Tokyo tanks
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Tokyo tanks were internally-mounted self-sealing fuel tanks used in the B-17 Flying Fortress bomber during World War II. Although nicknamed "Tokyo" tanks to dramatically illustrate the significant range (approximately 40% greater with combat weights) they added to the B-17, it was also an exaggeration in that no B-17 ever had the range to bomb Japan from any base in World War II.
These fuel tanks consisted of eighteen removable rubberized-compound containers, called cells, installed inside the main wings of the airplane, nine to each side. The main wings of the B-17 consisted of an "inboard wing" mounted to the fuselage and mounting the engines and flaps, and an "outboard wing" joined to the inboard wing and carrying only the ailerons. The Tokyo tanks were installed on either side of the joint (a load-bearing point) where the two wing portions were connected. Five cells, totalling 270 U.S. gallons (1020 litres) capacity, sat side-by-side in the outboard wing and were joined by a fuel line to the main tank delivering fuel to the outermost engine. The sixth cell was located in the space where the wing sections joined, with the remaining three cells located side-by-side within the inboard wing, and these four cells delivered 270 U.S. gallons of fuel to the feeder tank for the inboard engine. The same arrangement was repeated on the opposite wing. The Tokyo tanks added 1,080 U.S. gallons (4090 litres) of fuel to the 1,700 gallons (6440 litres) carried in the six regular wing tanks and the 820 gallons (3100 litres) that could be carried in a tank that could be mounted in the bomb bay, for a maximum toal of 3,600 gallons (13630 litres).
The first B-17s with factory-mounted Tokyo tanks were introduced to the Eighth Air Force in England in April, 1943, with the arrival of the 94th and 95th Bomb Groups, but in June 1943 they began to appear in great numbers as replacement aircraft, and from the beginning of July, 1943, all replacement aircraft were so equipped. All B-17F aircraft built by Boeing from Block 80, by Douglas from Block 25, and by Vega from Block 30 were equipped with Tokyo tanks, and the entire run of B-17Gs had Tokyo tanks.
Although the tanks were removable, this could only be done by first removing the outboard wing panel, and so was not a routine maintenance task. A drawback to the tanks was that there was no means of measuring remaining fuel quantity within the cells. Fuel was moved from the cells to the engine tanks by opening control valves within the bomb bay so that the fuel drained by gravity. Although the tanks were specified as self-sealing, vapor buildup within partially-drained tanks made them explosive hazards in combat.
Source: *Bishop, Cliff T. Fortresses of the Big Triangle First (1986), pp.50-51. ISBN 169487004
