time, Adolf Hitler was not particularly impressed by the V-2; he opined that it was merely an artillery shell with a longer range and much higher cost. During early September 1943, Braun promised the Long-Range Bombardment Commission that the A-4 development was "practically complete/concluded", but even by the middle of 1944, a complete A-4 parts list was still unavailable. Hitler was sufficiently impressed by the enthusiasm of its developers, and needed a "wonder weapon" to maintain German morale, so he authorized its deployment in large numbers. The V-2s were constructed at the Mittelwerk site by prisoners from Mittelbau-Dora, a concentration camp where 20,000 prisoners died. In 1943, the Austrian resistance group including Heinrich Maier managed to send exact drawings of the V-2 rocket to the American Office of Strategic Services. Location sketches of V-rocket manufacturing facilities, such as those in Peenemünde, were also sent to the Allied general staff in order to enable Allied bombers to perform airstrikes. This information was particularly important for Operation Crossbow and Operation Hydra, both preliminary missions for Operation Overlord. The group was gradually captured by the Gestapo and most of the members were executed. Technical details The A-4 used a 75% ethanol/25% water mixture (B-Stoff) for fuel and liquid oxygen (LOX) (A-Stoff) for oxidizer. The water reduced the flame temperature, acted as a coolant by turning to steam and augmented the thrust, tended to produce a smoother burn, and reduced thermal stress. Rudolf Hermann's supersonic wind tunnel was used to measure the A-4's aerodynamic characteristics and center of pressure, using a model of the A-4 within a 40 square centimeter chamber. Measurements were made using a Mach 1.86 blowdown nozzle on 8 August 1940. Tests at Mach numbers 1.56 and 2.5 were made after 24 September 1940. At launch the A-4 propelled itself for up to 65 seconds on its own power, and a program motor held the inclination at the specified angle until engine shutdown, after which the rocket continued on a ballistic free-fall trajectory. The rocket reached a height of or 264,000 ft after shutting off the engine. The fuel and oxidizer pumps were driven by a steam turbine, and the steam was produced by concentrated hydrogen peroxide (T-Stoff) with sodium permanganate (Z-Stoff) catalyst. Both the alcohol and oxygen tanks were an aluminum-magnesium alloy. The turbopump, rotating at 4000 rpm, forced the alcohol and oxygen into the combustion chamber at 125 liters