Michael Neufeld - The Rocket and the Reich

Von Braun’s program in 1933 had three main objectives. The first was development of engines based on aluminum alloys. Raketenflugplatz had begun using aluminum for the obvious purpose of saving weight and thus increasing the performance of launched vehicles. In the spring of 1933, troubled by the number of engine failures, von Braun went searching for expertise. “Solidly in Nebel’s footsteps, I grabbed the telephone directory and got in touch with welding experts, instrumentation firms, valve factories, and pyrotechnical laboratories.” He had learned something from the entrepreneurial methods of his former mentor. 68

Soon engine parts manufacturing was farmed out to various firms, and von Braun and his superiors made contact in April 1933 with a firm that specialized in aluminum anodizing ( Eloxieren: surface hardening through the electrolytic formation of an oxidization layer). This proved a crucial breakthrough in increasing the durability of engines. The firm had been working with Nebel, but Ordnance insisted that it cut off all contact with him. In turn that firm led von Braun and his superiors to a small manufacturer who would be the primary contractor for engine and alcohol-tank construction for three years: Zarges, in the southwest German city of Stuttgart. At first it would be a mutually agreeable relationship, but eventually the distance, secrecy considerations, and a desire for greater control over quality would result in a decision to manufacture in-house at Peenemünde. 69

Von Braun’s second objective was the fully automatic operation of ignition and tank pressurization. Proper ignition was a serious problem; if too much fuel or oxidizer reached the engine first and ignition was delayed, an explosion usually resulted. By the end of 1933 the problem was reasonably in hand, but it was never completely solved. Many experiments were also conducted to solve the old problem of how to pressurize the tanks. The weight-saving method of increasing liquid oxygen evaporation with small burning cartridges was tried, but putting gaseous oxygen in the fuel tank led to explosions. It became necessary to use compressed nitrogen or evaporated liquid nitrogen in the alcohol tank, although that meant a separate tank and system. Both forms of nitrogen were tried, but all the problems of tank pressurization remained. As the fuel drained from the tank, the gas would expand and the pressure would drop, resulting in a drop in the rate and pressure of propellant delivery to the engine over time. That meant a slow drop in thrust. Since the pressure of the burning gases in the rocket engine’s combustion chamber was about ten atmospheres in the engines of that time, it was necessary to force the propellants into the chamber with a pressure of a few atmospheres higher. That meant the fuel and oxidizer tanks had to withstand at least fifteen atmospheres of pressure (in practice even more), which made them heavy. As rockets got larger, the structural weight problem was magnified exponentially. The limits on tank pressure also limited combustion chamber pressure, which limited performance, because higher-pressure engines are more efficient. It was already clear that complicated turbopumps would have to be developed for larger missiles to get around those problems, a solution already discussed in the works of Oberth and the other pioneers. 70

Von Braun’s third objective was the design and construction of the rocket itself. By June 1933 the drawings were in hand for the first vehicle, the Aggregat-1 (“Aggregate” or “Assembly”), better known as the A-1. It was based on the 300-kg-thrust engine, and its unique feature was its method of stabilization, which derived directly from its origins in an artillery establishment. A liquid-fuel rocket cannot be spun on its axis like an artillery shell or a solid rocket because of the disturbing forces on the propellants in the tanks and lines. As a crude interim solution, Dornberger proposed that only part of the vehicle be spun. Thus the nose of the A-1 was a large gyroscope that stabilized it by brute force. (A gyroscope’s axis, like that of a top, will tend to remain fixed in space. If perturbed by an external force it will move or “precess” at a right angle to the force exerted. A gyroscope’s resistance to precession is directly dependent on its angular momentum, a product of its mass and rate of rotation.) Before launch, the gyroscope would be spun up to 9,000 rpm by an electric motor on the ground, then left to run solely on its momentum during the rocket’s brief flight. 71

But the A-1 was never to fly. “It took us exactly one half year to build…—and exactly one-half second to blow it up,” says von Braun, a bit hyperbolically. The late 1933 or early 1934 explosion was due to persistent difficulties with the fuel and oxygen valves, leading to delayed or hard ignition. Eventually the third A-1 was successfully started on the ground, but it was destroyed by the mechanical failure of the liquid-oxygen tank. Ordnance decided on a major redesign, entitled the A-2 (see Figure 1.1). Von Braun’s group separated the tanks and placed the gyro rotor between them. Moving the gyro to the middle had the advantage of bringing the center of gravity backward, thus shortening the moment arm of any deviations of thrust away from the rocket’s axis. That increased the stability of the rocket in the early part of the flight, when aerodynamic forces were weakest because of the rocket’s low velocity, although stability was actually decreased in the later part of the flight because the rocket’s center of gravity was closer to its center of aerodynamic pressure. Separating the tanks also stopped the problem of leakage into the fuel tank caused by vibration-induced cracking of the oxygen tank. 72

Figure 1.1 also shows a characteristic feature of the early German Army rockets. The engine was actually immersed in the alcohol tank, because it shortened the rocket and helped to cool the engine when the combustion chamber was so long. Difficulties in getting proper atomization and evaporation of the propellant droplets had driven von Braun and his co-workers toward longer and longer combustion chambers to give the propellant mixture more time to burn completely. 73Incomplete burning was one of the main causes of suboptimum engine performance. The 300-kg-thrust engine had an exhaust velocity of about 1,500 meters per second, whereas the theoretical maximum for a 75-percent-alcohol/liquid-oxygen rocket is a little over 2,000 m/sec at the combustion chamber pressures then feasible—10 to 13 atmospheres. In the equation of the rocket, exhaust velocity is one of the absolutely critical values determining performance; the higher it is, the more efficient the engine. For comparison purposes, the most efficient rocket engine in use today, the Space Shuttle Main Engine, has an exhaust velocity of around 4,500 m/sec using liquid hydrogen and liquid oxygen at a combustion chamber pressure of about 200 atmospheres.

All the problems with the A-1 and the redesign they necessitated meant many delays to the schedule of the program. At the time that von Braun completed his dissertation in April 1934, the A-2 was still months away from being finished. There was nothing unusual about such technical setbacks. In the course of a year and a half, the young physics doctoral student and his few assistants had significantly outstripped the existing amateur rocket technology. The systematic approach imposed by Ordnance had much to do with their success, but von Braun’s brilliance was no doubt a factor as well. For his efforts, he received high honors from his dissertation committee, headed by Erich Schumann, when he defended it at the beginning of June 1934. The subject was so secret that even the title was classified. Von Braun’s diploma carried a phony title instead: “Regarding Combustion Experiments.” 74