Germany Submarine
In October 1943, Walterwerke submitted a proposal to the Kriegsmarine for a submarine featuring a closed-cycle Walter system engine fueled by hydrogen peroxide. This engine was designed to operate effectively whether the submarine was on the surface or submerged. The various sections of the submarine hulls had to be manufactured in different shipyards, and sections for four submarines—designated U-4501 to U-4504—were produced before the war reached its conclusion.