What’s more, the cryogenic LH2 and LOX evaporate quickly at ambient pressure and temperature, meaning the rocket can’t be loaded with propellant until a few hours before launch. While that’s balmy compared to LH2, both propellant ingredients need special handling at these temperatures. To transform oxygen into its liquid state, it is cooled to a temperature of ‑297 degrees Fahrenheit (‑183 degrees Celsius). Oxygen Although it’s denser than hydrogen, oxygen also needs to be compressed into a liquid to fit in a smaller, lighter tank. This means cooling the hydrogen to a temperature of ‑423 degrees Fahrenheit (‑253 degrees Celsius). To get around this problem, turn the hydrogen gas into a liquid, which is denser than a gas. To have enough to power a large combustion reaction would require an incredibly large tank to hold it – the opposite of what’s needed for an aerodynamically designed launch vehicle. Gases – especially lightweight hydrogen – are low-density, which means a little of it takes up a lot of space. Hydrogen, the fuel for the main engines of rockets, is the lightest element and normally exists as a gas.

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