Kawasaki Ki-102 Otsu with Kawasaki I-Go
While the IJN put its focus on SAMs, the IJA’s resources went into developing Air-to-Surface missiles (ASMs). The culmination of these developments, begun in 1942 by the Koku Hombu, was the I-Go series of missiles. The majority of the research on the I-Go was carried out by Rikugun Kokugijutsu Kenkyujo located in Tachikawa. Once the preliminary work for the missiles was completed, the Koku Hombu reached out to Mitsubishi, Kawasaki and the Aeronautical Research Institute of Tokyo University to commence final development of the I-Go as they saw fit, using the initial data assembled by Rikugun. Sumitomo Communication Industry Co. Ltd. was the provider of the autopilot and the transmitter/receiver system for the first two I-Go missiles with T. Hayashi designing the former and K. Nagamori the latter.
The I-Go-1-A (Ki-147) was the Mitsubishi version of the I-Go. The final design of the Ki-147 was completed by the end of 1943. Work began on the missile using a basic airplane configuration and its construction was made of wood and metal. It was propelled by a rocket engine built by Nissan Jidosha KK which produced 240kg (529lb) of thrust with a burn time of 75 seconds, providing a top speed of 550kg (342mph). The warhead was substantial using 800kg (1,764lb) of explosive triggered by an impact fuse. Guidance was by radio from the carrying aircraft. The first Ki-147 missiles were completed in 1944 and by mid-year unguided test drops had commenced at Ajigaura, Atami and Shiruishi. The carrier aircraft was a modified Mitsubishi Ki-67-I Hiryu bomber. By October 1944, guided test drops of the Ki-147 had begun. Despite the testing, the Ki-147 did not enter production and only fifteen were built. The Ki-147 had a length of 5.8m (18.9ft), a span of 3.6m (11.8ft) and a launch weight of 1,400kg (3,086Ib).
The I-Go-1-B (Ki-148) was the Kawasaki I-Go. Smaller than the Ki-147, the Ki-148 used a HTP rocket motor that developed 150kg (331lb) of thrust, with an 80 second burn time. The wings were constructed of wood while the body and fins were made from tin. As a consequence of the smaller size, the warhead comprised only 300kg (661lb) of explosive and it used a direct-action fuse. For guidance, the Ki-148 used the same radio system as the Ki-147. Following wind tunnel testing with full- and half-size models, Kawasaki produced a number of missiles at their Gifu factory for testing to begin in late 1944. Ki-148 test launches were made from four modified Kawasaki Ki-48-ll Otsu bombers at Ajigaura in lbaraki Prefecture. By December 1944, up to 20 Ki-148 missiles were being launched per week from the bombers. Despite the relatively successful testing, the Ki-148 was never put into production and total deliveries of the pre-production/test Ki-148 missiles amounted to 180. Had the Ki-148 gone into service, the Kawasaki Ki-102 Otsu was to be the designated carrier aircraft.
Ki-148 had a length of 4.1m (l3.4ft), a span of 2.6m (8.5ft) and a launch weight of 680kg (1,499Ib).
The I-Go-1-C would be the final I-Go project. The Aeronautical Research Institute of Tokyo University decided to take a completely different approach to guidance. Deciding that anti-shipping would be the main use of the I-Go-1-C, the missile dispensed with the radio guidance method and instead employed a novel system that used the shockwaves produced by naval cannons as the means to direct the missile. In essence, the missile would guide itself to the target by sensing the shockwaves developed in the air by large naval cannons during firing. Since shockwaves travel outwards from the cannon, the missile could determine direction and adjust its flight path accordingly to bring it onto the target. The main benefit of the system was that the missile was a fire-and-forget weapon. As long as naval ships engaged in bombardment, the I-Go-1-C would be able to track and attack them on its own. Testing of the system got under way in 1945 and the initial results showed promise. However, the missile body was never built as the war ended before testing of the guidance hardware was complete. The proposed I-Go-1-C was to be 3.5m (11.4ft) long with a diameter of 1.6ft. Other specifications for the missile, such as its warhead size, rocket motor, performance and weight are still unknown. The I-Go-1-C is sometimes called the Ki-149 but there is no evidence to support the use of this name.
Since the Ki-147 and the Ki-148 achieved flight testing and both used the same radio guidance system, the procedures to launch and control the missiles were basically the same. The Ki-67 and Ki-48 bombers used in the testing were modified to accommodate the missile operator as well as the equipment needed to guide the weapon. Operationally, the missiles would be dropped at an altitude of 1,500m (4,922f1),11km (6.84 miles) from the intended target. By the time the missile was 5km (3.11 miles) from the target, the altitude varied between 30m to 150m (98ft to 492ft) depending on the preset of the altimeter. The operator would guide the missile via a joystick and just before it passed over the target, the missile would be put into a dive, bringing it down onto its target. The launching aircraft had to remain within sight of the missile and in most cases would be 4km (2.5 miles) from the target when the missile hit. While the handling characteristics of the weapons were found to be good, analysis showed that the missiles tended to fall either 300m (984ft) short of the target or 100m (328ft) past the target. The reason for was that the operator had to rely on his own vision and clear conditions in order to guide the missile. He was not provided with any form of special optics nor did the missile carry a means to mark itself in flight such as using burning flares or smoke which the operator could use to maintain sight of the weapon. The only measure of this kind ever employed was a taillight which was used at night so the operator could track the missile. Had the Japanese given further consideration to the operator’s needs, accuracy may have been improved. A factor against the use of the Ki-147 and Ki-148 was that the launch aircraft had to be within 11km (7 miles) of the target and had to remain in the area to proceed with the attack. With the heavy Allied air presence, getting to the launch range would have been a formidable task and this may have been a factor in the Ki-147 and Ki-148 failing to enter service.