The Mamoru was Nakajima’s seventh air-cooled design, which lead to its designation: N for Nakajima, K for air-cooled, 7 for the 7th design, and A for the major model number. Designed by Kiyoshi Tanaka, the Nakajima Mamoru engine was a 14-cylinder, air-cooled, two-row radial engine of 1870 hp. It was one of the largest 14-cyl. engines in the world and was to compete with early 18-cyl engines. The Nakajima model designation for this engine was NAK while it was an experimental project, in service it was known as the NK7, and known as the Ha-103 by the Army and “Mamori” or “Mamoru” by the Navy. According to unified designation code it was Ha-36. The meanings of these two Japanese words are very similar, Mamori translates as protection and Mamoru, translates as to guard, protect, defend and obey.
First run in 1941, the first application of the Mamoru was on the first prototype of the G5N1 Genzan. The G5N1 had been designed on the basis of the Douglas DC-4E as Japan’s first four-engine bomber, and proved to be a disappointment. These problems were compounded by the unreliability of the early Mamoru engines, which had to be de-tuned and left the G5N1 underpowered. The G5N1’s maiden flight was on 10 April 1941, and a further four prototypes were built with the Mamoru. In an attempt to salvage the project, two additional airframes were fitted with 1,530 hp Mitsubishi MK4B 12 “Kasei” engines and redesignated G5N2s. Although the Mitsubishi engines were more reliable than the original Mamoru 11s, further development was halted. Of the six completed Shinzans, four of them (two G5N2s and two G5N1s re-engined with the Kasei 12) were relegated for use as long-range Navy transports under the designation Shinzan-Kai Model 12 Transport G5N2-L.
The Nakajima Mamoru was also used on the Nakajima B6N Tenzan (heavenly mountain) carrier based attack aircraft. The Navy requested this aircraft based on the Kasei, but Nakajima’s Kenichi Matsumara insisted on using their Mamoru. The B6N first flew on 14 March 1941, demonstrating several problems, notably the poor engine reliability. With the delay of 2 years, by 1943 the engine had improved to the point where serial production was allowed to start, but after only 133 B6N1s had been delivered the Navy ordered the switch to the 1,850 hp (1380 kW) Mitsubishi MK4T Kasei 25. The rest of the 1,268 B6N2s were Kasei powered.
Two sub-models were built with minor changes, the Model 11 for the Navy, and the Model 12 for the Army. Both produced 1,850 hp. Production of the Mamoru was ended by the Navy after only a few hundred production examples were built.
Variants:
Mamori 11 NK7A (for IJN aircraft)
1,870 horsepower (1,390 kW) at 2600 rpm (take-off)
1,750 horsepower (1,300 kW), 2500 rpm at 1,400 metres (4,600 ft)
1,600 horsepower (1,200 kW), 2500 rpm at 4,900 metres (16,100 ft)
Mamori 12 Ha-103 (for IJA aircraft)
1,870 horsepower (1,390 kW) at 2600 rpm (take-off)
1,750 horsepower (1,300 kW), 2500 rpm at 1,400 metres (4,600 ft)
1,600 horsepower (1,200 kW), 2500 rpm at 4,900 metres (16,100 ft)
Applications:
Nakajima B6N 1 X 1,870 hp (1395 kW) Nakajima NK7A Mamoru 11
Nakajima G5N 4 X 1,870 hp (1395 kW) Nakajima NK7A Mamoru 11
Nakajima Ki-49 2 X 1,870 hp Nakajima Ha-103
Mitsubishi Ki-67 (prototype) 2 X 1,870 hp Nakajima Ha-103
Specifications:
Nakajima NK7A Mamoru type 11, 12
Type: 14-cylinder supercharged, air-cooled two-row radial piston engine
Bore: 155 mm (6.1 in.)
Stroke: 170 mm (6.7 in.)
Displacement: 44.9 lt (2,740 cu.in)
Diameter: 1380 mm (54.3 in.)
Dry weight: 870-903 kg (1,918-1,991 lb.)
Valvetrain: push rod operated overhead-valve
Supercharger: Two-speed single stage centrifugal
Cooling system: Air-cooled
Reduction gear: 0.578
Power output: takeoff: 1870 hp at 2600 rpm, cruise: 1750 hp at 2500 rpm at 1400 m
Compression ratio: 6.5
