John Becker’s
11-inch hypersonic wind tunnel. (NASA)
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.
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.” 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 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.
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. 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.
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.”
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. 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 the latter part of the war he was assistant chief of Stack’s
Compressibility Research Division. He specifically was in charge of Langley’s
16-Foot High-Speed Tunnel, which had fought its war by investigating cooling
problems in aircraft motors as well as the design of propellers. This facility
contributed particularly to tests of the B-50 bomber and to the aerodynamic
shapes of the first atomic bombs. It also assisted development of the Pratt
& Whitney R-2800 Double Wasp, a widely used piston engine that powered
several important wartime fighter planes, along with the DC-6 airliner and the
C-69 transport, the military version of Lockheed’s Constellation.
It was quite a jump from piston-powered warbirds to
hypersonics, but Becker willingly made the leap. The V-2, flying at Mach 5,
gave him his justification. In a memo to Langley’s chief of research, dated 3
August 1945, Becker noted that planned facilities were to reach no higher than
Mach 3. He declared that this was inadequate: “When it is considered that all
of these tunnels will be used, to a large extent, to develop supersonic
missiles and projectiles of types which have already been operated at Mach
numbers as high as 5.0, it appears that there is a definite need for equipment
capable of higher test Mach numbers.”
Within this memo, he outlined a design concept for “a
supersonic tunnel having a test section four-foot square and a maximum test
Mach number of 7.0.” It was to achieve continuous flow, being operated by a
commercially-available compressor of 2,400 horsepower. To start the flow, the
facility was to hold air within a tank that was compressed to seven
atmospheres. This air was to pass through the wind tunnel before exhausting
into a vacuum tank. With pressure upstream pushing the flow and with the
evacuated tank pulling it, airspeeds within the test section would be high
indeed. Once the flow was started, the compressor would maintain it.
A preliminary estimate indicated that this facility would
cost $350,000. This was no mean sum, and Becker’s memo proposed to lay
groundwork by first building a model of the big tunnel, with a test section
only one foot square. He recommended that this subscale facility should “be
constructed and tested before proceeding with a four-foot-square tunnel.” He
gave an itemized cost estimate that came to $39,550, including $10,000 for
installation and $6,000 for contingency.
Becker’s memo ended in formal fashion: “Approval is
requested to proceed with the design and construction of a model supersonic
tunnel having a one-foot-square test section at Mach number 7.0. If successful,
this model tunnel would not only provide data for the design of economical high
Mach number supersonic wind tunnels, but would itself be a very useful research
tool.”
On 6 August, three days after Becker wrote this memo, the potential
usefulness of this tool increased enormously. On that day, an atomic bomb
destroyed Hiroshima. With this, it now took only modest imagination to envision
nuclear-tipped V-2s as weapons of the future. The standard V-2 had carried only
a one-ton conventional warhead and lacked both range and accuracy. It
nevertheless had been technically impressive, particularly since there was no
way to shoot it down. But an advanced version with an atomic warhead would be
far more formidable.
John Stack strongly supported Becker’s proposal, which soon
reached the desk of George Lewis, NACA’s Director of Aeronautical Research.
Lewis worked at NACA’s Washington Headquarters but made frequent visits to
Langley. Stack discussed the proposal with Lewis in the course of such a visit,
and Lewis said, “Let’s do it.”
Just then, though, there was little money for new projects.
NACA faced a postwar budget cut, which took its total appropriation from $40.9
million in FY 1945 to $24 million in FY 1946. Lewis therefore said to Stack,
“John, you know I’m a sucker for a new idea, but don’t call it a wind tunnel
because I’ll be in trouble with having to raise money in a formal way. That
will necessitate Congressional review and approval. Call it a research
project.” Lewis designated it as Project 506 and obtained approval from NACA’s
Washington office on 18 December.
A month later, in January 1946, Becker raised new issues in
a memo to Stack. He was quite concerned that the high Mach would lead to so low
a temperature that air in the flow would liquefy. To prevent this, he called
for heating the air, declaring that “a temperature of 600ºF in the pressure
tank is essential.” He expected to achieve this by using “a small electrical
heater.”
The pressure in that tank was to be considerably higher than
in his plans of August. The tank would hold a pressure of 100 atmospheres.
Instead of merely starting the flow, with a powered compressor sustaining in
continuous operation, this pressure tank now was to hold enough air for
operating times of 40 seconds. This would resolve uncertainties in the
technical requirements for continuous operation. Continuous flows were still on
the agenda but not for the immediate future. Instead, this wind tunnel was to
operate as a blowdown facility.
Here, in outline, was a description of the installation as
finally built. Its test section was 11 inches square. Its pressure tank held 50
atmospheres. It never received a compressor system for continuous flow,
operating throughout its life entirely as a blowdown wind tunnel. But by
heating its air, it indeed operated routinely at speeds close to Mach 7.
Taking the name of 11-Inch Hypersonic Tunnel, it operated
successfully for the first time on 26 November 1947. It did not heat its
compressed air directly within the pressure tank, relying instead on an
electric resistance heater as a separate component. This heater raised the air
to temperatures as high as 900ºF, eliminating air liquefaction in the test
section with enough margin for Mach 8. Specialized experiments showed clearly
that condensation took place when the initial temperature was not high enough
to prevent it. Small particles promoted condensation by serving as nuclei for
the formation of droplets. Becker suggested that such particles could have
formed through the freezing of CO2, which is naturally present in air.
Subsequent research confirmed this conjecture.
