Introduction

As an approach to the concept of hypersonic flight, one may begin by thinking of a sequence of high-performing aircraft that have flown at successively higher speeds. At Mach 2, twice the speed of sound, typical examples included the F-104 fighter and the Concorde commercial airliner. Though dramatically rakish in appearance, they were built of aluminum, the most familiar of materials, and used afterburning turbojets for propulsion.

At Mach 3 and higher, there was the Lockheed SR-71 that cruised at 85,000 feet. The atmosphere at such altitudes, three times higher than Mount Everest, has a pressure only one-fiftieth of that at sea level. Even so, this airplane experienced aerodynamic heating that brought temperatures above 500°F over most of its surface. In turn, this heating brought requirements that dominated the problems of engineering design. Aluminum was out as a structural material; it lost strength at that high temperature. Titanium had to be used instead. Temperature-resistant fuels and lubricants also became necessary. Even so, this aircraft continued to rely on afterburning turbojets for propulsion.

At Mach 4, the heating became still more severe and the difficulties of design were more daunting. No version of the turbojet has served at such speeds; it has been necessary to use a ramjet or rocket. The X-7, a ramjet testbed craft of the 1950s, was built of steel and had better temperature resistance than the SR-71. Still, when it flew past Mach 4.3 in 1958, the heating became so severe that it produced structural failure and a breakup of the vehicle in flight.

Yet Mach 4 still counts as merely supersonic flight, not as hypersonic. For more than half a century analysts have defined hypersonic speeds as Mach 5 and higher.4 Only rocket-powered craft have flown so fast—and Mach 5 defines only the lower bound of the hypersonic regime. An important range of hypersonic speeds extends from Mach 20 to 25 and includes the velocities of long-range ballistic missiles and of satellites re-entering from orbit. Moreover, flight above Mach 35 was a matter of national concern during the Apollo program, for its piloted Command Module entered the atmosphere at such speeds when returning from the Moon.