'War Hammer' - The Kawasaki Ki-178 U~ōhanmā
Japanese ramming attacks on US bombers were pioneered by IJAAF squadrons flying Kawasaki aircraft. The Shinten Seiku Tai ('Heaven-Shaking Ones') units flew Ki-45 Toryu and Ki-61 Hien fighters against USAAF B-29s with some success ... but also plenty of pilot casualties. By the end of 1944, even the latest-model Ki-61-IIs were having trouble intercepting the fast American bombers. Stripping armaments reduced weight, improving climb rates but losses among experienced fighter pilots were mounting. In the meantime, the Kawasaki Kokuki Kogyo K.K. was ordered to pursue dedicated designs for tai-atari or 'body strike' attacks on enemy bombers.
Kawasaki had already been instructed to develop the rocket-powered Kokukyoku Shusui-shiki Kayaku ram-fighter as a rare joint Imperial Japanese Army/Imperial Japanese Navy project. However, Kawasaki design staff had little faith in the potential of this piloted missile. A more conventional side project was proposed to the IJAAF to improve the kill-rate of the ramming Shinten Seiku Tai - 'those who make the heavens shake'. Pursued under project name 'U~ōhanmā' ('War Hammer'), the concept was to make use of as many reclaimed Ki-61-I components as was practical. To take full advantage of the early-model Hien's lower-powered Ha40 engine, the aircraft's empty weight had to be slashed. That was going to be a challenged with the demanded inclusion of steel in the structure.
Bottom Initial Concept for the Kawasaki U~ōhanmā shown in the markings of the Shinten Seiku Tai of the 244th Sentai.
Most critically for its mission, the new ram-fighter was to have a welded-steel wing main spar. In theory, this heavy-weight spar would allow the ram-fighter to slice through a bomber's aluminum airframe with near impunity. The windscreen was also protected by a steel frame. The cockpit was to be very low-slung and covered by an easily-jettisoned canopy. The side-fuselage mounted radiators of the Ki-78 speed record aircraft were adopted as these were felt to be less vulnerable during an intentional collision. The cowling and engine mounts along with the aftmost fuselage/tailplane were to be reclaimed standard Ki-61-I components.
The IJAAF quickly rejected Kawasaki's initial proposal. A key reason was its relatively sophistical structure - in light of the recent destruction of Kawasaki's Gifu assembly plant by American bombers. Instead, the IJAAF requested a design which could make use of semi-skilled labour. This dictated the use of wooden components in place of lighter stressed-skin aluminum. The forward fuselage was based on the Ki-61 but, aft of that, the structure was primarily wood reinforced with welded-steel. This revised concept was accepted by the IJAAF as the somewhat dumpy Ki-178 U~ōhanmā.
Top An operational Ki-178 U~ōhanmā in the new markings of the Hagakure-Tai of the 244th Sentai on its tail fin. [2] This aircraft was flown by Shoi (Ensign) Katsumi Sakita on his final mission on 10 January 1946. [3]
For the Ki-178, the welded-steel wing leading-edge of the initial U~ōhanmā proposal was carried over but this approach was also extended to highly-raked vertical tailplane. [1] The cockpit canopy was much enlarged compared with the original concept but well-protected by angled, steel deflector bars. To this was added steel wing tip 'kaiken' - steel hooks meant to increase collision damage to aluminum-skinned bombers. Operational use of the Ki-178 verified the effectiveness of those 'kaiken' but put paid to the notion that welded-steel components would make the U~ōhanmā near invulnerable during tai-atari attacks. Indeed, the loss rate was little different from Ki-61s employed as rammers.
Prompted by combat experience and to speed production, the Ki-178 ko variant introduced a simplified undercarriage. This gear lacked hydraulic retraction and was extending by gravity alone. The Ki-178 ko was launched by railway sled with its undercarriage already retracted. Should the ram-fighter survive combat and regain an airfield, the undercarriage would be dropped for a conventional landing. Otherwise, pilots who managed to avoid incapacitation in their tai-atari attack would bail out or try to belly-land his fighter on level ground.
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[1] The horizontal tailplane were standard Hien components but reinforced by steel Difurekuta ('deflectors').
[2] The Hagakure-Tai are often described as a 'Special Attack Unit'. The name actually translates as something akin to 'Hidden Leaves Corps' - after Hagakure - the way of the Samurai.
[3] Sakita was postumously promoted to Chui (1st Lieutenant) and then to Tai-i (Captain) when awarded the 'Bukosho' (Rikugun Bukokisho or Military Merit badge).
