Michael Neufeld - The Rocket and the Reich

If the integration of university institutes into the rocket program was doubtful for the country, it was certainly beneficial to Wa Prüf 11’s technology development, including Thiel’s efforts in propulsion. While the fundamental design of the 25-ton motor was not changed by academic research, Professor Wewerka of the Technical University of Stuttgart made significant suggestions for overcoming turbopump design problems. In the realm of theory, Wewerka verified Thiel’s findings regarding the best angle of opening for expansion nozzles. To choose only one other important example, Professor Beck of the Technical University of Dresden (later Berlin) was instrumental in proposing and testing alternative designs for the A-4 injection system. A new design was certainly needed. The eighteen-pot motor could be made to work, but it was a “monstrosity and a plumber’s nightmare,” to use the words of Thiel’s later replacement, Martin Schilling. The eighteen small injection chambers required eighteen separate liquid oxygen lines. The piping for fuel flow to the cooling jacket and film cooling outlets was equally complex. After the propulsion group withdrew completely to Peenemünde in August 1940, one of the test stands at Kummersdorf was handed over to Beck’s institute full time for experiments. The alternate injection systems, called “mixing nozzles,” used a series of concentric ring slots, or a plate covered with injection holes. But the hoped-for quick success never materialized. 19

The energetic and imaginative Thiel also pursued alternatives to the basic 25-ton engine. Since 1937 he had experimented with higher chamber pressures, which promoted more efficient burning. He put some effort into developing a 25-atmosphere, 725-kg-thrust engine for the He 176 rocket plane. The existing technology did not, however, seem to allow much success in this area. Ultimately he had to settle for raising the A-4 motor’s pressure to thirteen atmospheres, which could be done without major changes to the design. He also looked anew at alternative propellants in 1941, because super-cold liquid oxygen was extremely inconvenient to use and in potentially short supply. Thus, when Thiel declared the basic eighteen-pot A-4 motor officially finished on September 15, 1941, it was far from clear that it was satisfactory for mass production. 20Nonetheless, in the five years since he had joined Dornberger’s rocket section, he and his assistants had created a revolution in rocketry. It was now possible to lift a missile with a takeoff weight of more than 12 tons and hurl it a couple of hundred kilometers. It was truly a remarkable accomplishment.

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