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Road to Mach 10: Lessons Learned from the X-43A Flight Research Program

Road to Mach 10: Lessons Learned from the X-43A Flight Research Program by Curtis Peebles. American Institute of Aeronautics and Astronautics, 2008, 250 pp.

At hypersonic speeds, defined as Mach 5 or higher, the compressive heating generated by a vehicle as it passes though the atmosphere is so intense that the air itself undergoes chemical changes. Such heating would destroy turbine or ramjet engines constructed of any known material. Although a variety of rocket-propelled vehicles have flown at hypersonic speeds (e.g., ballistic missile reentry vehicles, the X-15, and the space shuttle), no air-breathing vehicle until the X-43A has proved capable of sustained hypersonic flight.

At least in theory, the scramjet (supersonic combustion ramjet) can serve as a hypersonic jet engine. Conceived more than 50 years ago, it differs from the ramjet in that the latter’s inlet slows the air down to subsonic speed, while the scramjet inlet only decelerates the flow to supersonic speeds, which reduces heating. The scramjet introduced many complications, however, especially the problem of sustaining combustion in a supersonic flow.

After decades of analysis, wind tunnel tests, and concepts for flight research projects that never reached fruition, the National Aeronautics and Space Administration (NASA) began work on the Hyper-X project in 1996. Hyper-X consisted of the X-43A (a 12-foot-long unmanned research vehicle with a scramjet engine) and a rocket booster to push the X‑43A to hypersonic speeds at an altitude of approximately 100,000 feet. At that point, the vehicle would separate from the booster and start its scramjet engine. A B-52 bomber lifted the entire stack, releasing it over the Pacific Ocean off the coast of California.

Curtis Peebles drew on his unique vantage point as NASA project historian to write this book about the Hyper-X. Based on both internal and published documents, interviews with project participants, and the author’s own observations, Road to Mach 10 offers a insider’s detailed view of one of the most exciting flight research projects in several decades. As befits a book published by an organization of aerospace engineering professionals, this one is highly technical in places. Although he has not written an engineering textbook, Peebles assumes that the reader has a good background in the full range of aerospace technologies. If readers have difficulty with such sentences as “The computational-fluid-dynamics data were used to quantify the ground-to-flight scaling and unsteady-flow phenomena during the dynamic separation” (p. 64), then they may want to pass by this book.

Those undaunted by the required level of technical knowledge are rewarded with a detailed but readable story that begins with a background of scramjet research and continues to Hyper-X design, manufacturing, integration and checkout, and flight operations. The first flight tumbled out of control shortly after release from the B-52, but the second and third flights successfully demonstrated scramjet-powered flight at Mach 6.83 and 9.68, respectively. Peebles offers an excellent description of all the missions as well as the mishap investigation.

The book includes numerous good photographs; unfortunately, they are reproduced in black and white rather than color. More significantly, it omits line drawings of the vehicles and their systems. Specifically, at several points, the book describes intricate mechanisms, but the absence of supporting illustrations makes it difficult to visualize the systems.

Quibbles about illustrations aside, Road to Mach 10 is an outstanding recounting of an exciting and notable project. Individuals with a professional interest in modern flight research at the leading edge of technology will benefit greatly from reading it.

Kenneth P. Katz

Longmeadow, Massachusetts

"The views expressed are those of the author(s) and do not reflect the official policy or position of the US government or the Department of Defense."

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