Liquid hydrogen is used as a rocket propellant because it has the highest efficiency relative to the amount used, over any other known propellant. In combination with an oxidiser like liquid oxygen, it’s light and extremely powerful, burning at over 3,000 degrees Celsius (5,500 degrees Fahrenheit). There were, however, significant challenges and hazards to using liquid hydrogen when it was being developed in the Sixties. Both liquid hydrogen and liquid oxygen are cryogenic gases, meaning they only phase-change to liquid at an extremely low temperature. Liquid hydrogen needs to be stored at -252 degrees Celsius (-423 degrees Fahrenheit) and carefully insulated from all sources of heat to prevent it from boiling off and to stop it expanding and exploding the propellant tank. Liquid hydrogen can also seep through tiny cracks between welds in the tank, so some very technical engineering must be employed to create a rocket capable of safely using this kind of propellant.