Origins
The balloon campaign was the fourth attack the Japanese had made on the American mainland. The fūsen bakudan campaign was, however, the most earnest of the attacks. The concept was the brainchild of the Imperial Japanese Army's Ninth Army's Number Nine Research Laboratory, under Major General Sueyoshi Kusaba, with work performed by Technical Major Teiji Takada and his colleagues. The balloons were intended to make use of a strong current of winter air that the Japanese had discovered flowing at high altitude and speed over their country, which later became known as the jet stream.
The jet stream reported by Wasaburo Oishi blew at altitudes above 9.15 km (30,000 ft) and could carry a large balloon across the Pacific in three days, over a distance of more than 8,000 kilometers (5,000 mi). Such balloons could carry incendiary and high-explosive bombs to the United States and drop them there to kill people, destroy buildings, and start forest fires.
The preparations were lengthy because the technological problems were acute. A hydrogen balloon expands when warmed by the sunlight, and rises; then it contracts when cooled at night, and falls. The engineers devised a control system driven by an altimeter to discard ballast. When the balloon descended below 9 km (30,000 ft), it electrically fired a charge to cut loose sandbags. The sandbags were carried on a cast-aluminium four-spoked wheel and discarded two at a time to keep the wheel balanced.
Similarly, when the balloon rose above about 11.6 km (38,000 ft), the altimeter activated a valve to vent hydrogen. The hydrogen was also vented if the balloon's pressure reached a critical level.
The control system ran the balloon through three days of flight. At that time, it was likely over the U.S., and its ballast was expended. The final flash of gunpowder released the bombs, also carried on the wheel, and lit a 19.5 meter (64 ft) long fuse that hung from the balloon's equator. After 84 minutes, the fuse fired a flash bomb that destroyed the balloon.
The balloon had to carry about 454 kilograms (1,000 lb) of gear. At first the balloons were made of conventional rubberized silk, but improved envelopes had less leakage. An order went out for ten thousand balloons made of "washi", a paper derived from mulberry bushes that was impermeable and very tough. It was only available in squares about the size of a road map, so it was glued together in three or four laminations using edible konnyaku (devil's tongue) paste. Hungry workers stole the paste and ate it. Many workers were nimble-fingered teenaged school girls. They assembled the paper in many parts of Japan. Large indoor spaces, such as sumo halls, sound stages, and theatres, were required for the envelope assembly.
Similar, but cruder, balloons were also used by Britain to attack Germany between 1942 and 1944.
Read more about this topic: Fire Balloon
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