Michael Neufeld - The Rocket and the Reich

Competition for Kreiselgeräte was first formally discussed on November 9, 1937, when Dornberger and von Braun went to a meeting at the aviation instruments division of Siemens, the giant electrical concern. Among those attending was Siemens’s Dr. Karl Fieber, who remembers the “unusual declarations of secrecy with threats of the death penalty.” Also present were Karl Otto Altvater, director of the aviation instruments division, and Klaus Riedel, who had worked for the company for three or four years after the collapse of the Raketenflugplatz. Altvater, a retired U-boat captain and Naval Ordnance section chief in the 1920s, was Riedel’s uncle. He had given his nephew and two or three other rocket enthusiasts jobs when the amateur group fell apart. After the settlement of the Nebel–Riedel patent in July 1937, Klaus Riedel went with his friends to Peenemünde, where he in all likelihood suggested Siemens as an alternative source of gyroscope expertise. The firm had become increasingly involved in aircraft autopilots and navigation instruments since the 1920s and had purchased Boykow’s patents in this area in 1931. 38

During the November 9 meeting von Braun lectured on the A-3 guidance system, giving no indication that the rocket group had anything but confidence in it, and the Siemens people made no comment. Nevertheless, Dornberger indicated that Ordnance had “the conscious intent… to create a competitive line of development to Kreisegeräte Ltd.” Nothing came of the discussions right away. For the next two months the preparations for the launches on the Oie and the postmortems on the failures completely absorbed the attention of Dornberger and his assistants. Only toward the end of January 1938 was it possible to meet again. Now the tone was completely different. With hindsight provided by the A-3 launches, the Siemens representatives expressed their reservations about the Sg 33’s inadequate control forces and its inability to stop a rapid rolling of the vehicle. 39

Meanwhile, the discussions between the rocket group and Kreiselgeräte had produced some basic decisions. There was a need for systematic launch testing, a dramatic increase in the forces exerted by the jet vanes, and a refinement of the aerodynamic stability of the new vehicle. Kreiselgeräte must also make its control system simpler in most aspects, while more complicated in others. The company would remain with Boykow’s basic concept, one fundamental to most inertial guidance systems: a stabilized platform that remains fixed in space regardless of the movements of the rocket. Such a platform provides a reference for measuring the position or acceleration of the vehicle. But for the A-5 it was imperative to add the third gyro for the roll axis, which had been omitted from the Sg 33 for reasons of simplification. To reduce complexity, Peenemünde and Kreiselgeräte decided to eliminate the nitrogen jets that stabilized the platform, as well as the electricity supply that powered the gyros after liftoff. The A-5 would be equipped with gyros large and heavy enough to hold the platform steady by themselves, even though they would be slowed down by friction during the forty-five seconds of engine firing. That solution was clearly a temporary stopgap and was inadequate to the much more demanding requirements of the A-4.

Eliminated as well were the small accelerometer wagons that measured horizontal movement away from the trajectory. The A-5 guidance system, which Kreiselgeräte soon called Sg 52, would concentrate only on maintaining the proper attitude of the rocket. To get more sensitive control and perhaps even stability, more inputs were needed to keep the rocket from yawing, pitching, and rolling and to return it to its desired position. In contrast to the A-3s, the gyros on the stabilized platform of the Sg 52 would feed in signals giving the attitude of the vehicle with respect to the platform. It would be necessary also to have a feedback of the positions of the jet vanes that deflected the engine’s thrust. Those signals would then be mixed with the ones from the three rate gyros, which were not on the platform and which sensed the rate (velocity) of the rocket’s turning. The mixed inputs would then be transmitted to the vanes via a mechanical system of rotating rods and gears. 40

Most of the decisions about the new Kreiselgeräte system were made in short order, by the middle of January 1938. It took Dornberger and von Braun much longer to initiate a competing A-5 system with Siemens. The most relevant experience of Altvater’s group was with autopilots that kept airplanes traveling on specific headings at set altitudes. In such devices, a “course gyro” maintained the heading. In some versions, one or two other gyros, in conjunction with rate gyros for stability, also kept the aircraft from rolling around its longitudinal axis or pitching its nose up and down. Those autopilots had no stabilized platform. It was thus natural for Siemens to try also to use position gyros fixed to the body of the rocket, although a system of that type is inherently less accurate than a platform, because a movement of the vehicle in one axis shifts the orientation of the other axes. Only the short period of active guidance during engine firing—less than 45 seconds for the A-5—and a control system that effectively limited missile motion would make the errors tolerable. A rocket-fixed system would, however, have the virtue of greater simplicity.

Despite the similarities between such a system and autopilots, Altvater’s people had to start nearly from scratch in considering how to make it work on a rocket—and not just for the A-5, but above all for the A-4. Unlike an autopilot, a ballistic missile guidance system would have to work over a range of velocities from zero to about 5,400 kilometers per hour (3,350 miles per hour) and altitudes from sea level to 30 kilometers (100,000 feet), where the A-4 would burn out. Thus the external aerodynamic forces acting on the vehicle would be constantly changing, as would the rocket’s center of gravity and moments of inertia because of emptying fuel tanks; yet in 1938 very little was known about any of the crucial quantities that would be needed to design an A-4 system. The high vibration and acceleration of launch also imposed a much more demanding environment on the equipment. 41

In response to those difficult requirements Karl Fieber, one of Altvater’s key subordinates, proposed a system using only two position gyros of a new specialized type. His idea would become the basic principle of the A-4 operational guidance system and thus was a crucial innovation for the missile. One gyro, the “Horizont,” controlled the pitch axis; its name derived from aircraft artificial horizon gyros. The other, which Fieber called the “Vertikant,” measured movement in both the roll and the yaw axes (like the Horizont, it was a two-degree-of-freedom gyro, that is, free to move in two axes). Siemens eventually came to apply the term “Vertikant guidance” to the whole system. According to Fieber, he filed a patent application for the idea at the end of 1938. When the patent was awarded, it was so secret that he was not even allowed to retain a copy. 42

In Fieber’s system, the Horizont had a further purpose besides measuring the attitude of the vehicle in pitch. It also sent the signals that tilted the rocket over from the vertical to an angle of about 45 degrees. The guided phase of a simple ballistic missile trajectory has the same function as an artillery barrel. The projectile reaches a certain velocity at a certain angle of firing at the end of the barrel or at the burnout of the engine, after which it coasts under the influence of gravity to the target (see Figure 3.3). Velocity and angle of fire are the primary determinants of range for a given projectile. As Becker and the Ordnance rocket enthusiasts had recognized from the outset, the price for getting rid of the gun barrel’s limitations was the use of some complicated mechanism to stabilize and guide the rocket during burning. This mechanism must include some means to pitch the missile over as well, since a liquid-fuel rocket cannot easily be fired off an angled rail like a battlefield solid rocket. A missile using a liquid-fuel engine must take off vertically because its initial acceleration is low; if it were launched at an angle, the aerodynamic forces would be too weak to stabilize it against the disturbing forces of gravity and wind.