Only the Germans and the Americans seem to have used glide weapons in combat during World War II. The Japanese Army did work on a series of air-to-surface guided weapons designated “I-GO-1”, but never used any of them operationally.
The initial “I-GO-1A” was built by Mitsubishi. I-GO-1A was a radio-guided missile of partly wooden construction with straight wings and a rocket booster, intended for launch by a twin-engine bomber. Length was 5.77 meters (18 feet 11 inches), wingspan was 3.6 meters (11 feet 10 inches), and launch weight was 1,400 kilograms (3,090 pounds), including an 800 kilogram (1,765 pound) warhead. The I-GO-1A was tested in the fall of 1944, but never used in combat.
The “I-GO-1B” was a conceptually similar but smaller radio-guided weapon built by Kawasaki. The I-GO-1B had a length of 4.09 meters (13 feet 5 inches), a span of 2.6 meters (8 feet 6 inches), and a launch weight of 680 kilograms (1,500 pounds), including a 300 kilogram (660 pound) warhead. Test drops were performed in late 1944 and 180 of these weapons were built, but none were used in combat.
The Tokyo Imperial University also experimented with an “I-GO-1C” air-to-surface missile that homed in on the sound of gunfire from naval vessels. Some tests were apparently performed in the spring of 1945, but little came of this exercise and details of this particular weapon are very unclear.
• The one Japanese air-to-surface weapon that was fielded was the notorious piloted “Oka (Cherry Blossom)” bomb, which was carried by a bomber and used three solid rocket boosters to accelerate it towards its target.
By the summer of 1944, the war was clearly going against Japan, and in desperation the Japanese military began to adopt extreme tactics, beginning with missions offering those involved little chance of survival, and then moving on to deliberately suicidal tactics. While some of the suicide tactics were dismal failures, such as the kaiten suicide minisubmarines and explosive-laden power boats, the aerial suicide corps would prove very effective in causing tremendous damage to Allied ships. The suicide pilots became known as the “kamikaze (divine wind)”, after a great storm that had saved Japan from Mongol invasion centuries before.
In the summer of 1944, as such kamikaze tactics were evolving, Ensign Shoichi Ota of the Imperial Japanese Navy (IJN) proposed a suicide aircraft in the form of a rocket-boosted glider that would be launched by a twin-engine bomber. Ota worked with the aeronautical engineering staff of Tokyo Imperial University to come up with a design, which he then submitted to his superiors. Although some Japanese military commanders were appalled at the idea of ordering their men on suicide missions, Ota’s idea was approved. The engineering staff at the Yokosuka Naval Aeronautical Research Laboratory worked to convert the proposal into a flying design, implementing an aircraft that was to be easily flown by a poorly-trained pilot, as well as simple and cheap to build.
The program was assigned the designation the “Navy Suicide Attacker Model 11”. Initial test examples were available by end of September 1944. They were originally designated the “MXY7”, but this was quickly replaced by the simple and romantic name of “Oka”.
The Oka weighed 2,140 kilograms (4,718 pounds), including a huge 1,200 kilogram (2,646 pound) steel-jacketed armor-piercing warhead. The weapon was 6.07 meters (19 feet 11 inches) long, with small wings only 5.13 meters (16 feet 10 inches) long. Since the Oka was not designed to take off under its own power, a larger wing was not necessary. The bomb had a twin-fin tail assembly, and three solid rockets that could be fired by the pilot singly or in combination to provide boosts during the fall to the target to evade Allied fighters or perform the terminal attack. The weapon was primarily a glider, with a normal glide slope of about 5.5 degrees.
The pilot armed the weapon after launch by pulling a handle, which activated five detonators, one in the nose of the warhead and four on the rear. Fuzing could be set for instantaneous detonation, or a delay of up to 1.5 seconds to allow the warhead to penetrate into the bowels of a ship before exploding. The cockpit included armor protection to allow the pilot to press his attack to impact in the face of antiaircraft fire.
For flight training, a glider version was built, using water ballast to simulate the weight of the warhead and rockets. When the trainee completed the training portion of the flight, he would dump the ballast, allowing the lightened glider to land safely on a skid. At first, the trainer was a single-seat aircraft, but a tandem-seat version, with separate cockpits for instructor and trainee, was planned, with the designation of “Model 43 K-1 Kai”.
Even as the ten test items were being delivered, the IJN was ordering the Oka into production. A decision was made to have it manufactured at the Yokosuka Arsenal to help keep the weapon secret. Some officials also believed that civilian manufacturing firms would find the idea so shocking that they would think the Navy had gone mad. A force of volunteers, designated the “Jinrai Butai (Thunder God Corps)”, was organized to fly the weapons. The volunteers were screened to weed out first sons of families, only children, or men with family responsibilities. The “winners” of the selection process were then put through indoctrination and training.
The training and flight tests were conducted through the fall and into the early winter of 1944. However, deployment proved troublesome. Two batches of Okas were sent to the Philippines on carriers late in 1944, but both carriers were sunk by American submarines, along with their suicide weapons. A few Okas were eventually transported to Formosa, Okinawa, Singapore, and elsewhere in the shrinking Japanese empire.
The delays in putting the Okas into operation led to demoralization of the pilots of the Jinrai Butai, since it left them with far too much time on their hands to contemplate death. They had a particular reason for worry. Each Oka was to be carried into battle on a modified Mitsubishi G4M twin-engine bomber, known as “Betty” to the Allies, and released at high altitude. While the Oka was likely to be all but unstoppable once released, the Betty was notoriously vulnerable. Since the bombers had to fly at high altitude to launch the Okas, they would certainly be picked up by American radar and torn to pieces by American fighters. The pilots of the Jinrai Butai knew they were going to die, but they at least wanted their deaths to pay off.
The Okas finally went into service on 21 March 1945, in an attempt to attack a US Navy fleet that had been raiding the Japanese home islands. A formation of 15 Bettys and their escorting Zero fighters were simply slaughtered by US Navy Grumman Hellcat fighters. Only a few Zeroes made it back to base to report on the disaster. US Navy pilots reported that some of the Bettys they shot down seemed to be carrying some kind of winged missile.
The Okas were sent into action again when the Americans invaded Okinawa at the beginning of April. This time, the Bettys were sent in one at a time, in unpredictable directions, and they were able to release Okas. One scored a hit on one of the main turrets of the battleship USS WEST VIRGINIA.
Allied intelligence began to understand what the Japanese were up to. They called the Oka the “Baka”, Japanese for “fool”, in attempt to make a joke of it, but that was just rear-area posturing; US Navy sailors found nothing humorous about enemies who were willing to kill themselves as long as they could take some Americans with them. However, the Oka proved to be the least of their worries. Conventional kamikaze aircraft proved far more effective than the rocket bomb, due to the extreme vulnerability of the Betty bombers that launched the Okas. While Oka attacks were performed up to the end of June, the last hit they scored was on 11 May. The Betty mother ships were far too vulnerable, failing to return from their missions about 70% of the time.
The Japanese developed an improved Oka, the “Model 22”, which was about a third lighter and powered by the Tsu-11 “turbo-piston” engine — a dubious contraption featuring a compressor driven by a four-cylinder piston engine. The smaller Model 22 Oka was to be carried by the Yokosuka P1Y “Francis” twin-engine bomber, which was much less vulnerable than the Betty and could fly higher. The Model 22 was expected to have a range of about 64 kilometers (40 miles) in operation. About 50 were built, but none were ever used. The Japanese also considered other improved Oka versions, including one that was powered by a proper Ne-20 axial-flow turbojet derived from German BMW jet technology and catapulted from a ground launcher, but these concepts never went beyond the paper stage.
Although the kamikazes proved to be a highly effective weapon, the Oka turned out to be less a guided weapon than a misguided one. Okas were only credited with sinking one destroyer, damaging a destroyer and a minesweeper beyond repair, and damaging a half-dozen other vessels. This was an insignificant score in comparison with the total earned by the kamikazes, and certainly the Okas in no way justified the resources and effort placed into them, much less the supreme sacrifices of their pilots.
• A Soviet designer named Vladimir Vakhmistrov came up with an interesting paper design for a glider bomb in 1944. This scheme involved a twin-boom glider, with each boom tipped by a 1,000 kilogram (2,200 pound) bomb. A fighter was attached to the top of the glider on struts to provide propulsion and targeting, and the whole system took off on an undercarriage system that was to be released after takeoff. On release from the fighter, the glider would proceed to target using a gyroscopic autopilot. The scheme was never implemented, which was just as well, since gyroscopically-guided glide bombs never proved to be a good idea.