KURZWEG, HERMANN, AND SUPERSONIC AERODYNAMICS
Possessing the raw power to propel a missile over such distances was one thing; stabilizing and guiding it through velocities exceeding Mach 4.5 (four and a half times the speed of sound) was quite another. When Wernher von Braun began to investigate the best form for the A-3 in 1935, he knew essentially nothing about the supersonic aerodynamics of fin-stabilized bodies. The ballisticians, led by Becker and his mentor, Professor Carl Cranz, had experimented with spinning rifle bullets and artillery shells moving at those velocities; indeed, the fuselage of the A-3 was based on the infantry “S” bullet, because it was known that its shape worked at higher Mach numbers. But some artillery specialists told the rocket group that stabilizing a supersonic body with fins was impossible.21
That Army ballisticians could have made this statement indicates how little contact they had with the aerodynamics community, which was funded by the Transportation Ministry before 1933 and the Air Ministry thereafter. The areas of interest of the two communities were in any case far apart, since the aerodynamicists concentrated on airfoils and aircraft moving at the low speeds typical of the day. But from the late 1920s on the theory of winged bodies moving at supersonic and high subsonic velocities, where air begins to compress significantly, had made large advances, especially under the direction of the grand old man of aerodynamics, Professor Ludwig Prandtl of Göttingen. Practical experiments had also begun in small supersonic wind tunnels at this time. Although the first was built in Switzerland, Germany had a dominant place in this area too. Based on testing done in tunnels at Göttingen and Dresden, one of Prandtl’s rising stars, Dr. Adolf Busemann, first revealed in October 1935 that swept-back wings worked much better than straight wings at velocities approaching and exceeding Mach 1 (the so-called sound barrier). Swept wings delayed the onset of turbulence near Mach 1 and had much better lift and drag characteristics at high speeds. That discovery interested few aerodynamicists at the time, as it did not seem to have much practical application.22
In the meantime, the rise of the rocket aircraft program had put von Braun in touch with Busemann and the aerodynamics community. With the help of the Air Ministry, a few A-3 tunnel tests were made in 1935, but the real collaboration began with von Braun’s visit to the Technical University of Aachen on January 8, 1936. There he encountered Dr. Rudolf Hermann, an assistant professor who had constructed a Luftwaffe-financed supersonic wind tunnel. The test section of this tunnel, where the aircraft or missile models were placed, was square and measured only 10 centimeters (4 inches) on a side. During 1936 and early 1937 Hermann made basic A-3 measurements up to the tunnel’s maximum velocity, Mach 3.3. But the models had to be very small, and his task was only to modify a design already chosen. By enlarging the A-3’s highly swept-back fins, he was able to create an “arrow-stable” vehicle, but its form was far from ideal. If the A-3 had made it to high altitudes, reduced atmospheric pressure would have allowed the engine exhaust jet to expand, burning off the fins or the antenna ring around the ends of the fins. Actual flight testing also indicated that the A-3 was too stable, making it difficult for the guidance-and-control to push the vehicle back to the desired attitude when the aerodynamic forces were so overpowering.23
Long before the unfortunate outcome of those launches became known in December 1937, however, von Braun had decided that the rocket program needed its own supersonic wind tunnel complex if it was to take on the challenge of the A-4. It was not only inconvenient to go to Aachen, but the tunnel was too small, and ready access was not assured. The Air Ministry controlled the aeronautical research establishment; more important, the existing tunnels were greatly overbooked until the Ministry’s massive investments in new research facilities bore fruit in the early war years.24
Dornberger agreed with his able assistant that a supersonic tunnel would be a good idea. It certainly fitted his “everything-under-one-roof” concept for the new Peenemünde center, but “the cost frightened me; the estimate was 300,000 marks. I had enough experience with building to know that there wasn’t the least chance of remaining at that figure, especially with von Braun about. The supersonic tunnel was more likely to cost a million marks.” Dornberger decided to move, however, once Hermann had demonstrated in the autumn of 1936 that he could make the A-3 stable at supersonic velocities. He went to Becker, then chief of Testing Division, and asked for the tunnel. His boss agreed, but only on the condition that at least one other section make use of it. Curiously, Dornberger was not able to secure the support of Section 1 (ballistics and munitions), which his rocket group had so recently left, but he did convince the head of anti-aircraft artillery that the shape of shells could be refined in the tunnel. Becker issued the order on November 30, 1936, and in April 1937 Rudolf Hermann joined the staff of Peenemünde.25
The thirty-two-year-old aerodynamicist, described by Dornberger as “slender… with a lofty brow and light-brown, wavy hair brushed straight back,” was a talented and energetic engineering scientist. According to American records, he was also an “extremely ardent Nazi” in the later war years, although he did not join the Party until membership was reopened in mid-1937. When Dornberger and von Braun recruited Hermann away from academia, they doubtless appealed to his patriotism, but above all they stroked his ambition: He would have a chance to build the world’s most advanced supersonic wind tunnel. Hermann immediately began to recruit a staff and to plan for the facility. The heart of the “Aerodynamic Institute,” to be constructed in the middle of the laboratory and shop area of Peenemünde-East, would be a scaled-up version of the 10-by-10-centimeter tunnel built at Aachen. It would have a test section measuring 40 centimeters (about 16 inches) to a side and a maximum running speed of Mach 4.4, a world’s record equaled but not exceeded before the end of the war.26
The configuration and principles of the main Peenemünde tunnel are shown in Figure 3.2. It was of the open or “blow-down” type. A spherical vacuum reservoir with a diameter of about 12.5 meters (40 feet!) was emptied out by six pumps exerting 1,100 horsepower. When the quick-acting valve was opened, air rushed in from outside and filled the vacuum chamber in about 20 seconds, the maximum running time for an experiment. The velocity of the air through the test section was determined by the shape and size of the opening in the “Laval nozzle” through which the inrushing molecules must first pass. A “three-component balance” measured the lift and drag forces on the model; the airflow patterns around it could be photographed or measured with elaborate optical equipment. A smaller 18-by-18-centimeter tunnel was also built and connected to the same reservoir. By pumping continuously, this tunnel could be operated without interruption, although only for lower Mach numbers.27
The planning and construction of this expensive state-of-the-art facility took a long time, which must have resulted in considerable pressure on Hermann and his staff from the Ordnance Office. The aerodynamicists were not able to put the big tunnel into operation until about May 1939, and the small one came even later. Even then there were numerous startup difficulties. The design of the Laval nozzles that determined the Mach number was a problem of great theoretical complexity and strenuous trial-and-error correction. Even at the beginning of 1941 the nozzle for Mach 3.1 was still being refined, and the highest working speed was Mach 2.5. The nozzle for Mach 4.4 was not ready until 1942 or 1943. Another problem was condensation clouds that formed because the rapidly expanding air cooled dramatically after going though the throat of the nozzle. To get accurate results it was necessary to place a special air-drying silica-gel honeycomb across the mouth of the opening (see Figure 3.2). Until that system was finished in the spring of 1940, the effectiveness of Hermann’s facility was diminished by measurements of questionable reliability.28