Propulsion for the A-4 (V-2)

Soon after he began working for German Army Ordnance at Kummersdorf in late 1932, Wernher von Braun began experiment­ing with rocket engines, which developed burnthroughs, “igni­tion explosions, frozen valves, fires in cable ducts and numerous other malfunctions." Learning “the hard way," von Braun called in “welding experts, valve manufacturers, instrument makers and pyrotechnicists. . . and with their assistance a regeneratively – cooled motor of 300 kilograms [about 660 pounds] thrust and pro­pelled by liquid oxygen and alcohol was static tested and ready for flight in the A-1 rocket which had been six months a-building." Von Braun’s boss, Walter Dornberger, added that the “650-pound-thrust

chamber . . . gave consistent performance" but yielded an exhaust velocity slower than needed even after the developers “measured the flame temperature, took samples of the gas jet, analyzed the gases, [and] changed the mixture ratio."1

As the staff at Kummersdorf grew, bringing in additional exper­tise, engine technology improved. But only with the hiring of Wal­ter Thiel did truly significant progress occur in the propulsion field. Thiel was “a pale-complexioned man of average height, with dark eyes behind spectacles with black horn rims." Fair-haired with “a strong chin," he joined the experimental station in the fall of 1936. Born in Breslau in 1910, the son of an assistant in the post office, he matriculated at the Technical Institute of Breslau as an under­graduate and graduate student in chemistry, earning his doctorate in 1935. He had served as a chemist at another army lab before com­ing to Kummersdorf.2

Dornberger said Thiel assumed “complete charge of propul­sion, with the aim of creating a 25-ton motor" (the one used for the A-4, providing 25 metric tons of thrust). Because Thiel remained at Kummersdorf until 1940 instead of moving to Peenemunde with the rest of the von Braun group in 1937, testing facilities limited him to engines of no more than 8,000 pounds of thrust from 1936 to 1940. Although Thiel was “extremely hard-working, conscientious, and systematic," Dornberger said he was difficult to work with. Ambitious and aware of his abilities, he “took a superior attitude and demanded. . . devotion to duty from his colleagues [equal to his own]." This caused friction that Dornberger claimed he had to mol­lify. Martin Schilling (chief of the testing laboratory at Peenemunde for Thiel’s propulsion development office and, later, head of the of­fice after Thiel died in a bombing raid in 1943) noted that Thiel was “high strung." He said, “Thiel was a good manager of such a great and risky development program. He was a competent and dynamic leader, and a pusher. At the same time, he was no match to von Braun’s or Steinhoff’s vision and optimism." (Ernst Steinhoff was chief of guidance and control.) 3

A memorandum Thiel wrote on March 13, 1937, after he had been on the job about six months, gives some idea of the state of development of a viable large engine at that time. It also suggests the approach he brought to his task. Although he certainly lacked optimism at some points in his career at Kummersdorf and Peene – munde, he did not betray that failing in his memo. He referred to “a certain completion of the development of the liquid rocket" that had been achieved “during the past years," surely an overstatement in view of the major development effort that remained. “Combustion

chambers, injection systems, valves, auxiliary pressure systems, pumps, tanks, guidance systems, etc. were completely developed from the point of view of design and manufacturing techniques, for various nominal sizes. Thus, the problem of an actually usable liq­uid rocket can be termed as having been solved."

Despite this assessment, he listed “important items" requiring further development. One was an increase in performance of the rocket engine, using alcohol as its fuel. He noted that the engines at Kummersdorf were producing a thermal efficiency of only 22 per­cent, and combustion-chamber losses were on the order of 50 per­cent. Thus, about half of the practically usable energy was lost to incomplete combustion. The use of gasoline, butane, and diesel oil theoretically yielded an exhaust velocity only some 10 percent higher, but measurements on these hydrocarbon fuels showed ac­tual exhaust velocities no higher than those with alcohol. Thiel felt that “for long range rockets, alcohol will always remain the best fuel," because hydrocarbons increased the danger of explosion, pro­duced coking in the injection system, and presented problems with cooling.

He said the way to improved performance lay in exploiting the potential 50 percent energy gain available with alcohol and liquid oxygen. Fuller combustion could come from improving the injec­tion process, relocating the locus for mixing oxygen and fuel into a premixing chamber, increasing the speed of ignition and combus­tion, and increasing chamber pressure by the use of pumps, among other improvements. He knew about the tremendous increases in performance available through the use of liquid hydrogen, but he cited the low temperature of this propellant (-423°F), its high boil – 104 off rate, the danger of explosion, and huge tank volume resulting Chapter 3 from its low specific weight (as the lightest element of all), plus a requirement to insulate its tanks, as “strong obstacles" to its use (as indeed, later proved to be the case).

He made repeated reference to the rocket literature, including a mention of Goddard, but noted that “the development of practi­cally usable models in the field of liquid rockets. . . has far outdis­tanced research." Nevertheless, he stressed the need for coopera­tion between research and development, a process he would follow. He concluded by stating the need for further research in materials, injection, heat transfer, “the combustion process in the chamber," and “exhaust processes." 4

Despite Thiel’s optimism here, Martin Schilling referred in a postwar discussion of the development of the V-2 engine to the “mysteries of the combustion process." Thiel, indeed, said the

combustion process needed further research but did not discuss it in such an interesting way. Dornberger also failed to use such a term, but his account of the development of the 25-ton engine sug­gests that indeed there were mysteries to be dealt with. He pointed out that to achieve complete combustion of the alcohol before it got to the nozzle end of the combustion chamber, rocket research­ers before Thiel had elongated the chamber. This gave the alcohol droplets more time to burn than a shorter chamber would, they thought, and their analysis of engine-exhaust gases seemed to prove the idea correct. “Yet performance did not improve." They realized that combustion was not “homogeneous," and they experienced frequent burnthroughs of chamber walls.

Dornberger said he suggested finer atomization of both oxygen and alcohol by using centrifugal injection nozzles and igniting the propellants after mixing “to accelerate combustion, reduce length of the chamber, and improve performance." Thiel, he said, devel­oped this idea, then submitted it to engineering schools for research while he used the system for the 1.5-ton engine then under devel­opment. It took a year, but he shortened the chamber from almost 6 feet to about a foot. This increased exhaust speed to 6,600 and then 6,900 feet per second (from the roughly 5,300 to 5,600 feet per second in early 1937). This was a significant achievement, but with it came a rise in temperature and a decrease in the chamber’s cool­ing surface. Thiel “removed the injection head from the combus­tion chamber" by creating a “sort of mixing compartment," which removed the flames from the brass injection nozzles. This kept them, at least, from burning.5

In conjunction with the shortening of the combustion chamber, Thiel also converted the shape from cylindrical to spherical to en­compass the greatest volume in available space. This also served to reduce pressure fluctuations and increase the mixing of the propel­lants. Until he could use a larger test stand at Peenemunde, how­ever, Thiel was restricted in scaling up these innovations in the 1.5-ton engine to the full 25 tons. He thus went to an intermediate size of 4.2 tons that he could test at Kummersdorf, and he moved from one injector in the smaller engine to three in the larger one. Each had its own “mixing compartment" or “pot," and the clus­tering actually increased the efficiency of combustion further. But to go from that arrangement to one for the 25-ton engine created considerable problems of scaling up and of arranging the 18 “pots" that the researchers designed for the A-4 combustion chamber. At first, Thiel and his associates favored an arrangement of six or eight larger injectors around the sides of the chamber, but von Braun sug-

gested 18 pots of the size used for the 1.5-ton engine, arranged in concentric circles on the top of the chamber. Schilling said this was a “plumber’s nightmare" with the many oxygen and alcohol feed lines that it required, but it avoided the problems of combustion in – stability—as we now call it—that other arrangements had created.6

Cooling the engine remained a problem. Regenerative cooling used on earlier, less efficient engines did not suffice by itself for the larger engine. Oberth had already suggested the solution, film cool­ing—introducing an alcohol flow not only around the outside of the combustion chamber (regenerative cooling) but down the inside of the wall and the exhaust nozzle to insulate them from the heat of combustion by means of a “film" of fuel. Apparently, others in the propulsion group had forgotten this suggestion, and it is not clear that the idea as applied to the 25-ton engine came from Oberth. Several sources agree that diploma engineer Moritz Pohlmann, who headed the propulsion design office at Kummersdorf after August 1939, was responsible. Tested on smaller engines, the idea proved its validity, so on the 25-ton engine, there were four rings of small holes drilled into the chamber wall that seeped alcohol along the in­side of the motor and nozzle. This film cooling took care of 70 per­cent of the heat from the burning propellants, the remainder be­ing absorbed into the alcohol flowing in the regenerative cooling jacket on the outside of the chamber. Initially, 10 percent of the fuel flow was used for film cooling, but Pohlmann refined this by “oozing" rather than injecting the alcohol, without loss of cooling efficiency.7 Whether this procedure emanated from Oberth or was independently discovered by Pohlmann, it was an important inno­vation with at least the technical details worked out by Pohlmann.

106 Thiel’s group had to come up with a pumping mechanism to Chapter 3 transfer the propellants from their tanks to the injectors in the pots above the combustion chamber. The large quantities of propellant that the A-4 would use made it impractical to feed the propellants by nitrogen-gas pressure from a tank (as had been done on the earlier A-2, A-3, and A-5 engines). Such a tank would have had to be too large and heavy to provide sufficient pressure over the 65-second burning time of the engine, creating unnecessary weight for the A-4 to lift. This, in turn, would have reduced its effective performance. In 1937 Thiel had mentioned that there was a the need for pumps to increase the chamber pressure and that some pumps had already been developed. Indeed, von Braun had already begun working in the middle of 1935 with the firm of Klein, Schanzlin & Becker, with factories in southwestern and central Germany, on the develop­ment of turbopumps. In 1936 he began discussions with Hellmuth

Walter’s engineering office in Kiel about a “steam turbine" to drive the pumps.8

In the final design, a turbopump assembly contained separate cen­trifugal pumps for alcohol and oxygen on a common shaft, driven by the steam turbine. Hydrogen peroxide powered the pumps, con­verted to steam by a sodium permanganate catalyst. It operated at a rate of more than 3,000 revolutions per minute and delivered some 120 pounds of alcohol and 150 pounds of liquid oxygen per second, creating a combustion-chamber pressure of about 210 pounds per square inch. This placed extreme demands on the pump technology of the day, especially given a differential between the heat of the steam ( + 725°F) and the boiling point of the liquid oxygen (-297°F).9

Moreover, the pumps and turbine had to weigh as little as possible to reduce the load the engine had to lift. Consequently, there were problems with the development and manufacture of both devices. Krafft Ehricke, who worked under Thiel after 1942, said in 1950 that the first pumps “worked unsatisfactorily" so the development “transferred to Peenemunde." He claimed that Peenemunde also de­veloped the steam generator. Schilling suggested this as well, writing that for the steam turbine, “we borrowed heavily from" the Walter firm at Kiel. He said a “first attempt to adapt and improve a torpedo steam generator [from Walter’s works] failed because of numerous details (valves, combustion control)." Later, a successful version of the steam turbine emerged, and Heinkel in Bavaria handled the mass production. As for the pumps, there are references in Peenemunde documents as late as January 1943 to problems with them but also to orders for large quantities from Klein, Schanzlin & Becker.10

The problems with the pumps included warping of the pump housing because of the temperature difference between the steam and the liquid oxygen; cavitation because of bubbles in the propel­lants; difficulties with lubrication of the bearings; and problems with seals, gaskets, and choice of alloys (all problems that would recur in later U. S. missiles and rockets). The cavitation problem was especially severe since it could lead to vibrations in the com­bustion chamber, resulting in explosions. The solution came from redesigning the interior of the pumps and carefully regulating the internal pressure in the propellant tanks to preclude the formation of the bubbles.11

Ehricke also reported that development of “control devices for the propulsion system, i. e. valves, valve controls, gages, etc." pre­sented “especially thorny" problems. The items available from commercial firms either weighed too much or could not handle the propellants and pressure differentials. A special laboratory at Peene-

munde had to develop them during the period 1937 to 1941, with a pressure-reducing valve having its development period extended until 1942 before it worked satisfactorily.12

Technical institutes contributed a small but significant share of the development effort for both the engine and the pumps. A profes­sor named Wewerka of the Technical Institute in Stuttgart provided valuable suggestions for solving design problems in the turbopump. He had written at least two reports on the centrifugal turbopumps in July 1941 and February 1942. In the first, he investigated dis­charge capacity, cavitation, speed relationships, and discharge and inlet pressures on the alcohol pump, using water instead of alcohol as a liquid to pass experimentally through the pump. Because the oxygen pump had almost identical dimensions to those of the alco­hol pump, he merely calculated corrections to give values for the oxygen pump with liquid oxygen flowing through it instead of wa­ter through the alcohol pump. In the second report, he studied both units’ efficiencies, effects of variations in the pump inlet heads upon pump performance, turbine steam rates, discharge capacities of the pump, and the pumps’ impeller design. He performed these tests with water at pump speeds up to 12,000 revolutions per minute.13

Schilling pointed to important work that Wewerka and the Tech­nical Institute in Stuttgart had done in the separate area of nozzle design, critical to achieving the highest possible performance from the engine by establishing as optimal an expansion ratio as possible. This issue was complicated by the fact that an ideal expansion ra­tio at sea level, where the missile was launched, quickly became less than ideal as atmospheric pressure decreased with altitude. Wewerka wrote at least four reports during 1940 studying such 108 things as the divergence of a Laval nozzle and the thrust of the jet Chapter 3 discharged by the nozzle. In one report in February, he found that a nozzle divergence of 15 degrees produced maximum thrust. Ger­hard Reisig, as well as Schilling, agreed that this was the optimal exit-cone half angle for the A-4. In his account of engine develop­ment, Reisig, chief of the measurement group under Steinhoff until 1943, also gives Wewerka, as well as Thiel, credit for shortening the nozzle substantially. In another report, Wewerka found that the nozzle should be designed for a discharge pressure of 0.7 to 0.75 at­mosphere, and Reisig says the final A-4 nozzle was designed for 0.8 atmosphere.14

Schilling also pointed to other professors, Hase of the Technical Institute of Hannover and Richard Vieweg of the Technical Insti­tute of Darmstadt, for their contributions to the “field of power – plant instrumentation." Other “essential contributions" that Schil-

ling listed included those of Schiller of the University of Leipzig for his investigations of regenerative cooling, and Pauer and Beck of the Technical Institute of Dresden “for clarification of atomiza­tion processes and the experimental investigation of exhaust gases and combustion efficiency, respectively." In an immediate postwar interview at Garmisch-Partenkirchen, an engineer named Hans Lindenberg even claimed that the design of the A-4’s fuel-injection nozzles “was settled at Dresden." Lindenberg had been doing re­search on fuel injectors for diesel engines at the Technical Institute of Dresden from 1930 to 1940. Since 1940, partly at Dresden and partly at Peenemunde, he had worked on the combustion chamber of the A-4. His claim may have constituted an exaggeration, but he added that Dresden had a laboratory for “measuring the output and photographing the spray of alcohol jets." Surely it and other techni­cal institutes contributed ideas and technical data important in the design of the propulsion system.15

Along similar lines, Konrad Dannenberg, who worked on the combustion chamber and ignition systems at Peenemunde from mid-1940 on, described their development in general terms and then added, “Not only Army employees of many departments par­ticipated, but much of the work was supported by universities and contractors, who all participated in the tests and their evaluation. They were always given a strong voice in final decisions."16

One final innovation, of undetermined origin, involved the igni­tion process, which used a pyrotechnic igniter. In the first step of the process, the oxygen valve opened by means of an electrically activated servo system, followed by the alcohol valve. Both opened to about 20 percent of capacity, but since the propellants flowed only as a result of gravity (and slight pressure in the oxygen tank), the flow was only about 10 percent of normal. When lit by the ig­niter, the burning propellants produced a thrust of about 2.5 tons. When this stage of ignition occurred, the launch team started the turbopump by opening a valve permitting air pressure to flow to the hydrogen peroxide and sodium permanganate tanks. The perman­ganate solution flowed into a mixing chamber, and as soon as pres­sure was sufficient, a switch opened the peroxide valve, allowing peroxide to enter the mixing chamber. When pressure was up to 33 atmospheres as a result of decomposing the hydrogen peroxide, the oxygen and alcohol valves opened fully, and the pressure on the tur­bines in the pumps caused them to operate, feeding the propellants into the combustion chamber. It required only about three-quarters of a second from the time the valve in the peroxide system was elec­trically triggered until the missile left the ground.17

Even after the propulsion system was operational, the propulsion group had by no means solved Schilling’s “mysteries of the combus­tion process." The engine ultimately developed an exhaust velocity of 6,725 feet/second, which translated into a specific impulse of 210 pounds of thrust per pound of propellant burned per second (lbf-sec/lbm), the more usual measure of performance today. Quite low by later standards, this was sufficient to meet the requirements set for the A-4 and constituted a remarkable achievement for the time. As von Braun said after the war, however, “the injector for the A-4 [wa]s unnecessarily complicated and difficult to manufac­ture." Certainly the 18-pot design of the combustion chamber was inelegant. And despite all the help from an excellent staff at Peene – munde and the technical institutes, Thiel relied on a vast amount of testing. Von Braun said, “Thiel’s investigations showed that it required hundreds of test runs to tune a rocket motor to maximize performance," and Dannenberg reported “many burn-throughs and chamber failures," presumably even after he arrived in 1940.18

But through a process of trial and error, use of theory where it was available, further research, and testing, the team under von Braun and Thiel had achieved a workable engine that was sufficient to do the job. As late as 1958 in the United States, “The development of almost every liquid-propellant rocket ha[d] been plagued at one time or another by the occurrence of unpredictable high-frequency pres­sure oscillations in the combustion chamber"—Schilling’s “mys­teries" still at work. “Today [1958], after some fifteen years of con­centrated effort in the United States on liquid-propellant rocket development, there is still no adequate theoretical explanation for combustion instability in liquid-propellant rockets," wrote a no – 110 table practitioner in the field of rocketry.19

Chapter 3 That the propulsion team at Kummersdorf and Peenemunde was able to design a viable rocket engine despite the team’s own and later researchers’ lack of fundamental understanding of the com­bustion processes at work shows their skill and perseverance. It also suggests the fundamental engineering nature of their endeavor. Their task was not necessarily to understand all the “mysteries" (although they tried) but to make the rocket work. Their work con­stituted rocket engineering, not rocket science, because they still did not fully understand why what they had done was effective, only that it worked.

Even without a full understanding of the combustion process, the propulsion group went on to design engines with better injec­tors. They did so for both the Wasserfall antiaircraft missile and the A-4, although neither engine went into full production. Both fea-

tured an injector plate with orifices so arranged that small streams of propellants impinged upon one another. The streams produced oscillations in the engine (combustion instability), but the develop­ers found the correct angle of impingement that reduced (but never completely eliminated) the oscillations (characterized by chug­ging and screeching). They also designed a cylindrical rather than a spherical combustion chamber for the A-4, but it had a slightly lower exhaust velocity than the spherical engine.20

Under difficult, wartime conditions, in-house contributions and those from technical institutes and industry came together through discussions among the contributors at Kummersdorf and Peene – munde. The pooling of their expertise probably contributed in innu­merable ways to the progress of technological development, but the

process can only be partially documented. Certainly, technical re­ports written by both staff at Peenemunde and people at the techni­cal institutes contributed to the fund of engineering knowledge that Peenemunde passed on to the United States. Germans from Peene­munde immigrated to the United States after the war, carrying their knowledge and expertise with them; but in addition, much of the documentation of the engineering work done in Germany was cap­tured by U. S. forces at the end of the war, moved to Fort Eustis, Vir­ginia, and even translated. The full extent of what these documents contributed to postwar rocketry is impossible to know, but the infor­mation was available to those engineers who wanted to avail them­selves of it. Finally, many actual V-2 missiles, captured and taken to the United States, also provided a basis for postwar developments that went beyond the V-2 but started with its technology.