 Once exhaled, the gas would travel via an external tube to a canister containing carbon dioxide-absorbing chemicals to restore it to a pure oxygen state prior to being re-circulated.The 138 had an internal fuel capacity of 82 gallons, spread across 70 gallons in the lower tank and 12 gallons in the upper tank.A specially-developed fuel, known as S.A.F.4, was used for the altitude record flight, derived from standard grade Shell Ethyl aviation gasoline.