Hydrogen is also notoriously low density, even in liquid form. It embritles metals on contact. Hydrogen fires are invisible during the day (if you see rocket en

Hydrogen is also notoriously low density, even in liquid form. It embritles metals on contact. Hydrogen fires are invisible during the day (if you see rocket engineers walking around with broom pointed forward in front of them, that's why). Personally I count on hydrogen for high energy space propulsion (either chemical with oxygen or alone in a NERVA for NTR) as otherwise it's IMHO too much of a headache to work with in practice.