2nd Naval Gunpowder Arsenal Ruins

The 2nd Naval Gunpowder Arsenal (第二海軍火薬廠, Dai-ni Kaigun Kayaku Shō) was the Imperial Japanese Navy's principal facility for the research, development, and mass production of gunpowder, smokeless propellants, explosives, and ammunition. Located in what was then Hiratsuka City and Ōno Town (now both within Hiratsuka City), Kanagawa Prefecture, it occupied approximately 380,000 tsubo (about 1.26 million square meters) of land — making it one of the largest naval industrial complexes in the Kantō region.

Origins: The Anglo-Japanese Powder Venture (1905–1919)

The arsenal's origins trace to 1905, when the Japanese Explosives Manufacturing Company Limited (日本火薬製造株式会社, also recorded as 日本爆発物製造株式会社) was established with its head office in Britain by Sir W.G. Armstrong, Whitworth & Co., Nobel's Explosives Company and a third British firm, given as the Chilworth Powder Company in some accounts and as Vickers in others. The factory was built on a large tract of land in Hiratsuka, Kanagawa Prefecture, with the purpose of manufacturing British MD cordite — a smokeless propellant — to supply the Imperial Japanese Navy. Formal production of cordite began in December 1908.

The company constructed an elegant Western-style building around 1912, with the arched windows and turret fashionable at the time. This building survives today as the "Hachimanyama no Yōkan" (八幡山の洋館), a registered tangible cultural property.

Naval Acquisition and Expansion (1919–1941)

In 1919 (Taishō 8), based on provisions in the original Anglo-Japanese contract, the Imperial Japanese Navy purchased the entire facility for 3.8 million yen and designated it the Naval Gunpowder Arsenal (海軍火薬廠, Kaigun Kayaku Shō), placing it directly under the Naval Ministry (海軍省). The Navy's Technical Department (艦政本部, Kansei Hombu) managed its operations.

Under naval management, the facility expanded dramatically. Research into German-type propellants began in 1920, with experimental production commencing in July 1921 and full-scale production of the 13th Year Type (大正十三年式) propellant starting in April 1924. This represented a significant shift from British cordite technology to German-influenced tubular propellant formulations.

By the late 1930s, seven major plants were operating within the arsenal compound:

• No. 1: smokeless powder (無煙火薬) for guns and rockets
• No. 2: nitrocellulose (gun cotton) and mixed acids
• No. 3: acid manufacture and recovery, coal gas
• No. 4: plant design, installation, repair and utilities
• No. 5: machine-gun propellants and solvent recovery
• No. 6: storage and sorting of smokeless powder
• No. 7: nitroglycerin manufacturing

In 1929, an explosives division was relocated to Maizuru, Kyoto Prefecture, and in 1939 a branch facility was established at Funaoka, Miyagi Prefecture.

Reorganization as the 2nd Arsenal (1941)

On 21 April 1941, as part of a wartime reorganization to expand production capacity, the Naval Gunpowder Arsenal system was restructured into three numbered facilities:

• 1st Naval Gunpowder Arsenal (第一海軍火薬廠) — Funaoka, Miyagi Prefecture
• 2nd Naval Gunpowder Arsenal (第二海軍火薬廠) — Hiratsuka, Kanagawa Prefecture (the original and largest facility)
• 3rd Naval Gunpowder Arsenal (第三海軍火薬廠) — Maizuru, Kyoto Prefecture

The Hiratsuka facility, as the 2nd Arsenal, remained the headquarters and principal production center. It was connected to the national railway network via a dedicated spur line (第二海軍火薬廠専用線) branching from Hiratsuka Station on the Tōkaidō Main Line, which carried raw materials inbound and finished ordnance outbound.

Wartime Production and Workforce

At peak wartime capacity, the arsenal employed thousands of workers including naval personnel, civilian employees, conscripted laborers (徴用工), Women's Volunteer Corps members (女子挺身隊), and mobilized students (勤労動員学徒). Production encompassed the full range of naval propellants and explosives needed for the Pacific War, from torpedo propellants to anti-aircraft ammunition.

The arsenal also conducted research into new propellant formulations, seeking to improve the range and performance of naval guns and torpedoes. Its chemical research capabilities contributed to the broader network of IJN technical facilities in the Kanagawa region, which included the Naval Technical Research Institute (海軍技術研究所) and the Sagami Naval Arsenal (相模海軍工廠) for chemical weapons.

Destruction in the Hiratsuka Air Raid (July 1945)

On the night of 16–17 July 1945, the United States Army Air Forces launched a devastating incendiary attack on Hiratsuka. A force of 138 B-29 Superfortress bombers of the 20th Air Force, flying from the Marianas, dropped 447,716 incendiary bombs (about 1,173 tons) on the city between about 22:30 and 00:35. The naval arsenal was a primary target. The destruction of the city was catastrophic — about 8,000 of its 10,419 houses burned, and the death toll is given variously as 228 (US Strategic Bombing Survey), 237 (prefectural police) or 343 (Hiratsuka City).

Subsequent attacks followed: on 30 July 1945, 16 SB2C Helldivers and 24 F4U Corsairs struck industrial targets, killing 25 workers (mostly conscripted students); and on 13 August 1945, 61 aircraft attacked again. The arsenal was effectively rendered non-operational by war's end.

Postwar Legacy

Following Japan's surrender on 15 August 1945, the 2nd Naval Gunpowder Arsenal was formally dissolved on 30 November 1945 with the abolition of the Naval Ministry. The site was occupied by Allied forces until 1950.

After the occupation ended, the vast arsenal grounds were distributed among multiple entities:

• Yokohama Rubber Company (横浜ゴム) received the largest portion and established its Hiratsuka Manufacturing Plant, which continues to operate today
• Pilot Corporation (パイロット万年筆), Sankyo Pharmaceutical (三共製薬), and other companies received factory sites
• The Ministry of Agriculture established the National Institute of Fruit Tree Science on a section of the former grounds
• Hiratsuka City used portions for civic facilities including City Hall and the Central Library

Surviving structures include the original main gate pillars, a former shrine building (奉安殿), air raid shelter entrances, remnants of the railway spur line, and reinforced concrete industrial buildings. The dedicated railway spur was eventually abandoned; today its route through the city (Nishikichō 1-gō Line / 錦町1号線) survives as a curving city road.

The most significant surviving structure is the Western-style building of about 1912, which served through the naval period as an officers' club (Yokosuka Suikōsha). After the war, Yokohama Rubber used it as a facility building. In 2004, Yokohama Rubber donated it to Hiratsuka City; it was registered as a national tangible cultural property on 23 July 2004. After three years of careful disassembly and relocation, the building was re-erected in Hachimanyama Park (八幡山公園, adjacent to Hiratsuka Hachiman Shrine) and opened to the public in April 2009 as "八幡山の洋館" (Hachimanyama no Yōkan — the Western House on Hachiman Hill).

A memorial stone (平塚海軍火薬廠跡の碑) stands near the former main gate, approximately 1,060 meters northwest of Hiratsuka Station's north exit, beside the Yokohama Rubber plant fence along the main bus route, marking the site where over 380,000 tsubo of naval industrial land once produced the ammunition and propellants that fueled the Imperial Japanese Navy's operations across the Pacific.