Michael Neufeld - The Rocket and the Reich

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

The construction of this world-class aerodynamics institute at Peenemünde, with a staff of sixty in mid-1939 and two hundred in 1943, was another large stride on the road to massive in-house research and development capability. But the delay in putting the tunnels into operation meant that the critical aerodynamic innovations for the A-4 were largely obtained through educated guesses and improvised experiments. When the decision was made in January 1938 to build a new vehicle (the A-5) to test guidance systems, the question of the external form for a rocket again became pressing. Two problems in particular forced Hermann and his staff to improvise: fin design and the stability of rocket bodies as they passed through the sound barrier.

In designing the A-5, the body form was not the problem. Since the A-3 fuselage was considered adequate, and because extensive wind-tunnel testing was still unavailable, the body was made only slightly fatter on the A–5, which then became the model for the A-4. By contrast, no one knew how to design fins for supersonic flight. In 1937 Hermann had lured to Peenemünde as his chief assistant and head of research Dr. Hermann Kurzweg, an acquaintance from the University of Leipzig then working at the eminent optical company Zeiss. Kurzweg, an inactive but contributing member of the SS since 1934, tackled the A-5 fin problem in early 1938 with only the most limited information: a knowledge of supersonic aerodynamics, a rough estimate of the expected pressure distribution over the rocket body, and wind tunnel information on the characteristics of flat plates. (Another consideration must have been Dornberger’s injunction that the fins, when scaled to A-4 dimensions, be able to pass through a standard railroad tunnel.) To allow for the expansion of the exhaust jet at high altitude, where air pressure is low, Kurzweg made the rear of the A-5’s fins open at a much wider angle than those of the A-3. For good supersonic characteristics he swept the forward edge more strongly, kept the fins relatively thin, although thicker than on the A-3, and not quite as wide as they were long. The result was the first critical aerodynamic innovation for the A-4: a broad fin shape familiar from later pictures of that missile. 29

Lacking any way to get quick answers about the appropriateness of his design, Kurzweg resorted to homemade improvisations. One weekend he carved a rocket body out of a Peenemünde pine branch, inserted weights into holes to get the proper balance, and made hard rubber fins in the proposed shape, but in three different sizes. To obtain low-speed stability information he tried throwing the model off the roof of his house. When that proved unsatisfactory, the model was mounted on a wire through its center of gravity, and Kurzweg drove down the Berlin-Anklam highway at a speed of 100 km/h (about 60 mph). The largest and second-largest versions of the fins seemed to be stable, but not the smallest. Since it now seemed that the aerodynamics group had a workable A-5 concept in hand, wind tunnel testing was done in 1938 and 1939 at Aachen and in the subsonic installation at the Zeppelin Airship Construction Company in Friedrichshafen on Lake Constance. Kurzweg’s fin design came to be embodied in the first unguided A-5s launched from the Greifswalder Oie in October 1938. With small modifications, this shape was also used on the A-4. 30

But before the aerodynamics group could make a definite decision, a more systematic attack on fin form was necessary. In conjunction with the 1938 A-5 launches, which aimed to exercise the launch organization and demonstrate the aerodynamic stability of the rocket, Peenemünde-East sent aloft a number of models equipped with various fin shapes and propelled by small Walter hydrogen-peroxide motors. The exact purpose of those model experiments is unclear, but the most likely one was testing for an unguided anti-aircraft missile proposed by Wa Prüf 11. Inspired by that effort, the aerodynamics group then fired off forty subscale A-5 models using Walter motors in March 1939. Eight different fin shapes were given to the 1.6-meter-(5-foot)-high models, and the launches were systematically photographed. The results confirmed that, of the designs tested, Kurzweg’s original had the best subsonic stability characteristics. Later, extensive wind tunnel work at Peenemünde and Zeppelin substantiated this research for the whole velocity range and refined the shape for the A-4. It is a tribute to Kurzweg’s ability that he had so successfully defined the solution from the outset. 31