It had big drawbacks, however - it was highly cryogenic, and it had a very low density, making for large tanks. The technology was transferred to the Centaur rocket stage program, and by the mid-1960's the United States was flying the Centaur and Saturn upper stages using the fuel. It was adopted for the core of the space shuttle, and Centaur stages still fly today. In Russia hydrogen fuelled upper stages were designed and developed by the mid-1970's, but the Russians never seem to have found the extra performance to be worth the extra cost. The equilibrium composition of liquid hydrogen is 99.79 per cent parahydrogen and 0.21 per cent orthohydrogen.
Gaseous hydrogen is colourless.
Hydrogen is not toxic but is an extremely flammable material. The flammable limits of gaseous hydrogen in air are 4.0 to 75 volume percent. Hydrogen is produced from by-product hydrogen from petroleum refining and the partial oxidation of fuel oil. The metallic oxides catalyse the ortho-para transformation of freshly liquefied hydrogen. Last update 22 December 2000.