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G8N Renzan – Japan’s Long-Range Colossus

The Nakajima G8N Renzan was a long-range bomber developed by the Imperial Japanese Navy towards the end of the Second World War. One of the very first Japanese attempts at constructing a warplane with 4 engines, the G8N was absolutely huge, deriving many of its features from American planes such as the experimental Douglas DC-4E as well as captured Boeing B-17 Flying Fortresses.

Despite a relatively trouble-free development and assembly phase, the fate of this technological marvel, which even impressed American observers, would ultimately be determined by Imperial Japan’s changing fortunes as their initially promising Second World War campaign began to turn into a nightmare.

A Japanese G8N parked on the runway.
The G8N was a massive, long range, heavy bomber.

Contents

The Mitsubishi G7M1 and the Kawanishi K-100

By the end of 1942, the Japanese had enjoyed some considerable success employing twin-engined medium bombers such as the Mitsubishi G3M and G4M against Allied maritime forces, most notably sinking the British frigate the Prince of Wales and the battlecruiser Repulse off the coast of Malaya on December 10th 1941.

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Initially settling on the same configuration that had given them such good results, the Imperial Japanese Navy (IJN) was soon looking for a superior dual-engine replacement, leading to the production of the Mitsubishi G7M1, known as the Navy Experimental 16-Shi Attack Bomber, and the Kawanishi K-100, which was also referred to as the Navy Experimental 17-Shi Attack Bomber.

A He 177 A-2 in flight.
The G7M1 had a striking resemblance to the He 177 Greif (pictured).

These projects though were shortly abandoned after the IJN began to reconsider the number of engines they wanted in what was intended to be their flagship aerial menace.

G5N Shinzan

Impressed by the performance and range of 4 engined American craft, the IJN revised their specifications and soon required a long-distance bomber with the same amount of engines.

Working alongside Japanese submarines and battleship destroyers, the IJN anticipated that this cutting-edge new unit would play a key role in the destruction of the armada of US ships, which after the surprise Japanese assault on Pearl Harbor, were expected to make their way across the Pacific Ocean to exact revenge.

The Japanese had no experience constructing quad-engined bombers, but they did have an American-made example already in their inventory, the Douglas DC-4E, a four-engine pressurized experimental airliner they had bought in 1939.

The G5N Shinzan was another large bomber that led tto the development of the G8N.
The G5N Shinzan.

First operated in 1938, the DC-4E had originally been shelved by the Americans because it was too heavy, underpowered, and costly, and sold by Douglas as part of an effort to recoup their losses, with the understanding that it was to be used commercially by the Japanese.

The Japanese however had a different purpose in mind, instead transferring it to the workshops of the Nakajima Aircraft Company, where it was to be reverse-engineered into a long-range naval attack bomber.

Consequently, the Japanese produced the G5N Shinzan, codenamed ‘Deep Mountain,’ an enormous quad-engined bomber that was 101 feet and 9 inches long with a capacious wingspan of 138 feet and 2 inches, and which had a staggering maximum take-off weight of 70,500 pounds.

Taking to the skies for the first time on April 8th 1941 without incident, and brandished with the nickname ‘Liz’ after it came to the attention of the Americans, the Japanese soon discovered that their titanic new warplane suffered from many of the same problems as the Douglas DC-4E, being far too hefty while its four 14-cylinder Nakajima Mamori engines each with a horsepower of 1,870, proved far too weak to propel the giant in a satisfactory manner.

The DC-4E also had similar performance issues...
The DC-4E also had an issue with underpowered engines. Photo credit – Carl Malamud CC BY 2.0.

Similar to the humungous American Boeing XB-15 bomber, ’Liz’ and the 5 other G5N prototypes were repurposed as long-range transporters instead as the Japanese returned to the drawing board.

G8N Renzan

By 1943 following the seizure of 3 Boeing B-17 Flying Fortresses in the Philippines and the Dutch East Indies the previous year, the Japanese were a lot more knowledgeable about the intricacies of American bomber designs.

Unfazed by the failure of the G5N and armed with new technological insights, on September 14th 1943 the Japanese military announced they were looking for a bomber that had a top speed of 368 miles per hour, a maximum range of 4,605 miles, a warload of 8,816 pounds, could climb to an altitude of 26,245 feet in 20 minutes, and which was protected on all sides by an array of multi-directional armaments.

A captured Japanese B-17 in flight.
Japan managed to capture three B-17s and put their learnings into practice.

With all the criteria laid out, Nakajima commenced work on the G8N Renzan which was given the moniker ‘Mountain Range,’ with a total of 48 units and 16 prototypes expected to be delivered by September 1945.

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Accommodating up to 10 crew members the G8N Renzan, given the moniker ‘Rita’ by Allied command and also officially designated as the Navy Experimental 18-Shi Attack Bomber Renzan (Mountain Range) G8N1 by its Japanese creators, was fabricated using large thick plating for ease of production and had a length of 75 feet and 3 inches, a height of 23 feet and 7 inches, and was 6,000 pounds lighter than its precursor with an empty weight of 38,000 pounds.

On the other hand, the G8N’s maximum take-off weight was just below 70,900 pounds, clocking in slightly heavier than the G5N. Elsewhere, the G8N had a mid-mounted wing with a span of 106 feet and 9 inches and an area of 1205 square feet, which was aerodynamically enhanced with a laminar flow segment.

The four engines gave the G8N good performance for the size of the aircraft.
The G8N had four 2,000 horsepower engines – much more powerful than the G5N.

The most notable change though was the inclusion of four 2,000 horsepower Nakajima NK9K-L Homare 24 engines amplified with Hitachi 92 turbosuperchargers. Cooled by fans that rotated in the opposite direction of the propellors, they provided 25% more power than the disappointing Mamori engines of its predecessor, thus enabling the G8N to fly much faster with a top speed of 368 mph and a cruise speed of 230 mph.

Thanks to the new propulsion system it could also operate a lot higher, and was able to climb to 26,245 feet in 17 minutes and 34 seconds, while its altitude ceiling was 33,465 feet. Furthermore, its maximum range was over 2,000 miles better than the G5N at 4,639 miles.

With an offensive configuration very similar to that of the captured B-17, the G8N was armed to the teeth, possessing six 20 mm cannons placed in powered dorsal, ventral, and tail turrets in addition to four 13 mm machine guns installed in a powered nose turret and flexible waist mounts.

The G8N had a huge defensive array of weapons.
For enemy fighters the G8N would have been extremely dangerous to go near – the armament was deadly.

It boasted significantly more firepower than the G5N, which only had two mounted 20 mm guns and 4 7.7 mm machine guns, and also had the capacity to drop either four bombs of 551 pounds or two 4,409 pound explosives.

An Unfortunate End

After an 18-month period of development, Nakajima revealed the first example of a G8N Renzan in October 1944, which undertook its maiden voyage on October 23rd. Experiencing only minor issues with the turbosuperchargers, the G8N was deemed flightworthy in a battery of tests that were also constantly disrupted by enemy attacks, prompting a further 3 units to be crafted by December 1944, March 1945, and June 1945 at Nakajima’s Koizumi plant.

Yet like many promising aeroplanes past and future, the G8N arrived on the scene just a little too late. By the time it was being rolled out the nature of the war, and Japanese military strategy had shifted dramatically as the Axis powers found themselves increasingly on the defensive.

With the spectre of an Allied victory hanging over them, the Japanese were now fighting for their very survival, making the prospect of a long-ranged offensive craft all but obsolete.

In addition to this, accompanying economic hardship meant that the light alloys necessary to assemble the G8N were becoming incredibly scarce, while any metal the Japanese could get their hands on would be used purely for defensive planes.

The G8N had the hallmarks of good aircraft. She was introduced far too late into the war.
The cockpit of the G8N was fairly large and required a crew of 10 to operate.

On top of this, Japanese manufacturing capabilities had been disabled beyond repair with the bombing of industrial plants by fleets of Boeing B-29s flying in from American airfields located on the Mariana Islands.

Before the discontinuation of production, the Japanese toyed with several other ideas, envisioning that the gigantic G8N could potentially act as a mothership for the Okha 43 Special Attack Bomber, that another version called the Renzan-Kai Model 22 G8N2 could be propelled by Mitsubishi MK9A radial engines and even the possibility that the airframe of a third prototype, the G8N3 Renzan-Kai Model 23, could be made entirely out of steel.

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However after the atomic bombing of Hiroshima and Nagasaki forced the Japanese to surrender, all of these plans would be quickly forgotten. Of the 4 G8N Renzans built, which were never used in a military operation, one was destroyed on the ground following an American air raid, while another was seized and transported back to the US for assessment.

After conducting a series of test flights, evaluators were impressed by the mechanical makeup and performance of the G8N. The qualities of the G8N however, could not save it from humiliation, for shortly after the conclusion of US examinations it was scrapped, with sadly no surviving example left for posterity today. 

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Specifications

  • Crew: 10
  • Length: 22.935 m (75 ft 3 in)
  • Wingspan: 32.54 m (106 ft 9 in)
  • Height: 7.2 m (23 ft 7 in)
  • Empty weight: 17,400 kg (38,360 lb)
  • Gross weight: 26,800 kg (59,084 lb)
  • Max takeoff weight: 32,150 kg (70,879 lb)
  • Powerplant: 4 × Nakajima NK9K-L Homare 24 18-cylinder air-cooled radial piston engines, 1,500 kW (2,000 hp) each for take-off 1,380 kW (1,850 hp) at 8,000 m (26,247 ft)
  • Maximum speed: 593 km/h (368 mph, 320 kn) at 8,000 m (26,247 ft)
  • Range: 3,945 km (2,451 mi, 2,130 nmi)
  • Service ceiling: 10,200 m (33,500 ft)
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