Turbine 'Tony' - The Kawasaki Ki-208

[apophenia]

'Turbine Tony' - The Kawasaki Ki-208 Tsubame Jet Fighter

A shortage of engines necessitated the Kawasaki Kokuki Kogyo K.K. completing the final Ki.61-II airframes as radial-engined Ki-100s "Type 5 Fighter" (Go-shiki sentouki or Goshikisen). The result was surprisingly good but Kawasaki requested permission to use unfinished airframe components to develop a jet-propelled interceptor better capable of intercepting American B-29 bombers. This was prompted, in part, by an Imperial Japanese Army decision to remove domestic production of the German Messerschmitt Me 262 jet fighter from Kawasaki. Instead, the Me 262 copy would be built by rival Nakajima as the Ki-201 Karyu (Nakajima was already developing a smaller, twin-jet naval attack aircraft roughly based on the Me 262 - the Kikka). Kawasaki engineers believed that they could produce a simpler jet fighter more quickly than Nakajima.

Kawasaki had been asked to focus on production of the Kayaba Ku-4 Katsuodori - a diminutive ramjet-powered point defence interceptor. However, in July 1944, the IJA cancelled both the flying-wing Katsuodori and its ramjet engine. [1] That left Kawasaki free to concentrate on a turbojet fighter concept. Nakajima's jet aircraft were both to be powered by domestic turbojets inspired by drawings of the German BMW 003 - the 1,050 lbf Ishikawajima Ne-20 (Kikka) and 1,950 lbf Nakajima Ne-230 (Ki-201). [2] Ishikawajima Shibaura's engine had begun as a private venture. Kawasaki followed suit. The firm had bought rights to the Heinkel-Hirth HeS 08A and HeS 30 turbojets when the German RLM cancelled these early projects in April 1944. Arrangement was made to ship complete drawings and sample components of both engine types to Japan.

For their turbojet fighter design, Kawasaki engineers kept none of the radical airframe features of the Katsuodori but did retain that interceptor's prone pilot position. For what became known as the 'Nensho Rocketto 30' (or Jet Engine 30) fighter proposal, Kawasaki re-used the wings and undercarriage from the Ki-61 piston-engined fighter - now mid-positioned on an entirely new, wooden fuselage. Reversing the orientation of the Katsuodori, the jet engine would be mounted above the fuselage. That jet engine was to be Kawasaki's private venture Ne-30 - a domestic development of the German Heinkel-Hirth HeS 30 five-stage axial turbojet producing around 1,900 lbf of thrust. Optional rocket-boosting was offered with a tail-mounted Toko Ro.2 (KR10) based upon German liquid-fuelled rocket motor technology.

Top Initial 'Nensho Rocketto 30' proposal powered by a single Ne-30 turbojet. As illustrated, two options are depicted - the use of four imported MG151/20 guns (as on Ki-61-I); and a detachable rear fuselage housing a booster rocket.

Back from the Drawing Board - Kawasaki's Twin-Jet 'Swallow'

Unfortunately for Kawasaki, components and drawings arrived aboard the submarine I-29 [3] for only one engine type - the lower-powered Heinkel-Hirth HeS 08A. With a functioning HeS 30 engine still some way off, the IJA's  Inspectorate General of Aviation quickly rejected Kawasaki's single-jet fighter proposal. The 'Nensho Rocketto 30' proposal was still-born. Having rejected the 'Nensho Rocketto 30' proposal, the Inspectorate voiced its support for a Kawasaki twin-jet fighter - providing such an aircraft could be developed well in advanced of the planned Nakajima Ki-201. With the Inspectorate assured that Kawasaki could proceed quickly, work began at the Kawasaki drawing office on a completely new 'austerity' jet fighter design. The demand for speedy development dictated the use of two of the available but less powerful HeS 08A turbojets as well as other available components. Since there were still Ki-61-II Hien components available at Gifu, these became the focus.

The resulting twin-jet design was derived directly from the piston-engined Ki-61-II fighter airframe. But that is not to say that it was a simple transformation. The HeS 08A engines were to be slung beneath the Hien wings with the section of flaps in the path of the jet efflux behind replaced with heat-resistant steel panels. The main undercarriage was moved behind the main spar to accommodate a revised tricycle gear arrangement. [4] Based upon German experience, this tricycle undercarriage was regarded as essential - but these revisions displaced the Ki-61's original wing fuel tanks. That would result in a great deal of revision to the original Ki-61 fuselage structure.

Initially, the Ki-61 fuselage was to be left unchanged aft of the original firewall. Forward of that firewall would be the main fuel tank with an armaments bay in front of that. Early official assessments were unfavourable and the Ki-61 fuselage was revised to push the cockpit location further forward. This would improve the view from the cockpit but at another cost to future pilots. That cockpit would now be straddled by fuel tanks -  hardly ideal in combat but splitting the fuel supply into two did improve the fighter's c/g, enhancing its manoeuvrabiity. The opposite was true for the fighter's fixed armament which was concentrated in the nose. As ammunition supplies were depleted, the airframe became increasingly tail heavy and pilots had to trim to allow for this change in balance.

With other, piston-engined fighter projects underway, Kawasaki was well aware to the intense competition between airframe makers for the newest 30 mm aircraft cannons. Instead of joining that fray, the new jet fighter would have a very conservative fixed armament. The main gun would be the same 37 mm Ho-203 mounted in the Ki-45 twin piston-engined fighter. This would be replaced by the higher-performing Ho-204 autocannon when sufficient numbers became available. [5] Mounted above the Ho-203 would be twin 20mm Ho-5 (Type 2) machine guns. [6] Some design work was also done on optional wing guns (although this was not yet complete when the initial, twin-jet 'Nensho Rocketto 80A' concept was submitted for consideration. [7] The Inspectorate General of Aviation was sufficiently intrigued to request the construction of a working prototype based upon Ki-61-II components.

Bottom 'Nensho Rocketto 80A' - the Kawasaki Josho Tsubame (Rising Swallow) prototype as constructed. Note the interim Ho-203 nose armament installation but absence of 'cowl' 20 mm Ho-5 guns.

As completed, the 'Nensho Rocketto 80A' prototype was considered a demonstrator. Assigned the IJA designation Ki-208, the aircraft's engines lost their 'A' suffix, being officially designated Ne-80. The Ki-208 first flew from Gifu on 30 September 1945, performing much as expected. One issue was a longer landing run due to the reduced flap area and new tricycle gear. This had been predicted but a brake shoe redesign was needed to get around excessive wear. Concerns about sufficient tail area proved unfounded as did worries about the fabric covered elevators being scorched by jet exhaust. After two test flights, the prototype was fitted with a 37 mm Ho-203 cannon for aerial firing trials.

(To be continued ...)
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[1] Developed in collaboration with Kawasaki, the Ne 0 engine was a Sänger-style ramjet produced in 1943 by the Army's Tachikawa Aeronautical Technology Research Institute. Intended to produce 750 kgf thrust to propel the Katsuodori, the Ne 0 was flight-tested (under a Kawasaki Ki-48 bomber) but never achieved its planned output.

[2] The Nakajima Ne-230 was originally designated TR230. Related developments were the 2,000 lbf Ishikawajima Ne-130 (aka TR140) and the 2,910 lbf Mitsubishi Ne-330 (aka TR330).

[3] HeS 30 components and drawings had actually left Germany earlier aboard another submarine. Unfortunately for Kawasaki, the Ro-501 (ex-U-1224) was sunk by US forces off the Cape Verde islands.

[4] The new nose gear was inspired by that of captured US P-39 Airacobra fighters (then under test by the IJAAF's Koku Gijutsu Kenkyujo).

[5] By comparison, the Ho-204 had a higher rate of fire (400 rpm vs 120 rpm for the Ho-203) and a more powerful round (37 x 144mm vs 37 x 112R). The Ho-204 also had longer-barrel (50 inches vs 31 inches) and muzzle velocity (710-820 m/s vs 570 m/s). However, the Ho-204 was also substantially heavier than the Ho-203 (290 lbs vs 195 lbs).

[6] A 20 mm gun was considered a 'machine gun' by Japanese authorities. The term is, perhaps, most appropriate with the Ho-5 - a scaled-up Browning machine gun.

[7] Engine placement had eliminated the Ki-61's original wing gun location but underwing, pod-mounted guns were being considered for the B-29 interceptor role.

[Dr. YoKai]

 I really like the Josho Tsubame - reminds me a bit of a single fin He 280.

[apophenia]

Cheers Doc! That was exactly what I was going for 

[Sport25ing]

I would also like to see the Japanese version of the JV44, both jets and piston fighters 

[apophenia]

I would also like to see the Japanese version of the JV44, both jets and piston fighters 

So far, the 'ramu sentoki' haven't made it past the sketchpad stage ... so no 44th Sentai just yet 

[apophenia]

Considered successful overall, the Josho Tsubame (Rising Swallow) demonstrator was obviously underpowered. The Ne-80 (Nensho Rocketto 80) were never regarded as definitive powerplants but the unavailability of the proposed Ne-30A (based upon the Heinkel-Hirth HeS 30) threw Kawasaki's engine division back on its own resources. As a substitute, it was proposed to simply scale up the Ne-80 to provide additional power. Although a private venture, this required IJA approval - despite much of the needed material being standard, non-heat-resistant steel. Approval was given and work began on the new Ne-82 in dispersed locations.

Dispersal of Ne-82 development proved fortuitious. Just as Ne-82 work began in January 1945, a USAAF bombing raid completely devasted the Kawasaki engine factory at Akashi. Not only was the Ne-82 programme undisrupted, it actually benefitted to some degree. With piston engine development halted, recovered machinery could be relocated along with skilled machinists to assist in dispersed Ne-82 production. By early May 1945, an airworthy Ne-82 prototype was complete and installed under a Ki-45 fighter at Gifu for flight testing. These tests were considered successful but plans to re-engine the Ki-208 prototype with Ne-82s were interrupted.

Concerns about the original Ki-61-II tail surfaces being scorched by jet efflux proved totally unwarranted. But that did not mean that the tailplane was completely unaffected by the jets. With its engine and test armament removed, the Ki-208 demonstrator became a 'tail-sitter'. When moving the engineless prototype, assistants tried to help lift the tail by pushing up on the stabilizers. The starboard stabilizer immediately deflected, being held in place largely by the tubular connection of the elevators. The Ki-208 demonstrator's tailplane structure was fatigued to the point of failure by high loads resulting from jet wakes passing beneath the control surfaces. Fortunately, due to the unwarranted concern about scorching, production-model Ki-208s were to feature raised horizontal tailplanes.

The Kawasaki Tsubame (Swallow) twin-jet fighter entered production as the Ki-208 ko. These fighters had the raised tailplane but the 'ko' suffix signified the switch to higher-powered Kawasaki Ne-82 turbojet engines. There was no prototype Ki-208 ko as such. Eventually, the re-engined (and re-tailled) Ki-208 demonstrator would rejoin the programme as a development airframe but, by then, Ki-208 ko variants were being delivered to the 5th Sentai of the IJAAF.

Bottom An early-production Kawasaki Ki-208 ko fighter attached to the IJAAF Tsubame Operational Trials Unit at Gifu field, early September 1945

The 5th Sentai had been operating from Gifu airfield which was heavily bombed by USAAF B-29s between early June and late July 1945. By the time operational Ki-208 ko fighters arrived, the 5th Sentai was mostly flying from the surviving road network surrounding Kagamigahara. In some cases, aircraft were maintained inside repurposed buildings but, mostly, the Tsubames either sat in the open beneath camouflage nets or parked under tents mocked-up to resemble tradiational Japanese house roofs. Such ruses were fairly effective until USAAF escort fighters began low-level strafing on their return from bomber escort missions. Thereafter, Tsubame losses on the ground began to mount. Once airborne, fuel and ammunition shortages became bigger concerns. But, with its chunky 37 mm nose gun, the Ki-208 was crudely effective against even escorted low-flying B-29s.

IJA planners were still concerned with the threat of a USAAF return to high-altitude bombing. Low-flying B-29s dropping incidiaries had devastated traditional, wooden Japanese architecture. But it was feared that the Americans would eventually resume targeting Japan's war industries with HE bombs from higher altitudes. IJA piston-engined fighters were rarely fast-climbing enough to reach high-flying B-29s. The jet-powered Ki-208 ko had a distinct edge in level speed but did not climb spectactularly quickly. To address this worry, Kawasaki designers returned to the rocket-boosted solution recommended for their original, single-jet 'Nensho Rocketto 30' concept. In the new proposal, the Tsubame's aftmost fuselage section would be replaced with a new wooden structure with twin tail fins. The liquid-fuelled rocket motor's  steam-generator and turbo-pump would be accommodated in the original Ki-61 fuselage's rear section. The control valves and rocket combustion chamber would be mounted in the wooden aft section. [1]

Top Dual-power Ki-218 (Ki-208 otsu) as conceived with Toko-Ro.2 liquid-fuelled rocket motor in its tail. The Kawasaki Kokuki Kogyo K.K. logo is displayed on the vertical fins. On the nose is the hiragana character 'ne'. [2]

As built, the Ki-218 (Ki-208 otsu) employed the cut-down fuselage of the Ki-61-II kai. However, the nose, wings, jet nacelles, and undercarriage members were standard Ki-208 ko components. No Toko-Ro.2 rocket motor was ever installed - for none were available to Kawasaki. Flown as something of an 'empty shell', the Ki-208 served mainly to prove the aerodynamic qualities of the re-design. That is did - the tendency to 'hunt' directionally experienced by the Ki-208 ko was eliminated with the new tailplane. Eventually, the sole Ki-218 was used to test a tail-mounted solid-propellent rocket motor - the Mitsubishi Toku-Ro 1 Type 3 with a thrust of 530 lbf. These trials were considered successful but the explosion of a Toku-Ro 1 Type 3 motor shortly after take off would result in the destruction of the protoype in mid-November 1945. [3]

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[1] The interior of the wooden fuselage section nearest the combustion chamber were lined with mild steel to limit potential heat damage.

[2] The hiragana 'ne' was displayed on the flown Ki-208 prototype but its significance is unknown.

[3] The explosion and resulting fire damaged the tail surface control runs. Despite this, the test pilot was able to put his striken mount down in the Kiso River near Sakahogi.

[finsrin]

A most interesting history.      We can only wonder how rocket version would have performed once engine was reliable.

[jcf]

Noice

Hmm, next a Ki-78 based turbojet "research" aircraft? 

[apophenia]

... Hmm, next a Ki-78 based turbojet "research" aircraft?

No plans for a 'Jet Ken' but I did sketch out a dedicated Hagakure-Tai fighter.

The latter was really based on rearranged/cut-down Ki-61 components but would have some Ki-78 features - side-fuselage radiator ducts, aft-placed cockpit, etc. I'm hoping that there's time to render this concept before the end-of-month GB deadline.

Meanwhile, more on my 'Turbine Tony' notion ...

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Despite producing the Ki-208 ko under increasingly difficult conditions, designers at the Kawasaki Kokuki Kogyo K.K. continued to tweak their concept. Before the rocket-boosted Ki-218 (Ki-208 otsu) development had been rejected by the IJAAF, even more radical redesigns of the basic Tsubame concept were being proposed. One was the three-engined Ki-228 interceptor. As originally conceived, this was to be a faster-climbing Ki-208 boosted by a tail-mounted Tachikawa-Kawasaki Ne 03 engine - a Sänger-style ramjet. However, Ki-208 ko deliveries were already being slowed by Kawasaki's inability to deliver sufficient Ne-82 turbojets. As a result, what became the Ki-228 proposal was reconceived at an early stage.

The revised design reversed the original powerplant scheme. Main power would come from a single, tail-mounted Ne-82 turbojet. Once sufficient flying speed had been attained, twin underwing Ne 03 ramjets would be ignited to provide higher speed (and climb performance). The Ki-228 was intended to tackle B-29s - should the Americans resume high-altitude bombing attacks on Japan. A fixed armament of twin Ho-5 20 mm machine guns and two of its scaled-up developments - the 30 mm Ho-155-I cannons (evolved by the Nagoya Army Arsenal). Additional armament was being considered in the form of unguided anti-aircraft rockets mounted beneath the wings.

Like the rejected Ki-218, the three-engined fighter was to employ standard Ki-208 ko components for its nose, wings, and undercarriage members. The fuselage was to be based upon that of the Ki-61-II kai with its cut-down rear decking. The rear fuselage had to be substantially redesigned to accommodate the Ne-82 turbojet. [1] Much of the structure surrounding that jet engine was constructed from mild steel. However, the tailplane was to be the same wooden, twin-tailled unit as proposed for the Ki-218.

Planners of the Imperial Japanese Army regarded fulfilling the planned role as a high priority but technical assessments were less enthusiastic. The conclusion was that the Ki-228 would be underpowered on take-off - making the interceptor vulnerable to marauding US fighters. The assessment team was also highly skeptical of Tachikawa-Kawasaki power output claims for the small Ne 03 ramjets. Worse, the Ki-228 was judged to be incapable of carrying sufficient fuel for its missions. The interceptor would, therefore, need to carry a great deal of external fuel which would further degrade its performance. As such, the Ki-228 concept would be abandoned.

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[1] Internal equipment would also have to be re-arranged compared with the Ki-208 ko. This was as much avoid 'tail sitting' as it was to clear space for the turbojet.

[apophenia]

Knowing that the Ki-61 II part supply was dwindling, Kawasaki designed replacement components for their jet fighter programme. The Ki-208 ko KAI introduced a number of wooden access panels as well as some steel parts to replace aluminum originals. More radically, Kawasaki proposed a more comprehensive redesign of the Tsubame airframe, incorporating wood construction into major airframe sections.

Under an unofficial internal designation - Ki-228 (II) - Kawasaki conceived of a quicker to construct fighter based around a wooden wing and fuselage structure (over welded steel-tubing in the centre fuselage and cockpit areas). A wooden twin tail was introduced not unlike that of the original, three-engined Ki-228 (I) concept. In the extreme tail was to be mounted a Mitsubishi Toku-Ro 1 Type 3 solid-propellent rocket motor (as previously trialled in the one-off Ki-218 demonstrator). [1] The exact arrangement differed in the aftmost fuselage section being made from mild steel (as opposed to an entirely wooden structure as used on the Ki-218).

The cockpit of the Ki-228 (II) was also moved forward to address the Tsubame's incipient balance issues. Shifting the cockpit forward simplified placement of both fuel tanks and fixed armament closed to the aircraft's c/g. Fixed armament was to consist of twin 12.7 mm Ho-103 (Type 1) 'cowl guns' mounted ahead of the cockpit. The cannon armament - whose barrels ran alongside the cockpit - was made up of four 20 mm Ho-5 (Type 2) or of twin 37 Ho-204 or 40 mm Ho-301 guns.

Top Kawasaki's Ki-228 (II) Tsubame development concept with forward cockpit and reduced reliance upon strategic materials in its construction.

The IJAAF review panel was impressed by the potential savings in strategic materials with the Ki-228 (II) concept. However, the proposed fixed armament was considered too heavy for available power. In any case, supplies of Ho-204 guns was extremely limited and it was very unlikely that the Ho-301 would be made available to Kawasaki since that weapon was in great demand for higher-priority fighter types. That said, the official review concluded with the recommendation that Kawasaki introduce as much of the Ki-228 (II) construction approach as practical to the existing Ki-208 Tsubame production line.

The Ki-208 hei looked similar to the Ki-208 ko but adopted the wooden wing construction proposed for the aborted Ki-228 (II). The hei model was quickly eclipsed by the Ki-208 tei which introduced a primarily wooden central fuselage. Armament for the Ki-208 tei was to consist of twin 20 mm Ho-105 'cowl guns' and a single 30 mm Ho-155 cannon. In fact, due to shortages, many were delivered as Ki-208 bo fighters armed with a trio of Ho-105 guns. Although considered underarmed, the lightened Ki-208 bo regained some of the performance surrendered to heavier wooden construction. The similar Ki-208 ki would be the last of the Tsubames. The ki was to be armed with four Ho-105 guns (an optional search light being mounted in the extreme nose) but it is not clear whether any Ki-208 ki fighters actually entered service before Japan's collapse.

Bottom An operational Kawasaki Ki-208 tei Tsubame of the 1st Chutai, 5th Sentai. 'White 19' is in the standard scheme of green splotches over light grey/natural metal finish. Note the long 30 mm Ho-155 cannon barrel - a distinctive feature of the Ki-208 tei.

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[1] Originally, Kawasaki had been provided with Mitsubishi Toku-Ro 1 Type 3 motors to propel its Ki-148 I-Go Type 1 hei anti-shipping missile.