Fu-Go balloon bomb
Fu-Go was an incendiary balloon weapon deployed by Japan against the United States during World War II. It consisted of a hydrogen-filled paper balloon in diameter, with a payload of four incendiary devices and one high-explosive anti-personnel bomb. The uncontrolled balloons were carried over the Pacific Ocean from Japan to North America by fast, high-altitude air currents, today known as the jet stream, and used a sophisticated sandbag ballast system to maintain their altitude. The bombs were intended to ignite large-scale forest fires and spread panic.
Between November 1944 and April 1945, the Imperial Japanese Army launched about 9,300 balloons from sites on coastal Honshu, of which about 300 were found or observed in the U.S., Canada, and Mexico. The bombs were ineffective as fire starters due to damp seasonal conditions, with no forest fires being attributed to the offensive. A U.S. media censorship campaign prevented the Imperial Army from learning of the offensive's results. On May 5, 1945, six civilians were killed by one of the bombs near Bly, Oregon, becoming the war's only deaths by enemy action in the contiguous United States. The Fu-Go balloon bomb was the first weapon system with intercontinental range, predating the intercontinental ballistic missile.
Background
The balloon bomb concept was developed by the Imperial Japanese Army's Ninth Technical Research Institute, tasked with creating special weapons. In 1933, Lieutenant General Reikichi Tada started a balloon bomb program at Noborito designated Fu-Go, which proposed a hydrogen-filled balloon in diameter with a time fuse, capable of delivering bombs up to. The project was not completed and stopped by 1935.After the Doolittle Raid in April 1942, in which American planes bombed the Japanese mainland, the Imperial General Headquarters directed Noborito to develop a retaliatory bombing capability against the U.S. In mid-1942, Noborito investigated several proposals, including long-range bombers that could make one-way sorties from Japan to cities on the U.S. West Coast, and small bomb-laden seaplanes which could be launched from submarines. On September 9, 1942, the latter was tested in the Lookout Air Raid, in which a Yokosuka E14Y seaplane was launched from a submarine off the Oregon coast. Warrant Officer Nobuo Fujita dropped two large incendiary bombs in Siskiyou National Forest in the hopes of starting a forest fire and safely returned to the submarine. Response crews spotted the plane and contained the small blazes. The program was cancelled by the Imperial Navy.
Also in September 1942, Major General Sueki Kusaba, who had served under Tada in the original balloon bomb program in the 1930s, was assigned to the laboratory and revived the Fu-Go project with a focus on longer flights. The Oregon air raid, while not achieving its strategic objective, had demonstrated the potential of using unmanned balloons at a low cost to ignite large-scale forest fires. According to U.S. interviews with Japanese officials after the war, the balloon bomb campaign was undertaken "almost exclusively for home propaganda purposes", and the Army had little expectation of its effectiveness.
Design and development
By March 1943, Kusaba's team developed a design capable of floating at for up to 30 hours. The balloons were constructed from four to five thin layers of washi, a durable paper derived from the paper mulberry bush, which were glued together with konnyaku paste. The Army mobilized thousands of teenage girls at high schools across the country to laminate and glue the sheets together, with final assembly and inflation tests at large indoor arenas including the Nichigeki Music Hall and Ryōgoku Kokugikan sumo hall in Tokyo. The original proposal called for night launches from submarines located off of the U.S. coast, a distance the balloons could cover in 10 hours. A calibrated timer would release an incendiary bomb at the end of the flight. Two submarines were prepared and two hundred balloons were produced by August 1943, but attack missions were postponed due to the need for submarines as weapons and food transports.Engineers next investigated the feasibility of balloon launches against the United States from the Japanese mainland, a distance of at least. Engineers sought to make use of strong seasonal air currents discovered flowing from west to east at high altitude and speed over Japan, today known as the jet stream. The currents had been investigated by Japanese scientist Wasaburo Oishi in the 1920s. In late 1943, the Army consulted Hidetoshi Arakawa of the Central Meteorological Observatory, who used Oishi's data to extrapolate the air currents across the Pacific Ocean and estimate that a balloon released in winter and that maintained an altitude of could reach the North American continent in 30 to 100 hours. Arakawa further found that the strongest winds blew from November to March at speeds approaching.
Changing pressure levels in a fixed-volume balloon posed technical challenges. During the day, heat from the sun increased pressure, risking the balloon rising above the air currents or bursting. A relief valve was added to allow gas to escape when the envelope's internal pressure rose above a set level. At night, cool temperatures risked the balloon falling below the currents, an issue that worsened as gas was released. To resolve this, engineers developed a sophisticated ballast system with 32 sandbags mounted around a cast aluminum wheel, with each sandbag connected to gunpowder blowout plugs. The plugs were connected to three redundant aneroid barometers calibrated for an altitude between, below which one sandbag was released; the next plug was armed two minutes after the previous plug was blown. A separate altimeter set between controlled the later release of the bombs. A one-hour activating fuse for the altimeters was ignited at launch, allowing the balloon time to ascend above these two thresholds. Tests of the design in August 1944 indicated success, with several balloons releasing radiosonde signals for up to 80 hours. A self-destruct system was added; a three-minute fuse triggered by the release of the last bomb would detonate a block of picric acid and destroy the carriage, followed by an 82-minute fuse that would ignite the hydrogen and destroy the envelope.
In late 1942, the Imperial General Headquarters had directed the Navy to begin its own balloon bomb program in parallel with the Army project. Lieutenant Commander Kiyoshi Tanaka headed a group which developed a rubberized silk balloon, designated the B-Type. The silk material was an effort to create a flexible envelope that could withstand pressure changes. The design was tested in August 1944, but the balloons burst immediately after reaching altitude, determined to be the result of faulty rubberized seams. The Navy program was subsequently consolidated under Army control, due in part to the declining availability of rubber as the war continued. The B-Type balloons were later equipped with a version of the A-Type's ballast system and tested on November 2, 1944; one of these balloons, which was not loaded with bombs, became the first to be recovered by Americans after being spotted in the water off San Pedro, California, on November 4.
The final A-Type design was in diameter, and had a gas volume of and a lifting capacity of at operating altitude. The bomb payload most commonly carried was:
- four thermite incendiary devices, consisting of a steel tube long and in diameter with a mechanical impact fuse;
- one Type 92 high-explosive anti-personnel bomb, containing a block of picric acid or TNT surrounded by shrapnel rings;
- or alternatively to the anti-personnel bomb, one Type 97 incendiary bomb, containing three magnesium containers of thermite.
Offensive and defenses
Each launch pad consisted of anchor screws drilled into the ground in a circle the same diameter as the balloons. After anchoring an envelope, hoses were used to fill it with of hydrogen while it was tied down with guide ropes and detached from the anchors. The carriage was attached with shroud lines, and the guide ropes were untied. Each launch required a crew of 30 men and took between 30 minutes and one hour, depending on the presence of surface winds. The best time for launches was after a high-pressure front had passed, and wind conditions were best before the onshore breezes at sunrise. Suitable wind conditions were only expected for three to five days a week, for a total of about fifty days during the winter period of maximum jet stream velocity.
The first balloons were launched on November 3, 1944. Some balloons in each of the launches carried radiosonde equipment instead of bombs, and were tracked by direction finding stations to follow their progress. Two weeks after the discovery of the B-Type balloon off San Pedro, an A-Type balloon was found in the ocean off Kailua, Hawaii, on November 14. More were found near Thermopolis, Wyoming, on December 6 and near Kalispell, Montana, on December 11, followed by finds near Marshall and Holy Cross, Alaska, and Estacada, Oregon, later in the month. Authorities were placed on heightened alert, and forest rangers were ordered to report landings and recoveries. The balloons continued to be discovered across North America, with sightings and partial or full recoveries in Alaska, Arizona, California, Colorado, Hawaii, Idaho, Iowa, Kansas, Michigan, Montana, Nebraska, Nevada, North Dakota, Oregon, South Dakota, Texas, Utah, Washington, and Wyoming; in Canada in Alberta, British Columbia, Manitoba, Saskatchewan, and the Northwest and Yukon Territories; in Mexico in Baja California Norte and Sonora; and at sea. By August 1945, the U.S. Army had recorded 285 balloon incidents.
Most U.S. defense plans were only fully implemented after the offensive ended in April 1945. In response to the threat of wildfires in the Pacific Northwest during the summer months, the Army's Western Defense Command, Fourth Air Force, and Ninth Service Command organized the "Firefly Project" with Stinson L-5 Sentinel and Douglas C-47 Skytrain aircraft and 2,700 troops, including 200 paratroopers from the all-black 555th Parachute Infantry Battalion, who were deployed in 36 firefighting missions between May and October 1945. The Army used the U.S. Forest Service as a proxy agency, unifying fire suppression communications between federal and state agencies and modernizing the service through an influx of military personnel, equipment, and tactics. In the WDC's "Lightning Project", health and agricultural officers, veterinarians, and 4-H clubs were instructed to report any new diseases of crops or livestock caused by potential biological warfare. Stocks of decontamination chemicals, ultimately unused, were shipped to key points in the western states. Although biological warfare had been a concern for months, the WDC's plan was not formalized and fully implemented until July 1945. A sub-section of the project, "Arrow", provided for rapid air transportation of all balloon remains to the Technical Air Intelligence Center laboratory in Washington, D.C., for biological analysis. A U.S. investigation after the war concluded there had not been plans for chemical or biological payloads.File:Japanese fire balloon shotdown gun.jpg|thumb|Balloon shot down by P-38 Lightning fighters at Attu, Aleutian Islands, Alaska, on April 13, 1945
Army Air Forces and Navy fighter planes were scrambled on several occasions to intercept balloons, but they had little success due to inaccurate sighting reports, bad weather, and the very high altitude at which they traveled. In all, only about 20 balloons were shot down by U.S. and Canadian pilots. Attempts to track the radiosonde balloons produced 95 suspected signals, but they were of little use due to the very low proportion of balloons with transmitters, and observed fading of signals as they approached. Experiments on recovered balloons in February 1945 to determine their radar reflectivity were unsuccessful. In the "Sunset Project", initiated in early April and fully operational by June, the Fourth Air Force attempted to detect balloons with search radars at ground-controlled interception sites in coastal Washington, but the project detected nothing and was cancelled in early August.
Few American officials believed at first that the balloons could have come directly from Japan. Early U.S. theories speculated that they were launched from German prisoner of war camps or from Japanese-American internment centers. After bombs of Japanese origin were found, it was believed that the balloons were launched from coastal submarines. Statistical analysis of valve serial numbers suggested that tens of thousands of balloons had been produced. The mineral and diatom composition of sand from the sandbags was studied by the Military Geology Unit of the United States Geological Survey, which assessed its origin as Shiogama, Miyagi, or less likely, Ichinomiya, Chiba, only the latter being correct. Aerial reconnaissance of Shiogama in May 1945 showed what was mistakenly interpreted as inflated balloons and a possible launch area at the beach.