Wasserfall was essentially an anti-aircraft development of the V2 rocket, sharing the same general layout and shaping, but being about 1/4 the size and including an additional set of fins located at the middle of the fuselage to provide extra lift while manuvering.
Unlike the V2, Wasserfall was designed to stand ready for periods of up to a month and fire on command, therefore the volative liquid oxygen used in the V2 was inappropriate. The resulting engine, developed by Dr. Thiel, was based on Visol (vinyl isobutyl ether) and SV-Stoff, or Salbei, (90% nitric acid, 10% sulfuric acid), a hypergolic mixture that was forced into the combustion chamber by nitrogen gas released from another tank. The first models were being tested in March 1943, but a major setback occurred in August 1943 when Dr. Thiel was killed in the massive RAF bombing raids on Peenemünde.
Guidance was to be a simple radio control MCLOS system for use against daytime targets, but night-time use was considerably more complex because neither the target nor the missile would be easily visible. For this role a new system known as Rheinland was under development. Rheinland used a transponder in the missile for locating it in flight (as read by a radio direction finder on the ground) and a radar unit for tracking the target. A simple computer then guided the missile into the tracking radar beam, at which point the operator could see both "blips" on a single display and then guide the missile onto the target.
A second development was underway that used only a single cross-shaped radar radar beam that was rotated while pointing at the target. Like the Rheinland system the missile was first directed into the beam via the transponder, and from there would keep itself centered in the beam. It did this by listening to the radar signal, if it was off course it would hear pulses instead of a steady signal, and automatically place itself back in the middle of the beam. However the high supersonic speed of the Wasserfall meant that the accuracy of the system would have to be very high in order to get the missile close to its target, and it was generally accepted that some sort of terminal guidance system would have to be added.
The original design called for a 100kg warhead, but the accuracy concerns led to the design of a much larger 306kg one including a liquid explosive. For daytime use the operator would detonate the warhead by remote control, while night-time use was to be by a proximity fuse.
The first launch took place on the 8th of January 1944 and was a failure, with the engine "fizzling" and launching the missile to only 7km of altitude at subsonic speeds. The following February 1944 saw a successful launch which reached a speed of 2,772km/h in vertical flight. When the program was canceled on February 6th 1945 nearly 40 flights had been made.
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