The Nazi Rocket U-boat or Project Prufstand as it was referred to, was one of Hitler’s wonder weapons. A platform to launch long distance rockets from a submerged submarine, which was aimed at striking terror into the hearts of citizens living in Allied countries. The development only started too late in the war to make a major impact however, Germany did manage to produce one Project Prufstand Rocket U-boat.
Purposed Design
U-511 was modified to test the feasibility of launching artillery rockets, more specifically the Wurfkörper
42, at sea. In cooperation with Ernst Steinhoff, a member of the Peenemünde Army Research Center, the Kriegsmarine fitted a rack of six Wurfkörper 42 onto the deck of the U-boat. The sub was able to fire the rockets both at the surface and at a depth of 12 meters. Testing records of these rockets show that 
they were successfully deployed against Soviet harbors, but fail to mention if any damages or casualties were inflicted on the enemy.
The mounted rockets also had a major impact on underwater handling. This, and the fact that these
rocket were not equipped with any guidance systems gave very little practical uses for these rockets hence the project was abandoned, citing lack of seaworthiness.
The Bigger Plan
Admiral Karl Donitz continued to look at utilizing rockets on U-Boats and was interested in using the V-1 rocket as an anti-escort weapon. This would have been the first surface launched anti-ship weapon but technical challenges combined with the rivalry with the German Luftwaffe quickly cancelled this concept’s potential.
When development of the V-2 came to fruition, the same idea was again proposed, only this time it was taken much more serious. The threat of Allied invasion was growing larger by the day and Nazi Germany was looking for any advantage.
Development thus began on a suitable undersea transport medium since the V-2 rocket was too large to be fitted in the U-Boat’s hull. This plan called for the U-boats to have a Prufstand XII container in tow, carrying inside it a V-2 rocket and extra diesel for the long journey towards key targets, with New York being top of the list.
Firing this rocket from inside the container would mean ballasting the container in a vertical position, requiring the submarine to surface and crewmen to operate valves on the container to have the container pointed nose up. Unfortunately this procedure would place the submarine in a dangerous situation, but the launching procedure could be done from inside the submarine.
Being largely inaccurate, the V-2 bomb would not have hit a specific target but would impose psychological terror on any country. This was meant to demonstrate German capabilities to harm any Allied country and that not even mainland America was not safe from German attack.
It is not known whether Nazi Germany ever managed to successfully test a Prufstand rocket, since no recorded cases or eyewitness accounts exist to confirm this. It is however, speculated that a Rocket ship mission had been tested in February 1945, when the submarine U-1053 undergone diving trails. The timing on this trails where very peculiar, since U-boats where already a proven technology, arising suspicions of Rocket U-boat activity.
Fate
Ultimately the Rocket U-boat missions never succeeded, either having been destroyed on suspicion of attack, or having sunk during possible testing. This however does not put the Prufstand in the archives as a failed technology. Without realizing it at the time, Nazi Germany’s fascination with this project paved the way for several technologies, the first being rocket control.
The Wurfkörper 42 had no way of acquiring targets. It was a weapon of chance; point it in the right direction, calculate the trajectory and hope something happens. The V-2 rockets had a little bit more control, but still only a very crude system. A distance was set by supplying only enough fuel for said distance and a crude controller to keep it on track.
In the end Project Prufstand paved the way for modern ballistic missiles. As rockets became smaller and more powerful, it became feasible to incorporate them into the design of a submarine. The addition of dedicated missile silos enabled underwater launch of rockets.
As technology developed, both rockets and navigation systems became more advanced and throughout the years developed into what we know today as intercontinental ballistic missiles.
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