Eric Berger has an article up about what caused NASA’s SLS rocket launch to be scrubbed last week, and why the problems surrounding liquid hydrogen are so hard to solve:
Hydrogen is the most abundant element in the universe, but it is also the lightest. It takes 600 sextillion hydrogen atoms to reach the mass of a single gram. Because it is so tiny, hydrogen can squeeze through the smallest of gaps. This is not so great a problem at ambient temperatures and pressures, but at super-chilled temperatures and high pressures, hydrogen easily oozes out of any available opening.
To keep a rocket’s fuel tanks topped off, propellant lines leading from ground-based systems must remain attached to the booster until the very moment of launch. In the final second, the “quick-disconnects” at the end of these lines break away from the rocket. The difficulty is that, in order to be fail-safes in disconnecting from the rocket, this equipment cannot be bolted together tightly enough to entirely preclude the passage of hydrogen atoms—it is extremely difficult to seal these connections under high pressure, and low temperatures.
NASA, therefore, has a tolerance for a small amount of hydrogen leakage. Anything above a 4 percent concentration of hydrogen in the purge area near the quick disconnect, however, is considered a flammability hazard.