of 4.8×10⁷ foot-pounds. The small dive maneuver shown directly after launch is an artifact of the trajectory analysis; the simulation program was limited to a dynamic pressure during climb of 1,500 pounds per square foot (psf) and sought altitude solutions that would satisfy that criterion. More realistic boost trajectories will be developed early in HyTech Phase 2. Dynamic pressures at points A, B, C, and D are, respectively, 4,700 psf, 9,500 psf, 15,200 psf, and 26,300 psf. Note: The technical community commonly calculates temperatures in degrees Rankine, which is a scale that has the freezing point of water at 492° and the boiling point of water at 672° (i.e., 460° higher than on the Fahrenheit scale). Source: Air Force Hypersonic Technology Program Office, United Technologies Corporation (Pratt & Whitney), and Boeing North American.

miles. It may be possible at Mach 6 to use a composite structure that is not actively cooled, which could result in perhaps a 25 percent weight saving and, thereby, longer range, better acceleration, and lower cost. Alternatively, the longer range could be translated into a smaller, lighter missile. The terminal velocities of the munitions are also indicated (see the legend regarding the munitions).

Figure 1-2 shows altitude and Mach number corridors of flight for both missiles in terms of equilibrium air total temperatures and dynamic pressures. This figure also shows the separate trajectories of the munitions. Table 1-1 shows combuster maximum temperatures and static pressures for Mach 6 and Mach 8 missiles at a constant dynamic pressure.

In the United States and elsewhere, air-breathing propulsion systems that operate at these speeds are being studied, as are related technologies for a missile system. However, the integration of these technologies into a complete operational system is problematic. Can it be done? Yes, the committee believes it can. Whether it can be done at a reasonable cost and whether it should be done are more difficult questions facing decision makers.

The U.S. military could reap important benefits from affordable hypersonic systems. For example, a hypersonic missile capable of an average speed of Mach 6 (i.e., approximately one nautical mile per second at the planned operating altitudes) could strike a time-sensitive target 250 to 500 nautical miles

way in about four to eight minutes.⁵ Moving a mobile target in less than five minutes would be difficult (see Chapter 2). A subsonic Tomahawk missile, although very capable, would take 30 to 60 minutes to reach the same target, in which time a mobile target could be moved to a safe location. Also, the kinetic energy of a missile warhead that strikes a target on the ground at terminal speeds on the order of several thousand feet per second is significant, even without high explosives (see Figure 1-1). A hypersonic missile could strike and destroy buried, reinforced installations. Operation at hypersonic speeds also improves the survivability of the system.

On the surface, given these potential benefits, the decision to develop and field hypersonic weapons might seem obvious. In reality, the decision is not straightforward, which is one reason the committee was asked to undertake this study.

During the discussions that led to this study, it was evident that the time has come for the Air Force to decide whether or not hypersonic technology can lead to a militarily useful product within a reasonable time frame. The Air Force’s ongoing Hypersonic Technology (HyTech) Program is being managed at Wright-Patterson Air Force Base, Dayton, Ohio, by the Propulsion Directorate, which is part of the newly consolidated Air Force Research Laboratory in the Air Force Materiel Command. The HyTech Program is the subject of this study.

The committee operated under the following Statement of Task from the Air Force.

The following tasks are to be accomplished:

(1) Evaluate and make recommendations regarding the Air Force Hypersonics Technology Program. The NRC should focus its initial efforts on the technologies needed to demonstrate a hypersonic, air-breathing missile concept, using hydrocarbon-based propulsion technology for the Mach 8 regime, in time to achieve an initial operational capability of 2015 or sooner. Emphasize the underlying strategy and key components of the program, the critical technologies that have been identified by the Air Force and by other sources, as appropriate (e.g., advanced propulsion systems using ramjet and scramjet technologies); and the assumptions that underlie technical performance objectives and the operational requirements for hypersonic technology.

(2) Address the following specific questions:

a(i). Will the Hypersonic Technology Program, as planned by the Air Force Materiel Command (all references to the hypersonic program are directed at this specific program rather than broader contexts), lead to a capability which will meet operational requirements for hypersonic technology applications?

a(ii). What technologies (beside propulsion) should next be pursued, and in what priority, for a hypersonic air-to-surface weapon?

b. Are all the necessary technical components of a hypersonic Mach 8 regime propulsion technology program identified and in place, or if not, what is missing?

c(i). What are the salient uncertainties in the propulsion component of the hypersonic technology program, and are the uncertainties technical, schedule related, or bound by resource limitations as a result of the technical nature of the task (e.g., materials sources, qualifications of support personnel, or technology driven costs that affect affordability), to the extent it is possible to enunciate them?

c(ii). What are the salient uncertainties for the other main technology components of the hypersonic technology program (e.g., materials, thermodynamics, etc.)?

c(iii). Does the program provide a sound technical foundation for a weapon system program that could meet operational requirements as presently defined?

d. How does the Air Force hypersonic program interrelate with other Department of Defense hypersonic initiatives, e.g., the Defense Advanced Research Projects Agency’s Advanced Concept Technology Demonstration on hypersonic vehicles?

e(i). From an engineering perspective, what are reasonable milestone dates for a hypersonic missile system development program leading up to production, i.e., concept development, engineering and manufacturing development, etc. For example, with a 2015 target date for operational capability, does the current program have a coherent plan and road map to build and test a Mach 8 regime hydrocarbon-fueled scramjet engine?

e(ii). Are there foreign hypersonic technology applications that are significantly more developed than those of the United States, that, if acquired by the U.S. government or industry through cooperative venture, license, or sale, could positively affect the development process or schedule for Air Force hypersonic vehicles?

e(iii). Based on these assessments, the committee will make recommendations on the technical content and pace of the program.

f. Are there any evident implications for the Air Force support infrastructure for a hypersonic missile system? For example, will other technologies need to be developed in parallel to support a hypersonic vehicle and are those likely to pose significant barriers to eventual success in demonstrating the missile concept or in fielding a viable weapon system by 2015?

(3) To the extent possible, identify technology areas that merit further investigation by the Air Force in the application of hypersonics technology to manned or other unmanned weapon systems by 2015 or beyond.

The principal purpose of this study is to determine whether a hypersonic, scramjet-powered, hydrocarbon-fueled missile with speeds up to Mach 8 can be developed and can reach an initial operational capability by the year 2015. The secondary purpose is to identify technologies that merit consideration for other systems by 2015 or beyond.

At initial meetings, the committee developed a strategy to fulfill the Statement of Task. To complete the work of the committee within the time and resource constraints of the study, the committee focused on the specific questions set forth in the Statement of Task.

The committee members included experts with substantial experience in both research and technology development programs in a wide range of disciplines (e.g., high-speed aerodynamics; basic fluid mechanics; aeronautics; high-speed air-breathing and rocket-propelled vehicles; the development and testing of propulsion systems; military systems acquisition and operational issues; sensors; guidance and control; materials science and engineering; and advanced technology development and systems engineering). The committee also familiarized itself with the diverse work being done on enabling technologies by government agencies that could support the development of hypersonic systems for the Air Force, including the HyTech Program, Navy programs, Defense Advanced Research Projects Agency programs, and National Aeronautics and Space Administration (NASA) programs. The committee determined that an understanding of current work by industry that relates directly to scramjet propulsion and, more broadly, to hypersonic flight and access to space would also be necessary.

The data-gathering goals were met over the course of the committee’s five meetings through briefings by representatives of the Air Force and other government agencies describing existing programs and military needs and by industry representatives who responded to committee questions. These meetings are summarized below (see Appendix A for details).

• At the first meeting, in July 1997, the committee was briefed by Air Force officials at Wright-Patterson Air Force Base and their contractors on the technical content and pace of the HyTech Program. The committee was also briefed by a representative of the Defense Advanced Research Projects Agency on a program to demonstrate an advanced, low-cost, hypersonic missile concept and by representatives of NASA on its hypersonic programs.

• At the second meeting, in Irvine, California, in August 1997, the committee was briefed by Navy and NASA officials on their hypersonics technology programs and conceptual design results. The committee was also briefed by representatives of six companies that have research and technology development programs on engines or vehicles for hypersonic flight or access to space.

• The committee met a third time, in October 1997 in Washington, D.C., to be briefed by Air Force officials on the mission needs and operational requirements for a hypersonic missile. The committee was also briefed by industry representatives concerning the system-level issues associated with the development of a hypersonic missile.

• At the fourth meeting, in December 1997, the committee was briefed at the Air Force Research Laboratory (Phillips Laboratory) in Albuquerque, New Mexico, by Air Force officials who discussed international developments in hypersonic technology and the Air Force military space plane program.⁶ The committee also met with a representative of a company developing a

vehicle for commercial access to space and an independent expert who discussed lessons learned from past scramjet programs.

• During the committee’s fifth meeting, in January 1998 in Washington, D.C., a representative of the HyTech Program described the Air Force’s decision-making process in selecting one propulsion contractor.

While the committee continued to gather data, it also began writing the report. By consensus, the committee decided to resist the tendency to review the history or project the future of the development of hypersonic technology. Instead, the committee decided to respond to the Statement of Task and to organize the report around answers to the questions.

Chapter 2 contains the committee’s responses to Parts 1 and 2, which are answered in the following format: (1) the question is repeated, verbatim, from the Statement of Task; (2) the answer is summarized; (3) a detailed response, including requisite background information, is given to justify the summary answer and detail the committee’s reasoning. In Chapter 3, the committee addresses paragraph 3 in the Statement of Task in a discussion of applications, other than a Mach 8 missile, of hypersonic technology that merit further investigation for use after 2015. In Chapter 4, the committee summarizes its principal conclusions and recommendations.