The idea of the rocket U-boat was conceived as early as 1941, at Germany’s top secret rocket research center, Peenemunde. This led to some trials in the summer of 1942 whereby rocket launchers were mounted on the deck U-511, a Type IXC. The Schweres Wurfgerat 41 rocket launcher carried six 30cm Wurfkorper Spreng 42 rockets mounted on the upper deck. The tests were successful, enabling the U-boat to launch rockets both on the surface and when submerged up to a depth of 12 meters (40feet). Donitz expressed interest in the idea as an anti-escort weapon, but further development was hampered by the lack of a suitable guidance system capable of targeting surface ships. In the event, Peenemunde was stretched thin developing the V-1 rocket, and could not devote any time for this project. The rocket launcher also degraded the boat’s underwater performance and the idea was not pursued further.

Both surfaced and underwater rocket trials were carried out aboard U-511 during May and June 1942. German Heavy Rocket Launcher Type 41. These were mounted on wooden frames on the upper deck.

Courtesy of Kirk Steinhoff

In July 1943, the idea was again resurrected after Germany had successfully developed the V-1 rocket. The Vergeltungswaffen 1 (Vengence Weapon 1) was an unmanned flying bomb, powered by a pulse-jet with a range of 148 miles (238km). It was designed to be launched from a land installation but the idea of using U-boats as a launch platform was proposed. The notion was to provide the weapon with mobility, where it could strike almost any city in the world. The V-1 however was a Luftwaffe weapon and the idea was again rejected.

When the V-2 was developed in 1943, the idea of the rocket U-boat was again brought up. This time the viability was taken seriously, but it was not until late 1944 that Peenemunde was able to allocate any resources to it. Codenamed Project “Prufstand XII”, this time the target was New York.

The large size of the V-2 meant that it would not fit into the hull of a U-boat. Instead, the plan called for the V-2 to be mounted inside a large watertight cylindrical container, and was to be towed across the Atlantic. Upon reaching its launching position, the V-2 would be fueled, its guidance system set, and launched at New York.

Three containers were proposed to be towed across the Atlantic by a Type XXI U-boat. Each container displaced about 500 tons and would be kept submerged by the forward motion of the boat. In addition to the V-2 rocket, each container also held a reserve of diesel fuel intended to supply the U-boat in the course of its journey. In order to launch the missile, the ballast tanks in the container would be flooded, thus bringing it to a vertical position. After the guidance system was set, it was remotely launched from within the U-boat, where the missile would leave its container and head towards its target.

4th June 1942, U-511 Rocket Trials.

Courtesy of Kirk Steinhoff. A V-2 Rocket housed within a watertight container to be towed across the Atlantic.

German records indicate that orders for three containers were placed in December 1944 and at least one had been completed by the end of the war. Plans for the development of ICBM U-boats were also carried out, but these ended together with the war.

Why Germany undertook yet another ambitious project at the end of the war, when it was losing at all fronts is quite easily understood. Other than the apparent fact that Hitler had always been awed with “wonder weapons”, but the principal notion behind this was to bring the war to American soil. It was a psychological war, where the Third Reich wanted to impress upon American citizens that it was not safe anywhere, even on their own soil. The V-2 would have been inaccurate, and would certainly not even cause a dent in the allies war capability, but it would be a certain psychological victory.

Through intelligence, the allies knew a great deal about the missile program and had drawn up a contingency plan to thwart it. Codenamed Operation Teardrop, the US responded with four escort carrier groups to prevent any U-boat penetration into American coasts. Thus in March 1945, when a group of six Type IXC U-boats equipped with snorkels were detected sailing for American shores, they were promptly hunted with four of the six destroyed. Two got away, although apparently later, it was not the anticipated missile attack.

One of the war mysteries shrouded around the sinking of U-1053 on February 15 1945. She was a Type VIIC sunk during deep diving trials in the North Sea near Bergen, Norway. Why a Type VIIC would undergo diving trials at such a late point in the war is unknown. By that time, the Type VII was rendered obsolete by the German High command and all production halted, to be replaced by the Type XXI. Some authorities attributed this to rocket trials, although this is not entirely clear.

In a separate development, by the end of the war, the guidance system for targeting surface vessels with anti-ship rockets fired from submerged U-boats was complete and installed in the latest Type XXIs. Known as SP-Anlage, the device could accurately pinpoint the location of surface vessels and enabled anti-ship rockets to be fired up from below. The rockets however were still under development when the war ended. This project was named Project Ursel.

After the war, this concept led to the US and Soviet ballistic missile submarines of the 1950s.