The project was top secret and well concealed. Allied intelligence knew so little about the Seiran that it was not even assigned a customary English code name. It was manufactured in a detached experimental-aircraft assembly building on a corner lot of Aichi’s Eitoku plant on the river’s edge, a place akin to Lockheed’s celebrated Skunk Works. Training was carried out at dispersed secret bases.
A 1944 Allied Air Technical Intelligence Center report, now declassified, verifies that the Allies were aware of a “330-mph class” aircraft named M6A1 or Seiran (then translated as “clear day”). The half-page description noted that the aircraft was of “unusual design” and “for submarine use,” and that the nomenclature gave no hint as to its type but indicated only that it was a special aircraft made by Aichi. Allied intelligence obviously had no clue as to the mission Yamamoto had in mind for it.
Within the past few years, personal memoirs and interviews of the surviving Imperial Navy personnel involved in the Seiran and sen-toku projects have begun to appear in Japanese publications. Backed up by memoirs and chronicles by U.S. servicemen on the scene immediately after the war, these accounts collectively present a comprehensive history of the unique development and demise of the special task force who tried to realize Yamamoto’s vision.
Along with long range and endurance for a deep strike mission from the sea, the maximum speed attainable to evade fighter interceptors was critical to the success of the all-or-nothing mission. Without floats, the aircraft had a maximum design speed of 348 mph. This was comparable to the top speeds of fighters like the Grumman F6F Hellcat: 380 mph. Even with floats attached, the Seiran attained a respectable maximum speed of 295 mph at an altitude of 17,160 feet, although it couldn’t have outrun most fighters.
The Atsuta engine was a type 31 or 32 water-cooled, inverted V-12 rated at 1,340 horsepower (1,400 hp maximum), manufactured by Aichi and based on the German Daimler-Benz DB601A. The propeller, based on a Hamilton Standard, just cleared the inner diameter of the sub’s hangar tube. The sub’s hangar had facilities for heating the engine coolant and the lubricating oil. By pumping them into the aircraft’s engine just prior to launch, the crew could warm it up without actually running it. Ultimately, the decision favoring the aerodynamically clean water-cooled engine over the larger diameter air-cooled engine prevalent in the Japanese aircraft of the time was based on the loading of the long, slender torpedo under the fuselage. The inverted V-12 engine ensured that the propeller arc was well forward of the torpedo’s warhead.
Next came the task of tucking the aircraft into the 11.5-foot-diameter hangar tube. One of Ozaki’s engineers came up with the idea of rotating the wings forward 90 degrees around the wing spar root and then folding them rearward against the sides of the fuselage. Unfolding the wings and connecting all the control-surface linkages and fuel lines from the wing tanks would have to be done in seconds to stay within the three to four minutes of an aircraft’s launch preparation time. This operation was to be performed by four launch crew members on deck—most likely in darkness. The hydraulic wing-fold mechanism was powered by a source in the sub’s hangar. The deck crew would connect a hydraulic line from the sub to a receptacle in the access panel under the fuselage at the wing root and power the unfolding actuators when the aircraft was clear of the hangar. All linkages, via various articulating mechanisms, were hooked up automatically. The deck crew merely had to hand-crank a set number of turns on the foldaway handles to seat the wing locking pins, two on each wing. Indicator arms, painted red and glow-in-the-dark phosphorescent, withdrew into the aircraft to show that the pins were set.
Three empennage tips, folded for hangar stowage, were flipped up and locked into place. The floats, when used, were mechanically rolled out of separate stowage tubes on both sides of the hangar tube as the aircraft was emerging. They were snapped onto the pylons and then onto the underside of the wings with four quick-locking pins at each float’s eight attach points.
The foldaway design continued inside. A 13-mm machine gun mounted behind the navigator’s seat was stowed upside down in a recess in the fuselage and rolled rightside up into shooting position, facing aft to confront pursuers. For this manipulation, the aft end of the cockpit canopy was rolled upside down and away into the same stowage space below. In the up position the gun mount was hooked up to the gunner’s seat and aimed by the gunner, who used a pedal to adjust the seat vertically.
As this was to be a dive bomber, it had to be equipped with an aerodynamic dive brake to control the speed and angle of dive. Ozaki’s team employed a double-slot design that combined the flap with a dive brake, an idea Aichi had successfully developed for its carrier-based attack bomber, the B7A2 Ryusei (Allied code name “Grace”). The combination flaps extended fully to lower the landing speed to 78 mph. Only the smaller flaps deployed in the dive-brake mode at continuously selective angles down to the maximum 90 degrees. The pilot would select flap or dive brake mode and flip a switch at the top of the control stick while on approach or in dive bombing.
For the one-shot raid the Japanese navy would use the largest bomb in its inventory—the 1,764-pound general-purpose bomb with a steel penetrating head—or a 1,808-pound torpedo. The back seater’s primary duty was precision navigation to the target and back to the sub. This navigator-bombardier-gunner would sit in a swivel seat, usually facing forward. His panel was equipped with a set of navigation and communication gear well beyond those of most Japanese single-engine bombers.