Development of an engine for the A4 ballistic missile was begun in June 1936. Walter Thiel, a gifted and systematic researcher, was responsible for the engine design. He had great difficulties in obtaining stable combustion, and preventing burn-through of the chamber walls. Various injector patterns were studied in a 1.5 tonne thrust chamber. His research finally reduced the combustion chamber length from 2 m to 30 cm, while the exhaust velocity was increased from 2000 m/s to 2100 m/s, and eventually reached 2280 m/s. However the reduction in the cooling area of the chamber also increased problems in preventing hot spots and burn through. This was finally solved by using a conical throat exit and a mixing chamber ahead of the burning chamber.
The 1.5 tonne thrust engine was initially run at 15 bar pressure, versus the 50 bar desired. But whenever the combustion chamber pressure was increased, burn-throughs occurred, as well as forcing increases in the mass of the pumps and tanks. Therefore finally the decision was taken to leave the chamber pressure at 15 bar.
The next step was to make a 4.5 tonne thrust by clustering three of the 1.5 tonne engines as preburners. However Thiel still had burn-throughs in test runs. Poehlmann suggested the use of film cooling, which finally solved the problem. For the 25 tonne thrust engine, Thiel simply used 18 x 1.5 tonne thrust chambers, feeding a common mixing chamber. The Model 39 engine design delivered to von Braun in January 1938 was so compact, that the length of the A4 could be cut in half. This engine was on the test stand in early 1939.
A workable engine had been developed, but it was complex, suitable for prototypes only, and the engineers involved did not have the experience to turn it into something designed for mass production. Continuous changes on the engine also affect other parts of the rocket, resulting in drawing changes simultaneous with the effort to mass-produce detailed parts. By the end of 1942 the final engine configuration had to be frozen for production. The 18 preburners were retained, but a lightweight nozzle throat was developed, using film cooling and glass wool insulation instead of double-walled regenerative liquid cooling. The screw-on aluminium chamber head was replaced by a welded steel head.
In August 1943, with only four months to go before the start of the government's mandated production of 900 A4 missiles per month, the engineers declared the missile was simply not ready for production. Thiel and his team declared that in fact development of the A4 could never be finished before the war's end. They recommended that plans to put it into production should be stopped.
Thiel, at the verge of a nervous breakdown, led this engineering 'revolt', although Eberthard Rees was the spokesman. They declared they would stop work at Peenemuende and retire to their universities. Von Braun argued against this position, demanding that production continue. Dornberger suffered a crisis of confidence in the rocket team as a result of this fight, but decided to continue trying to get the missile in production and fielded with the Germany Army.
On 17 August the Royal Air Force staged a massive bombing raid on Peenemuende, killing Thiel. The raid itself set back development only six weeks, but it led to a decision to disperse production of the missile, and move flight and training launches to Poland. These set back the program even more, and production deliveries did not finally commence until September 1944, ten months behind the original schedule. The Model 39 design was copied by the Russians as the RD-100, and an improved version, the RD-101 was developed there. In France and America, further development was abandoned in favor of the single-chamber Model 39b.
Thrust (sl): 264.900 kN (59,552 lbf). Thrust (sl): 27,010 kgf. Engine: 931 kg (2,052 lb). Chamber Pressure: 15.00 bar. Area Ratio: 3.29. Thrust to Weight Ratio: 34.156820622986. Coefficient of Thrust vacuum: 1.4886700933446. Coefficient of Thrust sea level: 1.26933676001127.
AKA: Model 39.
Unfuelled mass: 931 kg (2,052 lb).
Diameter: 0.76 m (2.49 ft).
Thrust: 311.80 kN (70,095 lbf).
Specific impulse: 239 s.
Specific impulse sea level: 203 s.
Burn time: 68 s.
Number: 4300 .