Category Facing the Heat Barrier: a History of Hypersonics

German Work with High-Speed Flows

At the Technische Hochschule in Hannover, early in the twentieth century the physicist Ludwig Prandtl founded the science of aerodynamics. Extending earlier work by Italy’s Tullio Levi-Civita, he introduced the concept of the boundary layer. He described it as a thin layer of air, adjacent to a wing or other surface, that clings to this surface and does not follow the free-stream flow. Drag, aerodynamic friction, and heat transfer all arise within this layer. Because the boundary layer is thin, the equations of fluid flow simplified considerably and important aerodynamic com­plexities became mathematically tractable.1

As early as 1907, at a time when the Wright Brothers had not yet flown in public, Prandtl launched the study of supersonic flows by publishing investigations of a steam jet at Mach 1.5. He now was at Gottingen University, where he built a small supersonic wind tunnel. In 1911 the German government founded the Kaiser-Wil – helm-Gesellschaft, an umbrella organization that went on to sponsor a broad range of institutes in many areas of science and engineering. Prandtl proposed to set up a center at Gottingen for research in aerodynamics and hydrodynamics, but World War I intervened, and it was not until 1925 that this laboratory took shape.

After that, though, work in supersonics went forward with new emphasis. Jakob Ackeret, a colleague of Prandtl, took the lead in building supersonic wind tunnels. He was Swiss, and he built one at the famous Eidgenossische Technische Hoch – schule in Zurich. This attracted attention in nearby Italy, where the dictator Benito Mussolini was giving strong support to aviation. Ackeret became a consultant to the Italian Air Force and built a second wind tunnel in Guidonia, near Rome. It reached speeds approaching 2,500 miles per hour (mph), which far exceeded those that were available anywhere else in the world.2

These facilities were of the continuous-flow type. Like their subsonic counter­parts, they ran at substantial power levels and could operate all day. At the Tech­nische Hochschule in Aachen, the aerodynamicist Carl Wiesenberger took a differ­ent approach in 1934 by building an intermittent-flow facility that needed much less power. This “blowdown” installation relied on an evacuated sphere, which sucked outside air through a nozzle at speeds that reached Mach 3-3-

This wind tunnel was small, having a test-section diameter of only four inches. But it set the pace for the mainstream of Germany’s wartime supersonic research. Wieselberger’s assistant, Rudolf Hermann, went to Peenemunde, the center of that country’s rocket development, where in 1937 he became head of its new Aerody­namics Institute. There he built a pair of large supersonic tunnels, with 16-inch test sections, that followed Aachen’s blowdown principle. They reached Mach 4.4, but not immediately. A wind tunnel’s performance depends on its nozzle, and it took time to develop proper designs. Early in 1941 the highest working speed was Mach 2.5; a nozzle for Mach 3-1 was still in development. The Mach 4.4 nozzles were not ready until 1942 or 1943-3

The Germans never developed a true capability in hypersonics, but they came close. The Mach 4.4 tunnels introduced equipment and methods of investigation that carried over to this higher-speed regime. The Peenemunde vacuum sphere was constructed of riveted steel and had a diameter of 40 feet. Its capacity of a thousand cubic meters gave run times of 20 seconds.4 Humidity was a problem; at Aachen, Hermann had learned that moisture in the air could condense when the air cooled as it expanded through a supersonic nozzle, producing unwanted shock waves that altered the anticipated Mach number while introducing nonuniformities in the direction and velocity of flow. At Peenemunde he installed an air dryer that used silica gel to absorb the moisture in the air that was about to enter his supersonic tunnels.5

Configuration development was at the top of his agenda. To the modern mind the V-2 resembles a classic spaceship, complete with fins. It is more appropriate to say that spaceship designs resemble the V-2, for that missile was very much in the forefront during the postwar years, when science fiction was in its heyday.6 The V-2 needed fins to compensate for the limited effectiveness of its guidance, and their design was trickier than it looked. They could not be too wide, or the V-2 would be unable to pass through railroad tunnels. Nor could they extend too far below the body of the missile, or the rocket exhaust, expanding at high altitude, would bum them off.

The historian Michael Neufeld notes that during the 1930s, “no one knew how to design fins for supersonic flight.” The A-З, a test missile that preceded the V-2, had proven to be too stable; it tended merely to rise vertically, and its guidance system lacked the authority to make it tilt. Its fins had been studied in the Aachen supersonic tunnel, but this problem showed up only in flight test, and for a time it was unclear how to go further. Hermann Kurzweg, Rudolf Hermanns assistant, investigated low-speed stability building a model and throwing it off the roof of his home. When that proved unsatisfactory, he mounted it on a wire, attached it to his car, and drove down an autobahn at 60 mph.

The V-2 was to fly at Mach 5, but for a time there was concern that it might not top Mach 1. The sound barrier loomed as potentially a real barrier, difficult to pierce, and at that time people did not know how to build a transonic wind tunnel that would give reliable results. Investigators studied this problem by building heavy iron models of this missile and dropping them from a Heinkel He-111 bomber. Observers watched from the ground; in one experiment, Von Braun himself piloted a plane and dove after the model to observe it from the air. The design indeed proved to be marginally unstable in the transonic region, but the V-2 had the thrust to power past Mach 1 with ease.

A second test missile, the A-5, also contributed to work on fin design. It sup­ported development of the guidance system, but it too needed fins, and it served as a testbed for further flight studies. Additional flight tests used models with length of five feet that were powered with rocket engines that flew with hydrogen peroxide as the propellant.

These tests showed that an initial fin design given by Kurzweg had the best subsonic stability characteristics. Subsequently, extensive wind-tunnel work both at Peenemunde and at a Zeppelin facility in Stuttgart covered the V-2 s complete Mach range and refined the design. In this fashion, the V-2’s fins were designed with only minimal support from Peenemunde’s big supersonic wind tunnels.7 But these tunnels came into their own later in the war, when investigators began to consider how to stretch this missile’s range by adding wings and thereby turning it into a supersonic glider.

Once the Germans came up with a good configuration for the V-2, they stuck with it. They proposed to use it anew in a two-stage missile that again sported fins that look excessively large to the modern eye, and that was to cross the Atlantic to strike New York.8 But there was no avoiding the need for a new round of wind – tunnel tests in studying the second stage of this intercontinental missile, the A-9, which was to fly with swept wings. As early as 1935 Adolf Busemann, another
colleague of Prandtl, had proposed the use of such wings in supersonic flight.9 Walter Dornberger, director of V-2 development, describes witnessing a wind-tunnel test of a model’s stability.

Подпись:The model had “two knifelike, very thin, swept-back wings.” Mounted at its center of gravity, it “rotated at the slight­est touch.” When the test began, a techni­cian opened a valve to start the airflow. In Dornberger’s words,

“The model moved abruptly, turning its nose into the oncoming airstream.

After a few quickly damping oscillations of slight amplitude, it lay quiet and stable in the air that hissed past it at 4.4 times the speed of sound. At the nose, and at the edges of the wing supports and guide mechanism, the shock waves could be clearly seen as they traveled diagonally backward at a sharp angle.

As the speed of the airflow fell off and the test ended, the model was no longer lying in a stable position. It made a few turns around its center of gravity, and then it came to a standstill with the nose pointing down­ward. The experiment Dr. Hermann had wished to show me had succeeded perfectly. This projectile, shaped like an airplane, had remained absolutely stable at a supersonic speed range of almost 3,500 mph.”10

Work on the A-9 languished for much of the war, for the V-2 offered problems aplenty and had far higher priority. But in 1944, as the Allies pushed the Germans out of France and the Russians closed in from the east, Dornberger and Von Braun faced insistent demands that they pull a rabbit from a hat and increase the V-2 s range. The rabbit was the A-9, with its wings promising a range of465 miles, some three times that of the standard V-2.11

Peenemunde’s Ludwig Roth proceeded to build two prototypes. The V-2 was known to its builders as the A-4, and Roth’s A-9 now became the A-4b, a designation that allowed it to share in the high priority of that mainstream program. The A-4b took shape as a V-2 with swept wings and with a standard set of fins that included slightly enlarged air vanes for better control. Certainly the A-4b needed all the help it could get, for the addition of wings had made it highly sensitive to winds.

The first A-4b launch took place late in December 1944. It went out of control and crashed as the guidance system failed to cope with its demands. Roth’s rock­eteers tried again a month later, and General Dornberger describes how this flight went much better:

“The rocket, climbing vertically, reached a peak altitude of nearly 50 miles at a maximum speed of2,700 mph. [It] broke the sound barrier without trouble. It flew with stability and steered automatically at both subsonic and supersonic speeds. On the descending part of the trajectory, soon after the rocket leveled out at the upper limit of the atmosphere and began to glide, a wing broke. This structural failure resulted from excessive aerodynamic loads.”12

This shot indeed achieved its research goals, for it was to demonstrate success­ful launch and acceleration through the sound barrier, overcoming drag from the wings, and it did these things. Gliding flight was not on the agenda, for while wind – tunnel tests could demonstrate stability in a supersonic glide, they could not guard against atmosphere entry in an improper attitude, with the A-4b tumbling out of control.13

Yet while the Germans still had lessons to learn about loads on a supersonic aircraft in flight, they certainly had shown that they knew their high-speed aerody­namics. One places their achievement in perspective by recalling that all through the 1950s a far wealthier and more technically capable United States pursued a vigorous program in rocket-powered aviation without coming close to the A-4b’s perfor­mance. The best American flight, of an X-2 in 1956, approached 2,100 mph—and essentially duplicated the German failure as it went out of control, killing the pilot and crashing. No American rocket plane topped the 2,700 mph of the A-4b until the X-15 in 1961.14

Hence, without operating in the hypersonic regime, the Peenemunde wind tun­nels laid important groundwork as they complemented such alternative research techniques as dropping models from a bomber and flying scale models under rocket power. Moreover, the Peenemunde aero dynamic is t Siegfried Erdmann used his cen­ter’s facilities to conduct the world’s first experiments with a hypersonic flow.

In standard operation, at speeds up to Mach 4.4, the Peenemunde tunnels had been fed with air from the outside world, at atmospheric pressure. Erdmann knew that a hypersonic flow needed more, so he arranged to feed his tunnel with com­pressed air. He also fabricated a specialized nozzle and aimed at Mach 8.8, twice the standard value. His colleague Peter Wegener describes what happened:

“Everything was set for the first-ever hypersonic flow experiment. The highest possible pressure ratio across the test section was achieved by evacuating the sphere to the limit the remaining pump could achieve. The supply of the nozzle—in con­trast to that at lower Mach numbers—was now provided by air at a pressure of

about 90 atmospheres__ The experiment was initiated by opening the fast-acting

valve. The flow of brief duration looked perfect as viewed via the optical system.

Beautiful photographs of the flow about wedge-shaped models, cylinders, spheres, and other simple shapes were taken, photographs that looked just as one would expect from gas dynamics theory.”15

These tests addressed the most fundamental of issues: How, concretely, does one operate a hypersonic wind tunnel? Supersonic tunnels had been bedeviled by con­densation of water vapor, which had necessitated the use of silica gel to dry the air. A hypersonic facility demanded far greater expansion of the flow, with consequent temperatures that were lower still. Indeed, such flow speeds brought the prospect of condensation of the air itself.

Conventional handbooks give the liquefaction temperatures of nitrogen and oxygen, the main constituents of air, respectively as 77 К and 90 K. These refer to conditions at atmospheric pressure; at the greatly rarefied pressures of flow in a hypersonic wind tunnel, the pertinent temperatures are far lower.16 In addition, Erdmann hoped that his air would “supersaturate,” maintaining its gaseous state because of the rapidity of the expansion and hence of the cooling.

This did not happen. In Wegener’s words, “Looking at the flow through the glass walls, one could see a dense fog. We know now that under the conditions of this particular experiment, the air had indeed partly condensed. The fog was made up of air droplets or solid air particles forming a cloud, much like the water clouds we see in the sky.”17 To prevent such condensation, it proved necessary not only to feed a hypersonic wind tunnel with compressed air, but to heat this air strongly.

One thus is entitled to wonder whether the Germans would have obtained useful results from their most ambitious wind-tunnel project, a continuous-flow system that was designed to achieve Mach 7, with a possible extension to Mach 10. Its power ratings pointed to the advantage of blowdown facilities, such as those of Peenemunde. The Mach 4.4 Peenemunde installations used a common vacuum sphere, evacuation of which relied on pumps with a total power of 1,100 horse­power. Similar power levels were required to dry the silica gel by heating it, after it became moist. But the big hypersonic facility was to have a one-meter test section and demanded 76,000 horsepower, or 57 megawatts.18

Such power requirements went beyond what could be provided in straightfor­ward fashion, and plans for this wind tunnel called for it to use Germany’s largest hydroelectric plant. Near Kochel in Bavaria, two lakes—the Kochelsee and Wal – chensee—are separated in elevation by 660 feet. They stand close together, provid­ing an ideal site for generating hydropower, and a hydro plant at that location had gone into operation in 1925, generating 120 megawatts. Since the new wind tunnel would use half of this power entirely by itself, the power plant was to be enlarged, with additional water being provided to the upper lake by a tunnel through the mountains to connect to another lake.19

In formulating these plans, as with the A-4b, Germany’s reach exceeded its grasp. Moreover, while the big hypersonic facility was to have generous provision for drying its air, there was nothing to prevent the air from condensing, which would have thrown the data wildly off.20 Still, even though they might have had to learn their lessons in the hard school of experience, Germany was well on its way toward developing a true capability in hypersonics by the end of World War II. And among the more intriguing concepts that might have drawn on this capability was one by the Austrian rocket specialist Eugen Sanger.

Eugen Sanger

Born in 1905, he was of the generation that came of age as ideas of space flight were beginning to germinate. Sanger’s own thoughts began to take shape while he was still in grammar school. His physics teacher gave him, as a Christmas present, a copy of a science-fiction novel, AufZwei Planeten (“On Two Planets”). “I was about 16 years old,” Sanger later recalled. “Naturally I read this novel avidly, and thereafter dreamed of doing something like this in my own lifetime.” He soon broadened his readings with the classic work of Hermann Oberth. “I had to pass my examination in mechanics,” he continued, “and had, therefore, made a particular study of this and related subjects. Then I also started to check and recalculate in detail everything in Oberth’s book, and I became convinced that here was something that one could take seriously.”

He then attended the Technische Hochschule in Vienna, where he tried to win a doctoral degree in 1928 by submitting a dissertation on the subject of rocket – powered aircraft. He did not get very far, later recalling that his professor told him, “If you try, today, to take your doctor degree in spaceflight, you will most probably be an old man with a long beard before you have succeeded in obtaining it.” He turned his attention to a more conventional topic, the structural design of wings for aircraft, and won his degree a year later. But his initial attempt at a dissertation had introduced him to the line of study that he pursued during the next decade and then during the war.

In 1933 he turned this dissertation into a book, Raketenflugtechnik. It was the first text in this new field. He wrote of a rocket plane burning liquid oxygen and petrol, which was to reach Mach 10 along with altitudes of 60 to 70 miles. This con­cept was significant at the time, for the turbojet engine had not yet been invented, and futurists, such as Aldous Huxley who wrote Brave New World, envisioned rock­ets as the key to high-speed flight in centuries to come.21

Sanger’s altitudes became those of the X-15, a generation later. The speed of his concept was markedly higher. He included a three-view drawing. Its wings were substantially larger than those of eventual high-performance aircraft, although these wings gave his plane plenty of lift at low speed, during takeoff and landing. Its tail surfaces also were far smaller than those of the X-15, for he did not know about the

Eugen Sanger

Rocket aircraft of Eugen Sanger. Top, the Silbervogel. Bottom, the Amerika-Bomber that was to use a skipping entry. Note that both were low-wing monoplanes. (Courtesy of Willy Ley)


Eugen SangerEugen Sanger

stability problems that loomed in supersonic flight. Still, he clearly had a concept that he could modify through further study.

In 1934, writing in the magazine Flug (“Flight”), he used an exhaust velocity of 3,700 meters per second and gave a velocity at a cutoff of Mach 13- His Silbervogel, Silver Bird, now was a boost-glide vehicle, entering a steady glide at Mach 3.5 and covering 5,000 kilometers downrange while descending from 60 to 40 kilometers in altitude.

He stayed on at the Hochschule and conducted rocket research. Then in 1935, amid the Depression, he lost his job. He was in debt to the tune of DM 2,000, which he had incurred for the purpose of publishing his book, but he remained defiant as he wrote, “Nevertheless, my silver birds will fly!” Fortunately for him, at that time Hitlers Luftwaffe was taking shape, and was beginning to support a research establishment. Sanger joined the DVL, the German Experimental Institute for Aeronautics, where he worked as technical director of rocket research. He did not go to Peenemunde and did not deal with the V-2, which was in the hands of the Wehrmacht, not the Luftwaffe. But once again he was employed, and he soon was out of debt.

He also began collaborating with the mathematician Irene Bredt, whom he later married. His Silbervogel remained on his mind as he conducted performance studies with help from Bredt, hoping that this rocket plane might evolve into an Amerika – Bomber. He was aware that when transitioning from an initial ballistic trajectory into a glide, the craft was to re-enter the atmosphere at a shallow angle. He then wondered what would happen if the angle was too steep.

He and Bredt found that rather than enter a glide, the vehicle might develop so much lift that it would fly back to space on a new ballistic arc, as if bouncing off the atmosphere. Stones skipping over water typically make several such skips, and Sanger found that his winged craft would do this as well. With a peak speed of 3-73 miles per second, compared with 4.9 miles per second as the Earth’s orbital velocity, it could fly halfway around the world and land in Japan, Germany’s wartime ally. At 4.4 miles per second, the craft could fly completely around the world and land in Germany.”

Sanger wrote up their findings in a document of several hundred pages, with the title (in English) of “On a Rocket Propulsion for Long Distance Bombers.” In December 1941 he submitted it for publication—and won a flat rejection the fol­lowing March. This launched him into a long struggle with the Nazi bureaucracy, as he sought to get his thoughts into print.

His rocket craft continued to show a clear resemblance to his Silbervogel of the previous decade, for he kept the basic twin-tailed layout even as he widened the fuselage and reduced the size of the wings. Its bottom was flat to produce more lift, and his colleagues called it the Platteisen, the Flatiron. But its design proved to be patentable, and in June 1942 he received a piece of bright news as the government awarded him a Reichspatent concerning “Gliding Bodies for Flight Velocities Above Mach 5.” As he continued to seek publication, he won support from an influential professor, Walter Georgii. He cut the length of his manuscript in half Finally, in September 1944 he learned that his document would be published as a Secret Com­mand Report.

The print run came to fewer than a hundred copies, but they went to the people who counted. These included the atomic-energy specialist Werner Heisenberg, the planebuilder Willy Messerschmitt, the chief designer Kurt Tank at Focke-Wulf, Ernst Heinkel of Heinkel Aircraft, Ludwig Prandtl who still was active, as well as Wernher von Braun and his boss, General Dornberger. Some copies reached the Allies after the Nazi surrender, with three of them being taken to Moscow. There their content drew attention from the dictator Josef Stalin, who ordered a full trans­lation. He subsequently decided that Sanger and Bredt were to be kidnapped and brought to Moscow.

At that time they were in Paris, working as consultants for the French air force. Stalin sent two agents after them, accompanied by his own son. They nevertheless remained safe; the Soviets never found them. French intelligence agents learned about the plot and protected them, and in any case, the Soviets may not have been looking very hard. One of them, Grigory Tokaty-Tokayev, was the chief rocket sci­entist in the Soviet air force. He defected to England, where he wrote his memoirs for the Daily Express and then added a book, Stalin Means War.

Sanger, for his part, remained actively involved with his rocket airplane. He suc­ceeded in publishing some of the material from his initial report that he had had to delete. He also won professional recognition, being chosen in 1951 as the first president of the new International Astronautical Federation. He died in 1964, not yet 60. But by then the X-15 was flying, while showing more than a casual resem­blance to his Silbervogel of 30 years earlier. His Silver Bird indeed had flown, even though the X-15 grew out of ongoing American work with rocket-powered aircraft and did not reflect his influence. Still, in January of that year—mere weeks before he died—the trade journal Astronautics & Aeronautics published a set of articles that presented new concepts for flight to orbit. These showed that the winged-rocket approach was alive and well.23

What did he contribute? He was not the first to write of rocket airplanes; that palm probably belongs to his fellow Austrian Max Valier, who in 1927 discussed how a trimotor monoplane of the day, the Junkers G-23, might evolve into a rocket ship. This was to happen by successively replacing the piston motors with rocket engines and reducing the wing area.24 In addition, World War II saw several military rocket-plane programs, all of which were piloted. These included Germany’s Me – 163 and Natter antiaircraft weapons as well as Japan’s Ohka suicide weapon, the

Cherry Blossom, which Americans called Baka, “Fool.” The rocket-powered Bell X-l, with which Chuck Yeager first broke the sound barrier, also was under develop­ment well before war’s end.25

Nor did Sanger’s 1944 concept hold military value. It was to be boosted by a supersonic rocket sled, which would have been both difficult to build and vulner­able to attack. Even then, and with help from its skipping entry, it would have been a single-stage craft attaining near-orbital velocity. No one then, 60 years ago, knew how to build such a thing. Its rocket engine lay well beyond the state of the art. Sanger projected a mass-ratio, or ratio of fueled to empty weight, of 10—with the empty weight including that of the wings, crew compartment, landing gear, and bomb load. Structural specialists did not like that. They also did not like the severe loads that skipping entry would impose. And after all this Sturm und drang, the bomb load of 660 pounds would have been militarily useless.26

But Sanger gave a specific design concept for his rocket craft, presenting it in suf­ficient detail that other engineers could critique it. Most importantly, his skipping entry represented a new method by which wings might increase the effectiveness of a rocket engine. This contribution did not go away. The train of thought that led to the Air Force’s Dyna-Soar program, around I960, clearly reflected Sanger’s influence. In addition, during the 1980s the German firm of Messerschmitt-Boel – kow-Blohm conducted studies of a reusable wing craft that was to fly to orbit as a prospective replacement for America’s space shuttle. The name of this two-stage vehicle was Sanger.27

Facing the Heat Barrier: a History of Hypersonics

It is a pleasure to note the numerous people who helped me with this book. My personal involvement in hypersonics dates to 1982. I wrote a number of free-lance articles, along with three book-length reviews, before beginning work on the present book in 1999. During these past two decades, several dozen people kindly granted interviews in the course of these assignments. This book draws on discussions with the following:

J. Leland Atwood, Robert Barthelemy, George Baum, Fred Billig, Richard Booton, Peter Bradshaw, William Cabot, Robert Cooper, Scott Crossfield,

Paul Czysz, William Dannevik, Anthony duPont, James Eastham, John Erdos, Maxime Faget, George Gleghorn, Edward Hall, Lawrence Huebner, Antony Jameson, Robert Jones, Arthur Kantrowitz, James Keller, George Keyworth, William “Pete” Knight, John Lumley, Luigi Martinelli, Robert Mercier, Parviz Moin, Gerhard Neumann, Louis Nucci, Philip Parrish, John Pike, Heinz Pitsch, Jerry Rising, Anatol Roshko, Paul Rubbert, Ron Samborsky, Robert Sanator, George Schairer, David Scott, Christian Stem – mer, Arthur Thomas, Steven Weinberg, and Robert Williams.

In the NASA History Division, NASA Chief Historian Steven Dick served effec­tively as my editor. NASA-Langley has an excellent library, where I received par­ticular help from Sue Miller and Garland Gouger. In addition, Dill Hunley, the historian at NASA-Dryden, hosted me for a week-long visit. The archivist, Archie DiFante, gave similar strong support during my visits to Maxwell Air Force Base. The Science and Technology Corporation, administered my work under subcon­tract, for which I give thanks to Andrea Carden, Carla Coombs, Sue Crotts, Marion Kidwell, and George Wood.

Dennis Jenkins provided me with documents and answered a number of ques­tions. The artists Don Dixon and Chris Butler, who helped me on previous book projects, provided valuable assistance on this one as well. In addition, as for previous books, Phyllis LaVietes served as my secretary.

This book reflects my interest in the National Aerospace Plane effort, which I covered as a writer beginning in 1985. It is a pleasure to recall my ongoing friend­ships with Robert Williams, who gave me access to sources; Fred Billig, who helped me learn the trade of hypersonics; and Arthur Kantrowitz, who was present at the beginning. These three stand out among the dozens of people with whom it has been my privilege to conduct interviews and discussions.


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.1

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.2

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.3

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.

Specifically, the hypersonic regime is defined as the realm of speed wherein the physics of flows is dominated by aerodynamic heating. This heating is far more intense than at speeds that are merely supersonic, even though these lesser velocities have defined the performance of the SR-71 and X-7.

Hypersonics nevertheless was a matter of practical military application before the term entered use. Germany’s wartime V-2 rocket flew above Mach 5,5 but steel proved suitable for its construction and aerodynamic heating played only a limited

role in its overall design.6 The Germans used wind-tunnel tests to ensure that this missile would remain stable in flight, but they did not view its speed regime as meriting a name of its own. Hsue-shen Tsien, an aerodynamicist at the California Institute of Technology, coined the term in 19467 Since then, it has involved three significant areas of application.

The first was the re-entry problem, which came to the forefront during the mid – 1950s. The Air Force by then was committed to developing the Atlas ICBM, which was to carry a nuclear warhead to Moscow. Left to itself, this warhead would have heated up like a meteor when it fell back into the atmosphere. It would not have burned up—it was too massive—but it certainly would have been rendered use­less. Hence, it was necessary to devise a heat shield to protect it against this intense aerodynamic heating.

The successful solution to this problem opened the door to a host of other initia­tives. The return of film-carrying capsules from orbit became routine, and turned strategic reconnaissance of the Soviet Union into an important element of national defense. Piloted space flight also became feasible, for astronauts now could hope to come back safely. Then, as the engineering methods for thermal protection were further improved, thoughts of a space shuttle began to flourish. They took shape as a reusable launch vehicle, the first of its kind.

Hypersonic technologies also became important as policy makers looked ahead to an era in which the speed and performance of fighters and bombers might increase without limit. This expectation led to the X-15. Though designed during the 1950s, this rocket-powered research airplane set speed and altitude marks that were not sur­passed until the advent of the shuttle. Aerodynamic heating again defined its design requirements, and it was built of the nickel alloy Inconel X. It routinely withstood temperatures of 1200°F as it flew to Mach 6,8 and reached altitudes high enough for some of its pilots to qualify as astronauts.

Only rocket engines could propel a vehicle at such speeds, but hypersonic pro­pulsion has represented a third important area of application. Here the hope has persisted that innovative airbreathing engines—scramjets—might cope with intense aerodynamic heating while offering fuel economy far surpassing that of a rocket. Other work has emphasized airbreathing rockets, which could give improved perfor­mance by eliminating the need to carry liquid oxygen in a tank. These concepts have held their own importance. They lay behind the National Aerospace Plane (NASP) program of 1985-1995, which sought to lay groundwork for single-stage vehicles that were to use both types of engine and were to fly from a runway to orbit.

The Air Force historian Richard Hallion has written of a “hypersonic revolu­tion,” as if to place the pertinent technologies on par with the turbojet and liquid – propellant rocket.9 The present book takes a more measured view. "Work in hyper – sonics had indeed brought full success in the area of re-entry. Consequences have included strategic missiles, the Soviet and American man-in-space programs, the

Corona program in strategic reconnaissance, Apollo, and the space shuttle. These activities deterred nuclear war, gained accurate estimates of the Soviet threat, sent astronauts to the Moon and brought them home, and flew to and from space in a reusable launch vehicle. This list covers many of the main activities of the postwar missile and space industry, and supports Hallions viewpoint.

But in pursuing technical revolution, engineers succeed in actually solving their problems, as when the Apollo program sent men to the Moon. These people do not merely display brilliant ingenuity while falling short of success. Unfortunately, the latter has been the case in the important area of hypersonic propulsion.

The focus has involved the scramjet as a new engine. It has taken form as a prime mover in its own right, capable of standing alongside such engines as the turboprop and ramjet. Still, far more so than the other engines, the scramjet has remained in the realm of experiment. Turboprops powered the Lockheed Electra airliner, P-3 antisubmarine aircraft, and C-130 transport. Ramjets provided propulsion for the successful Bomarc and Talos antiaircraft missiles. But the scramjet has powered only such small experimental airplanes as the X-43A.

Why? From the outset, the scramjet has faced overwhelming competition from a successful alternative: the rocket. This has strongly inhibited funding and has delayed its development to a point at which it could be considered seriously. On paper, scramjets offer superior performance. They therefore drew attention in the mid-1980s, during the heyday of NASP, at a time when Air Force officials had become disenchanted with the space shuttle but faced huge prospective demand for access to space in President Reagans Strategic Defense Initiative. For once, then, scramjets gained funding that served to push their development—and their perfor­mance fell well short of peoples hopes.

Within this book, Chapter 1 covers the immediate postwar years, when America still had much to learn from the Europeans. It focuses on two individuals: Eugen Sanger, who gave the first proposal for a hypersonic bomber, and John Becker, who built Americas first hypersonic wind tunnel.

Chapter 2 covers the first important area of hypersonic research and develop­ment, which supported the advent of strategic missiles during the 1950s. The focus was on solving the re-entry problem, and this chapter follows the story through flight tests of complete nose cones.

Chapter 3 deals with the X-15, which took shape at a time when virtually the whole of Americas capability in hypersonics research was contained within Beckers 11-inch instrument. Today it is hard to believe that so bold and so successful a step in aviation research could stand on so slender a foundation. This chapter shows how it happened.

Chapter 4 introduces hypersonic propulsion and emphasizes the work of Anto­nio Ferri, an Italian aerodynamicist who was the first to give a credible concept for a scramjet engine. This chapter also surveys Aerospaceplane, a little-known program of

paper studies that investigated the feasibility of flight to orbit using such engines.

The next two chapters cover important developments in re-entry that followed the ICBM. Chapter 5, “Widening Prospects for Re-Entry,” shows how work in this area supported the manned space program while failing to offer a rationale for a winged spacecraft, Dyna-Soar. Chapter 6, “Hypersonics and the Shuttle,” begins by outlining developments during the mid-1960s that made it plausible that NASA’s reusable space transporter would be designed as a lifting body and built using hot structures. In fact, the shuttle orbiter came forth as a conventional airplane with delta wings, and was built with aluminum structure covered with thermal-protect­ing tiles. This discussion indicates how those things happened.

Chapter 7, “The Fading, the Comeback,” shows how work with scramjets did not share the priority afforded to the topic of re-entry. Instead it faded, and by the late 1960s only NASA-Langley was still pursuing studies in this area. This ongoing effort nevertheless gave important background to the National Aerospace Plane— but it was not technical success that won approval for NASP As noted, it was the Strategic Defense Initiative. Within the Strategic Defense Initiative, the scramjet amounted to a rabbit being pulled from a hat, to satisfy Air Force needs. NASP was not well-founded at the outset; it was more of a leap of faith.

Chapter 8, “Why NASP Fell Short,” explains what happened. In summary, the estimated performance of its scramjet engine fell well below initial hopes, while the drag was higher than expected. Computational aerodynamics failed to give accurate estimates in critical technical areas. The ejector ramjet, a key element of the propul­sion system, proved to lack the desired performance. In the area of materials, metal­lurgists scored an impressive success with a new type of titanium called Beta-21 S. It had only half the density of the superalloys that had been slated for Dyna-Soar, but even greater weight savings would have been needed for NASP.

Finally, Chapter 9 discusses “Hypersonics After NASP.” Recent developments include the X-33 and X-34 launch vehicles, which represent continuing attempts to build the next launch vehicle. Scramjets have lately taken flight, not only as NASA’s X-43A but also in Russia and in Australia. In addition, the new topic of Large Eddy Simulation, in computational fluid mechanics, raises the prospect that analysts indeed may learn, at least on paper, just how good a scramjet may be.

What, in the end, can we conclude? During the past half-century, the field of hypersonics has seen three major initiatives: missile nose cones, the X-15, and NASP. Of these, only one—the X-15—reflected ongoing progress in aeronautics. The other two stemmed from advances in nuclear weaponry: the hydrogen bomb, which gave rise to the ICBM, and the prospect of an x-ray laser, which lay behind the Strategic Defense Initiative and therefore behind NASP.

This suggests that if hypersonics is to flourish anew, it will do so because of developments in the apparently unrelated field of nuclear technology.

1 F-104: Gunston, Fighters, pp. 120-126. Concorde: Heppenheimer, Turbulent, pp. 202-203, 208.

2 Crickmore, SR-71, pp. 89-91, 95-99, 194.

3 Ritchie, “Evaluation.”Steel: Miller, X-Planes, p. 119.

4 See, for example, Anderson, History, pp. 438-439.

5 Top speed of the V-2 is given as 1,600 meters per second (Dornberger, V-2, p. xix) and as 1,700 meters per second (Naval Research Laboratory, Upper, cited in Ley, Rockets, pp. 596-597); the speed of sound at the pertinent altitudes is 295 meters per second (Kuethe and Chow, Founda­tions, p. 518).

6 Ley Rockets, p. 243; Neufeld, Rocket, pp. 85-94.

7 Tsien, “Similarity”

8 1200°F: NASA SP-2000-4518, diagram, p. 25.

9 Hallion, Hypersonic.