United States Air Force Scientific Advisory Board Report on Why and Whither Hypersonics Research in the US Air Force SAB-TR-00-03 December 2000 Cleared for Open Publication This report is a product of the United States Air Force Scientific Advisory Board Committee on Why and Whither Hypersonics Research in the US Air Force. Statements, opinions, recommendations, and conclusions contained in this report are those of the committee and do not necessarily represent the official position of the US Air Force or the Department of Defense. United States Air Force Scientific Advisory Board Report on Why and Whither Hypersonics Research in the US Air Force SAB-TR-00-03 December 2000 Cleared for Open Publication (This Page Intentionally Left Blank) ii Foreword This report summarizes the deliberations and conclusions of the 2000 Air Force Scientific Advisory Board (SAB) study on Why and Whither Hypersonics Research in the US Air Force. In this study the committee describes the operational requirements of a hypersonic system and presents a research program for air breathing hypersonics to meet the operational requirements. We define a program resulting in an operational air breathing hypersonic space launch system in about 2025. This program includes several exit ramps and potential options. The exit ramps would lead to either an operational rocket-based reusable launch system or continuation of the expendable course the Air Force is currently on. A Red Team Panel was part of the study team and provides alternatives to the air breathing hypersonic systems to meet the operational requirements. The study results represent an outstanding collaboration between the scientific and operational communities and among government, industry, and academia. The Study committee wishes to thank the many individuals who contributed to the deliberations and the report, as listed in Appendix A. In addition to Scientific Advisory Board members, many ad hoc members devoted their time. The team would also like to thank all the organizations that gave presentations to our panel and hosted us as listed in Appendix D. The Air Force Academy provided outstanding technical writers—Capt Susan Hastings, Capt David Jablonski, and Capt Matthew Murdough— who provided fantastic support in preparing this report. Lt Col Dan Heale from the Air Force Research Laboratory served as an outstanding executive officer for the Investment Panel as well as provided a liaison role with Air Force Materiel Command. The Study committee would like to recognize the SAB Secretariat and support staff, in particular Maj Doug Amon, and the ANSER team, especially Ms Kristin Lynch, who provided invaluable administrative and logistical assistance in pulling together the myriad of inputs into this final report. Their efforts are greatly appreciated. Finally, this report reflects the collective judgment of the SAB and hence is not to be viewed as the official position of the United States Air Force. Dr. Ronald P. Fuchs Study Chairman September 2000 iii (This Page Intentionally Left Blank) iv Executive Summary Since the 1960s, the Air Force has had operational hypersonic systems in the form of intercontinental ballistic missiles (ICBMs), launch vehicles, and reentry vehicles. This report addresses another type of hypersonic system, the sustained-flight hypersonic systems characterized by airbreathing hypersonic propulsion systems. The Air Force Scientific Advisory Board (SAB) was asked to assess the operational utility of such systems. To ensure that the usual unbridled enthusiasm the SAB has for new technology did not overwhelm the results, the study incorporated its own red team to identify and assess alternatives. This report is a consensus of the entire study team’s recommendations. While the Air Force has always had enthusiasm for higher speed, airbreathing hypersonic flight efforts have suffered from a series of fits and starts over the past 40 years. During the same time, hypersonic efforts for ICBMs, reentry vehicles, and launch vehicles have prospered. Why is this so? The primary reason is probably that the efforts for airbreathing hypersonics have focused on getting somewhere in less time, and a clear requirement making essential use of such a time advantage has not been established. This has resulted in great difficulty in determining a valid operational concept because 1. The hypersonic system concepts were often complex and took more time to get ready for flight than subsonic systems, thus minimizing the speed advantage. 2. The hypersonic system concepts were usually extremely expensive to develop and acquire, thus calling into question the cost-effectiveness of timely response. 3. The timelines were fragile—that is, for a fixed range, the feasible hypersonic speed region (say Mach 5 to Mach 15) would make a difference only over a narrow window of time relative to that which is possible with high supersonic flight. For instance, as shown in Figure ES-1, for a target at 1,000 nautical miles (nm) there is only a 13-minute window where a reasonable hypersonic speed range would make any difference. If more than 20 minutes time of flight is acceptable, supersonic or subsonic speed is adequate; if less than 7 minutes is needed, Mach numbers higher than 15 would be required. 50 40 M1 M3 30 M5 20 M15 Time (min.) 13 min.min 10 0 00 0 1 300 500 700 900 ,300 500 ,700 900 1,10 1 1, 1 1, Range (nm) Figure ES-1. Hypersonic Speed Windows v 4. Most hypersonic concepts, to take advantage of their fast response, require an intelligence, surveillance, and reconnaissance system that won’t exist for a long time. 5. Many argue that the decision timelines are so long relative to the time of flight that there is little need for hypersonic flight. (This argument presumes that the decision timeline can be shortened—there is no clear evidence that this is likely. Interestingly the same argument can be made to support hypersonics—that is, the decision makers want even longer timelines, so shorter times of flight are desirable.) So, has anything changed? Yes. The greatest change is that hypersonic flight for other than getting somewhere faster now appears to be a valid need of the Air Force. The Air Force published Vision 2020: Global Vigilance, Reach and Power stating a desire for “controlling and exploiting the full aerospace continuum.” If that vision implies frequent, routine, on-demand operations into and within space, the enabler for this vision is an affordable, responsive, reliable, robust space launch capability. Getting to orbit requires Mach 25 flight—and all speeds between 0 and Mach 25. This interpretation of the vision cannot be fulfilled within the likely Air Force investment program using expendable launch vehicles (ELVs); reusable launch vehicles (RLVs) will be necessary to make routine space operations affordable. Airbreathing hypersonic systems are one of the two concepts that show promise of allowing the realization of these capabilities— the other being rocket systems. On the other hand, if the vision simply implies doing more of the same things done today, the Air Force can probably live with ELVs indefinitely. What’s missing is a clear statement of the mission needs and operational requirements for space control, space warfighting, responsive launch, and other missions that might demand a reusable launch system. This will be a critical enabler for making the Air Force vision a reality, as hypersonics could be the next great step in the transformation of the Air Force into a completely integrated aerospace force. We recommend that Air Force Space Command develop appropriate clear statements of its requirements for space launch. These documents must be the basis for steering the investment program as described below. Until these requirements are defined, hypersonic technology programs should be monitored and funded at about their current levels, because these technologies will be required for any resulting program. However, to preclude a continuing slip in the ultimate system availability, a funding wedge for a hypersonic program should be inserted in the budget starting in 2003 or 2004. When the mission requirements and concept of operations (CONOPS) are defined, key questions will need to be answered before a decision can be made on which technologies will provide an affordable approach to satisfying these requirements. The first question is whether an RLV or expendable launch vehicle is needed. The next is what type of propulsion system should be used. The National Aeronautics and Space Administration (NASA) has been emphasizing a rocket-based single-stage-to-orbit approach that now looks technically risky and might not result in the kind of operational capability the Air Force needs. Two-stage-to-orbit approaches have much lower technical risk, but may be more costly from both acquisition and operational standpoints. The data to make definitive decisions about these key questions and about many more do not exist, nor is there a reasonably paced program in place to provide those data. While it may seem unreasonable, short of another Apollo or Manhattan Project, we are about 25 years from an operational system enabling routine space operations for the Air Force. Furthermore, this capability is slipping away at almost 1 year per year because current levels of vi funding are insufficient to make significant progress. In addition, much of the nation’s hypersonic talent is reaching, or has passed, retirement age. In this report we define a program resulting in an operational airbreathing hypersonic space launch system in about 2025. This program includes several exit ramps and potential options. The exit ramps would lead to either an operational rocket-based reusable launch system or continuation of the expendable course the Air Force is currently on. Early-year investments are those already in the Air Force budget. This program would require annual investments of $30 million to $50 million during the latter half of the Six-Year Defense Plan, leading to a moderate-risk decision on a space launch engineering and manufacturing development (EMD) program in 2008.