Japanese piloted bomb showed desperation and resolve

Ensconced in museums around the world, surviving examples of the World War II Yokosuka Ohka piloted bomb tell a story of desperation and resolve as Japan faced increasingly grim forecasts for the outcome of the war during 1944.

With huge naval armadas constricting the once far-flung Japanese sphere of influence, some Japanese planners sought new ways to combat invasions by sea.

Ensign Mitsuo Ohta, a transport pilot, conceived a rocket-boosted airframe with a large explosive payload and a pilot to guide it into an Allied warship. The piloted bomb would be launched from a mother ship aircraft.

Aided by the Aeronautical Research Institute of the University of Tokyo, Ohta had drawings of the suicide bomb to show by August 1944.

Japanese navy officials liked the idea and developed plans from Ohta’s concept.

Called MXY7 and given the name Ohka, or Cherry Blossom, the craft employed three solid-propellant rocket motors in the aft fuselage that could be ignited in unison or one at a time to provide acceleration to the target.

Ohka Model 11 had a top speed of 403 mph with rocket power at an altitude above 11,000 feet. Its terminal dive speed was said to be 576 mph. Its range was about 23 miles. The explosive charge in the nose was 1,200 kilograms, or 2,646 pounds — more than a ton of destruction.

When launched within range of a ship, such performance in a small package was worrisome to the Allies.

But the weak link in the Ohka project was its reliance on relatively slow and easy-to-intercept bombers as mother ships, as friend and foe would learn during the battle for Okinawa in 1945.

Rapid development led to production of prototypes by September 1944, with unpowered flights the following month. Some development flights were unpiloted.

Additionally, glider trainer variants called K-1 were built with skids for landing, a feature not necessary for the intended one-way flight of combat versions.

The K-1s carried water ballast in place of the warhead and powerplant, and some ballast could be released to make landing speeds more manageable in the trainers.

As all nations in the war faced strategic materials shortages, the Ohka was designed to be built largely of wood and readily-available metallic alloys. Simplicity of construction was factored into the design to enable its mass production by semi-skilled workers.

Put into production in September 1944 before testing was completed, 755 Ohkas were built by March 1945.

On March 21, 1945, a force of 16 Mitsubishi G4M twin-engine bombers carried Ohkas toward the U.S. fleet near Okinawa. Their mission became a rout when American fighters intercepted the bombers too far away to launch their flying bombs. The bomber crews jettisoned the Ohkas in an effort to spare the bombers for future sorties.

Ohka attacks in April had some success, with a near miss on the battleship USS West Virginia on April 1 and the loss of the destroyer USS Mannert L. Abele off Okinawa on April 12.

Navy gunners fired on the fast-approaching Ohka, but its pilot succeeded in hitting the destroyer near its waterline. The ensuing explosion broke the destroyer in two. Two more U.S. destroyer picket ships were hit by Ohkas that day.

The U.S. Navy was concerned about the Ohkas in the battle for Okinawa and what they could portend for the anticipated invasion of the Japanese home islands.

A 1945 Navy report said Ohka “presents the most difficult target our surface forces and aircraft have encountered in the war to date. It is also potentially the most dangerous antishipping weapon to be devised, being a guided missile with the best possible control — a human being.”

The Navy said proximity fuzes — one of the best weapons against fleet attacks by aircraft — would be triggered by an Ohka at about 30 to 45 feet proximity, but because of the high speed of the diminutive flying bomb, perhaps four times as many of the proximity shells would need to be fired by antiaircraft gunners to achieve destruction.

The small and partially wooden Ohka gave it stealth capability before the term was in vogue.

Navy officials noted: “Current experiments indicate that radar ranging on Baka will be approximately 30% of that on a fighter aircraft.”

Baka was a nickname given to the Ohka by Americans. It means “idiot” or “fool” in Japanese.

The best answer to defeating Ohka was to intercept its carrier aircraft as far from the surface fleet as possible. The availability of mother ships for the Ohka was seen as a limiting factor in its use, but in the summer of 1945 the Navy rightly forecast future developments of Ohka that might use ship or shore launching and greater range.

In Japan, the limitations of the Model 11 Ohka were noted. A response was the Ohka Model 22, using a jet engine and a faster mother ship, the Navy Yokosuka P1Y Ginga twin-engine bomber.

The Ohka 22 carried a half-size explosive warhead of 1,323 pounds. It was much slower than the Ohka 11, but had a range said to be more than 80 miles, compared to the Model 11’s 23 miles.

Ohka 22 never saw service, and other proposed developments of the design promised greater speed and range, but went unbuilt as Japan’s fortunes rapidly declined in the summer of 1945 up to the end of hostilities that August.