Facing the Heat Barrier - NASA's History Office

Facing the Heat Barrier: A History of Hypersonics
Cherry Blossom, which Americans called Baka, “Fool.” The rocket-powered Bell
X-1, with which Chuck Yeager first broke the sound barrier, also was under development
well before war’s end. 25
Nor did Sänger’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.
Sänger 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 Sänger 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 1960, clearly reflected Sänger’s
influence. In addition, during the 1980s the German firm of Messerschmitt-Boelkow-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 Sänger. 27
NACA-Langley and John Becker
During the war the Germans failed to match the Allies in production of airplanes,
but they were well ahead in technical design. This was particularly true in
the important area of jet propulsion. They fielded an operational jet fighter, the
Me-262, and while the Yankees were well along in developing the Lockheed P-80 as
a riposte, the war ended before any of those jets could see combat. Nor was the Me-
262 a last-minute work of desperation. It was a true air weapon that showed better
speed and acceleration than the improved P-80A in flight test, while demonstrating
an equal rate of climb. 28 Albert Speer, Hitler’s minister of armaments, asserted
in his autobiographical Inside the Third Reich (1970) that by emphasizing production
of such fighters and by deploying the Wasserfall antiaircraft missile that was in
development, the Nazis “would have beaten back the Western Allies’ air offensive
against our industry from the spring of 1944 on.” 29 The Germans thus might have
prolonged the war until the advent of nuclear weapons.
Wartime America never built anything resembling the big Mach 4.4 wind tunnels
at Peenemunde, but its researchers at least constructed facilities that could compare
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First Steps in Hypersonic Research
with the one at Aachen. The American installations did not achieve speeds to match
Aachen’s Mach 3.3, but they had larger test sections. Arthur Kantrowitz, a young
physicist from Columbia University who was working at Langley, built a nine-inch
tunnel that reached Mach 2.5 when it entered operation in 1942. (Aachen’s had
been four inches.) Across the country, at NACA’s Ames Aeronautical Laboratory,
two other wind tunnels entered service during 1945. Their test sections measured
one by three feet, and their flow speeds reached Mach 2.2. 30
The Navy also was active. It provided $4.5 million for the nation’s first really
large supersonic tunnel, with a test section six feet square. Built at NACA-Ames,
operating at Mach 1.3 to 1.8, this installation used 60,000 horsepower and entered
service soon after the war. 31 The Navy also set up its Ordnance Aerophysics Laboratory
in Daingerfield, Texas, adjacent to the Lone Star Steel Company, which had
air compressors that this firm made available. The supersonic tunnel that resulted
covered a range of Mach 1.25 to 2.75, with a test section of 19 by 27.5 inches. It
became operational in June 1946, alongside a similar installation that served for
high-speed engine tests. 32
Theorists complemented the wind-tunnel builders. In April 1947 Theodore von
Karman, a professor at Caltech who was widely viewed as the dean of American
aerodynamicists, gave a review and survey of supersonic flow theory in an address
to the Institute of Aeronautical Sciences. His lecture, published three months later
in the Journal of the Aeronautical Sciences, emphasized that supersonic flow theory
now was mature and ready for general use. Von Karman pointed to a plethora of
available methods and solutions that not only gave means to attack a number of
important design problems but also gave independent approaches that could permit
cross-checks on proposed solutions.
John Stack, a leading Langley aerodynamicist, noted that Prandtl had given a
similarly broad overview of subsonic aerodynamics a quarter-century earlier. Stack
declared, “Just as Prandtl’s famous paper outlined the direction for the engineer in
the development of subsonic aircraft, Dr. von Karman’s lecture outlines the direction
for the engineer in the development of supersonic aircraft.” 33
Yet the United States had no facility, and certainly no large one, that could reach
Mach 4.4. As a stopgap, the nation got what it wanted by seizing German wind tunnels.
A Mach 4.4 tunnel was shipped to the Naval Ordnance Laboratory in White
Oak, Maryland. Its investigators had fabricated a Mach 5.18 nozzle and had conducted
initial tests in January 1945. In 1948, in Maryland, this capability became
routine. 34 Still, if the U.S. was to advance beyond the Germans and develop the true
hypersonic capability that Germany had failed to achieve, the nation would have to
rely on independent research.
The man who pursued this research, and who built America’s first hypersonic
tunnel, was Langley’s John Becker. He had been at that center since 1936; during
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