The Kaiten (“Heaven Shakers”) were manned torpedoes intended for suicide attack. They were based on the Model 3 Long Lance torpedo. The design was completed in the spring of 1943, but senior officers initially rejected the concept because there was no way for the pilot to escape.
At one point the inventors, Ensign Nishina Sekio and Lieutenant (junior grade) Kuroki Hiroshi, wrote a petition in their own blood begging the high command to reconsider.
This was a traditional way in Japanese culture to show one’s makoto (“sincerity”). Approval of the prototype design finally was granted in February 1944, on condition that the weapon have an escape mechanism for the pilot, but production remained slow.
This changed after the loss of Saipan in June 1944. A crash program was begun to manufacture the weapon and train its crew, under the cover name “Circle Six Metal Fitting.”