Army Navy Collaboration
Air-to-Air rocket projectiles in use by the IJN have been known and documented for some time, but much less is known about the Army's testing and use of such weapons. A degree of cooperation is suggested by the fact that the Army and Navy had a rocket exchange plan in effect from 1943 although it was not until March 1944 that there was any actual collaboration. Each service pursued a slightly different approach with the Army specialising in spin stabilisation and the Navy in fin stabilisation. An agreement was reached that whichever technique was perfected first would be adopted as the standard methodology by both services.
The Navy fin-stabilised rocket achieved the earliest success but the Army objected that the design depended on the use of specialist Navy arsenal equipment and Army arsenals could not be adapted to its manufacture. As a result the Army continued with development of their own spin stabilised projectile. Ground deployment of the Army rocket reached the operational stage when the rockets were used during the defence of the Phillipines. Aerial deployment was pursued as described below with eventual limited operational testing of the projectiles but was eventually discontinued in favour of conventional gunnery and specialist aerial bombing techniques using Ta-dan ordinance (which will be explored in a future article).
The Army Aerial Rocket Projectile Programme
The Army research into rocket projectiles, to be launched from rails under the wings of fighter aircraft and intended to attack large aircraft or surface vessels, began in 1943. Three types of rocket were to be developed:-
Ro-San Dan (Ro-3 ordinance): weighing about 5 kg (for attacking aircraft)
Ro-Go Dan (Ro-5 ordinance) : weighing about 30 kg (for attacking aircraft)
Ro-Shichi Dan (Ro-7 ordinance): weighing about 300 kg (for attacking ships)
The testing of Ro-San Dan and Ro-Go Dan were completed in April 1944 and October 1944 respectively. Both types of rocket were manufactured by the Osaka Rikugun Zoheisho (Osaka Army Arsenal) and the Tokyo Dai-2 Rikugun Zoheisho (Tokyo 2nd Army Arsenal). The launching rails were manufactured by Tachikawa Heiki (Tachikawa Weapon). The initial velocity of Ro-San Dan was about 200 m/sec, and its effectiveness was assessed to be equal to that of the Type 88 Anti-Aircraft Gun (八八式七糎半野戦高射砲 Hachi-hachi-shiki nana-senti-han Yasen Koshahō) which fired a 75mm projectile of just over 6 kg in weight (a variant of this gun was installed in the Ki-109 converted Hiryu bomber interceptor). Ro-Go Dan had a larger explosive charge. Ro-Shichi Dan was also manufactured by both arsenals, but the launching rails were manufactured by Nakajima. It had an explosive charge of about 50 kg.
Before the testing of Ro-San Dan had been completed, the Koku Kosho (Air Factory) was ordered to convert the Type 1 fighter (Ki-43 Hayabusa) to be equipped to launch the rockets. By March 1944 the Air Factory had converted a total of 40 Hayabusa to be able to launch the rockets. In November 1943, after the testing of Ro-San Dan was complete the Air Factory had been further ordered to also convert Ki-44 and Ki-45 Kai aircraft types to launch the weapons. With the Hayabusa the Air Factory attached the launching rails to the wing locations for the drop tank racks. The rocket used was the Ro-3 Dan, and was hastily developed. Originally an attempt was made to equip the Ki-43-II with three Ro-3 Dan launchers. However the centreline location envisaged was found to be impracticable because the rocket could not be launched to safely clear the propeller so the final configuration was for Ro-3 Dan launch rails under each wing. This launch configuration was perfected by March 1944. Subsequently launching tests were made with the prototype converted aircraft, but they were judged to be ineffective and further development was cancelled. The only known photograph of a Ro-3 Dan under the wing of a Hayabusa (top image) shows the aircraft of an Army test unit and comes from the book 'Michi no Ken' (Unknown Sword) by Yoji Watanabe. The rocket projectiles appear to be somewhat similar in appearance to British style aerial rockets. The second image is a drawing from Japanese military archives illustrating the underwing launch rails for the Ro-3 Dan on the Hayabusa.
The Singapore Connection
The 1st Yasen Hójú Hikótai based at Singapore was the Army Field Air Replacement Unit for the SE Asia theatre. It consisted of a fighter unit, light bomber unit, medium bomber unit and reconnaissance unit. Essentially a combination of operational training unit and personnel pool for the 3rd and 4th Air Armies under the administration of the 3rd Air Army, the unit was equipped with various types of aircraft and also engaged in the air defence of Singapore against the first B-29 incursions. Personnel for this unit were drawn from both advanced flying training regiments (Kyôiku Hikôrentai) and the operational flying training units (Rensei Hikôtai).
Minoru Akimoto recorded that "Ki-44 aircraft in Singapore trained in the use of rockets against the B-29." At least two personal records of well known Army pilots mention this rocket development and testing.
In his 'Japanese Army Air Force Aces 1937-45' for Osprey, Henry Sakaida mentions the involvement of Major Yasuhiko Kuroe in air-to-air rocket testing:
"In January 1944, he was recalled to Japan to become a test pilot with the Army Flight Inspection Department, his new assignment involving the evaluation of both rockets and experimental high altitude twin-engined fighters."
In the recent Revi monograph* on the Ki-44 Shoki, author Martin Ferkl mentions that Captain Ryótaró Jóbó of the 1st Yasen Hójú Hikótai "flew a Ki-44 experimentally fitted with 44mm rockets under the wing. According to his recollections, due to high velocity of the bombers, he was always able to make only one pass. He would attack from below, however only one pass was never enough to bring a huge bomber down. Moreover, the rocket launch mechanism was prone to frequent malfunction." The reference to 44mm rockets is improbable and perhaps a mistranslation relating to the type of aircraft.
(*Very interesting material in this book, some of which appears identical to information posts made by yours truly at j-aircraft.com based on my own primary research - although not credited as such.)
Although of poor quality (from a newspaper) the photograph shows Ki-44 aircraft lined up at a Singapore airfield post-surrender, bearing the distinctive tail insignia of the 1st Yasen Hójú Hikótai.
A further snippet of information comes from a previously classified report on Japanese-German military collaboration:-
"However, in early 1945 the Japanese Army reportedly was using an aircraft-launched 87mm rocket which is reminiscent of the Panzerschreck; two rockets were carried under each wing."
Confirmation of the operational use of the rockets by Army aircraft comes from the AAF HQ Intelligence Report 45-101 on Japanese Fighter Tactics B-29 supplement. The crew of a B-29 reported that an Oscar approached from 5 o'clock low and fired a rocket at a range of 1,000 yards out. The rocket exploded with a red and yellow flash, mixed with white smoke and emitting streamers. It burst 100 feet behind, 100 feet below and 200 yards to the right of the targetted B-29 aircraft. The illustration reproduced here is taken from this intelligence document.
This article was prepared with the kind and generous collaboration of Ryusuke Ishiguro, whose principle contribution and assistance is gratefully acknowledged. Without the diligent investigative work of Ishiguro-san the details of the Army's air-to-air rocket programme would have remained obscure.
Image credit: Ki-43 Illustration Japanese Military Archive via Ryusuke Ishigaru; Ki-43 Photograph 'Michi no Ken' by Yoji Watanabe; Ki-44 Photograph Geoff Thomas via Ian K Baker; Rocket Attack Illustration AAF HQ Intelligence Report 45-101 B-29 Supplement.