During World War II, Momsen served as Commander, Submarine Squadron 2 (ComSubRon 2) and Commander, Submarine Squadron 4 (ComSubRon 4). While Momsen was ComSubRon 2 in the U.S. Pacific Fleet, captains under his command reported their Mark 14 torpedoes were not functioning properly. When fired from the preferred perpendicular angle of impact, the torpedoes did not always explode. However, when fired to hit at acute angles, the torpedoes usually exploded. When officers of Momsen's own squadron complained, he decided to find out why. He took torpedoes to the shallow waters and sheer cliffs of the Hawaiian Island of Kahoolawe and fired until he got a dud. Then, risking his own life, he dove into the water to find the unexploded torpedo. With help, he recovered the dangerous live torpedo and brought it on board. A small problem with the firing pin inside the primer cap of the warhead was causing the duds: it was becoming crushed, rather than firing the warhead.