Here I am as a young man fabricating the structural fuel tanks of aircraft 6061 T-6 aluminum (0.063" wall). Tanks are heli-arc weld with two internal fuel baffles (to minimize fuel sloching during manuvers).
The internal space on outside of tanks will be later filled with hi-density urethane foam as impact protection (kind of like the inside of a motor cycle helmet).
If you want to 'fill it up', thats 35 gallons.
