Editing Fu-Go balloon bomb (section)

== Design and development ==
By March 1943, Kusaba's team developed a {{convert|20|ft|m|adj=on}} design capable of floating at {{convert|25,000|ft|m}} for up to 30 hours.{{sfn|Mikesh|1973|p=6}} The balloons were constructed from four to five thin layers of ''[[washi]]'', a durable paper derived from the [[paper mulberry]] (''kōzo'') bush, which were glued together with ''[[konjac|konnyaku]]'' (Japanese potato) 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.{{sfn|Coen|2014|pp=26–27}} The original proposal called for night launches from submarines located {{convert|600|mi|km}} off of the U.S. coast, a distance the balloons could cover in 10 hours. A calibrated timer would release a {{convert|11|lb|kg|adj=on}} incendiary bomb at the end of the flight.{{sfn|Coen|2014|p=19}} Two submarines ([[Japanese submarine I-34|''I-34'']] and [[Japanese submarine I-35|''I-35'']]) 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.{{sfn|Mikesh|1973|p=6}}

Engineers next investigated the feasibility of balloon launches against the United States from the Japanese mainland, a distance of at least {{convert|6,000|mi|km}}.{{sfn|Coen|2014|pp=26–27}} 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 [[Japan Meteorological Agency|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 {{convert|30,000|to|35,000|ft|m}} 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 {{convert|200|mph|kph}}.{{sfn|Coen|2014|pp=20–22}}

<gallery mode="packed">
File:342-FH-3B23426 (18160066205).jpg|Fu-Go carriage, with labeled ring, electrical circuits, fuses, ballast, and bombs
File:342-FH-3B23434 (18160038915).jpg|Top view of carriage assembly, with control device removed
File:342-FH-3B23427 (17537483644).jpg|Altitude control device, with central master [[aneroid barometer]] and backups
File:342-FH-3B23433 (17537463564).jpg|Bottom view of carriage fuses
File:342-FH-3B23428 (18160060525).jpg|Recovered Fu-Go at the moment a blowout plug is detonated
</gallery>

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 barometer]]s calibrated for an altitude between {{convert|25,000|and|27,000|ft|m}}, below which one sandbag was released; the next plug was armed two minutes after the previous plug was blown. A separate altimeter set between {{convert|13,000|and|20,000|ft|m}} 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 (the maximum time allowed by the batteries). 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.{{sfn|Coen|2014|pp=33–40}}

[[File:342-FH-3B23429c (17972344110).jpg|thumb|right|B-Type rubberized silk balloon, with outline of a human for scale]]

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 {{convert|30|ft|m|adj=on}} rubberized silk balloon, designated the B-Type (in contrast to the Army's A-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.{{sfn|Coen|2014|pp=32–33}} 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, Los Angeles|San Pedro, California]], on November 4.{{sfn|Coen|2014|p=42}}

The final A-Type design was {{convert|33|ft|m}} in diameter, and had a gas volume of {{convert|19,000|cuft|m3}} and a lifting capacity of {{convert|300|lb|kg}} at operating altitude.{{sfn|Coen|2014|p=33}} The bomb payload most commonly carried was:

* four {{convert|11|lb|kg|adj=on}} [[thermite]] [[incendiary device]]s, consisting of a steel tube {{convert|16|in|cm}} long and {{convert|4|in|cm}} in diameter with a mechanical impact fuse;
* one Type 92 {{convert|33|lb|kg|adj=on}} high-explosive [[Anti-personnel weapon|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 {{convert|26|lb|kg|adj=on}} incendiary bomb, containing three [[magnesium]] containers of thermite.{{sfn|Mikesh|1973|pp=21, 58-61}}